GB2355039A

GB2355039A - Drilled pile

Info

Publication number: GB2355039A
Application number: GB9923636A
Authority: GB
Inventors: Duncan Cuthill; Martin Leon Kobiela; Philip Anton Strong
Original assignee: Liberty Offshore Ltd
Current assignee: Liberty Offshore Ltd
Priority date: 1999-10-06
Filing date: 1999-10-06
Publication date: 2001-04-11
Anticipated expiration: 2019-10-06
Also published as: GB2355039B; GB9923636D0

Abstract

A method of installing a pile 10 in a subsea substrate is provided, which method comprises the steps of providing a pile 10 with a drill bit 14 at one end, and rotating both the pile 10 and the drill bit 14 relative to a substrate to form a bore. Also provided is a subsea pile 10 in the form of an elongate member 12 with a drill bit 14 fixed at one end thereof. In certain embodiments, the elongate member 12 may be tubular, allowing fluids to be supplied to the drilling face via the pile. Such fluids may be drilling mud, sea water, or a settable material for fixing the pile in the bore. In preferred embodiments of the invention an upper portion of the pile may be adapted to be laterally flexible.

Description

2355039 DRILLED PILE The present invention relates to a drilled pile,

particularly for use in subsea applications. The pile is suitable for use as a mooring termination point or anchor on the seabed. The invention further relates to a method of installing such a pile.

A variety of pile systems and anchoring systems are known in the art; however, such systems suffer from a number of disadvantages. Conventional drilled piles either require the pre-drilling of a hole, removal of the drilling apparatus, and insertion of the pile; or are provided as an integral pile\drilling member, with the drilling member being removed from the pile after drilling. Either of these systems is relatively costly and time-consuming to install.

Further, such conventional piles are constructed so as to be resistant to bending forces, as will be experienced by the pile on application of any non-axial loading.

Accordingly, piles are conventionally formed of heavy rigid tubing of relatively large diameter; the relatively large diameter of the tubing is also useful in transferring lateral forces into the surrounding soil. At least in an integral pile/drilling member, the large diameter of the pile also facilitates accommodation of the drilling member and associated drilling apparatus within the relatively large diameter pile bore. However, handling of such large 2 diameter heavy pi les _J s dif f icult and, more signif icantly, the drilling of a larae diameter bore to accommodate the 4 4 -pens' P,ie s time-consuming, and thus ex- ive.

Lit is among the objectives of embodiments of the preseni invention to obviate or alleviate these and other disadvantages of known piling systems.

According to a firs- aspect of the presen-_ invention, there is provided a me7hod of installing a pile in a subsea substrate, the method comprising the steps o-f:

providing a pile with a drill bit at an end thereof; and -otat--ng and a(dvancing the pile and the drill bit rela-Live to a substraze to form a bore.

According to a second aspect of the present invention, is there is provided a subsea pile comprising an elongate member with a drill bi: fixed at one end thereof.

--be present -provides an integral pi-e\drJllinq means which may be used to drill a bore into which the pile advances as the bore is formed. Typically, -L1--e drill bit is not recovered, ti-iereby maki-ng the method fast and relatively inexpensive.

The pile and the drill bit are ro!:ated together, thereby removing the need for bearing arrangements to enable --he drill bi-_ to rotate separately of the pile. In certain embodiments, the pile may comprise a section of conventional drill pipe, with a drill bit at an end thereof. As the pile is rotated as it is advanced through the drilled bore, the friction between the bore wall and 3 the pile is relatively low, in contrast to arrangements in which a pile is advanced into a drilled bore without rotation.

Preferably, the pile is an elongate tubular member.

This permits fluids, such as drilling fluid or "mud", to be supplied to the drill bit and drilling face via the pile.

Other drilling fluids, including sea water, may be utilised; sea water offers the advantage that the fluid need not be recovered, providing a considerable simplification in the pile drilling apparatus.

Preferably, the method further comprises the step of securing the pile in the bore. This may be achieved by passing a settable material into the bore, between the pile member and the bore wall, and allowing the material to set is in the bore. Conveniently, the settable material is cement, grout or the like, and will be delivered to the section of the bore which passes through the firmer soil formations which are generally found at depth below the surface soil layers. Use of the preferred tubular member to deliver a settable material downhole provides a simple and ef f icient means to f ix the pile in the bore. The absence of any separate drilling member within the pile of the present invention also avoids the risk of the drilling member being cemented within the pile. Alternatively, the pile may be cemented or grouted in the bore by delivering cement via a separate arrangement of conduits and hoses, and a cement fill-up device may be used to divert slurry into cement hoses, which are directed into the bore 4 externally of the pile.

In other embodiments of the inven-Lion it may be possible --o secure the pile i-n the bore by oEher means, for examp-e by providing radially extending members on the Qile. Of course, in some embodiments the soil formations may swell-I or collapse to grip the pile, or the intended function of the pile may not require the p_-'j7_e to withstand elevated axial loads, such tha-1- cementing or the provision of securing members is not required.

