Mooring of the Veslefrikk B Close to the Wellhead Platform

ABSTRACT The station keeping system on the Veslefrikk B floating production and quarters platform is a thruster assisted twelve point chain system with pile anchors. The existing system on the converted drilling rig has been upgraded to comply with a set of design and operating criteria related to the personnel bridge, a number of flexible lines and safety of operation criteria in both intact and damaged conditions. The design criteria have been developed to comply with Norwegian regulations for the petroleum activities, and emphasis has been put on design against defined progressive limit states. INTRODUCTION The Veslefrikk field development concept comprises a floating production platform, complete with living quarters, utilities, and processing facilities. Derrick and other drilling equipment are installed on a fixed wellhead platform in a water depth of 174m. The two platforms are interconnected by a personnel bridge and a number of flexible pipes and cables for transfer of liquids, gas, electric power and signals, see Fig. 1. The platform orientation as shown in Fig. 2 was chosen as a compromise between aspects associated with marine operations, helicopter approaches, pipeline routings and prevailing wind directions in relation to fire and blowout scenarios. The floating platform, Veslefrikk B, hereafter referred to as VFB was originally built in South Korea as a drilling platform named West Vision. The conversion into a floating production platform took place in Stavanger during 1988 and 89, Ref.l. The original station keeping system was a traditional eight point 76mm chain system, designed in accordance with the Norwegian Maritime Directorate's regulations for mobile drilling units (NMD). In addition the platform was equipped with eight 3900 HP azimuth thrusters. In connection with the conversion the station keeping system had to be upgraded to comply with the regulations for the offshore activities as defined by the Norwegian Petroleum Directorate (NPD). With the selected concept configuration it was obvious that reliability of the station keeping system would be essential both for safety and regularity of operations. Also because this is the first field development where a floating platform is permanently located in the close vicinity of a fixed structure it was necessary to scrutinise design methods and criteria. At the same time it was an objective to make optimal use of all existing systems and equipment. When the eight point 76 rom chain system was found to be insufficient, it was therefore decided to add one more mooring line per corner rather than to increase the chain dimensions. It was also decided to define an operating philosophy based on a certain usage of the existing thrusters. OPERATIONAL PHILOSOPHY AND REQUIREMENTS The production life of the Veslefrikk field is 20 years. Consequently, the installations are also designed for 20 years of operation. However, elements can be planned under the assumption that they willbe changed out because of limited design life. It was established that use of thrusters and active winching could be considered, provided that an acceptable degree of reliability for the operations could be shown.