OFFSHORE PLATFORMS

Abstract: A combination of a deck portion (A) and a base portion (B) for an offshore platform, including a plurality of mating devices each of which comprises an extendable stabbing pin (14) mounted on siad deck portion and capture means (27) for said stabbing pin mounted on said base portion, and resilient means (32, 132) associated with said mating devices and arranged, during operation of said devices, to oppose a reduction in the separation of the said portions with a total spring rate that is greater at small separations than at large separations.

Description: This invention relates to offshore platforms, and especially to the assembly of an offshore platform by mating a deck portion being carried by a barge or the like onto a base portion that is already standing secured to the bed of a sea or other body of water, or floating, with its top at or slightly above water level. EP-A-65695 discloses a combination of a deck portion and a base portion for an offshore platform, including a plurality of mating devices each of which comprises an extendable stabbing pin mounted on one of the said portions and capture means for said stabbing pin mounted on the other of the said portions, and resilient means associated with said mating devices and arranged, during operation of said devices, to oppose a coming together of the said portions with a total spring rate that is greater when they are closer together than when they are further apart. The present invention is characterised in that said resilient means comprises first compression spring means having a first spring rate and second compression spring means having a second spring rate greater than said first spring rate, said first and second spring means being arranged to become effective in that order during operation of the said mating devices, and in that the force which compresses the said first spring means is transmitted from the said deck portion to the said base portion through the said stabbing pins, while the force which compresses the said second spring means is transmitted from the said deck portion to the said base portion without passing through the said stabbing pins. The invention is especially applicable where the said base portion has previously been installed on, for example, the sea bed, in comparatively deep and exposed water, and a deck structure is to be lowered onto it from a barge or the like. With suitable arrangement of the said mating devices, the stabbing pins can be extended into engagement with their said capture means and then serve to guide the deck portion into correct mating alignment as it is lowered onto the base portion, while the increasing spring rate of the said resilient means ensures a smooth transfer of the weight of the deck portion from the barge onto the base portion. Preferably the stabbing pins are mounted on the deck portion and the capture members on the base portion. A distal end portion of each stabbing pin and a corresponding portion of its associated capture means may be so shaped as to tend to produce a self-centering action. The distal end portion of the stabbing pin is advantageously convex and the corresponding portion of the capture means concave, and they preferably have conical surfaces. The said deck portion and base portion may include tubular leg members that are arranged to abut end to end when the said portions are fully mated, and each said mating device is then advantageously disposed within a said leg member of the base portion and the corresponding leg member of the deck portion. Preferably, when the stabbing pin is retracted the mating device lies completely within the space envelope of the two legs, where it will neither be exposed to accidental damage nor obstruct other operations carried out within the vicinity of either the deck portion or the base portion. Two embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a schematic view of a platform during assembly: Figure 2 is a schematic plan view of the arrangement of the mating devices in the platform of Figure 1; Figure 3 is an axial cross-sectional view of the stabbing member, and associated parts, of a first mating device of a first embodiment of the invention; Figure 4 is a cross-section on the line IV-IV of Figure 3; Figure 5 is an axial cross-sectional view of a capture means, and associated parts, of the said first mating device; Figure 6 is a cross-section on the line VI-VI of Figure 5; Figure 7 is an axial