WO2000023660A1

WO2000023660A1 - A method and apparatus for underwater piledriving

Info

Publication number: WO2000023660A1
Application number: PCT/GB1999/003379
Authority: WO
Inventors: James Daniel Duggan
Original assignee: Duggan Marine Equipment Limited
Priority date: 1998-10-20
Filing date: 1999-10-20
Publication date: 2000-04-27
Also published as: AU6218499A; GB9822769D0

Abstract

In a method of sub-sea pile-driving in ultra-deep water a pile (200) releasably suspended from a deck structure (32) carried by a suspension rope (2) from a floating craft is lowered until a base structure (236) rests upon the sea bed (53). A mooring rope (276) paid out from the craft is connected, via a mooring chain (270) releasably connected to and partially fleeted upon the structure (236), to a releasable mooring collar (258). A guide tube (38) receives a pile-driving hammer of which the upper surface is exposed to the ambient sea, and a pile-driving anvil which is at the top of the pile (200). Defined by the tube (38), the hammer and the anvil is a water-tight chamber whereby raising of the hammer in the tube induces a negative pressure in the chamber which ensures that substantially all of the hydrostatic force on the hammer is employed in striking the anvil and thus driving-in the pile (200).

Description

A METHOD AND APPARATUS FOR UNDERWATER PILEDRIVING According to a first aspect of the present invention, there is provided a method of pile driving under water, in which hydrostatic pressure of the ambient water provides part of the pile-driving force.

According to a second aspect of the present invention, there is provided apparatus for use in pile driving under water, comprising a pile-driving hammer which is displaceable between an upper condition and a lower, pile-driving condition and whereof a top surface is exposed to the ambient water, and a chamber below said hammer and bounded by a bottom surface of said hammer and substantially inaccessible to the ambient water.

Owing to these aspects of the invention, a very high pile-driving force can be obtained at water depths greater than 1,000 metres.

According to a third aspect of the present invention, there is provided a method of pile driving, comprising creating suction under a pile-driving member to increase downward resultant force on the member.

According to a fourth aspect of the present invention, there is provided apparatus for use in pile driving, comprising a pile-driving member and suction-creating means serving to create suction under said member to increase downward resultant force on the member.

Owing to these aspects of the invention, if the member is a pile-driving hammer, it is possible, for a shorter driving stroke of the hammer, to produce the same driving force on a pile as a pile-driving hammer without suction created thereunder.

If the member is an anvil which is used in pile-driving under water, with the anvil resting upon the top of a pile and being struck by a pile-driving hammer, it is possible to avoid ambient water under hydrostatic pressure being present between the anvil and the pile and thus counteracting the pile-driving force of the hammer.

It is particularly advantageous, especially in sub-sea pile driving below 1,000 metres, if a top surface of the hammer is exposed to the water.

According to a fifth aspect of the present invention, there is provided apparatus comprising a rotary valve operable between first and second conditions, a change-over member in the form of a rack, a pinion driven by said rack and whereby said rack can change said valve between said first and second conditions, and actuating means arranged to bring said rack alternately into a first position corresponding to said first condition and a second position corresponding to said second condition.

Owing to this aspect of the invention, there is provided a very simple mechanism for producing very accurate changeover of a rotary valve.

According to a sixth aspect of the present invention, there is provided apparatus comprising a pile-driving hammer displaceable between an upper condition and a lower, pile- driving condition, retractable lifting means arranged to lift said hammer from the lower condition to the upper condition, and a releasable ratchet device effective to retain said hammer against downward movement from a raised position which said hammer can occupy in said upper condition.

Owing to this aspect of the invention, the lifting means can be retracted without being followed by the hammer.

According to a seventh aspect of the present invention, there is provided apparatus comprising a pile-driving anvil for supporting upon an upper end of a pile, and a guide tube encircling said anvil and extending upwardly therefrom, said anvil being downwardly displaceable relative to said guide tube and being formed with a laterally outward protrusion which limits downward displacement of said guide tube relative to said anvil.

Owing to this aspect of the invention, in circumstances in which the guide tube is lowered after each pile-driving stroke, the laterally outward protrusion prevents the guide tube from being lowered excessively; moreover, in a version in which suction is created in the guide tube above the anvil, the laterally outward protrusion prevents sucking of the anvil into the guide tube.

