GB2177744A - Compliant tower - Google Patents

Abstract

A compliant offshore tower (1) is secured by tubular long piles (13) extending to near water-level and fixed to the tower only at their upper ends. The major portion (15) of the length of the piles is thin-walled, while a minor portion (14) which is for driving into the seabed (2) is thick-walled. Preferred length and thickness ratios are given. The pile is driven at the upper end of the thick-walled portion (14) by means of a hammer (21) working within the thin-walled portion (15). The piles are prefabricated and floated out to site as part of the tower, in one embodiment whole piles (13) are preassembled and are driven complete, being fixed axially to the tower only after driving; in another embodiment the thin-walled part (15) is prefixed to the tower, and only the thick-walled part (14) is driven and then fixed to the thin-walled part (15). <IMAGE>

Description

SPECIFICATION Compliant tower This invention relates to towers used in offshore production for the support of working etc facilities at sea level from the sea bed.

The invention relates to such towers used in deep water where depths of for example 800 metres can be encountered.

All towers of this character comply to a certain extent with the environmental forces acting on them, the most important of which is wave force. Recently towers have been deliberately built to give compliance or flexibility and one series of proposals has used a mechanical universal joint to secure the tower to the bed; another has centred on the idea of giving compliance by extending the piles which anchor the tower to the sea bed up to near the sea level where they are rigidly secured to the tower.

Although this latter mode of construction has advantages it brings considerable difficulties in its train also. Primarily there is the problem of the need to fabricate the lengthy piles at the time that they are being driven, usually by the addition of welded "add-ons".

This enables them to be driven from the surface using conventional hammers, but this can only be done if the piles are thick and therefore strong enough to carry the shock.

These piling systems where the piles extend to near the top of the tower have so far had two characteristic conformations, one where comparatively small diameter piles have been grouped within the legs at the corners of the frame; another where a number of larger diameter piles have been grouped in the central area of the frame.

In our invention we adopt a totally different approach which as will be seen avoids not only the fabrication problems of the previous "long pile" systems but overcomes also cer- tain additional problems which have not yet been mentioned.

In our invention, a compliant tower has at least one long pile which is to extend over a substantial portion of the length of the tower, the pile having a lower portion which is used for driving into the ground and is thickerwalled than the majority of the length of the pile which is comparatively thin-walled. The pile is attached rigidly to the tower a substantial distance up the tower, comparatively close to sea level, but is left axially free of the tower below the rigid attachment.

In one version there is an internal ledge at or adjacent to the intersection between the two types of wall. The internal ledge is to be acted on by an underwater hammer of the type known as a pencil hammer, in which a body containing a hydraulic power pack for a vibrating driving shoe is suspended by an umbilical cord containing electric power supply means and control cables. Such a hammer is suspended within the thin-walled portion of the long pile. Its shoe acts on the internal ledge and is actuated to drive at least a substantial portion of the thick-walled part of the pile into the sea bed, before the rigid attachment of the pile to the tower.

In another version the thick-walled portion of the pile fits inside the thin-walled portion and the two are not fixed together until after driving of the thickwalled portion using an underwater hammer (as described above) suspended within the thin-walled portion and acting on the upper end of the thick-walled portion. The thick-walled portion is initially axially slidable relative to the thin-walled portion, and the latter is therefore rigidly attached to the tower before driving; as it is driven the thickwalled portion slides downwardly out of the fixed thin-walled portion. The two portions are then fixed together, for example by having an overlap with the upper end of the thick-walled portion remaining partially within the lower end of the thinwalled portion.The gap between the portions at the overlap may then be filled with a grout or similar material to fix them together, or the portions may be swaged together by forcibly expanding the (inner) thickwalled portion outwardly to contact and press against the inside of the other portion at the overlap.

In another aspect of the invention we provide a method of securing an offshore tower to the sea bed, which consists in positioning on the tower at least one long pile which is substantially the same length as the tower and which has two portions, the portion adjacent the base of the tower having a wall thicker than that of the remaining portion, positioning the tower in relation to the sea bed, driving the thick-walled portion of each pile using an underwater hammer within that pile, and securing a top portion of each pile to the tower.

