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Method for laterally steering streamer cables

Method for laterally steering streamer cables description/claims

The invention relates generally to marine seismic prospecting and, more specifically, to devices and methods for controlling the lateral position of streamer cables towed underwater behind survey vessels.

In marine seismic exploration, instrumented cables, known as streamers, are towed underwater by a survey vessel. The streamers are outfitted with a variety of electronic devices, including hydrophones that detect seismic signals transmitted into the water and reflected off geologic structures beneath the sea floor.

Devices known as cable-leveling birds are attached to a streamer at intervals along its length to control the streamer's depth. The birds are equipped with adjustable diving planes, generally referred to as wings, each having a pitch axis about which the wings can be pivoted by a motor to generate the lift needed to maintain the cable at a desired depth. Most commonly, the birds are rotatably attached to the streamer and weighted to hang pendulously from the cable with the pitch axis of the wings below the cable. These birds are effective depth-control devices.

It is not uncommon for a survey vessel to tow six or eight or more streamers of lengths up to 12 km. Because the costs of lost survey time and of replacing a damaged or lost streamer are so high, it is important that the streamers not become entangled during their deployment. Entanglement is more likely to occur in the presence of strong cross currents or while the survey vessel is turning to make another pass across the survey zone. To help avoid entanglement in turns, for example, each streamer is often operated at a different depth. While this technique provides some measure of entanglement control, it also subjects the cables to potentially strong shear layers of current that vary considerably with depth, possibly increasing the risk of entanglement. Generally the most satisfactory way to avoid entanglement with conventional systems is to steer the vessel through wide turns and to overspace the streamers from each other. But these techniques increase cost and reduce the precision of the seismic image.

Paravanes and other devices are used to separate the streamers at their head ends close to the survey vessel. But lateral streamer control and streamer position predictability diminish as cable tension lessens down the lengths of streamers. The wake created by the seismic vessel creates a phenomenon known as “trouser” effect on the array. The streamers fan out port and starboard, creating a large void in the seismic coverage directly aft of the vessel. The streamers assume the shape of trousers. These voids must be resurveyed on subsequent passes known as “in-fill.” In-fill can increase the cost of seismic surveying by up to 20%. Lack of repeatability in processes and positional inaccuracies can reduce the quality of the seismic data and increase the cost by necessitating in-fill. Thus, there is a need to provide a technique for lateral streamer positioning to reduce the cost of operation and to improve the quality of the resultant seismic image.

Today's state-of-the-art seismic vessels have the capacity to deploy, tow, and recover up to 18 streamers. Existing deployment schemes limit the degree to which streamers can be simultaneously deployed, which greatly increases the cost of operation. There is a need to provide for lateral streamer control during the deployment and recovery phases to support simultaneous streamer operation without entanglement.

Accordingly, a method embodying features of the invention is provided for laterally steering a streamer. A bird assembly of the kind typically operated in a depth-controlling orientation in which one or more wings are pivotable about one or more pivot axes that lie generally in a horizontal plane is operated in another way. The bird assembly is operated in an orientation in which the one or more pivot axes of the one or more wings lie largely in a vertical plane to steer the streamer laterally.

In another aspect of the invention, a method for laterally steering a streamer cable with a cable-leveling bird having a pair of wings normally operated underwater with the pair of wings pivotable about horizontal axes comprises converting the cable-leveling bird, which is rotatably attachable to a streamer cable, into a cable-steering bird by ballasting the bird so that it operates underwater in an orientation in which each of the wings pivots about a pivot axis that lies largely in a vertical plane to steer the attached underwater cable laterally.

In yet another aspect of the invention, an underwater cable-steering device comprises a connector attachable around the periphery of an underwater cable section and rotatable about the cable. A control device includes a body connected to the connector external of the cable. The control device includes a first wing at one side of the body and a second wing at an opposite side of the body. The first wing pivots about a first axis, and the second wing pivots about a second axis. The two axes may coincide, but do not intersect the cable. The control device also includes means for pivoting the wings about their pivot axes. Ballast means is provided to help ballast the steering device to maintain the pivot axes largely vertical.

In another aspect of the invention, an underwater cable-steering device comprises a connector attachable around the periphery of an underwater cable section and rotatable about the cable. A control device includes a body connected to the connector external of the cable. The control device includes a shaft extending through the body and defining a pivot axis. A first wing portion is connected to one end of the shaft at one side of the body, and a second wing portion is connected to the other end of the shaft at an opposite side of the body. The two wing portions may be unitarily formed as a single wing. The pivot axis does not intersect the cable. Ballast means is provided to help ballast the steering device to maintain the pivot axis largely vertical.

In still another aspect of the invention, an underwater cable-steering device comprises a connector attachable around the periphery of an underwater cable section and rotatable about the cable. A control device includes a body connected to the connector external of the cable. A first shaft extends from a first side of the body. A first wing attached at an end of the first shaft can pivot about a first axis defined by the axial rotation of the first shaft. The first axis does not intersect the cable. The control device also includes means for pivoting the wings about their pivot axes. Ballast means is provided to help ballast the steering device to maintain the first axis largely vertical.

These features and aspects of the invention, as well as its advantages, are better understood by reference to the following description, appended claims, and accompanying drawings, in which:

FIG. 1 is an isometric view, partly cutaway, of a cable-steering device embodying features of the invention on a section of streamer cable;

FIG. 2 is a top view of the cable-steering device of FIG. 1;

FIG. 3 is a cross section of the cable-steering device of FIG. 1 taken along lines 3-3;

FIGS. 4A-4C are schematics representing various ways of ballasting a cable-steering device as in FIG. 1;

FIG. 5 is a front elevation view of a cable-steering device as in FIG. 1 with buoyant tank ballast;

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