Read Combined%20SCR%20mooring.pdf text version

Brendan Hogg

e-mail: MCS, Galway Technology Park, Parkmore, Galway, Ireland

Annette M. Harte

e-mail: Department of Civil Engineering, National University of Ireland, Galway, Ireland

A Combined Riser Mooring System for Deepwater Applications

For deepwater developments, riser and mooring system requirements become a significant factor in the cost of the overall field development. Therefore, methods for reducing the riser and mooring design requirements and minimizing the cost of these systems become increasingly important. This paper presents the design methodology for a combined riser mooring (CRM) system and demonstrates the feasibility of the system for a test case application in a deepwater development offshore West of Africa. The CRM system offers significant benefits over the independent riser and mooring systems, namely reduced riser dynamics, reduced vessel offsets, a smaller seafloor footprint and system installation prior to the arrival of the FPSO. DOI: 10.1115/1.1834621

Frank Grealish

e-mail: MCS, Galway Technology Park, Parkmore, Galway, Ireland


In deepwater applications, the selection of both the riser and mooring system represents a significant factor in the overall feasibility and cost of the system. The use of spread moored floating, production, storage, and offloading vessels FPSOs with steel catenary risers SCRs are being favored for deepwater applications with mild environmental conditions such as West of Africa. For these proposed applications, SCRs are hung from a spread moored FPSO along the side of the vessel's hull or at the stern of the vessel with the first and second order motions of the vessel governing the design and configuration of the risers system. The spread moored FPSOs with SCRs have a number of drawbacks, some of which are outlined below: · The dynamic motions of the FPSO are transferred directly to the SCR at the hang-off positions. This leads to fatigue life issues in the riser touchdown point on the seabed, which govern the cross-sectional design of the pipes. · With increasing water depth, the length of mooring line required and the cost of the mooring system also increase; hence there are significant savings to be made by optimization of the mooring system. · The potential for interference between the SCRs, mooring lines of the FPU, and mooring lines of neighboring platforms. A typical FPSO is the centerpiece of a deepwater field with wellhead platform and offloading buoy mooring lines in close proximity. · The minimal ability for a spread moored FPU to weather vane into the direction of the environment if the dominant environmental conditions are on the side of the vessel. · Long installation times for SCRs, resulting in a large delay between the arrival of the FPU on site and the first oil date. An innovative way to provide a solution to some of the above shortcomings of the two conventional systems is a concept combining the functional requirements of both systems. This CRM system will integrate the functions of both mooring and riser systems using existing technology in use in the offshore industry. The system will reduce the amount of hardware required to successfully moor the vessel and export production fluid from a deepwater field development. This combined riser mooring system must meet the design requirements for both the mooring and riser systems and solve some of the present shortcomings of the conventional systems.

Contributed by the OOAE Division for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received May 2003; final revision, April 2004. Associate Editor: H. R. Riggs.

The CRM system is illustrated in Fig. 1 attached to the stern of the vessel with conventional mooring lines attached to the bow. The hybrid system is comprised of SCRs connected to a subsea buoy with flexible jumpers located between the buoy and the vessel. This configuration is similar to a number of proposed lazy wave steel riser systems. However, in addition the lazy wave configurations the CRM system has tethers, connecting the subsea buoy to the stern of the vessel. The system utilizes the full potential restoring force capability of the SCRs, which are positioned in tandem with mooring lines of minimal length. Previous proposed lazy wave systems and conventional SCR systems aim to reduce the hang-off angle of the SCRs in order to minimize the forces exerted by the SCRs on the vessel. By increasing the hang-off angle of the SCRs, significant horizontal restoring forces can be provided by the SCRs. The subsea buoy anchored to the seabed provides an interface to which all the components of the CRM system are connected. The buoy provides a decoupling effect between the motions of the vessel due to wave loading and the motions of the SCRs. The design of the subsea buoy has been driven by both operational and installation considerations. The main advantages of the combined riser mooring system are as follows: · A reduced potential for interference between SCRs as the mooring lines of the SCRs are located at the bow or stern of the FPU. · Decoupling of vessel motions transferred to the SCRs result in reduced wall thickness requirements and increased fatigue life in the riser touchdown and hang-off zones. · A reduced seabed footprint compared with convention mooring systems. This can be an important issue in congested field developments. · The ability for a spread moored FPU to weather vane under beam environmental conditions. · Reduced installation times for SCRs, resulting in a short time between the arrival of FPU on site and the first oil date. · All the components required of the system are existing components, which have been installed and proven in various field developments. In order to successfully evaluate the CRM system, a design process must be developed to assess both the riser and mooring requirements of the system.

