WO2000014338A1

WO2000014338A1 - Mechanical pipeline span prop with suction anchor foundation

Info

Publication number: WO2000014338A1
Application number: PCT/GB1999/002940
Authority: WO
Inventors: John Stephen Baross; Robin Stuart Colquhoun
Original assignee: John Stephen Baross; Robin Stuart Colquhoun
Priority date: 1998-09-08
Filing date: 1999-09-06
Publication date: 2000-03-16
Also published as: GB2341409B; BR9906939A; GB2341409A; GB9920491D0

Abstract

A submarine pipeline freespan support, installable by special recoverable mechanical unit operated by WROV, incorporating a system of devices to enable a contiguous pile-column consisting of a hollow square section or other suitable rolled or fabricated section to be used. The pile is driven into the seabed by creating an under pressure within it by evacuation of water from it by means of a pump or by venting the water into a one atmosphere chamber. In cases where seabed soil conditions preclude this method of foundation, an articulated base can be fitted to the column which is still deployed by the same mechanical unit.

Description

Mechanical Pipeline Span Prop with Suction Anchor Foundation

This invention is a system of devices to enable a contiguous pile-column consisting of a hollow square section or other suitable rolled or fabricated section to be used as a simple submarine pipeline freespan prop for supporting submarine pipelines at points where there are unacceptable free spans due to seabed undulation. That is to say, it is a pipeline "prop".

It is particularly appropriate for use in deepwater locations beyond the depth at which it is feasible for divers to operate. Should the device be used in water depths accessible to divers then some or all of the tasks performed by WROV (Working Remotely Operated Vehicle), as described below, may be performed by divers. The device is characterised by the fact that its foundation at the seabed takes the form of suction pile. It is also characterised by the arrangements for its installation and attachment to and support of the pipeline.

A suction pile is an open-bottomed closed-topped can, typically but not necessarily made of fabricated steel plate, which is inserted into the clay or sand seabed by pumping out the internal water to create an internal underpressure ("suction") which lowers the can until the can top is approximately level with the seabed. The anchor mobilises the passive pressure of the seabed soil to resist the lateral forces and moments applied to it and mobilises pile-soil friction to resist vertical loads either upwards or downwards. A suction pile is normally referred to as such when it has a high aspect ratio reminiscent of a pile. At lower aspect ratios it may be referred to as a suction anchor.

Standard suction piles and suction anchors are being used increasingly in the offshore oil and gas industry. Standard suction anchors are the subject of one or more patents held by others and such patents are neither challenged nor claimed by the present application. A person or company wishing to exploit the present invention may be liable for royalty to the patent holder of standard suction piles and suction anchors in addition to any royalty for use of the present invention.

A specific embodiment of the invention will now be described by way of an example with reference to the accompanying drawings in which:

Fig 1 Shows the general arrangement of a typical manifestation of the invention. Figs 2 to 8 Show the installation sequence of the prop:

Fig 2 The prop is manoeuvred alongside the pipeline.

Fig 3 The buoyancy is released and the pipeline engaged in the cantilever bracket.

Fig 4 The prop is lowered to the seabed and penetrates under self weight.

Fig 5 The foundation is driven into the sea bed.

Fig 6 Hydraulic jacks stroked to increase pipeline support. Fig 7 Installation device is disconnected and recovered to surface for re-use.

Fig 8 Prop in situ supporting the pipeline.

Referring to the drawings, the main part of the device consists of a pile (1) made of steel SHS (square hollow section) or other suitable rolled or fabricated section which is open at the lower end and closed at the upper end. The lower part of this pile (2) constitutes a suction pile and is embedded in the seabed soil after installation of the device. The suction pile is relatively narrow and has a relatively high aspect ratio (for example the diameter may typically be less than half a metre and the height/ diameter ratio may typically be 4.0. The upper part of the pile (3) constitutes a column which stands proud of the seabed after installation of the device. The suction pile is integral with, and has the same section as, the upper column. The column has a rack (4) up one side. A sleeve (the "lower sleeve") (5) fits around the pile (1) and is free to move up and down on the pile subject to the restraint of a pawl (6). The sleeve (5) can be fixed in position with the aid of lock (7). This lock is designed to be operated by WROV. The lower sleeve (5) carries a cantilever bracket (8) to support the pipeline freespan. A latch (9) is provided to attach the bracket positively to the pipeline.

A second sleeve (the "upper sleeve") (10) fits around the pile (1) and is free to move up and down the column subject to the restraint of a pawl (11). The upper sleeve, or installation tool, (10) has a pair of cantilever bearing structures (12) which support a pair of jacks (13) and to which are attached a pair of buoyancy tanks (14). The jack pistons (15) support the lower sleeve (5) via a system of yokes (16) and tierods (17). There is a seacock (18) on the pile head. This seacock is designed to be operated by WROV. A hose (19) runs from a QCDC (quick-connect-disconnect) (20) on the pile head to a valved tee (21) on a manifold pipe (the "lower manifold") (22) joining the bottoms of the two buoyancy tanks (14). The QCDC is designed to be disconnected by WROV. The lower manifold (22) is fitted with a seacock (23) which is designed to be operated by WROV. The tops of the two buoyancy tanks (14) are joined by a second manifold pipe (the "upper manifold") (24). The upper manifold is fitted with a vent cock (25) which is likewise designed to be operated by WROV. The top of the pile (1) is fitted with a padeye (26) for attachment of the line (27) from a deployment winch mounted on the installation vessel. The shackle (28) connecting the line (27) to the padeye (26) is designed to be released by WROV. A pennant (29) is attached at one end to the shackle (28) and at the other to the upper sleeve (10). The upper sleeve (10) together with the jacks (13) and buoyancy tanks (14) can be lifted up and away from the pile (1) by the line (27). The upper sleeve (10) also carries a fixed buoyancy unit (30) of syntactic foam or other suitable material and a protective frame (31).