In one version of the method the thick and thin-walled portions of each pile are positioned on the tower fixed rigidly together, then driven after positioning of the tower with the underwater hammer acting upon the pile at a position adjacent the transition between the thin and thick-walled portions, following which the top portion of the pile is attached rigidly to the tower, leaving a major part of the pile below the position of attachment substantially axially free of the tower.

It is an advantage of this version of the method that the piles may be completely fabricated on land.

In another version of the method the top portion only of each pile is initially attached rigidly to the tower. The thick-walled portion of the pile is positioned axially slidably within the thin-walled portion and driven, after positioning of the tower, by the underwater hammer acting on its upper end. The thick and thin walled portions are then fixed rigidly to gether; for example by swaging i.e. forcible outward expansion of part of the thick-walled portion overlapping within the thin-walled portion, by grouting in a gap between the portions at the overlap, or by another suitable method.

This version of the method has the advantage that the mass driven by the hammer is only that of the thick-walled portion, not of the whole pile, so the driving is particularly efficient. It also avoids problems of strain, caused by the driving, at the transition between the thick and thin walled portions; this is because the two portions are not joined until after the driving.

The piles may thus be floated out to the position of use preloaded onto the tower structure: furthermore piles fitted in that way may be used during the transport of the structure to site as additional buoyancy chambers for the structure. The tower may also have a buoyancy tank for use when secured and vertical and the head end of such a buoyancy tank is a particularly suitable position for the attachment of the top end of the piles.

The piles may be in any position in relation to the tower structure, but it is particularly preferred that they are associated with and comparatively close to the corner legs of the frame of the structure of the tower, for example a pair of such piles being provided adjacent each such leg and disposed in a line extending at 45 to the side surface of the tower, with each of the pair of piles associated with one leg being equidistant from that leg and directly opposed across it. In the corner position they have the additional advantage of being at maximum moment from the centre of the frame, both from the point of view of resisting sideways (lateral) loads and torsional (twisting) loads on the tower. Furthermore since the piles are external of the legs, their diameter and number is rendered almost independent of the size of the leg.

The invention is applicable both to guyed and unguyed towers. At least in the context of the North Sea users or designers have been constrained to use guys, which restrain the tower but because of their yielding nature allow a response of the tower to the prevailing environmental forces. However with the degree of flexibility which is afforded by tbe present proposed structure (obviously a thin wall pile being substantially more flexible than a thick walled pile of the same length) this invention should be usable also for unguyed structures at least in suitable environments.

Furthermore the use of piles which for the majority of their length are thin walled reduces the loading on the pile itself, increasing the capacity of the frame structure to carry the load.

Particular embodiments of the invention will now be described by reference to the accompanying drawings wherein: Figure 1 is a side view of a guyed tower embodying the invention; Figure 2 is a diametrical section through a pile at its lower end portion; Figure 3 is a section on the plane Ill-Ill of Figure 1; Figure 4 is a detail showing the attachment of the top end of the piles to a leg; Figure 5 is a side view of a support and guidance system for the piles; and Figure 6 is a diametrical section through a pile at its lower end portion, to illustrate a second embodiment of the invention.

Figure 1 shows a tower 1 extending from the sea bed 2 to above sea level 3 in deep water. The depth may be anything up to and even exceeding 800 metres. But the embodiment illustrated would probably not be economically viable for depths less than about 300 metres of water. The tower is to support production equipment shown above sea level at 4 and is supported by guys 5 going to the sea bed. Wind forces 6 and/or wave and current forces 7 acting on the tower will cause its centre line to be displaced sideways as illustrated very approximately by dotted lines 1 a and this is accommodated by tautening of the upstream guys and slackening of the downstream ones. The distance of displacement of the head of a tower of say 500 metres length might be as much as 15 metres each way.

There are also vertical loads on the tower which may be counter balanced by buoyancy tanks 8 fitted to the tower comparatively near the sea surface, but low enough down to avoid the worst of wave forces.

To allow for the lateral sway of the tower it is made compliant and the invention is directed to the means and method by which this is done. The tower consists of conventional tubular braced structure generally indicated at 9 which has at each corner a leg 10.

There is also conventional internal bracing 11 (Figure 3) and guides for conductors 12. The section in Figure 3 also shows the buoyancy tanks 8 surrounding each of the legs 10. This figure also shows the upper end portion of long piles 13, a pair of which is associated with each of the legs 10.

