Deepwater moorings now easier to deploy
Deepwater mooring poses significant technical challenges for the naval architect. As moorings go deeper, issues surrounding the mechanical and practical demands on the design and deployment of mooring systems come to the fore. Brian Green of First Subsea Ltd comments on the advances made in this sector of the industry.
O
ne area of specific concern to operators is the time taken to make-up and deploy deepwater
mooring lines, resulting in long installation times and high vessel costs. Over the last decade, the move to deepwater
oil and natural gas production beyond 1000m (3281ſt) has gathered pace as shallow water reserves have become uneconomical. In the Gulf of Mexico alone deepwater oil production has almost doubled. Recent projects such as the Tahiti Spar in 1280m (4200ſt) of water, Tunder Hawk DeepDraſt semisubmersible floating production unit (FPU) moored at 1800m (6050ſt), and the Perdido Spar at 2438m (8000ft), are part of the growing trend in deepwater and ultra-deepwater production. Deepwater mooring lines can be either
polyester rope or spiral wire. Wire and chain mooring systems rely on the weight of the mooring lines to hold the surface production unit on station. However, polyester rope taut-leg mooring systems use the elasticity of the rope to provide the restoring force needed to hold the Spar, for example, in place. Te higher the elasticity the greater the line will stretch and absorb higher dynamic loads. Generally polyester rope is preferred where marine architects are looking for a ‘softer’ mooring. The following Thunder Hawk FPU mooring case study illustrates a typical deepwater mooring scenario.
Polyester rope mooring The Thunder Hawk DeepDraft Semi in the Tunder Hawk field is located 240km south-east of New Orleans. Designed and constructed by SBM Atlantia, the Tunder Hawk FPU has a process capacity of 45,000bopd and 70mmcf/d of gas. Murphy Exploration & Production Company is the field operator and operates the Thunder Hawk FPU.
Thunder Hawk showing the underwater mooring lines (Credit: SBM Atlantia Inc). The Thunder Hawk semisubmersible
FPU is moored in Mississippi Canyon Block 736, in a water depth of 1847m (6060ſt). It is anchored with a polyester rope-chain spread mooring system connected to 12 driven piles by Ballgrab ball and taper, subsea mooring connectors. Tunder Hawk’s mooring lines are arranged
in four clusters, one for each corner of the FPU, of three lines each (Figure 1). SBM Atlantia’s manager for the Mooring Installation, Engineering Group, Mark Slider, said: “We chose polyester rope mooring because of
Offshore Marine Technology 3rd Quarter 2010
the water depth and the type of platform. Te FPU motions will be better for the risers with polyester rope than with steel wire rope. Polyester gives us a ‘soſter’ mooring system than steel wire rope and consequently, the FPU motions are more compliant and riser friendly.” Lankhorst Ropes and mooring specialist
Offspring International, its worldwide sales agent, supplied the polyester rope and project managed its deployment. GAMA 98 polyester rope was selected to meet the SBM Atlantia requirement for a rope with a minimum
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