EP2550403A1

EP2550403A1 - Movable platform, assembly of a movable platform and a barge, method of installation, method of exploitation, method of re-use

Info

Publication number: EP2550403A1
Application number: EP11712044A
Authority: EP
Inventors: Johannes Brusik; James David Craig; Rob Burghard; Franciscus Jacobus Slangen; Willem Bal; Mark Erik Riemers
Original assignee: Centrica Infrastructure BV; IV-Oil&Gas BV; Heerema Fabrication Group BV; Suction Pile Tech BV; Centrica Production Nederland BV
Current assignee: Centrica Infrastructure BV; IV-Oil&Gas BV; Heerema Fabrication Group BV; Suction Pile Tech BV; Centrica Production Nederland BV
Priority date: 2010-03-22
Filing date: 2011-03-21
Publication date: 2013-01-30
Also published as: NL2004443C2; WO2011119025A1

Abstract

Movable platform comprising a deck structure with at least one leg, wherein the legs are fitted with suction piles for anchoring in a sea bottom, wherein the legs are adjustable between a transport position wherein the legs substantially extend above the deck structure of the platform for transport of the platform on a barge and an installation position wherein the legs substantially extend below the deck structure of the platform for installation of the platform on the sea bottom, wherein each leg comprises an isolated stiffening structure at the connection with the suction pile, for artificially heightening the sea bottom and wherein in the transport position at least a part of the stiffening structure extends above the underside of the deck structure.

Description

Title: Movable platform, assembly of a movable platform and a barge, method of installation, method of exploitation, method of re-use

The invention relates to a movable platform for use on an offshore location.

Movable platforms for exploiting minor gas and/or oil fields offshore are known. Minor gas and/or oil fields usually have a relatively short life time, i.e. less than 20 years. Known movable platforms may use suction piles as anchoring and are mainly designed for relatively shallow water depths, for example up to approximately 20 m or 30 m water depth, mild environmental conditions, for example in relatively sheltered areas or in open areas with relatively calm environmental conditions, and a relatively low- weight topside, for example up to approximately 1500 tonnes. Such platforms may be used as unmanned platforms and/or as satellite platforms.

Movable platforms usually comprise a deck structure with legs that are adjustable with respect to the deck structure. The legs are usually adjustable between a transport position wherein the legs substantially extend above the deck structure and an installation position wherein the legs substantially extend below the deck structure. During transport from an onshore location to the location offshore for exploiting the gas and/or oil field, the deck structure is usually transported on a barge and the legs are in the transport position. At the offshore location, the legs are lowered and anchored in the sea bed. Thereafter, the deck structure is lifted from the barge after which the barge is removed between the legs. Usually the legs are equipped with suction piles or suction anchors which can be anchored in the sea bed and which can be removed from the sea bed after the exploitation of the minor gas and/or oil field has terminated. Then, the platform can be transported on a barge to a further location onshore, for example for maintenance, and/or to a further location offshore, for example for the exploitation of another minor gas and/or oil field. Minor gas and/or oil fields in larger water depths, for example more than approximately 20 m water depth, and/or severe environmental conditions usually remain unexploited since it may be too expensive and/or financially too risky to design, build, install and exploit a fixed platform for such a location. In particular when the topside is relatively heavy, e.g. in case the platform serves as a host platform, it may not be profitable to provide a fixed platform for such minor gas and/or oil field having a relatively short life time.

An object of the invention is to provide for a movable platform for exploiting minor gas and/or oil fields in relatively large water depths and/or in relatively harsh environmental conditions and/or with a relatively heavy topside and with a life time of approximately 20 years.

Thereto, the invention provides for a movable platform comprising a deck structure with at least one leg, wherein the legs are fitted with suction piles for anchoring in a sea bottom, wherein the legs are adjustable between a transport position wherein the legs substantially extend above the deck structure of the platform for transport of the platform on a barge and an installation position wherein the legs substantially extend below the deck structure of the platform for installation of the platform on the sea bottom, wherein each leg comprises an isolated stiffening structure at the connection with the suction pile, for artificially heightening the sea bottom and wherein in the transport position at least a part of the stiffening structure extends above the underside of the deck structure.

