1. Field of the Invention
The invention is generally related to the laying of pipeline offshore and particularly to the near vertical laying of pipeline offshore in deep water.
2. General Background
The laying of offshore pipelines has been done for many years utilizing the technique referred to as S-lay. In the S-lay technique, joints of pipe are added to the pipeline in a horizontal position on the deck of a lay barge. The pipeline then curves over the stern of the barge, angles down toward the seabed, curves back to horizontal and lays on the seabed. The profile of the pipeline from the lay barge to the seabed is in the form of a long "S" which leads to the name of S-lay
Although S-lay has been the method of choice for virtually all pipeline installed to date, there are physical limitations on the use of this technique. Chief among these is water depth. As the water depth increases, the ability to move the vessel on anchors becomes more and more difficult and the horizontal component of the pipe tension becomes greater and greater. The offshore pipeline industry has been aware of this problem for years and has as a solution, accepted the concept of near vertical lay, called J-lay, as the system of choice for deep water pipe laying. It should be understood that the definition of deep water, when referring to the use of J-lay, is a direct function of pipe diameter. This relationship is a result of the minimum water depth required for pipe of a certain diameter to achieve the proper flex during the vertical laying operation. There is also a maximum practical depth for specific pipe diameters. As an example, pipe having a diameter of 6.625 inches requires a minimum water depth, with no water in the pipe, of 124 feet. At the opposite end of the scale, pipe having a diameter of 42 inches requires a minimum water depth, with no water in the pipe, of 1,337 feet.
Considerable work has been done over the years on the theoretical aspects of the J-lay concept, but very little work has been done on the actual hardware and equipment needed for this type of system. Most of the systems proposed have utilized existing semi-submersible drilling units. These units, which are capable of being modified for this service, were not built to be used as pipe layers and can not be made to be very efficient during pipe laying operations.
Another problem area in laying pipeline offshore is the storage, transportation, and transfer of pipe to the lay vessel. In normal operations, the line pipe for the pipeline is transported to the field in forty foot long joints. The transport vessel, which is usually a small material barge or a special purpose pipe haul boat, is tied to the side of the lay vessel as the pipe is transferred to the lay vessel. Transfer of the pipe one joint at a time may take several days. The transfer of pipe from between vessels subject to sea induced motion is hazardous to personnel and equipment under good sea conditions and becomes impossible to do safely under bad conditions.
Applicants are aware of a system that utilizes a ramp that can be adjusted from horizontal to vertical. It uses large tensioners to grip the pipe and apply the necessary amount of tension to the system. This system has never been used for deep water pipeline construction.
Offshore pipe laying systems, those in use(S-lay) and theoretical proposals(J-lay), have certain things in common. The systems may or may not use pipe add ons that have been multi-jointed outside the system to provide pipe joint lengths greater than the normal forty foot pipe length. Theoretical J-lay systems have proposed joint lengths of eighty feet or more. A single station is used to accomplish the welding, NDT(non-destructive testing), and pipe coating of joints. In J-lay, it is necessary to have a means to transfer the pipe from the horizontal position on the lay vessel to a near vertical position on the lay line. It is also necessary to have some means of lowering the pipe as the lay vessel moves forward. A particular problem in this area has been the issue of devising an efficient manner of transferring the load of the pipeline to a holding mechanism so that the lowering mechanism can then be raised in preparation for receiving the weight of the pipeline after addition of the next pipe joint. Copending application Ser. No. 08/154,320 filed Nov. 18, 1993 discloses an apparatus for near vertical laying of pipeline that includes a tower on the lay barge that supports the weight of the flooded pipeline and uses a travel block and pedestal arrangement that allows for a ready weight transfer of the pipeline from the travel block to the pedestal during lowering of the pipeline after a new pipe joint has been added. A problem with the system is that the major loads of the pipeline are carried through compression in the tower during lowering. This puts large loads above the vessel deck on a continuous basis and raises the center of gravity of the system above the normal center of gravity of the vessel.