Design Methodology

The individual components of the CRM system have dual functions, which lead to a certain amount of iteration with the design NOVEMBER 2004, Vol. 126 Õ 273

Journal of Offshore Mechanics and Arctic Engineering Copyright © 2004 by ASME

11. Perform a mean, low frequency, and wave frequency analysis using the mooring model. Establish extreme offsets of the vessel for all loading directions. 12. Repeat Steps 7 and 8 using the new offsets established in Step 11. The offshore analysis software programs, Flexcom-3D and Ariane are used to calculate all system and component responses. Vessel Extreme Excursions. The response of the overall mooring system to mean environmental forces is calculated by a balance of mean wave drift, current and wind induced static forces. The low frequency response of the moored vessel is derived from the time domain integration of governing second order response equations. The wave frequency response is derived using appropriate RAOs combined with wave elevation. Maximum vessel excursions and maximum mooring line tensions are essentially the main design criteria for the mooring system. The maximum excursion is taken as the mean offset plus the combined wave frequency and low frequency vessel motions 2 . Maximum excursion is determined in the following manner:

Fig. 1 Schematic of selected CRM system

X max X mean X LF X max X mean X WF




if X LF if X WF





max sig max max

(2) methodology to develop the optimum system. The design methodology is separated into two parts, which are analyzed with two different analysis tools. The riser design of the CRM is analyzed using the finite element program, Flexcom-3D 1 , and the mooring of the full system is analyzed using the mooring analysis program, Ariane 2 . The conventional mooring lines at the bow of the vessel are not modeled in the FE analysis for the riser design and, similarly, for the mooring design the full CRM system is not modeled. The CRM system is reduced to two nonlinear springs for the purposes of the mooring analysis. The approach to the design and analysis of the CRM system is summarized in the following steps: 1. Establish the suitability of the riser sizes for the functional requirements of the production system i.e., riser sizing and crosssectional design . 2. Verify that the static response of the riser components in the CRM configuration is acceptable. 3. Verify that the static response of the mooring components in the CRM configuration is acceptable. 4. Establish an optimized buoyancy requirement of the buoy system based on a balance of forces for the static system. 5. Develop finite element models for the CRM portion of the entire mooring system. 6. Establish restoring force versus offset relationships for the CRM system for near, far, and cross directions. 7. Establish preliminary maximum offset values as a percentage of water depth for the vessel. Using these offsets, perform current and dynamic analyses for the CRM system in accordance with the extreme load case matrix. Establish the extreme responses for key parameters for all riser and mooring components of the CRM portion of the system. 8. Iterate on the design of the system to ensure that the responses of all components are within allowable limits. 9. Develop a mooring system model using Ariane. The conventional mooring lines at the bow of the vessel are modeled explicitly. The CRM system at the stern of the vessel is modeled using two nonlinear springs. The restoring force effects of the CRM system are accounted for by including the restoring force versus offset relationships developed for the CRM system Step 6 . 10. Establish the contribution to loading on the vessel from the current acting on the CRM portion of the system by performing current analyses using the finite element model. Include this loading contribution as an external force term in the mooring model. 274 Õ Vol. 126, NOVEMBER 2004 where X mean mean vessel excursion X max maximum vessel excursion X LF ( max) probable maximum low frequency vessel excursion X LF ( sig) significant low frequency vessel excursion X WF ( max) probable maximum wave frequency vessel excursion X WF ( sig) significant wave frequency vessel excursion Extreme Case Analysis of Mooring Components. The maximum line tension is the mean offset plus the combined wave frequency and low frequency tensions. Maximum line tensions are determined in the following manner 2 : T max T mean T LF T max T mean T WF where T mean mean line tension T max maximum line tension T LF ( max) probable maximum low frequency line tension T LF ( sig) significant low frequency line tension T WF ( max) probable maximum wave frequency line tension T WF ( sig) significant wave frequency line tension In calculating the low frequency response, a sufficient number of 3 hour simulations each with a different random seed number are performed so that reliable statistical results can be derived. The cumulative average and standard deviation from a number of simulations is monitored until steady values are reached. The wave frequency dynamic line tension response is calculated directly by performing a short duration high frequency simulation incorporating line dynamics in the region of expected peak response. For the CRM system, the vessel extreme offset due to mean loads and second order motions is derived from an Ariane analysis. A Flexcom-3D regular wave dynamic analysis of the CRM is then performed using this extreme offset as a mean offset for the regular wave analysis. The result from this analysis then gives the extreme tensions in the CRM tethers, accounting for all offsets.

