GB2479453A - A system for protecting subsea installations - Google Patents

Abstract

The present invention concerns a system for protecting subsea installations including a protective subsea cover (1) with at least one mud mat (3 of Fig 3) along an edge of the cover, and a plurality of concrete stabilizing ballast blocks (10) for resting on top of the mud mat (3 of Fig 3) and for holding the protective cover (1) in place at a seabed. The concrete stabilizing ballast blocks (10) include at least one shape for preventing at least a sliding motion in relation to the protective cover (1) or other stabilizing ballast blocks, thus preventing independent motion of the stabilizing ballast blocks (10) and the protective cover. Furthermore, the invention concerns ballasting blocks (10) and a protective subsea cover.

Description

System for protecting subsea installations The present invention relates to a system for protecting subsea installations, ballasting blocks and a protective subsea cover. The system includes a protective subsea cover made as a shell structure held in place and stabilized by stabilizing ballast blocks. Each block includes at least one interlocking geometry or shape for preventing a sliding motion between the protective cover and the ballast blocks.

Cables, pipelines, wellheads, flow lines, spools and other seabed located equipment herein after called cables, require in many cases some sort of protection against in particular impacts from anchors, other falling equipment, fishing equipment, trawis, trawl doors etc. For this purpose it is known to place the cables in a ditch and/or cover them with a tunnel assembled of longitudinal elements, herein after called covers or shell structures, forming a cavity for the cables below. In the case of wellheads or other less extending elements, the cover may have more of a dome shape or any shape where the cover is surrounding the element to be protected.

Longitudinal covers typically have a downward concave shape. The cover may be secured with ballast made from moulded concrete blocks and/or piles of rock, in a so called rock dump.

A method and an apparatus for covering tubing at a seabed can be found in Norwegian patent application NO 1998 3727. The publication shows a cover with mud mats, and it is suggested to fill a layer of gravel on top of the cover.

Norwegian patent NO 307002 also describes a protective shell structure that may be completely or partly covered with a mass for holding and stabilizing the shell structure. Dumping large quantities of mass on top of a structure is expensive, and the expense increases with increasing depth.

It is a purpose with the present invention to provide a system for protecting installations on a seabed that combines a cover in the form of a shell structure and ballast elements of a manageable size without compromising strength and the ability to withstand mechanical influence from for instance trawis or anchors. For the purpose of this disclosure, "ballast blocks", "concrete elements" or just "blocks" all meant to cover the ballasting elements of the invention. In this disclosure, "concrete" has been used through out the specifi-cation, but any other material with similar properties may well be used. A problem when using concrete blocks of a limited number and of a manageable size is that the protective cover they are meant to hold easily can slide away and the concrete blocks may be torn away, one by one, leaving the ballast quite ineffective. The protective cover may also easily slide away if the concrete blocks only are placed on top of mud mats along the side of the cover. Accordingly the present invention further suggests a system with a plurality of blocks and a cover where the blocks are designed with features for mechanical locking/interlocking connection to other blocks or to the protective cover for mechanically preventing the protective cover and the at least one block to be able to move independently from each other in at least a horizontal plane. This is achieved with interlocking structures on at least the concrete blocks. It is the purpose of the present invention to provide a solution where all the blocks act together to hold the protective cover.

The complete mass or number of ballasting blocks can be reduced compared to ballasting for protective covers where each ballasting element is allowed to move independently. This is a considerable advantage during the installation of such systems. Furthermore, inspection and maintenance is easier as this may be performed by lighter equipment and ROVs. The performance of the result-ing structure is also more predictable as the ballasting will be more consistent.

The system with ballast for and subsea protection covers may also be used during the installation phase of subsea equipment and for temporary stabilization of covers until permanent stabilization is installed. Ballast needed for permanent stabilization of the cover is according to the present invention installed separately, after the cover is installed. When blocks are placed individually, they may typically be guided on guide posts or by other structures.

The protection covers are geometrically shaped or prepared with attachments to lock the blocks to each other and/or to the protection cover for one or more degrees of freedom (d.o.f.). Degrees of freedom that can be locked are: tree translational d.o.f. (2 independent horizontal d.o,f, one vertical do.f.) three rotational d.o.f. (rotation around two independent horizontal axes and rotation around the vertical axis.) The purpose of this is to effectively engage more ballast weight than what can be achieved by just placing the blocks side by side, or on top of each other. The system may also be used to engage a larger area of the seabed than the cover mud mat to minimize pressure towards seabed and to reduce settlements.