cross-sectional view of the said first mating device with its stabbing member and capture means interengaged; Figure 8 is an axial cross-sectional view of the stabbing member, and associated parts, of a second mating device of the said first embodiment; Figure 9 is an axial cross-sectional view of the capture means, and associated parts, of the said second mating device; Figure 10 is an axial cross-sectional view of the said second mating device with its stabbing member and capture means interengaged; Figure 11 is an axial cross-sectional view of the mating device, and associated parts, used in a second embodiment of the invention; Figure 12 is a view similar to Figure 11 showing the parts during mating; Figure 13 is a view similar to Figure 12 showing a later phase of mating; Figure 14 shows the parts fully mated; and Figure 15 is a view similar to Figure 12 but showing the parts misaligned. Referring first to Figure 1, a deck portion, which here is a generally rectangular integrated deck of tubular or plate girder steel construction indicated generally by the reference letter A, is arranged to mate onto a base portion, which here is a tubular steel jacket indicated generally by the reference letter B, to form a platform. The deck has twelve legs, comprising four corner legs 10 and eight inner legs 11 (see Figure 2), the lower end of each of which is arranged to abut end-to-end with a respective corner or inner leg 12, 13 of the jacket. Many other possible arrangements of legs may be envisaged, for example eight legs in two groups of four, or sixteen in two groups of eight. Referring now to Figure 3 to 7, within each of the deck corner legs 10 is a stabbing pin indicated generally by the reference numeral 14, comprising a cylindrical body 15 with a convex conical bottom end cap 16 and with an outwardly extending flange 17 at its upper end. A stop collar 18 secured to the inside of the deck leg 10 encircles the cylinder 15 near the bottom end of the deck leg. A bearing 19 fixed to the flange 17 slidably engages the inner surface of the deck leg 10. Bearings 20 and 21 fixed to the stop collar 18 and a further collar 22 respectively slidably engage the cylinder 15. The bearings 19 to 21 permit the stabbing pin 14 to move axially while maintaining it co-axial with the deck leg 10. The bearings 19 to 21 are plain bearings. A hydraulic ram 23 is connected at its lower end to a bracket 24 on the top of the cylindrical body 15 and at its upper end to a bracket 25 on a crossbar 26 fixed within the deck leg 10. When the hydraulic ram 23 is fully retracted, as shown in Fig. 3, the conical cap 16 is at the level of the bottom end of the deck leg. The cylinder of the hydraulic ram 23 is attached to the crossbar 26 and the piston rod is attached to the stabbing pin 14. The hydraulic ram 23 is connected by pipes (not shown) to a source of hydraulic power (not shown). Within the jacket corner leg 12 (Figure 5) is a capture funnel indicated generally by the reference numeral 27 that comprises a cylindrical tube 28, the internal diameter of which is somewhat larger than the external diameter of the stabbing pin 14. At the upper end of the tube 28 the funnel 27 spreads out to form a concave secondary capture cone 29, the extreme diameter of the funnel being somewhat less than the internal diameter of the legs 10 and 12. Two flanges 30 and 31 extend outwards from the tube 28. The capture funnel 27 stands on, but is not secured to, the top end of resilient means in the form of a compression spring stack 32. The spring stack 32 is long compared with the stroke of the ram 23, and is preferably made of elastomeric material with spacers so that it can deform and be compressed axially fairly easily. As shown, the spring stack 32 comprises a stack of separate elastomeric members which may each be of generally annular form. Within the capture funnel 27, a concave primary capture cone 33 stands on, and is preferably secured to, the top of the spring stack 32. The tube 28 is encircled at its upper part by a resilient bearing 34 in the form of a collar of elastomeric material secured to the outside of the tube. The resilient bearing 34 serves mainly to resist horizontal loads, and also to stabilise the capture funnel 27 against radial displacement and tilting. There is a thickened rim 35 around the upper end of the jacket leg 12. The operation of the mating device provided in the corner legs 10 and 12 is as follows: The jacket B is installed at