IBP2082 08 INTEGRATED TLWP-FPSO SOLUTION FOR …

_____________________________ 1 Product Development Mgr. – SBM Atlantia 2 Lead Ocean Engineer – SBM Atlantia 3 Lead Structural Engineer – SBM Atlantia

IBP2082_08 INTEGRATED TLWP-FPSO SOLUTION FOR DEEPWATER

FIELD DEVELOPMENT OFFSHORE BRAZIL Neil Williams1, Homayoun Heidari2, Sean Large3

Copyright 2008, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2008, held between September, 15-18, 2008, in Rio de Janeiro. This Technical Paper was selected for presentation by the Technical Committee of the event according to the information contained in the abstract submitted by the author(s). The contents of the Technical Paper, as presented, were not reviewed by IBP. The organizers are not supposed to translate or correct the submitted papers. The material as it is presented does not necessarily represent Brazilian Petroleum, Gas and Biofuels Institute’ opinion, nor that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2008 Proceedings.

Abstract A development strategy for some deepwater fields offshore Brazil consists of a TLWP (Tension-Leg Wellhead Platform) connected by a fluid transfer line to a nearby FPSO. The TLWP provides dry tree riser support, drilling capability, manifolding, test separation, and multiphase pumping, while all other functions including full processing, accommodations, gas compression, power generation, water and chemical injection, storage and offloading, and gas export are provided by the FPSO. In one such scenario, the TLWP and FPSO could be connected using SBM’s GAP mid-water fluid transfer line technology, with SBM Atlantia’s FourStarTM hull concept for the TLWP. The FourStarTM is designed to be quayside- or floatover-integrable, thus eliminating the need for a heavy lift installation vessel; while being structurally and hydrodynamically superior to other TLP alternatives. This paper presents a description of the technical development of the FourStarTM TLP and an overview of the GAP technology, and discusses the characteristics of an integrated TLWP-GAP-FPSO field development solution for a typical application offshore Brazil.

1. Introduction An integrated field development solution consisting of a TLWP connected by a mid-water flowline to a nearby FPSO, is shown in Figure 1. The TLWP provides dry tree riser support, drilling capability, accommodations, power generation, manifolding, test separation, and multiphase pumping, while full processing, gas compression, water and chemical injection, storage and offloading, and gas export are provided by the FPSO. In the figure, the TLWP and FPSO are connected using SBM’s GAP mid-water fluid transfer line technology. Compared to flowlines, the GAP offers improved flow assurance, and the bundled line arrangement eliminates flowline clashing issues. Furthermore, it allows longer distances between the floater and the FPSO, is insensitive to water depth, and leads to a less congested subsea field layout. The horizontal load imposed on the TLWP by the GAP is balanced by a catenary mooring system on the opposite side of the TLWP, as shown in the figure. For the TLWP component of this integrated system, SBM Atlantia have recently developed the FourStarTM TLP, a new generation of TLP that offers an efficient and economic solution for handling large payloads. The FourStarTM TLP hull possesses the unique feature that the columns are battered towards the geometric center of the vessel (Figure 2). This configuration enables the platform to support large payloads, while maintaining stability during the various stages of integration, transportation and installation (e.g., during quayside hull-topside integration and wet-tow transportation to the installation site, and topside float-over integration), as well as enabling a more straightforward removal at field end-of-life. The enhanced stability provided by the battered column geometry allows self-supported hull installation and tendon lock-off without the need for a heavy lift vessel or supplemental buoyancy. The battered-column design results in an ‘open structure’ at the water level that yields better hydrodynamic performance, and the larger footprint of the platform results in a more economical tendon system and improved pontoon access for flowline installation. This paper discusses various components of the proposed integrated TLWP-FPSO field development solution, including the GAP system, with a focus on the development of the FourStarTM TLP. The unique characteristics of the

Rio Oil & Gas Expo and Conference 2008

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FourStarTM concept are discussed, and its advantages over a conventional vertical column TLP are highlighted. The application of the proposed integrated field development solution is demonstrated through an example design.

Figure 1: Integrated TLWP-GAP-FPSO Deepwater Field Development Solution.

Figure 2: Typical FourStarTM TLP Hull.

