A New Approach to Subsea Intervention

ABSTRACT In March 1987 representatives of Statoil and the Asea Brown Boveri Group could witness the completion of a successful sea test of a new subsea intervention system. These tests were a determined step towards a new approach in the subsea maintenance area. A flexible and versatile system has been proposed and developed. The system, which is of a modular design, consists of a carrier vehicle, tool packages, launching system and surface control & power systems. The intervention tasks cover replacement of heavy subsea installation and production. This paper deals with the development of this system including tools packages for a broad range of maintenance tasks. INTRODUCTION Within the framework of technology cooperation between the Norwegian oil company Statoil and the European Industrial group Asea Brown Boveri, a subsea intervention development project was initiated 1985. The project group recognized the advantage of carrying out subsea interventions from an inexpensive monohull vessel as opposed to drillings rigs. Comparisons of representive operations performed from both types of vessels leaned strongly in favour of the monohull vessel with increased operational availability, reduced mobilization time, costs etc. On the basis of this rationale the two companies designed and built Wellman, a diverless, versatile, easy-to-handle and cost effective intervention system. Encouraged by the system?s good performance during the sea trials in Norway, the two companies are now making a further developed version, the Wellman Mk II, available to the oil industry. This paper highlights the design philosophy, system configuration and versatility to illustrate the Wellman cost effectiveness. DESIGN PHILOSOPHY Based on previous studies, the joint development team reached the following conclusions at an early stage :*The intervention system should be operated from an inexpensive monohull support vessel.*No diver assistance should be required*To replace a subsea installed component, a two-trip operation would allow substantial simplification of the intervention system, if compared to a one trip design. The additional time required was judged to be marginal and in many cases the service crew would anyhow prefer to inspect the faulty component at the surface before installing a new one.*The intervention system should be landed and docked on the subsea installation before any subsea operation starts.*The tool packages should be accurately positioned around the component to be replaced using preferably a 2-point dedicated docking arrangement. Reaction forces should be transmitted to the subsea structure.*The intervention system should be negatively buoyant suspended from the surface to minimize handling requirements. Guidelines and standard guide posts were the preferred choice.*The system should impose no or only minor changes to protection equipment and its arrangement. The project team further concluded that:–For all subsea tasks a common work base should provide power, communication, navigation and optional equipment such as manipulators, thruster, cameras etc.–Much emphasis should be placed on simple, but robust tools, tool packages deployment and system handling.–The basic unit with its replaceable tool package should be easy to handle and operate. Fast exchange of tool packages was considered essential.–The common work base should facilitate both horizontal and vertical component replacement in situ.