The present invention relates to a cleaning apparatus for an elongate member, and in particular, an apparatus for cleaning seismic cables towed behind a survey vessel at sea.

Marine seismic data acquisition is normally done by towing multiple, typically 6 to14, sensor cables 1 behind a vessel 2, as is shown schematically in Figure 1. Each sensor cable is typically 4 to12 km long and has hydrophone sensors 3 and/or other types of sensors equally spaced along the cable. The sensor cable is made up of several sections typically 150 metres long allowing the length of the sensor cable to be changed. The sensor cable is connected to the vessel by a lead-in cable 4 providing electrical power to the sensor cable. It is also used for transmitting data signals to and from the sensor cable. The lead-in cable is typically 1000 to 1500 metres long.

When an array of sensor cables is towed, particularly in tropical waters and at certain times of the year, the cables will become covered by marine growth, such as algae and barnacles. Such marine growth will increase towing resistance and ultimately cause the cable to break because of the elevated tension this causes in the cable. Additionally, the marine growth will cause turbulent water flow along the sensor cable, which generates noise. Such noise will significantly reduce the data quality recorded by the sensor cables.

Because of the problems described above, any significant marine growth on sensor cables must be prevented from forming on the cables and any marine growth present on cables must be removed. There are many different anti-fouling and anti-fungi coatings and paints available to prevent marine growth on ships and other objects exposed to water. However, none of these existing products has been found to solve the problems with marine growth on sensor cables.

Cleaning marine growth off a sensor cable is complicated by the fact that the sensor cable typically is submerged at 6 to 30 metres below the surface, towed through the water at 4 to 5 knots and has multiple external devices attached to the cable. These external devices are typically "birds" used for depth control and lateral steering, acoustic positioning devices, recovery systems etc. Their presence makes any mechanical cleaning operation significantly more complicated compared to cleaning a continuous cable with uniform diameter. The most common way to solve the problem with marine growth on sensor cables used for marine seismic surveys is to use a mechanical device to

scrape/remove any algae and barnacle attached along the cable. There are several such methods and apparatuses that are known for cleaning marine growth from sensor cables. These typically involve the use of a cleaning device which is located around the cable and which provides sufficient drag to cause it to be pushed along the cable as it is towed through the water. Such devices are known from, for example, US 7145833, US 7409919 and US 2006/0054186.

These disclosures each describe devices that are able to pass over external devices attached to the cable. For example, US 7145833 discloses a device in the form of a cylinder with internal brushes and which is arranged to pass around external devices. Since birds and other external devices are of a significant size and typically have elongate projections such as fins, it will be appreciated that these devices must be large and/or complicated both in regards to construction operation.

According to the present invention there is provided a cleaning apparatus for removing foreign matter from a marine seismic cable as it is moved along the cable, the apparatus comprising a means for slidably attaching the apparatus to the cable, a sensor for determining the presence of obstructions, and means for steering the apparatus, whereby the apparatus may be detached from the cable when an obstruction is encountered, be steered past the obstruction, and be re-attached to the cable.

Thus, the invention is based on the use of a cable cleaning apparatus which may be used whilst the cable is deployed in the water. It may be selectively attached to the cable and be moved along the cable (in either direction) to scrape/brush away any algae, barnacle or other marine growth attached to the surface of the cable. The apparatus is preferably pulled along the cable by means of a line attached to it. The distal end of that line may be attached to a winch on board the ship that is towing the cable. However, alternatively, the drag that the apparatus induces may be used to move it in the opposite direction (away from the towing ship), or internal propulsion could be provided.

The term "steer" as used herein means the control of the direction of travel of the apparatus in the water, including vertical and/or horizontal components of direction.

Where the apparatus is towed, the towing line/cable is preferably also used to transmit power (electrical, hydraulic or pneumatic) and control signals (electrical or optical) to/from the apparatus. However, it is possible for there to be an internal power supply and/or for control to be via radio communications.

The means for attaching the apparatus to the cable may comprise a clamp or clamps arranged to pass around the cable whilst allowing relative movement, at least in the longitudinal direction, between the cable and clamp(s), thus, the apparatus may slide along the cable. The attaching means may be arranged to automatically lock the apparatus to the cable.

In order to remove foreign matter, the apparatus may have cleaning means, such as brushes, scrapers, water jets or a combination of these. These may be located so as to be able to remove any marine growth attached to the cable when the apparatus is clamped onto and pulled along the cable. Although they may be provided separately, one preferred arrangement is for the clamps to comprise the cleaning means, e.g. a brush and/or scraper. For example, the clamp(s) may comprise moveable jaws and the brush/scraper may be located on them. In this way it is possible to provide a brush/scraper that encircles the cable when the apparatus is attached thereto.

