WO2002047970A1

WO2002047970A1 - Low motion semisubmersible floating production system

Info

Publication number: WO2002047970A1
Application number: PCT/US2001/048210
Authority: WO
Inventors: Richard Bartholomew D'souza; Patrick James Kelly; David Leslie Barton; William Lewis Greiner
Original assignee: Halliburton Energy Services, Inc.
Priority date: 2000-12-15
Filing date: 2001-12-14
Publication date: 2002-06-20
Also published as: AU2002229043A1

Abstract

A system includes a semisubmersible (20) with an optimized low motions hull form (82), a plurality of steel catenary risers (30) connected to and extending from the semisubmersible, and a semi-taut mooring system (40) attached to the semisubmersible and extending to the ocean floor (34). The system maintains a motions envelope capable of preventing the steel catenary risers from failing either from fatigue or structural failure. The system is optimized by obtaining the meteorological ocean data at the location of the subsea oil and gas field, determining the number and sizes of steel catenary risers, determining the motions envelope required to prevent failure of the steel catenary risers, and optimizing the combination of the semisubmersible hull form, semi-taut mooring system, and steel catenary risers to maintain the motions of the semisubmersible within the motions envelope required of the steel catenary risers. The low motions semisubmersible incorporates center columns (86a-b) larger that the corner columns (84a-d), and bouyancy elements or pontoons which are wider at the center than the ends.

Description

LOW MOTION SEMISUBMERSIBLE FLOATING PRODUCTION SYSTEM

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of 35 U.S.C. 111(b) provisional application Serial No. 60/255,806 filed December 15, 2000, and entitled "Low Motion Semisubmersible

Floating Production System" hereby incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable. BACKGROUND OF THE INVENTION

The present invention relates to apparatus and methods for producing hydrocarbons from one or more subsea wells from a semisubmersible, and more particularly to a production system which includes a semisubmersible, steel catenary risers extending from the semisubmersible to the ocean floor, and a semi-taut mooring system, and still more particularly to the optimization of the semisubmersible, steel catenary riser system, and semi-taut mooring system to achieve a low motion envelope which will avoid the fatigue and/or structural failure of the steel catenary risers in the weather and sea environment at the geographic location of the wells.

The offshore drilling, completion, workover and production of wells for the production of hydrocarbons occurs from a platform supported at the surface. Such platforms include fixed platforms supported from the ocean floor or vessels, such as semisubmersibles or ships that float on the surface. See for example U.S Patents 5,277,521; 5,791,819; 4,646,672; and 5,435,262. Fixed platforms necessarily are used in shallow water where their legs may extend from the platform to the ocean floor. In deep water, vessels support the platform for conducting operations. See U.S. Patent 4,983,073, hereby incorporated herein by reference.

Increased activity in the offshore drilling, completion, workover and production of wells has caused such activity to occur in deep water, defined herein as water having a depth greater than 500 meters. Deep water poses unique problems associated with conducting underwater operations. One of the principal problems with deep water operations is extending one or more strings of pipe from the ocean floor to the platform where one end of the pipe string is fixed to the ocean floor while the upper end of the pipe string is attached to a floating vessel subject to motion due to the environment.

Floating vessels have six degrees of motion, three of which are linear and three of which are rotational. Linear motion includes surge, which is fore and aft movement, and sway, which is port to starboard (sideways) movement. Heave is vertical movement. Rotational motion includes roll, which is angular movement between port and starboard, and pitch, which is angular movement between fore and aft. The last degree of movement is yaw, which is the changing of the heading of the vessel. These degrees of freedom of motion of the vessel must be limited otherwise the risers connecting the floating platform to the ocean floor will fatigue and/or structurally fail. In other words, the risers can only withstand so much motion or they will fail.

The motions of the vessel are caused by excitation due to the environment at the particular geographic location in which the vessel is operating. The environment excites the vessel causing it to move in the six degrees of motion. The environment includes the number, amplitude, and frequency of the waves and swells and it also includes currents and winds. This environment is continuously studied to understand its physical properties, which are published as meteorological ocean data for a given geographic location in a body of water. For example, the Gulf of Mexico exhibits moderately large wave heights along with strong currents with loop currents. The North Sea, on the other hand, has greater wave heights but weaker currents. Also, the Gulf of Mexico has higher wind speeds due to hurricanes than does the North Sea. Other environments such as West Africa and portions of South America exhibit small wave heights and low wind speeds but experience very long period swells.

