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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Report No. 338 February 2003



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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Report No: 338 February 2003


Erik Hjelde Bob Hemmings Egil Olsen Ove Tobias Gudmestad Kjell orvald Sørensen Michael Hall TotalFinaElf Exploration Norge AS Shell Exploration ExxonMobil International Statoil ASA Norsk Hydro asa ConocoPhillips Chairman

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


e objective of this document is to present the experienced gained by the industry in the period 1998­2002 in a "state-of-art" review of the technical challenges and other assessment issues considered in order to identify the best disposal option for disused offshore concrete gravity substructures within the OSPAR Maritime Area. OSPAR Decision 98/3 provides the regulatory framework for decommissioning all offshore structures. In respect of gravity based concrete structures the Decision states that "e dumping, and the leaving wholly or partly in place, of disused offshore installations within the maritime area is prohibited", but adds that "...if the competent authority of the Contracting Party concerned is satisfied that an assessment ...shows that there are significant reasons why an alternative preferable to reuse or recycling or final disposal on land, it may issue a permit for...a concrete be dumped or left wholly or partly in place...". e part of the concrete platform where such alternative disposal options may be assessed would be the concrete substructure; ie the load bearing structure supporting the topside facilities. No derogation possibility exists for the topside facilities. ere are altogether 27 concrete platforms located within the maritime area of the OSPAR Convention, in Norwegian (12), British (12), Dutch (2) and Danish (1) sectors of the North Sea. Between the adoption of Decision OSPAR 98/3 and July 2002, decommissioning of 4 concrete platforms has been considered. Related studies have been carried out and completed and they represent most of the knowledge gained by the industry since 1998. e two North Sea operators who have presented decommissioning proposals on behalf of the their co-ventures, have considered the following main disposal options for four disused offshore concrete platforms: · Removal for onshore disposal · Removal for deep water disposal · Partial removal (cut down the structure down to -55m to respect the IMO Guidelines) · Leave in place is report highlights the main findings on the four key elements in the comparative assessment of each disposal option: · Technical feasibility · Safety for personnel · Environmental impact · Cost is review identifies several uncertainties associated with the removal of both first and second-generation concrete gravity structures such that a case-by-case evaluation will be required to assess the specific circumstances for each installation. e first generation of offshore concrete gravity platforms installed in the 1970s were not designed or constructed for future removal operations. Although provisions for removal were incorporated into the design of later, second-generation concrete platforms, these may not be fully effective because the obstacles to and hazards associated with removal were not appreciated. An important development over the period of this review has been the introduction of a comprehensive programme of consultation involving a wide range of stakeholders, experts and other users of the sea to view the question of decommissioning from as many angles as possible. is consultation and engagement process has been pivotal in arriving at balanced conclusions in respect of the major decommissioning activity that has taken place between 1998 and 2002.

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International Association of Oil & Gas Producers

e need for monitoring of concrete substructures left in place is highlighted. Concrete structures left in the marine environment will degrade slowly and may be expected to remain standing for 500 to 1000 years. Shorter-term contamination of the marine environment due to residual oil in storage chambers and pipe-work is not expected to be significant. Future liability is addressed where the responsibility remains with present owners unless otherwise agreed with the regulators. It is particularly the long-term liability that is of concern for both the industry and the authorities.


© 2003 OGP

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Table of contents

1 2 Introduction................................................................................................................................................................ 4 Description of concrete gravity platforms ......................................................................................................... 5 2.1 Design.................................................................................................................................................. 5 2.2 Construction........................................................................................................................................6 2.3 Installation...........................................................................................................................................7 3 Population of concrete gravity platforms............................................................................................................ 8 3.1 Concrete gravity platforms in the OSPAR Maritime Area.................................................................... 8 3.2 Concrete gravity platforms outside the OSPAR Maritime Area............................................................9 4 International regulatory requirements for decommissioning ...................................................................... 10 5 Decommissioning alternatives ..............................................................................................................................11 5.1 Removal .............................................................................................................................................. 11 5.2 Removal for deep water disposal .........................................................................................................16 5.3 Partial removal ....................................................................................................................................16 5.4 Leave in place......................................................................................................................................19 6 Safety.......................................................................................................................................................................... 20 7 Environmental impact ............................................................................................................................................ 22 7.1 Re-float for onshore disposal .............................................................................................................. 22 7.2 Deepwater disposal ............................................................................................................................ 22 7.3 Cutting to -55 metres.......................................................................................................................... 23 7.4 Leave in place..................................................................................................................................... 23 7.5 Long-term fate of concrete structures ................................................................................................. 23 8 Monitoring ................................................................................................................................................................ 24 9 Liability....................................................................................................................................................................... 25 10 Cost............................................................................................................................................................................ 26 11 Decommissioning experience and future plans ............................................................................................... 27 11.1 Recent work on disposal of concrete platforms ................................................................................... 27 11.2 Future decommissioning plans........................................................................................................... 28 12 Public consultation.................................................................................................................................................. 29 13 Conclusions .............................................................................................................................................................. 30 Appendix 1­ Concrete gravity platforms within the OSPAR Maritime Area......................................... 31 Appendix 2­ Concrete gravity platforms outside the OSPAR Maritime Area ...................................... 33 Reference List .......................................................................................................................................................... 34

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International Association of Oil & Gas Producers



In 1996, the International Association of Oil and Gas Producers - OGP, (then the Oil Industry International Exploration and Production Forum - E & P Forum) published a report (E&P Forum report number 240) on decommissioning offshore gravity-based concrete structures, from the perspective of the international regulatory regime in force at that time. At its Ministerial level conference in 1998 Contracting Parties to the OSPAR Convention agreed a new and binding Decision (Decision (98/3) on disposal of disused offshore installations. At the heart of this Decision was the recognition that re-use, recycling or final disposal on land will generally be the preferred option for decommissioning offshore installations. Nonetheless, recognising the particular problems associated with the decommissioning large concrete structures, the decision also set out conditions whereby these structures might be left in place (wholly or partially) or dumped at sea, including a detailed consultation mechanism that would engage all contracting parties. e final decision on decommissioning would still reside with the national competent authority. e objective of this document is to update the earlier 1996 report, taking into account knowledge and experience gained by the industry in the period 1998 to 2002 and in the light of the new regulatory conditions for the North East Atlantic, focusing in particular on the issues and risks associated with the decommissioning options considered.


© 2003 OGP

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

2 Description of concrete gravity platforms

2.1 Design

A concrete gravity platform is one that is placed on the seabed and by its own weight is capable of withstanding the environmental forces it may be exposed to during its lifetime. Most of the platforms are additionally stabilised by skirts that penetrate into the seabed. ese platforms are huge in size and weight. Some of them are among the most impressive structures ever built. e weights of the concrete substructures range from 3,000 tonnes to 1,200,000 tonnes, and support topsides weighing from between 5,000 to 52,000 tonnes. Some of the concrete substructures have oil storage ranging from 400,000 to 2,000,000 barrels (approximately 50,000 to 270,000 tonnes) (see Appendix 1 and 2 for further details). Main purpose of most concrete gravity platforms was to provide storage facilities for oil at the offshore location at a time when no, or few export pipelines were available for transport of oil from the oil fields to shore. e aim was to provide sufficient storage capacity in the platform base storage cells to enable continued production from the field. e stored oil would then typically be pumped from the platform storage cells via an offloading system to shuttle tankers. Concrete structures were also designed to provide sufficient support for topsides loads of more than 50000 tonnes e requirement for new fixed concrete structures with offshore storage capabilities has gradually decreased with the development of offshore pipeline infrastructure and the introduction of new technology including sub sea engineering, flexible risers and based on Floating Production Storage and Offloading installations (FPSOs). One advantage of the concrete gravity based structures compared with conventional piled steel jacket structures, was that they could be floated/towed out to the installation site and installed with the topsides already in place. e installation could thus to a great extent be completed onshore/inshore before tow-out to the field, thereby minimising offshore hook-up and commissioning work. Since the 1970s, several concrete platform designs have been developed. Most of the designs have in common a base caisson (normally for storage of oil) and shafts penetrating the water surface to give support for the topside structures. e shafts normally contain utility systems for offloading, draw down and ballast operations, or they serve as drilling shafts. e most common concrete designs are: · Condeep (with one, two, three or four columns) ­ see Figure 2.1 · ANDOC (with four columns) ­ see Figure 2.2 · Sea Tank (with two or four columns) ­ see Figure 2.3 · C G Doris ­ see Figure 2.4 · Ove Arup ­ see Figure 2.5 e first concrete gravity platform to be installed in the North Sea was a C G Doris platform, the Ekofisk Tank, in Norwegian waters in June 1973. During summer 1975, three other concrete platforms were installed, two Condeeps and another C G Doris platform; all placed in the UK sector of the North Sea. After these first successful installations of concrete gravity platforms, a number of different designs was developed. e last concrete platform was installed in 1999.

