A Hub Class Spar Production Platform for the Ultra-Deepwater Gulf of Mexico. Richard S. Mercier Shell Deepwater Development Systems Inc.

Transcription

1 13 th Annual Floating Production Systems Conference A Hub Class Spar Production Platform for the Ultra-Deepwater Gulf of Mexico Richard S. Mercier Shell Deepwater Development Systems Inc. Denby G. Morrison Shell E&P Technology Co. Wes E. Schott Wes Schott International Bobby E. Cox, Paul L. Dorgant, C. Tracy Harris, Jack J. Kenney, and Keith C. Oxley Shell Deepwater Development Systems Inc. Abstract The production of hydrocarbon reserves from the ultra-deepwater (> 4000 ft water depth) Gulf of Mexico with floating production systems poses considerable challenges. Most notable among these are exposure to Loop current eddies, deepwater station keeping, deployment of drilling and production risers, and accommodation of satellite wells. Shell has designed a spar production platform concept that addresses these challenges. This paper describes the spar design, focusing on the features that help reduce the technical and economic risks associated with ultra-deepwater developments. Introduction For the past decade Shell has been rapidly expanding its presence in the deepwater Gulf of Mexico. Most visible in this expansion has been the series of Tension Leg Platforms (TLPs) it has designed, built, and installed in the 3,000 to 4,000 ft water depth range. Starting with Auger, installed in 1993, the Mars, Ram/Powell, and Ursa TLPs have followed in quick succession, and a fifth TLP is in the late stages of design. These TLPs are designed to handle very large production volumes, both from wells located directly below the platform as well as from subsea well tiebacks from nearby satellite fields. The TLPs also act as nodes on the deepwater pipeline network. The processing facilities required to handle such large volumes of production are substantial. The facilities payload is a major consideration for the design of the floating structure. In water depths deeper than 4,000 ft (referred to as ultra deepwater), the cost of TLPs increases dramatically with water depth because of the resulting increase in the weight of the tendons. The tendon weight increases not only because of increase in length, but also because of the increase in cross section required to keep the heave and pitch natural frequencies above the wave frequencies. The increase in the tendon weight requires more buoyancy to support it, which in turn increases the environmental loads, further compounding the required increase in tendon strength. For ultra-deepwater field developments where direct vertical well access is required, an attractive alternative to the TLP is the spar production platform. Semisubmersibles and ships are not moored with tendons but their large heave motions make it very costly to accommodate vertical 1

2 risers. Because the hull and mooring for a spar are less coupled than for a TLP, the rate of increase in structure cost with water depth for a spar is less than for a TLP. Shell has been developing spar-type floating production facilities since 1968 (Graaf, 1971). In the Gulf of Mexico migrating to deeper water also means higher exposure to Loop Current eddies. For TLPs this means larger spacing is required between the vertical risers to avoid interference in Loop Currents as well as reduced operability for deploying risers (particularly drilling risers). Deep draft spars with internal wellbays offer a number of advantages for riser deployment and avoidance of riser interference. But these advantages can be offset by the station keeping costs if there are requirements for vertical well access while under the influence of a Loop Current eddy. In summary, the production of hydrocarbon reserves from the ultra-deepwater Gulf of Mexico with floating production systems poses considerable challenges. Most notable among these are exposure to Loop current eddies, deep water station keeping, deployment of drilling and production risers, and accommodation of satellite wells. Shell has designed a spar production platform concept that addresses these challenges. This paper describes the spar design, focusing on the features that help reduce the technical and economic risks associated with ultra-deepwater developments. Functional Requirements The functional requirements used as the basis for the design of the hub class production platform are summarized in Table 1. The platform is designed to accommodate a very large number of steel catenary risers (SCRs) for subsea production from up to five satellite wells and from a total of eight import/export pipelines (four gas lines and four oil lines). Although the design is targeted for 6,000 ft water depth, the feasibility of applying the design in the 4,000 ft to 8,000 ft water depth range was also investigated. Table 1: Functional Requirements for Hub Class Platform Water Depth 6,000 ft Nominal Topsides Payload 25,000 short tons CG of Topsides Payload +/- 5 ft Production Quarters Capacity 80 men Additional Quarters Capacity 80 men Direct Vertical Access Yes Rig Capability Full (drilling & workover) Peak Liquid Rate MSTPBD Water Cut % Gas-Oil Ratio 500 2,000 SCF/STBO No. of Production Riser Slots 16 No. of Production SCRs 10 (2 per subsea tieback) No. of Umbilical Pulltubes 10 (2 per subsea tieback) No. of Export SCRs 4 Dedicated Drill Slot Yes Future Payload Capacity Water Injection Yes Gas Lift Yes No. of Import 4 2