Narious mooring systems have been developed to limit the motions envelope of the vessel. Narious factors are considered in selecting a mooring system configuration, such as water depth and meteorological ocean data. A mooring system attaches the vessel to the ocean floor and acts as a type of spring to assist with position keeping. The floating platform has an equilibrium position where the vessel is at rest when no environmental force is being applied to the platform, i.e. the point at which the platform is stationary with no wind, wave, swells or currents being applied. When an environmental force is applied to the vessel, the vessel moves away from its equilibrium position causing additional tension on a portion of the mooring system such that the mooring system provides a restoring force, like a spring, to move the vessel back to its equilibrium position. The more the vessel moves away from its equilibrium position, the greater the restoring force that the mooring system applies to the vessel. In effect, the mooring system serves as a naturally stabilizing positioning system for the floating vessel. See for example U.S. Patents 4,281,613; 4,493,282; RE 32,119; 5,704,307; and 5,884,576.

One type of mooring system which has traditionally been used for mooring floating vessels, such as semisubmersibles and ships, is a catenary system. See U.S. Patents 5,222,453 and 4,422,401, hereby incorporated herein by reference. A catenary mooring system includes a plurality of cables consisting of chains, synthetic lines, wire or any combination thereof, which drape under their own weight from the vessel down to the ocean floor and has a ground tackle segment which touches down along the ocean floor forming a horizontal or leader portion along the ocean floor. This horizontal portion is designed to have a length such that no matter what kind of environmental force is applied to the vessel, there will be no vertical force vector applied to the anchor. It is preferred that this length of ground tackle not be lifted off of the ocean floor, so as to provide a margin of safety to the anchor holding capability. The ground tackle is attached at its lower end to a conventional drag embedment anchor, similar to anchors typically used for the temporary mooring of a ship. The conventional drag embedment anchor only holds if there is a zero vertical force on the anchor, i.e. no uplift at the touchdown point of the anchor. This type of anchor only holds when a horizontal force vector is applied, since a vertical force vector will cause the anchor to disengage the sea floor, allowing the release of the mooring system. With respect to a catenary, a mathematical formula describes the shape of its free- hanging configuration. A catenary mooring system hangs its cables, consisting of chain, synthetic line, wire or any combination thereof, naturally from the floating vessel with their other ends fixed to drag embedment anchors, as described above. The cables have no bending properties of any significance and merely hang using the force of gravity. Sometimes in floating platforms, a catenary mooring system with localized intermediate buoyancy is used. In this type of a system, the chain or wire cable extends from the leader portion to a buoy at its upper end, forming a buoyancy point. Another length of chain or wire cable, called a pennant, extends from the buoyancy point to the fixed platform. This allows the leader portion and anchor to be laid first, and then the platform may be connected later to the platform at the buoyancy point.

Another type of mooring system is the taut leg mooring system. The taut leg mooring system includes a positive anchor in the ocean floor, such as a piling driven into the ocean floor or a suction anchor. A positive anchor is one that will not only withstand a horizontal force vector on the anchor, but also a vertical force vector. It can be any kind of an anchor that is designed to withstand a significant vertical force. The anchor is fixed to the ocean floor. The taut leg mooring system includes a cable consisting of chain, synthetic line, wire or any combination thereof that extends from the positive anchor to the floating vessel. The cable is under tension so as to extend virtually in a straight line between the positive anchor and the platform. Restoring force is generated by stretch of the cable. Stiffness of this type of mooring system is typically high compared to a catenary mooring system, wherein the restoring force is generated by picking up or laying down cable on the ocean floor. The high stiffness of the taut leg mooring is good for minimizing the motion of the platform. Taut leg moorings are typically only feasible when the wave frequency motions of the vessel are small compared with the length of the cable. A low motions semisubmersible therefore allows optimization of cable sizes for a taut leg system.

A semi-taut mooring system is a third type of system combining the attributes of taut leg and catenary mooring systems. The semi-taut mooring system includes cables consisting of chain, synthetic line, wire or any combination thereof, extending from the semisubmersible to fixed anchors on the ocean floor. A semi-taut mooring system maintains some cable on the ocean floor in its undisturbed position, so restoring force is generated by a combination of stretch in the cable and picking up line from the ocean floor. Vertical forces do occur at the positive anchor point under environmental loading. The semi-taut mooring system can effectively moor semisubmersibles exhibiting larger motions than could be moored by a taut leg mooring system, while providing much more restoring force than could a catenary mooring system.

The platform also includes one or more marine risers extending to the ocean floor, depending upon the particular operations being conducted, such as drilling, completion, workover, intervention or production. These marine risers extending to the ocean floor are subjected to the motion of the floating vessel.