According to an eighth aspect of the present invention, there is provided a method of operating under water, wherein an hydraulic pump and an hydraulic motor in an hydraulic circuit with said pump utilize the ambient water as their hydraulic fluid.

According to a ninth aspect of the present invention, there is provided apparatus for use in operating under water, comprising an hydraulic circuit having an open inlet and an open outlet for receiving and venting ambient water as its hydraulic fluid, said circuit having an hydraulic pump downstream of said inlet and an hydraulic motor downstream of said pump and upstream of said outlet. Owing to these aspects of the invention, it is possible to avoid the need to construct the hydraulic circuit to resist hydrostatic pressures, since the water can already be at ambient hydrostatic pressure, and to avoid the need to seal the hydraulic circuit, which would conventionally contain oilic fluid, against the ingress of ambient water. According to a tenth aspect of the present invention, there is, provided a method comprising driving a pile into a bed beneath water and utilising a chain as a shock absorber between the pile, which serves as an anchor, and a mooring rope, with the chain taking a configuration in which it extends substantially vertically downwards at the mooring rope and extends substantially horizontally at the pile.

According to an eleventh aspect of the present invention, there is provided a combination comprising a mooring rope for connection to an object floating on water and for extending downwardly in the water, a pile for serving as an anchor at a bed beneath the water, and a chain connected at one end zone to said mooring rope and at an opposite end zone thereof to said pile. Owing to these aspects of the invention, it is possible to reduce the tendency for pile anchors to be pulled upwards and thus possibly out of the bed in taut leg moorings, since the chain naturally tends to occupy a catenary configuration in which its lower end applies a generally horizontal force to the pile anchor. In addition, the use of the chain discourages any contact between the mooring rope and the bed and thus any consequent abrasion of the rope. In order that the invention may be clearly and completely disclosed, reference will now be made, by way of example, to the accompanying drawings, in which: -

Figure 1 is a side elevation of a sub-sea pile-driving apparatus, with the pile being shown in full lines at the beginning of pile-driving and being shown fragmentarily in chain lines at the end of piling,

Figure 2 shows a detail of Figure 1,

Figure 3 shows a vertical, axial section through part of the apparatus and the pile, Figure 4 shows a section taken on the line IV-IV of Figure 3,

Figure 5 shows a section taken on the line V-V of Figure 3,

Figure 6 shows a detail of Figure 2, Figure 7 shows another detail of Figure 2, and Figure 8 shows another detail of Figure 1. Referring to the drawings, the pile-driving apparatus includes a non-rotating, suspension rope 2 of synthetic fibre and whereby the apparatus is lowered from a modified floating craft (not shown) . This rope 2 is connected via a shackle connection 4 and a nitrogen-charged shock absorber 5 to the apparatus. The shock absorber 5 comprises a liquid- containing piston-and cylinder device 6 connected to accumulators 7 containing nitrogen gas. An umbilical cable 8 for supplying electrical and hydraulic power to the apparatus enters a bellmouth 10 on a tower 12 which supports a closed circuit television camera 14 and a floodlight 16. The cable 8 is connected to an hydraulic and electrical junction box 18 which is mounted on the tower 12 and supplies electrical power via a cable 20. Also supported by the tower 12 are hydrostatic pressure compensating pots 22 hydraulically connected to the junction box 18. The loading of the pots 22 can be adjusted to compensate for the differing hydrostatic pressures at differing working depths.

