Marine seismic exploration is generally conducted by towing a marine seismic streamer cable at a given depth through the ocean or other body of water. However, in some locations due to congestion on the surface or a requirement to detect compressional and shear waves a marine seismic bottom cable is used.
The current practice in marine seismic data collection is to require marine seismic cable of a longer length than was required in the past. For streamer cable, this means a length which could exceed four miles and for bottom cable the length could be equally as long and with an operating depth in excess of 4000 feet. In addition, some seismic data collection techniques require a plurality of marine streamer cables being towed behind a ship at the same time.
A marine cable includes sections of marine cable, usually about 300 feet long, connected together by couplers. In some marine cable designs, each 600 feet, an instrumentation package is connected between sections of marine cable. Each section and each instrumentation package, if it is to be interchangeable, must be able to absorb the cumulative drag force upon the cable when towed in water and if a bottom cable, it must also be able to absorb the weight of the cable between the ship and the bottom.
A marine cable section can absorb this tension force by including longitudinally within the cable section stress members which connect to the couplers to transfer the drag and weight forces on that cable section to the next section of marine cable until finally at the connection to the ship, the drag force on the cable is received as a cumulative tension force on the cable.
In the case of the instrumentation package connected between two couplers, the drag and weight forces are transferred from the trailing coupler, which is linked to stress members of the trailing section of seismic cable, through the body of the instrumentation package to the connecting coupler of the next section of marine cable.
Longer cable or cable which can operate at greater depths place new demands on marine cable operations. The additional length can be a safety hazard. It takes time to retrieve the marine cable from water. If a storm approaches the crew of the ship could be placed in danger while the marine cable is retrieved. In some cases, because there is not enough time to retrieve the cable the marine cable may have to be towed until the weather subsides or in an emergency, abandoned from the ship with a marker buoy attached to the cable.
The present methods for marine cable retrieval limit the speed at which cable can be retrieved from the water without damaging the cable. In addition marine cable which meets new requirements for additional length or greater operating depth face even more sever limitations on the speed at which the marine cable can be retrieved.
The problem is the tension on the cable as the cable is wound on the cable storage reel. This can be explained using the retrieval of a streamer cable. A streamer cable, especially if it is being towed with other equally long cables, is retrieved during towing to keep the cables separated and also to speed the departure of the ship from the area. However, the towing does exert a drag force on the cable which is proportional to the length of the cable. This force is seen at the cable reel as a longitudinal tension force on the cable.
Streamer cable sections can be as much as 4 inches in diameter. They are usually made with an outer layer of polyurethane tube which is supported by spacers and the tube is filled with oil or some other nonconducting liquid to provide buoyancy to the marine cable.
The streamer cable is fragile and flexible at the surface. When the cable is wound on a cable storage reel, the longitudinal tension force on the cable, as the cable goes on the storage reel, is also converted to a transverse force which squeezes the cable. This transverse force which squeezes the cable also migrates down within the layers of the cable on the storage reel as the cable winds on the reel because the longitudinal tension force acts to tighten the cable as the cable is wound on the storage reel. The friction between the layers of wound cable acts to retain the longitudinal tension force. This causes the transverse force to build up as the cable is wound on the cable reel because each layer of wound cable also exerts a force on the underlying cable. Consequently, the transverse force is cumulative. The lower layers of cable on the reel are subjected to an increasingly greater force squeezing across the diameter of the cable. Under this cumulative transverse force, the cable could rupture, spilling oil and collapsing, damaging the internal components of the cable.
A bottom cable has a similar construction as the streamer cable and undergoes the same process when placed on a storage reel, the major difference is that a large component of the tension on the bottom cable at the cable storage reel is the weight of the cable between the ship and the bottom. This weight can be substantial, especially, when it is added to the drag force on the cable as it is moved upward through the water to produce the cumulative longitudinal force on the cable at the cable storage reel.
The tension on the cable at the storage reel could be reduced by: reducing the towing speed of the streamer cable as it is being retrieved; slowing the rate at which the cable storage reel winds up cable; or using sheaves.
As mentioned, reducing the towing speed is one method now used to retrieve streamer cable. However, to reduce the tension to an acceptable level at the cable reel, the longer the cable, the more the towing speed must be reduced. If towing speed is reduced too much, it can lead to the tangling of multiple streamer cables. This will also delay the departure on the ship from the area in the event of a storm or a change in exploration plans.
Reducing the take-up speed on the cable storage reel is another method to compensate for longer cable. Especially, bottom cable where tension on the cable storage reel is a combination of the weight of the cable and the drag force on the cable. However, when the cable storage reel retrieval speed is reduced, it increases the time necessary to retrieve the cable. In addition, when retrieving bottom cable, since a major portion of the longitudinal force is the weight of the cable suspended beneath the ship, a reduction in speed will not appreciably reduce the longitudinal tension force at the cable storage reel.
The problem could be solved if the cable storage reel could be isolated from the longitudinal tension force on the cable. Sheaves have traditionally been used with cable to perform this function. However, sheaves have proven not to be effective in reducing tension forces on a marine cable at the cable reel because: (1) the diameter of the marine cable can vary along its length due to the various types of instrumentation packages which can be placed on the cable, (2) connecting couplers which link streamer cable sections and instrumentation packages together to form a streamer cable may have a diameter greater than the streamer cable, and (3) in order for sheaves to reduce the tension applied at the cable reel, it must exert a force transverse to the length of the streamer cable. However, current designs of marine cable can not withstand a transverse force on the outer surface because the outer surface is flexible. The outer surface is supported by spacers and a fluid within the cable. Consequently, sheaves can not exert enough force at a single point on the cable to counter balance the longitudinal tension force upon a marine cable at the cable reel.
One approach to solving this problem is found in U.S. Pat. No. 4,828,223, Cable Handling Apparatus, issued on May 9, 1989 to Russel and Gjestrum. The Cable Handling Apparatus overcomes many of the above mentioned disadvantages of using sheaves by using a series of sheaves which encounter the marine cable at several different locations along its length. The invention can also accommodate varying diameters of the cable with each sheaves in turn absorbing some of the tension caused by the drag force on the cable. However, a device of this nature has a complex pneumatic control system. In addition, it is large and bulky thereby taking up considerable space aboard a ship.
A simpler approach is desired to reduce the longitudinal tension at the storage reel or to isolate the longitudinal tension force from the cable storage reel during cable retrieval operations so that marine cable retrieval can be performed quicker and within tension levels at the storage reel which will not damage marine cable.