A Deep Draft Semisubmersible with a Retractable Heave Plate

Abstract

Gulf of Mexico deep water oil fields present challenges to the offshore industry. Large fields require heavy drilling and production topsides which favor inshore deck installation, hook-up and commissioning in order to reduce time, costs and risks associated with carrying out such operations offshore. Harsh environments require that the motions of the structure be small to allow the use of dry trees and SCRs. A new system, called the DPS 2001, has been developed, which is a deep draft semisubmersible with a retractable heave plate. The system combines the advantages of a semisubmersible with the operational motion advantages of a truss spartype floater. Minimal motions of the DPS 2001 allow the use of dry trees with drilling/workover capability. The truss/heave plate is in a retracted configuration during fabrication and towing; this allows the deck to be stalled and commissioned inshore. The truss/heave plate is lowered and locked at a deep draft after the DPS 2001 is towed to the installation site. Because of the heave plate, the DPS 2001's motions are significantly less than that of conventional semisubmersibles and ship-type hulls. INTRODUCTION The DPS 200101 is a floating drilling and production platform designed specifically for oil and gas development in deep water harsh environments. Three different versions of the DPS 2001system are currently being developed by CSO Aker:DPS 2001-4: a fourcolumn version of the system for 8,000 - 10,000 st topsides,DPS 2001-3: a three-column version of the system for 5,000 - 8,000 st topsides, andDPS 2001-5: a five-column version of the system for use with a topsides up to 40,000 st. A patent is pending for the DPS 2001. The DPS 2001-5was presented at the 2001 DOT conference. The DPS 2001-3will be presented in the near future. The DPS 2001-4is the focus of the present paper. The DPS 2001-4is shown in Fig 1. The upper hull consists of horizontal pontoons and four corner columns and provides buoyancy. The lower hull consists of a heave plate and a truss that supports the heave plate. The heave plate provides large added mass and significantly reduces the platform's motions. The DPS 2001-4is anchored to the sea bottom with a spread mooring system consisting of chains, steel strand or polyester ropes and suction anchors. The upper hull is similar to a typical semisubmersible hull. The lower truss and heave plate unit is similar to the truss structure employed on truss spars. The system is an integration of proven components and embodies the best features of both a semisubmersible and a truss spar. The hull and the truss/heave plate unit can be fabricated in a number of yards in Asia or Europe. Upon completing the hull and truss/heave plate, it will be drytransported to the GOM. The hull and truss/heave plate will be in a retracted configuration during fabrication and transportation (Fig 2).

Conceptual design of a deep draft semi-submersible platform with a moveable heave-plate

“…The deep draft can decrease the platform heave motion. Bindingsbø (2002) indicated that the heave and pitch RAOs of the deep draft semi-submersible platform are 50% less than those of the original semi-submersible platform; Halkyard (2002) improved the structure of deep draft semi-submersible platform, and demonstrated the structure of the DPS platform (Fig.3), which is similar to the Truss-spar platform (Fig.4). The heave-plate of a DPS platform, which generates huge added mass and viscous damping in platform heave motion, can rise to the bottom of the hull under transit conditions, and descend to 100 m below the sea surface under operating and survival conditions as a rigid member connected to the hull.…”

Floating Offshore Platform Design

“…Deep draft semi-submersibles and those with heave mass traps have been shown to have suitable motions for dry trees, but as of this writing, they have not been employed (Le Guennec et al, 2002;Halkyard et al, 2002). The response for two ship shaped hulls of differing length and displacement are shown in fig.…”

“…Deck installation requires an offshore heavy lift, floatover, or a scheme for self-installation (LeGuennec et al, 2002;Halkyard et al 2002). While tall semi-submersibles have stability problems while submerging, a permanently sited unit would not require any deck variable load during installation and certain permanent deck payload may be absent.…”

Dynamic performance of a semi-submersible platform subject to wind and waves

“…As the semi-submersible platform has been widely used in recent years, the studies in this field are increasing: Morch and Moan (1985) compared the calculated and measured the wave-induced motion of a moored semi-submersible platform; Soylemez (1995) tested the motion of a twinhulled semi-submersible platform; Wu (1997) simulated the motion response of semi-submersible platform and mooring forces in regular waves by numerical method; Soylemez (1998) considered the combined action of wind, waves and current on semi-submersible platform, and simulated platform motion response with the method of time domain analysis, with the numerical results agreeing with their experiment; Bindingsbø (2002) studied the hydrodynamics of semi-submersible platform, his numerical results show that the hydrodynamics of a deep draft semi-submersible platform is better than that of the normal type; Halkyard et al (2002) designed a retractable heave plate for deep draft semi-submersible platform, which overcame transitional difficulties and optimized the hydrodynamic performance by using a dry tree. Engineers and scientists in China also did lots of researches on semi-submersible platform: Chen (1997) calculated the motion response and mooring line forces of a moored semi-submersible under wave action; Wang (2007) analyzed the motion performance of four kinds of semi-submersible platforms, and selected the suitable configuration for South China Sea environment.…”