GB2524272A

GB2524272A - Underwater platform

Info

Publication number: GB2524272A
Application number: GB1404860.7A
Authority: GB
Inventors: Stuart Parkes; Ian Andrew Mcleay
Original assignee: BIBBY MARINE SURVEY SERVICES Ltd
Current assignee: BIBBY MARINE SURVEY SERVICES Ltd
Priority date: 2014-03-18
Filing date: 2014-03-18
Publication date: 2015-09-23
Also published as: GB201404860D0; WO2015140526A1

Abstract

A combination comprises a towing vessel 14 configured to travel across the surface of a body of water, and a towed platform 10 configured to be towed underwater by the towing vessel as the towing vessel travels across the water surface. A towing cable 12 connects the towing vessel and the towed platform and a winch 18 on the towing vessel determines the length of paid-out towing cable. The towed platform is negatively buoyant and has thrust means 26 on board for controlling only its lateral position. A platform for use in the combination is also disclosed.

Description

DESCRIPTION

UNDERWATER PLATFORM

The present invention relates to underwater survey platforms and in particular to underwater platforms which rely on a surface vessel for primary propulsion Remotely operated vehicles (ROVe) have been in use for many years and are used to carry out a variety of tasks underwater, particularly in hostHe circumstances or at depths which would make it difficult for a diver to operate.

ROVs are typicay provided with propeflers to control the verdcal position (depth) of the vessel in the water and other propellers for controlling the forwardlrearward motion and lateral motion of the vessel, completely independently from the surface vessel The position and movement of ROt/s can be controlled accurately and many ROVs have onboard sensors which automatically control the various propellers to maintain the vessel in a chosen 3 dimensional position.

However, ROVs suffer from a number of key disadvantages for certain applications. Large and powerful work class ROV5 are extremely expensive and rely upon a very large and even more expensive surface vessel which can also limit access to work areas The working window of a small ROV is often very limited in circumstances where there are strong tides. This limits the amount of time for which the ROV can be engaged in its assigned tasks. Whilst ROVs are particularly suitable for carrying out tasks at a specific location, their maximum speed is relatively low compared to a surface vessel As a result, known ROVe are not particularly suited to circumstances where a task is carried out over a large area or where the ROV has to travel a considerable linear distance in carrying out its task. An example is where an ROV is being used to sense the position of a cable, pipeline or the like located on or just below the seabed, by means of specialist sensors As a consequence of the potentially small working window and relatively low speed of available ROVe, the survey of a cable or pipeline can take a long time and prove to be very expensive.

It is an object of the present invention to provide an underwater platform which overcomes or alleviates some of the problems of the prior art.

in accordance with a first aspect of the invention, there is provided a combination of a towing vessel configured to travel across the surface of a body of water, a towed platform configured to be suspended below the towing vessel as the towing vessel travels across the water surface, a towing cable connecting the towing vessel and the towed platform and a winch on the towing vessel for determining the length of paid-out towing cable, the towed platform being negatively buoyant and having thrust means on board for controlling only its lateral position.

The above arrangement relies on the surface towing vessel for both forward movement and for controlling its vertical position (depth) within the water column. The thrust means mounted on the towing vessel are configured only to control the lateral position of the towed platform.

The use of a towing vessel in combination with the underwater and vertically suspended towed platform significantly increases the working window in circumstances where cross tides are involved.

Moreover, the use of a towing vessel allows the towed platform to be effective at speeds which exceed those achievable by suitable known ROVs. As a consequence, the present invention is particularly suited for carrying out tasks such as sensing of cables or pipelines on or just beneath the seabed and in other circumstances where the task is to be performed over a relatively large area.

Furthermore, since the towed platform requires thrust means only for controlling its lateral position (since its forward motion and depth are controlled by movement of the towing vessel and the length of the towing cable), the complexity and cost of the towed platform can be significantly reduced as compared with known ROVs.

Preferably, the towing vessel is powered.

The towed platform may comprise one or more accessories mounted on board.

In one embodiment, an inductive coil sensor is mounted on the towed platform. The sensor is preferably displaceable between a deployed position and a retracted position.

For example, the sensor may be pivotaDy mounted on the towed platform.

There may be powered drive means for displacing the sensor between its deployed and retracted posffions.

The combination may further comprise means for detenining the depth of the towed platform and/or the spacing of towed platform f.rom the bed within the body of water.

