GB2222425A

GB2222425A - Dredging system for backfilling

Info

Publication number: GB2222425A
Application number: GB8919909A
Authority: GB
Inventors: Ian James Murray
Original assignee: MURRAY IAN ENGINEERING Ltd
Current assignee: MURRAY IAN ENGINEERING Ltd
Priority date: 1988-09-03
Filing date: 1989-09-04
Publication date: 1990-03-07
Anticipated expiration: 2009-09-04
Also published as: GB8919909D0; GB2222425B; GB8821884D0

Abstract

A backfilling operation is described in which material is removed from a position remote from the pipeline (10) and transfers the material to a position in which it supports the pipeline (10). The apparatus (12) comprises at least one pumping chamber (26a, 26b, 26c) which is filled with material from the seabed. The chamber (26a, 26b, 26c) is then connected to a source of compressed gas to evacuate the material from the chamber (26a, 26b, 26c) via a conduit (18) and deposit the material the pipeline (10). <IMAGE>

Description

Dredqinq System This invention relates to a method of and apparatus for backfilling subsea pipelines.

Oil and gas pipelines laid on the seabed are subject to scouring action which removes localised areas of seabed material. This can result in unsupported lengths of pipeline spanning voids, which in turn can lead to structural failure of the pipeline. It is conventional to counter this problem by backfilling and/or covering the pipeline, and this is commonly done by depositing broken stone, gravel or aggregate on top of the pipeline. This procedure requires special vessels to transport and accurately deposit the backfill material, and is very costly.

An object of the present invention is to provide a dredging system which may be used to backfill subsea pipelines using material from the seabed.

The invention accordingly provides, in one aspect, a method of backfilling a subsea pipeline, comprising operating submerged pumping apparatus to transfer material from the seabed at a position remote from the pipeline to a position in which it supports the pipeline.

Preferably the pumping apparatus is connected with a compressed gas supply located at the surface of the sea, the compressed gas therefrom having an active part in the operation of the pump. The pumping apparatus preferably acts by drawing the material from the seabed into a pumping chamber under the influence of hydrostatic pressure, and said compressed gas then evacuates the chamber through an outlet.

Preferably also, the material used is taken from a depth of 5 to 10 metres below the seabed, and a recess or trench may be preformed by the pumping apparatus to expose material from that area before the material is deposited around the pipeline.

From another aspect, the invention resides in backfilling apparatus comprising a pump assembly adapted to rest upon the seabed, the pump assembly comprising at least one pumping chamber, a first inlet pipe extending from the pumping chamber so as to be immersed in use, in the material of the seabed, a second inlet pipe connected to a compressed gas supply, and an outlet pipe extending from the pumping chamber.

Preferably there is a plurality of pumping chambers, suitably three in number, having their outlet pipes feeding a common outlet conduit.

Preferably also, the apparatus further comprises a discharge tube extending from the outlet, the tube being adapted to be directed, in use, from a surface vessel to discharge the dredged material onto the seabed around the pipeline.

An embodiment of the invention will now be described, by way of example only, with reference to the drawings, in which: Fig. 1 is a pictorial view of a dredging system in accordance with the invention being used to backfill a pipeline; Figs. 2A and 2B illustrate the sequence of operations in backfilling the pipeline; Fig. 3 is a schematic side view of a pump assembly used in the system of Fig. 1; and Figs. 4A, 4B and 4C illustrate the operation of the pump.

Referring to Fig. 1, a subsea pipeline 10 requires to be surrounded with backfill material. The system of the present embodiment comprises a pump assembly 12 and a discharge tube 14 which are both towed by a vessel 16 and which are interconnected by a flexible pipe 18. The pump assembly 12 acts to remove a sand/gravel/water mixture from the seabed for delivery through the discharge tube 14 to cover the pipeline 10.

The pump assembly 12 sits on the seabed and is pulled along it by a towing cable 13. The discharge tube 14 is cable suspended from a load handling system 15 which may include means for close following of the pipeline 10, and heave compensation.

The preferred sequence of operation is shown in Fig. 2. In a first pass (Fig. 2A) the vessel removes surface material from the seabed forming a trench 20 parallel to the pipeline 10, the spoil being dumped to the opposite side as a ridge 22. In a second pass (Fig. 2B) the trench 20 is deepened by removing material from a lower level and depositing this around the pipeline 10 as a backfill 24. With a suitable discharge head, the backfill material can be placed both under the pipeline for support and over the pipeline for protection.