By providing the legs with an isolated stiffening structure at the connection with the suction piles, the legs can be provided with additional stiffness, as if the sea bottom is heightened. An unstiffened length of the legs, for example the length between the stiffening structure and the connection of the leg with the deck structure, can remain relatively limited. Due to the stiffening structure, the legs may be sufficiently stiff to withstand the relatively harsh environmental conditions at a relatively large water depth and/or to limit displacement and/or distortion of the platform during harsh environmental conditions at a relatively large water depth. Without such a stiffening structure, the platform should not be able to operate safely in the relatively harsh environmental conditions at the relatively large water depth, for example at the offshore location denoted F3-FA in the Dutch part of the North Sea.

The stiffening structures are isolated meaning that the stiffening structures are not interconnected to each other and therefore the legs are not interconnected to each other below the deck structure, contrary to known stiffening frames. By providing the additional stiffness to the lower end of the legs, the platform can be installed in relatively large water depths, e.g. more than approximately 20 meters and up to approximately 60 meters or more, preferably between approximately 30 meters and approximately 50 meters.

By providing isolated stiffening structures, spacing between the legs remains open such that a barge can be positioned between the legs for transporting the platform with the legs elevated. By arranging the platform such that in transport position at least a part of the stiffening structure extends above the underside of the deck structure, the draft of the platform in transport position, when the deck structure is connected to the floating barge, may remain limited such that transport through harbours and shallow waters may remain possible, which may increase the transportability of the platform.

Advantageously, the stiffening structure is approximately as high as the width of the suction pile. The width of the suction pile can e.g. be the diameter of the suction pile. Also, the stiffening structure may also be higher than the width of the suction pile, e.g. up to twice the width of the suction pile. By providing a relatively high stiffening structure, the lower end of the leg can be stiffened over a relatively large height, thus allowing the platform to be installed in relatively large water depths. By providing the height of the stiffening structure approximately as large as the width of the suction pile to which it is connected, a relatively stiff structure can be obtained for providing additional stiffness to the lower end of the leg. The legs can thus be provided with additional stiffness over a relatively large length and the unstiffened length of the legs can remain relatively limited to provide sufficient overall stiffness of the platform, in particular during harsh environmental conditions in relatively large water depth.

In a preferred embodiment, the stiffening structure comprises a truss structure. For optimal stiffness and load distribution, a truss structure can serve as a reliable stiffening structure. The stiffening structure therefore clearly distinguishes from known stiffening beams placed on the suction piles which clearly do not provide additional stiffness to the lower end of the leg in order to artificially heighten the sea bottom and/or to limit the unstiffened length of the leg.

By further providing a stiffened wall section on the leg of the platform, preferably at the connection with a truss of the truss structure with the leg, for example at an upper side of the truss structure, the stiffening structure may provide sufficient stiffness and the loads can be relatively reliable distributed into the leg. The stiffened wall section may for example comprise an additional wall thickness and/or may comprise stiffening ribs or flanges at the inner side of the leg.

After installation at the offshore location, the leg will be connected to the deck structure. When removing the platform from the offshore location, e.g. after exploitation of the gas and/or oil field has been terminated, this connection may be undone. By arranging the connection of the leg with the deck structure of the platform to direct the load in a horizontal force

component and a vertical force component, the load on the deck structure can be relatively well analyzed. By splitting the load in a horizontal component and a vertical component, the deck structure can be designed appropriately thereby saving weight and thus costs.

For example, by using a grout pack, the load can be efficiently split in a horizontal force component and a vertical force component. The horizontal force can be lead into the deck structure, while the vertical force component can be received by the leg itself. This may lead to a more predictable dynamic and/or static load pattern on the deck structure. Alternatively and/or additionally, an elastic structure may be provided, such as rubber pads, at the connection of the leg with the deck structure. Also, a bolt arrangement may be provided for splitting the load. Typically, the bolts are arranged in an upward direction at the connection of the leg with the deck structure and will receive vertical force components. The elastic structure and/or the grout pack typically may receive the horizontal force components.

By connecting the leg at an upper side of the deck structure, an optimal unstiffened length of the leg can be obtained. The leg may be connected with a removable connection, such as a bolt connection and/or a wedge connection, instead of the known-in-the-art welded connection. First, the welded connection needs to be cut when removing and/or relocating the platform.

By providing a sleeve structure at the connection of the leg with the deck structure, the leg can smoothly be moved through the sleeve structure as a guide structure during displacement of the leg between the transport position and the installation position.

