MODEC, a leading contractor in FPSO vessels, faced a challenge. The company was converting a vessel into an FPSO (floating production, storage and offloading) and as part of this wanted to equip the FPSO Cidade de Santos MV20, with a new mooring chain tensioning system. Stay up-to-date with industry news, sign up for our newsletter
There were several reasons for choosing this design. It would be easier to route the rope and pull through the underwater fairlead, which had a very limited bending radius. The mooring winch could be made smaller, so claiming less deck space. And one winch could do the whole pull-in.
MODEC wanted a centralized mooring winch fore and aft to pull and tension 24 mooring lines. These would replace the usual configuration of a chain jack at each mooring cluster. The FPSO Cidade de Santos MV20 uses a Spread Mooring System.
The PH 350T/230T mooring winch used by the Cidade de Santos MV20 under tension. The high-strength Samson 120 mm Quantum™ 12 rope made with Dyneema® is lightweight, and its patented Samson DPX™ fiber technology provides superior abrasion and cut resistance, but with a higher coefficient of friction than other high modulus polyethylene ropes.
Willie Ng, sales manager of PH Hydraulics & Engineering, one of Asia's leading manufacturers of FPSO deck equipment and a long-time supplier to MODEC, calculated that with a pull force of 350mT, the FPSO Cidade de Santos MV20 would need a wire rope with a diameter of approximately 96mm, and a minimum break load of 796mT. With a minimum D/d ratio of 18 for low rotation wire rope, the required bending diameter would be 1728mm. However the diameter of the FPSO Cidade de Santos MV20’s underwater fairlead was only 1500mm.
PH had a solution, though. What MODEC needed was a pull-in rope that would be as strong as a steel wire rope, but with a bending diameter of under 1500mm, so it could be used with the existing underwater fairlead. There was only one answer: a synthetic rope made with Dyneema®. These are lighter and stronger than their steel wire rope equivalents and – crucially, for MODEC and the FPSO Cidade de Santos MV20 project – they can endure a much lower bending radius than the equivalent steel wire rope.
The order went to Gaylin, which supplied PH and MODEC with a Samson 120 mm Quantum™ 12 rope made with DPX™ fiber technology, this has an MBL of 887 mT and shows much lower strength-loss when bent than a comparable steel wire rope under the same conditions.
The FPSO Cidade de Santos MV20’s Samson 120mm Quantum™ 12 rope has an MBL of 887mT and shows much lower strength-loss when bent than a comparable steel wire rope. It also passes winch rollers easily and is seven times lighter than its steel wire rope equivalent.
As well as meeting the Fairlead/ bending diameter criteria, the rope also met all the other specifications. It took up less deck space, one winch could do the job at each end, and the rope was much easier to handle and position on the winch’s rollers. It was one-seventh of the weight of a similar-strength steel wire rope.
In addition, the braided rope is intrinsically torque neutral, so it will not induce any twist in the mooring chains. This additional effect is highly appreciated by the installation companies. All boxes checked.
The FPSO Cidade de Santos MV20’s 114mm mooring chain is held under tension by the Samson 120mm Quantum™ 12 rope, made with Dyneema®.
The Samson 120mm Quantum™ 12 rope, made with Dyneema®, is slightly abraded after completion of two-thirds of the on-site tensioning operation. This generally doesn’t reduce the strength of the rope, and the surface fluffing it has developed will actually protect the rope from further abrasion.
Tensioning mooring chains with steel wire rope raises a number of issues:
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