3. FourStarTM TLP Development Overview The FourStarTM TLP concept was developed to provide an efficient field development solution in applications where the SeaStar TLP was no longer the optimal solution, primarily because of the payload capacity limitations that are inherent in the monocolumn design. Furthermore, the FourStarTM is designed to be stable throughout the various stages of integration, transportation and installation. The stability provided by the battered column geometry allows self-supported hull installation and tendon lock-off without the need for a heavy lift vessel or supplemental buoyancy. SBM Atlantia began the development of the FourStarTM concept in 2006. Since that time, FourStarTM systems have been configured for full drilling and production (dry-trees), drilling only (dry-trees) and production only (wet-trees) applications. Systems have been developed for payloads ranging from 10,000 tonnes to 45,000 tonnes for Gulf of Mexico, W. Africa, SE Asia and Brazil metocean environments and water depths up to 2000 m. The concept has been found to be a scalable and efficient field development solution for a wide range of drilling and/or production applications.


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3.1. System Verification The FourStarTM design has been verified by a series of comprehensive wave basin model test programs performed at the Offshore Technology Research Center at Texas A&M University. Two sets of model tests have been performed on the battered column geometry, for payloads between 10,000 and 36,000 tonnes. Tests have been performed on both wet-tree and dry-tree systems. The tests were performed at 1:50 scale, and utilized the latest Gulf of Mexico metocean criteria. Figure 3 shows one of the wave basin models during outfitting. For the dry-tree configuration, the top-tensioned risers were included in the physical model in order to provide more realistic estimates of the hydrodynamic behavior of the TLP. Figure 4 shows an underwater section of a dry-tree model with the tendons and (equivalent) top-tensioned risers visible. The main objectives of the wave basin model tests were to:

� Provide a baseline for the calibration of the numerical tools, � Study the wave runup on the battered columns, � Validate platform airgap, and to � Investigate response of the TLP in various frequency-bands (high-, wave-, and low-frequency).

Figure 5 shows a FourStarTM model during wave testing in a hurricane seastate. In both cases the measured responses showed good agreement with the theoretical estimates and confirmed the performance of the FourStarTM as a stable drilling and production platform. The underwater section of the FourStarTM hull was also tested in the Low Speed Wind tunnel at Texas A&M University to determine the overall current loading. Of special interest was the flow around the base node geometry. These tests also confirmed the effectiveness of rounding the edges of the columns and pontoons in reducing hydrodynamic drag. The FourStarTM hull in the wind tunnel during testing is shown in Figure 6.

Figure 3: Wave Basin Model During Outfitting. Figure 4: Underwater View of Dry-Tree Wave Basin Model.

Figure 5: FourStarTM TLP in Gulf of Mexico Figure 6: Wind Tunnel Test on the Underwater Section Hurricane Seastate. of FourStarTM TLP Hull. 3.2. Structural Design The FourStarTM deck structural design follows the same principles used for the decks of fixed jacket platforms common in the Gulf of Mexico. The structure is configured for float-over or single/multiple quayside lift and “full” onshore commissioning. In the case of a dry-tree TLP with drilling capabilities, the locations of the major truss rows are


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determined by the wellbay skid beam spacing. The openness of the truss framing is maximized to provide flexibility for equipment and piping layouts. The girder spans are optimized to minimize steel weight. A typical dry-tree FourStarTM TLP deck structure is shown in Figure 7.

Figure 7: Typical FourStarTM TLP Deck Structure. The primary goals of the hull structural design were optimum load transfer through the outer shell of the structure, and a hot-spot-free stress distribution. It was also a priority to design with constructability in mind. Carrying the load in the outer shell enables the internal framing to be simplified considerably as it is designed primarily to resist hydrostatic loading and eliminates to a large extent the need for additional back-up structure. As shown in Figure 2, the rectangular columns are positioned diagonally, with the strong axis oriented to resist the large bending moement inherent in the battered column design. This batter starts immediately above the (continuous) pontoon top plate. The corner radii and transitions are designed as true quarter-cylinder sections. The rectangular shape of the columns, along with the large radius at the pontoon-column-basenode connection, results in a smooth transition, and hence a hot-spot-free base node connection. This approach also eliminates the need for any horizontal gusset plates at the basenode, although there is a vertical gusset at the pontoon-column-basenode intersection where multiple load paths meet. Figure 8 presents the FEA model showing the hull outer shell. The internal framing, including bulkheads and flats is shown in Figure 9. A close-up of the column-pontoon-basenode intersection is shown in Figure 10.

Figure 8: FEA Model of FourStarTM Hull Showing Outer Shell.


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Figure 9: FEA Model of FourStarTM Hull Figure 10: Close-Up of Pontoon-Column- Showing Internal Framing. Basenode Connection.