By arranging a lower end of the deck structure to accommodate at least a part of the stiffening structure during transport, the stiffening structure can extend above the underside of the deck structure in the transport position. For example, a lower end of the deck structure may be provided with only a segment of a guide structure. A clamping element can be provided for connecting the leg with the lower end of the deck structure. The clamping element may for example comprise the corresponding segment of the guide structure to approximately fully enclose the leg.

Further, the invention relates to an assembly of a movable platform and a barge.

Also, the invention relates to a method for installing the movable platform, to a method for exploiting a minor oil and/or gas field and to a method for re-using the movable platform. Since the platform may have a lifetime of approximately 20 years and the minor oil and/or gas field usually has a shorter life time, typically 6— 8 years, the platform can be re-used for other minor oil and/or gas fields on different locations.

Further advantageous embodiments are represented in the subclaims.

The invention will further be elucidated on the basis of an exemplary embodiment which is represented in a drawing. The exemplary embodiment is given by way of non-limitative illustration of the invention.

In the drawing:

Fig. la shows a schematic view of a platform according to the invention in the transport position;

Fig. lb shows a schematic view of the legs of the platform during transport;

Fig. 2 shows the platform of Fig. 1 during installation;

Fig. 3 shows the platform of Fig. 1 in the installation position; and

Fig. 4 shows a barge with a sea fastening system for the platform.

It is noted that the figure is only a schematic representation of an embodiment of the invention that is given by way of a non-limiting example. In the figures, the same or corresponding parts are designated with the same reference numerals.

Fig. la shows a movable platform 1 comprising a deck structure 2 and legs 3. In the embodiment shown in Fig. 1, the platform 1 has four legs 3. The deck structure 2 is approximately rectangular shaped, each corner is provided with one leg 3. Each leg 3 is fitted with a suction anchor 4 for anchoring in the sea bottom. Suction anchors or suction piles are well known and will not be described further.

The deck structure 2 is relatively heavy and may comprise two, three, four, five, six or more deck levels. For example, the deck structure 2 may comprise a cellar deck, a mess deck, a main deck, an intermediary deck a top deck and a weather deck. Such a deck structure usually weighs more than 2000 tonnes, for example about approximately 4000 - 5000 tonnes or more. The platform is advantageously used as a host platform and may be manned. Possible, satellite platforms or other peripheral structures may be connected to the platform at the offshore location.

The legs 3 are adjustable between a transport position, shown in Fig. la, and an installation position, shown in Fig. 3. Fig. 2 shows the legs 3 in transition between the transport position and the installation position. In the transport position, the legs 3 substantially extend above the deck structure 2 of the platform 1. In the transport position, the deck structure 2 can be mounted on a barge 5 and can be transported over water. In the installation position, the legs 3 substantially extend below the deck structure 2 of the platform 1 when the platform 1 is installed on the sea bottom.

At the underside of the leg 3, the leg 3 comprises an isolated stiffening structure 6. The stiffening structure 6 is provided at the connection of the leg 3 with the suction pile 4. The stiffening structure 6 provides additional stiffness to the underside of the leg 3 over the length of the stiffening structure and thus an artificial heightening of the sea bottom is obtained when the platform 1 is installed on the sea bottom, thereby allowing the platform 1 to be installed in larger water depths in relatively harsh environmental conditions, for example on the F3-FA location in the Dutch section of the North Sea or similar locations.

The stiffening structure 6 is an isolated stiffening structure, meaning that the stiffening structures are not interconnected to each other. A single stiffening structure 6 corresponds to a single leg 3 with a suction pile 4. By providing an isolated stiffening structure 6, the spacing between the legs 3 remains open to position the barge 5 in between for transport, as can be seen in Fig. la.

As can be seen in Fig. la, at least a part of the stiffening structure 6 extends above the underside 7 of the deck structure 2 in the transport position. Due to the isolated stiffening structure 6, the legs can be elevated sufficiently such that the stiffening structure 6 can be positioned partially in the deck structure 2. In the transport position, the legs 3 can thus be elevated more, so the draft of the floating combination of the barge with the platform with the legs may remain limited, for example to approximately 6 meters or more e.g. up to 15 meters or more. This may increase the transportability of the floating combination, the floating combination may also be transported when the water depths along the tow route or in the offshore field location are limited.

