GB2081414A

GB2081414A - Pipe anchoring system

Info

Publication number: GB2081414A
Application number: GB8025934A
Authority: GB
Original assignee: Pilgrim Engineering Developments Ltd
Current assignee: Pilgrim Engineering Developments Ltd
Priority date: 1980-08-08
Filing date: 1980-08-08
Publication date: 1982-02-17

Abstract

An assembly for anchoring a pipeline 10 to the sea bed comprises a collar 12 which is fixed around the pipeline 10. Two struts 13 connect the collar to two piles 11 driven into the sea bed. Each strut 13 has at one end a collar 16 secured around its respective pile 11. The other ends of the struts 13 are pivoted to opposite ends of two balance beams 22 which are pivoted at their midpoints on the collar 12. The struts 12 include hydraulic jacking arrangements 19 which are stressed to apply a predetermined stress to the pipe line to restrain the latter against creep. <IMAGE>

Description

SPECIFICATION Pipe anchoring system The present invention relates to a system for anchoring pipes, especially sub-sea oil or gas pipelines.

It has been found with the earlier sub-sea oil pipelines that with the passage of time progressive creeping of the pipeline has occurred. The rate of creeping may be up to 70mm per year. More recent installations incorporate a Z-shaped expansion bend adjacent the platform supporting structure to accommodate the creep phenomenon. In many instances creep has brought the pipeline into contact with the bracing structure of the platform making it necessary temporarily to cut away some of the braces.

The object of the present invention is to provide a pipe anchoring system which can be used effectively to restrain a pipeline against creep.

According to the present invention in a first aspect there is provided an assembly for anchoring a portion of a pipeline, comprising a collarforfixing around the pipeline and first and second struts, each strut being attached at one of its ends to the collar and having at its other end an attachment means for attaching the strut to a respective fixed member, each strut including a stressing means for stressing each strut to a predetermined value to prestress the pipeline.

With this arrangement it is not necessary to cut the struts accurately to size. Evenly balanced stressing of the struts can be achieved by the hydraulic stressing means and connecting the stressing means to a common source of hydraulic pressure.

Advantageously the struts are connected to the collar by means of a pivoting member mounted on the collar. With this arrangement, if one of the struts is damaged by dragging of a ship's anchor, for example, the tension on the other strut is released, thus preventing eccentric loading on the pipeline.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, of which: Figure 1 shows a plan view of a pipe-anchoring system in accordance with the invention; Figure 2 shows a plan view of a second pipeanchoring system in accordance with the invention; Figure 3 shows on a larger scale an end view, partly in section, of a pipe-clamping collar of the assembly of Figure 2; Figure 4 shows a side elevation, partly in section, of the pipe-clamping collar of Figure 3; Figure 5 shows a detail of the collar of Figures 3 and 4 on an enlarged scale; Figure 6shows on an enlarged scale a plan view of a balanced beam of the assembly of Figure 2; Figure 7shows on an enlarged scale an elevation, partly in section, of a pintle and its bearings;; Figure 8 shows on an enlarged scale a vertical section through a bearing of the pintle of Figure 7; Figure 9 shows on an enlarged scale a ball joint and hydraulictensioning arrangementofthe assemblies of Figure 1 and 2; and Figures 10 to 14 show various stages in assembly of a collar of the assembly of Figure 1 onto a pipeline.

Referring to Figure 1, this shows a simple version of an assembly for clamping a section of a 915mm (36 inch) diameter steel pipeline 10 to the seabed in accordance with the invention. Two 1.9or (75 inch) diameter heavy steel piles 11 are driven into the seabed opposite one another on either side of the pipeline 10 and about 5m from it. A longitudinallysplit steel collar 12 is fitted around the pipeline and bolted to clamp it to the pipeline at a position about 12m from the piles. Two steel struts 13 connect the collar 12 to the piles. At their ends adjacent the collar, the struts are connected to lugs 14 on the collar by pin joints 15.At their other ends they have annular rings 16 which are divided across a diameter and are provided with bolting flanges 17 for receiving hydraulically tensioned bolts 18 for clamping the rings 16 around the piles 11. Between the ring 16 and the main part of each strut is a ball joint and hydraulic jacking arrangement 19. The ball joint provides articulation and the jacking arrangement which can exert a force of 560 tons and has a stroke of 0.2m enables the strut to be stressed.

Figure 2 shows a modified version of the pipe anchoring assembly of Figure 1. It differs from the version of Figure 1 in that ball joints 20 are provided in the struts at the collar end and in that the struts are not connected directly to the collar. In other respects the assemblies are the same and the same reference numerals have been used.