a desired location on the sea-bed, and the deck A is manoeuvred over it while being supported by a barge or the like C (Figure 1). The deck is so positioned that each deck corner leg 10 is approximately coaxial with its respective jacket corner leg 12, with the conical cap 16 of the stabbing pin 14 separated from the primary capture cone 33 by a distance slightly shorter than the stroke of the ram 23. The ram 23 is fully retracted. That is the position shown in Fig. 1. It will be appreciated, however, that because of the action of wind and waves it will not normally be possible to align the deck legs and the jacket legs exactly, except transiently. The rams 23 of two diagonally opposite corner legs are then extended, extending the stabbing pin 14. That may be done quickly when the legs 10 and 11 are sufficiently closely aligned. Each conical cap 16 either enters the tube directly or is guided into it by the secondary capture cone 29, and then seats in the primary capture cone 33, as shown in Figure 7. The stabbing pin 14 is kept co-axial with the deck leg 10 by the bearings 19 to 21. The stabbing pin 14 preferably pushes down the primary capture cone 33 and begins to compress the spring stack 32 so that the cap 16 and the cone 33 will remain in engagement even if the barge C carrying the deck A is lifted by a wave. The same operation is now carried out with the other two diagonally opposite corner legs. The mating devices of the inner leg combinations 11 and 13 are now brought into operation. These mating devices are illustrated in Figures 8 to 10 and are similar in all essential respects to those of the corner legs 10 and 12, with the important exceptions that the spring stacks 132 of the jacket inner legs 13 are of a substantially larger spring rate than the spring stacks 32 of the corner legs, and the collar 122 is formed as a secondary mating cone for engagement with the secondary capture cone 129, so that the force which compresses the spring stacks 132, upon engagement of the members 122 and 129, does not pass through the stabbing pins 114 as it does in the case of the pins 14 of the corner legs 10, 12. Parts of the apparatus shown in Figures 8 to 10 which are similar in structure or function to corresponding parts in Figures 3 to 7 are given the same reference numerals with 100 added and will not be described again. When the mating devices of all of the inner leg combinations 11, 13 have been operated, the deck is now lowered by ballasting the barge C, while keeping all of the rams 23 and 123 extended. Part of the weight of the deck is now transmitted from the legs 10 through the rams 23 and the primary capture cones 33 to the spring stacks 32, which are compressed, and the engagement of the stabbing pins 14 with the capture cones keeps the legs 10 and 12 approximately aligned. At this time, however, there is no compression of the spring stacks 132 in the legs 13, as the secondary mating cones 122 have not yet engaged the secondary capture cones. Lowering continues in this fashion, with the force in the rams 23 gradually increasing, until about 30 percent of the deck weight has been transferred. When about 30 percent of the weight of the deck has thus been taken up by way of the mating devices in the corner legs, the secondary mating cones 122 in the inner legs engage the secondary capture cones 129 and the next about 20 percent of the load transfer is taken up by the stiffer spring stacks 132. At approximately 50 percent of load transfer the mating surfaces of the leg tubulars make contact. Ballasting then continues with no relative motion between the deck and the jacket until about 80 percent of the deck weight has been transferred, whereafter the barge is separated from the deck by the actuation of drop blocks which do not form part of the present invention. The leg tubulars may now be welded together to complete the operation of assembling the deck and jacket. Further details of the structure of the spring stacks 32 and 132 and the resilient bearings 34 and 134, in a typical case, are as follows. The spring stack 32 is manufactured as a series of standard rubber layer elements, each incorporating steel plate reinforcement to form a stack height of 6.47m. The steel plate is arranged to operate as an internal bearing around a central guide pin, and provide clearance to avoid rubber bulging inwards and binding on the guide pin. The rubber is bonded to the reinforcement during vulcanisation, which would