The lower part of the deck structure 2 is arranged to accommodate at least a part of the stiffening structure 6. The upper part of the stiffening structure 6 may be received in the deck structure 2, as shown in Fig. 1. The lower part of the deck structure 2 is kept free, such that the upper part of the stiffening structure 6 can extend above the underside 7 of the deck structure 2. The underside 7 of the deck structure 2 usually comprises the cellar deck. For example, a corner of the cellar deck 7 can be arranged to comprise only a segment of a guide structure for guiding the leg 3, e.g. during displacement from the transport position to the installation position. The guide structure may comprise a sleeve element. When the leg 3 is lowered, shown in Fig. 2 and Fig. 3, the guide structure can be completed by a clamping element 8 that for example comprises the remaining segment of the guide structure. The leg 3 can then be further lowered in the guide structure to the installation position. After installation of the leg 3 in the sea bottom, the clamping element 8 can be used for connecting the leg 3 to the deck structure 2. The clamping element 8 may thereto be provided for example with wedges.

During transport the legs 3 are advantageously kept in a slightly inclining position with respect to each other, as schematically shown in Fig. lb. It is noted that the inclination in Fig. lb is for sake of clarity exaggerated. The inclination angle is relatively small, approximately up to approximately 1 to approximately 5 degrees. The legs 3 are only fixed to the deck structure 2 at two locations 9a and 9b. Location 9a is for example situated at the top deck, location 9b is for example situated at the intermediary deck. Such a position of the legs 3 during transport may be advantageous for example for stability.

The platform 1 is usually manufactured and commissioned on a site onshore. From the onshore location, the platform 1 is transferred onto a barge 5 and prepared for transport to the offshore location, e.g. a minor oil and/or gas field that may be exploited using the platform 1. The platform 1, in particular the deck structure 2 is fastened on the barge 5 using a sea fastening system 10. The barge 5 with sea fastening system 10 is shown in Fig. 4. Preferably, the sea fastening system 10 is an automated sea fastening system, using hydraulic clamps and a remote control. The deck structure 2 can then be fastened and/or loosened from the sea fastening system 10 automated, without workmen needing to remove the sea fastenings and/or to cut the welds. The automated sea fastening system 10 only provides for clamping devices that can be remotely controlled, so no welds are provided.

At the offshore location, the legs 3 can be lowered until the suction piles 4 anchor in the sea bottom, as can be seen in Fig. 2. When the legs 3 are anchored in the sea bottom, the deck structure 2 can be removed from the barge 5. Since an automated sea fastening system 10 is used, the deck structure 2 can be removed from the barge 5 with limited assistance of workmen on the sea fastening system. Once the deck structure 2 is removed from the barge 5, the deck structure 2 can be lifted until the final installation height, shown in Fig. 3. The platform 1 is then installed and can be hooked-up to infrastructure, such as a pipeline and/or a wellhead. Thereafter, the production of the oil and/or gas field may start. After exploitation of the minor oil and/or gas field, the platform 1 can be decoupled from the oil and/or gas field and can be put on transport, using a barge 5 with a sea fastening system 10, to another offshore location. At the other offshore location, the platform 1 can be installed, hooked-up and the production from the oil and/or gas field at the other offshore location may start. The stiffening structure 6 is preferably approximately as high as the width of the suction pile 4, here the diameter of the suction pile 4. By stiffening a relative large section of the underside of the leg 3, the leg 3 can be considered as if the sea bottom has been artificially heightened. An unstiffened length L of the leg 3, in this example the length between an upper end of the stiffening structure 6 and the connection of the leg 3 at the top deck, can remain appropriate to provide sufficient stiffness to withstand the relatively harsh environmental conditions in a relatively high water depth for which the platform 1 may be designed. Preferably, the leg 3 is firmly, but removable, connected to the deck structure 2 at the top deck, i.e. the upper side of deck structure 2, to provide for an optimal unstiffened length L to withstand the harsh environmental loads.

Advantageously, the stiffening structure 6 comprises a truss structure. The truss structure comprises several trusses that are welded to each other. Usually, at the connection of the truss structure with the leg 3 at location 6a, a wall section of the leg 3 is additionally stiffened. The wall section can be additionally stiffened by providing a larger wall thickness and/or by providing stiffening ribs and/or flanges at the inner side of the leg 3.

