IBP1333 12 DRY TREE SEMISUBMERSIBLE PLATFORM: FROM

Copyright 2012, Brazilian Petroleum, Gas and Biofuels Institute - IBP This Technical Paper was prepared for presentation at the Rio Oil & Gas Expo and Conference 2012, held between September, 17-20, 2012, 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 final paper submitted by the author(s). 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, or that of its Members or Representatives. Authors consent to the publication of this Technical Paper in the Rio Oil & Gas Expo and Conference 2012 Proceedings.

______________________________ 1 Ph.D., Principal Engineer – Aker Solutions 2 Ph.D. in Fluid Mechanics – Aker Solutions 3 Ph.D., Vice President – Aker Solutions 4 Bachelor in Mechanical Engineering, Senior Engineer – Aker Solutions

IBP1333_12 DRY TREE SEMISUBMERSIBLE PLATFORM: FROM ‘TECHNOLOGY ACCEPTED’ TO ‘PROJECT READY’

Roger Lu1, Tao Wang2, Magne Nygård3, Rolf Bendiksen4

Abstract Development of new technologies for application of dry tree solutions on semi-submersible hull production platforms opens up new flexible and cost efficient solutions for oil and gas fields in deep and ultra deep water, see Ref. 3 and 4. Aker Solutions has brought the technology through the last steps to a project ready stage. Top tension risers allow for shifting the Christmas tree from seabed to topside, and enables drilling and production from the same unit. By shifting the wellhead and Christmas tree from seabed to platform deck, well maintenance is simplified, and makes the Dry Tree Semi relevant to consider for the Brazilian pre-salt reservoirs. Easy access to the wells for maintenance gives a potential for a higher recovery rate for the field. Future development of ultra deep water oil and gas fields demands solutions overcoming the limitations set by the TLP and the Spar buoy. Combining knowledge from already proven solutions, and a limited addition of new technology, these limitations are overcome with Aker Solutions’ Dry Tree Semi. It is designed and proven for Gulf of Mexico environmental conditions. It is verified for conditions with a significant wave height of 14 meters, and able to withstand 100-year extreme weather and 1000-years survival conditions. A conventional semi-submersible platform, but with a deeper draft to further reduce the already low motions, makes the basis for the concept. Top tensioned risers (TTR) with vertical motion compensation stroke of up to 10 meters, makes it possible to shift the Christmas tree from seabed to platform deck. The deep draft design of the hull reduces the wave force excitation on the pontoons, reducing both the roll and pitch dynamics. The Dry Tree Semi represents several major benefits, including low sensitivity to water depth, reduced hull and topside weight, and flexibility in transportation, mooring, and decommissioning. 1. Global Configuration Aker dry tree semi (DTS) is based on a conventional hull form with a deck box and ring pontoon. Figure 1 illustrates a typical DTS developed for the Gulf of Mexico operating environment. Top tensioned risers (TTRs) are supported by motion compensating tensioners, which are mounted on the lower level of the deck box structure. See Figure 2.

Rio Oil & Gas Expo and Conference 2012

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45 m

32 m

21.8 m51.8 m

21.8 m

19.0 m

Figure 1. Aker Dry Tree Semi Overview and Principal Dimensions

Pontoon Elevation

Seabed EL -8,000’

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Figure 2. One Top Tensioned Riser Supported by Deck-Mounted Motion Compensating Tensioner Compared with conventional drilling Semis that have 20~25 m draft, and compared with wet tree production Semis that have approximately 30~40 m draft, the DTS hull extends further down so that the wave excitation forces on pontoon is reduced, and heave motions are reduced accordingly. Optimal draft depends predominantly on three factors: 1.1. Operating environment Gulf of Mexico, North Sea, Offshore North-western Australia are known for their harsh environments. Typical GoM 100-year significant wave height is in the range of 14~16 m, with peak period of around 15 sec. On the other hand, oil regions such as Offshore Brazil, Southeast Asia, and Offshore West Africa have benign wave environment but present with swell conditions. Therefore, a DTS application for these benign environments may be sized differently than for the harsh environments. In general, a moderate draft Semi may be applicable for a Brazil installation to attain similar motions to a deeper draft Semi for GoM.