be carried out in a large flat bed press. Shaping is included both on the internal and external diameters to minimise bulging and maximise tear resistance. Proposed sizes for the spring stack 32 are 104 elements 1250 mm OD, 525mm ID with a rubber thickness of 58 mm to provide a total stack height of 6.47 m and axial stiffness of 1250 Tonne/m. The specification is as follows: Size 1.25 OD x 0.525m ID Height 6.47m (104 elements x 0.058m, excluding reinforcements) Material Natural Rubber 60 IRHD + 2 Stiffness 1250 Te/m Deflection 1.9m (maximum) Strain 0.30 maximum Stress 24 MN/m² Shape Factor 3.125 The spring stack 132 would utilise the same elements as specified for the stack 32 and operate under similar conditions. Proposed sizes for the stack 132 consist of 61 elements 1250mm OD, 525mm ID with a rubber thickness of 58mm to provide a total stack height of 3.9m and axial stiffness of 2125 Tonne/m. The specification is as follows: Size 1.25m OD x 0.525m ID Height 3.9m (61 elements x 0.058m, excluding reinforcements) Material Natural Rubber 60 IRHD + 2 Stiffness 2125 Te/m Deflection 0.25m (maximum) Strain 0.07 maximum Stress 5.16 MN/m² Shape Factor 3.125 The resilient bearing 34 is manufactured as a set of four complete rings, stacked vertically. Each ring element is 2.2 m OD and 0.5m high. Moulding is in an autoclave, the elastomer ring being bonded to a back flange for attachment to the capture cone body. The lateral stiffness of the assembly is 12500 Tonne/m. The bearing 34 is designed to accommodate 0.2 m of lateral deflection and form an integral part of the total composite lateral stiffness of the jacket leg ends. This feature minimises overloading of the jacket leg, the bearing deflection being limited by external stops on the capture cone body. The specification is as follows: Size 2.2m OD x 1.3 m ID Height 2.0m (4 elements x 0.5m) Material Natural Rubber 75 IRHD + 2 Stiffness 12500 Te/m Deflection 0.2m maximum Strain 0.44 The requirements for resilient bearing 134 are similar to, but much less severe than, those specified for bearing 34, most of the lateral support being handled by the latter bearings. The specification is as follows: Size 1.9m x 1.3m ID Height 1.0m (2 elements x 0.5m) Material Natural Rubber 75 IRHD + 2 Stiffness 12500 Te/m Deflection 0.034m Strain 0.10 A second embodiment of the invention will now be described with reference to Figures 11 to 15. In this embodiment the general arrangement of the deck portion A and jacket or base portion B of the platform, and of the twelve legs to be mated, is the same as is illustrated in Figures 1 and 2. However, instead of employing different mating devices in the corner legs and inner legs respectively, in this second embodiment all of the mating devices are the same, and one of them is illustrated in detail in Figures 11 to 15. Thus, within each of the deck legs 201 is a stabbing pin indicated generally by the reference numeral 203, comprising a hollow cylinder 204 with a convex conical bottom end cap 205 and with an outwardly extending flange 206 at its upper end. A stop collar 207 secured to the inside of the deck leg 201 encircles the cylinder 204 near the bottom end of the deck leg, and a secondary mating cone 208, in the form of a collar with a convex frustoconical lower surface, is secured within the deck leg 301 substantially at the bottom end thereof and encircles the cylinder 204 below the stop collar 207. A bearing 209 fixed to the flange 206 slidably engages the inner surface of the deck leg 201. A bearing 210 fixed to the stop collar 207 and a bearing 211 fixed to the secondary mating cone 208 slidably engage the cylinder 204. The bearings 209 to 211 permit the stabbing pin 203 to move axially while maintaining it co-axial with the deck leg 201. The bearings 209 to 211 are plain bearings. A hydraulic ram 212 is connected at its lower end to a bracket 213 on the top of the conical end cap 205 and at its upper end to a bracket 214 on a crossbar 215 fixed within the deck leg 201. When the hydraulic ram 212 is fully retracted, as shown in Fig. 11, the conical cap 205 is at the level of the secondary mating cone 208 and the crossbar 215 is just above the top of the stabbing pin 203. The lowest parts of the conical cap 205 and the secondary mating cone 208 are level with or slightly above the bottom end of the deck leg 201. The cylinder of the hydraulic ram 212 is attached to the crossbar 215 and the piston rod is attached to the stabbing pin 