max max


sig sig

if T LF if T WF

max max


max max

(3) (4)

Test Case Application

The design criteria used for the evaluation of the CRM system for a test case application in a deepwater field is presented in this Transactions of the ASME

Table 2 Summary of CRM design SCRs and jumpers SCR 12 in. production 10 in. production 12 in. water injection 10 in. water injection 10 in. gas injection Umbilical Buoy particulars Diameter of individual buoys Length of buoy Mass of structure Total buoyancy of structure Net buoyancy Tethers Fig. 2 Schematic of selected CRM system Section Grade Upper tethers Spiral strand Number 6 3 2 1 1 3 Wall thickness 19.1 mm 15.9 mm 19.1 mm 15.9 mm 18.3 mm N/A 6m 36 m 1100 tonne 4173 tonne 3073 tonne Lower tethers Circular braided polyester 4 160 mm 1295 m 7436 kN Upper Chain R4 K4 studless 100 mm 50 m 9864 kN layout at 5 deg

section. The test case application chosen for the detailed evaluation of the CRM system is a full field development in 1400 m water depth in Angola Block 18 area offshore West of Africa. The full field development includes all risers required for the field i.e. production lines, fluid injection lines, and umbilical. Offshore West of Africa is a deepwater field with benign environmental conditions, which favor the use of the CRM system. The configuration chosen for this application consists of a CRM system attached to the stern of the vessel and two groups of conventional mooring lines attached to the bow, as illustrated in Figs. 1 and 2. The CRM and conventional mooring system provide the mooring requirements of the vessel with the CRM system also transferring production and operational fluids between the seabed and the vessel. The vessel considered for the study is a ship shaped very large crude carrier based FPSO, with a two million barrel storage capacity. Relevant vessel dimensions for such an FPSO were used for the analysis along with wind/current coefficients, wave quadratic transfer functions QTFs , areas, and response amplitude operators RAOs . The environmental conditions used for the design of the mooring and riser system are typical extreme swell, sea, wind and current conditions for a West of Africa location offshore Angola. Directional environmental conditions are considered that contributes to the optimization of the combined riser and mooring design. The sea states occurring off the coast of West of Africa are best represented by a JONSWAP wave spectrum. The corresponding associated regular wave conditions for the sea and swell conditions are presented in Table 1 along with the current profiles. The typical current to be used for beam current transverse to the vessel heading sea mooring analyses is 1.0 m/s. The water depth for the area under consideration in this study is 1400 m. Water depth variations due to tidal effects are considered to be minimal and were not considered in this study. The wind data in this region is not expected to have a large effect on the CRM design, with maximum vessel excursion driven by current profiles rather than wind conditions. The following extreme wind data for 100-year conditions was assumed:

Number 6 Diameter 100 mm Length 210 m MBL 8500 kN Conventional mooring lines Section Grade Diameter Length MBL Configuration Lower Chain R4 K4 studless 100 mm 2100 m 9864 kN Middle Wire Spiral strand 110 mm 1425 m 9941 kN 2 3

i Head/quarter conditions--14 m/s ii Beam conditions--16 m/s This is the 10 min average wind speed at 10 m above the MWL.