Traditionally, during drilling and completion operations, joints of marine riser, connected by mechanical connectors, are assembled to extend from the platform to the ocean floor. Such a marine riser, being a column, is placed in tension to prevent the riser from failure through buckling. If the riser string goes into compression, it will fail. In deep water, a riser column is very long, thereby requiring a substantial amount of tension to be placed on the riser to avoid failure.

One type of marine riser used to avoid the impact on the vessel caused by the need to assemble and re-assemble the marine riser and/or the substantial amount of tension required in deep water, is a free standing riser, which includes a plurality of buoyancy cans attached to the riser string at various locations, whereby the buoyancy cans place the string in tension. For example, see U.S. Patent 6,082,391. A connector is attached to the top of the free standing riser for attachment to a tie back string that extends from the platform. The tie back riser is lowered and connected to the top of the free standing riser, and the tie back riser is placed in tension in the conventional manner. Typically, the tie back riser only extends a couple of hundred meters below the water surface so that in an emergency situation, such as in a severe storm, the tie back riser can be disconnected, so as to avoid damage to the free standing riser. The tie back riser, being shorter in length, requires less time to remove and less storage space on the vessel. Further, it requires less tensioning equipment. A free standing riser is particularly beneficial for batch production jobs performed on a template where the vessel substantially stays in place, allowing the free standing riser to be attached to one wellhead for a period of time and then moved to another wellhead. Disconnecting the tie back riser from the free standing riser causes minimal impact to the vessel. Free standing risers may also be used for drilling, workover or intervention in a completed well.

Another type of riser is a flexible riser, such as that manufactured by Coflexip or Wellstream. The wall of the flexible pipe includes oriented steel bands together with layers of plastic, allowing the riser to be more flexible than regular steel pipe. However, flexible riser is relatively expensive and must be ordered in advance due to production requirements.

Still another type of riser, which can extend between the ocean floor and platform, is a steel catenary riser. See for example U.S. Patents 6,062,769; 5,639,187; and 5,702,205. A steel catenary riser is a metal pipe that extends from the vessel to the ocean floor so that one end lies flat on the ocean floor. A steel catenary riser is installed similarly to a flow line or pipeline. Sections of pipe are joined to the steel catenary riser and lowered to the ocean floor. The steel catenary riser is similar to a pipeline with one of its ends lifted off the ocean floor. It has been determined that in very deep water bending of steel catenary risers comes within an acceptable range because of the relative magnitude of the water depth versus the diameter of the pipe. The steel pipe hung off from the vessel forms a shape very analogous to that of a cable in a catenary mooring system. Steel catenary risers are simpler and cheaper to construct, offer a wider range of sizes in deep water relative to flexible pipe, and are a viable alternative to vertically tensioned steel marine risers. A steel catenary riser has the further advantage in that it can be constructed at almost at any location. The steel catenary riser has a touchdown point on the ocean floor and has its lower end connected to apparatus located on the ocean floor. Such apparatus may include a manifold that gathers production from a plurality of wells or a pipeline for transporting hydrocarbons to a storage vessel. The motion of the vessel tends to subject the steel catenary riser to the possibility of fatigue and/or structural failure. Therefore, it is necessary to limit the envelope of motion of the steel catenary riser to avoid these modes of failure.

The present invention overcomes the deficiencies of the prior art. SUMMARY OF THE INVENTION

The system of the present invention includes a semisubmersible which is optimized for low motions, a plurality of steel catenary risers connected to and extending from the semisubmersible to the ocean floor, and a semi-taut mooring system also attached to the semisubmersible. The combination of the semisubmersible, steel catenary risers, and semi-taut mooring system is optimized to provide a low motions envelope of the semisubmersible to avoid failure of the steel catenary risers. In particular, the semisubmersible includes a hull form with a plurality of columns, each with one end attached to the platform and another end attached to a buoyancy member. The semisubmersible design features four corner columns and two or more center columns, with the center columns having a greater cross-sectional area than the comer columns. Further, the buoyancy members are preferably pontoons with that portion of the pontoon attached to the center columns being larger than that portion of the pontoons attached to the corner columns. Adjacent pontoons are connected either by cross pontoons or by other structural cross connections.

