WO1979000108A1

WO1979000108A1 - A system for depositing sediment and/or protecting an installation on the floor of a body of water

Info

Publication number: WO1979000108A1
Application number: PCT/GB1978/000010
Authority: WO
Inventors: O Larsen
Original assignee: Hartley D; O Larsen
Priority date: 1977-08-24
Filing date: 1978-08-23
Publication date: 1979-03-08
Also published as: EP0020321A1

Abstract

This invention relates to a device which deposits sediment and/or protects an installation on the floor of a body of water. A ridge-like heavy, sectioned structure in its cross-section is divided in pairs of supporting symmetrical parts (1) and, resting on these parts, symmetrical movable parts (3) which are prevented from sliding downwards, but may be removed upwards along the supporting parts (1). Upon such removal the supporting parts make up a secondary shield, until the movable parts (3) have been re-positioned.

Description

A SYSTEM FOR DEPOSITING SEDIMENT AND/OR PROTECTING AN INSTALLATION ON THE FLOOR OF A BODY OF WATER

The invention relates to a system for depositing sediment and/or protecting pipelines, cables and the like en the floor of a body of water. A ridge-formed system consisting of a plurality of mutually displaceable compenents for depositing sediment and/or protecting an installation against fishing gear, ships' anchors, and the like, is already known.

The present invention as defined in Claim 1 presents a system which is so flexible that it can adapt to very rugged seabed contours and at the same time maintain a continuous surface to further the sediment depositing effect and to present no obstacle to fishing gear passing over the system.

The upper surface of the flexible system according to Claim 2 has no discontinuations, even with different settlements of adjacent modules. The fastening according to Claim 3 presents a simple and cheap method of connecting the movable parts to the supporting parts.

The hooked connection according to Claim 4 allows for release of the ties in case of removal of the movable parts over top of the structure. A perforation according to Claim 5 reduces the lifting forces on the structure.

Flaps according to Claim 6 ensure a steady contact between structure and seabed, so that fishing gear always slides over the structure without hooking its edges. The system according to Claim 7 is able to absorp very great impacts frcm dropped objects.

The unprolongable ties according to Claim 8 prevent the top of the protective cover from coming to rest on the installation to be protected. The construction material according to Claim 9 ensures flexibility and tight contact with the seabed. The method according to Claim 11 allows for immediate start of production anywhere, either near the site of installation or in the nearest harbour, and possibly for saving of transport of sand and stones from shore. The method according to Claim 12 gives a very high production rate per day. The method according to Claim 13 presents a maximum laying speed. By the method according to Claim 14 the laying of the protective cover to a certain extent is independent of weather conditions.

The method according to Claim 15 presents a rational, cheap laying of the covers when they can be laid together with the cable or the pipeline. The materials according to Claim 16 allow for adjusting the duration of dissoluticn of the materials to duration of the laying procedure.

In the following description reference will be made to the drawing in which: Fig.1 is a cross-section taken along the line I-I of Fig.2,

Fig. 2 is a plan view of a preferred embodiment of the invention. Fig. 3 is a longitudinal section taken along the line II-II of Fig. 1, Fig.4 is a cross-section taken along the line IV-IV in Fig.5 of an alternative embodiment of the invention, Fig.5 is a longitudinal section along the line III-III of Fig. 4,

Fig. 6 is a cross-section of a catamaran type laybarge provided with jackup legs. Two different water leeel situations are shewn in the figure.

In particular on uneven and/or hard clayey or rocky seabed, flexibility of the protective structure is important. This may be obtained for instance by means of a short length of the modules in which the cover is sectioned, and by flexibility between the individual components of the structure, cf. Figs.1-5.

By means of flexible borders consisting of narrow, pivotally hinged members4, a tight contact with the seabed is obtained, even if this is uneven or becomes eroded. Trawl- and other fishing gear therefore always slides over the structure without catching hold of its edge. The members 4 are kept together by wire means 6 passing through holes in the members 4, so that the vertical jumps between adjacent members are limited, and they form a continuous surface. The slidable primary shield 3 appropriately may be triangular and supported on the gently sloping edges of the triangular upper surfaces 2, of the supporting parts 1 which form the secondary shield of the structure. Even if unevenness of the seabed or erosion cause a mutual vertical displace ment of two adjacent modules 1, 3 will turn about the two supporting sides and always remain in tight contact all the way along these.