The collar 12 of Figure 2 has, instead of the lugs 14, journals 21 on which pivot two balance beams 22, one of which lies above the collar and the other of which lies below the collar 12. The struts are connected to two pintles 23 extending between the two beams parallel to the pivot axis of the journals 21 and situated at opposite ends of the beam equidistant from the pivot axis. This arrangement has the advantage that under emergency conditions when one of the struts is damaged, for example by dragging of a ship's anchor, the load is instantaneously released on the other strut so that there is no eccentric loading on the pipe which might cause buckling and even failure of the pipe.

The balance beam and pintle arrangement of the embodiment of Figure 2 will now be described in greater detail with reference to Figures 3, 6, 7 and 8.

In Figure 6 the left-hand half shows the upper balance beam 22 and right-hand half shows the lower balance beam 22. The two balance beams are made of 100mm thick mild steel plate. Each beam has a central aperture 24, and two apertures 25 equidistant from the central aperture 24 on either side of it. The apertures 24 and 25 receive the bearings which permit the pivoting movement of the beam relative to the collar 12 and of the pintles 23 relative to the beam. The aperture 24 in the lower beam receives the journal 21 which is on the under-side of the collar 12. The bearing thus formed may be assembled, packed with reases and sealed with a watertight seal before lowering to the sea bed.

The journal 21 of the upper beam is formed on a separate ring 26 which is assembled on the beam, packed and sealed to form a cartridge type bearing prior to installation. The ring 26 is secured on a tapered boss 27 on the upper side of the collar 12, by means of bolts 28.

Similarly the bearings between the pintles 23 and the balance beam are of the cartridge type. They are shown in detail in Figures 7 and 8.

The lower bearing for the hinge pintle 25 comprises an inner ring 29 which is fixed on a tapered portion 30 of the pintle 23 and provides the journal 31, and an outer ring 32 which provides the bearing bush 33 and has a stepped and tapered outer surface 34 which fits in the correspondingly stepped and tapered aperture 25. The inner and outer rings are assembled on the pintle, sealed and secured in place by means of a plate 34 and bolts 35, to form a cartridge type bearing. The bearing is subsequently secured to the balance beam by bolts 36 when the pintle is installed on the beam 22.

The upper bearing is shown in detail in Figure 8.

The upper portion 37 of the pintel 23 is tapered to receive a ring 38 with a correspondingly tapered bore 39. The ring 38 has an outer cylindrical surface 40 which is covered with Glacier "DX" anti-friction bearing material 41 (available from Glacier Metals).

The antifriction material is secured in position on the ring 38 with epoxy or locktite (locktite is a trade mark) adhesive before the ring 38 is fitted on the tapered portion 37 of the pintle 23. The bearing material 41 forms the journal of the bearing. The bearing bush is formed by the cylindrical wall of the aperture 25 in the upper balance beam 22. The bearing surfaces 25 and 41 are lubricated by grease from a reservoir which is formed by a well 42 in atop cover plate 43 and connected by bores 44 in the cover plate 43, pintle 23 and balance beam 22 to the bearing surfaces. The bearings are sealed against leakage of grease by 'O' ring seals 46 which are located in peripheral grooves in the cover plate and pintle and bear upon collars 47 secured to the upper balance beam 22 by screws 45.

The grease reservoir is covered by a corrigated nitrile rubber diaphragm 48 attached to the cover plate 43 at its periphery by screws 49. The di aphragm is subjected to the hydrostatic pressure prevailing at the depth of ocean at which the pipeline is installed. Thus the pressure of the grease exactly balances the pressure at which seawater is trying to enter the bearings.

The ball joint and hydraulic jacking arrangement 19 will now be described in greater detail with reference to Figure 9.

One half of the pile clamp 16 is welded to a pair of spaced plates 50 for supporting the ball joint and jacking arrangement 19. A base plate 51 is welded between the support plates 50 and has four threaded sockets 52 disposed in a square array. The sockets 52 each receive an end portion of a threaded stud 53. A cylinder member 54 of the hydraulic jack rests upon the base plate 51. The cylinder member 4 has an inner portion 55 with four bores 56 through which the studs pass and an outer portion 57 of reduced external dimension with external cavities 58 to accommodate the studs and nuts 59 on the stud for clamping the cylinder member to the base plate. The interior of the cylinder 54 houses a piston 60 connected by a piston rod 60a to a platform 61 which carries the ball joint.The platform 61 has four holes 62 which allow it to slide freely on the studs 53. The ends of the cylinder 54 are closed by plates 63 and 64, the piston rod passing through a bore in the plate 64. The joints between the plates and the cylinder member and the piston rod are fitted with "0" rings 67 to seal the chambers 65 and 66 on either side of the piston.

