Since the marine seismic industry has progressed to towing multi-equipment, including two or more streamers, from one vessel, there has been a substantial increase in the operational down time whenever it is necessary to make repairs to a single streamer. Retrieving only a single streamer from among a plurality of towed equipment requires that the vessel maintain a nominal forward motion throughout. Utilizing present methods and apparatus, reeling in a single streamer while the vessel maintains a nominal forward motion results too frequently in damage to the streamer, i.e., holes, pinched wires and crushed bulkheads. Thus, the present industry norm dictates that the vessel be backed up while retrieving a streamer. This requires that all towed equipment be picked up, substantially increasing the effort and operational down time.
Streamers are complex cables usually comprised of a fluid filled jacket having a diameter of three or more inches. Some of the newer cables utilizing fiber optics may have diameters closer to two inches. There are various and numerous types and builders of seismic cables with associated varieties of includable equipment and means for their attachment. Generalizing with respect to this variety, cables are typically comprised of multiple electric lines, steel stress members, bulkheads, interconnects, hydrophones, program plugs, bird collars, and other equipment, predominantly encased in a polyurathene or PVC jacket. The typical jacket wall thickness is about 1/8 of an inch
Streamer lengths typically run between 2000 to 5000 yards. Each length is comprised of multiple sections. A section may run approximately 50 yards. Sections are connected by rigid connectors, several inches in length. Some connectors may exceed a foot in length. The connectors are held to the jacket by attachment means such as punch lock bands. Sections frequently contain, coupled within their length, other equipment, such as program plugs or bird collars. The plugs are comprised of rigid cylinders approximately twelve to fifteen inches long and are fixed within the jacket by attachment means such as punch lock bands. Bird collars are usually screwed around the circumference of the jacket, intermediate the sections.
A seismic cable must be handled with care. The length of the housing for certain of the included equipment dictates that the cable not be wound upon a drum with a curvature too great to accommodate certain straight cylindrical portions. The punch lock band attachment means, as well as the connectors themselves, the bulkheads, program plug housings and other included equipment, offer sharp corners, edges or points. They can penetrate or tear the jacket if inappropriately handled or juxtaposed under pressure. The cable, as mentioned above, is filled with a fluid for pressure purposes, for ballast and to satisfy other equipment needs such as that of the hydrophones. Holes in the jacket require that the jacket be repaired or replaced.
A seismic cable retrieved while the vessel maintains a nominal forward speed will be retrieved under significant tension The tension is such that layers of the streamer, when successively wrapped around a main cable reel, may frequently manage to puncture the jacket, pinch wires or crush bulkheads and equipment on the underlying layers. It is because of this potential for harm to the cable when wrapped under tension that the present industry norm dictates that seismic cables be retrieved while slowly backing the vessel through the water.
It is an object of the present invention to provide a method and apparatus for retrieving marine seismic cables wherein a single cable can be safely retrieved while the vessel maintains a nominal forward motion. Maintenance of nominal forward motion obviates the need to retrieve all towed equipment, thereby saving work and operational down time.
It is a further object of the invention to provide a feedback indication of the tension under which the cable is being retrieved. If the tension exceeds a threshold level, the reeling process can be slowed or stopped and/or the vessel's rate of forward motion can be decreased.
It is further object of the invention for the retrieval apparatus to function to facilitate a level wind for the main reel.