The means for determining the depth of the towed platform and/or the spacing of towed platform from the bed of the body of water may comphse sonar means.

The sonar means may be mounted on the towed platform and/or on tha towing vessel.

Preferably, the winch comprises a heavecompensated winch, Preferably, the towing cable comprises an umbilical cable, Preferably, the umbilical cable comprises electrical cable for supplying electricity to the towed platform. Preferably, umbilical cable also comprises means for transmitting signals and data between the towed platform and the towing vessel.

The towed platform preferably comprises cable connection means for connecting one end of the towing cable to the towed vessel, The cable c.onnection means preferably pivotally connects the towing cable to the towed platform.

The thrust means for controlling only the lateral position of the towed platform may comprise one or more propellers.

In accordance with a second aspect of the present invention, a platform adapted to be towed underwater is negatively buoyant and comprises means for connection to a towing cable and thrust means mounted on the platform for controlling only its lateral position.

Preferably, one or more accessories are mounted on the platform.

For example, an inductive ooU sensor may be mounted on the vesseL Preferably, the sensor is displaceable between a deployed position and a retracted position. For example, the sensor may be pivotaHy mounted on the towed platform The platform preferably comprises powered drive means for displacing the sensor between its deployed and retracted positions.

The platform may comprise means for determining ts depth and/or its spacing from the bed wtthin a body of water in which it is towed, The platform may comprise sonar means for determining its altitude and/or its spacing from the bed of the body of water.

Preferably, the means for connection to a towing cable pivotally connects the towing cable to the platform In one embodiment, the thrust means comprises one or more propeliers.

By way of example only, a specific embodiment of the present invention will now be described with reference to the accompanying drawings, in which: Fig. 1 is a sohemetic view of an embodiment of subsea survey system in accordance with the present invention; Fig. 2 is a perspective view of an underwater platform which forms part of the subsea survey system of Fig. 1, with its sensor in a retracted, raised position; Fig. 3 is a perspective' view* of the frame of the underwater platform of Fig. 3: Fig. 4 is a flow diagram illustrating one mode of operation of the subsea survey platform of Fig. 1, and Fig. 5 is a flow diagram illustrating a modified mode of operation of the subsea survey platform of Fig. I Referring firstly to Fig. 1, a sub*sea survey system comprises an underwater platform 10 which is connected by a cable 12 to a surface vessel such as a boat 14. The underwater platform 10 is towed behind the boat 14 as the boat travels across the surface 16 of a body of water and the depth of the platform Is controfled by adjusting the length of the cable 12 by means of a winch 18 mounted on the boat 14. The winch 18 is a conventional heave-compensated winch which pays out or winds in the cable to compensate for the change in vertical position of the boat 14 arIsing from the swell of the sea.

The length of cable 12 required to tow the platform 10 at a given depth will depend on the depth of the water, speed of the boat 14, as the drag on the platform 10 and hence the shape of the cable will have an effect. Consequently, the winch 18 is under the control of a computer located on the boat 14, illustrated schematically at C, which receives a signal corresponding to the speed of the boat 14 and which is programmed to adjust the length of the paid-out cable accordingly In order to maintain the platfon-n 10 at a desired vertical position.

As best seen in Fig. 3, the platform 10 comprises an aluminium alloy frame 20 comprIsing a generally oval, lower sub-frame 21 and a generally oval upper sub-frame 22. The two sub-frames 21, 22 are of the same overall dimensions and are mounted at opposite ends of a central column 24, such that the sub-frames 21, 22 are parallel with each other, with the upper sub-frame 22 positioned directly above the lower sub-frame 21. As can be seen from Fig. 3, the sub-frames 21, 22 and the central column 24 are constructed from many different frame members bolted together, whereby they form an open lattice or grid, which reduces both the weight and the drag of the platform 10 and which facilitates repair in the event of damage.

The platform 10 further comprises four identical conventional, reversible thrusters 26 (only three of which are visible in the drawings), each comprising a propeller 28 rotatable within a tubular shroud 30 by means of an associated electric motor 32. The thrusters 26 are fixed and operable to control the lateral position of the platform 10, but not its vertical position.

