Crane Control System With Active Heave Compensation and Constant Tension Modes Onboard the Vessel Stena Wel/servicer

Abstract

1. Abstract As a consequence of decreasing oil prices and increasing subsea activity, cost reduction in installation and maintenance of underwater equipment becomes more and more important to the oil companies. Most underwater operations involve crane support at the seafloor. The Field Support Vessel (FSV) Stena Well servicer is equipped with a 130 ton twin crane. The hydraulic crane is controlled by a computerized Crane Control System including Manual, Active Heave Compensation and Constant Tension modes on all winches. Using well-equipped dynamic positioned monohull vessels designed for minimized movements in open sea, with a crane system including the above mentioned functions, rather than anchored semi-submersibles, enables the offshore operators to reduce cost, save time and to extend the weather window for subsea operation at no extra risk. 2. Introduction Oil prices in the mid-eighties encouraged the offshore industry to search for less expensive methods of producing oil from offshore fields. Examples of such methods are:Introduction of early production through floating production platforms.Development of enhanced oil-recovery methods.Introduction of well-stimulation techniques for new and excising wells.Development of sub sea production systems. There is a worldwide trend to move towards subsea production environment in order to cut costs. As the number of offshore subsea installations will increase, the need for cost efficient systems to deploy, recover and maintain this equipment is obvious. To meet the requirements for cost efficiency in installing and maintaining new and exciting production equipment, Stenas objective was to design a monohull vessel which is able to compete in the market for underwater construction work. Up till now, small semi submersibles have been generally considered to be the only possible working platforms for such operations. A stable platform has been essential for performing underwater work offshore. Roll, pitch and heave motions are smaller for semi submersibles than for monohull maintenance and service vessels. A semi-submersible however, is more expensive and less flexible than a monohull vessel and usually requires support vessels to serve it (e.g. anchor handling). A monohull FSV would certainly be a tempting alternative for the offshore industry for jobs that today require semi-submersibles, if no greater risk is introduced. For a monohull vessel to be able to perform maintenance and installation of underwater offshore equipment, sufficient lifting capacity with heave compensation is necessary. During the summer of 1989 a computerized crane control system was installed on the Stena Well servicer. The crane system enables safe and efficient lifting capacity of up to 130 ton loads by computer control of winches and crane. The vessel is now fully able to compete with semi submersibles for operations involving critical load handling close to the seafloor or close to installations on the seafloor. Being the world leader in manufacturing dynamic positioning systems, Simrad Albatross crane control systems, and in particular, computer control systems for active heave compensation fits well into the companies technology and product spectrum. The now developed and installed system onbard Stena Well servicer is the most advanced and efficient active heave- compensating system installed onboard a monohull vessel in the North Sea.