FIELD OF THE INVENTION

The present invention relates to offshore support structure, particularly for use in the shallow water and methods installing an offshore support structure for use in a body of water with safe and quick installing the offshore support structure at the offshore field site. BACKGROUND OF THE INVENTION

Offshore structures have been provided which have platform structures mounted thereon to support various types of drilling and production units. Many of these offshore structures are exceedingly large, massive, and expensive. Many wells are drilled at offshore locations from a jack-up drilling rig or a semi-submersible drilling rig, and after the drilling process has been completed, a platform supported by some sort of support structure is still necessary for the production of the hydrocarbons. These support structures are likewise quite expensive. It is thus desirable to reduce the cost of offshore support structures, so that the cost of placing a field into production is minimized. It would then be possible that some less productive, or marginal, offshore fields could be placed into production of hydrocarbon.

Conventional designs of shallow water platform are extension of conventional jacket technologies, and are not optimized for site specific conditions. These designs much focus on in-place design condition and ignoring the possible optimization for the pre-services design conditions. The resulted design intends to be heavier in weight, more difficult in fabrication and installation and is noneconomic particularly for shallow water marginal field.

US 5498107 disclosed an apparatus and method for supporting the conductor pipe, the access platforms, the wellhead and the wellhead equipment of an offshore well located in a depth of water above a sea bottom after a drilling rig has been removed from the location, where a movable drilling rig is used to drive the conductor pipe into the mudline of the water bottom and temporarily support the conductor pipe while the offshore well is drilled and completed with a wellhead, through the conductor pipe, comprising means for holding the conductor pipe in tension from the drilling rig; a caisson driven into the mudline adjacent to the conductor pipe; a plurality of at least three anchor piles driven into the mudline around the caisson; a plurality of anchor cables; means for attaching to the anchor piles and to the caisson; means for tightening the cables to a desired degree of tension; a plurality of conductor braces mounted to the caisson between the caisson and the conductor pipe for supporting the conductor pipe at intervals along its length above the mudline; and wellhead access means mounted to the caisson for access to the wellhead.

There are certain disadvantages to the methods and apparatus described by US 5498107. Among others, such offshore support structure requires high breaking strength cables to support the caisson, and could collapse when the highly tensioned cables slacked over time which is a natural behavior of cable materials. In addition, due to its complexity, a prolong period is required to construct such structure with unconventional skills and heavy equipment to install at the predetermined site, inducing long offshore operation time, high installation cost and offshore installation risk. Therefore, there is a need for an offshore support structure which can further optimize the structure configuration and avoid the use of cable as permanent structural members, and minimizes offshore construction duration, cost and risk.

The present invention overcomes these and other deficiencies of the above-mentioned drawbacks by developing an improved offshore support structure which can be constructed by using light weight lifting machinery, can be shipped with conventional means of transport or self-transported and can be installed within a shorter offshore time using the conventional techniques and equipment, and thus permits the production of inexpensive as well as reusable offshore platform.

SUMMARY OF THE INVENTION

The present invention provides an offshore support structure for use in a body of water comprising a topside structure located above a surface of the body of water, a base support structure located below the surface of the body of the water having a topside support caisson connected to the topside structure, a plurality of diagonal bracing members extended from the topside support caisson, a plurality of water depth adjusters connected to the diagonal bracing members to regulate the height of the offshore support structure according to a depth of the seabed; and each water depth adjuster is attached to a mudmat as a support foundation of the offshore support structure.

In one of the preferred embodiment of the present invention, the base support structure forms a 3D space frame structure wherein each face of the frame being defined by the topside support caisson, diagonal bracing members and water depth adjusters.

In another preferred embodiment of the present invention, the framework extends from the surface of the body of water to the seabed.

In one of the embodiment of the present invention, the framework is a trigonal pyramidal structure and the framework is formed from a plurality of elongated cylindrical support columns. In one of the preferred embodiment, the mudmat is buoyant mudmat for the offshore structure to floating on the surface of the water and providing a self-transporting of the offshore support structure based on the buoyancy on surface of the water to the predetermined offshore field site and the buoyant mudmat is a buoyancy tank in this process.