The connection between the leg 3 and the deck structure 2, in particular at the top deck, may be arranged to direct the load, usually environmental loads, in a horizontal force component and a vertical force component to the deck structure and the leg. For example, the connection may comprise a grout pack suitable for receiving and directing the horizontal force components to the deck structure. Also, the connection may comprise an elastic structure, such as rubber pads, to receive and direct a horizontal force component to the deck structure. The connection may also comprise a bolt arrangement suitable for receiving and directing vertical force components to the leg and/or the deck structure. For example, the connection at the top deck may comprise a bolt arrangement and a grout pack. The connection at the cellar deck may comprise a wedge arrangement and rubber pads. During lowering of the leg 3, these connections may provide for sufficient movement. After the leg is lowered and installed, the bolts may be fastened and the wedges may be clamped to provide a firm fixed connection. When removing the platform 1 from the offshore location, the bolts and/or the wedges may be loosened.

A leg 3 can also be arranged to accommodate conductors and/or risers, small to medium size diameter piping, in. The top end of the leg 3 containing the conductors may thus be differently arranged in order to accommodate the end parts of the conductors. The part of the conductors that extend in the seabed may be positioned outside and/or inside the suction pile perimeter.

As can be seen in Fig. 1 - Fig. 3, on top of the top deck, a weather deck is provided accommodating a Christmas tree.

Many variants will be apparent to the person skilled in the art. The invention is explained for the exploitation of a minor gas and/or oil fields, but it is apparent to the person skilled in the art that the movable platform can also be applied for supporting electrical transformer stations for wind farms, or other applications. All variants are understood to be comprised within the scope of the invention as defined in the following claims.

Claims

1. Movable platform comprising a deck structure with at least one leg, wherein the legs are fitted with suction piles for anchoring in a sea bottom, wherein the legs are adjustable between a transport position wherein the legs substantially extend above the deck structure of the platform for transport of the platform on a barge and an installation position wherein the legs

substantially extend below the deck structure of the platform for installation of the platform on the sea bottom, wherein each leg comprises an isolated stiffening structure at the connection with the suction pile, for artificially heightening the sea bottom and wherein in the transport position at least a part of the stiffening structure extends above the underside of the deck structure.

2. Movable platform according to claim 1, wherein the stiffening structure is approximately as high as the width of the suction pile.

3. Movable platform according to claim 1 or 2, wherein the stiffening structure comprises a truss structure.

4. Movable platform according to any one of the preceding claims, wherein the stiffening structure further comprises a stiffened wall section on the leg.

5. Movable platform according to any one of the preceding claims, wherein a connection of the leg with the deck structure of the platform is arranged to direct the load in a horizontal force component and/or a vertical force component to the deck structure and/or the leg.

6. Movable platform according to claim 5, wherein a grout pack provides for the splitting of the load in a horizontal force component and/or a vertical force component.

7. Movable platform according to claim 5 or 6, wherein an elastic structure provides for the splitting of the load in a horizontal force component and a vertical force component.

8. Movable platform according to claim 7, wherein the elastic structure comprises rubber pads.

9. Movable platform according to any one of the claims 5— 8, wherein a bolt arrangement provides for the splitting of the load in a vertical force component.

10. Movable platform according to any one of the preceding claims, wherein the leg is connected to the deck structure at an upper side of the deck structure.

11. Movable platform according to any one of the preceding claims, wherein the connection of the leg to the deck structure comprises a sleeve structure.

12. Movable platform according to any one of the preceding claims, wherein a lower end of the deck structure is arranged to accommodate at least a part of the stiffening structure during transport.

13. Movable platform according to claim 12, wherein a clamping element is provided for the connection of the leg with the lower end of the deck structure.

14. Assembly of a movable platform and a barge, wherein the movable platform is provided as a platform according to any one of the claims 1— 13.

15. Assembly according to claim 14, wherein the barge is provided with an automated sea fastening system for fastening the deck structure and/or the legs of the platform to the barge during transport.

16. Method for installing a movable platform according to any one of the claims 1— 13 at an offshore location, comprising commissioning the platform onshore, transporting the platform on a barge to the location offshore, lowering the legs from the transport position to the installation position, removing the deck structure from the barge and lifting the deck structure.

17. Method for exploiting a minor oil and/or gas field at an offshore location, comprising manufacturing a movable platform according to any one of the claims 1 - 13, installing the platform according to the method of claim 16, hooking-up the movable platform at the offshore location to the oil and/or gas field, starting production of the oil and/or gas field.

18. Method of re-using a movable platform according to any one of the claims 1— 13, comprising decoupling the movable platform from the oil and/or gas field at a first offshore location, transporting the movable platform on a barge to a second offshore location, exploiting the oil and/or gas field at the second offshore location according to the method of claim 17.