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International Association of Oil & Gas Producers

Figure 2.1: A typical Condeep design

Figure 2.2: A typical ANDOC design (Anglo Dutch Offshore Concrete)

Figure 2.3 A typical Sea Tank Design

Figure 2.4

A typical concrete gravity platform designed by Doris Engineering

Figure 2.5

platform where the base is of concrete with storage capacity on which a steel jack-up rig is fixed



e lower part of the concrete gravity structure including the skirts, is built in a dry dock. When the lower part of the caisson or storage tanks had been fabricated and has reached a certain height, the concrete substructure is floated out of the dry dock and moored at an inshore deep-water site where the pouring of concrete continues. As the construction advances the structure is more or less continuously ballasted down to maintain a workable height for slip-forming activities. e outfitting of the shafts then takes place before the deck structure is installed. e topsides on some concrete substructures are installed inshore, in components, by a heavy lift vessel before being towed offshore. On others, the deck structure and modules are installed as a complete unit onto the concrete substructure in sheltered inshore waters. e concrete substructure is ballasted with water so that only about 5 metres of the columns protrude above water. Barges then position the complete topsides over the concrete columns. e concrete substructure is then de-ballasted and gradually the weight of the topsides is transferred onto the concrete substructure.


© 2003 OGP

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

A number of incidents has shown the deep ballasting operation to be very critical as extreme water pressure is applied to the concrete substructure. One concrete substructure collapsed during such an operation in 1991. e implosion that followed as it sank caused the concrete substructure to be completely broken up. Other structures have shown severe cracking without reaching a catastrophic stage. Such uncertainties in question be an important issue when addressing the technical challenges of potential re-floating during decommissioning. ese factors are discussed later in Section 5.1. A distinct benefit of installing the complete topsides with modules on the concrete substructure in sheltered waters is that most of the hook-up and commissioning work is performed before towing the complete platform to its final location offshore. is has meant that the platform could be operational very shortly after it was safely installed.



Concrete gravity platforms installed prior to 1979 were equipped with a simplified installation system consisting of a combined water depletion and grout system. is system was used for drainage of water under the platform and in the skirt compartments during platform installation. Following platform installation, the system was used for placing grout under the platform, thereby securing full contact between the platform underside and the seabed. Water and grout return lines were also installed. ese were used for draining out the displaced water, while injecting grout under the platform and enabled the installation team to check that the grout had been distributed evenly under the platform. e grout thus ensured that the contact pressure was equally distributed over the foundation area. ere is, however, uncertainty as to whether the grout would stick to the underside of the platform during a removal attempt, or whether it would fall off when the platform lifts off from the seabed. A sudden loss of the grout may have an adverse effect on the stability of the platform (see also Section 5.1.2). From 1979, platforms installed in the North Sea (so-called second generation installations) were equipped with a more sophisticated installation system involving separate water removal system for use in the installation phase. is system was not filled with grout during the grouting operation but was sealed off. It was intended that this system could be used to inject water under the platform in a controlled way during a possible re-float operation, in order to assist in loosening the platform from the seabed.

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International Association of Oil & Gas Producers


Population of concrete gravity platforms


Concrete gravity platforms in the OSPAR Maritime Area

e OSPAR region covers the whole of the North East Atlantic area including the North Sea. All together there are 27 concrete platforms in this maritime area (see Appendix 1 for details).

ere are 12 concrete gravity base plat- forms in Norwegian waters in water depths from 70 to 330 metres. e earliest, the Ekofisk Tank, was installed in 1973. e largest concrete platform ever built is the "Troll Gas" platform installed in 1995. e UK sector has 12 concrete platforms, the majority of which were installed in Figure 3.1: Number of concrete platforms in the OSPAR Maritime Area the 1970s. e last concrete structure to be installed was the "Harding" platform in 1995 (concrete base only). Two concrete platforms are located offshore the Netherlands and one offshore Denmark. e "Arne South" platform in the Danish Sector was installed in 1999 and is the last concrete platform to be installed in the OSPAR Maritime Area. Of the 27 concrete platforms in the North Sea, 16 have facilities for oil storage within the base of the structure. Figures 3.1, 3.2 and 3.3 show respectively, the type (in terms of first or second generation), location of the concrete gravity platforms within the OSPAR Maritime area as well as the number in different water depths. e notation "second generation" indicates that removal was addressed as a design condition during design and construction.

Figure 3.3 Number of Concrete Platforms per depth interval in the OSPAR Maritime Area


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area




















Figure 3.2 Locations of Concrete Platforms in the OSPAR Maritime Area


Concrete gravity platforms outside the OSPAR Maritime Area

Concrete has also been used for platform construction in other parts of the world, albeit to a lesser extent than the North Sea: notably in Australia, where three structures were installed in the mid-1990s; the recently installed Malampaya concrete structure in the Philippines; the massive Hibernia platform offshore Canada, and two small structures in the shallow waters of the Baltic. ese latter two platforms at Schwedeneck See are currently being decommissioned, and removal is expected in the near future. Details of the platforms are provided in Appendix 2.

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International Association of Oil & Gas Producers


International regulatory requirements for decommissioning

Decommissioning procedures for disused offshore installations are generally set out in national legislation, with accompanying guideline and practice documents. Internationally, there are a number of agreements relating to aspects decommissioning, principally addressing partial removal and disposal at sea. e IMO Guidelines and Standards for the removal of Offshore Installations adopted by IMO Contracting States in 1989, set out conditions for removal of installations with the aim of protecting navigation and the safety of other legitimate users of the sea. In essence the guidelines suggest that where complete removal is not possible, partial removal should leave an unobstructed water column of 55 metres. e London Convention 1972 (formerly known as the London Dumping Convention) is an agreement that regulates dumping material at sea (including offshore installations). e 1996 Protocol to the London Convention 1972 categorises offshore installations as platforms or other man-made structures at sea, and although the Protocol is not in force, the Contracting Parties to the 1972 Convention have adopted Guidelines for assessing disposal options. In addition to being signatories to the London Convention, States littoral to the North East Atlantic are also signatories to the OSPAR Convention. Annex 3 to the agreement contains the provision relating to the prevention and elimination of pollution from offshore installations. Although this Annex sets out general conditions, subsequent measures agreed by Contracting Parties have tightened the regime as regards disposal at sea. In particular, Decision 98/3 contains a virtual prohibition of disposal for all installations with a limited and small number of exceptions including large concrete structures. Any proposal for disposal at sea (including leaving in place is subject to an extensive international consultation exercise, but with the final decision resting with the national competent authority (taking into account the views of other Contracting States).