Global Analysis Results

Selection Configuration. The final configuration selected for the full field development in Angola Block 18 is presented below. The main components of the selected CRM system are as follows: · 13 SCRs and 3 umbilicals terminating at a subsea buoy structure. · 16 jumpers running from the buoy up to the FPSO. · A buoy structure comprised of four individual buoys. · Four lower tethers mooring the buoy to the seabed, supported by suction anchors. · Six upper tethers in two groups anchoring the FPSO to the buoy. · Six conventional mooring lines attached to the bow of the vessel. In order to minimize any adverse effect of the environment, in particular the swell condition, on the FPSO and CRM system, the FPSO is oriented with the bow toward the SSW direction i.e. towards the prevailing swell direction with the initial plane of the CRM configuration oriented in the NNE direction. A schematic of the configuration selected for this project is presented in Fig. 2. The main properties for the CRM system are summarized in Table 2. The finite element model for the extreme analysis was constructed by grouping the SCRs and jumpers into seven equivalent lines for the purposes of construction of a manageable size model, illustrated in Fig. 3. A detailed refinement of the riser layout on the buoy was not undertaken for the purposes of this study as it is more a detailed design issue and not a critical parameter of the CRM system. The modeling of the buoy structure is shown in the inset of Fig. 3. NOVEMBER 2004, Vol. 126 Õ 275

Table 1 Summary of environmental data Current profiles 95% nonexceedence 10-year peturn period Wave data Associated conditions H max (m) T H max (s) Direction Omni SW-SE 95% 4.5 12.2 5 yr 7.0 14.4 Surface 0.30m/s 1.5 m/s 10 yr 7.4 14.7 Mudline 0.15 m/s 0.45 m/s 100 yr 8.7 15.6

Journal of Offshore Mechanics and Arctic Engineering

Fig. 3 Snapshot of FE model of CRM system

The extreme response of the CRM system was analyzed with dynamic time domain regular wave analyses and was carried out for near, far, and cross-loading directions in order to determine the feasibility of the CRM configurations. An iteration on the CRM design under dynamic loading was performed until the relevant acceptance criteria were satisfied. SCRs. The selected SCR designs are based on hoop/collapse/ buckle propagation calculations from API 2RD 1998 3 . The buckle propagation calculations show that buckle arrestors are required. The SCR hang-off angles at the subsea buoy were selected based on the global mooring stiffness of the system and von Mises stress criterion in the risers. The maximum von Mises stress utilization occurring in the intact conditions is 0.63 302.2 MPa in the 10 in. gas injection riser, which is well within the allowable value of 0.8. The largest damaged condition utilization is similarly well under the allowable utilization of 1.0. The variation in SCR angle to the vertical at the connection point for the various dynamic conditions is about 8 deg for the intact cases and 14 deg for damaged cases between the min. near angle and max. far angle . The dynamic amplification factors are relatively small typically less than 1.16 confirming that the dynamic loads on the SCRs are relatively small. Flex Joint. The flex joint connection between the SCRs and the CRM buoy is an important detail and a major design consideration for the system. At the equilibrium position, the angles between the SCRs and the buoy structure vertical axis are 1.4 deg i.e. SCR hang-off angle with respect to the global vertical is 21 deg and the buoy rotation is approximately 20 deg . For the intact dynamic conditions, the maximum variation in flex joint angle is 10.6 deg. Similarly, for the damaged dynamic conditions, the maximum angle variation is 11.6 deg. These angles are largely driven by rotations of the buoy as the dynamic rotations of the SCRs are minimal. One key benefit of the proposed buoy design is that in the near and far offset directions the buoy will rotate due to reduced/ increased tether tensions in a direction that is beneficial to the flex joint, e.g., in the far direction, the SCR is stretched out so its hang-off angle increases. At the same time, the increased tether tension causes the buoy to rotate toward the FPSO, thereby keeping the flex joint rotation relatively small. The angle variations governing the flex joint designs are not necessarily determined by the extreme normal operational conditions. Variations in internal fluid production riser empty were evaluated resulting in flex joint variations of up to 16.3 deg for the mean hang-off angle. For damage buoy conditions i.e., the 276 Õ Vol. 126, NOVEMBER 2004