0 Preferably, at least an upper portion of the pile is adapted to flex in response to lateral- loading. The pile may be formed of standard oilfield tubulars, such as drill.

p --pe; driI7 pipe is designed for straiaht-"'orward and secure connection and has a low -bending stiffness but high tensile strength and resistance to fatigue failure. Further, the resistance of the -Qlie to lateral forces may be varied along its lengt, such t-hat the bending characteristics and bending rad-ii of the pile may be controlled. For examp le, a lower portion of the pile may be relatively rigid, and may be grouted or cemented in the bore. The relatively high rigidity may be achieved by using heavier section drill oiiDe, drill collars or even bore casing to form the pile. The pile may thus be adapted for specific applications, and particular soil and rock formations, as would be known to the pile installer from previous surveys, knowledge or experience. A pile having such a flexible upper portion will withstand axial 'Loads in a comparable manner to a conventional pile, but when subjected zo lateral forces the upper portion of the pile will bend, and effectively translate the lateral forces to axial forces which are withstood by the lower portion of the pile. This enables the pile to be more versatile and durable than laterally rigid piles. Conventional piles may be installed with force-dispersing collars, in order to transmit lateral forces and stresses away from the pile into the surrounding medium; such arrangements are generally unnecessary with a laterally flexing pile. Conventional piles are also typically of large diameter, for similar reasons; the present invention enables piles to be made more slender.

As well as reducing the bulk and expense of the pile, the provision of a smaller diameter pile facilitates drilling of the bore to accommodate the pile; if the diameter of a is pile in accordance with an embodiment of the invention is even only half the diameter of a corresponding conventional pile, the bore drilled to accommodate the pile is only one quarter of the area of the bore necessary to accommodate the conventional pile.

Preferably, the pile is provided with a mooring line connection. Most preferably, the mooring line connection includes a flexible member which may be in the form of a wire rope, chain or the like. If a mooring line is not to be connected prior to installation of the pile, the flexible member may be secured to the body of the pile by cable ties or other restraining means; this avoids the risk of the connection obstructing the installation of the pile. A portion of the connection may be buoyant, to facilitate location of ihe connection following instal lation of the pile. Alternatively, a mooring line connection may be rotatably mounted on the pile.

The pile and drill bit may be driven by connection to a drill string or otner elongate member. The drill string may be substantially conventional and preferably the connection between the string and the pile may be broken from surface once the pile is installed to the correct depth, such that the drill string may be retrieved.

Conveniently, -his.may be achieved by an appropriate termination assembly, for example, a low -orque back-off disconnection system. Alternative termination assemblies as known in the art may also be utilised. Most preferably, the termination assembly is positioned at a level relative to the substrate surface such -hat the pile, or a portion of the pile, does not nave to be recovered once the pile is no ionger in use.

T'--'-- drill string mav extend to and be driven from surface, for example by a rotary motor provided on an installation vessel. Alterna:ively, the pile may be driven by a motoi-, s-,icl-i as a conventional "downhole" motor, suspended from a drilling vessel on a drill string or sus-oended from a barge or the like by a flexible member such as a crane line and c=Dled to a fluid supply by an appropriate hose. 'I'his latter arrangement allows the drilling operation to be carried out without requiring the provision of a relatively expensive floating drilling unit, there being no requirement to provide rotary transmission 7 from surface. It is envisage that the drive assembly may be coupled to some means of opposing reactive torque generated by the motor, which would otherwise tend to cause the assembly to rotate in the opposite direction to the desired direction of rotation of the drill bit. This could be a guide-frame located on the seabed, with a key way that interacts with anti-rotation keys on the assembly.

The drill bit may take any suitable form, depending on the soil conditions and other requirements, such as the bore length and diameter. As the drill bit is likely to be left in the bore with the pile, that is be non-recoverable, the bit will therefore only be required to drill a single relatively short bore and may therefore be of less robust construction, and therefore less expensive, than a is conventional drill bit. The drill bit may be of conventional form or may be formed integrally with the end of the pile, for example by welding, depositing or otherwise forming cutting structures on the end of the pile, such as blades provided with tungsten carbide aggregate thereon. Typically, the drill bit will be of a diameter greater than the pile. Alternatively, the drill bit will be of smaller diameter than the pile, fluid erosion or applied weight being utilised to displace softer sediments to accommodate the pile.

According to a further aspect of the present invention, there is provided a pile comprising an elongate member with a drill bit at one end thereof, wherein the pile has at least an upper portion adapted to f lex in response to application of lateral loading thereto.