© 2003 OGP

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


Decommissioning alternatives

In the specific case of the OSPAR region, while the regulation allows for disposal at sea as a decommissioning option, the option only relates to the concrete substructure. Topsides need to be removed to land unless there are exceptional or unforeseen circumstances or where the topside support structure is an integral part of the sub-structure. is is frequently the case for concrete gravity structures. Any recommendation to dispose of a concrete substructure at sea needs to be supported by a detailed comparative assessment of the disposal options. e following sections set out the main issues that need to be considered in determining the best disposal option for a concrete substructure.



As explained in Section 2.1, the first generation of offshore concrete gravity platforms installed in the seventies were not designed or constructed for a future removal operation. Later concrete platforms were designed with removal in mind, but the extent of the challenges and possible obstacles and hazards that might occur may not always have been fully appreciated in the original design. Hence, the uncertainties identified in the first generation concrete platforms may also be valid for the second-generation concrete substructures.


Removal method

For large concrete gravity platforms, the most likely removal method will, in essence, be to reverse the method of installation. However, there are a number of issues that the installation operation did not need to consider but that would require consideration upon removal. All concrete platforms located in the North Sea today have been installed by controlling the level of water ballast within the concrete substructure. When on location, a careful increase of the water level allowed safe and accurate positioning of the platform. An adjustment of the relative water levels in the cells of the caisson allowed an on-bottom correction to achieve a true vertical position of the platform. On most of the structures significant amounts of cement grout were injected under the base slab of the platforms to ensure a uniform distribution of loads on to the seabed. In principle, a reverse installation could also minimise the offshore work by allowing removal of all the topside facilities to shore. ese can then be removed in a sheltered location where the weather conditions allow a more efficient execution of work. However, studies have shown that weight increases during the operating phase may require a significant amount of the topside loads to be removed before engaging in a re-float operation. is is because limited buoyancy may be available to lift the structure from the seabed. A weight uncertainty also arises due sand produced from the reservoir trapped in the storage cells, the possible adherence of under-base grout and soil, marine growth and the absorption of water in the cement matrix. e exact weight of the topsides may also add to the uncertainties as considerable amount of equipment have been added during an operational life often more than 30 years. To secure an adequate weight tolerance for the re-float operation, a number of offshore lifts may thus be required prior to removal in order to reduce the overall weight. All piping penetrations through the concrete hull below water level have to be closed to ensure a watertight structure. Any excessive leaks will jeopardise the platform's ability to remain afloat in all phases until it is safely located in a dry dock for final deconstruction. is period could last for up to three years after initial removal. Essential equipment required during the re-float phase will be the water ballast systems and pipe connections inside the concrete substructures. Originally, these systems ensured a

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gradual filling of water ballast to ensure a controlled touchdown on the seabed. On the first generation of platforms, these ballast systems where typically only designed as installation aids and not maintained or grouted up after the structure was in place. On some structures it will be necessary to inject water under the base slab to mobilise additional upward force to be able to pull the base skirts out of the subsoil. is water injection has to be carefully monitored in parallel with the water de-ballasting during the re-float operations. For safety reasons it is preferred that the re-float operations are performed with no personnel on the platform. e towing route to shore will have to be carefully evaluated to ensure sufficient draught during the towing operation. Some structures may have such deep draughts that the inshore sheltered areas that they can enter may be limited.


Technical uncertainties

Each of the platform designs described in Section 2 has its own features depending on the service for which they were intended. e feasibility of a removal operation will depend on clarification of a number of uncertainties that will exist, even if the concrete platform was initially designed with future removal in mind. Studies recently undertaken have identified the following main common uncertainties and difficulties related to the removal of concrete gravity base structures. ese are: · Sealing and testing of penetrations · Structural integrity in re-float phase · Under-base grout · Sudden uncontrolled release · Under base injection · Mechanical systems Sealing and testing of penetrations Sealing of penetrations and cracks in the concrete substructures are seen as major concerns. e problems include limited or no access to penetrations and cracks, inability to test a sealed penetration, difficulties in detecting and sealing cracks etc. Conductor penetrations in drill shafts may be particularly difficult to address. Although cracks may have been sealed during the operational phase of the installation, these may re-open and cause leaks during re-floatation and towing operation as the loads change. Structural integrity in re-float phase During the re-float operations the concrete platform may need to be de-ballasted to a greater extent than during the installation. Additional uplift forces to overcome friction and suction in the seabed may be required. It may also be difficult to empty one cell or buoyancy compartment during de-ballasting. is will require additional de-ballasting in the remaining compartments to compensate for the non-emptied cell(s). is, in turn, may give high differential pressures in the compartments, that may lead to total collapse if the structural strength is exceeded. A preventive measure would be to introduce compressed air into the cells. is would assist in maintaining the overall structural integrity and mitigate the stresses in certain structural elements. However, this must be carefully evaluated as it may introduce a risk of overstressing vital structural parts. An excessive "pop-up" to a level where the air pressure exceeds the


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

ambient water pressure could introduce severe structural consequences, as well as being a hazard to personnel and vessels involved in the re-float operation. An excessive differential loading between the cells may cause collapse of internal walls on some types of structures. is concern is also applicable if the platform experiences excessive tilt during the re-float phase. As each individual concrete structure has its own characteristics, a thorough structural analysis checking all applicable load cases will be required to eliminate these uncertainties. e current applicability of the codes used in the original design and any experience gained, have to be duly considered. Over the last 30 years, the design codes have introduced more stringent structural strength requirements. All structural analysis for removal operations should therefore be based on conservative assumptions reflecting any deterioration and any uncertainties that affect the design. e safety factor should not be lower than specified in current design codes for construction, installations and operations. is structural check will also be necessary for second-generation concrete platforms having re-float as a load condition in the original design. Allowance must be made for designs that did not fully recognise the challenges and possible obstacles that might occur during a refloat operations; often taking place over 30 years after installation. Under base grout On some platforms, grout was injected under the slab to ensure a uniform soil pressure after installation. Also, during completion of the production wells, grout was injected and is expected to have been spread underneath and become attached to the slab. Prior to a refloatation there is no method available to assess the amount of grout under the base slab, or whether or not the grout will remain attached to the base. If a re-floatation is carried out and a large amount of grout is attached to the underside, inshore deconstruction is not advisable, since there is no method to remove the grouting from the underside within an acceptable risk. Both mechanical equipment and explosives have been evaluated for use in detaching the grout. However, it should be noted that use of such methods might cause a sudden release of a large amount of grout and cause instability of the substructure causing it to sink. Sudden uncontrolled release After release from the seabed, the concrete platform could have unbalanced buoyancy that could cause an uncontrolled release from the seabed. Uncertainties in platform weight and centre of gravity, soil resistance, under base grout lost before, during or after re-float, and possible soil suction may contribute to unbalanced buoyancy. Some platforms have an accumulation of drill cuttings inside the concrete shafts. Deposits of produced sand in the storage compartments also add to the uncertainty in knowing the exact weight of the structure. is could lead to an unpredicted instability and pitching of the structure after being released from the seabed. Under base injection Injection of water under the base slab will require certain strength in the upper soil layers under the platform. Exceeding this threshold will result in failure of the soil, causing channelling or "piping" thus allowing water to escape preventing a pressure build-up under the base. Placing gravel around the base of the substructure could in some instances reduce the risk for developing channelling in the soil.