203. The hydraulic ram 212 is connected by pipes (not shown) to a source of hydraulic power (not shown). Within the jacket leg 202 is a capture funnel indicated generally by the reference numeral 216 that comprises a cylindrical tube 217, the internal diameter of which is somewhat larger than the external diameter of the stabbing pin 203. At the upper end of the tube 217, the funnel 216 spreads out to form a concave secondary capture cone 218, the extreme diameter of the funnel being just less than the internal diameter of the legs 201 and 202. Two flanges 219 and 220 extend outwards from tube 217. The capture funnel 216 stands on, but is not secured to, the top end of a first compression spring 221. The first spring 221 is long compared with the stroke of the ram 212, and is preferably made of elastomeric material with spacers so that it can deform and be compressed axially fairly easily. As shown, the first spring 221 is made up of a stack of separate elastomeric members which may each be of generally annular form. Within the capture funnel 216, a concave primary capture cone 222 stands on, and is preferably secured to, the top of the first spring 221. The tube 217 is encircled above the middle flange 219 by a resilient bearing 223 in the form of a collar of elastomeric material which may be secured to the outside of the tube or to the inside of the jacket leg 202. The resilient bearing 223 may be stiffened by one or more metal tubes, coaxial with the leg 202, embedded in the elastomer, and is advantageously wound in a spiral with alternate turns or metal and elastomer. The resilient bearing 223 serves mainly to resist horizontal loads, and also to stabilise the capture funnel 216 against radial displacement and tilting. An annular second compression spring 224 in the form of a sleeve of elastomeric material encircles the tube 217 below the middle flange 219 and is supported by a flange 225 on the inside of the jacket leg 202. The second spring 224 is much stiffer than the first spring 221, and may have metal disc annuli embedded in it to increase its stiffness. The resilient bearing 223 and/or the second spring 224 may instead be in the form of a plurality of discrete blocks spaced apart or contiguous around the circumference of the tube 217. If either the resilient bearing 223 or the second spring 224 is in the form of discrete blocks and has metal reinforcement, then the metal is in the form of corresponding sectors of the reinforcement for the equivalent annular arrangement. There is a thickened rim 226 around the upper and of the jacket leg 202. The operation of the mating device of Figures 11 to 15 is as follows: The jacket B is installed at a desired location on the sea-bed, and the deck A is manoeuvred over it while being supported by the barge or the like C. The deck is so positioned that each deck leg 201 is approximately coaxial with its respective jacket leg 202, with the conical cap 205 of the stabbing pin 203 separated from the primary capture cone 222 by a distance slightly shorter than the stroke of the ram 212. The ram 212 is fully retracted. That is the position shown in Fig. 11. It will be appreciated, however, that because of the action of wind and waves it will not normally be possible to align the deck leg 201 and the jacket leg 202 exactly except transiently. The ram 212 is then extended, extending the stabbing pin 203. That may be done quickly when the legs 201 and 202 are sufficiently closely aligned, and if as in this embodiment there are a plurality of pairs of legs 201 and 202 with mating devices then the stabbing pins 203 may be extended separately as and when the opportunity arises for each, but it is preferred to extend all of the stabbing pins together. The conical cap 205 either enters the tube 217 directly or is guided into it by the secondary capture cone 218, and then seats in the primary capture cone 222, as shown in Fig. 12. The stabbing pin 203 is kept co-axial with the deck leg 201 by the bearings 209 to 211. The stabbing pin 203 preferably pushes down the primary capture cone 222 and compresses the first spring 221 slightly, so that the cap 205 and the cone 222 will remain in engagement even if the barge C carrying the deck A is lifted by a wave. The deck is then lowered, while keeping the ram 212 extended. Part of the weight of the deck is then transmitted from the leg 201 through the ram 212 and the primary capture cone 222 to the first spring 221, which is compressed, and