As shown in Fig.5, 1 may be further divided in a supporting lower portion 1 and 2, and a loose upper portion 7 resting on either 2 or 3. 3 may be prevented from sliding downwards for instance by means of at least one tie connecting a point of the underside of 3 with the top of 1, for instance with the axle 5 of the hinge between the symmetrical halves of 1. The tie will allow for 3 to move upwards over top of 1 in case a dragging anchor hooks the edge of 3. The connection between the tie and 3 and/or the top of 1 may be some type of hook that releases its grasp when the tie is tightened after 3 has passed over the top of 1.

For neutralization of hydraulic pressure differences between over- and underside, and to further sedimentation under the structure, appropriate portions of this, for instance the transverse joints between adjacent modules 1, and/or between 2 and 3, may be perforated.

To prevent the top of the protective cover from sinking so far that its underside cones to rest on the installation to be protected, the top hinge between the symmetrical halves of 1 may be designed so that the acute angle between the halves cann open up only to a certain point. A simple way of obtaining this is to place the axle 5 below the centre of the halfcircular mating parts of the hinge, of. Figs. 1 and 4.

Normally the construction material is concrete. Also other materials are applicable. To obtain plasticity, 4 and possibly 3 and 2 may be formed of longitudinally extending cushions made of for example plastic cloth filled with sand or stones, possibly mixed with asphalt with permanent plasticity, so that it will always follow after, if the seabed along the edges of the structure is eroded. To optimize the shape of the cushions to resemble the stable configuration shown in Figs. 1-5, the underside of the cushions may be provided with horizontal stays. The invention may be used for protection of for example submarine pipelines near offshore platforms from objects falling overboard from the platforms. To increase the capacity of the cover to absorb the impact of a dropped object hitting the cover, opposite symmetrical halves 1 of the structure may be interconnected by prolongable tie means 16 above (Fig.1) or below the installation 15 to be protected. The kinetic energy of the falling object thereby is reduced with the energy required to prolong the tie 16 when the falling object depresses the top of the cover and conse quenty spreads the sides of the cover. The tie may be made of elastic material, e.g. neoprene or natural rubber, or of plastic, e.g. nylon, aluminium, steel or other material. Lowering of the top of 1 to the level of the top of 15 may be prevented by an excessive strength of 16 or by means of additional ties with higher strength than that of 16.

Depending en the distance from shore, the covers may be fabricated on shore and transported to the installation site on a surface vessel. Or the fabrication may take place onboard a mothership. If the construction material for instance is concrete or sediment filled cushions, the sand and stones involved may possibly be taken from the sea bottom in the near of the site of installation.

Concrete modules preferably are fabricated by a machine that puts the concrete under pressure and vibration at the same time, whereby the production rate per day can be increased very considerably. Small uncomplicated elements to protect for instance a cable or pipeline of minor diameter may be laid at the same time as the cable or pipeline is laid. If cables are laid pairwise, the elements may have two parallel longitudinally extending notches for the cables on their undersides and be placed on top of the cables during the laying operation. The elements may be attached to the cables by means of ties that become dissolved by the water shortly after the laying, so that the elements get freed from the cables. The ties may for example be made of polyvimylalcohol coated with a layer of cellulose the thickness of which is adjusted to the time it takes to lay the cables. The elements may also be placed on a single cable to be laid, if cable and elements are hanging from an auxiliary wire that is wound off from the laybarge in the same way as the cable is laid. The attachment of the cable and elements to the auxiliary wire may be performed by the same kind of dissolvable tie as described above. Larger covers may be laid in deeper water by means of an unmanned, remotely controlled submersible. Normally a mothership at the site of installation is required for attaching the covers to the submersible. The mothership is supplied with a crane capable of lifting the submersible up to the deck through a moonpool, or outboard the stern, and afterwards lσwering the submersible with the attached covers into the water. Alternatively the mothership may have an opening under water through which the submersible can pass. The mounting of the covers to the submersible in this case may be performed by divers under water, or take place in a hold of the ship that can be made dry during the mounting of the covers.

