GB2165571A - Installation of piles for offshore structures - Google Patents

Abstract

A method of installing a pile for an offshore structure consists of grooving the soil, during pile driving, by wedge shaped grooving tools (16, 17 and 18) on the external surface of a pile (14), then injecting grout through grouting ports (e.g. 19) into the grooves so formed, thus to ensure a uniform flow of good quality grout along the wall of the pile. Although the full circumference of the pile surface may not be in contact with grout, the installation of a long pile grouted in this manner should produce confidence of adequate grouting, since the grooves define paths of reduced resistance to grout flow, and provide a defined minimum area of bond between the pile and the grout. The grooves may alternatively be formed by waterjetting. <IMAGE>

Description

SPECIFICATION Installation of piles for offshore structures Many substructures for offshore platforms (e.g. for the drilling for and production of oil and/or gas) are secured to the underlying seabed by tubular steel piles extending down from the base of the substructure into the subsea soil. Such substructures will hereinafter be referred to as "jackets", which is a term commonly used in offshore engineering.

Recent experience with driven piles in calcareous and other similar foundation materials has led to a preferenceforthe use of grouted piles in drilled sockets. Such drilled and grouted piles take a considerable time to install and are therefore expensive; and, because of installation equipment requirements, can seriously restrict the possible geometry of jackets standing on such materials.

The problem with calcareous materials arises because these materials exhibit brittle behaviour in shaft adhesion. It is envisaged that this invention will be well suited to soils exhibiting brittle behaviour without strength loss arising from cyclic degradation.

The concept of grouting driven piles has been considered. The industry is not presently satisfied that the injection of grout into an annulus of disturbed soil or rock around a driven pile will result in sufficiently predictable wetting of the pile surface.

This annulus of weakened material may have been created by the disturbance of the soil or rock during driving. There is tendency for the grout to flow along a path of least resistance, and this preferred path is likely to increase in size on account of erosion caused by the flow of grout. There is also a possibility that grout may flow into the formation.

An objective of the present invention is to grout, with confidence, a sufficient area of the skin of a driven pile to ensure that good quality grout lies between the exterior surface of the pile and the soil, so that the required load carrying capacity is developed between the pile and the soil into which is it driven.

The invention provides a method of installing a pile for an offshore structure, comprising the steps of: Driving the pile into subsea soil, Forming a plurality of longitudinal grooves in the soil surrounding the pile, such grooves being spaced apart around the external surface of the pile, Connecting a supply of grout to the grooves, and Supplying grout to the plurality of grooves so that specific parts of the external surface of the pile are adhered by grout to the subsea soil.

Conveniently the grooves are diametrically opposed about the circumference of the pile.

Preferably the grooves are formed at two or more locations axially along the pile, and the grout is supplied to each of these locations along the pile.

In this last mentioned form it is further preferred that grout is supplied simultaneously to each of the grooves at one axial location.

In one preferred form there is an individual supply of grout to each groove.

The invention also provides a pile for an offshore structure, and having grooving tools on its external surface for installation in accordance with the method defined above.

The invention includes a platform having a pile installed according to the method defined above, or incorporating a pile as defined above.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a schematic view of an offshore platform.

Figure 2 is a diagram of a pile of that platform, and Figure 3 is a cross section taken through the top of the pile shown in Figure 2.

As shown in Figure 1 a jacket 10 (forming the substructure for a platform e.g. for the drilling for and production of oil and/or gas) has legs 11, and stands on a seabed 12. The jacket is secured to the seabed by tubular steel piles 14, which extend deep into the subsea strata. Typically the water depth may be 1 50m, and the plies may extend over 1 00m into the subsea strata.

In most types of subsea strata, the piles have conventionally been inserted into the soil by driving them through pile guides 15 attached to the legs of the jacket. In calcareous soils this method of installing the piles is unsatisfactory, as insufficient adhesion may be obtained between the conventionally driven piles and the subsea strata to secure the jacket 10 to the sea bed 12.

An alternative method of pile installation is to drill out an elongated socket in the subsea strata, to insert a pile into the socket, and then to grout up the annular space between the bore of that socket and the external surface of the pile. Although this method may be very effective in certain types of calcareous soils, it is time consuming, and causes considerable delays in bringing the platform "on stream".

A specific embodiment of the method consists of grooving the soil, during pile driving, by wedges shaped grooving tools 16 on the external surface of the pile 14 (or by other means such as water jetting), then injecting grout simultaneously into the grooves so formed, thus to ensure a uniform flow of good quality grout along the wall of the pile. Although the full circumference of the pile surface may not be in contact with the good quality grout ("wetted"), the installation of a long pile grouted in this manner should produce confidence of adequate grouting, at a lower overall costthan drilled and grouted piles.