Concrete blocks with interlocking features are known from other subject areas, but these blocks have not been designed for ballasting purposes on top of a mud mat of a protective cover.

If the structure to be protected such as cables is placed on mud, sand, gravel or other unconsolidated sea beds, scouring due to hydro dynamic effects removing material from underneath the cables and the protective structure may also be a problem. To prevent scouring, the ballast blocks of the present invention may also include mattresses. The mattresses may typically be made of several longitudinal concrete elements typically linked to the block with steel wires or synthetic ropes. These longitudinal elements are able to prevent scouring as they will impose a constant force on the sea bed and will follow the seabed if scouring nevertheless occurs.

The following issues may be considered as advantages with the present invention: U settlements of the covers without increasing cover mud mat width are reduce; U installation of ballast weights is simplified; U concrete blocks are inexpensive and can be field made; making notches or recesses for placing locking devices etc. is simple; and U making holes for bolts or other fixing devices or other purposes can be made simple and cost effective without machining etc. (made in moulding process.) The present invention concerns a system for protecting subsea installations including a protective subsea cover with at least one mud mat along an edge of the cover, and a plurality of concrete stabilizing ballast blocks for resting on top of the mud mat and for holding the protective cover in place at a seabed. The concrete stabilizing ballast blocks include at least one shape for preventing at least a sliding motion in relation to the protective cover or other stabilizing ballast blocks, thus preventing independent motion of the stabilizing ballast blocks and the protective cover.

The interlocking shape in the concrete stabilizing ballast block may include a recess. The protective cover may include at least one first protrusion extending substantially in a vertical and longitudinal direction along and adjacent the at least one mud mat. The at least one first protrusion may be adapted to extend into the recess in the ballast block thus holding the block in relation to the cover in a substantially horizontal plane and preventing the block from sliding away from the mud mat.

The interlocking shape in the first concrete stabilizing ballast block may include at least one recess for interlocking connection to a similar second ballast block lying along side or on top of the first ballast block.

The interlocking connection between the first and the second ballast block may include a separate interlocking body installed in the recesses of the ballast blocks.

The at least one shape for preventing at least a sliding motion in relation to the protective cover or other stabilizing ballast blocks may include a recess for passing a longitudinal element extending between the ballast blocks. The longitudinal element is typically a rope, a wire or a metal rod.

Several interlocking, interconnecting ballast blocks may completely surround the protective subsea cover, thus preventing independent motion between the ballast blocks and the protective subsea cover.

There may be one mud mat and at least one concrete stabilizing ballast block at each side of the protective cover.

Furthermore the invention concerns a concrete stabilizing ballast block for holding a protective cover in place at a seabed. The ballast block includes at least one geometrical feature for interlocking the element for maintaining the element in position in relation to the protective cover.

Furthermore the invention concerns a concave longitudinal subsea protective cover with at least one mud mat in a longitudinal direction in relation to the cover i with at least one first protrusion extending substantially in a vertical and longitudi- nal direction along and adjacent the at least one mud mat, and wherein the pro-trusion is adapted to extend into a recess in a concrete stabilizing ballast block.

The subsea protective cover may be made of FRP or any other suitable material.

The protective cover may further include at least one integrated concrete block fixed to and extending substantially the entire length of the cover in a longitudinal direction.

Short description of the enclosed figures:

Fig 1 is a perspective view of a protective cover and ballast blocks according to the invention; Fig. 2 is a perspective view of a first embodiment of a ballast block according to the invention; Figs. 3A-3E are cross sections of various embodiments of ballast blocks, placed on a protective cover according to the invention; Fig. 4 is a perspective view of a second embodiment of a ballast block according to the invention; Fig. 5 is a perspective view of two ballast block according to the invention, linked with a linking element; and Figs. 5 to fig. 9 are perspective views showing various embodiments of locking devices for locking the ballast blocks to each other.

Detailed description of embodiments of the invention with reference to the drawings: Fig 1 shows a protective cover I in the form of a shell structure. The protective cover 1 is shown as a cover for a single structure on a seabed, having walls surrounding the structure. The protective cover is typically made of FRP (Fibre Reinforced Polyester). Mud mats (not shown) form a foundation adapted to rest on a seabed. Ballast blocks 10 are located all the way around the structure. The ballast blocks 10 are interconnected and form a complete frame around the structure. The ballast blocks are modular, and can be deployed, one by one. The ballast blocks may be physically interconnected to each other and / or to the protective cover such that a force applied substantially parallel to a horizontal plane to any one block or the cover will be distributed to the entire unit. The entire unit in this context is all the ballast blocks and the protective cover.