3 Loop Currents and Mitigation of Vortex Induced Vibrations The oceanographic events that have the most impact on the design of Gulf of Mexico platforms are hurricanes and Loop Current eddies. Loop Current eddies are cyclonic features shed from the Yucatan current as it loops in and out of the Gulf before becoming the Gulf Stream. The eddies have very strong currents that extend from the surface to depths of about 2,000 ft. Once formed these eddies slowly migrate over a period of months toward the western portion of the Gulf then dissipate as they encounter the shelf and interact with the bottom. Figure 1 shows a typical condition in the Gulf of Mexico with the Loop Current intruding into the eastern Gulf (shown as the gray-shaded feature on the lower right portion of the figure) and an eddy that it has recently shed starting to migrate westward. Also shown in the figure are the water depth contours in meters. This figure illustrates the spatial scale of these current features and reinforces the observation that exposure to Loop Current eddies is highly dependent on location in the Gulf. In the 4,000 to 8,000 foot range of water depth the exposure varies from less than 5% of the time in the western Gulf to over 40% of the time in the central Gulf. Since most of the deepwater development is concentrated in the central Gulf, exposure to Loop Current eddies is a significant concern for design and operation of drilling and production platforms. Spar platforms are particularly vulnerable to Loop Eddy currents because they have deep drafts and attract more current load. In strong currents, the frequency of vortex shedding from the spar may also overlap with the natural frequency of the sway and/or roll response and feed energy into these resonant response modes. Large amplitude vortex induced sway and roll vibrations will be set up which will cause the effective drag coefficient inline with the current to increase dramatically, thereby causing large tensions in the mooring system. Without some mechanism for mitigating the vortex-induced vibrations, the mooring tensions may be larger than those experienced under hurricane conditions. To mitigate these vibrations special measures are taken to suppress the strength of the loads imparted by the vortex shedding process. The most commonly used technique is helical strakes. The sharp edge of the strakes controls the location at which vortex shedding will occur locally. Wrapping the edge around the platform ensures that the vortex shedding will not occur at the same azimuthal location along the full length of the spar, effectively reducing the strength of the transverse forcing. But since the strakes themselves add projected area to the spar, the reduction in the transverse forcing comes at the expense of increased forcing in the in-line direction. The technique that Shell has devised to mitigate the effects of vortex shedding is to divide the upper hull into two sections and separate the two sections with a gap. The gap in the hull isolates the boundary layers in the top and bottom portions so that vortex shedding on the upper portion does not occur at the same time as on the lower portion. This method of reducing the lateral forcing does not increase the in-line forcing. Another major advantage is that it doesn t require attaching special structures to the outside of the hull that could interfere with the hull fabrication and load-out or the attachment of appurtenances. The gap feature is a very efficient technique for mitigating the effects of vortex-induced vibrations in Loop Current eddies. It reduces the mooring loads in Loop Current eddies to a point where these currents do not control the design of the mooring system. Furthermore, it makes it feasible to employ an active mooring system to control the platform offset in Loop Current eddies so that riser deployment, drilling, and well workover operations can continue uninterrupted. This capability is particularly important for ultra-deepwater developments in the central and eastern Gulf. 3

4 Hurricane Design Criteria With the gap feature to limit loads and responses in Loop Current eddies, the design of the spar is for the most part controlled by extreme hurricane events. Table 2 lists the 100-year and year return period Gulf of Mexico hurricane criteria employed in the design. The spectral models employed are the JONSWAP model for the wave spectrum and the API model for the wind spectrum. Table 2: Representative Gulf of Mexico Hurricane Design Criteria 100-Year Hurricane 1000-Year Hurricane Significant Wave Height (ft) Spectral Peak Wave Period (sec) Spectral Peakedness Parameter, γ Hour Wind Speed at 10 m Elevation (ft/sec) Surface Current (ft/sec) Depth to Zero Current (ft) Current at Half Depth of Profile (ft/sec) Spar Hull and Mooring Design The general features of the hub class spar floating production system designed to satisfy the functional requirements and oceanographic criteria described above are illustrated in Figure 2. The spar supports a standard deck on the hull via a module support frame. The upper hull provides the buoyancy supporting the payload. The gap in the hull provides mitigation of vortexinduced vibrations in strong currents. The lower hull is ballasted with high-density material to act as a counterweight and provide static rotational stability. The truss acts as the lever between the counterweight and the center of buoyancy. There is a rectangular well bay through the center of the hull that accommodates the vertical risers for the direct vertical access (DVA) wells. Each riser is attached to its own buoyancy module that provides the required top tension. The risers are free to stroke vertically in the well bay but are restrained horizontally by a series of guides at various elevations. The hulls are ring-stiffened plate structures with radial bulkheads from the wellbay to the outer shell. The main truss has four vertical members in a square arrangement. Diagonal bracing and horizontal framing between the vertical members provide the required strength in shear and bending. The horizontal framing, which is provided at three elevations, supports the guides for the risers. The mooring for the hub class spar is a 12-line chain-wire-chain system anchored with suction piles. Starting from the anchor, each line consists of 2,000 ft of 4.5 inch ground chain, followed by 8,200 ft of 4 inch wire rope ending with about 330 ft of 4.5 inch platform chain to the fairlead. The fairleads are located above the base of the upper hull. Due to the need to be able to perform riser deployment and well workover operations in Loop Current eddies, the mooring system is actively controlled. Each line has its own chain jack 4