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Mechanical systems e de-ballasting operations as well as any under base water injection will require mechanical systems that are proven to be fully reliable in all functions and operations. e original systems are very likely to have deteriorated after many years in seawater unless they have been properly maintained and tested during the in-service life of the platform. Demanding requirements on the durability and reliability of the system were not fully accounted for during the design, as they would stay idle for decades prior to use, without the opportunity to test the system. e original carbon steel piping may, therefore, have to be changed before the system can be used. Part of the piping embedded in concrete may have to be flushed and smaller diameter, flexible or expanding piping inserted into the old and deteriorated pipelines. Prior to the operations, any parts used for removal must be thoroughly inspected, tested and commissioned. However, it may often be difficult to inspect or even impossible to replace these systems. e only alternative is then to install an external ballast piping system linking each buoyancy compartment together that would be located outside the concrete substructure. is will involve additional risks with extensive use of divers. A new buoyancy system would require penetrations to be made in the storage tanks that would introduce potential new points of leakage. An external system would also be exposed to dropped objects and impact from collision with support vessels. Such operations have not been executed before and could add a considerable cost to the project. Methods and procedures need to be developed and tested inshore before a conclusion can be drawn on their feasibility. It is also questionable if such solutions will give the required reliability needed to launch a re-float operation within the acceptance criteria. Case-by-case evaluation Finally, it is important to note that each platform will have its own and unique problems (for example weight increases, stability, cracks, structural strength, high probability of leakage etc), and that each platform therefore should be considered on a "case by case" basis. Only indepth studies for each installation can conclude whether its re-floatation is possible or not. Appropriate risk analysis is a tool that can be used to establish the risk level compared to the acceptance criteria set for similar offshore operations.



A towing operation to a sheltered inshore location needs to be considered before a full removal is considered acceptable. e major differences between an installation tow and a removal tow are related to the risk of: · Grout attached to the underside of the base slab can fall of and hit a live pipeline; · Grout falling resulting in instability of the platform and causing it to sink; · Major leakage may occur in sealed penetrations and cracks, causing the platform to sink during an offshore or an inshore phase of the towing route (it could hit an offshore live pipeline, block the entrance to a harbour etc). Towing points on the concrete platform also need to be thoroughly inspected and tested and, if necessary, replaced before a re-floatation and towing operation is attempted.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


Inshore/onshore deconstruction

e inshore and onshore deconstruction phase for a typical concrete substructure is estimated to take two to four years. erefore, the concrete substructure needs to be kept floating for at least two to three years. e concerns for the inshore deconstruction phase are basically the same as for the re-floatation/ towing operation, however, there are differences, as described below. Detachment of grout from the underside of the base slab while the floating substructure is being cut into small pieces represents an unsafe working site for personnel. Sudden loss of grout is likely to cause instability of the substructure resulting in a tilt, and in a worst case scenario, the sinking of the substructure. If uncontrollable leaks arise due to failure of previously sealed penetrations, in-service deterioration of the piping system and structure, or unpredictable loss of grout and soil from the underside of the base slab, it could have catastrophic consequences resulting in loss of life. If the structure sinks at an inshore location the environmental consequences may be more severe than if it occurs at an offshore location. e increased consequences include the assumption that more fuel will be required onboard the structure to keep the temporary buoyancy system and other temporary systems running required for the deconstruction work, and that the distance from the installation to shore will be only a few hundred metres. On the other hand, it is assumed that any inshore releases can be managed more effectively by use of pumps etc. Concrete substructures that have been used for oil storage would be require cleaning to remove any free oil that could be released prior to its onshore disposal and possibly before any re-float operation is carried out. Of particular concern are the storage cells of the platforms where no access is possible except via a piping system. Concrete is a semi permeable material and it should be assumed that oil has penetrated into the pores of the concrete walls. e extent of oil contamination of the concrete walls inside the storage cells is, however, considered to be relative small as the concrete material is normally very dense. Furthermore, a layer of wax is likely to be deposited on the concrete walls, limiting the oil penetration into the wall. It may be very difficult to remove the oil contained in the concrete pores by water flushing, steam cleaning or other cleaning methods. us the reuse potential of this concrete material may be limited to for example for use as road hardcore or landfill.


Reuse at another location

If a concrete platform can be safely removed from its present location within the acceptance criteria set, a reuse at another location would then be evaluated. However, a number of criteria have to be fulfilled at its new location such as: satisfactory soil condition, water depth, environmental conditions, fulfilling current design codes and level of safety. Reuse of the concrete substructure as, for example bridge foundation or quay support, could be a practical solution compared to an expensive deconstruction work. Each platform would have to be assessed for the particular re-use opportunities that may present themselves.

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5.2 Removal for deep water disposal

5.2.1 Removal method

e activities in this alternative are essentially the same as those discussed in Section 5.1 "Removal". e main difference is that for this alternative the complete topsides (including the main support frame) would need to be removed before the re-float of the concrete substructure takes place. Alternatively, the topsides could be removed when the structure is afloat at an inshore sheltered area, and then the concrete substructure could be towed to an approved deep-water site for disposal. As much of the internal and external steelwork as practicable is likely to be removed for reuse or recycling onshore. Following the re-float operation, the concrete substructure will be towed to an approved deep-water location. By taking water out of the cells and then submerging the substructure by pumping water into the columns an "implosion" could occur, which would effectively demolish the concrete. For some concrete substructures this method would not be possible due to the design features. In those cases it is likely that the complete substructure would hit the seabed and be severely deformed and disintegrate.

5.2.2 Technical uncertainties

e technical uncertainties in the re-float stage are essentially those valid for the removal and onshore disposal alternative described in Section 5.1.2.

5.2.3 Towing operation

e risks associated with the towing operation are the similar to those for towing to an inshore location. However, the towing route to a deepwater location for disposal may be substantially longer than for removal to land and the weather conditions encountered might be more severe. us the length of good weather periods may be critical.

5.3 Partial removal

Partial removal of a concrete substructure represents a removal of parts of the substructure to such an extent that it fulfils the Guidelines given by the International Maritime Organization (IMO), namely to leave a free water column of 55 metres above the remaining structure for safety of navigation (see also Section 4).

5.3.1 Removal method

Mechanical Means is option presupposes that all the topsides and the external/internal steel works are removed and taken to shore for recycling or deconstruction before the deconstruction of the concrete part commences. Offshore deconstruction alternative entails cutting the concrete substructure into pieces at the offshore location. e concrete pieces are likely to be left next to the remaining substructure. Alternatively they may be lifted on to a vessel and transported to shore for recycling or deconstruction. e internal steel outfitting in the shafts would be removed in reverse installation order to the greatest extent possible. However, it may not be possible to remove some of the outfitting before the concrete structure has been deconstructed down to the level of the actual outfitting. e only controlled method of cutting reinforced concrete is by using cutting tools such as diamond wire or saws controlled by divers. Use of explosives has been evaluated, but studies concluded that it could not be the preferred option as it is not possible to guarantee that present methods will successfully cut the heavily reinforced, pre-stressed structure at the first


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

attempt. e environmental consequences (noise and possible disturbance of fish and marine mammals) may also be reasons for not using explosives. Mechanical cutting the concrete shafts could either be done from inside a dry shaft or from the outside. An external cofferdam would be required if making the cuts from the inside to prevent ingress of water. Personnel would be required to operate the cutting machinery inside the shaft. For the concrete substructures of column and caisson types, the shafts can be cut to obtain the required depth. However, if the top of the caisson reaches into the >55-metre zone, parts of the caisson would have to be removed. is would represent extensive additional underwater work. For concrete substructures with no shafts, the preferred cutting method would be to cut the substructure down to -55 metres, piece by piece, either lifting away each piece or toppling them outwards. e actual cutting operations would require extensive underwater works that ideally should be performed by remotely operated means. However, extensive use of divers in various operations would almost certainly be required. Initiating structural collapse is option pre-supposes the use of explosives to initiate structural collapse of the concrete structure. e explosives may be placed on the outer surface and/or the inner surface of the structure. e platform is expected to remain as a "pulverised" heap of concrete and reinforcement on the seabed, and may represent a hazard for bottom trawls. To make the site over-trawlable, the remains of the structure may be re-distributed on the seabed and/or rock may be dumped to cover the remnant structure. Rock dumping may also reduce minor leaching of hydrocarbons to the water column (from residuals attached to the structure and any accumulated drill cuttings). All possible precautions would have to be taken to limit the effect that the explosives would have on fish and other sea mammals present in the area. e time of the year selected for the operation, the type of explosives and the position of the explosives on the structure etc, will be important to limit the effect on the marine environment. However, despite all precautions taken, it is inevitable that some fish would be killed within a few hundred metres of the explosion.

