Abstract: During laying of a pipeline on a surface located under water, on a vessel a pipe is fitted to a pipe string of the pipeline, in each case during a fitting step. Herein during the fitting step the vessel is moved substantially continuously in lengthwise direction of the pipe string while a pipe is fitted to the pipe string by means of positioning means moved relative to the vessel and by means of welding means moved relative to the vessel. According to the invention, a train of positioning means, welding means and clamping means is moved during an operational phase relative to the vessel in lengthwise direction of the pipe string in principle together with the pipe string and during a resetting phase set between two successive operational phases this train is displaced in the direction of the prow of the vessel relative to the pipe string and relative to the vessel.

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.