The system is optimized by first obtaining the meteorological ocean data for the location of the subsea oilfield. The steel catenary riser numbers and sizes are determined based on functional requirements and a motions envelope is determined based on the structural properties of the steel catenary risers. The combination of the semisubmersible, semi-taut mooring system, and steel catenary risers are then optimized whereby the motions of the semisubmersible are maintained within the motions envelope of the steel catenary risers. Where the platform includes drilling or workover capability, the system may be used in combination with a free standing riser extending to the ocean floor.

One advantage of the present invention includes the configuration of a hull form capable of supporting large topside payloads, such that for the given environmental conditions at the production site, low motions (heave, pitch and roll), are achieved. The motion performance achieved is on the order of 20% to 40% less than that of current competitive designs of floating production system semisubmersible hulls (see Figure 2 for a comparison of the semisubmersibles' heave response amplitude operators). The shape and proportions of the hull's pontoons and columns are designed to minimize the heave motions in normal and extreme sea states, thus mitigating both fatigue and extreme loading on the steel catenary risers. The hull form also provides a large hydrostatic rotational stiffness, reducing quasi-static roll and pitch motions in storm conditions induced by a combination of steady wind and mooring system moments, again reducing loading on the steel catenary risers. These results are achieved while still maintaining a high pay load to displacement ratio, a key measure of the efficiency of a semisubmersible's hull. The reduced platform motions may also allow the semisubmersible to perform drilling and production operations in more severe environments. By minimizing the mass at the corners of the platform, stresses induced by waves interacting with the hull are reduced. Further, the hull shape is configured to achieve omni-directional response in wind and waves. This flexibility permits the steel catenary risers to be supported at any location on the hull pontoons with no penalty for vessel orientation relative to extreme weather or location of subsea wells/manifolds.

A further advantage of the present invention includes its minimal platform motion response due to the optimal low motion hull form, thus permitting use of an optimized semi- taut mooring system. The mooring system is configured to optimize surge, sway and yaw motions, again with the goal of reducing platform motions to a range acceptable to the steel catenary risers. The tuning together of this combination of features, i.e. a low motion semisubmersible hull form, an optimized semi-taut mooring system, and the characteristics of the steel catenary risers, results in a low cost, site optimized floating production system capable of efficiently producing hydrocarbons from deep water oil and gas fields.

Still another advantage of the present invention is that the semisubmersible hull incorporates structural features which eliminate the need for vertical bracing, which allows a fully outfitted production deck to be mated to the hull by means of a "float-over" procedure. This permits employment of an efficient, modular approach to fabricating the production system and eliminates the need for extensive offshore hook-up and commissioning and expensive, weather sensitive operations.

Other objects and advantages of the invention will appear from the following description. BRIEF DESCRIPTION OF THE DRAWINGS For a detailed description of the embodiment of the invention, reference will now be made to the accompanying drawings wherein:

Figure 1 is a schematic elevation view of the installed configuration of the production system of the present invention; Figure 2 is a heave response amplitude operator graph, showing the heave amplitude versus period for the low motion semisubmersible hull as compared to floating production system semisubmersibles' heave response amplitude operators typical of platforms in current use; Figure 3 is a side elevation view of the preferred semisubmersible of the present invention;

Figure 4 is a section view taken at the water's surface looking down at the semisubmersible shown in Figure 3; and

Figure 5 is a schematic elevation view of the installed configuration of a production and drilling system of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the description that follows, like parts are marked throughout the specification and drawings with the same reference numerals. The drawing figures are not necessarily shown to scale; certain features of the invention may be shown in exaggerated scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness.

The present invention relates to methods and apparatus for performing operations in a field of one or more offshore subsea wells. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein, h particular, various embodiments of the present invention provide a number of different constructions and methods of operation in conducting offshore operations, including but not limited to drilling, completion, production, workover, and well and/or subsea intervention. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any ^suitable combination to produce desired results. Reference to "up" or "down" will be made for purposes of description with "up" meaning "toward the water's surface" and "down" meaning "toward the ocean floor". Referring initially to Figure 1, system 10 includes a vessel, such as a semisubmersible

20, a plurality of steel catenary risers 30 and semi-taut mooring system 40. Semisubmersible 20 includes a platform 22, a plurality of support columns 24 and two or more buoyant members 26, also called pontoons. Equipment 28, 29 is disposed on platform 22 for conducting subsea operations and processing of well fluids, preferably for a field 32 of one or more producing wells extending from the ocean floor 34 down into the earth. Semisubmersible 20 floats at water's surface 36.