the engagement of the stabbing pin 203 with the capture cone keeps the legs 201 and 202 approximately aligned. That continues, with the force in the ram 212 gradually increasing, until the secondary mating cone 208 engages and seats on the secondary capture cone 218, which is supported by the second spring 224 abutting its middle flange 219 now that the first spring 221 is compressed, as shown in Fig. 13. Because the first springs 221 are both long and soft, only a minor proportion of the weight of the deck is ever carried by the rams 212. As the lowering of the deck continues, weight will be transferred from the deck leg 201 through the secondary mating cone 208 and the capture funnel 216 to the second spring 224. The secondary mating cone 208 and the capture funnel 216 will maintain a closer alignment of the legs 201 and 202 than the stabbing pin 203 and the primary capture cone 222. Because of the stiffness of the springs 224, they rapidly come to take a substantial proportion of the weight of the deck A with the result that fluctuations in the upthrust on the barge caused by waves and the like are absorbed with only small movements of the legs 201 and eventually the bottom of the deck leg 201 seats on the top of the jacket leg 202, as shown in Fig. 14, in a controlled manner and with sufficiently accurate alignment. A small separation of the axes of the deck leg 201 and the jacket leg 202 can be accommodated because of the thickness of the rim 226 on which the deck leg seats. Because the second spring 224 acts between the legs 201 and 202 by way only of the secondary mating cone 208, the capture funnel 216, and the flange 225, the ram 212 is never subjected to the full load on the second spring and the degree of compression of, and hence the force taken by, the second spring at the moment when the legs 201 and 202 can be accurately predetermined. The legs 201 and 202 are then welded together and it may then be possible to dismantle the mating device and to remove at least part of it from inside the leg. Referring now to Fig. 15, if the stabbing pin 203 is extended when there is angular misalignment between the deck leg 201 and the jacket leg 202, or if the deck tilts after the stabbing pin has engaged the primary capture cone 222, then the primary capture cone and the capture funnel 216 will tilt to accommodate the misalignment. As seen in Fig. 15, the resilient bearing 223 compresses on one side, and the first spring 221 compresses unevenly so that its top face tilts, with the result that the capture funnel tends to pivot in a sense to align itself with the stabbing pin 203. As shown in Fig. 15, even a very large misalignment of 5 o can be accommodated at that stage, but it is preferred that the angular alignment should be no more than 1 o out of true. As the deck is lowered, the possible misalignment is progressively reduced because the vertical separation between the resilient bearing 223 and the top of the first spring 221 increases, so that a smaller angular displacement corresponds to the same lateral displacement. Also, where a platform has several pairs of legs 201 and 202 with mating devices, as in the present embodiment, a tilt of the deck corresponds to a difference in the separation between different legs 201 and their respective springs 221 and 224 that tends to right the deck, and becomes greater as the second springs 224 are engaged. The hydraulic ram 212 may be replaced by a pneumatic ram or by some other suitable driving means. The mating surfaces of the cap 205, the secondary mating cone 208, and the primary and secondary capture cones 221 and 218 are preferably, as shown in the drawings, conical surfaces, convex and concave, co-axial with the legs 201 and 202 and of equal cone angles, but any other surfaces that will provide the desired self-centring engagement may be used instead. The mating surfaces are advantageously provided with a low-friction coating to assist centring. The following dimensions are suitable for the arrangement shown in Figures 11 to 15. The legs 201 and 202 may be about 2 metres in diameter, the ram 212 may have a stroke of about 3 metres, and the first spring 221 may have an uncompressed length of about 6 metres. It is to be clearly understood that throughout this specification when the terms "resilient" and "resiliency" are used, the important characteristic being described is the ability of the material concerned to elastically deform, thereby dampening motion.