Suitably, the trench 20 is excavated 10-20 metres from the pipeline 10, and 5-10 metres of material is removed in the first pass. The upper layer usually contains a high degree of silt, while the lower material typically contains 60-808 solids (by volume) which is more suitable for backfill.

Turning to Fig. 3, the pump assembly 12 comprises three pumping chambers 26a, b and c secured together in triangular formation. Each pumping chamber 26 has a downwardlyprojecting intake pipe 28. Outlet pipes 30 extending from the chambers 26 join together to feed into the flexible pipe 18. The pump assembly 12 is actuated by compressed air from a compressor on the vessel 16 via a supply hose 32, distributor valve 34, and conduit 36.

The operation of one chamber 26 is shown in Fig. 4. The pump is allowed to sit on the seabed so that the open end of the intake pipe 28 is immersed in seabed material. Owing to the hydrostatic pressure acting on the seabed, the seabed material (which is typically a fluid sand/gravel/water mixture) flows into the chamber 26 via intake pipe 28 and check valve 38, as seen in Fig. 4A. Air displaced by this inflow is vented via conduit 36 and distributor valve 34.

When the chamber 26 is full (Fig. 4B) the intake valve 38 automatically closes under its own weight and that of the contents. At this point, compressed air is supplied via the distributor valve 34 and conduit 36 to force the chamber contents out through the outlet pipe 30 via an outlet check valve 40 (Fig. 4C). When the chamber 26 is almost empty, the distributor valve changes to allow the compressed air within the chamber 26 to be discharged to atmosphere. As soon as the internal pressure has been released, the cycle restarts.

The use of a pump assembly 12 having three chambers 26 is preferred, since this allows virtually continuous operation, but any number of chambers from one upwards may be used.

The pump may suitably be operated at about 3 cycles/minute.

It will be noted that there are no moving parts in the underwater assembly apart from the check valves 38 and 40, which may be simple rubber members. The pump is therefore highly reliable in the hostile environment of a sea water/abrasive mixture.

It will be appreciated that the distributor valve 34 may be any suitable assembly which cyclically connects the working chambers to the compressed air supply and to atmosphere.

The system of the present invention allows dredging at any desired depth. It is particularly suited to dredging sand and gravel in open sea, and can work in rough water conditions as the weight of the pump is mainly on the seabed and the towing cable is relatively slack.

The power consumption is low, since most of the energy used is derived from the hydrostatic pressure at the seabed.

Modifications and and improvements to the foregoing embodiment may be made within the scope of the invention.

MURGITROYD AND COMPANY CHARTERED PATENT AGENTS MITCHELL HOUSE 333 BATH STREET GLASGOW G2 4ER

Claims

CLAIMS 1. A method of backfilling a subsea pipeline, comprising operating submerged pumping apparatus to transfer material from the seabed at a position remote from the pipeline to a position in which it supports the pipeline.
2. A method according to Claim 1, further comprising drawing the material from the seabed into a pumping chamber, connecting the pumping apparatus to a compressed gas supply on the surface of the sea and evacuating the chamber by means of the compressed gas through an outlet in the chamber to transfer the material to the position in which it supports the pipeline.
3. A method accoding to Claim 1 or Claim 2, further comprising first removing a top layer of the seabed and subsequently transferring material exposed by removing the top layer to the position in which it supports the pipeline.
4. A method according to Claim 3, wherein the top layer which is removed first is substantially 5 to 10 metres thick.
5. A method according to any of the preceding Claims, wherein the remote position is substantially 10 to 20 metres from the pipeline.
6. Backfilling apparatus comprising a pump assembly adapted to rest upon the seabed, the pump assembly comprising at least one pumping chamber, a first inlet pipe extending from the pumping chamber so as to be immersed in use, in the material of the seabed, a second inlet pipe connected to a compressed gas supply, and an outlet pipe extending from the pumping chamber.
7. Apparatus according to Claim 7, wherein there are three pumping chambers.
8. Apparatus according to Claim 6, wherein there are at least two pumping chambers.
9. Apparatus according to Claim 7 or Claim 8, wherein the outlet pipes feed a common outlet conduit.
10. Apparatus according to any of Claims 6 to 9, further comprising a discharge tube extending from the outlet and being adapted to be directed, in use, from a surface vessel to discharge material onto the seabed around a pipeline.
11. Backfilling apparatus substantially as hereinbefore described with reference to the accompanying drawings.
12. A method of backfilling a subsea pipeline substantially as hereinbefore described with reference to the accompanying drawings.