These piles are prefabricated structures in the sense that they may be fabricated at the same time as the tower, on land. they have two principal portions. As is best seen from Figure 3, a lower end portion of each pile 13 is of a thick walled construction 14 while the remainder of its length 15, which is the major part of its length, is comparatively thin walled.

In this embodiment there is inserted at the transition between the two a shear collar 16 which offers an upwardly facing ledge 17 on the inside of the pile. In the embodiment shown this ledge is annular.

The ratio of wall thicknesses between thick and thin-walled portions may for example be in the range 3:2 to 4:1, preferably about 2:1.

Solely by way of example, the pile 13 may have a diameter of approximately 2 metres and a wall thickness in the thick portion 14 of 75 mm but a wall thickness in the thin wall portion 15 of about 38 mm i.e. the wall thickness of the thin wall portion may be approximately one half of the wall thickness of the thicker wall portion. The thick wall portion 14 is to be driven into the sea bed preferably to an extent that part of the thick wall portion remains projecting above the sea bed, so that it acts as a particularly strong pin or anchor for the base of the tower against torsional force on that base.

At intervals down the length of each leg 10 for example each frame or each alternate frame there is provided a pair of pile guides 18 in the form of sleeves attached to the legs by shear plates 19 and having at their upper end guide funnels 20 for the reception of the pile during construction. The tower is fabricated on land with the long piles 13 in position in the guides and guided also in sleeves in the buoyancy tanks to be described in more detail later. They or some of them may be sealed off during transportation and be-used as additional flotation tanks for that period.

The tower is then positioned on site in its final vertical position and the piles are driven in succession by lowering into each of them in turn a pile driver in the form of an underwater hammer also known as a pencil hammer 21, Figure 2. This is shown diagrammatically as having a body portion and a head or driving shoe 22 which engages the ledge 17. The body 21 contains a hydraulic power pack for driving the hammer and it is controlled through an umbilical cord from the barge or other vessel from which the erection operation is being directed. Using this hammer the lower portion 14 is driven the appropriate amount into the sea bed at which stage the top end of the long piles 13 should be approximately level with and slightly projecting above the top end of the buoyancy tank structures 8.

This is best seen in Figure 4; these have at their upper end a pair of sleeves 23 which are massively attached to the leg 10 through welded shear plates 24 and which have at their upper ends guide funnels 20. The piles pass through these sleeves 23.

When the pile has been driven it is securely attached by a known technique, such as conventional grouting, to the sleeve 23 and hence rigidly and massively to the leg 10 over a length of for example 15 to 20 metres at a position comparatively close to the sea level, as is indicated diagrammatically by the block 25 in Figure 1. It is not axially attached anywhere else.

In the conformation shown here which is a preferred conformation the piles 13 are disposed one pair adjacent to each of the corner legs 10 of the frame and in a line which is at 45" to the side surfaces of the frame. This conformation with the piles outside the leg allows their dimensions to be almost independent of those of the leg, allows any number within reason of such piles to be fitted, gives the maximum resistance both to lateral and torsional force on the tower.

Figure 6 is illustrative of a preferred embodiment of the invention showing a diametrical section through a portion of a long pile 30 at its top end and near the sea bed 2. The various thicknesses and lengths of the parts of the pile 30, its function, position and final manner of attachment to the tower are all as described earlier for the piles 13 of the first embodiment; the differences lie in the structure of the pile 30 itself and in the manner of installation.

As seen in the Figure, the pile 30 has a lower, thick-walled portion 31 having an external diameter slightly less than the internal diameter of an upper, thin-walled portion 32, so that the lower portion 31 can slide inside the upper portion 32 when the two are not fixed together. The thick-walled portion 31 is to be driven into the sea bed 2.

The tower is fabricated on land. The thinwalled portion 32 of each pile 30 is then positioned on the tower and securely attached to it at a portion, e.g. 25, that will be comparatively close to sea level when the tower is on site; below this position the thin-walled portion 32 is not axially secured to the tower.

This operation is done on land, in contrast with the first embodiment. The thick-walled portion 31 is not fixed to the rest of the pile 30 at this stage.