Fig. 4 Buoy design

flooded compartment locking of the flex joint may occur i.e., the rotation limit reached with the joint designed to absorb the additional bending loads. Buoy Design. The CRM buoy ties all the components of the CRM system together, hence the sizing and orientation of the buoy is a critical design factor. The buoy was positioned below the wave zone at a distance of 186 m from the stern of the vessel to provide suitable mooring stiffness. The net buoyancy of the buoy was 3073 tonnes, which was required to support the SCRs and provide tension to the lower tethers for all extreme and damaged conditions. A suitable length of buoy was selected to allow the numerous SCRs and jumper lines to be arranged to avoid interference issues. In order to have better control over the roll of the buoy, four separate buoys were used with two bulkheads per buoy, as illustrated in Fig. 4. The use of the four separate buoys also removes the problem caused by the damaged buoy conditions. Ballasting and flooding of individual buoyancy compartments is used for the SCR installation procedure. The orientation of 20 deg to the horizontal in the equilibrium position was selected to simplify the installation procedure to be used for the SCRs. The location of the upper and lower tether connections on the CRM buoy are to balance the moments for the SCRs and the upthrust of the buoy and minimize rotation. The range of buoy roll is 22.8 deg for the extreme intact conditions and 30.7 deg for extreme damaged conditions. The yaw and pitch of the buoy is negligible in the near and far directions. In the cross direction the maximum yaw is 22.8 deg for the intact Case, and is 23.1 deg for the damaged case. The critical condition for buoy rotation is the impact of abnormal conditions, i.e. flooding of one or more compartments in the buoy. The maximum roll of the buoy due to damaged conditions is 24.2 deg JumpersÕTethers. Both the upper and lower tethers consist of a group of lines to provide the required level of redundancy for mooring system design. The lower tethers are grouped into two sets of two circular braided polyester lines with allowable factors of safety of 0.48 and 0.64 for intact and damaged conditions, respectively. The upper tethers are grouped into two sets of three spiral strand wire lines with allowable factors of safety of 0.6 and 0.8, respectively. Transactions of the ASME

Table 3 A comparison of CRM with conventional CRM system Extreme response Max. effective tension kN Max. bending moment kN m Fatigue Touch down point years Hang-off location years 2856 102 466.7 10000 Conventional system 4553 131 130.3 4.8

The largest tensions occurring due to environmental loading in the upper tethers is in the far offset direction. A maximum value of 2463 kN for intact and 3031 kN for damaged occurs in this direction. The cross direction is more critical for the lower tethers and yields the largest tensions of 3598 kN for intact and 3739 kN for damaged. All of the effective tension values are below the maximum allowable tensions. In addition, note that in most cases the dynamic amplifications in the tether tensions are relatively small. The jumpers were designed to ensure that their minimum bending radii were within allowable limits and no interference occurred between the jumpers. The nominal hang-off angle of the jumpers to the FPSO in the equilibrium position is 15.5 deg. No bend stiffeners were required at the vessel hang-off. The tension values are not excessive and therefore variations in jumper details do not cause an appreciable change in the global stiffness of the system. Comparison With Conventional. A conventional SCR option was designed for the same deepwater application and the results compared with the CRM system. The wall thickness required for the conventional system is larger than that of the CRM SCRs due to the more onerous fatigue conditions. The conventional SCRs were attached to the side of the vessel and hung in a catenary configuration to the seabed. For the extreme analysis the largest effective tension in the SCRs for the CRM system is 2856 kN occurring in the 12 in. water injection riser. The maximum tension in the conventional system is 4553 kN in the same riser. This is to be expected due to the benign motions of the CRM SCR support point buoy versus the relatively severe dynamic motions of the conventional SCR support point FPSO . Some of the differences in tension are also accounted for by the extra wall thickness used for the conventional SCRs. The rotations of the SCRs relative to the subsea buoy in the CRM system are relatively small at 10 deg. This is compared to the 20 deg for the variation between the vessel and the SCR for the conventional option. The difference is due to the relatively small movements and rotations of the buoy compared with the large dynamic motions of the vessel. Clearly the CRM SCRs is a substantially less onerous application for flex joints than the conventional SCR. The fatigue life of the risers was analyzed at two main points: the riser touchdown point in the near offset direction and the upper connection point in the far offset direction. The effective tension and bending moment variations for the SCRs in the CRM application are less than that of the conventional application. A fatigue life analysis of the risers was carried out for two critical cases using a fatigue life postprocessor, Life-3D 1 . Dynamic analyses were carried out in the frequency domain using the finite element tool, Freecom-3D 4 applying the 95% nonexceedence return period wave Jonswap f p 0.97, a 0.00143 and g 4) from Table 1. The fatigue curve used was the API X'-curve and the stress concentration factors SCFs used in the fatigue life calculations was taken to be 1.23 along the entire SCR. The fatigue life of both systems is presented in Table 3 with the fatigue life of the CRM risers being significantly higher than the conventional SCR applications. Journal of Offshore Mechanics and Arctic Engineering