The platform 10 also carries three identical, conventional cable pipeline detectors 34 mounted edge-to-edge, forming a detector head 36 which is wider than the frame 20. Each of the detectors 34 In the embodiment shown Is a Teledyne TSS Dualtrack detector, which uses pulse Induction technology for detecting cables, pipelines and the like. The detector head 38 is mounted on two support arms 38 which are in turn mounted to a respective V-shaped mounting frame 40,42, each of which is pivotally mounted on a horizontal pivot 44 in one of two mounting plates 46 mounted on opposite sides of the platform 10. The mounting frames 40,42 can be pivoted about their respective pivots 44 by means of a dual-acting hydrauhc ram 48, whereby the detector head 36 can be moved between a vertical, inoperative position shown in Fig. 2 and a horizontal, deployed position shown in Fig. 1, in which it is able to detect cables, pipelines and the like located on, or just below, the sea bed as the platform is towed by the boat 14.

The platform 10 may be provided with additional equipment, such as an altimeter, compass, gyroscope, level sensor, video camera and illumination light (not visible in the drawings) and a sonar (illustrated at 50) for detecting the distance of the platform from the sea bed. It will also be observed that the towing cable 12 is connected to the upper end of the central column 24 of the frame 20.

by means of a cable termination mounting 52 which is pivotally connected to the upper end of the central column 24 of the frame 20.

It is Important to note that the platform 10, including any accessories mounted on it, should be heavily negatively buoyant and that its vertical position is adjusted solely by operation of the heave-compensated winch 18. If necessary, ballast weights can be attached to the platform 10 to ensure that It Is negatively buoyant (I.e. denser than the water).

The towing cable 12 is a so-called umbilical cable, whereby if desired one or more of the accessories mounted on the platform (e.g. the thrusters 26, the hydraulic rams 48, the video camera, the light and the detector head 36) may be controlled from the boat 14 and signals from one or more of the accessories (e.g. the altimeter, compass, gyroscope, video camera, level sensor, detector head 36 and sonar 50) may be transmitted to monitoring equipment on the boat. The umbilical cable 12 also has one or more electric power cables for operating the accessories on the platform 10.

The platform 10 is designed to be towed at a predetermined distance from the sea bed, typically I or 2 metres. As the sea bed both varies in depth and is not uniformly flat, further adjustment of the winch 18 is necessary in order to maintain the platform at the desired distance from the sea bed as the platform passes over it.

As mentioned previously, the sonar unit 50 will provide information on the topography of the sea bed slightly ahead of the platform 10. This information is transmitted to the monitoring equipment on the boat 14 via the umbilical cable 12 and is in turn supplied to the winch control computer C which, together with Information relating to the speed of the boat and algorithiis relating to the drag of the platform 10 and cable 12, operates the winch 18 to maintain the platform 10 at the desired spacing from the sea bed. The platform 10 is towed directly beneath the boat 14 and any layback will be taken into account by the winch control to Introduce a suitable time lag before operating the winch 18 to adjust the vertical height of the platform in response to the detection of the topography of the sea bed at a location ahead of the platform. Alternatively, or in addition, the topography of the sea bed ahead of the platform 10 may be detected by a sonar 54 on the hull of the boat 14.

Fig. 4 is a flow diagram illustrating one mode of operation of the sub-sea survey system. In Fig. 4 and in the following description, "Step" will be abbreviated to "5".

The routine starts at Slot). At S102, the desired spacing of the platform from the sea bed is set on the computer C located on the boat 14 and at $104 the spacing of the platform from the sea bed is measured, for example by means of the sonar unit 50. The signals returned to the sonar unit are supplied to the computer Con the boat 15 via the umbilical cable 14 and at 8106 the computer C determines whether the measured spacing from the sea bed is equal to the set spacing, within predetermined tolerances. If the platform is determined to be at the correct spacing from the sea bed, the routine returns to 5104 at which the spacing from the sea bed Is measured once again.

If the platform is determined not to he at the correct spacing from the sea bed, the routine moves to 5108. where it is determined whether the measured spacing is greater than the set spacing. If it is determined at 5108 that the measured spacing is greater than the set spacing (La the platform is spaced too tar from the sea bed), at 3110 the computer C operates the winch iSto pay out the umbilical cable 12. On the other hand. if it is determined at 5108 that the measured spacing is less than the set spacing (iCe. the platform is too close to the sea bed). at 5112 the computer C operates the winch 16 to wind in the cable 12, in either case, the routine then returns to 5104, where the spacing of the platform from tho sea bed is repeated.