In one embodiment, system 10 is a production system used for the production of field 32. Production system 10 includes a plurality of steel catenary risers 30 extending from semisubmersible 20 to the ocean floor 34. Riser 38 shows one type of steel catenary riser. Steel catenary riser 38 is a steel pipe having its upper end 42 connected to pontoon(s) 26 by connector 44. This connection could also occur at the platform 22, but is most commonly made at the pontoon 26. Steel catenary riser 38 is hung or draped from semisubmersible 20 down into the water with a generally horizontal leader portion 46 touching down at 48 on ocean floor 34 and extending to connector 50 at its lower terminal end 52. For exemplary purposes only, steel catenary riser 38 is shown connected to subsea production manifold 54 serving as a collection point for a plurality of the producing wells in field 32. A choke manifold (not shown) may be included which would include flow lines extending from the trees of individual wells to production manifold 54. Steel catenary riser 38 is made up of sections of steel pipe and other conduits joined together during the installation operation.

It should be appreciated that the production system 10 will include a plurality of steel catenary risers 30 extending from semisubmersible 20 to ocean floor 34 and that such risers 30 may be used for a variety of operations in the conventional production of hydrocarbons from field 32. For example, steel catenary risers 30 may include one or more risers, such as riser 38, for the transporting of produced hydrocarbons from a collection point, such as manifold 54, to platform 22. Typically such hydrocarbons are passed through a separator, such as separator 29, where oil, gas, and water are separated from the produced fluids. Another use for a steel catenary riser 30 includes steel catenary riser 56, having its upper end 58 connected by connector 60 to pontoon(s) 26 and hung or draped down to ocean floor 34, where its lower terminal end 62 is connected by connector 64, such as for example to a pipeline 66. It should be appreciated that export riser 56 may be used for transporting either oil or gas which has been separated from produced fluids by separator 29 for transportation to pipeline 66 or to a storage facility, such as a floating production storage and off loading unit similar to a large tanker. Failure of any riser could cause significant environmental pollution and severely disrupt production operations resulting in serious commercial liability. Maintaining the integrity of these risers is of vital importance to the production system. It should also be appreciated that steel catenary risers 38 may be used for water or gas injection risers where water or gas is used to apply pressure to the formation to assist in production. Such water or gas injection risers (not shown) will also be connected to pontoon(s) 26, or possibly the platform 22, and draped to ocean floor 34, similarly to that of steel catenary risers 38, 56.

Depending upon field 32 and the location of individual subsea producing wells, there may be a plurality of steel catenary risers 30 based on the subsea configuration of field 32. For example, there may be production risers, water injection risers and export risers. Further, control line umbilicals will also be suspended from semisubmersible 20 for the control of individual wells and subsea manifolds. It should be appreciated that steel catenary risers 30 may be used for a variety of purposes including bringing fluids to and from platform 22, with such fluids having various pressures and with individual steel catenary risers 30 being dimensioned for their particular use.

The environment, such as the seas and weather, at the particular location of field 32 will cause an excitation of semisubmersible 20 such that semisubmersible 20, as a floating vessel, will move in six degrees of freedom, namely surge, sway, heave, roll, pitch, and yaw. These motions of semisubmersible 20 place moments and forces on the connections of steel catenary risers 30 to pontoon(s) 26, such as connector 44 for riser 38 and connector 60 for riser 56. The amplitude and frequency of the motions of semisubmersible 20 must not fatigue and/or structurally load the steel catenary risers, so as to cause them to fail. The motions of semisubmersible 20 must be maintained within a limited or low motions envelope to ensure that the steel catenary risers' predetermined fatigue life is not exceeded and that they do not fail structurally. Once the number and size of the steel catenary risers 30 is known, and upon setting a preferred or predetermined fatigue life for these risers, the low motions envelope required for the movements of semisubmersible 20 may be determined, since the number of motion cycles which can be absorbed by the risers and connectors and the maximum motion amplitude will be known.

Knowing the low motions envelope required for the steel catenary risers, system 10 is designed and optimized for limiting the motions of system 10 to within the motions envelope of these risers. Motions may be optimized for the meteorological ocean data for the specific geographic location of field 32. This meteorological ocean data defines the environment of the geographic area of field 32 that will excite system 10. Such data will include data on the amplitude and frequency of the waves and swells, as well as the currents and winds at the geographic location of field 32. Once the environment of the particular geographic location of field 32 is known, system 10 is optimized to maintain the motions of semisubmersible 20 within the motions envelope in view of the local meteorological ocean data.