In shallower water where the influence of the waves is even more critical for the laying operation, the covers may be laid for instance by means of a special catamaran type barge as shown in Fig.6. The symmetrical halves 8 of the barge are interconnected by portal frames 11 on which cranes 10 can move transversely and longitudinally, and by means of a frame 14 lift a row of modules 12 from the supply barge 13 and lower them to rest over the pipeline 15 on the bottom, when the supply barge 13 has been removed. The laybarge is supplied with jack-up legs 9. During the laying of each row of modules, the barge is raised above the water surface to minimize the influence of the waves. When the length of protection corresponding to the length of the laybarge has been laid, the legs 9 are jacked up, the barge is moved one length forward, is raised above water surface, etc.

Claims

C L A I M S :

1. A system for depositing sediment and/or protecting an installation on the floor of a body of water, comprising an elongate structure of rigid and/or flexible material, said structure including a longitudinally extending centre portion which, in use, is placed over the floor, and two longitudinally extending side portions the upper surfaces of which, in use, diverge away from each other to said floor, said structure being so heavy that no anchoring of the structure is necessary, and wherein said structure is sectioned in modules each of which in its cross-section is divided in a pair of symmetrical supporting parts resting on the floor, and - resting on these supporting parts - at least one pair of symmetrical movable parts which are prevented from sliding downwards, but may be moved upwards along the surface of the supporting parts.

2. A system according to Claim 1, wherein the upper face of the structure is divided in triangular plates that mutually support eachother.

3. A system according to Claim 1 or 2, wherein said movable parts are prevented from sliding downwards by means of tie means fastened to points of the undersides of the movable parts and by the other ends to the ridge of the structure.

4. A system according to Claim 3, wherein said tie means are fastened by means of hooks that release their grasps in case the corresponding movable parts are moved over the ridge of the structure.

5. A system according to any preceding claim, wherein the lower edge of said movable parts on either side of the structure is provided with a flexible border of transversely extending narrow members that are pivotally hinged to said movable parts and mutually connected by longitudinally extending ties.

6. A system according to any preceding claim, wherein opposite halves of the supporting parts are interconnected by prolongable ties.

7. A system according to Claim 6, wherein prolongation of said prolongable ties is limited by unprolongable ties that are longer than the original length of the prolongable ties.

8. A system according to any preceding claim, wherein at least part of the structure is fabricated from plastic material.

9. A system according to Claim 8, wherein said plastic consist of cushions of cloth filled with sedimentary material.

10. A method of fabricating the system according to Claim 1, wherein fabrication takes place onboard a mothership, and wherein the materials for fabrication possibly are taken from the seabed.

11. A method of fabricating the system according to Claim 1, wherein the protective structure is made of concrete and fabricated under simultaneous pressure and vibration.

12. A method of laying the system according to Claim 1, wherein a row of modules at a time are laid by means of an unmanned, remotely controlled submersible that in order to be loaded with another row of modules each time either is lifted up to the deck of the mothership or through an opening in the mothership sails into a hold of this.

13. A method of laying the system according to Claim 1, wherein a row of modules at a time are laid by means of a catamaran type laybarge provided with jack-up legs and with cranes that can move in the longitudinal and transverse directions, and wherein said laybarge is jacked up above the water surface on said legs during the laying operation.

14. A method of laying the systam according to Claim 1, wherein the individual modules during the laying operation by means of water-dissolvable ties are fastened on top of the cables or pipelines to be laid, and/or are hanging from an auxiliary wire.

15. A method according to Claim 14, wherein the water-dissolvable ties consist of polyvinylalcohol coated with a layer of cellulose of suitable thickness.

16. A system for depositing sediment and/or protecting an installation on the floor of a body of water substantially as herein described with reference to any of the drawings.

17. Methods of fabricationg and using a system for depositing sediment and/or protecting an installation on the floor of a body of water substantially as herein described with reference to any of the drawings.