A modtfication to the routine of Fig. 4 is shown in Fig. 5. One factor to bear in mind is that the location where the topography is monitored may be some distance ahead of the actual location of the platform, for example if the sea bed topography is monitored using the sonar 54 on the hull of the towing boat 14.

The routine of Fig. 5 is largely the same at that of Fig. 5. However, 5104 of Fig. 4 is replaced with S104, in which the topography of the sea bed at a location in front of the platform is detected. In addition, steps S110 and SilT are introduced before steps 110 and l2 respectively, in which the computer C calculates a delay before actuating the winch, to take into account that the platform is located behind the location where the sea bed is being monitored, The invention is not restricted to the details of the foregoing embodiments,

Claims

Claftos 1. A combination comprising a towing vessel configured to travel across the surface of a body of water, a towed platform configured to be towed underwater by the towing vessel as the towing vessel travels across the water surface, a towing cable connecting the towing vessel and the towed platform and a winch on the towing vessel for determining the length of pai&out towing cable, the towed platform being negatively buoyant and having thrust means on board for controlling only its lateral position.
2, A combination as claimed in claim 1, wherein the towing vessel is powered.
3. A combination as claimed in claim I or claim 2, comprising one or more accessories mounted on the towed platform.
4. A combination as claimed in claim 3. comprising an inductive loop sensor mounted on the towed platform.
5. A combination as claimed in claim 4, wherein the sensor is displaceable between a deployed position and a retracted position.
6. A combination as claimed in claim 5. wherein the sensor is pivotally mounted on the towed platform.
7. A combination as claimed in claim 5 or claim 6, comprising powered drive means for displacing the sensor between its deployed and retracted positions.
8. .A. combination as claimed in any of the preceding claims, comphsing means for determining the depth of the towed platform andior the spacing of towed platform from the bed of the body of water.
9. A combination as claimed in claim 8, comphsing sonar means for determining the depth of the towed platform and/or the spacing of the towed platform from the bed of the body of water,
10. A combination as claimed in claim 9, comprising sonar means mounted on the towed platform. g11. A combination as claimed in claim 9 or claim 10, comprising sonar means mounted on the towing vesseL 12. A combination as claimed in any of the preceding claims, wherein the winch comphses a heavecompenseted winch, 13. .A combination as claimed in any of the preceding claims, wherein the cable comprises an umbifical cable, 14. A combination as daimed in daim 13, wherein the umbifical cable comprises an electrical cable for supplying electricity to the towed platform.15. A combination as claimed in claim 13 or claim 14. wherein the umbilical cable oomprisss means for transmitting signals between the towed platform and the towing vesseL 16. A combination as olaimed in any of the preceding claims, comprises cable conne.ction means for connecting one end of the towing cable to the towed ptform.17. A combination as claimed in claimn 16, wherein the connection means pivotafly connects the towing cable to the towed platform.18. A combination as claimed in any of the preceding claims, wherein the thrust means comprises a plurality of propellers.19. A platform adapted to be towed underwater, the platform being negatively buoyant and comprising means for connection to a towing cable and thrust means mounted on the vessel for controlling only its lateral position.20. A platform as claimed in claim 19, comprising one or more accessohes mounted on the towed platform.21. A platform as claimed in claim 20, comprising an inductive loop sensor mounted on the towed platform.22. A platform as claimed in claim 21, wherein the sensor is displaceable between a deployed position and a retracted position.23. A platform as claimed in claim 22, wherein the sensor is pivotally mounted on the towed platform.24. A platform as claimed In claim 22, or claim 23, comprising powered drive means for displacing the sensor between its deployed and retracted positions.25. A platform as claimed in any of claIms 19 to 23, comprising means for determining its depth and/or its spacing from the bed of the body of water in which it is towed.26. A platform as claimed in claim 24, comprising sonar means.27. A platform as claimed In any of claims 20 to 26, wherein the means for connection to a towing cable pivotally connects to the towing cable.28. A platform as claimed in any of claIms 20 to 27, wherein the thrust means comprises a plurality of propellers.29. A combination substantially as herein described with reference tot and as illustrated in, the accompanying &awings.30. A platform adapted to be towed underwater, substantially as herein described with reference to, and as illustrated in, the accompany drawings.