Technical Challenges and Impacts of Extending Spar and Semisubmersible Platform Capabilities to 4,500 m Water Depth

Abstract

Spar and semi-submersible platforms are proven deepwater production platforms. They have been used extensively in the Gulf of Mexico (GoM) to produce large deepwater developments. Spars have progressed from 600 m to 2,600 m water depth since 1995 and semi-submersibles from 450 m to 2,500 m since 1985. Topsides and hull sizes have grown to accommodate increasingly larger topside, riser, and mooring payloads. Extending these platforms to 4,500 m represents an almost 80% increase in water depth. The main technical challenges relate to the technical feasibility and installation of risers and mooring systems and their impact on the size and global performance of the hulls. Both aspects are examined in this paper for applications in the GoM. The first aspect addresses technical challenges associated with designing production, injection, and export risers to handle the large hydrostatic pressure and limitations and/or gaps with regard to current lay vessel capabilities to install risers in 4,500 m water depths. The second aspect addresses the relative sensitivity of hull sizes and global performance of semi-subs and spars at these water depths for a common topsides and subsea architecture basis of design. This paper presents the impact of extending design, fabrication, and installation of spar and semi-sub floating platforms to 4,500 m water depth. Field development scenarios, design basis and topside parameters (load, area, and height) are used to size hull, mooring, and risers to define the overall system. Proven, conventional hull designs and sub-systems technologies are used for both 2,500 m and 4,500 m water depths. Platform sizes and global responses in 2,500 m and 4,500 m water depths are compared. Technology gaps in meeting basis of design are identified. Trends, observations and relative sensitivities of the hull forms are captured. Enhancing and enabling technologies for risers and moorings are discussed. The 4,500 m hull sizes and key dimensions are compared with existing platforms to address availability of yards and vessels for construction and installation phases. Challenges related to fabrication, topside integration, mooring and risers installation in 4,500 m water depth for the two hull types are also addressed and contrasted.