5 located on the top of the hull. Two separate control rooms and hydraulic power units are located on the lower deck on either side of the spar, each one operating six mooring lines. Vertical Riser System The riser systems considered for the spar are patterned after the risers on the Ursa TLP. The production risers are dual casing with a 13-5/8 inch outer casing and a top tension of 1620 kips in 6000 ft water depth (tension factor of 1.25). The drilling riser is 21 inch outer casing with a top tension of 2,000 kips. To limit the risk of interference between risers it is important to consider their spacing and overall arrangement. Arrangement of the risers in a close-packed matrix pattern minimizes the required wellbay space but complicates the deployment of risers in the middle of the pattern. Such a layout typically requires use of a guideline system to eliminate the risk of interference during deployment. Alternatively, the risers may be arranged in a more open perimeter pattern that increases the wellbay space requirements relative to a matrix pattern but simplifies riser deployment. By deploying the riser and approaching the well head from outside the pattern the risk of interference is minimized. To assess which of these arrangements is best it is necessary to determine the minimum allowable spacing between the risers. A number of situations can control this minimum spacing: interference between installed risers caused by spar motions in extreme storm conditions, interference due to relative motion between installed risers while subje cted to Loop Current eddies, interference while deploying a new riser adjacent to an installed riser in a Loop Current eddy, and interference while deploying a new riser adjacent to an installed riser in operational seastates with significant wave height up to 12 ft. During deployment the mooring system must draw the riser back to nearly vertical over the wellhead. Adequate separation between the risers must be maintained. If no guideline system is used the well pattern must be flared at the sea floor. If a single arm guideline system is used the need for flaring is eliminated. In the design of the production risers it is important to consider the stress concentration at the keel of the spar and at the subsea wellhead. Shell-proprietary designs have been developed for these elements. Steel Catenary The numerous steel catenary risers are hung off from baskets located at various points on the hull. For installation of the SCRs there are a number of possible ways to route pull-in lines directly down the face of the hull. Substantial deck space has been reserved with access hatches on the lower deck levels. Complimentary winch locations can be located on the top of the hull for additional pull-in capability. 5

6 The SCR sizes analyzed for the subsea satellite wells include 12-inch carrier with 8-inch pipe-inpipe and 8-inch pipe with 3-inch foam insulation. For the import/export lines 18-inch SCRs were considered. In all cases helical strakes would be installed over the top 500 ft of the SCR for suppression of vortex-induced vibrations in strong currents. The large number of SCRs has a significant impact on the mooring stiffness. As the SCRs are attached the tension in the mooring lines must be adjusted to compensate for the offset force. Analysis of the SCR performance under motions imposed by the hull indicated no problems with loss of tension, excessive top rotation angles, interference with the hull, over-stressing or fatigue. Indeed the spar was found to be very accommodating from the point of view of SCR installation, performance, and maintenance. Motion Characteristics Table 3 summarizes the predicted motions of the spar without any vertical risers and SCRs attached, and illustrates the relative importance of the mean and dynamic components for each response. The motions are referenced to a body-fixed coordinate system. The 1-year winter storm event has a significant wave height of 14 ft. The natural periods of the spar motions are 350 seconds in surge/sway, 150 seconds in roll/pitch, 75 seconds in yaw, and 28 seconds in heave. The reported heave motions are upper bound values due to conservative assumptions made in the modeling of the resonant response. The pitch motions are relatively large, up to 13 degrees maximum in the 100-year hurricane. These motions in turn have a significant impact on the horizontal accelerations at deck level. The pitch motions can be reduced somewhat by adding more ballast at the keel to limit the mean pitch component. Figure 3 compares the motions of the spar with that of a TLP and a semisubmersible with similar payload in 6,000 ft water depth. The figure shows how the 3-hour maximum motion responses vary with increasing seastate, as characterized by the significant wave height, Hs. Each plot contains two curves for the spar one for the condition where no vertical risers or SCRs are attached and one for the condition where all 16 vertical risers and 18 SCRs are attached. The offset response for the spar and the TLP are similar because both platforms have vertical production risers. To limit the moment on the wellheads the horizontal offset at the top of the risers must be constrained. The semisubmersible on the other hand does not have top tensioned vertical risers its mooring is designed to accommodate the demands of steel catenary risers only. In the all risers condition, the large number of SCRs plays a significant role in assisting the mooring system. Without production risers attached the spar experiences larger pitch motions than the semisubmersible. TLPs of course have insignificant pitch motions. The buoyancy modules exert a considerable righting moment and reduce the spar s pitch motions to a level comparable to a semisubmersible. The spar with its deep draft upper hull experiences vertical accelerations that are smaller than a TLP. The TLP springing response is very important for significant wave heights in the 10 to 20 ft range where the associated wave period is two to three times that of the heave natural period. The vertical motion of the semisubmersible is substantially larger than that of the spar in all seastates. 6