Fig. 2 shows a first embodiment of a ballast block 10 according to the invention.

The block includes a first inclined front portion 13 adapted to point away from the protective cover. The inclined front portion 13 reduces the risk of fishing equipment getting snagged / caught in the block, with unfavorable consequences for both the fishing equipment and the protective structure. The block 10 furthermore includes a second inclined rearward portion 11 adapted to be placed against a wall on the protective cover. A third lower wall 15 is adapted to rest against the seabed and the mud mat. The lower wall 15 may be substantially flat, or may include pro-trusions 14. The protrusions 14 may go into a corresponding u-shaped channel in the mud mat or into a suitable recess 18 in the top surface of a corresponding element if the elements are stacked on top of each other in a staggered pattern Figs. 3A-3E show various embodiments of ballast blocks 10, placed on a pro-tective cover I according to the invention. All the embodiments show a recess 16 in the lower wall 15 for receiving an upward directed protrusion or edge 6 along a mud mat 3. The protrusion or edge 6 extending into the recess 16 prevents the mud mat 3 from sliding away from the block 10. As can be seen, the blocks 10 may easily be placed on top of the mud mat from above.

Figs 3A and 3B show a top edge 7 for contact with an upper portion of the ballast block 10 for further holding the block 10 in relation to the cover 1. The ballast block may include a shape 19 adapted to the top edge 7. The shape 19a includes an upward extending lip, extending into the top edge 7. The top edge may be inte-grated in the cover during production, or may be attached to the cover at a later stage.

Similarly, a top edge 7 is shown on fig 3B, but in this embodiment, the top edge 7 extends into a recess 19b in the block 10.

In Fig 3C, a top edge is formed as a lip portion 8 integrated in, and forming a part of the protective cover itself, extending into a recess I 9c in the block 10.

Common for the embodiments 3A, 3B and 3C is that the block 10 must be tilted when installed or removed. The advantage of these embodiments is that the ele-ments are fixed more rigidly than when only placed on top of an upward extending body as shown in figs 3D and 3E. The top edges 7 or the lip 8 holds the block in relation to the protective cover in an additional degree of freedom, and gives a higher effective vertical force on the mud mat as all the weight of each element will be transferred to the cover.

Fig 3D shows an embodiment where the block 10 is adapted for a protective cover o with integrated ballast.

All the figures 3A to 3E show that the block 10 furthermore includes a second inclined rearward portion shown as rearward portion 11 on fig 3E, adapted to be placed against a wall on the protective cover. The wall and the rearward portion 11 have substantially the same angle a in relation to the mud mat 3 and the lower wall l5oftheblockl0.

Fig. 4 shows a block 10 that also includes a side wall, gripping geometries 12 for easier attachment to a ROV or other equipment intended to handle the block 10, and mattress elements 21 linked to the block 10 with wires 20. Fig 4 shows eight mattress elements 21, but any number of elements may be used as needed, de-pending on the seabed etc. The Mattress elements 21 and the block 10 may be moulded in the same operation, or may be formed as elements that are linked together after moulding. The mattress elements 21 may be made of a different material than the main block. Rubber mats, nets, artificial seaweed etc. may also be used to prevent scouring problems that may occur in strong currents at the seabed. By adding mattress elements to the ballast, separate installation of mattresses can be avoided, resulting in considerable cost saving. Further, to prevent scouring, only a few concrete elements are needed, not a large mattress.

The mattress elements may also increase overtrawlability by covering sharp edges that may be snag points for fishing equipment.

The gripping geometries 12 may be integrated parts i.e. lifting attachment points or locking devices.

Fig. 5 -fig. 9 show various solutions for locking the ballast blocks 10 to each other.

These solutions may include locking devices where separate elements are used for fixation purposes. In fig 5 a separate "dog bone shaped" linking element 30 are placed in corresponding cut outs 29 in the ballast blocks.

Fig. 6 shows an element 31 for extending into a hole in a neighboring block, fig. 7 shows an element 32 for extending into a neighboring block, fig. 8 shows a fixing bracket 33 and fig. 9 shows a version where a wire or synthetic rope 34 runs in recesses 35 in the top surface of the blocks.