The tower is then floated out and positioned vertically on site; the entire thick-walled portion 31 is "telescoped" slidably inside its respective thinwalled portion 32. An underwater hammer 21 (as previously described) is then lowered into each pile and the shoe 22 of the hammer acts on the thick-walled portion 31 at its top end, driving it downardly into the sea bed through the open lower end of the upper portion 32 of the pile. Because only the thick-walled portion 31 is being driven, rather than the entire pile, the driving is done particularly efficiently. When the portion 31 is being driven, rather than the entire pile, the drivingis done particularly efficiently.

When the portion 31 has been driven sufficiently deep, but a part 33 e.g. 10-20 m in length, of its top end is still within the upper portion 32, driving ceases and the portions 31 and 32 are fixed rigidly together. In Figure 6 this has been done by filling the cylindrical gap between the upper part 33 of the thickwalled portion 31 and the inside of the thinwalled portion 32 with a grout 34. It is also possible to swage the two portions together by lowering an expandable mandrel into the open top part 33 of the thick-walled portion 31, and then expanding the part 33 into con tact and bonding with the inner surface of the thin-walled portion 32, which for this purpose would be locally reinforced.

From the point of view of the method securing the tower, the efficiency of having prefabricated piles which may be floated out as part of the assembly is clear as is also the weight and mass saving of having piles which while being thick enough to drive at their lower end, are thin for the majority of their length (an example of the ratio of axial length of the thick wall: thin wall portions is in the range of 1:3 more preferably 1:5 to 1:10, although the exact ratio will depend on water depth and soil conditions in any particular proposed place of use).

In different embodiments of tower there may of course be different numbers of piles per leg and different dispositions of the piles in relation to the -leg. In particular the piles need not necessarily extend parallel to the axis of the tower.

Claims (14)

1. An offshore compliant tower comprising a tower structure extending generally vertically from the seabed to above sea level, hollow long piles comprising a minor and lower portion of their length and a major and upper portion of their length, the lower portion having a wall thickness greater than that of the upper portion, the lower portion being embedded in the seabed and the upper end of the upper portion being axially fixedly secured to said tower structure.

2. A tower as claimed in claim 1 wherein the ratio of lengths major portion:minor portion is in the range 3:1 to 10:1.

3. A tower as claimed in claim 1 wherein the wall thickness of the minor part is about twice that of the major part.

4. A tower as claimed in any one of the preceding claims wherein a shear ring is interposed between said major and minor parts and offers a ledge within said pile.

5. A tower as claimed in any one of the preceding claims wherein the tower structure is a rectangle in plan and has four corner legs, a pair of said piles being provided adjacent each said leg and being disposed in planes at about 45Q to adjacent sides of the rectangle.

6. A tower according to any one of the preceding claims for use in water of between 300 and 800 metres depth.

7. A tower as claimed in claim 6 wherein the ratio of the length of the major portion to that of the minor portion is between 3:1 and 10:1 and the ratio of the first wall thickness to the second wall thickness is about 2:1.

8. A method of securing an offshore tower to the seabed comprising the steps of: providing hollow long piles to extend from the seabed to adjacent water-level, said piles having a major portion of their length and a minor portion of their length, the minor portion being of a wall thickness greater than that of the major portion, driving the minor portion at least partly into the seabed by applying driving force substantially only to said minor portion and fixing the tower axially in relation to said driven minor portion, whereby the tower is axially secured to said piles only at an upper end portion of said major portion.

9. A method as claimed in claim 8 including using a hammer within the major portion to apply driving force to an upper end of the minor portion.

10. A method as claimed in claim 9 including axially moving the minor portion relative to the major portion during said driving, said fixing step comprising fixing the minor and major portions together against relative axial movement.

11. A method as claimed in claim 10 wherein said major portion is secured at its upper end to the tower before said driving step.

12. A method as claimed in claim 9 wherein the whole of the piles is to be axially movable relative to the tower at said assembling stage, the piles being fixed in said fixing stage at a top portion of said major portion.

13. A method according to any one of claims 6 to 12 including assembling the long piles with the lower, with at least the minor portion to be axially movable relative to the tower; floating the tower with the piles to its site; positioning the tower vertically and then carrying out the driving and fixing steps.

14. A method as claimed in claim 13 including using at least one of said piles as a buoyancy chamber during said floating step.