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1.2. Acceptable tensioner strokes Riser tensioners of about 8 m strokes have been successfully deployed in Dry Tree Production Units in recently years. Tensioners of longer strokes have been widely used for drilling applications, and their application for Production is being earnestly evaluated. Longer-stroke tensioner is a proven technology as far as hydraulic design is concerned. When a large number of long-stroke tensioners, e.g. 10.5 m, are installed in a wellbay area of a Production and Drilling unit, the layout will be expanded both horizontally and vertically. Global configuration thus becomes a balancing act of Semi size (and motions) and tensioner stroke (and size). 1.3. Execution strategy Execution strategy often times dictates construction constraints. For instance, how topsides modules are fabricated and integrated with the Hull may determine the overall platform configuration and sizing. Semi-submersible platforms do have the feature of integrating the hull and topsides facilities in a protected, inshore environment due to its moderate draft. Any particular design shall maximize such benefit by utilizing the existing capacity of the Integration Yard. The Hull draft is selected mostly from a motion point of view, columns are sized for stability, freeboard is chosen based on air gap requirements, and pontoons are designed for achieving suitable heave added mass, in-hull ballast capacity, and global structural strength. Figure 1 shows an example of an Aker Dry Tree semi tailored for Gulf of Mexico. The column width is 21.8 m, and a column center-to-center distance of 73.6 m. This gives a pontoon clear length of 51.8 m between nodes. Pontoon width is tapered to fit the column size at its ends, while reduced width are used for most of the pontoon. Freeboard is 32 m to the top of the column, or about 21.5 m to the bottom of the deck box. The mooring system is a taut chain-polyester rope-chain line with three lines at each of the four corners of the Semi-submersible. The polyester rope diameter is about 250 mm. The platform and anchor chain diameter is about 152 mm. Required mooring sizes will be less for offshore Brazil applications. 2. Riser Systems Top tensioned risers are supported by deck-mounted, motion-compensating tensioners. Tensioners are designed to ensure risers do not exceed design strength when the Hull moves up, and do not buckle when the Hull moves down. Stiffness of the riser tensioners, when added to the water plane stiffness of the hull, may bring the system heave period into the range of dominant wave energy, e.g. below 18 sec. The solution is to design the tensioners with relatively low stiffness, for instance, around 250 kN/m. For a multiple of 12 TTRs, the combined stiffness will be about 3000 kN/m. This is significantly less than the water plane stiffness of the shown Semi. The riser tensioner system is a passive compensation system based on air pressure and air volume control. Softer tensioner is realized by use of cassette-based, larger-volume cylinders. Figure 3 shows a typical RAM-style tensioner used for the DTS. Optimal designed is found from numerical simulation of load variations vs. cylinder and Air Pressure Vessel (APV) sizes (Figure 4). The resulting tensioner design has 850 mt nominal tension load, 250 kN/m nominal stiffness, and 10.5 m stroke range. The tensioning system cassette is designed to stay within an envelope of approximately 4.9 m x 4.9 m (Figure 6). Supported by the double bottom deck, the system ensures that the production trees / BOP will always be inside the deck box protected well bay area. The ram cylinder, accumulators and centralizer tube will extend down below the deck box. A guide structure under the deck box is built to accommodate the lower tensioner centralizer roller assemblies, and to transfer any lateral loads into the deck box structure. The structure will also act as a protection and wave breaker for the extreme environment cases (100-yr and 1000-yr waves). Upper centralizer assembly is integrated in the tensioner support frame. The centralizer rollers run on 4 sets of guide strips attached to the centralizer tube. In addition to working with the centralizer rollers, the guide strips act as anti-rotation device between the centralizer tube and the tensioner cylinder assembly. Tesnioner assembly uses a special cylinder to accumulator connection manifold developed to eliminate the amount of flexible connection for hooking up the cylinder assembly. The hard pipe system will have to include a break-out spool between cylinder manifold and the air manifold system at the double bottom deck.


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The design also considers cylinder failure. Should that happen, the pressures in the remaining three cylinders will be automatically adjusted, and the failed cylinder will be replaced while the other three cylinders hold up the riser tension for a sufficiently long period of time. The tensioning system remains in full operation.

Figure 3. A typical RAM-Style Riser Tensioner

APV volume (m3)

Load

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Figure 4. Simulation of Tensioner Load Variation vs. Cylinder Diameter and APV Volume TTR interfaces also include a keel guide structure at the pontoon level. Risers slide in the longitudinal (vertical) direction, but are restrained in lateral direction to eliminate an otherwise significant mount of bending on the tensioner centralizer. Figure 5 illustrate one such keel guide structure.

Figure 5. Keel Guide Support Structure

The production Semi also supports other risers, most commonly steel catenary risers for tie-in from future fields, for export of oil and gas, and/or for purposes of e.g. water injection, gas lifting, etc. These risers are typically supported on porches hanging off the pontoon outside, and installed by a Construction Vessel. Upper termination of SCR is either a flex joint or a stress joint. Inboard of the porches, the risers are routed along pontoons and columns to the topsides. Flexible risers or many other riser systems may also be used.