Semi-taut mooring system 40 includes a plurality of cables 70, preferably two or more cables 70 attached to each corner of semisubmersible 20, having upper ends 72 attached to semisubmersible 20 and hung or draped downwardly with lower terminal ends 74 attached to fixed anchors 76. Cable 70 may be a chain, synthetic line, wire or any combination thereof. Fixed anchor 76 may be a piling 78 driven into ocean floor 34, a suction anchor, or any other type of anchor that can withstand significant vertical forces. Tension is applied to line 70 at its connection to semisubmersible 20 to move cable 70 out of its equilibrium position, i.e. a catenary position. When the top end of the cable is moved away from anchor 76, the cable 70 does not have a zero tangent at ocean floor 34 but forms an angle 80 with the horizontal, thereby creating a force vector x in the horizontal direction and a force vector y in the vertical direction. Positive anchor 76 is designed to withstand the vertical force vector in the y direction. Semi-taut mooring system 40 provides a restoring force, when combined with the characteristics of semisubmersible 20 and the steel catenary risers 30, for maintaining system 10 within the motions envelope required for the steel catenary risers. Semi-taut mooring system 40 is particularly tuned for water depth D with depth D being at least 500 meters and taking into account the meteorological ocean data for the particular location of field 32.

The restoring force provided by line 70, created by movement of semisubersible 20, is further refined by taking into account the impact of steel catenary risers 30. Since steel catenary risers have stiffness, this stiffness provides a restoring force for semisubmersible 20. Steel catenary risers provide a tension and exhibit similar properties to that of a catenary mooring system. Thus, steel catenary risers 30 provide a restoring force due to their properties as a catenary. Accordingly, in determining the design and particularly the tension of semi-taut mooring system 40, the mooring capabilities of steel catenary risers 30 are taken into account. Thus, as an environmental force is applied to semisubmersible 20 and semisubmersible 20 is moved away from its equilibrium position, semi-taut mooring system 40 and steel catenary risers 30 provide a restoring force, causing semisubmersible 20 to return to its equilibrium position, so as to maintain the excursions of semisubmersible 20 within the desired motions envelope.

Most importantly, semisubmersible 20 is designed, such that, when combined with semi-taut mooring system 40 and steel catenary risers 30, the motions of semisubmersible 20 are maintained within the motions envelope dictated by the structural properties of the risers. In particular, the meteorological ocean data for the particular location of field 32 is studied with respect to the design and structure of semisubmersible 20. In contemplating the design and structure of semisubmersible 20, heave response amplitude operators are determined to model the dynamic response of semisubmersible 20 to the waves. Referring now to Figure 2, a heave response amplitude operator is illustrated for the low motion semisubmersible 20 compared with two examples of the prior art. A response amplitude operator is the ratio between the response amplitude of the motion (e.g. heave) and the wave amplitude for specific wave periods. The response amplitude operator for a specific vessel configuration is determined through either model testing or hydrodynamic analysis. The heave response at the lower periods is small as compared to the heave response at the longer periods such as above 20 seconds, for example. The heave increases substantially as the period between waves reaches the heave natural frequency of semisubmersible 20. The heave natural frequency of semisubmersible 20 is determined by the mass and stiffness of semisubmersible 20 in the water. Referring now to Figures 3 and 4, semisubmersible 20 is shown having a hull 82 designed and optimized for low motions. Hull 82 consisting of four corner columns 84a, 84b, 84c, and 84d and two or more center columns 86a, 86b. The upper ends of columns 84, 86 support platform 22. The lower ends of comer columns 84a, 84c and center column 86a are attached to buoyancy member(s) or pontoon(s) 88a and the lower end of corner columns 84b, 84d, and center columns 86b are affixed to buoyancy member(s) or pontoon(s) 88b. Cross pontoons 90a, 90b extend between the buoyancy members 88a and 88b, respectively. The individual cross-sectional areas 92 of corner columns 84a-d are smaller than the individual cross-sectional areas 94 of center columns 86a, b. Cross-sectional areas 92, 94 of corner columns 84 and center columns 86 form a planar area. Pontoons 88a, 88b have an overall length 96, an overall height 98, and a width at the ends 100 that is smaller than the width at the center 106.

Semisubmersible 20 is a column stabilized unit, which is a free floating object where the water plane at the water line is solely on columns 84, 86. The area that is exposed to the waves of the sea is the water plane area of these columns. Hull 82 includes columns 84, 86 and pontoons 88, so as to provide the buoyancy required to support platform 22. Platform 22 has a height 110 with the hull 82 providing a free board 112 of platform 22 above water surface 36 and a draft 114. The form of hull 82 is configured to achieve an omni-directional response to the wind and seas of the environment of the location of field 32. This allows semisubmersible 20 to support steel catenary risers 30 at any location on pontoons 88 with no penalty for vessel orientation due to the environment. Thus, it does not make any difference where steel catenary risers 30 are oriented from semisubmersible 20.