Connected below the shock absorber 5 by way of a universal joint 24 is a spreader 26 which is of H-form as seen in plan view and at the four corners of which are mounted respective suspension cables 28 the lower ends of which are attached by bolt-and-eye connections 30 to a hammer assembly deck structure 32 which carries the tower 12, a second tower 34 and a hammer assembly 36. The hammer assembly 36 includes a guide tube 38 which is in the form of a vertical cylindrical casing and closely receives a pile- driving, vertically reciprocable, cylindrical hammer 40. A horizontal bar 42 extends diametrically through, and is fixed to, the hammer 40, from which it protrudes at its respective ends for co-operating with respective hydraulic ram caps 44 and 46. The bar 42 runs vertically in diametrically opposite vertical slots 48 in the guide tube 38. Co-axially encircling and fixed to the guide tube 38 is an annular collar 50 formed with diametrically opposite, upward opening recesses in which are received respective shock-absorbing blocks 52 of high- density rubber which project upwards from the collar 50 and are mounted directly below the bar 42 to avoid damage to the guide tube 38 by the bar 42 as the bar 42 arrives at its lower end position in the slots 48, especially in the event that the pile is being driven into a very soft sea bed 53. The ram caps 44 and 46 are fixed to the respective upper ends of two diametrically opposite pistons 54 of hammer-raising hydraulic rams 56 of which the cylinders 57 are fixed to the structure 32. The ram cap 44 serves to change over a rotary four-way valve 58 which controls the reciprocation of the pistons 54. The ram cap 44 serves to displace a change-over vertical bar 60 between upper and lower end positions (of which the lower is seen in Figure 6) , in which the bar 60 is retained by respective snap-acting detents 62, the bar 60 and the detents 62 being mounted upon the tower 34 by way of bolt-in-slot connections 61 allowing limited vertical reciprocation of the bar 60. Fixed to the bar 60 is a rack 64 which co-operates with a pinion of a gear train 66 which drives the movable part of the rotary valve 58, the arrangement being such that the stroke of the rack is equivalent to an exactly 90° turning of the rotary part of the valve 58. The bar 60 has horizontal, upper and lower lugs 68 and 70 fixed thereto and positioned therealong such that the ram cap 44 changes over the valve 58 upon arriving in its own upper and lower end positions, respectively. In this way, there is achieved a substantially instantaneous change-over of the valve 58 at each end of each stroke of the pistons 54. Ambient sea water is used as hydraulic fluid for operating, via the valve 58, the hydraulic rams 56 and, via a second, four-way, rotary valve 72, two collapsible pawls 74. The hydraulic fluid is drawn in at an intake filter 76 and is supplied via a hose 78 to an hydraulic pump 80 driven by an electric motor 82 itself supplied from the electrical cable 20. From the pump 80 extends an outlet hose 84 which terminates at the valve 58. In one condition of the valve 58, the hose 84 is connected to a hose 86 which branches into two hoses 86A and 86B extending to the respective upper ends of the cylinders 56 to cause the piston 54 to be lowered from the position shown in Figure 6. The fluid expelled from the lower ends of the cylinders 57 passes via branches 88A and 88B of a hose 88 and a non-return valve 90 and a hose 92 to the valve 58 and thence is discharged back into the sea through a discharge pipe 94. Upon arrival of the ram cap 44 at the lower end of its stroke, it changes over the valve 58 so that the hose 84 is now supplying seawater to the hose 92 and thence into a delay pot 96, which ensures that the hammer 40 has dropped before raising of the pistons 54 commences. The pot 96 has an outlet 98 which is exposed to the hydraulic fluid once the pressure of the fluid has forced a spring- loaded piston 100 past the outlet 98. The fluid passes from the outlet 98 to the hose 88 via a non-return valve 102. The pot 96 is of such capacity that upward movement of the pistons 54 does not commence until the maximum stroke of the hammer 40 would have