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3. Wellbay Layout The wellbay area is an interior module in the middle of the platform, and designed to meet riser layout, operation, and performance requirements. The arrangement is illustrated in Figure 6. The well slots are arranged in a 3 x 5 pattern, with surface spacing of 5.6 m (18.5 ft) center-to-center. The center slot is dedicated to drilling. The laydown areas adjacent to the drilling slot are accessible by cranes. Fluids are transferred via flexible hoses from the trees to the two production manifolds to the south and north. The manifolds run full length in the EW direction of the wellbay. Adjacent to the manifolds are areas for APVs. Figure 7 shows an elevation view of the platform, with riser tensioners at nominal, downstop and upstop positions. The riser motions (relative to Hull) are absorbed by the tensioners for up to 100-year extreme GoM hurricane conditions. In a survival condition of 1000-year GoM hurricane, the tensioner cylinders reach their design stroke limit. The downstop position is immediately above the deck box, and the upper stop position defines the bottom elevation of the drilling rig. The range between the upper and lower stops is about 10.5 m, conforming to the calculated stroke ranges. The wellbay of the Aker Dry Tree Semi is spaced out both vertically and horizontally. The spatial arrangement allows for easy access of personnel, material handling, etc. This is considered an important feature of the total design. In Aker’s DTS concept, one appreciates that each and every component of the system is extracted / extrapolated from proven technologies, yet that the total system is novel in the sense of the large number of voluminous, large-stroke tensioners placed side by side. The production trees have to be protected at all times with a hatch system, which means the stroke has to be contained inside the well bay module. To support the riser tensions, a truss frame structure is developed with the upper chords at the level of the bottom of the deck box, and the lower chords about 10 m below the upper chords. The riser loads are about 10000 mt in operating conditions associated with a 10-year Winter Storm condition in the Gulf of Mexico and 12000 mt in 100-year hurricane. Larger loads might be obtained in survival events like the 1000 years hurricane. The support structure on one hand makes maximum use of the strong deck box structure system, and on the other hand provides protection to sideway wave slamming in the 1000-year GoM events. For vertical and horizontal impact loads, the structure is designed to sustain minimum yielding.

5.6m x 5.6mWell slots(typical)

Figure 6. Riser Arrangement in the Wellbay Area


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EL. 270.0 ft

EL. 107.0 ft

EL. 115.0 ftDrill floorTop of Skid base

EL. 93.0 ftTop of Cap Beam

EL. 83.0 ft

EL. 107.0 ft

EL. 115.0 ftDrill floorTop of Skid base

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Flare boom270 ft longtriangular

section tapered @

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Platform NorthPlatform North

Figure 7. Riser Tensioners at Bottom-out, Nominal, and Top-up Positions

It is noted that offshore Brazil environmental loading is lower than for Gulf of Mexico. As a result, design loads for the extreme and survival conditions will not be as severe. Opportunity for optimization thus arises in terms of risers and tensioners, and Hull and Mooring, or a combination of all. 4. Operating Load Balance Table 1 shows load balance for the operating condition. It is seen that the topsides payload is 30000 mt, including the process equipment, utility equipment, drilling module, tensioner packages, module structural and deck box structural steels. The weight of the Deep Draft Semi hull is about 37000 mt including contents. For vertical payloads, top tensioned risers contribute 10000 mt, and steel catenary risers and umbilicals amount to 5200 mt. Vertical component of the mooring line tension below fairleads is about 4000 mt, and mooring inboard of the fairleads weights about 1600 mt. Total ballast is about 38200 mt, mostly in pontoon and node compartments, with a small amount in lower columns. Platform displacement is 126000 mt. It is noted that ballast makes about 30% of the total displacement. This is a significant amount compared with conventional Semis. An application for deepwater offshore Brazil will allow for a shallower draft hull, reducing the overall size of the hull for the same functionality as for Gulf of Mexico.

