GB2052610A

GB2052610A - Improvements in or relating to marine structure

Info

Publication number: GB2052610A
Application number: GB8015452A
Authority: GB
Original assignee: HOLLANDSCHE BETON BV Mij
Current assignee: HOLLANDSCHE BETON BV Mij
Priority date: 1979-05-09
Filing date: 1980-05-09
Publication date: 1981-01-28
Also published as: NL8002710A; NO801361L; NL185166C; GB2052610B; NL185166B

Abstract

A marine structure, floating or standing on the sea bed, has at least one part 10 which protrudes above the surface of the sea and which is protected by a fender comprising three steel rings 1, 2, and 3 arranged with their centres lying on a common axis and maintained axially spaced apart by steel bracing members 4. The fender encircles and is spaced from the protruding part 10 in the vicinity of sea level and is connected to the protruding part by flexible ropes 5, 6 of high tensile strength which are secured to the ring 3 at spaced positions around its periphery. If a ship should collide with the fender, the fender will be deflected inwardly towards the protruding part 10 and a substantial proportion of the energy arising from the impact will be absorbed by strain imparted in ropes that are remote from the point of impact. <IMAGE>

Description

SPECIFICATION Improvements in or relating to marine structures This invention relates to marine structures and is particularly concerned with marine structures of the kind which form or can form a floating body and/or are or can be sunk to stand on the bed of the sea or other body of water (hereinafter referred to as the sea) and which, when floating or standing on the sea bed, have at least one part which protrudes above the surface of the sea and which is usually, but not necessarily, slimmer than the underlying part of the structure. Marine structures of this kind will hereinafter, for convenience, be referred to as "marine structures of the kind described".The invention is especially, but not exclusively, concerned with those marine structures of the kind described that are generally referred to as offshore gravity platforms and which include a hollow base, and, upstanding from the base, at least one upper structure slimmer than the base, which platforms can form a floating body for movement from one place to another and which, when required, can be sunk to stand on the sea bed with the or each upper structure protruding above the surface of the sea.

When an offshore gravity platform is standing on the sea bed with its slimmer upper structure or slimmer upper structures protruding above the surface of the sea, there is always a risk that a ship or other floating vessel may collide with the protruding upper structure or one of the protruding upper structures of the plafform. Such a collision may seriously damage the platform, and in some circumstances, may cause at least the upper part of the platform to collapse, possibly with loss of human life. This risk has always been recognised by marine structure engineers and it has been proposed to provide the or each upper structure of an offshore gravity platform with protection against collision by a ship or other vessel.Such protection usually takes the form of a device which encircles an upper structure at or slightly above sea level and which is generally called, and will hereinafter be referred to as, a fender.

It is an object of the present invention to provide a marine structure of the kind described in which the part or at least one of the parts protruding above sea level is encircled by an improved fender which will substantially eliminate, or at least reduce to an acceptable level, the effect on said protruding part of impact by a ship or other floating vessel.

In the marine structure according to the invention the or each fender comprises a substantially rigid ring of circular or non-circular shape which encircles and is spaced from the assoc;ated protruding part of the structure at and/or slightly above and/or slightly below sea level and, connecting the ring to the protruding part, a plurality of flexible elongate members of high tensile strength which are secured to the ring at spaced positions around its periphery and which are secured to the protruding part above and below the ring at spaced positions around the protruding part, the arrangement being such that, if a ship or othe floating vessel should collide with the fender, the fender will be deflected inwardly towards the protruding part and at least a substantial proportion of he energy arising from the impact will be absorbed by strain imparted in flexible elongate members that are remote from the point o, impact.

The fender is preferably of substantially the same shape as that of the orotruding part of the structure and the flexible elongate members are preferably of such lengths that, in its normal position, the fender is symme',ricaiWy positioned with respect to the prntruding pert.

Preferably two flexible elongate members of high tensile strength are secured at each of the spaced positions around the ring of the fender, one of these two flexible members being attached to the protruding part of the structure at a position above the ring and the other of the two members being attached to the protruding part at a position below the ring. Preferably, aiso, the positions of attachment of the flexible elongate members on the ring and on the protruding part of the structure are uniformly spaced.

Some of the plurality of flexible elongate members secured to the protruding part of the marine structure above and below the ring may be arranged to extend tangentially of the protruding part in Such a way that, if a ship or other floating vessel should collide with the fender, the fender will also be caused to rotate at least partially with respect to the protruding part.

It will be appreciated that, in addition to being able to absorb at least a substantial proportion of the energy arising from an impact by a ship, the flexible elongate members must aiso support the weight of the fender. As the flexible elongate members, it is preferred to employ ropes of nylon or other suitable synthetic material because they are substantially immune to corrosion by sea water.

