Performance Benefits of the MinDOC Hull at the Mirage Field in a Post-Katrina/Rita Environment

Abstract

The MinDoc3 platform, a dry tree capable, spread-moored deep draft floater featuring three columns braced together, was selected for Mirage field development because of flexibility, functionality and ability to fabricate in the US Gulf Coast. This paper presents the MinDOC's global performance and discusses impacts from hull form, mooring design and riser interaction and makes comparisons to previous design standards. The Mirage Field is located in Mississippi Canyon 941/942 in 4000 ft WD. The weather criteria includes the impact of hurricanes Katrina/Rita, and allows for maximum wave heights and robust mooring considerations. The MinDOC's global performance has been modeled experimentally and numerically with good correlation. VIM behavior was determined experimentally, and suppression is achieved by only straking the interior portion of the upper columns, which simplifies horizontal construction. The MinDOC was chosen for its payload, robustness and low motions. The mooring system is a relatively typical 3x4 semi-taught chain-polyester-chain system with a radius of approximately 6700 ft. The MinDOC hull form global motions are sufficiently low to permit the use of Top-Tensioned risers and Dry Trees. The low motions reduce riser stroke, which also reduced tensioner stiffness, and reduce the coupling impacts on the global motions. The low motions were achieved by a conscious effort to concurrently design the hull, risers, tensioners and mooring systems. Introduction MinDOC hull design was initiated in late 2006 with design and fabrication commencing nearly in parallel. The initial design site is for the Mirage field, located in Mississippi Canyon in Blocks 941/942. The location is near the edge of the shelf and the mooring system water depth varies from 3,822 ft in the north, to 4,288 ft in the south. The mooring pattern consists of 12 lines arranged in three bundles spaced 120° with each bundle having four lines grouped in two pairs. The platform bow orientation is 156 degrees. The leg numbering starts with the SE most line, counting clockwise around. Each mooring leg is chain-polyester-chain and consists of 5" R4 studless top chain, 9.75" Polyester and 5" R4 studless bottom chains. The nominal radius is approximately 6700 ft from the fairlead to the anchor chain touchdown. The total nominal length of each line is 7,500 ft. (Figure 1).