5.3.2 Technical uncertainties

e various methods proposed for cutting the concrete substructures down to -55 metres are considered to be theoretically feasible although there are a number of critical operations that would need to be proven. No experience exists today of cutting such heavily reinforced prestressed concrete structural members under water. e traditional tools used on land such as diamond wire or saw have not been exposed to underwater conditions such as the North Sea. Studies have revealed that prior to launching any offshore works, extensive development and testing of equipment will be required to prove its practical feasibility and efficiency. Diamond wire tool e most likely cutting technique is a diamond wire tool. Different contractors have advanced this as a feasible method. However, the tool will need to be fabricated and tested before a clear conclusion can be drawn on the capability of such a tool to cut reinforced concrete under compression. In the past there have been difficulties with the diamond wire tool, especially if the material to be cut is a "composite" material and under compression. Most of the load-bearing sections

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in any concrete substructure, including the concrete shafts, consist of high strength concrete with an inner and outer dense layer of steel reinforcement and pre-stressing tendons in steel ducts. e pre-stressing tendons ensure that the concrete section remains in compression at extreme wave loads to avoid cracking in the concrete. e pre-stressing tendons were installed in purpose-built ducts in the shafts, tensioned and bonded to the structure by injection of grout in the annulus between the tendons and the duct walls. If the bonding between the cable and the grout is not properly performed, an enormous amount of energy could be released when the pre-stressing tendons are cut. e effect on the concrete of such a release of energy is not fully understood. Another problem, which has been experienced in the past, is controlling the tension in the diamond wire. Any over-tensioning will cause the diamond wire to break. Excess transverse feed velocity of the wire or the presence of vibrations in the tool/ wire could result in over-tensioning the wire. If the wire breaks during the final cuts, the wire has to be cut and abandoned, since the gap created by the wire will close due to shear leg effects or effect of the tension wires. us, a new cut has to start above or below the previous cut. Weaknesses have also been revealed in some of the diamond wire types making them unsuitable for cutting steel material. Diamond saw tool A diamond cutting saw is more likely to be used when access is restricted to only one face of the concrete section to be cut. Studies have shown that the diameter of a diamond saw could reach 3.5 metres to be able to cut structural elements with thickness 80 to 100 cm. is cutting tool would require heavy support to be fixed to the concrete surface to guide the cutting tool in a controlled manner. Jamming of the diamond saw is also very likely for the same reason as described for the diamond wire tools. Explosives e ability of explosives to cut thick (up to one metre) concrete walls effectively underwater with substantial amounts of pre-stressing and reinforcing steel is not well proven and involves many uncertainties. e firing of explosive charges to topple the structures is a "point of no return" and is likely to result in an unplanned situation from which it may be impossible or extremely difficult and dangerous to recover. Explosives may, however, be used to make the final cut to enable the toppling or bending of a cut section outwards to reach the -55 metre requirement. Structural stability For the non-shaft concrete substructures, the cutting operation of structural members will weaken the structural integrity gradually. By removing structural members the ability to withstand wave forces will be reduced. If it is not possible to complete the work in one summer season, it is very likely that the winter storms will deteriorate the structural strength further; to such an extent that it will be hazardous to send divers back to resume the work the following summer. e storage tanks will also be problematic to deconstruct, since there are no practical methods to divide the structure into smaller parts underwater. e other concern with this disposal option is the stability of the section for the period after the final cuts are made until a heavy lift vessel lifts off the section. e cuts have to be planned and performed in such a way, as to maintain the stability of the section as long as possible. us, three or four sections of the circumference of the legs have to remain intact until a sufficient weather window is forecasted. Holes therefore have to be


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

pre-drilled into the concrete walls by divers or remote operated vehicle (ROV) to be able to insert the diamond wire cutting tool and perform the cuts of the sections. As mentioned above, the critical period will be when making the final cuts. If the cutting tool fails during these final cuts the cut section may be lost if the weather worsens.


Leave in place

e leave-in-place alternative presupposes that the topsides are removed and taken to shore for disposal and if considered a hazard, external steelwork would also be removed to shore.


Work to be done

e modules and support frames, forming the topsides, would be removed first. Before removing the deck, the accessible steelwork inside the platform would be removed as far as practicable. On some concrete substructures the support frame consists of concrete beams, often forming part of the main structure. In such cases it is likely that these structural parts would remain with the concrete substructure. Flushing and cleaning of any oil storage tanks would be performed to reduce the content of hydrocarbon and other residuals to a minimum. e internal walls in the storage tanks would not be exposed to the sea outside, but would remain protected inside the storage tanks for natural degradation to take place as the concrete structure slowly deteriorates. Environmental impact assessments are required to demonstrate that any impacts arising are within acceptable limits. e necessary navigation aids would be installed on the substructure in accordance with applicable national and international requirements. e navigation system would be designed in an easily maintained package with back-up systems (for example by means of a helicopter but not dependent on a helicopter deck). A programme for maintaining a reliable navigation system would be designed, agreed with the competent authorities and introduced. Debris around the concrete substructure would be recovered, where practicable and brought to shore.

5.4.2 Technical uncertainties

Removal of topsides would include known technical operations, but could still be very challenging requiring detailed planning and control to prevent major unforeseen events. e hydrocarbon and other residues left in the storage cells of the concrete substructures present an additional challenge. e design allows the cells to be flushed through a complex pipe work system, but on some platforms, this system provides the only access into the cells. Rigorous inspection and conventional cleaning methods by scraping or through use of solvents are either not feasible or environmentally unattractive. Both alternative access and cleaning techniques would need to be developed or a thorough assessment performed to demonstrate the acceptability of any potential impacts to the environment by the gradual release and natural degradation process as the structure slowly deteriorates. See also Section 7.

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Health and safety of the workforce is crucial in any decommissioning work of offshore installations. e level of safety should be the same as during installation and operations and work should be carried out in accordance with the principle that the risk for the workforce should be as low as is reasonably practicable. ere is very little experience of managing hazards and risks associated with offshore (and onshore) decommissioning of gravity concrete structures. In each of the three large-scale removal operations of steel structures conducted in the North Sea, there has been a fatality. ese incidents all occurred when the structures were being dismantled in shore or on land. Nonetheless it is clear that the risks to personnel both in the conduct established operations and arising from the substantial technical and environmental uncertainties (for example cutting, use of divers, lifting, towing) are significant and must be a major factor in defining the best `disposal' alternative for an individual installation. To place the importance of safety in a `Regulatory Context', the UK Health and Safety Executive indicates that the risk of fatality for an individual shall not be greater than 1×10-3 per year (1 in 1000) and shall be as low as reasonable practicable. In practice a personnel risk level considerably lower than this will be sought for in all decommissioning activities in accordance with the principle that risks shall be as low as reasonably practicable. Refloat for onshore disposal ere is no experience to date in relation to removal and onshore disposal of concrete platforms. Evaluations made in the planning of the Ekofisk 1 Disposal [3] and the Frigg Field Cessation Plan show that there is a significant risk to personnel in removing the concrete substructures, even though personnel may not be on the structure during the re-float. If a serious problem developed during the refloat or towing, it would be necessary to undertake remedial works to remove the substructure in a damaged condition. e predicted fatalities in that situation could be considerably higher than predicted for a straightforward refloat operation. Additional risks are introduced if the complete topsides are removed offshore prior to the refloat operation. is risk may be less if the topsides are lifted off in an inshore sheltered area. However, that reduced risk would be offset by an increased risk of having personnel dismantle the concrete substructure whilst floating and dependent on the continued integrity of the ballasting systems for the extended deconstruction period and the handling of material to shore.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Deepwater disposal During the tow out to the deep-water location it is assumed that no personnel would be onboard the platform. Preparations for sinking the concrete substructure are likely to be made with people located on a nearby vessel. However, it may be necessary to put people on board in the event of a failure of the mechanical systems initiating the sinking process. A deep-water disposal of a concrete substructure would eliminate high risk to personnel during inshore and onshore deconstruction phases. Cutting down to -55 metres Studies have shown that cutting the concrete shafts is likely to involve high risk to personnel. Even though much of the underwater work can be done by remotely operated vehicles (ROV), extensive use of divers must be assumed. Diver interventions are likely to be required to reduce the down time. Mechanical failures may require the work to be stopped and the equipment brought up to the supporting vessel for repair. If any unplanned events take place requiring additional works to meet the -55-metre requirement, the risk to personnel would obviously increase. Leave in place e topside removal phase will present certain risks where limited experience is available. e installation of the deck structure with modules was often done by means of a "deck-mating" with limited offshore lifts. Offshore removal cannot be achieved by reversing this process. Removal of exterior steelwork will also expose personnel to risk although remote techniques will be preferred. e impact of cleaning and inspection will need to be addressed as techniques are developed. Ongoing monitoring and maintenance of navigation aids will also need consideration.