The substantially rigid ring may be of any convenient shape and form but, with a view to limiting the effect of wave action on the fender, preferably the ring is a lattice work structure built up of a plurality of elongate metallic members. In a preferred embodiment, the lattice work ring is in the shape of a flattened diabio built up of three separately formed circular rings arranged with their centres lying on a common axis and maintained axially spaced apart by rigid bracing members, the outer two rings being of the same external diameter as one another and the intermediate ring being d smaller external diameter than the other two rings. Preferably the rings and spacing members are made of steel.

The invention will be further illustrated by a description, by way of example, of a preferred fender for use in an offshore gravity platform with reference to the accompanying drawings, in which: Figure 1 is a side view of the fender mounted on an upper structure of the offshore gravity platform; Figure 2 is a plan view of the fender; and Figure 3 is a diagrammatic plan view of the fender illustrating how the fender serves to protect the upper structure from a collision by a ship.

Referring to Figures 1 and 2, the fender comprises three steel rings 1, 2 and 3 arranged with their centres lying on a common axis and maintained axially spaced apart by steel bracing members 4. The central ring 3 is of smaller external diameter than the outer rings 1 and 2.

Secured to the central ring 3 at sixteen uniformly spaced positions around the ring are thirty two nylon ropes 5, 6. The nylon ropes 5 are secured to a collar 7, fixed to an upper structure 10 of the platform, at uniformly spaced positions around the collar and the ropes 6 are secured to a collar 8, fixed to the upper structure at a position below the fender and sea level, at uniformly spaced positions around the collar.

As will be seen on referring to Figure 3, when a ship collides with the fender, as indicated by arrow A, the fender is deflected inwardly towards the upper structure 10 and the energy arising from the impact is substantially absorbed by the strain imparted in the ropes 5, 6 that are remote from the point of impact, as indicated by arrows B.

Claims

1. A marine structure of the kind which forms or can form a floating body and/or is or can be sunk to stand on the bed of the sea and which, when floating or standing on the sea bed, has at least one part which protrudes above the surface of the sea, wherein the protruding part or at least one of the protruding parts of the structure is protected by a fender comprising a substantially rigid ring of circular or non-circular shape which encircles and is spaced from the associated protruding part of the structure at and/or slightly above and/or slightly below sea level and, connecting the ring to the protruding part, a plurality of flexible elongate members of high tensile strength which are secured to the ring at spaced positions around the protruding part, the arrangement being such that, if a ship or other floating vessel should collide with the fender, the fender will be deflected inwardly towards the protruding part and at least a substantial proportion of the energy arising from the impact will be absorbed by strain imparted in flexible elongate members that are remote from the point of impact.

2. A marine structure as claimed as in Claim 1, wherein the or each fender is of substantially the same shape as that of the protruding part which it encircles and the flexible elongate members are of such lengths that, in its normal position, the fender is symmetrically positioned with respect to the protruding part.

3. A marine structure as claimed in Claim 1 or 2, wherein two flexible elongate members of high tensile strength are secured at each of the spaced positions around the ring of the fender, one of these two flexible members being attached to the protruding part of the structure at a position above the ring and the other of the two members being attached to the protruding part at a position below the ring.

4. A marine structure as claimed in Claim 3, wherein the position of attachment of the flexible elongate members on the ring and on the protruding part of the structure are uniformly spaced.

5. A marine structure as claimed in Claim 3 or 4, wherein some of the plurality of flexible elongate members secured to the protruding part of the structure above and below the ring are arranged to extend tangentially of the protruding part in such a way that if a ship or other floating vessel should collide with the fender, the fender will also be caused to rotate at least partially with respect to the protruding part.

6. A marine structure as claimed in any one of the preceding Claims, wherein the ring of the or each fender is a lattice work structure built up of a plurality of elongate metallic members.

7. A marine structure as claimed in Claim 6, wherein the or each lattice work ring is in the shape of a flattened diablo built up of three rings arranged with their centres lying on a common axis and maintained axially spaced apart by rigid bracing members, the outer two rings being of the same external diameter as one another and the intermediate ring of smaller external diameter than the other two rings.

8. A marine structure as claimed in Claim 7, wherein the rings and spacing members are made of steel.

9. A marine structure as claimed in any one of the preceding Claims, wherein the flexible elongate members are ropes of nylon or other synthetic material.

10. A marine structure of the kind which forms or can form a floating body and/or is or can be sunk to stand on the bed of the sea and which, when floating or standing on the sea bed, has at least one part which protrudes above the surface of the sea, wherein the protruding part or at least one of the protruding parts of the structure is protected by a fender substantially as hereinbefore described with reference to and as shown in the accompanying drawings.