Fig. 5 Schematic of mooring system model

Mooring Analysis Results

The mooring analysis of the CRM system was carried out using the mooring analysis program, Ariane. Three-hour time domain irregular sea simulations were run, considering both 3D first order motions and low frequency second order effects. The vessel is moored in two different ways. The aft of the vessel is moored using the CRM system and the bow is moored with a conventional 2 3 mooring line configuration. The layout of the CRM system and the mooring lines are shown in Fig. 5 with the CRM system shown as two nonlinear springs and the six conventional mooring lines modeled explicitly. A summary of the results from the extreme mooring analyses is presented in Table 4. The maximum vessel excursions are presented in the near far and cross offset directions, for both intact and damaged mooring conditions most loaded line broken . Plots of in-line and transverse stiffness for the overall system are presented in Figs. 6 and 7. The maximum utilization value for tensions in the mooring lines is 0.45 for the intact system and 0.58 for the damaged system. These utilizations compare well with allowable factors of 0.6 and 0.8. This is with a dynamic amplification factor of 1.2 included in the values for maximum dynamic tension as a percentage of the minimum breaking load MBL . Preliminary line dynamics analyses indicate that the selected value is reasonable. To evaluate damaged conditions, the effects of various combinations of upper and lower tether failure of the CRM system on the mooring performance characteristics are analyzed. The mooring stiffness of the system does not vary with the different damaged tether conditions and therefore there is no change in the global response of the system from intact conditions. The main issues in the evaluation of the damaged conditions are as follows: · Increased tensions in the remaining tethers tether groups were designed for this critical damaged condition . · A small degree of yaw of the buoy caused by asymmetric loading.

Table 4 Summary of mooring results. Note: The far damaged condition is equal to far intact as there is no change in the mooring stiffness of the CRM under damaged conditions. Direction Intact Near Far Cross Damaged Near Far Cross Vessel offset m 16.0 12.8 82.5 55.2 12.8 132.4 Vessel yaw deg 0 0 5.1 1.3 0 3.4

NOVEMBER 2004, Vol. 126 Õ 277

terial characteristics as the CRM system mooring lines. The lines were orientated at 45 deg 5 deg to the bow­aft plane of vessel with a pretension of 2500 kN. Figures 6 and 7 present a comparison between the conventional mooring system and the CRM system for in-line and transverse stiffness characteristics. The in-line stiffness of the full CRM system is stiffer than the conventional system. The full CRM system stiffness in the transverse direction is lower than the stiffness of the conventional 12 line mooring system, which results in offsets that are about 20% larger for the CRM system. The riser section component of the CRM transverse stiffness curve is only 13%, which allows for a certain amount of weathervaning of the vessel in beam seas. This is due to the difference in transverse stiffness being provided between the bow and stern of the vessel.