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Environmetal impact

Concrete and steel are not intrinsically polluting. With the exception of residual hydrocarbons leaking to the environment, the impacts of decommissioning large concrete gravity structures both at the site of oil and gas production and inshore at dismantling locations will largely relate to physical disturbance, and interference with amenities and other users of the sea.


Re-float for onshore disposal

e environmental impact of the removal with onshore disposal will be most dominant during the inshore and onshore deconstruction. e onshore deconstruction of the huge concrete substructure will cause aesthetic impacts such as visual effects, noise, smell and dust. Noise is considered the most dominant factor. e sources of noise could be: · Chipping of concrete with a hydraulic chisel hammer · Crushing of concrete in a crushing mill · Drilling and blasting concrete · Noise from cranes and diesel engines During the rather long period the concrete substructures may have to remain afloat during deconstruction (two to three years), there will be a risk that the substructure could sink at its inshore location. ere is a high probability of not being able to re-float the substructure subsequently. Environmental studies have shown that unlike steel structures, the significant energy consumption (and consequent discharges of CO2 required to bring ashore and the recover of the steel embedded within offshore concrete substructures, generally exceeds the energy consumption and discharges required to replace that steel using iron ore.


Deepwater disposal

Disposal of the concrete substructures in deep water may cause minor environmental impact due to leakage of oil from temporary tanks used for pumps necessary to control the buoyancy of the structure during re-float, towing and sinking operations. If the concrete substructure has been used for oil storage, residual sludge and other deposits inside the storage tanks may have some local environmental impact at the disposal site. Even though extensive flushing/ cleaning of the storage tanks will have been performed prior to the re-float operation and tow to the deep water site, residual sludge and other deposits with a high wax content will remain inside the tanks. During the sinking process, it is likely that the platform will be more or less pulverised or severely deformed due to overpressure and impact when it hits the sea bottom. e surfaces of the inner storage tanks will immediately be exposed to seawater. However, since the residuals are assumed to be relatively immobile (due to high wax content) and will be contained in pores in the inner walls of the storage tanks, a very slow leaching of hydrocarbons from the surfaces to the seawater is anticipated. A seabed inspection and environmental survey will normally be performed prior to leaving the deep-water site. Deep-water disposal will eliminate major environmental impacts onshore during the deconstruction phase.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


Cutting to -55 metres

is alternative may expose the oil storage tanks to the open sea if their distance below the sea surface is less than 55 metres. Otherwise, the environmental impact would be the same as described below for the leave-in-place option.


Leave in place

Long-term impact on the marine environment from any contents of the concrete substructure left in place for natural decay should be included in the environmental impact assessment. In those cases where concentrations exceed agreed thresholds, preparation for disposal should include measures to remove them or to reduce the quantities of these contaminants to an acceptable level. Flushing and cleaning of concrete substructures used for the storage of crude oil may be required to reduce the content of hydrocarbon and other contaminants to an acceptable level. Residual quantities of oil will remain adhered to the internal walls of the concrete storage tanks and will not be exposed to the sea until the structure eventually breaks up. Leaving a concrete substructure in place may limit the fishing activity in the vicinity of the substructure. If debris in the area around the substructure is recovered, the chances of snagging fishing gear should be considerably reduced. Removal of external steelwork on the substructure will reduce debris littering the nearby seabed in future.


Long-term fate of concrete structures

Ultimately all the components and contents of a concrete substructure dumped at sea, partially removed or left in place will corrode, decay, disintegrate and collapse onto the seabed. Studies on long-term stability [12] have considered the long-term effects of seawater and pressure on concrete and concrete strength. Other aspects considered have been the mechanisms associated with corrosion of reinforcement and bacterial attack in compartments where hydrocarbons are stored. e overall conclusion of these studies is that concrete is a very durable material where changes are measured in hundreds of years. Initially concrete cover in the splash zone (the sea surface open to the action of wave erosion) is likely to break away from the reinforcement after 100 years, but the remaining strength will support the legs for several hundred years more until leg collapse is assumed to occur after 500 to 1000 years. e base cells could remain largely intact unless breached by falling debris over 1000 years and taking considerably longer to substantially disintegrate. Once surface navigation aids can no longer be maintained, other electronic or physical means of marking the remains would need to be considered.

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Concrete substructures left in place will be equipped with navigation systems that fulfil both national requirements as well as the International Maritime Organization requirements to secure safe navigation for users of the sea. e navigation aids will be designed to ensure a high level of reliability. ey will incorporate back-up systems should be serviced at regular intervals. To assist fishermen, some operators may introduce the position of the concrete substructure into the "Fish SAFE" programme, presently in operation in the UK. Regular surveillance would be carried out to check that the navigation aids are operational. It is envisaged that the navigation aids will be designed in such a way as to allow them to be changed out from a helicopter, thus obviating the need to man the platform for this purpose. e responsibility for the maintenance of the navigation aids remains with the owners, unless otherwise agreed with the authorities. During the regular surveillance of the navigation aids it would be appropriate to make a visual inspection of the general condition of the concrete substructure visible above the water surface. Any unexpected deterioration should be evaluated to check if it represents any hazard to the users of the sea. For structures dumped at sea, occasional monitoring may be required to confirm the location and condition of the structure on the seabed.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area



e owners of installations at the time of decommissioning will normally continue to be the owners of any residues, unless otherwise agreed with the authorities. e owners (in most cases the licensees to a production licence) will be jointly and severally liable for damage caused wilfully or inadvertently in connection with a disused facility left in place. Any claims for compensation by third parties arising from damage caused by any remains will be a matter for the owners and the affected parties and will be governed by the general law. Given the long term over which concrete structures are likely to persist in the marine environment after decommissioning, there are unresolved considerations concerning liability that require resolution.

