Offshore pipelines are commonly laid from a continuously moving vessel known as a lay barge. The separate lengths of pipe in the pipeline may be welded together on shore and then coiled on a reel and placed on a lay barge for the pipe laying operation. Alternatively, lengths of the pipe may be placed on the lay barge and welded together on the barge just prior to laying the pipeline. In either case, to lay the pipeline one end of the line is passed over rollers and then down an elongate cradle or "stinger" which extends rearwardly and downwardly from the stern of the lay barge to the bed of the body of water. As the barge moves forward, the forward motion of the barge causes the pipeline to be pulled along the rollers and down the stinger and thereby continuously laid on the bed of the body of water.
Pipelines larger than about 12 inches in diameter are commonly coated with a thick layer of concrete to weight the pipeline down in the water. Such concrete coatings are applied to individual lengths of the pipe with the coating extending the full length of the pipe except for several inches near each end. Prior to applying the concrete coating, the pipe lengths are usually coated with a material, such as epoxy, to protect the pipe from corrosion. This coating material is also terminated several inches from each end. Successive lengths of pipe are joined together by welding, leaving a gap in the coating material and in the concrete covering at each joint. It is the usual practice to apply a coating to the pipe in this gap to protect the pipe against corrosion, for example with a hot fusion bonded epoxy and/or a shrink sleeve, and to fill the gap between the ends of the concrete coatings so as to provide a smooth, uninterrupted covering for the pipeline. The pipeline can then move smoothly over the rollers and down the stinger to the sea bed.
The gaps are filled with various materials by various techniques. One procedure is to place a metal sleeve or mold over the gap and fill the sleeve with molten mastic which solidifies as it cools. However, in most cases the pipeline must be in a condition for handling immediately after the gaps are filled, so that the laying of the pipeline can proceed without delays. The mastic filling does not set to a sufficiently strong material within the required time to allow further processing of the pipe without additional reinforcement to protect the mastic. Therefore, in conventional filling operations where mastic is used, the sleeve is left in place, so that it remains on the pipe on the ocean floor. This is objectionable, since the sleeve can snag fishnets, and cause other damage to marine operations. Moreover, the filling of the gaps with mastic in this way is highly labor intensive, and therefore expensive.
A more effective method is to fill the gap with a fast cure material inside a removable, preferably reusable, mold which is removed before the pipe is advanced into the water. Various polymer concretes, such as those disclosed in U.S. Pat. No. 4,608,208, or polyurethanes have been used in this way. The removable mold surface is coated with a release agent to prevent the fast cure material from sticking to the mold surface. Such a mold release adds expense and processing time, and is inefficient if improperly applied. Furthermore, some of the release material remains on the surface of the fast cure material and enters the water with the pipe as a potential environmental contaminant.