These and o--her aspects of the presen-- invention will now be described, by way of example, only with reference to the accompanying flaures, in which:

Figure i s',I-iows a side elevation of a pile according to an embodiment of t,-)e oresent invention during ihe drilling phase of installation; Figure 2 shows t-he pile of FJqure 1 d,-,--'ng the grouting phase; Figure 3 shows the pile of Figure during the disconnection of the drill string; F-'.gure 4 shows the pile of Figure 1 during the connection of the mooring line; Figure 5 shows the pile of --Figure 1 when connected to a mooring line; a=.

Figure 6 shows a mooring line conne=-ion in accordance with a further embodimen-- of the present invention.

Figures I t--o 5 of the drawings show a p4le in accordance with an embodiment of the 33resent invention.

The pile -10 comprises a tubular elongate member 12, in this example formed predominately of standard join-Ls of drill pipe, with a rotary drill bit 14 connected to the lower end of the member 1-2, in the illustrated example the diameter of the drill Di 14 being greater than the member 12. A drill collar 1-6 is provided at the lower end of the member 12 above the drill bit 14. A connection assembly 18 is Qrovided at the upper end of the pile 10 and releasably connects the pi-ie 10 to a rotary drill string 20. The 9 drill string 20 extends to the surface and an appropriate installation vessel (not shown) provided with a motor for driving the string 20.

Also connected to the connection assembly 18 is a cable 22 at the free end of which is a mooring line coupling 24. The other end of the cable 22 is coupled to the pile 10, the intermediate portion of the cable 22 being releasably secured to the drill string 20 by means of a number of cable ties 26.

The installation of the pile 10 is carried out as follows. The pile 10 and drill bit 14 are rotated together by means of the drill string 20, and driven into the bore thereby generated in the seabed (Figure 1). Cutting or drilling fluid is pumped from the surface through string 20 and the member 12 to the drill bit 14. The cutting fluid passes from jetting nozzles in the bit 14, cooling the bit and washing cuttings away from the end of the bore. The cutting fluid and the entrained cuttings pass up between the member 12 and the bore wall to discharge into the sea.

In this example, the pile is driven to a depth where the end of the pile 10, marked by the connection assembly 18, is below the surface of the seabed, to ensure that the upper end of the pile does not constitute a subsea obstruction which would have to be removed after use.

once the pile has been drilled to the desired depth, as shown in Figure 2, cement 28 is pumped down the drill string 20 from surface, and passes through holes (not shown) in the pile 10 and drill bit 14, and into the borehole surround_ng the -oile 10. The fill level of the cement 28 mav be determined as appropriate for the specific application and soi-conditions, and in -E'-is example it will be seen, that the bottom half of the pile is cemented in the bore. ThIs section of the pile is located in firmer soil sediments, below the softer surface sediments.

Once the cement 28 has set, and the pile 10 is secured in the hole, the dril-; string 20 is disconnected from the pile 10 a7L zhe connection assembly 18 (Figure 3) by rotating the string 20 coun--er- clockwise. The cable ties 26 break as the drill stay 20 retracts, releasing the cable 22. The coupl. Lng 24 at the end of the cable 22 may be kept clear of- the dri-lied hole and the seabed by means of a buoyant attachment.

As shown in Figure 4 a mooring line 30 is then connected to zhe cable 22. The mooring line 30 may be guided and connected by a remotely operated vehicle or by other means.

Figure 5 illus-rat-es the -response of the pile 10 when subjected to laEeral mooring line loads once the installation is complete; lateral forces applied via the mooring -line 3C deflect zhe flexible upper portion of the pile 10 above.he section secured with cement 28. Such deflection may be elastic or in some applications plastic, causing permane= deformation of the pile. The deflection of the pile 10 displaces some of the surrounding softer surface sediments 32 and causes -the lateral forces to be translated to axial forces which are resisted by the cemented lower portion of the pile.

Finally, Figures 6a and Gb show cross-sections of a combination drill string disconnection\mooring line connection subassembly 118, in accordance with a further embodiment of the present invention. In Figure 6a the upper drill string 120, used to support and rotate the pile during installation, is provided with a termination assembly 133 for transferring torque from the drill string to the pile 110, the string termination assembly 133 including a male ratchet thread 135 for engaging a corresponding female thread. Disconnection of the drill string 120 occurs at a releasable joint 134, which in this particular example is a low torque left-hand back-off disconnection system or safety joint.

The termination subassembly 118 includes a guide comprising a funnel 136 and a length of plastic pipe 138.

This serves the function, shown in Figure 6b, of aligning a mooring connection assembly 140 with the termination assembly 133, and of course prevents the surrounding soft soil from covering the safety joint 134 after the withdrawal of the drill string 120. The mooring connection assembly 140 comprises the mooring line 130, a mooring line socket 140, and a male thread ratchet assembly 142.