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e cost of bringing a concrete substructure to shore for reuse represents a considerable proportion of the total cost of decommissioning. is needs to be considered in an economic evaluation considering the benefit to society of reusing the substructures, for example as bridge foundations. Significant economic risks would arise during a re-float operation due to the uncertainties involved. is is particularly the case for the first generation concrete installations that were not designed for removal, but could be valid also for the second generation platforms as the challenges of full removal were not properly understood during the design. e cost of cleaning up the seabed after structural failure of the installation, during re-float or towing is likely to be extremely high. More than half of the cost of a decommissioning event may be expended before obtaining sufficient confidence that a successful re-float can be performed within the set acceptance criteria. is will also be reflected in the cost for deepwater disposal, even when the cost of deconstruction inshore is not incurred. ere will also be cost associated with installation and maintenance of navigational aids if a structure is left in place. e cost associated with leaving a concrete substructure in place will be related to cleaning of the facilities of hydrocarbons if the substructure has been used for oil storage. External steelwork attached to the concrete structure is likely to be removed. e cost estimates presented for the Ekofisk Tank and the three concrete substructures on the Frigg Field predict significant cost levels for removal and onshore deconstruction of concrete substructures. ey vary from about 2000 MNOK or 155m to 4000 MNOK or 310m (assuming an exchange rate of 12.90 NOK per ) depending on the type of installation in question. e cost of removing the topside facilities including the support frame, either offshore or inshore is an additional cost.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


Decommissioning experience and future plans

11.1 Recent work on disposal of concrete platforms

A number of studies has been undertaken place since 1998 when OSPAR Decision 98/3 was introduced. Extensive studies have investigated the feasibility of removing concrete gravity based substructures. e Cessation Plans for the Ekofisk I (operated by Phillips Petroleum Company Norway in Stavanger) and the Frigg Field (operated by TotalFinaElf Exploration Norge AS, in Stavanger), where there are respectively one and four concrete substructures, have both been subject to detailed assessments in accordance to the framework given in Annex 2 of OSPAR Decision 98/3. Further information about these two cessation plans with comprehensive reference lists of performed studies can be found on the following web sites: · Ekofisk 1 Cessation: · Frigg Field Cessation: ese concrete substructures represent typical offshore concrete gravity platforms in operation in the North Sea today. e results of these in-depth studies have identified key problem areas related to each of the above-mentioned platforms, and have provided valuable input to this report. e two fields are located as follows On the Norwegian Continental Shelf

Ekofisk Tank GBS, located on the Ekofisk Field [3],[4] Frigg/TCP2, located on Frigg Field [5]

On the UK Continental Shelf

Frigg/CDP1, located on Frigg Field [5] Frigg/TC1, located on Frigg Field [5]

As a result of their studies, the two operators of the four disused concrete platforms have on behalf of the owners submitted a recommendation to the competent national authorities to leave the substructures in place. Norway has performed the consultation process requested by OSPAR regarding leaving in place the Ekofisk Tank GBS with its protective barrier. OSPAR Contracting Parties did not raise any significant objection. e Norwegian Storting (Parliament) has given the final approval for leaving in place the Ekofisk Tank with its Protective Barrier. e three concrete substructures on the Frigg Field are at present under consideration by the Norwegian and UK authorities.

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Another important source of knowledge has been obtained through a number generic studies. e most recent include: In Norway Summary report for Phase I and II: "Removal of Offshore Concrete Structures", rev. 4, dated 16.01.02, Dr. tech. Olav Olsen, Oslo [6]. In UK Joint Industry Project: UKCS Decommissioning Study", report No. 4017-ER, dated 10 January 2002, W.S. Atkins, Aberdeen [7]. e Dr Tech. Olav Olsen study looks at re-floatation and onshore deconstruction of specific concrete installations. e WS Atkins study looks at different disposal options such as leave in-place, partial removal etc., including safety, environmental and technical issues related to the different options.

11.2 Future decommissioning plans

It is difficult to predict the exact time when an offshore installation will be decommissioned. e main uncertainties are often the reservoir behaviour towards the end of production, as well as the oil and gas price. An alternative use for the platform could also prolong its operational lifetime. At the time of preparation of this report, it expect that the major phase of decommissioning will take place between 2010 and 2020, but some structures are designed for operation until at least 2050 (see Appendix 1).


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area


Public consultation

e decommissioning of offshore oil and gas platforms, including gravity based concrete structures, is controlled by a regulatory process set out by the relevant national Governments having offshore activities in the OSPAR Maritime Area. For example the requirements for public consultations in Norway and UK have subtle differences but many of the key principles are common (see references [9], [10] and [11]). Part of this regulatory process includes the statutory consultation of various parties for their views on the recommended disposal option. It has also become industry practice to go beyond what is required by regulation. A much broader range of interested parties is invited to comment at an early stage when disposal options are being developed. e stakeholders' views are sought on issues raised and on how the assessment is conducted. e industry has seen the importance of an open, transparent and inclusive decision-making process since the Brent Spar incident in 1995. All recent large-scale platform decommissioning now follow the pattern where technical options are developed in parallel with a dialogue and consultation process with a wide group of stakeholders. During the process of establishing a recommended disposal option for both the Ekofisk Tank and the Frigg concrete platforms, an extensive communication strategy towards the various stakeholder groups was adopted. e principle was to invite the stakeholder participation at an early stage of the process. After having identified the stakeholders with an interest in the decommissioning process, they were asked to comment on development of scope of work and raise any issues or concerns they would wish to see addressed. A number of additional studies were initiated as a result of constructive proposals received, which are now part of the respective Cessation Plans presented to the Authorities. Up to two or three years may pass before a recommendation for disposal can be presented and the stakeholders should be kept engaged and informed throughout this phase. A variety of tools to communicate and involve interested parties may be used including meetings, letters, websites, telephone calls, information bulletins, interactive events and presentations. Offshore trips may be organised to allow the stakeholders to obtain an impression magnitude of the structures and the challenges in decommissioning an offshore installation. When a recommended disposal option has emerged, further contact with the stakeholders should be made explaining the reason behind the recommendation. e commitment to keep in close contact with the Stakeholder groups does not stop when a Cessation Plan has been submitted to the authorities. e intentions should be to keep the stakeholders informed about the progress until the approved decommissioning programme is completed. Figure 12.1 illustrates that practice for public consultation adopted for the first four decommissioned concrete installations in the OSPAR Maritime Area.

Figure 12.1: Principles adopted for recent Public Consultation of Concrete Gravity Platforms in the North Sea

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In the period 1998 to 2002, detailed consideration has been given to the decommissioning of large concrete gravity based platforms in the North Sea. ese considerations have drawn extensively upon the numerous studies as well as evaluations undertaken as part of the decommissioning projects for the Ekofisk 1 and the Frigg Field. ese studies have indicated: · Each concrete gravity platform is unique and, as such, decommissioning of concrete gravity platforms needs to be considered strictly on a case-by-case basis. Individual concrete substructures have their own particular history and design features, and will require specific studies to investigate the issues and risks associated with the different decommissioning alternatives. · e first generation of offshore concrete gravity platforms installed in the 1970s were not designed or constructed for future removal operations. Although provisions for removal were included in the design of later concrete platforms, it appears that these may not be fully effective because the obstacles and hazards were not fully recognised. Hence, the uncertainties identified in first generation concrete platforms may also be applicable to the second-generation platforms. · Uncertainties associated with decommissioning include: structural integrity of the concrete installation when it is released from the seabed; weight and buoyancy of the re-floated structures; safety and issues associated with-long term liability. · Effective consultation mechanisms have been developed to engage stakeholders and other users of the sea in considering the options for decommissioning. · A comprehensive environmental impact assessment (EIA), undertaken by independent parties, is a vital element when considering the implications of different disposal alternatives. e environmental impact assessment should include consideration of the long-term impact on the marine environment from any contaminants that may be left in the substructure. It is important to allow the stakeholders to review and comment upon both the proposed scope of work for the EIA and the subsequent outcome from the assessment. · Concrete structures left in place in the marine environment are extremely durable, will degrade very slowly and may be expected to remain standing for 500 to 1000 years. · Contamination of the marine environment in the vicinity of the decommissioned installation is not expected to be significant, especially given strict controls on cleaning during decommissioning. · Costs of decommissioning will be significant irrespective of the ultimate outcome of the consideration of a full range of options. For example, the cost of removal and onshore deconstruction of a concrete platform is estimated to be in the range 2000 MNOK/ 155m to 4000 MNOK/310m, depending on the type of platform (excluding the cost of removal and disposal of the platform topsides). More than half this cost may be expended before obtaining sufficient confidence that an operation to re-float the substructure would be successful.


