Abstract: The laying of a pipeline on a surface located under water is to a great extent continuously performed because the pipes are arranged in line with a pipe string by means of at least two alternately used pipe carriers.

Abstract: A buoyant vessel which alone can support a heavy superstructure has a well within which a floating body is disposed. The floating body can displace sufficient water in the well to support the superstructure by itself. Valves are provided for transferring water into and out of the body to lower and raise the superstructure with respect to the water surface on which the vessel is floating.

Abstract: An artificial island structure which is constructed on land can be conveyed to and placed in a vertical standing position on an underwater base site by loading the island structure, while horizontal, on a floating buoyant carrier, transporting the carrier with island structure thereon to a large displacement buoyant vessel, journalling one end of the carrier to the vessel, transporting the vessel with carrier and island structure to a position over the underwater base site, ballasting the carrier until it swings to an upright position, suspending the upright island structure from the vessel, disconnecting the carrier from the island structure, and lowering the island structure onto the underwater base site.

Abstract: An apparatus for digging a trench under a pipeline or cable which has been placed on the sea bottom. A carriage moves along the pipeline. Two forward digging chains are mounted on the carriage so as to be disposed on each side of the pipeline, and so that their ends define two points on a line extending beneath the pipeline or cable. The digging chains are arranged one behind the other in the direction of movement of the carriage. Each digging chain is movable from a first position of incline with respect to the direction of movement of the carriage to a second position substantially at right angles thereto. The digging chains are shaped and positioned so as to dig the trench with an undercut slope at both sides.