The steering means of the apparatus preferably comprises one or a plurality of hydrofoils which are preferably controllable to provide horizontal and/or vertical components of force as the apparatus moves relative to the water.

Thus, preferably they are arranged to provide a depth (vertical) control system (wings). Additionally or alternatively, variable buoyancy may be employed.

In either case it is possible to remotely or automatically control the depth of the towed apparatus as this is towed through the water.

Lateral (horizontal) control may be provided by hydrofoils arranged as rudders.

Steering may also be provided by propulsion devices, such as propellers, water jets and/or thrusters.

The sensor is preferably able to detect any external devices attached to the cable. Such external devices will be "birds" and similar devices as described above. Such external devices can also be "wings" or "rudders" attached to in-line depth control and steering modules.

The system used to detect external devices on the cable can be a sonar system, it can be a camera system using image recognition, it can be a laser scanner, it can be based on sensors or devices integrated in the cable or the external devices or it can be a combination of the mentioned sensor systems or other sensor systems. Thus, a plurality of sensors may be provided. These may each be arranged to detect obstructions at opposite ends of the apparatus to allow it to travel in opposite directions along the cable.

The sensor(s) are preferably able also to determine the relative location and/or orientation of the cable with respect to the apparatus.

Although it is possible for the apparatus to be controlled entirely remotely, preferably the apparatus comprises a control unit which is connected to the means for attaching, the sensor(s) and the means for steering. The control unit is preferably arranged to operate the means for attaching so as to detach the apparatus from the cable when an obstruction is encountered and/or to attach the apparatus when an obstruction has been passed. The control unit may also be arranged to control the steering means so as to steer the apparatus past an obstruction whilst it is disconnected from the cable and/or to steer the apparatus back to the cable to permit reattachment.

Thus, in a preferred embodiment, the sensor system will give a signal to an operator and/or to the controller to unlock the cleaning apparatus from the cable when the cleaning apparatus still is a safe distance from the external device. The cleaning apparatus will then move vertically and/or horizontally a safe distance away from the cable to bypass the external device. The same sensor system will detect when the cleaning apparatus is clear of the external device and be used to guide the cleaning apparatus onto the cable again. Repositioned onto the cable, the cleaning apparatus will be secured thereto and the cleaning operation will continue.

It will be appreciated that the control unit may be autonomous or it may facilitate the apparatus being remotely operated, for example, from the vessel towing the cable.

The invention also extends to a corresponding method and so, viewed from another aspect, there is provided a method of removing foreign matter from a marine seismic cable comprising providing an apparatus that removes the matter as it is moved along the cable, the method comprising: slidably attaching the apparatus to the cable: determining the presence of an obstruction; detaching the apparatus from the cable when an obstruction is encountered; steering the apparatus past the obstruction; and re-attaching the apparatus to the cable.

Whilst the invention is described herein in relation to seismic cables, it may be applied to the cleaning of other elongate cables or members in a marine/aquatic environment. Preferably, the method comprises the use of the apparatus of any of the previously described preferred features. It also extends to a computer software product which causes the controller to perform the methods described herein.

An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings:-

Figure 1 is a schematic view of a vessel towing a seismic sensor cable;

Figure 2 is a schematic side view of a cleaning apparatus according to an embodiment of the invention;

Figure 3 is a cut-away end view of the cleaning apparatus showing a cable clamp in the open configuration;

Figure 4 corresponds to Figure 3, but shows the cable clamp in the closed configuration;

Figure 5 is a side view of the cleaning apparatus being pulled along a cable;

Figure 6 is a similar view to Figure 5, showing the cleaning apparatus approach an obstruction;

Figure 7 is a similar view to Figure 5, showing the cleaning apparatus detached from the cable;

Figure 8 is a similar view to Figure 5, showing the cleaning apparatus bypass the obstruction;

Figure 9 is a similar view to Figure 5, showing the cleaning apparatus reattached to the cable.

Figure 1 illustrates marine seismic data acquisition being performed by towing up to twenty-four sensor cables 1 (one shown) behind a vessel 2. Each sensor cable is 4 to12 km long and has depth control/steering units 3 equally spaced about 300 to 500m apart along the cable. These are known devices often referred to as "birds" which have projecting hydrofoils such as wings and rudders and/or flotation components. They may be remotely or automatically controlled in order to steer the cable both laterally and vertically. A typical cable 1 will also comprise other external devices such as acoustic positioning devices, recovery systems, etc., but these have been omitted for clarity.