It will be seen that the foregoing embodiments of the invention provide relatively simple and inexpensive alternatives to conventional piles and installation methods. In the preferred embodiment of the present invention, the provision of a flexible pile allows the 12 orientation of the load applied to the pile tobe controlled: under 'Lateral loading the pile has a tendency -_o form an arc, substantially aligning the axis of the part of tne pile subject:o load with the orientation of that load. This also allows the bending stresses to be relatively evenly applied across a relatively long section of the pile, as opposed to the use of a stiff or inflexible p il e which results th e bending stresses being concentrated in a small section of the pile. A pile in accordance wilth t'he D-eterred embodiments of the present invention mighE thus:vpically deflect under load by 5 - 80 degrees, as opposed to a 0.1 to 5 degree deflection of a conventional pile. Tising a --':'Iexible pile, and oml'tting a separate drill string wi7_1-in the pile, allows use of piies in accordance wi:- embodiments of the inventioi- with an ou-side diameter,OD) tvpically 3 - 6 times less tnan a conventional pile to provide similar load-bearing capabilities. For example, a conventional pile of 15 - 3011 OD may be replaced wiL'-, a pile in accordance with an embodiment of the invention formed of 5" drillipipe.

Although the invention has been described primarlly to with reference subsea applications, it will be understood by -those of skill -in the art that the inventlon is not limited thereto.

13

Claims

1. A method of installing a pile in a subsea substrate, the method comprising the steps of:

providing a pile with a drill bit at an end thereof; and rotating and advancing the pile and the drill bit relative to a substrate to form a bore.

2. The method of claim 1, wherein the drill bit is not recovered.

3. The method of claim 1 or 2, wherein the pile is an elongate tubular member and drilling fluid is supplied to the drill bit via the pile.

4. The method of any of the preceding claims, further comprising the step of securing the pile in the bore.

5. The method of claim 4, wherein the pile is secured in the bore by passing a settable material into the bore, between the pile member and the bore wall, and allowing the material to set in the bore.

6. The method of claim 5, wherein the settable material is delivered to the bore via an internal bore in the pile.

14

7. The method of any of the preceding claims, wherein the pile is at least partially formed from standard oilfield tubulars.

8. Th-e method of any of zhe preceding claims, wherein the pile and drill bit are rotated by a driven elongate member connected thereto.

9. The method of claim 8, wherein the connection between the elongate member and the pile is Droken from surface once the pile is installed to the correct depth, such that the elongate member may be retrieved.

10. The method of claim 9, wherein the connection is positioned a-- a level relative to the substrate surface such than the pile does noz have to be recovered once the pile is no longer in use-

11. The method of any of claims 8 to 10, wherein the elongate member extends no and is rotationally driven from surface.

12. Th-e method of any of claims 8 to 10, wherein the elongate member is rotationally driven by a motor 20 positioned below the surface of the water.

13. A subsea pil-e comprising an elongate member with a drill bit fixed at one end thereof.

14. The pile of claim 13, wherein the pile is an elongate tubular member.

15. The pile of claim 13 or 14, wherein at least an upper portion of the pile is adapted to flex one or both of elastically and plastically in response to lateral loading.

16. The pile of claim 13, 14 or 15, wherein the pile comprises standard oilfield tubulars.

17. The pile of claim 15 or 16, wherein the resistance of the pile to lateral forces varies along its length, such that the bending characteristics and bending radii of the pile may be controlled.

18. The pile of claim 17, wherein a lower portion of the pile is relatively rigid.

19. The pile of any of claims 13 to 18, wherein a lower portion of the pile is adapted to be grouted or cemented in the bore.

20. The pile of any of claims 13 to 19, wherein a lower portion of the pile comprises relatively high rigidity members, including one or more of heavy section drill pipe, drill collars and bore casing.

21. The pile of any of claims 13 to 20, wherein an upper 16 portion of the pile is adapted no bend when subjected to lateral forces and nranslate the lateral forces to axial forces which are substantially withstood by the lower portion of the pile.

22. The pile of any of claims 13 to 21, wherein the pile is provided with a mooring line connection.

23. The pile of claim 22, wherein the mooring line connection includes a flexibie member.

24. The pile of claim 23, wherein a portion of the 10 conn=:ion is buoyann, to facilitate location of the connection following installation of the pile.

25. The pile of any of claims 13 to 24, in combination with an elongate member for extending to surface, and a connection between the elongate member and the pile which is is adapted to be broken from surface once the pile is installed to the correct depth, such that the elongate member may be retrieved.

26. The combination of claim 25, wherein the connection is a low torque backoff disconnection system. 20

27. The combination of claim 25 or 26, wherein the elongate member is adapted no be rotationally driven from surface.

17

28. The combination of claim 25 or 26, further including a motor for location below the sea surface.

29. A pile comprising an elongate member with a drill bit at one end thereof, wherein the pile has at least an upper portion adapted to flex in response to application of lateral loading thereto.