3. GAP System Overview The Gravity Actuated Pipe (GAP) system is a patented SBM product that transports production fluids between the TLWP and the FPSO. The main body of the GAP consists of a large diameter carrier pipe supporting a number of flowlines and umbilicals, with the carrier pipe serving as the structural support. The ends of the carrier pipe are supported by tether chains hung from the dry-tree unit (TLWP) and the FPSO. Clump weights or buoyancy are mounted at the ends of the carrier pipe to generate vertical loads. These loads act on the angled tether chains to keep the carrier pipe under tension, which limits the hog/sag of the pipe bundle. Flexible jumpers carry the production fluids to/from the FPSO/TLWP. The mid-water GAP system provides an attractive alternative to flowlines that travel along the seabed. The main advantages of the GAP are:

• Improved flow assurance due to higher sea-water temperatures • Lower discharge pressures due to shorter flowline lengths • Avoidance of seabed topography and congestion issues

The first deployment of the GAP system occurred recently in the Kikeh field offshore Malaysia in the South China Sea in a water depth of 1320 m. In this application the dry-tree unit was a Spar. An overview of the design of the Kikeh GAP has been discussed by Brown et al. (2008) and Lemoel et al. (2008). A schematic of the Kikeh GAP is presented in Figure 11.

Figure 11: Schematic of Kikeh GAP.


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4. Design Example This section discusses the global sizing and configuration of an integrated TLWP-GAP-FPSO system for a particular application offshore Brazil. Typically, the TLWP provides dry tree riser support, drilling capability, manifolding, test separation, and multiphase pumping, while all other functions including full processing, accommodations, gas compression, power generation, water and chemical injection, storage and offloading, and gas export are provided by the FPSO. 4.1. Functional Specifications A hypothetical scenario for a gas field development offshore Brazil using a TLWP in 1,100 m water depth is considered. The TLWP should support up to 10 production wells, with 4 subsea tiebacks, plus a self-supporting 1,600 HP API drill rig of 1,000 kip hookload capacity with an accommodations module. The production flow should be conveyed from the TLWP through multiphase transfer lines to the FPSO for processing, storage and offloading. MEG from the FPSO will be added to the multiphase stream to prevent hydrate formation. The process capacity on the FPSO should accommodate 35,000 barrels of oil plus condensate per day, 1,000 m3 per day water processing, 5,000 m3 per day water injection, and 10,000,000 m3 per day gas compression. A maximum gas rate of 600,000 m3 per day per well should be allowed. Estimated peak production is 5,000,000 m3 of gas plus 200 m3 of condensate per day. Electrical power shall be supplied to the TLWP by the FPSO through an electrical cable. 4.2. Metocean Criteria The metocean data used in the study are shown in Table 1, and are typical of the environments offshore Brazil.

Hs Tp γ Vw (10 min) Vc (0) Vc (0.25*d) Vc (0.5*d) Vc (0.75*d) Vc (d) (m) (secs) (m/sec) (m/sec) (m/sec) (m/sec) (m/sec) (m/sec)

1yr 5.7 12.7 1.6 19.6 1.0 0.8 0.7 0.3 0.110 yr 6.9 14.6 1.6 23.9 1.2 1.0 0.8 0.4 0.1100 yr 7.8 15.4 1.7 30.9 1.5 1.2 0.9 0.5 0.2

Hs Tp γ Vw (10 min) Vc (0) Vc (0.25*d) Vc (0.5*d) Vc (0.75*d) Vc (d)

(m) (secs) (m/sec) (m/sec) (m/sec) (m/sec) (m/sec) (m/sec)

1yr 1.5 12 1.0 19.6 1.3 1.0 0.8 0.4 0.1

10 yr 1.5 12 1.0 23.9 1.8 1.1 0.9 0.5 0.2

100 yr 1.5 12 1.0 30.9 2.0 1.3 1.0 0.6 0.3

Wind-Wave Dominated Seastates

Current Dominated Seastates

Table 1: Metocean Criteria. 4.4. Global Sizing The global sizing load case matrix is shown in Table 2. In the table, the safety conditions A, B, and S refer to normal, extreme and survival, respectively, while H and L refer to the high- and low-design water levels. The various seastate conditions are made up as follows:

• Operating = 1 yr wind-wave + 1 yr current • Extreme Wind-Wave = 100 yr wind-wave + 10 yr current • Extreme Current = 10 yr wind-wave + 100 yr current • Reduced Extreme = 10 yr wind-wave + 10 yr current

In the present analysis, wind wave and current are assumed collinear.