Prior to overboarding of the device the seacock (18) is opened, the buoyancy tank valving is closed with the tanks full of air at atmospheric pressure, and the lower sleeve (5) is set at its lowest possible position on pile (1) and the upper sleeve (10) is set at a position a little way above it. The device is deployed to the seabed by the vessel winch line (27). As it approaches the seabed, its progress is monitored by a WROV. The descent is stopped while there is still sufficient clearance from the seabed to ensure that heave from the surface motion of the vessel does not result in bottom impact. The device is manoeuvred until it hangs vertically immediately adjacent to the pipeline. The azimuth of the device is adjusted with the aid of the WROV. When the pile is hanging in the correct position and orientation the WROV releases the lock holding the lower sleeve (5) at the lowest extremity of the pile (1) and the two sleeves (5) and (10) slide up the pile (1) under the influence of the combined buoyancy from the buoyancy tanks (14) and fixed buoyancy unit (30) until the bracket (8) comes into contact with the underside of the pipeline and the latch (9) is automatically closed by this contact. The lower sleeve (5) now functions as a template for the installation of the pile into the seabed.

Installation of the pile (1) into the seabed is initiated by continuing to lower on the winch line. The pile penetrates the seabed under the influence of its own weight, the water within it being forced out through the open seacock (18). When the pile has come to rest with its weight in equilibrium with the resisting friction of the seabed soil, the WROV closes the seacock (18). The WROV then opens the tee valve (21) on the lower manifold (22) and the water within the pile is forced through the hose (19) and into the buoyancy tanks (14) under the influence of the differential between ambient pressure and the pressure within the buoyancy tanks. This causes the pile to penetrate further into the seabed. The WROV observes painted graduations on the side of the pile to ascertain the depth of penetration throughout this process. When the pile has penetrated to the intended depth, the WROV closes the tee valve (21) and the pile comes to a standstill. The hydraulic jacks within the upper sleeve (installation tool) are stroked to increase pipeline support.

The WROV now opens the seacock (23) and the vent (25) to complete the flooding of the buoyancy tanks leaving the upper sleeve assembly with nett negative buoyancy. The WROV now disconnects the shackle (28) from the pile head padeye (26) leaving the winch line (27) attached solely to the upper sleeve assembly via the pennant (29). Finally the WROV disengages the tierods (17) from the lower sleeve (5) thus enabling the winch line (27) to recover the upper sleeve assembly to the surface vessel. When the upper sleeve assembly has arrived aboard the vessel, the buoyancy tanks are emptied of water and the upper sleeve assembly is ready for use with the next prop.

The method of creating an under pressure in the pile as described may be substituted by use of a suitable pump located either on the WROV or within the installation tool.

The method of latching the clamp to the pipeline by use of buoyancy as described may be substituted by turning the clamp upside-down and latching it by lowering it onto the pipeline. The installation tool can be designed to be positioned at the top of the column, separate from the clamp, during installation of the pile and thereafter to "walk" the column to latch with the clamp prior to stroking the jacks to increase pipeline support.

A baseplate may be added to the column when seabed soil conditions (e.g. rock or coral) prohibit the installation of the pile.

Claims

(1) A system of devices to enable a contiguous pile-column consisting of a hollow square section or other suitable rolled or fabricated section to be used as a simple submarine pipeline freespan prop.

(2) A system of devices as claimed in Claim 1 wherein a hollow steel pile-column is fitted with one or more external racks so that it can carry a pipeline support bracket with a ratchet pawl which engages the racks.

(3) A system of devices as claimed in Claim 1 or Claim 2 wherein the pipeline support bracket is fitted with a clamp which automatically captures and locks onto the pipeline.

(4) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 wherein a mechanical unit suspended from the winch line of the support vessel is used to deploy and install the prop and to clamp the support bracket to the pipeline and to transfer the weight of the pipeline freespan to the prop by engaging and jacking up the bracket, the different functions of the mechanical unit being activated by an attendant WROV (Working Remotely operated Vehicle) via an interface panel.

(5) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 wherein the pile is driven into the seabed partly with the aid of its own weight and the weight of the mechanical unit including supplementary ballast if required and partly by the extraction of water from within the pile.

(6) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 wherein the water is extracted from within the pile either by venting to one or more 1 atmosphere chambers on the mechanical unit or by pumping with a pump located either on the mechanical unit or on the attendant WROV, there being a hose connection between the pile-column head and the interface panel.

(7) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 wherein the mechanical unit is fitted with jacks and locks which enable it to "walk" the pipe support bracket up or down the pile-column should this be necessary.

(8) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 wherein a valve at the pile-column head hose connection can be closed to seal off the pile- column interior and the hose can be disconnected from the pile- column head and the mechanical unit can be recovered to the surface by the winch line of the support vessel ready for deployment of the next freespan prop. (9) A system of devices as claimed in Claim 1 or Claim 2 or Claim 3 or Claim 4 or Claim 5 or Claim 6 or Claim 7 or Claim 8 wherein the pile-column is shortened and fitted with an articulated baseplate to allow it to be used as a pipeline freespan prop at locations where the seabed is rock or other material which is too hard to be penetrated and where the seabed is sloping in one or both directions.