Table 1. Vertical Load Balance

Vertical Loads Weight Topsides payload (incl. deck box structure)

30000 mt

Hull weight 33800 mt Hull content 3200 mt TTR vertical tensions 10000 mt SCR and umbilical vertical tensions

5200 mt

Mooring line vertical tensions 4000 mt Mooring onboard 1600 mt Ballast 38200 mt Displacement 126000 mt


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5. Global Performance Figure 8 shows the motion RAOs of the demonstrated DTS. Heave natural period is about 21.5 sec with all Top Tensioned Risers installed. At dominant wave periods around 15 sec, the heave response is about 0.38 m/m, and pitch response about 0.21 deg/m. Maximum heave motion in 100-year wave is about 5.0 m for a GoM condition, and maximum pitch about 7 deg. The same DTS would have about 3.3 m heave motion for an offshore Brazil condition. Calculated tensioner stroke for the most critical riser located at the corner of the wellbay is about 10.2 m (bracketed between 4.1 m up stroke and 6.1 m down stroke for Gulf of Mexico conditions. In a survival 1000-yr hurricane condition, the tensioner could both bottom out and top up, and the riser stress is still slightly below material yield. Impact loads will occur, varying from maximum at one corner of the wellbay to minimum at the opposite corner. The tensioner cylinders and the tensioner support structure are designed to withstand these impact loads, allowing for local yielding damage, but maintaining global structural integrity. For a typical Brazilian environment, the heave response is about 34% smaller than for a GoM condition, or about 3.3 m. Tensioner stroke range is expected to be in the range of 7.6 m. This allows for further optimization of the tensioning system as compared to the one designed for GoM environment. The survival events could also easily be designed without allowing for the tensioners of bottoming out or topping up. This will be decided based on operators philosophy.

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Figure 8. Motion RAOs


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6. Key Technology Elements Aker Dry Tree Semi has been applied in different sizes in studies with various Operators. The current concept presented herein is model tested, and recently went through a Technology Qualification program led by DNV. The findings were documented in DeepStar CTR 10406 Report (Ref. 2). The most critical element identified for the Qualification is the long-stroke production riser tensioning system. Although use of long-stroke drilling riser tensioning system is well proven, the implication of using a multitude of voluminous tensioners in a permanent production facility requires further understanding. To meet this challenge call, Aker is currently undertaking a large-scale model testing program. It is expected that, when the program is conducted in 2012, Aker Solutions will be able to render a good understanding of functional principles for long stroke RAM-style tensioners, and reliably design and apply such assembly of tensioning system for Aker DTS for harsh and mild conditions. Some of the common operational issues will be examined, including removal of one cylinder for repair and maintenance. The test is also to demonstration that the tensioner-cassette is easy to maintain and has good reliability. It is expected that the test program will sort out and solve technical uncertainties related to both the tensioner and its control system, and ultimately raise the confidence level on the product from both supplier’s and buyer’s perspectives. Wellbay layout and safety is a design aspect, but is considered an essential element of the product concept. So is the tensioner inspection, maintenance, and repair plan. For high current conditions in Gulf of Mexico such as “loop currents”, hull “Vortex Induced Vibration” (VIM) is a phenomenon with high focus. Hull VIM may be important considering the draft is deep, and the columns are relative long with respect to its diameter, enhancing the 2D fluid effect. Currently, conservative motion amplitude is assumed. This will be no issue for Brazilian waters. 7. Conclusions and Path Forward After several years of engineering development and model testing, Aker Dry Tree Semi is considered a mature product for the overall concept, while identified; critical components are being further enhanced to secure a best possible application to the Aker Dry tree Semi. The product is, however, considered ready for a “pre-engineering” or “concept selection” stage for Gulf of Mexico. A Brazilian application will be less demanding on the technology than for Gulf of Mexico. The ongoing further enhancement is split in two

• The first part is to refine global engineering, develop a wellbay layout model, design critical riser-supporting structures, and perform a large-stroke tensioner model test.

• The second part is to analyze all critical load cases, analyze riser interference, analyze structures including global squeeze-pry loads, and perform a structural test of survival impact loads.

Attention is also focused on application of Aker DTS in mild environments, and develop optimal solutions for moderate-stroke tensioners with a moderate-draft Semi. 8. References AKER SOLUTIONS, Dry Tree Semi Floating Production and Drilling System Study Report, May 2011. DET NORSKE VERITAS, Technology Qualification for Aker Solutions Dry Tree Semi, August 2011. LEE, M-Y., POLL, P, ZENG, J. Dry Tree Alternatives for Drilling and Production in Ultra-Deepwater Gulf of Mexico, Proceedings of DOT Conference, 2011. ZENG, J., WANG, T., WANVIK, L., RASMUSSEN, S., LØKEN, R. Advances in Dry Tree Semi technology for Deepwater Field Develoment, Proceedings of DOT Conference, 2010.

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IBP1333 12 DRY TREE SEMISUBMERSIBLE PLATFORM: FROM