A method of assembling an offshore support structure for use in a body of water comprising determining the size of the offshore support structure to be assembled according to a depth of the seabed, forming and fabricating an one-piece base support structure of the offshore support structure by welding a topside support caisson to a plurality of diagonal bracing members and subsequently by welding to a plurality of water depth adjusters to the diagonal bracing members, attaching a mudmat to each water depth adjuster and mounting a topside structure to the base support structure vertically to form the offshore support structure.

A method of installing an offshore support structure for use in a body of water comprising transporting a topside structure and a base support structure of the offshore support structure to a predetermined offshore field site by a towing flattop barge, lifting the base support structure from the towing barge and installing the base support structure onto a seabed of the a predetermined offshore field site, mounting the topside structure onto the base support structure by lifting to form the offshore support structure and commissioning the offshore facility. In an alternative embodiment, a method of installing an offshore support structure for use in a body of water comprising loading out the offshore support structure having a topside structure and a base support structure to float onto the water line wherein a plurality of buoyant mudmats attached to a bottom end of the base support structure, wet towing the offshore support structure by its own buoyance and stability to a predetermined offshore field site and filing the buoyant mudmats with sea water to sink the base support structure of the offshore support structure assisted by a 300 ton lifting crane, installing the base support structure onto a seabed of the a predetermined offshore field site and commissioning the offshore support structure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. Figure 1 illustrates an offshore support structure for use in a body of water in accordance of an embodiment of the present invention.

Figure 2 illustrates a perspective view of offshore support structure for use in a body of water in accordance of an embodiment of the present invention.

Figure 3 illustrates a side view of offshore support structure for use in a body of water and located between surface of the body of water and seabed in accordance of an embodiment of the present invention. Figure 4 illustrates a topside structure of offshore support structure for use in a body of water in accordance of an embodiment of the present invention.

Figure 5 illustrates a mudmat of offshore support structure for use in a body of water in accordance of an embodiment of the present invention. Figure 6 illustrates a top view of offshore support structure for use in a body of water in accordance of an embodiment of the present invention.

Figure 7 illustrates an offshore support structure for use in a body of water in accordance of another embodiment of the present invention.

Figure 8 illustrates a perspective view of offshore support structure for use in a body of water in accordance of another embodiment of the present invention. Figure 9 illustrates a top view of offshore support structure for use in a body of water in accordance of another embodiment of the present invention.

Figure 10 illustrates a side view of offshore support structure for use in a body of water and located between surface of the body of water and seabed in accordance of another embodiment of the present invention.

Figure 11 illustrates a side view and a top view of the buoyant mudmat of offshore support structure for use in a body of water in accordance of another embodiment of the present invention.

Figure 12 illustrates a method of installing an offshore support structure for use in a body of water in accordance of an embodiment of the present invention.

Figure 13 illustrates a method of installing an offshore support structure with buoyant mudmats for use in a body of water in accordance of another embodiment of the present invention.

DETAILED DESCRIPTIONS OF THE INVENTION

The present invention will now be described in detail in connection with specific embodiments with reference to the accompanying drawings. Unless the context requires otherwise, throughout the specification and claims which follow, the word "comprise" and variations thereof, such as, "comprises" and "comprising" are to be construed in an open, W

inclusive sense that is as "including, but not limited to". Reference throughout this specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity. It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B." Certain terms are used throughout the following description and claims refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name but not function. The drawing figures are not necessarily to scale. Certain features and components herein may be shown exaggerated in scale or in somewhat schematic form and some details of conventional elements may not be shown in interest of clarity and conciseness.

The present invention relates to offshore support structure, particularly for use in the shallow water and method installing an offshore support structure for use in a body of water in one piece. Figure 1 illustrates the offshore support structure (100) for use in a body of water having a topside structure (110) located above a surface of the body of water, a base support structure (112) located below the surface of the body of the water having a topside support caisson (114) connected to the topside structure(110), a plurality of diagonal bracing members (116) extended from the topside support caisson (114), a plurality of water depth adjusters (118) connected to the diagonal bracing members (1 6) to regulate a height of the offshore support structure (100) according to a depth of the seabed and each water depth adjuster (1 8) is attached to a mudmat (120) as a support foundation of the offshore support structure. Figure 2 illustrates a perspective view of offshore support structure (200) for use in a body of water in accordance of an embodiment of the present invention. The base support structure forms a 3D space framework (210) wherein each face of the framework being defined by the topside support caisson (212), diagonal bracing members (214) and water depth adjusters (216). The diagonal bracing members (214), typically constructed of tubular steel members, consists of three main inclined and radially spaced legs approximately from forty-five (45) degrees to sixty (60) degrees (218) from horizontal, the upper ends of which are rigidly connected to a segment of vertical pipe i.e. topside support caisson (212). The offshore support structure of the present invention (200) provides a working surface for production well drilling, completion, and producing activities.