The sensor cable is made up of several sections, typically 150 metres long, allowing the length of the sensor cable to be changed. The sensor cable is connected to the vessel by a lead-in cable 4 providing electrical power and data connections. The lead-in cable is typically 1000 to1500 metres long. The cables 1 , 4 are deployed and recovered using winches (not shown) mounted at the rear of the vessel 2.

Figure 2 shows a schematic side view of a cleaning apparatus 10 for use in cleaning cable 1. It comprises a main housing 11 onto which are mounted front and rear cable clamps 12, 13, depth control wings 14, first and second sensors 15, 16 and control unit 17. A towing eye 18 is mounted to a reinforced portion of the upper surface of the housing 11.

The cable clamps 12, 13, which will be discussed in more detail below with reference to Figures 3 and 4, serve to selectively attach the apparatus 10 to cable 1 and also to provide means for cleaning it. By means of these clamps, the apparatus is attached around cable 1 , whilst being able to slide along it. The apparatus 10 may be allowed to be pushed towards the distal end of the cable 1 by means of the drag it creates as the cable is towed or it may be pulled towards the proximal (vessel 2) end by means of a cable attached to towing eye 18.

Wing 14 allows vertical components of force to be applied to the apparatus

10 as it moves through the water and vertical wings (i.e. fins/rudders) may additionally be provided to allow lateral forces to be applied in a similar manner. The wings are operated by servo motors which are controlled by signals from controller 17, which is a conventional microprocessor-based unit.

Also connected to the controller 17 are the sensors 15, 16. These are designed to detect the presence of obstructions, such as birds, along the cable 1. In the present embodiment, optical (i.e. camera-based) sensors are provided. These provide inputs to suitable image processing and pattern recognition software in the controller 17. This combination enables an obstruction to be identified and its distance from the apparatus 10 determined in real time. It also allows the cable to be detected and its relative position to be determined. It will be appreciated that the sensors 15 and 16 are provided on opposite ends of the apparatus to allow detection when running it in either direction.

Figures 3 and 4 illustrate the assemblies 20 that form each of the cable clamps 12 and 13. One of these assemblies is mounted at each end of housing 10, though only one is shown (with the housing partially cut away) in the figures for clarity.

Each assembly comprises a pair of pivoting jaws 21 , 22 each connected to a piston 23 which is received in a hydraulic cylinder 24. The interior faces of the jaws are provided with nylon or wire brushes and/or scrapers 25 and they pivot about a common pivot pin 26.

It will be noted that in Figure 3, the jaws 21 , 22 are open and are displaced from cable 1. The hydraulic actuators are contracted in this configuration and by extending them, the jaws may be closed. The supply of hydraulic fluid, and hence the position of the jaws, is controlled by means of controller 17.

Figure 4 shows the same assembly with the hydraulic actuator extended and the jaws 21 , 22 closed around cable 1. It will be seen that this brings brushes/scrapers 25 into contact with cable 1 , whilst allowing relative longitudinal movement between the apparatus 10 and the cable.

Figures 5 to 9 show a sequence of views as the apparatus 10 moves along cable 1 towards and then past an obstruction in the form of bird 3. It is shown being pulled by a towing line 26 attached to towing eye 18. It will be noted that sensors 15 and 16 are arranged to detect obstructions ahead and behind the apparatus, depending on its direction of travel.

Figure 5 shows the apparatus 10 some distance from the bird 3 as it is being pulled along by line 26. As it does so, brushes/scrapers 25 (not shown in these figures) remove barnacles and other debris from the cable 1.

In Figure 6 the apparatus is shown having detected the presence of bird 3 with the controller having determined that it has come as close to the bird as is desirable. Consequently, it causes the hydraulic actuators to open both of jaws 12, 13 and, as shown in Figure 7, actuates the wings 14 to increase their angle of attack and provide lift to the apparatus 10. This causes the apparatus 10 to rise above the level of the bird 3 and pass above it as shown in Figure 8.

Subsequently, the sensors 15, 16 are used to locate the cable 1 and the wings 14 are used to provide negative angle of attack, and hence a downwards force, to the apparatus 10. This deflects the apparatus 10 back down towards cable 1 so that its open jaws of clamps 12, 13 will re-engage with the cable and may then be closed around it as previously described with reference to Figures 3 and 4. Where vertical wings are provided, these may be used to provide lateral guidance to assist in mating with the cable. The apparatus then resumes cleaning and the sequence is then repeated from Figure 5 as each obstruction is encountered.