Claims: A combination of a deck portion (A) and a base portion (B) for an offshore platform, including a plurality of mating devices each of which comprises an extendable stabbing pin (14, 114; 203) mounted on one of the said portions and capture means (27, 127; 216) for said stabbing pin mounted on the other of the said portions, and resilient means (32, 132; 221, 224) associated with said mating devices and arranged, during operation of said devices, to oppose a coming together of the said portions with a total spring rate that is greater when they are closer together than when they are further apart, characterised in that said resilient means comprises first compression spring means (32; 221) having a first spring rate and second compression spring means (132; 224) having a second spring rate greater than said first spring rate, said first and second spring means being arranged to become effective in that order during operation of the said mating devices, and in that the force which compresses the said first spring means is transmitted from the said deck portion to the said base portion through the said stabbing pins, while the force which compresses the said second spring means is transmitted from the said deck portion to the said base portion without passing through the said stabbing pins. Apparatus as claimed in claim 1, wherein each of said mating devices includes a resilient bearing (34, 134; 223) arranged to cushion relative lateral movement of said stabbing pin (14, 114; 203) and said capture means (27, 127; 216). Apparatus as claimed in claim 1 or 2, wherein each of said mating devices includes a hydraulic ram (23, 123; 212) for extending said stabbing pin (14, 114; 203). Apparatus as claimed in any of claims 1 to 3, wherein, in each of said mating devices, the distal end portion (16; 205) of said stabbing pin (14; 203) and the corresponding end portion (33; 222) of said capture means (27; 216) are formed to produce a self-centering action therebetween. Apparatus as claimed in claim 4, wherein said end portions of said stabbing pin and capture means are convex and concave respectively. Apparatus as claimed in any preceding claim, wherein in each of the said mating devices the stabbing pin (14; 114; 203) is mounted on the said deck portion (A) and the capture means (27, 127; 216) on the said base portion (B). Apparatus as claimed in any preceding claim, wherein the said stabbing pin (14, 114; 203) and capture means (27, 127; 216) of each of the said mating devices are housed in respective tubular leg members (10-13; 201, 202) on the said platform portions, which leg members are arranged to abut end to end when the said portions are fully mated. Apparatus as claimed in claim 7, wherein when its stabbing pin (14, 114; 203) is fully retracted each of the said mating devices lies completely within the space envelope of its two associated tubular leg members (10-13; 201, 202). Apparatus as claimed in any preceding claim, wherein at least one of the said mating devices includes only the said first spring means (32) and the or each remaining mating device includes only the said second spring means (132). Apparatus as claimed in claim 9, including a substantially rectangular array of said mating devices, the said first spring means (32) being included only in the four corner mating devices (10, 12) of the said array and the second spring means (132) in all of the remaining mating devices (11, 13) located inwardly of the corners. Apparatus as claimed in any of claims 1 to 8, wherein each of the said mating devices includes both said first spring means (221) and said second spring means (224). Apparatus as claimed in any preceding claim, wherein said first and second spring means (32, 132; 221, 224) each comprise a stack of annular resilient members. Apparatus as claimed in claim 12, wherein the annular resilient members of said spring means (32, 132; 221) are maintained in alignment by a guide pin which passes through a central aperture in each said member. Apparatus as claimed in claim 12 or 13, wherein a primary capture member (33; 222) of said capture means (27; 216) operatively engages the top of the stack of resilient members comprising said first spring means (32; 221). Apparatus as claimed in claim 14, wherein a secondary capture member (29; 218) of said capture means (27; 216) encircles, but is not attached to, said primary capture member (33; 222). Apparatus as claimed in claim 15, wherein said secondary capture member (29; 218) sits upon, but is not attached to, the top of the stack of resilient members comprising said first spring means (32; 221). Apparatus as claimed in claim 16, wherein said second spring means (224) encircles, but is not attached to, said secondary capture member (218).

Inventors: BRITTON, THOMAS NYLE, HAWLEY, PHILIP RAYMOND, KYPKE, DEAN ALLEN, WHITE, GEORGE JOSEPH
Applicants: CONOCO (U.K.) LIMITED
Application year: 1986
Main classification: E02B17
IPC codes: E02B17