Intact One Tendon Removed One Comp. Damaged

Operating A B H, L 0o, 22.5o , 45o

Extreme Wind-Wave B S H, L 0o, 22.5o , 45o

Extreme Current B S H, L 0o, 22.5o , 45o

Reduced Extreme B H, L 0o, 22.5o , 45o

Safety ConditionSeaState Water Level Env. Heading

Table 2: Global Load Case Matrix.


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4.3. Topsides Layout and Payload Summary The topsides layouts shown in Figure 12 are based on the functional specifications in Section 4.1. The deck structure is nominally 50 m x 50 m x 2 levels, with a top-of-steel (TOS) to TOS distance of 9.2 m. The payload summary for the example TLWP is shown in Table 3.

Figure 12: Production Deck and Drilling Deck Topsides Layouts.

Budget Weight Budget Weight

(tonnes) (tonnes)

Topsides Payload Riser & Mooring Loads

Deck Primary Steel 2800 Riser Loads 3300

Deck Secondary Steel 300 GAP Load (Vertical) 350

Production Deck Equipment 2200 GAP Load (Horizontal) 150

Riser Tensioners & Jumpers 300 Mooring Load (Vertical) 350

Drilling Payload Mooring Load (Horizontal) -150

Drilling Module Steel 1000 Other Loads

Drilling Deck Equipment 1500 Weight Control Margin 350

Drilling Variable Loads 2000 Marine Growth 150

Total Payload 14600 tonnes

Weight Category Weight Category

Table 3: Payload Summary for TLWP.


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4.5. System Configuration The hull configuration is shown in Table 4, while the tendon and mooring system sizings are presented in Table 5. The weight balance for the system is shown in Table 6. In the operating case, the total tendon pretension is approximately 35% of the hull displacement. Hull Dimensions Tendon & Mooring SystemColumn Length 15.0 m Number of Tendons 8Column Width 10.7 m Tendon OD 36 in.Column Corner Radius 3.0 m Wall thickness 1.4 ~ 1.5 in.Pontoon Height 9.8 m Tendon Dry Weight (each) 850 tonnesPontoon Width 6.1 m Tendon Wet Weight (each) 120 tonnesTop of Column (TOC) 42.7 mDraft 30.5 m Number of Mooring Lines 2Freeboard 12.2 m Arrangement + 15o from GAP axisEdge-to-Edge Column Spacing at TOC 50.0 m Mooring Line Range 1500 mC-C Column Spacing at TOC 61.0 m Fairlead Chain 150 m, 4.5" R5C-C Column Spacing at Keel 74.7 m Wire 1500 m, 4.25" Bridon XtremeBatter Angle 15.1 deg. Anchor Chain 250 m, 4.5" R5 Table 4: FourStarTM Hull Configuration. Table 5: Tendon and Mooring Systems.

TLWP Operating Weight Balance

Hull Displacement 38,300 tonnes

Hull Structure 7500 tonnesDeck Structure 3100 tonnesDrilling Package 4500 tonnesProduction Equipment 2500 tonnesRiser & Mooring Loads 4000 tonnesTrim & Permanent Ballast 2500 tonnesWeight Control Margin 350 tonnesMarine Growth 150 tonnes

Total Tendon Pretension 13,700 tonnes

Table 6: TLWP Weight Balance 4.5. Stability Analysis As discussed previously, one of the key development drivers for the FourStarTM concept was stability of the platform during the different stages of integration, transportation, and installation. Figure 13 shows the panel model used in StabCad to analyze the stability of the platform. The following stages were analyzed:

• Integration of hull and topsides at quayside by heavy lift or by floatover in a sheltered region.

• Wet tow of the integrated hull and topsides to installation site.

• TLWP at maximum draft during installation just prior to tendon lock-off.

The analysis results confirm that the FourStarTM is stable from throughout all stages of integration, transportation and installation, and final removal. Additionally, a free-floating motion analysis using AQWA has confirmed that the TLWP exhibits acceptable motions during wet-tow of the integrated unit to the installation site. Figure 13: StabCad Stability Analysis Model. 4.6. Global Performance Analysis The global performance analysis results presented in this section are obtained using a frequency-domain diffraction analysis approach with calibration factors derived from model tests to account for nonlinear response effects. The formulation also includes margins / allowances for project uncertainties and as-built variations. Statistical extreme value analysis is used to estimate design responses. The objective of this approach is to obtain conservative design


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responses that can subsequently be confirmed by a more detailed coupled global performance analysis. A typical diffraction analysis model is shown in Figure 14. Only the portion of the wetted surface of the hull up to the mean water level is modeled.

