This paper presents a discussion of the riser tensioning system and the coupled analysis and design of the hull / riser system for the Holstein Spar Project. The dual casing risers are tied back to the spar using individual hydro-pneumatic tensioners, creating a coupled system between the risers, mooring and hull. Vessel response varies with the number of risers tied back to the spar. Riser strength and fatigue performance was evaluated using a coupled analysis approach, including the effect of the variation in top tension with vessel motion. Fatigue analysis was performed for first and second order vessel motions, hull vortex-induced motions, and riser vortex-induced vibrations. Global motions analysis was carried out using the time-domain software MULTISIM. In addition to hydrodynamic and wind loads, this program included coupled riser effects such as the variation of tension force versus stroke and frictional effects due to hydro-pneumatic cylinders and keel guides. Since the risers are tied back through tensioners, the hull motions have a stronger effect on riser tensions than the traditional buoyancy can mounted system. The motions and riser tension calculations were validated with model tests both for extreme and fatigue environmental conditions. Five tensioning concepts were evaluated and the hydro-pneumatic tensioning system was chosen. The trade offs between tensioner stiffness and required stroke and their impact on riser system performance are discussed in this paper.
The Holstein Field is located in Green Canyon Blocks 644 and 645 in the Southern Gulf of Mexico, 190 miles south of New Orleans, Louisiana in 4,344 feet of water. BP Exploration & Production Inc. and Shell Offshore Inc. each own 50% working interest in the property, with BP the operator. Several papers have been written describing various aspects of the technical development of the Holstein field. This paper presents a discussion of the riser tensioning system and the coupled analysis and design of the hull / riser system.
The Holstein field is developed using a truss spar with a 16-line steel mooring system. The topsides payload is 51,000 kips. The diameter of the spar is 149 feet (excluding VIM strakes). The development includes 15 wells; six of which are predrilled to TD from a MODU, and nine wells predrilled from a MODU through the 13-3/8" casing string. The spar is designed to accommodate up to three additional wells. The nine non-TD wells predrilled from the MODU will be drilled to TD through the production risers. The production risers are dual casing, consisting of an outer casing with a nominal 14-7/8" outer diameter (OD) and 0.594" wall and an inner casing with a nominal 11-7/8" OD and 0.550" wall. Some of the production risers may be converted to water injection during the field life. The term "combo riser" is used to describe the combined drilling / production / water injection functions of the top tensioned risers (TTRs). Unlike the buoyancy cans used as the tensioning system for previous spar TTRs, the TTRs for the Holstein spar are tensioned using individual hydro-pneumatic tensioners.