The framework further extends from the surface of the body of water (314) to the seabed (316) as illustrated in Figure 3. For the purposes of shallow water application such as water depth approximately 60 meters, the offshore support structure made from steel preferably with topside structure(310) with a weight of approximately 300 metric tonnes and the base support structure (312) with a weight approximately 340 metric tonnes. Depending on the depth of the seabed, a typical length of this type of offshore support structure with a topside structure having a height of approximately 3.5 metres, a base support structure located below the surface of the body of the water having a topside support caisson having a length of approximately 30 metres connected to the topside structure, a plurality of diagonal bracing members extended from the topside support caisson until a plurality of water depth adjusters having a length of approximately 41 metres. Each water depth adjuster is attached to a mudmat having a length of approximately 10 metres with skirt of approximately 2 to 2.5 metres as a support foundation of the offshore support structure.

Figure 4 illustrates a topside structure (410) of offshore support structure for use in a body of water in accordance of an embodiment of the present invention. The topside structure (410) is preferably designed with two topside deck levels (412). Each topside deck size is approximately 10 m x 10 m, and its total design weight is approximately 300 tons. The material of the topside structure is conventional steel with yield strength of 50 ksi (345 MPa) or similar. A welding connection is used between the topside structure and the base support structure. The topside deck can accommodate equipment and facilities of many different functions as needed. Referring now to Figure 5, the mudmat (500) of the offshore support structure for use in a body of water in accordance of an embodiment of the present invention is formed. The mudmat (500) comprises of mudmat plate (520), skirt plate and inside shear plates (530), and mudmat support frame (540). These plates are conventional steel material with 50 ksi (345 MPa) yield strength or similar. Mudmat size is approximately 10m x10m square (or 10 m in diameter). The skirt plate is approximately 2 m to 2.5 m depth. The mudmat with skirt plate efficiently increases the vertical bearing capacity and lateral sliding resistance. In addition, the mudmat with skirt plate eliminates the loss of strength of shallow sand layer during seismic activity due to sand liquefaction. Conventional welding connection is used between water depth adjuster and mudmat. Mudmat acts as permanent support foundation for the offshore support structure for the in-place service condition. This concept eliminates the need of foundation pile, thus provide saving on steel, offshore installation operation and schedule. In addition, it also reduces offshore risks due to the elimination of the piling and grouting operation as well as eliminating the risk of penetrating shallow gas deposit. With the use of mudmat to replace foundation pile, the offshore support structure also known as platform becomes removable and reusable if necessary. For removal of the offshore support structure, water can be injected through hot stab receptacle and injection system (510) into the gap between the bottom of the mudmat and the soil by various means to separate the two. After the cutting of the conductors and risers if any, the offshore support structure is then lifted off the seabed either in one piece or in multiple pieces per design and available equipment. When the mudmats are buoyant mudmats, deballasting the buoyance tanks of the mudmats prior to the step of injecting water into the gap of the mudmats and soil. The offshore support structure can be reused after removal with minimal modification if the water depth and topside functions are similar. The offshore support structure is re-installed to the next site by similar transportation and installation method as described herein. The concept of using mudmat as foundation to replace the pile foundation also reduces the dynamic response of the platform to seismic activities and eliminates the weakest link of a conventional piled platform at pile top-to-base structure connection, thus greatly improve the platform's seismic resistant capability. Furthermore, such mudmat may be disposed for attachment anywhere on base support structure or its related structure, and can be of any configuration necessary for a particular function, such as for example, rectangular or triangular.

Figure 6 illustrates a top view of offshore support structure for use in a body of water in accordance of an embodiment of the present invention. The framework is preferably a trigonal pyramidal structure (610) and forming a three support column symmetrical tripod. The framework formed from a plurality of elongated cylindrical support columns. The length L _h from the water adjuster (612) to the centre point of the framework (614) measured horizontally is approximately 19.5 metres while the distance L _d from water adjuster (612) to another water adjuster (616) is approximately 29.25 metres. By the use of the framework of present design which minimizes the number of framing members, the present structure lends itself to ease of relocation and reuse at a variety of sites. Nominal variations in water depths and topside weight may be accommodated by simply extending or shortening the topside support caisson, water depth adjusters, L _h , L _d and the sizes of the structural members.