been completed. The non-return valves 90 and 102 ensure that the delay pot 96 becomes effective only upon the pistons 54 reaching the lower ends of their strokes. The fluid arriving in the hose 88 from the pot 96 is supplied via the branches 88A and 88B to the lower ends of the cylinders, so that the fluid is expelled through the branches 86A and 86B to the valve 58 and thence discharged through the pipe 94, until the ram cap 44 strikes the upper lug 68 to change over the valve 58. Connected to the hose 88 is an accumulator 104 to act as a surge absorber to accommodate any residual fluid in the bottoms of the cylinders 57. Connected to the hose 84 is an accumulator 106 to prevent overload of the pump 80 at change-over of the valve 58. Branching from the hose 84 is a hose 108 leading to the valve 72 which is mounted on the tower 12. The valve 72 is changed over by a device similar to the device 60 to 70 which changes over the valve 58, except that here the vertical bar 110 is continuously urged into its upper end position by a spring 112 and that only a single lug 114 is fixed to the bar 110, the detents (62) being dispensed with. Again, the bar 110 changes-over the valve 72 through a rack 116 and a gear train 118 and is mounted on the tower 12 by way of bolt-in-slot connections 120. A horizontal stop 122 mounted upon the tower 12 so as to be accurately vertically adjustable provides an exact limit for the upper position of the rack 116 and thus accuracy of angular movement of the rotary valve part of the valve 72. In one condition of the valve 72, the fluid supplied by the hose 108 passes into a hose 124 and then through a Y-junction 126 and respective hoses 128 and 130 to the respective pawls 74. Each pawl 74 is substantially according to the collapsible pawl disclosed in British Patent 1367358 and has a mounting bracket 132 fixed to the outside of the guide tube 38, an S-shape arm 134 pivotally mounted on the bracket 132 so as to be turnable about a first horizontal axis, a U-shaped tooth 136 embracing a middle portion of the S-shaped arm 134 and pivotally connected thereto so as to be turnable about a second horizontal axis parallel to the first horizontal axis, and an hydraulic piston-and-cylinder device 138 operable to force the pawl 74 from an inwardly over- centre condition illustrated in the drawings towards an outwardly over-centre condition (not illustrated) . The hoses 128 and 130 extend to respective upper chambers of the devices 138. Respective lower chambers of the devices 138 are connected via respective branch hoses 140 and 142 and a Y- junction 144 to a hose 146 which extends to the valve 72, whence extends a discharge pipe 148 to the ambient sea. In the other condition of the valve 72, the hose 108 is connected via the hose 146 and the hoses 140 and 142 to the respective lower chambers of devices 138 to force the pistons of the devices 138 outwardly and upwardly to collapse the pawls 74 against the action of respective springs 150, the fluid in the upper chambers of the devices 138 being expelled through the hoses 128 and 130 and the hose 124 to the discharge pipe 148. The ram cap 46 strikes the lug 114 as it approaches its lower end condition and so produces change-over of the valve 72 from the condition in which the pawls are held in their inwardly over-centre conditions (and are thus not collapsed) into the condition of the valve 72 in which the pawls are brought into their outwardly over-centre conditions in which they are immediately collapsed by the downward force of the hammer 40 on the teeth 136 which are engaged in respective notches 152 in the hammer. Immediately upon upward movement of the ram cap 46 from its lower end position, the spring 112 acts to change-over the valve 72 into the condition in which the fluid is being pumped into the upper chambers of the devices 138 to bring the pawls 74 back into their inwardly over-centre conditions to await the return of the hammer 40 and thus the notches 152 into their initial positions, whereupon the springs 150 urge the teeth 136 into the notches 152 to engage and hold the hammer in that position so that it does not follow the pistons 54 downwards upon retraction of those pistons.