Fig. 6 In-plane mooring stiffness

The overall conclusions for the evaluation of the CRM system for deepwater applications are presented as follows: 1. The feasibility of using the CRM system as an alternative to conventional riser and mooring systems for deepwater applications from floating productions units has been demonstrated. The CRM system was shown to be a viable solution for the full field development of Angola Block 18. 2. The mooring capability of the CRM system has been shown to be largely consistent with a conventional spread moored system for this application. The key differences being as follows: i. Extreme offsets: The CRM gives smaller offsets in the inline direction and larger in the transverse direction. This is optimal from the viewpoint of the riser system, as the riser system can absorb much larger transverse offsets than inline offsets. ii. Damaged conditions: The CRM offsets for damaged conditions do not increase compared to intact offsets. For the conventional systems offsets typically increase by 50% to 80%. 3. The use of SCRs as part of the CRM system has been shown to represent a very benign application for SCRs, in particular when compared to conventional SCRs hung-off directly from the FPSO. The key advantages for the SCRs in this application are reduced dynamics minimal fatigue issues , allowing the use of a thinner wall pipe. 4. The design of the subsea buoy for the CRM system has been driven by both operational and installation considerations. Four separate buoys supported in a diaphragm structure is selected as an optimum solution for this application with the buoy orientated to minimize roll. The four buoys will each have three compartments, which can be separately independently ballasted/ deballasted to give optimum buoy rotations for all phases of the development and that also give substantial redundancy for buoy damage conditions e.g. one compartment flooded . 5. The analyses performed in this study have shown that the CRM can be installed at either end of the FPSO, i.e. either facing into or away from the predominant environmental loading direction for the West of Africa. 6. The CRM system almost completely decouples the riser construction from the FPSO fabrication, thereby reducing interfaces and scheduling risks. 7. All interference issues with the CRM system can be resolved through optimization of the design. The potential for interference between the SCRs can be resolved through varying the hang-off angles of the risers at the subsea buoy structure. 8. Horizontal movement of the buoy during installation stages does not cause excessive stresses in the SCRs. 9. The study has shown that the installation of the CRM system is feasible without exceeding any design parameters and the system can be installed using standard equipment and procedures. A Transactions of the ASME

The variation in the mooring stiffness of the system due to the internal fluid conditions was a 10%­15% reduction in inline stiffness and a 15% increase in transverse stiffness when considering the production risers empty. The global stiffness of the CRM system was also evaluated for various stages of phased installation of the SCRs. Stiffness Sensitivities. While the CRM system has a high stiffness in the longitudinal direction, the transverse stiffness of the system is low. Most of the transverse stiffness of the overall system comes from the six conventional mooring lines at the bow of the vessel. A number of sensitivity analyses were carried out on the base case system in order to evaluate the maximum transverse stiffness possible for the configuration selected. The following issues were evaluated and optimized for their effect on the transverse stiffness: · Location of the subsea buoy relative to FPSO. · Increasing pretension in six conventional lines thus pulling the equilibrium position of the vessel in the far direction . · Modification of the mooring line configuration for the six conventional lines. Comparison With Conventional. A conventional 12 line mooring system was used as a basis of comparison. The mooring system was orientated in a 4 3 layout with the mooring lines consisting of a chain­wire­chain configuration with the same ma-

Fig. 7 Transverse mooring stiffness

278 Õ Vol. 126, NOVEMBER 2004

key advantage of the CRM system is that it allows for the preinstallation of most components prior to the arrival of the FPSO. This study has achieved a certain level of optimization for the overall CRM system. Another round of iterations on the overall design is recommended to develop a fully optimized design. This evaluation assessed the CRM system in a single configuration. The use of the CRM system in alternative configurations e.g. connected to both ends of the vessel or to the vessel beam and for other field developments needs to be assessed to verify its applicability to a wide range of applications. In addition, there are a number of potential deepwater features/ issues that would need to be assessed with respect to the CRM system, such as the feasibility of incorporating pipe-in-pipe risers into the system or actively heated lines. However, many of these issues apply to general deepwater applications, including conven-

tional systems. Further investigation would be required into the installation of such components and the detailing over the subsea buoy for the CRM system.


The authors would like to acknowledge the contribution of SaiBOS Services for the development of installation procedures.


1 MCS International, 1999, ``Flexcom-3D, Users' Manual,'' MCS International, Galway, Ireland. 2 Bureau Veritas, 1992, ``Users' Guide--Ariane 3.0,'' Bureau Veritas. 3 American Petroleum Institute, 1995, ``RP for Design and Analysis of Station Keeping Systems for Floating Structures,'' API RP 2SK, 1st ed., Washington, DC. 4 MCS International, 1999, ``Freecom-3D, Users' Manual,'' MCS International, Galway, Ireland.

Journal of Offshore Mechanics and Arctic Engineering

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