In an alternative embodiment of the offshore support structure of the present invention, a buoyant mudmat (710) is introduced and attached to each water depth adjuster as illustrated in Figure 7. The buoyant mudmat (710) enables the offshore support structure to float on the surface of the water and self-transporting of the offshore support structure based on the buoyancy of the buoyant mudmat on surface of the water to the predetermined offshore field site. The buoyant mudmat is designed to meet the towing requirement such as the damaged stability requirement therefore sufficient compartmentation inside the buoyant mudmat is provided. Figure 8 and Figure 9 illustrate a perspective view and top view of offshore support structure for use in a body of water with buoyant mudmat. The length L _h of the water adjuster (910) to the centre point of the framework (912) measured horizontally is approximately 19.5 metres while the distance L _d from water adjuster (910) to another water adjuster (914) is approximately 29.25 metres. Figure 10 depicts a side view of offshore support structure for use in a body of water and located between surface of the body of water and seabed with buoyant mudmat. Depending on the depth of the seabed, a typical length of this type of offshore support structure with a topside structure (1010) having a height of approximately 3.5 metres, a base support structure located below the surface of the body of the water having a topside support caisson (1012) having a length of approximately 30 metres connected to the topside structure (1010), a plurality of diagonal bracing members extended from the topside support caisson until a plurality of water depth adjusters (1014) having a length of approximately 41 metres. Each water depth adjuster is attached to a mudmat (1016) having a height of approximately 6 metres with skirt (1018) of approximately 2 to 2.5 metres as a support foundation of the offshore support structure. Referring now to Figure 11 , the buoyant mudmat (1110) of the offshore support structure for use in a body of water in accordance of an embodiment of the present invention is a buoyancy tank. The buoyant mudmat (710) (shown in Figure 7) comprises of shell plate (1112), mudmat plate (1114), skirt plate (1116), and inside bulkhead plate (1118). The compartments shown in top view of Figure 11 are only for illustration, the bulkhead plates (1118) divides buoyant mudmat into a plurality compartments preferably four compartments. The number of compartments is designed for wet tow stability to account when the buoyancy tank damaged. During a ballasting process, a plurality of valves (1120) located at a lower section buoyant mudmat floods in seawater to lowering down the offshore support structure to seabed, and a valve (1122) located at an upper section of buoyant mudmat to vent out the trapped air in the buoyant mudmat. During the re-floating process for the removal of the offshore support structure for reuse, a reverse process is implemented. Compressive air is pumped into the each compartment of the buoyant mudmat through the valve (1122) located at an upper section of buoyant mudmat, and the sea water is released through the plurality of valves (1120) located at a lower section buoyant mudmat. Hot stab receptacle (1 24) as an interface of injection system which is designed to inject water into underneath of mudmat plate to release the soil suction during removing process of the offshore support structure from the seabed. All buoyancy tank and mudmat plates are made of conventional steel with yield strength of 50 ksi(345 MPa) or similar. Diameter of typical buoyancy tank is approximately between 6 m to 8 m. Its height is approximately between 6m to 8 m. The size of the buoyancy tank depends on the water depth, topside weight, and environmental loads, so the final size is optimized to meet the design requirement. All connections to water depth adjusters are welded connections. During wet tow, each buoyancy tank is empty providing buoyance to support the offshore support structure (also known as platform) on surface of water. When the offshore support structure (also known as platform) arrives at predetermined site, each buoyancy tank is ballasted by seawater via control valves and the offshore support structure (also known as platform) is lowered onto seabed with the assistance of a crane for lowering stability purpose. The requirement of such crane and craning operation is relatively low and operation time is short as compared to nominal offshore lifting operation. Buoyant mudmat provides the flexibility of self-transporting an integrated offshore support structure (topside structure and base support structure) from fabrication yard to the offshore field site. The buoyant mudmat of the present invention is able to provide sufficient buoyancy and stability of all conditions for the transport. The offshore support structure is fabricated in more conventional on land method. In the event that the integrated offshore support structure is to be fabricated in graving dock, the entire fabricated integrated offshore support structure is self-floated out from the graving dock. This reduces the cost incurred to loadout of the topside structure and base support structure of the integrated offshore support structure. The use of the buoyant mudmat eliminates the need of a transport barge, and the need of installing the topside side offshore, thus reduces costs and offshore schedule.

