Large Diameter Steel Catenary Riser Solutions for Semi-submersible Platforms in Offshore Northwest Australia

Abstract

Abstract The offshore northwest Australian sector is dominated by large gas plays and as the sector matures there is an increasing move towards deepwater developments. Riser solutions are key concept selection drivers and often high gas throughputs have led to a drive towards large riser diameters to limit the associated subsea infrastructure. The need for high availability in LNG projects leads to a requirement for high reliability, permanently moored Offshore Facilities. A representative development scenario employs large diameter (18" to 24") import and export Steel Catenary Risers (SCRs) supported by a Semi-submersible platform in 1,400 m water depth. This is potentially the deepest development offshore Australia. It is a relatively novel solution for the region given that the riser solution in the region has been primarily limited to flexible risers. SCR designs are governed by their fatigue performance in the touch down zone (TDZ). Feasibility of the large diameter SCRs is challenged by the Australian sea conditions dominated by southern ocean swells. Several options are considered to enhance the fatigue life of the SCRs. These include increasing the Semi draft to reduce heave motions, aligning the riser plane in a direction normal to the predominantly northeastern swell environment, using upset ends at critical locations to reduce curvature response, lazy wave configuration to decouple vessel motions from TDZ response, and weighted SCR to reduce TDZ response. These options are considered either individually or in a combination thereof. The technical feasibilities of and the challenges associated with these options are discussed. Some of the possible challenges include the manufacturability of upset ends, feasibility of top end terminations and installation of buoyancy modules. A relative cost comparison is also made between the different options. The metocean conditions in offshore Australia are compared with US Gulf of Mexico (GoM), where SCRs have been widely used. Their implications for SCR fatigue and extreme response are explored. Findings that are critical to the riser design include riser porch vertical velocity at extreme conditions, relative contribution of surge/sway versus heave motions to fatigue damage, contribution of vortex induced motions (VIM) to fatigue damage, and effect of long term currents on riser VIV response. Given the relative novelty of SCR applications with a Semi-submersible in offshore Australia, and the challenges associated with fatigue performance of large diameter SCRs in that harsh environment, this paper will serve as a valuable guide for future riser design and field development in the region.