By connecting the chamber 65 or 66 to a suitable supply of hydraulic pressure the platform 61 can be raised or lowered relative to the base plate 52 to extend or contract the strut 13 and place in under tension or compression. Locking nuts 68 are provided on each stud on either side of the platform 61 and are screwed down against either side of the platform to lock the platform in position while the loading pressure required to load the strut to 560 tons is maintained.

The ball joint comprises a hollow cylindrical casing 70 which is secured to the platform 61 by screws 71. A diaphragm plate 72 is cast or forged solid with the casing 70. Two stud ends 73 extend axially in opposite directions from the diaphragm plate. They may be the ends of a continuous stud inserted through the plate or they may be turned solid with the plate. Two convex part-spherical bearing members 74 are screwed on the studs 73 and are arranged to form part of a spherical surface when in position. Two cup members 75 having concave part-spherical bearing surfaces which match in curvature the bearing members 74 are held in contact with the bearing members by studs 76 which pass through holes 77 in the diaphragm with clearance sufficient to allow the desired mobility of the ball joint.The arrangement is filled with light grease and sealed with a flexible corrugated diaphragm 78 to equalise the internal pressure with that of the sea. A screw-threaded spigot 79 on the upper cup member 75 engages a screw threaded bore 80 in the end of the pipe that forms the main part of the strut 13.

The ball joint 20 adjacent the collar 12 is similar to that described above and is shown in Figure 5 with the same reference numerals.

The pipe clamping collar 12 will now be described in further detail with reference to Figures 3,4 and 5.

The collar 12 is a heavy casting or forging of mild steel. It is formed in two halves divided from one another across a diametral plane. The two halves of the collar 12 have along their longitudinal edges bolting flanges 82 with drilled holes 83 for receiving hydraulically tensioned studs 84. Inflatable "0" ring seals 81 may be provided in circumferential grooves on the inside of the collar 12 to provide a seal against the pipe if it is desired to inject a sealant into the interface between the collar and the pipe after the collar has been fitted to the pipe.

On the inside of the collar are several arcuate recesses 85 which receive gripping segments 86.

The gripping segments are rolled to shape and machined in a jig to form deep grooves, divided by walls which terminate in 60 knife edges 87. The edges are flame hardened and tempered in place to give a yield pressure of 100 tons per square inch. The segments are assembled in the recesses 85 which are turned in the bore of the clamp halves. The walls of the recesses 85 are machined with ribs 88 which can be deformed over the segments to grip the segments and retain them firmly in place.

To secure the flanges together the hydraulic studs 84 are inserted in the flanges, working nuts 89 are applied to their ends and a hydraulic tensioning devices 90 are applied over the working nuts. The bolts are tensioned simultaneously using a common supply of hydraulic pressure and the load is maintained by screwing down the working nuts by hand tommy bar before the hydraulic tensioning device 90 is released.

Figures 10 to 14 show the sequence of operation of the assembly jig for use by the divers in their standard assembly frame. Figure 7 shows the first sequence in the sub-sea assembly of the split pipe-clamping collar 12 on the pipeline 10. The buried pipeline is exposed in the area to be clamped by water jetting followed by removal of the concrete sinker and the bitumen protective coating from the pipeline.

The two collar halves are bolted to hinged side plates 91 which are jacked open by an upper hydraulic actuator 92. All the ancillary equipment such as studs, nuts and stud tensioners are housed as sub-assemblies in the upper and lower bolt trays 93. The stud tensioners 90 and the upper and lower hydraulic actuators are connected by flexible hoses to a valve manifold (not shown) which, in turn, is connected by flexible hose to the high-pressure pump in the divers' boat.

The next stage is shown in Figure 11. The halves of the pipe-clamping collar are swung together so that their flange faces are parallel. This is achieved by hydraulic control of the upper hydraulic actuator 92.

Figure 12 shows the next stage in which the lower hydraulic actuator 94 is first pinned to its anchor block 95 before both hydraulic actuators 92 and 94 are operated to draw the two collar halves together gripping the pipe 10. The studs 84 are slid through the holes 83 in the flanges and the working nuts 89 are screwed on and nipped up using a special tubular hexagonal spanner 98 (Figure 14).