Mounted on the tower 12 by way of a support 154 is a post 156 which carries a vertical row of electrical detectors 158 which individually detect the presence or absence of an actuating member 160 fixed to the bar 42 so as to indicate the extent of movement of the hammer 40 relative to the structure 32. Mounted on the tower 12 parallelly to the post 156 is a post 162 carrying the camera 14 and the floodlight 16.

The bar 42 is fixed to and supported on the hammer 40 by means of gusset plates 164. In its top surface 166 which is exposed to the hydrostatic pressure of the ambient sea, the hammer 40 is formed with an internally threaded blind bore 168 for receiving an eye bolt (not shown) for use in handling the hammer during maintenance or repair of the apparatus. At its lower end, the hammer 40 is provided with annular grooves receiving annular high-performance seals 170 providing substantially water-tight sealing between the hammer and the guide tube 38. The bottom surface of the hammer bounds a cylindrical chamber 172 which, except when the hammer is bearing upon an anvil 174, exists between the hammer 40 and the anvil 174. The anvil 174 includes a replaceable cap 176 the upper surface of which directly bounds the chamber 172 and the cap 176 is formed with grooves receiving annular high-performance seals 178 providing water-tight sealing between the anvil 174 and the guide tube 38. The anvil 174 also includes a flanged block 180, with a flange 182 at its lower end, so that a co-axial lateral recess 184 is provided between the cap 176 and the flange 182. Projecting internally of the guide tube 38 at the lower end thereof are segmental inserts 186 bolted to the end of the guide tube and onto which the end cap 176 can come to bear to prevent the anvil 174 from dropping out of the guide tube when a pile is not present below the anvil to support the same. The flange 182 prevents the anvil 174 from being sucked right into the guide tube 38 during the return stroke of the hammer 40. Extending through the anvil 174 from the chamber 172 to the exterior is a conduit 188 terminating in a non-return valve 190 to permit escape of air from the chamber 172 during assembly together of the guide tube 38, the hammer 40 and the anvil 174. Towards its lower end, the guide tube 38 is firmly fixed to the deck structure 32 by way of a collar 192. Fixed to the underneath of the structure 32, particularly by way of stiffening webs 194, is a skirt 196 carrying pile-retaining devices 198 for a tubular steel pile 200 the upper end zone of which is slidingly received in the skirt 196. The devices 198 retain the pile 200 during its descent from the floating craft to the sea bed. Fixed around the upper end zone of the pile 200 is a collar 202 upon which bears the lower end of the skirt 196 to support the deck structure 32 temporarily upon the pile. Throughout its length, the pile 200 is reinforced by webs arranged in a cruciform configuration 204. In addition, the upper end of the pile 200 is reinforced with a spider 206. Arranged axially of the cruciform configuration 204 at the upper end zone of the pile 200 is a female guide pipe 208 to receive a male guide pipe 210 for centering the upper end of the pile relative to the pile-driving apparatus. The devices 198 include respective bolts 212 which are urged radially inwards by respective compression springs 214 and which are radially outwardly retractable by respective hydraulic piston-and-cylinder devices 216 supplied with hydraulic fluid from one of the pots 22 via branch hoses 218 and 220, a Y-junction 222 and a hose 224. The bolts 212 are arranged to extend through diametrically opposite holes through the skirt 196 and into a horizontal, annular groove 226 in the top end zone of the pile 200. The pile-retaining positions of the devices 198 are shown in Figure 3; the bolts 212 are retracted immediately before commencement of pile- driving and remain retracted until back up at the floating craft. The deck structure 32 is vertically guided by tubular guiding posts 228 braced at their tops, above the deck structure 32, by a rolled steel channel surround 230 fixed to the tops of the posts 228. The structure 32 has extended guiding plates 232 fitted at each corner and by which the posts 228 are located. The posts 228 are releasably connected by way of hydraulically operated releasing devices 234 to a base structure 236. The devices 234 are supplied with hydraulic fluid from the other of the pots 22 via a hose 238, a group of junctions 240 and branch hoses 242, 244, 246 and 248. The devices 234 are arranged to retract pins 250 of articulations 252 between the base structure 236 and the respective guiding posts 228 to enable recovery of the posts 228 and the apparatus items upward thereof after installation of the pile 200. The base structure 236 has centrally fixed thereto a guide sleeve 254 for guiding the pile 200 through the base structure. The inner periphery of the guide 254 is in the form of a ring of leaf springs, so that a band 256 encirclingly fixed to the pipe 200 may pass through the guide 254.

In the present example, the pile 200 is to be employed for mooring purposes and therefore a mooring collar 258 is provided round the pile. The collar 258 consists of a half- collar 260 and two quarter-collars (one of which is seen and referenced 262) hinged, as at 264, to the half-collar 260. The other ends of the quarter-collars are overlapped and interconnected by a pin inserted through aligned bores in the overlapped portions. The pin is releasable from the bores, by means of an hydrophonically-operated releasing device 266 to allow the quarter-collars to swing around their hinges 264 relative to the half-collar 260. The device 266 is itself operated by a hydrophonic device 268. Articulated to the mooring collar 258 is a mooring chain 270 which extends up to the base structure 236 upon which it is fleeted within containment walls 272 of the base structure 236, the other end of the chain being shackled at 274 to a synthetic fibre mooring rope 276. The mooring rope 276 is preferably neutrally buoyant, for example made of polyethylene, to provide a taut leg mooring, for which reason the chain 270 is provided to give additional shock absorption to the mooring, to alleviate shock loads or overloading in storm conditions. Fixed to a pair of the posts 228 is a cantilever 278 at the outer end of which are fixed upper and lower pairs of lugs 280 and 282 within which respective links of the chain 270 are retained by means of respective pins retractable by hydraulically-operated releasing devices 284 and 286 connected via branch hoses 288 and 290 respectively to a Y- junction 292 and then, via a hose 294 and the group of junctions 240, to the hose 238, whereby the devices 234, 284 and 286 are operated simultaneously. The hose 238 is paid out from a constant-tension retrieving spool 296 upon which the hose 238 is wound. The hydrophonic device 268 serves for release of the mooring in the event of an emergency and to recover the mooring rope 276 and the items such as 258 and 270 connected thereto, after drilling operations have been completed. The apparatus described with reference to the drawings is particularly intended for fixing pile anchors to the sea bed 53 in ultra-deep waters. The system which is lowered to the sea bed consists of an anchor pile 200 with the items 258, 270 and 276 attached, the deck structure 32 and the items carried thereby including the base structure 236 suspended from the deck structure. At the sea bed, the pile 200 is hammered-in by remote control from the surface craft. The deck structure 32, the posts 228, the towers 12 and 34 and the hammer assembly are then recovered using the rope 2, and the mooring rope 276 and the chain 270 are buoyed-off. The operation is repeated until all of a group of piles are driven in, located in their predetermined positions to await the arrival of a rig. The pile anchor insertion procedure is as follows.