Figure 14: Panel Model for Diffraction Analysis. It is noted that detailed riser analysis was beyond the scope of the present study. Therefore, in order to ensure a workable riser system the maximum intact platform offset is limited to 7% of water depth in 100-yr (extreme) conditions. Additionally, the straight-line riser angle is limited to 2º in the drilling (operating) environment. At the preliminary design stage only the platform intact load cases outlined in Table 2 are considered. Also, an averaged (H+L)/2 design water level was considered. Table 7 presents the results of the global performance analysis for the example TLWP for the intact condition. It can be seen from the table that all design responses are within acceptable limits, and that the tendon design is optimized. The system pretension is governed by maximum offset and not by minimum tension considerations. The operating environment governs the tendon design. The extreme wind-wave environment governs the platform freeboard, while the extreme current case governs the maximum offset.

Wave Heading Max. Tension Min. Tension Max. Offset Offset / Min. Deck Clearance Max. Deck Acc. Straight-Line Angle(deg) (tonnes) (tonnes) IR UF (m) Water Depth (m) (g's) (deg.)

0.0 2433 1310 0.71 0.91 36.3 3.3% 4.6 0.07 1.922.5 2471 1270 0.72 0.93 39.0 3.5% 4.6 0.06 2.045.0 2471 1265 0.72 0.93 38.4 3.5% 4.6 0.064 2.0

0.0 2560 1196 0.53 0.80 28.4 2.6% 1.5 0.096 1.522.5 2630 1126 0.56 0.82 29.3 2.7% 1.5 0.089 1.545.0 2637 1114 0.57 0.82 29.0 2.6% 1.5 0.093 1.5

0.0 2335 1690 0.44 0.74 71.6 6.5% 7.8 0.019 3.722.5 2359 1707 0.45 0.74 76.8 7.0% 7.4 0.015 4.045.0 2356 1698 0.45 0.74 75.9 6.9% 7.5 0.017 3.9

Tendon DesignSeaState

Operating

Extreme Wind-Wave

Extreme Current

Table 7: Global Performance Analysis Results for Platform Intact Condition.

5. Conclusions An integrated field development solution consisting of a TLWP, GAP and FPSO suitable for applications offshore Brazil is discussed. This paper has presented an overview of the development of the various components of the integrated system, focusing on FourStarTM TLWP and the unique features of the concept. Finally, the global sizing and configuration of an integrated TLWP-GAP-FPSO system for an example application offshore Brazil have been presented.

6. References BROWN, P., ROODUIJN, E., LEMOEL, M., Kikeh Development: Design of the World’s 1st Gravity Actuated Pipe (GAP) Fluid

Transfer System, Proc. Offshore Technology Conference, OTC 19471, Houston, USA, May 2008. HEIDARI, H., WILLIAMS, N., LARGE, S., RANGARAJU, N., BYRNE, J., Development of a Battered Four Column TLP Concept,

Proc. OMAE Conference, San Diego, USA, June 2007. HEIDARI, H., WILLIAMS, N., LARGE, S., BYRNE, J., Development of a New Self-Stable Dry-Tree GOM TLP with Full Drilling

Capabilities, Proc. OMAE Conference, Estoril, Portugal, June 2008. LEMOEL, M., BROWN, P., JEAN, P., SHEPHEARD, B. Design of the World’s 1st Gravity Actuated Pipe (GAP) for Murphy’s

Kikeh Deepwater Development, East Malaysia. Proc. OMAE Conference, Estoril, Portugal, June 2008. WILLIAMS, N., HEIDARI, H., LARGE, S., RANGARAJU, N., BYRNE, J., FourStarTM: A Novel Battered Column TLP Concept,

Proc. ISOPE Conference, Lisbon, Portugal, July 2007. WILLIAMS, N., HEIDARI, H., LARGE, S., A Deepwater Dry-Tree Gulf of Mexico Solution: The FourStarTM TLP, Proc. ISOPE

Conference, Vancouver, Canada, July 2008.

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IBP2082 08 INTEGRATED TLWP-FPSO SOLUTION FOR …