The percentage of surface to be "wetted" and the design and number of the wedges may be adjusted for site specific material behaviour. It is recognised that by using wedges 16 to groove the soil, the process of locally crushing the soil may reduce its capacity to transfer load from the grout to the undisturbed soil. The use of square ended grooving blocks (not shown) may be found to be betterthan wedges in some soils.

Driving is not normally a problem in calcareous soils. However an increased resistance to driving might be expected due to the wedges. The levels of the wedges (i.e. their axial positions along the length of the pile) must be chosen to avoid simultaneous arrival at possible hard layers. For instance wedges 16, 17 and 18 are spaced apart along the pile 14 so that they do not reach hard rock layers, (e.g. 21,22 and 23) simultaneously.

The essential feature of this method is that the grooves provides paths of reduced resistance to grout flow, and provide a defined minimum area of bond between the pile and the grout.

The length of the pile and the nature of the soil surrounding the pile may dictate that several levels of grouting must be performed to ensure that losses of grout into the soil do not reduce the capacity of the bond. The number of levels of grouting may be selected in conjunction with knowledge of the grout density and viscosity to avoid excessive grout loss into the soil.

The tendency of the grout to flow into the formation may be reduced if required by limiting the grout head. This would be achieved by grouting the length of the pile in stages. The use of low density grout would further reduce this tendency. Similarly, any tendency for grout to flow circumferentially round the pile between the grooves would be reduced by the simultaneous grouting of all grooves at each particular level.

In the event of groove collapse, the size of the debris is likely to be such that the pieces may simply be incorporated in the grout as aggregate.

Positive displacement pumps are used in conjunction with separate flow lines to each groove, so to maximise the likelihood of an even flow along the grooves.

A single grouting line is provided to each groove at each level, typically terminating at a grout port 19.

Four of these lines are used to achieve the simultaneous grouting of all grooves at a particular level to reduce the likelihood of preferential grout flow, and to maximise the reliability of surface "wetting" while minimising grout quantities. The use of interconnected grouting lines is an option which may be used for specifically suitable soil types.

Connection of the grouting lines to the surface is achieved by a single stabbing tool 24, approximately the size of an underwater hammer insert, fitted with hoses and couplings. This tool would incorporate an orientation locating deivce, and could be lowered by crane through the pile guides 15, which were originally used to restrict sideways movement of the pile during driving. The stabbing tool 24 has self sealing couplings 25, arranged to engage grouting lines 26 (as shown in Figure 3). The grouting lines within the pile are shielded by protectors 27.

A stabbing tool suitable for connecting the grouting lines is described in our co-pending UK Patent Applications 28424/84 and 3638/85.

As an alternative to the use of grouting lines, direct injection of grout could be effected as the pile is being installed.

There might be a need for grout monitoring in the grooves, in order to enable certification of the completed platform.

The use of grouted piles in calcareous soils means that end bearing might be limited. This is due to the fact that to obtain full end bearing capacity it may be necessary to displace the pile vertically downwards by four to five times the displacement required to obtain peak shaft friction.

The reduction is cost gained by using this type of pile installation method (as opposed to using drilled and grouted piles) can be estimated to obtain a tangible measure of its viability. The order of magnitude is likely to be equivalent to saving the cost of a vessel on hire at $1 00,000/day used for 32 piles each taking 5 additional days per pile if drilling and grouting techniques were to be used. Thus the cost saving gained from grouting driven piles might amount to $16,000,000 on one platform.

Claims (8)

1. A method of installing a pile of an offshore structure, comprising the steps of: Driving the pile into subsea soil, Forming a plurality of longitudinal grooves in the soil surrounding the pile, such grooves being spaced apart around the external surface of the pile, Connecting a supply of grout to the grooves, and Supplying grout to the plurality of grooves so that specific parts of the external surface of the pile are adhered by grout to the subsea soil.

2. A method as claimed in claim 1 in which the grooves are diametrically opposed about the circumference of the pile.

3. A method as claimed in claim 1 or claim 2 in which the grooves are formed at two or more locations axially along the pile, and the grout is supplied to each of these locations along the pile.

4. A method as claimed in claim 3 in which grout is supplied simltaneouslyto each of the grooves at one axial location.

5. A method as claimed in any one of the preceding claims in which there is individual supply of grout to each groove.

6. A method substantially as hereinbefore described with reference to the accompanying drawings.

7. A pile for an offshore structure, and having grooving tools on its external surface to form axially aligned grooves in the subsea strata into which the pile is driven, at least some of the grooving tools having associated grout ports arranged to introduce grout into the respective grooves.

7. A pile for an offshore structure, and having grooving tools on its external surface for installation in accordance with the method of any one of the proceeding claims.

8. A pile substantially as shown in Figures 2 and 3 of the accompanying drawings.

9. A platform having a pile installed according to any one of claims 1 to 6, or incorporating a pile as claimed in claim 7 or claim

8.

Amendments to the claims have been filed, and have the following effect: *(a) Claims 7 above have been deleted or textually amended.

*(b) New or textually amended claims have been filed as follows