On arrival at the offshore location, the system illustrated in Figure 1 and complete with the pile 200 and the base structure 236 is lowered to the sea bed by deployment of the rope 2. The umbilical cable 8 and the mooring cable 276 are paid out under control at the same time. The hanging weight of the system is noted prior to touch-down on the sea bed. When the base structure 236 contacts the sea bed, the hanging weight decreases. The payout is eased-off when the hanging weight decreases by, say, 50%. The devices 198 are operated to retract the bolts 212, so that hammering can commence. At this point, the apparatus is in substantially the condition shown in the drawings. The hydraulic pump 80 pumps sea water into the delay pot 96. When the piston 100 has revealed the outlet 98, sea water enters the lower ends of the cylinders 57, so that the bar 42 and the hammer 40 are driven upwardly. As the ram cap leaves its lowermost position, it allows the spring 112 to change over the valve 72 to produce retraction of the piston-and-cylinder devices 138. The upward movement of the hammer 40 is against the very high pressure of the sea water, particularly on the exposed top surface 166 of the hammer 40, since the guide tube 38 is open at its top, and the seals 170 and 178 cause the creation of a substantially liquid-free vacuum in the chamber 172 formed between the anvil 174 and the hammer 40. As the rams 56 approach their fully extended conditions, the ram cap 44 changes over the valve 58 to cause the hydraulic pump 80 to pump hydraulic fluid into the upper ends of the cylinders 57 to cause the pistons 54 to lower. At the same time, the spring-loaded piston 100 expels hydraulic fluid from the delay pot 96 into the discharge pipe 94. The hammer 40 then attempts to lower under the very high downward force thereon but this is prevented by the automatic engagement of the pawls 74 in the notches 152. As the ram cap 46 nears its lowermost position, it strikes the lug 114 to change over the valve 72, which results in operation of the piston-and- cylinder devices 138 to collapse the pawls 74, so that the hammer 40 is freed to descend and strike the anvil 174 and so drive-in the pile 200. As the ram cap 44 approaches its lowermost position (substantially simultaneously of course with the ram cap 46) , the ram cap 44 strikes the lug 70 to change the valve 58 back into its initial condition, in which the hydraulic pump 80 pumps sea water into the pot 96. The hammer 40 drops under its own weight and the additional weight of the hydrostatic pressure of the ultra-deep water, to impact with great force upon the anvil 174 on the top of the pile 200. The bar 42 is of course carried down with the hammer 40. Now, the cycle of operations is resumed.

With each pile-driving blow, the hanging weight increases and is eased off each time (up to the 50% reading, say) by pay-out of the cable 2. The cable 2 can be marked, especially to indicate total insertion of the pile. The longitudinal position of the ring 256 on the pile 200 has previously been set according to sea bed conditions as indicated by an earlier survey (the band 256 is provided only if there is mud in the upper part of the sea bed) . As total insertion of the pile is approaching, this band 256 passes through the leaf-sprung guide 254 in the base structure 236 and impinges upon the mooring collar 258 through which the pile extends but which has been temporarily slung upon wires beneath the base structure 236. The impact of the band 256 upon the mooring collar 258 snaps the supporting wires thereof and forces the mooring collar 258 into the mud. This arrangement is used to minimise the bending moment on the pile 200 which might otherwise occur later if the mooring collar 258 were to be on the pile at a level significantly above the harder strata of the sea bed. When total insertion of the pile 200 has been achieved, the pins 250 and those of the devices 284 and 286 are released. The released apparatus is then raised to the surface and the umbilical cable 8 fully rewound. By winding-in the mooring rope 276 sufficiently, the ground chain 270 is pulled clear from the base structure 236. The bight of the mooring rope 276 is attached to a buoy and put overboard. A new base structure 236 with the items carried thereby, a new pile 200, and new mooring items 258, 270 and 276 are connected to the raised apparatus, the surface craft is relocated and the installation process repeated.

