Device for Weighting of Streamer Cables

The invention concerns a device for weighting of streamer cables.

During seismic surveys at sea a seismic vessel tows behind it an array of seismic energy sources which transmit seismic signals, and recording equipment for these signals in the form of hydrophones which are arranged in a so-called streamer or streamer cable. This streamer cable, which contains a row of hydrophones, should be kept at a determined depth in the water and should hold a course which is as rectilinear as possible, i.e. so that all the hydrophones are at more or less the same depth. For this purpose the cable must be balanced, in order to achieve the required balance between buoyancy and weight and the cable's depth position is determined by means of guiding wing devices called "birds", which are attached to the cable at regular intervals.

To date, in order to balance streamers weights have been fitted to the outside, and this weighting has consisted of lead pieces or lead plates which are taped to the cable manually. The positioning of these lead pieces has on the whole been based on a rough estimate and, in order to obtain the best possible adjustment of the cable, it has often been necessary to go out in a boat in order to tape on new weights or to move weights.

This method of balancing a streamer has a number of disadvantages. Firstly, the work of taping on the lead pieces is extremely time-consuming, while at the same time the tape dissolves or is destroyed by exposure to the water, causing weights to fall off, resulting in maintenance work and continual renewal of the tape. These materials are expensive, particularly tape of the required quality for use in seal water. Moreover, there is also a regular need to remove the weight, e.g. during transfer from one area to another. This involves a great deal of time-consuming and laborious effort for unwinding tape. It has to be unwound since cutting it off would be too risky as the flexible, soft streamer is easily damaged. The most serious disadvantage, however, is that loose tape or deformed tape generates severe noise and this noise has an extremely unfavourable effect on the recording equipment which is fitted inside the cable. This technical disadvantage combined with the major costs involved in constant renewal of the adhesion of the lead pieces has produced an obvious need for improvement in this stage of the process. In this connection it can also be mentioned that it can be a complicated process to tape on a piece of lead from a small boat in the water.

The object of-the present invention, therefore, is to provide a weighting device specially designed for streamer cables, whereby the necessary balancing or adjustment of the cable in local conditions can be performed quickly and simply, and where the device will not cause significant or intrusive noise which will affect the seismic recording.

This object is achieved by the invention in a weighting device, which can be of various sizes and weights, and is clipped or hooked on to the cable at the required intervals, a task which is easy and quick to perform and also can be performed relatively easily from a small boat which is sent out to carry out a final adjustment of a streamer. The device in accordance with the invention will retain its shape and it has been shown that with the construction possibilities this offers, the noise problems due to weighting are avoided to a considerably greater extent than previously.

Even though the device in accordance with the invention will be initially more expensive than the conventional lead pieces, it has proved to be a considerably more economical method since the device in accordance with the invention will be able to be used over a considerable period of time and will generate significantly less maintenance expenses. Another important advantage is that adjustments will be much simpler to perform since the weight can be moved to some extent and otherwise can be simply hooked on and taken off.

The invention will be described in more detail with the aid of embodiments illustrated in the accompanying drawings wherein:

FIGS. 1 and 2 are an end elevational view and a side elevational view respectively of a first embodiment for the device in accordance with the invention;

FIG. 3 is a section sketch of another embodiment for the invention;

FIG. 4 is a side elevational view of third embodiment of the device in accordance with the invention and;

FIG. 5 is a cross-sectional view taken along line 5--5 of FIG. 4.

The streamer cable for which the weighting device is intended is not illustrated in the drawings, apart from FIGS. 4 and 5 where part of such a cable is shown by reference number 1. The cable is a flexible cable, connected by a plurality of cable lengths and conductors are threaded through the cable and at regular intervals hydrophones are fitted for reception of seismic signal reflections. The embodiment of the device in accordance with the invention which is illustrated in FIGS. 1 and 2 consists of a circular body 2 of relatively narrow width as illustrated in FIG. 1. On one side of circular body 2 is an opening 3 which has been cut at an angle as best illustrated in FIG. 2. The width of this opening is smaller than the width or diameter of the streamer which causes weight ring 2 to be pressed on to the streamer. Normally this could be done simply by squeezing the streamer, which is relatively soft, but it can be an advantage if the material of the ring 2 is rather springy, i.e. that the material of the body 2 is made of a material with a certain amount of elasticity, so that it can be expanded to temporarily increase the width of opening 3 and at least part of the diameter of the inner surface of the body to facilitate fitting it onto the streamer after which The inner surface and opening return to their normal conditions for attachment to the cable. An example of such a material is brass. Naturally a softer material can also be used, but then the ring will have to be clamped tightly around the streamer. The slanted insertion opening 3 has been given a shape which enables the ring also to turn with the opening upwards without falling off the streamer cable for this reason. It will be advantageous to fit such an element in the vicinity of the connection point on the streamer for the so-called "birds" and to position it in this area without it having to be attached in any other way.

FIG. 3 illustrates another embodiment, where the device in accordance the invention is composed of two parts 4 and 5 which are pivotally linked by a hinge 6 on one side. On the other side, in one part, e.g. 4, a spring-loaded peg 7 is fitted which protrudes, but which can be pressed in. The other half 5 of the device can be bevelled at the end, so that when it passes over peg 7 it will press it in, until peg 7 is in line with a hole in the end part of part 5 into which peg 7 is inserted, thus locking the parts firmly together around the streamer. If the weighting has to be removed or moved the peg must be pressed in to allow the parts to be released and the ring pulled off.

The device in accordance with the invention does not necessarily have to be circular in shape with a relatively narrow width, but the outer shape can also be constructed in a streamlined way in order to reduce noise as much as possible. It can thus also have a sleeve-shaped form. An example of such a construction is illustrated in FIG. 4, which shows a sleeveshaped body with rounded end parts. In this embodiment two parts 4' and 5 are connected by a pivot joint 6' for relative pivotal movement but in this case they are connected by means of a hook or hookshaped object which is fitted in milled grooves which are indicated by reference number 11. In each of these grooves 11 is fitted a lever 8 which revolves around a shaft 9 which is attached at one end section of part 4, while a pin or peg 10 is fitted at the adjacent end section of part 5. As illustrated in the section drawing of FIG. 4, a locking of the sections will be achieved by depressing the left part of the lever 8 causing its other end part to grip under the peg 10, thereby causing locking. Given appropriate dimensions, this locking, i.e. this lock position can quite simply constitute the only locking, but if desired blocking devices can also be installed in order to ensure that the parts remain locked. When it is opened, the left part of the arm 8 is pulled outwardly, so that it is raised, thus releasing the parts from each other.

Many other modifications will be possible within the scope of the invention, in that the invention will be independent of the locking mechanism, whether the weighting device is composed of one, two or more parts and whether these are linked or hooked together in several places. A possible variant will also be two mutually loose parts which are connected by a kind of bayonet lock. The material in the device can also vary, even although metals such as brass are preferred at present. The invention is thus intended to cover all such variants.