The hull form includes a distribution of buoyancy in columns 84, 86 and pontoons 88, which provide the required stability for semisubmersible 20 and, in particular, prevents any capsizing of semisubmersible 20. The water plane area is maximized to provide stability for semisubmersible 20, which resists roll or pitch, while also minimizing the water plane area to reduce the excitation of semisubmersible 20 by the seas. The larger water plane area makes the hull form stiffer and consequently more stable. However, a reduction in the cross-section of columns 84, 86 lessens the exposure of the hull form to wave action. The excitation of the waves is proportional to the volume of the buoyancy such that the buoyancy volume is minimized if the cross-sectional area of the columns is reduced. Semisubmersible 20 is made effective in part by optimizing the size of the columns, i.e. their cross-sectional area and buoyancy. The buoyancy is primarily located in the pontoons but may also be located partially within or on the columns. It should be appreciated that although only six columns are shown, additional columns may be employed. Further, columns may be rectangular, circular or a combination thereof in cross- sectional shape. The buoyancy distribution and the water plane area of the columns are optimized together with steel catenary risers 30 and semi-taut mooring system 40 to maintain the motions of semisubmersible 20 within the motions envelope required by the steel catenary risers. This optimization has been achieved by a design resulting in a simple and easily constructed hull. The hull form is tailored to meet the low motions envelope, in view of the meteorological ocean data for the particular location of the field 32.

The hull form is designed to respond to the meteorological ocean characteristics so as to maintain the motion of semisubmersible 20 within the motions envelope to avoid damaging the steel catenary risers. However, the characteristics of the hull form must also meet certain other requirements. There must be a certain amount of stability to avoid capsizing, as well as adequate buoyancy to support the weight of platform 22. Further, semisubmersible 20 must include reserve stability in case of damage and must maintain minimum free board 112. There must also be a certain wind loading for semisubmersible 20, platform 22, and the production facilities 28 and 29 they support. Thus, semisubmersible 20 must be designed to meet these other requirements, in addition to meeting the steel catenary riser's motions envelope.

In most preferred embodiments, there are at least six or more columns 84, 86 with cross-sectional area 94 of center columns 86 being larger than cross-sections 92 of corner columns 84. Further, the medial portion of pontoons 88 include hull profiles 102 located as shown outboard of the pontoon, or inboard of the pontoon or a combination thereof, thereby causing the medial portions of pontoons 88 to be larger than the ends of pontoons 88 adjacent to corner columns 84. Although cross connections of pontoons 88a, b are shown to be cross pontoons 90a, b, it should be appreciated that tubular cross braces with or without vertical diagonals may be substituted for cross pontoons 90. Further, cross connections can occur either between pontoons 88a, b or between the bottom or lower ends of columns 84.

In developing the optimized combination of semisubmersible 20, steel catenary risers 30, and semi-taut mooring system 40, the meteorological ocean data for the location of field 32 is studied in view of the motions envelope required the structural properties of the steel catenary risers 30. The mooring effect of steel catenary risers 30 for water depth D is calculated and measured. The number and sizes of corner columns 84 and center columns 86 is optimized for water plane area and vessel motions. Pontoon arrangements and sizes are considered to optimize motions. Overall buoyancy distribution and water plane areas are then further optimized to ensure stability, while minimizing excitation from the seas. Hull form is optimized to achieve an omni-directional response with respect to the wind and waves and to minimize excitation. Semi-taut mooring system 40 is combined with semisubmersible 20 and the mooring effect of steel catenary risers 30 to tune semi-taut mooring system 40 with a preferred amount of tension, such that the combination of semisubmersible 20, steel catenary risers 30, and semi-taut mooring system 40 maintains the motions of semisubmersible 20 within the motions envelope required by the steel catenary risers. The fine tuning of this combination optimizes system 10 for a particular location and/or environment. The skills required for this optimization include a team of experts on risers, mooring systems, and hull forms, all of whom are those skilled in the art.