Claims

1. A method of pile driving under water, in which hydrostatic pressure of the ambient water provides part of the pile-driving force.

2. A method according to claim 1, and performed at water depths greater than 1,000 metres.

3. A method according to claim 1 or 2, and further comprising creating suction under a pile-driving member to increase downward resultant force on the member.

4. A method according to claim 3, wherein a top surface of the pile-driving member is exposed to the water.

5. A method according to claim 1 or 2, wherein a top surface of a pile-driving member is exposed to the water.

6. A method according to any preceding claim, wherein an hydraulic pump and an hydraulic motor in an hydraulic circuit with said pump utilize the ambient water as their hydraulic fluid.

7. A method according to claim 6 as appended to claim 4 or 5, wherein said hydraulic motor raises said pile-driving member against said hydrostatic pressure.

8. A method according to any preceding claim, wherein, after pile driving, a chain is utilised as a shock absorber between a pile, which serves as an anchor, and a mooring rope, with the chain taking a configuration in which it extends substantially vertically downwards at the mooring rope and extends substantially horizontally at the pile.

9. Apparatus for use in pile driving under water, comprising a pile-driving hammer which is displaceable between an upper condition and a lower, pile-driving condition and whereof a top surface is exposed to the ambient water, and a chamber below said hammer and bounded by a bottom surface of said hammer and substantially inaccessible to the ambient water.

10. Apparatus according to claim 9, and further comprising suction-creating means serving to create suction under said hammer to increase downward resultant force on the hammer.

11. Apparatus according to claim 9 or 10, and further comprising a rotary valve operable between first and second conditions, a change-over member in the form of a rack, a pinion driven by said rack and whereby said rack can change said valve between said first and second conditions, and actuating means arranged to bring said rack alternately into a first position corresponding to said first condition and a second position corresponding to said second condition.

12. Apparatus according to any one of claims 9 to 11 and further comprising retractable lifting means arranged to lift said hammer from the lower condition to the upper condition, and a releasable ratchet device effective to retain said hammer against downward movement from a raised position which said hammer can occupy in said upper condition.

13. Apparatus according to any one of claims 9 to 12 and further comprising a pile-driving anvil for supporting upon an upper end of a pile and for striking by said hammer, and a guide tube encircling said anvil and extending upwardly therefrom for guiding said hammer, said anvil being downwardly displaceable relative to said guide tube and being formed with a laterally outward protrusion which limits downward displacement of said guide tube relative to said anvil .

14. Apparatus according to any one of claims 9 to 13 and further comprising an hydraulic circuit having an open inlet and an open outlet for receiving and venting ambient water as its hydraulic fluid, said circuit having an hydraulic pump downstream of said inlet and an hydraulic motor downstream of said pump and upstream of said outlet.

15. Apparatus according to claim 14 as appended to claim 12, wherein said hydraulic motor serves to drive said lifting means to raise said hammer.

16. In combination, an apparatus according to any one of claims 9 to 15, and a mooring rope for connection to an object floating on the water and for extending downwardly in the water, a pile for serving as an anchor at a bed beneath the water, and a chain connected at one end zone to said mooring rope and at an opposite end zone thereof to said pile.

17. A method of pile driving, comprising creating suction under a pile-driving member to increase downward resultant force on the member.

18. A method according to claim 17 and performed at water depths greater than 1,000 metres.

19. A method according to claim 17 or 18, wherein a top surface of the pile-driving member is exposed to the water.

20. A method according to any one of claims 17 to 19, wherein an hydraulic pump and an hydraulic motor in an hydraulic circuit with said pump utilize the ambient water as their hydraulic fluid.