To assemble an offshore support structure for use in a body of water of the present invention, the global dimensions of the offshore support structure and the sizes of the members are determined according to a depth of the seabed, the topside weight and the environment primarily. By welding a topside support caisson to a plurality of diagonal bracing members and subsequently by welding to a plurality of water depth adjusters to the diagonal bracing members, one-piece base support structure of the offshore support structure is formed and fabricated. Then, a mudmat is attached to each water depth adjuster. Finally, a topside structure is mounted to the base support structure vertically to form the offshore support structure.

In operation, a method of installing an offshore support structure for use in a body of water of the present invention begins with transporting a topside structure and a base support structure of the offshore support structure to a predetermined offshore field site by a towing barge as illustrated in Steps 1 and 2 of Figure 12. When it reaches to the offshore field site, a base support structure is lifted from the towing barge and the base support structure is lowered and set onto seabed of the predetermined offshore field site as shown in Step 3 of Figure 12. Preferably, a minimum 400-ton floating crane is used to lift the base support structure from the barge and set onto seabed. Subsequently, the topside structure is lifted and stabbed onto the base support structure, eventually connected to the based support structure via welding process to form the offshore support structure (in Step 4 of Figure 12) and commissioning is performed on the offshore support structure. This method is applied when an ordinary mudmat is used.

Alternatively, when installing an offshore support structure for use in a body of water with buoyant mudmat, the offshore support structure having a topside structure and a base support structure is first fabricated in a graving dock as illustrated in Step 1 of Figure 13. Then, flooding the graving dock and have the offshore support structure to float on the water line wherein a plurality of buoyant mudmats attached to a bottom end of the base support structure as depicted in Step 2 of Figure 13. The offshore support structure is then wet towing to a predetermined offshore field site as shown in Step 3 of Figure 13. To lower the offshore support structure on to the seabed, each buoyant mudmat is ballasted with sea water by opening control valves as illustrated in Step 4 of Figure 13. Preferably, 300 tons floating crane is used to hold the offshore support structure, while the buoyancy tanks are being flooded and the offshore support structure is lowered. The requirement of such crane and craning operation is relatively low and operation time is short compare to nominal offshore lifting operation. Finally, the offshore support structure is installed onto seabed of the predetermined offshore field site and commissioning is performed on the offshore support structure. For removal of the platform, the buoyance tank is deballasted by injecting compressed air into the buoyance tanks to provide uplift force, Water is then injected into the gap between the bottom of the mudmat and the soil by various means to separate the two. After the cutting of the conductors and risers if any, the platform is then lifted off the seabed either in one piece or in multiple pieces per design and available equipment. The removed offshore support structure is wet towed to predetermined site. The offshore support structure can be reused after removal with minimal modification if the water depth and topside functions are similar. The offshore support structure can be re-installed to the next site by similar transportation and installation method as described herein. The concept of using mudmat foundation to replace the pile foundation also reduce the dynamic response of the offshore support structure to seismic activities and eliminate the weakest link of a nominal piled offshore support structure at pile top-to-base structure connection, thus greatly improve the offshore support structure's seismic resistant capability.

The offshore support structure of the present invention described herein has a number of notable advantages. Among others, the offshore support structure of the present invention is fast and flexible to install and most important eco-friendly and higher project economy with reusability. Furthermore, the offshore support structure of the present invention is lighter in total weight since piles are not required. In addition, the offshore support structure of the present invention is versatile and minimizes offshore transport and installation to reduce risk of offshore operation and schedule. Moreover, the cost incurred for constructing and installing the offshore support structure is relatively low as compared to the conventional offshore support structure. Last but not least, unconventionally skilled labors and heavy equipment are not required to form the offshore support structure of the present invention. The foregoing detailed description and examples are merely illustrative of the preferred embodiments. They are by no means meant to be the exclusive description of the inventive concept hereby disclosed. It will be recognized by one of ordinary skill in the art that certain aspects of the practice of the invention are readily susceptible to modification or practice by alternative, known means.