7 The lateral accelerations shown in Figure 3 are those that are experienced at deck level. With all risers attached, the lateral accelerations of the spar are comparable to those of the TLP and semisubmersible. Without attached risers, the spar experiences substantial horizontal deck accelerations in hurricane conditions. Fortunately, Gulf of Mexico platforms are evacuated when there is a hurricane threat. Table 3: Typical Spar Motions (Without Attached) 1-Year 100-Year 1,000-Year Winter Storm Hurricane Hurricane Fairlead Offset (ft) Mean RMS Hour Maximum Fairlead Heave (ft) Mean RMS Hour Maximum Inclination (degrees) Mean RMS Hour Maximum Deck Acceleration (% g) Mean Vertical RMS Vertical Hour Max Vertical Mean Horizontal RMS Horizontal Hour Max Horizontal Conclusions The hub class spar floating production system described in this paper has a number of features that make it particularly suitable for deployment in the ultra-deepwater Gulf of Mexico. These include a gap in the upper hull for effective mitigation of hull vortex-induced vibrations in strong currents without accompanying increase in hull drag characteristics, so that design of the mooring system is governed by survival of rare hurricane events rather than the more commonly occurring Loop Current eddies, an active mooring system to enable deployment of vertical risers in Loop Current eddies and seas up to 12 ft significant wave height, and accommodations for a large number of steel catenary risers for subsea tiebacks, and import and export pipelines. 7

8 The spar system is robust to changes in oceanographic environment, and scales easily with changes in payload and water depth. Reductions in payload, which also typically involve reductions in the deck wind area, are accommodated primarily by changes in hull diameter and fixed ballast. Reductions in water depth, all else equal, lead to reductions in the material cost of the mooring and riser systems with little impact on the hull. For environments that are more severe than the Gulf of Mexico it is necessary to increase the air gap and the heave natural period to accommodate the more intense and longer period seas. The gap feature in the hull greatly expands the flexibility of the spar to handle high current environments since this technique for mitigating vortex-induced vibrations of the hull results in very little increase in the effective drag area. This keeps the necessary increase in the capacity of the mooring system to a minimum. A design tool has been developed to assist in optimizing the global design of the spar for different functional requirements and oceanographic criteria. This tool is being used to guide and focus current work on design improvements that have the greatest potential for reducing system cost or expanding overall flexibility. With the Gulf of Mexico hub class spar concept described herein as a starting point, Shell is now designing a similar hub class spar for service in one of the world s harshest environments, the Northern North Sea. References Graaf, G. (1971), Spar-Type Floating Production Facility, US Patent No. 3,572,041. 8

9 13 th Annual Floating Production Systems Conference Figure 1: Map of Gulf of Mexico Showing Depth Contours in Meters and Characteristic Loop Current With Detached Eddy (Gray-Shaded). 9

10 Upper Hull Truss Mooring Lower Hull Top Tensioned Vertical Steel Catenary Figure 2: General Features of Hub Class Spar. 10

11 Hour Max. Offset (feet) Semi Spar - All Spar - No TLP Hour Max. Inclination (deg) Spar - No Semi Spar - All TLP Hs (feet) Hs (feet) 3-Hour Max Vertical Accel. (% g) Semi TLP Spar Hs (feet) 3-Hour Max. Lateral Accel. (% g) TLP Spar - All Hs (feet) Spar -No Semi Figure 3: Comparison of Motions for Spar, TLP and Semisubmersible 11