21. A method according to any one of claims 17 to 20, wherein, after pile driving, a chain is utilised as a shock absorber between a pile, which serves as an anchor, and a mooring rope, with the chain taking a configuration in which it extends substantially vertically downwards at the mooring rope and extends substantially horizontally at the pile.

22. Apparatus for use in pile driving, comprising a pile- driving member and suction-creating means serving to create suction under said member to increase downward resultant force on the member.

23. Apparatus according to claim 22, wherein the member is a pile-driving hammer displaceable between an upper condition and a lower, pile-driving condition.

24. Apparatus according to claim 22, wherein the member is an anvil which is used in pile-driving under water, with the anvil supported upon the top of a pile and being arranged to be struck by a pile-driving hammer.

25. Apparatus according to any one of claims 22 to 24 and further comprising a rotary valve operable between first and second conditions, a change-over member in the form of a rack, a pinion driven by said rack and whereby said rack can change said valve between said first and second conditions, and actuating means arranged to bring said rack alternately into a first position corresponding to said first condition and a second position corresponding to said second condition.

26. Apparatus according to claim 23, or claim 25 as appended to claim 23, and further comprising retractable lifting means arranged to lift said hammer from the lower condition to the upper condition, and a releasable ratchet device effective to retain said hammer against downward movement from a raised position which said hammer can occupy in said upper condition.

27. Apparatus according to claim 24, or claim 25 as appended to claim 24, and further comprising a guide tube encircling said anvil and extending upwardly therefrom for guiding a pile-driving hammer for striking said anvil, said anvil being downwardly displaceable relative to said guide tube and being formed with a laterally outward protrusion which limits downward displacement of said guide tube relative to said anvil .

28. Apparatus according to any one of claims 22 to 27 and further comprising an hydraulic circuit having an open inlet and an open outlet for receiving and venting ambient water as its hydraulic fluid, said circuit having an hydraulic pump downstream of said inlet and an hydraulic motor downstream of said pump and upstream of said outlet.

29. Apparatus according to claim 28 as appended to claim 26, wherein said hydraulic motor serves to drive said lifting means to raise said hammer.

30. In combination, an apparatus according to any one of claims 22 to 29, and a mooring rope for connection to an object floating on water and for extending downwardly in the water, a pile for serving as an anchor at a bed beneath the water, and a chain connected at one end zone to said mooring rope and at an opposite end zone thereof to said pile.

31. Apparatus comprising a rotary valve operable between first and second conditions, a change-over member in the form of a rack, a pinion driven by said rack and whereby said rack can change said valve between said first and second conditions, and actuating means arranged to bring said rack alternately into a first position corresponding to said first condition and a second position corresponding to said second condition.

32. Apparatus comprising a pile-driving hammer displaceable between an upper condition and a lower, pile-driving condition, retractable lifting means arranged to lift said hammer from the lower condition to the upper condition, and a releasable ratchet device effective to retain said hammer against downward movement from a raised position which said hammer can occupy in said upper condition.

33. Apparatus comprising a pile-driving anvil for supporting upon an upper end of a pile, and a guide tube encircling said anvil and extending upwardly therefrom, said anvil being downwardly displaceable relative to said guide tube and being formed with a laterally outward protrusion which limits downward displacement of said guide tube relative to said anvil .

34. A method of operating under water, wherein an hydraulic pump and an hydraulic motor in an hydraulic circuit with said pump utilize the ambient water as their hydraulic fluid.

35. Apparatus for use in operating under water, comprising an hydraulic circuit having an open inlet and an open outlet for receiving and venting ambient water as its hydraulic fluid, said circuit having an hydraulic pump downstream of said inlet and an hydraulic motor downstream of said pump and upstream of said outlet.

36. A method comprising driving a pile into a bed beneath water and utilising a chain as a shock absorber between the pile, which serves as an anchor, and a mooring rope, with the chain taking a configuration in which it extends substantially vertically downwards at the mooring rope and extends substantially horizontally at the pile.

37. A combination comprising a mooring rope for connection to an object floating on water and for extending downwardly in the water, a pile for serving as an anchor at a bed beneath the water, and a chain connected at one end zone to said mooring rope and at an opposite end zone thereof to said pile.

38. A combination according to claim 37 and providing a taut leg mooring.