Referring now to Figure 5, an alternative system 120 is shown. In system 120, semisubmersible 20 includes platform 22, which serves as a base for production, drilling, completion, workover, and intervention operations. In field 32, there may be wells that have not yet been drilled or producing wells that require workover or intervention. An example of one such well is well 122, having a well head and BOP stack 124 with connector 126. Free standing riser 130 includes a riser 128 with its lower terminal end connected to wellhead 124 by means of connector 126 and one or more buoyancy cans, such as top buoyancy can 132 and possibly other buoyancy cans 134 mounted on the exterior of riser 128. Buoyancy cans 132, 134 place free standing riser 128 in tension. At the upper end of riser 128 is connector 136. The upper end of riser 128 extends within a couple hundred meters of surface 36. Thus, the free standing riser 128 may be 300 or more meters long. For the drilling, workover, or intervention of well 122, free standing riser 128 is attached to connector 126 and extends upwardly due to buoyant cans 132, 134. A tie back riser 140 is lowered from platform 22 and its lower end is connected to connector 136 and thus to free standing riser 128. Tie back riser 140 is placed in tension in a conventional manner by tension device 138 disposed on semisubmersible 20.

Free standing riser 130 is particularly beneficial for batch production wells on a template where semisubmersible 21 remains on location and is able to tie back to free standing riser 130, connected to a particular well using tie back riser 140. Tie back riser 140 is preferably only a couple of hundred meters long so that it may easily be retrieved and stored on platform 22. Thus, disconnecting free standing riser 130 from semisubmersible 20 has minimal impact. Further, upon the completion of the drilling, workover, or intervention of well 122, free standing riser 130 may be disconnected from wellhead 124 and raised at platform 22 to be moved to another well in field 32. This avoids the retrieval of the free standing riser onto platform 22. Thus, not only is there a great saving in time but also savings in storage space on platform 22. Further, such an amount of pipe would add significant weight to semisubmersible 20, thus requiring additional buoyancy. Further, platform 22 would require a large deck area with a commensurate amount of handling required for that amount of pipe.

Free standing risers are not commonly used with catenary-moored semisubmersibles, in part because of the wide range of motions of the semisubmersible. Thus, by maintaining semisubmersible 20 within a low motions envelope, a free standing riser may be used to provide drilling, completion, workover, and intervention capability to the production system. In particular, it may be used to drill and complete subsea wells and for the workover or intervention of producing wells. While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the drawings and detailed description are not intended to limit the invention to the particular form disclosed, or on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention as defined by the appended claims.

Claims

1. A production system comprising: a low motions semisubmersible; a plurality of steel catenary risers connected to and extending from the semisubmersible; and a semi-taut mooring system attached to the semisubmersible.

2. The production system of claim 1, wherein said semisubmersible includes a low motions hull form which, when combined with said steel catenary risers and semi-taut mooring system, maintains a motions envelope capable of preventing failure of said steel catenary risers.

3. The production system of claim 2, wherein said hull form includes at least six or more columns, each having one end affixed to a production platform and another end attached to a buoyancy member.

4. The production system of claim 3, wherein said columns include four comer columns and two or more center columns, each said center columns having a greater cross-sectional area than each said corner columns.

5. The production system of claim 3, wherein said columns include four comer columns and two or more center columns, the center portion of said buoyancy members attached to said center columns being larger than the other portion of said buoyancy members attached to said corner columns.

6. The production system of claim 5, wherein adjacent buoyancy members are connected by cross connections.

7. The production system of claim 5, wherein adjacent buoyancy members are connected by cross brace tubes.

8. The production system of claim 1, wherein said semi-taut mooring system includes at least eight cables consisting of chain, synthetic line, wire or any combination thereof, extending from said submersible to fixed anchors, said cables being semi-taut.

9. The production system of claim 8, wherein said cables form an angle with the ocean floor at the fixed anchor when the vessel is offset.

10. The production system of claim 8, wherein said cables maintain a vertical force vector at said fixed anchor when the vessel is offset.

11. The production system of claim 1, further including a tie back riser connected to a free standing riser extending from said semisubmersible to the ocean floor.

12. A method of optimizing an offshore production system for a subsea field comprising: obtaining the meteorological ocean data for a location of the subsea field; determining the number and sizes of a plurality of steel catenary risers which extend from a semisubmersible to the ocean floor; determining the motions envelope which will prevent the steel catenary risers from failing from fatigue or from structural failure; and tuning the combination of the semisubmersible hull form, semi-taut mooring system, and steel catenary risers whereby the motions of the semisubmersible are maintained within the motions envelope required to prevent failure of the steel catenary risers in view of the meteorological ocean data.

13. The method of claim 12, wherein said semisubmersible is located at a water depth greater than 500 meters.

14. The method of claim 12, wherein said semi-taut mooring system provides a vertical force vector at a fixed anchor in the ocean floor when the vessel is offset.