The high-pressure hydraulic supply to the stud tensioners 90 is now controlled from the diver's boat causing the knife edges 87 on the inside of the collar to bite into the outer surface of the pipe. The edges penetrate about 0.05mm into the pipe surface but fatigue testing has shown that no lowering of the fatigue strength of the pipe results. The pressure is then released and the gaps between the flange faces of the collar are measured. The assembly of the collar on the pipe is made when the pipe is at a pressure significantly below its working pressure. It is necessary therefore that the correct gap between flanges is achieved in order that the clamping is satisfactory when the pipe is at the working pressure.This is achieved by calculating the desired gap from the measurements taken after the initial tensioning and having the diver insert the appropriately sized shims between the flanges. Afinal pressurizing of the stud tensioner 90 stretches the studs 84 to the desired tension. The pressure is maintained while the tension nuts 89 are nipped up using a tommy bar in the holes drilled in the outside of the nut. The rotation of the nut is noted and if satisfactory the tensioning operation is complete and the pressure can be released, the tension in the studs being retained by the tension nuts 89.

The assembly jig is then lifted away as shown in Figure 13 after the dowels and bolts 96 which secure the collar to the side plates 91 have been removed.

Tension wires 97 secure the lower part of the jig assembly to the upper part. Figure 14 shows the, collar after the jig assembly has been lifted away. To complete the installation the pile clamping rings and struts are placed in position. It is not practical to position the piles accurately on the seabed. Therefore steel tape measurements of the distances of each pile from the collar are taken by the divers and these measurements are used to cut the struts to size. The pile clamping rings are assembled with the struts and lowered into position. The pivot pins 15 are inserted to join the struts 13 to the collar 12. The pile clamps are secured to the piles using the hydraulic bolts 18 or cross-pins which pass through the piles. Hydraulic pressure is applied to the jacks 19 to tension the struts as described above, the tension being locked into the struts using the nuts 68.

When using the balance beam assembly shown in Figure 2 two sub-assemblies may first be made at the surface. The first sub-assembly comprises the lower balance beam 22 fitted to the lower journal of the pipeline clamping collar 12. The second subassembly comprises the upper balance beam the upper half of the pipeline-clamping collar and the pintles 23. The struts 13 are connected to the pintles 23. The collar and the pile clamping rings are clamped in position as described above. The balance beam is set in a position perpendicular to the pipeline and the struts are tensioned using the hydraulic jacking arrangement 19.

Claims

1. An assembly for anchoring a portion of a pipeline, comprising a collar for fixing around the pipeline and first and second struts, each strut being attached at one of its ends to the collar and having at its other end an attachment means for attaching the strut to a respective fixed member, each strut including a stressing means for stressing each strut to a predetermined value to prestress the pipeline.

2. An assembly according to claim 1 in which each strut is attached to the collar through a pivoting joint.

3. An assembly according to either preceding claim in which each attachment means comprises a pile-engaging collar for attachment to a fixed member comprising a cylindrical pile driven into the earth.

4. An assembly according to any preceding claim in which each strut is connected to its attachment means through a pivoting joint.

5. An assembly according to any preceding claim in which each stressing means comprises a hydraulic jack.

6. An assembly according to any preceding claim in which the struts are connected to the collar by means of a pivoting member mounted on the collar, the struts being attached to the pivoting member at opposite sides of its pivot axis.

7. An assembly according to claim 6 in which the pivoting member comprises first and second parallel beams mounted on opposite sides of the pipelineengaging collar, the pivot axis being perpendicular to and passing through the longitudinal axis of the said collar, the beams being joined at each side of the pipeline by parallel strut-carrying members.

8. An assembly according to any preceding claim in which the struts are free to pivot about respective axes parallel to each other and extending perpendicularly to the longitudinal axis of the pipelineengaging collar.

9. An assembly according to any preceding claim in which the pipeline-engaging collar comprises a pair of half-collars, meeting on a diametral plane and having hydraulic bolts for securing together the half-collars.

10. An assembly according to any of claims 3 to 9 in which the pile-engaging collars each comprise a pair of half-collars meeting on a diametral plane and having hydraulic bolts for securing together the half-collars.

11. A portion of a pipeline anchored to first and second fixed members by an assembly according to any preceding claim.

12. A portion of a pipeline anchored to first and second fixed members by an assembly according to either of claims 6 and 7, the struts extending along lines not passing through the pivot axis for exertion of a turning moment on the pivoting member by the stress of the first strut in the event of failure of the second strut or its fixed member.

13. A portion ofa pipeline according to claim 12 in which the fixed members are each disposed equidistantly from the longitudinal axis of the pipeline on a line which intercepts the longitudinal axis perpendicularly, the struts extending in the plane that contains the said line and the said axis.

14. A pressure-equalising lubrication system for a device having a bearing surface subjected to a hydrostatic pressure, comprising an enclosed lubricant reservoir, having a surface comprising a portion which is a flexible diaphragm, and a lubricant-supply passage from the reservoir to the bearing surface, the diaphragm being subjected to the hydrostatic pressure to pressurise the lubricant at the surface to that pressure.