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Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Appendix 1

Concrete Gravity Platforms within the OSPAR Maritime Area

Platform function Oil Storage Operator Water depth (m) 70 Instal. date 1973 Topsides weight (Te) 33,400 Substructure weight (Te) incl. ballast 273 700 896 900 229,200 254,000 651,000 583,500 784,000 208,000 320,000 434,000 358,000 788,000 661,500 Oil storage (bbl) 1,000,000 Designed ror removal no Planned decom date 1999

Concrete gravity based structures in the Norwegian Continental Shelf

Field Type

Ekofisk Tank+ Protective barrier Frigg TCP2 Statfjord A Gullfaks A Gullfaks B Gullfaks C Draugen Oseberg A Statfjord B Statfjord C Sleipner A Troll Gas


Phillips Norway TotalFinaElf (Norway) Statoil Statoil Statoil Statoil Shell (Norway) Norsk Hydro Statoil Statoil Statoil Statoil

Condeep Condeep Condeep Condeep Condeep Condeep Condeep Condeep Condeep Condeep Condeep

Production Production/ Drilling/Quarter Production/ Drilling/Quarter Drilling/Quarter Production/ Drilling/Quarter Production/ Drilling/Quarter Production/ Quarter Production/ Drilling/Quarter Production/ Drilling/Quarter Production/ Drilling/Quarter Production/ Drilling/Quarter

103 145 134 142 217 250 109 145 145 83 330

1977 1977 1986 1987 1989 1993 1988 1981 1984 1992 1995

22,900 41,300 47,500 27,000 52,000 28,000 37,000 42,200 48,100 37,000 25,000

no 1,200,000 1,195,000 no 2,000,000 1,400,000 no 1,900,000 1,900,000 no no

no no yes yes yes yes yes yes yes yes yes

2004 2010 2016 2016 2016 2016 2020 2010 2014 2035 2046

Concrete gravity based structures in the UK Continental Shelf

Field Type Platform Function Production/ Drilling Production Drilling/ Production Drilling/ Production Drilling/ Production Drilling/ Production Drilling/ Production Production/ Drilling Production Drilling/ Production Drilling Production Current use: Riser platform Operator Water Depth (m) 98 103 151 135 150 139 141 142 43 110 Instal. date 1975 1976 1977 1978 1978 1975 1978 1976 1989 1995 Topsides Weight (Te) 4,850 7,840 19,294 39,000 25,678 23,424 29,874 23,097 6,250 23,000 Substructure Weight (Te) incl. Ballast 415,700 162,000 228,611 584,000 294,655 165,664 287,542 177,809 58,500 134,300 Oil Storage (bbl) no no 838,200 1,000,000 1,000,000 1,100,000 600,000 1,100,000 no no Designed for Removal no no no no no no no no yes yes Planned decom date 2004 2004 2009 2009 2010 2011 2011 2011 2014 2015

Frigg CDP1 Frigg TP1 Dunlin A Ninian Central Cormorant A Brent B Brent C Brent D North Ravensburn Harding (34m base caisson) Beryl A MCP01

Doris Sea Tank Andoc Doris SeaTank Condeep SeaTank Condeep Arup Technip

TotalFinaElf (Norway) TotalFinaElf (Norway) Shell Kerr-McGee Shell Shell Shell Shell BP BP

Condeep Doris

ExxonMobil TotalFinaElf (UK)

117 94

1975 1976

20,000 13,000

494,000 376,000

900,000 no

no no

2018 2020

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Concrete gravity based structures in Denmark and Netherlands

Field Type Platform Function Drilling/ Production Operator Water Depth (m) 61 Instal. date 1999 Topsides Weight (Te) 7,100 Substructure Weight (Te) incl. Ballast 100,000 Oil Storage (bbl) 550,000 Designed for Removal Planned decom date 2011

South Arne

Blocks 5604/29 + 5604/30 Denmark Block F/3 Netherlands

Amerada Hess Denmark NAM Netherlands


Drilling/ Production 71.4x81.4m concrete caisson Wellhead




49 200 (excl. steel columns)





Block Q/1 Netherlands

Unocal Netherlands



650 including legs





Note: SouthArne: Halfweg:

Concrete Gravity Base with a steel lattice drilling tower Concrete base with a four leg jack-up which can be disconnected and refloated


© 2003 OGP

Disposal of disused offshore concrete gravity platforms in the OSPAR Maritime Area

Appendix 2 Concrete Gravity Platforms outside the OSPAR Maritime Area

Concrete gravity based structures outside the OSPAR Maritime Area

Field Type Platform function Operator Water depth (m) 61 Instal. date 1996 Topsides weight (Te) 800 Substructure weight (Te) incl. ballast 44,200 Oil storage (bbl) Designed ror removal Planned decom date


Gippsland Basin, SE Australia Gippsland Basin, SE Australia WA-14-L - NW Shelf - Western Australia Baltic Sea, DCS Baltic Sea, DCS Offshore Newfoundland Production

ExxonMobil Australia ExxonMobil Australia ExxonMobil Australia RWE-DEA Germany RWE-DEA Germany Mobil Canada

West Tuna











Schwedeneck-See Schwedeneck-See Hibernia

Production Production Drilling/ Production

26 16 80

1984 1984 1997

1,300 1,300 37,000

16,000 14,000 900,000

no no 1,300,000

Yes Yes

2002 2002

© 2003 OGP


International Association of Oil & Gas Producers

Reference List

1 2 3 4 5 6 7 8 9 10 11 12 OGP (EX. E&P FORUM) REPORT: "Decommissioning of Concrete Gravity Based Structures" Report no. 10.13/ , 240, June 1996 OSPAR Decision 98/3 on the Disposal of Disused Offshore Installations, issued in July 1998. Ekofisk 1 Disposal: Impact Assessment, Environmental and Societal Impacts, dated 22 October 1999. Ekofisk Tank Substructures, A summary of Disposal Option Assessments, dated 22 March 2001. Frigg Field Cessation Plan, Second Draft, dated November 2001. Summary report for Phase I and II: "Removal of Offshore Concrete Structures" , rev. 4, dated 16.01.02, DR. TECH. OLAV OLSEN, Oslo Joint Industry Project: UKCS Decommissioning Study", report No. 4017-ER, dated 10 January 2002, W.A. ATKINS, Aberdeen "Guide to the classification of environmental quality in ords and coastal waters" , issued by the Norwegian State Pollution Agency, SFT 97.03. Norwegian Act of 29 November 1996 No. 72 relating to petroleum activities e United Kingdom Petroleum Act 1998 "Guidance Notes for Industry - Decommissioning of Offshore Installations and Pipelines under then Petroleum Act 1998" issued in 2000. , "Durability of high-strength offshore concrete structures" , presented at the 5th International Symposium on Utilisation of High strength/high performance Concrete, June 1999, Sandeord, Norway, by SIDSEL M. BECH, DR. TECH. OLAV OLSEN, NORWAY AND JAN ERIK CARLSEN, Selmer ASA, Norway.


© 2003 OGP

What is OGP?

e International Association of Oil & Gas Producers encompasses the world's leading private and state-owned oil & gas companies, their national and regional associations, and major upstream contractors and suppliers.


· To work on behalf of all the world's upstream companies to promote responsible and profitable operations. Mission · To represent the interests of the upstream industry to international regulatory and legislative bodies. · To achieve continuous improvement in safety, health and environmental performance and in the engineering and operation of upstream ventures. · To promote awareness of Corporate Social Responsibility issues within the industry and among stakeholders.


· To improve understanding of the upstream oil and gas industry, its achievements and challenges and its views on pertinent issues. · To encourage international regulators and other parties to take account of the industry's views in developing proposals that are effective and workable. · To become a more visible, accessible and effective source of information about the global industry - both externally and within member organisations. · To develop and disseminate best practices in safety, health and environmental performance and the engineering and operation of upstream ventures. · To improve the collection, analysis and dissemination of safety, health and environmental performance data. · To provide a forum for sharing experience and debating emerging issues. · To enhance the industry's ability to influence by increasing the size and diversity of the membership. · To liaise with other industry associations to ensure consistent and effective approaches to common issues.

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