New fiber mooring lines for Modec

MODEC, a leading contractor in floating production, storage, and offloading (FPSO) vessels, faced a mighty challenge: how to convert the Cidade de Santos MV20 into an FPSO with a new mooring chain tensioning system?

Fact file: FPSO Cidade de Santos MV20

History: built 1973; converted 2008–9; first oil 2010
Operating sites: Uruguá and Tambaú fields, Brazil
Client: Petrobras
Purpose: collects and processes natural gas from the Tambaú field, then sends processed gas to the Mexilhão field, 170 km from Uruguá
Processing capacity: 350 million cubic feet of gas per day; 35,000 barrels of oil per day

Storage capacity: approximately 700,000 barrels
Did you know: first FPSO to process more gas than oil
Water depth: 1,300 meters
Mooring type: SOFEC spread mooring

With a spread mooring system in place on the vessel, MODEC decided on a centralized mooring winch fore and aft to pull and tension 24 mooring lines – replacing the usual configuration of a chain jack at each mooring cluster. In turn, the FPSO needed very strong ropes that could handle a small bending diameter. There were several reasons for this: first, it would be easier to route the rope and pull it through the underwater fairlead. Second, the mooring winch could be made smaller, saving space on deck. Finally, one winch would be able to do the whole pull-in at each end.

Wanted: A smaller bending diameter

Willie Ng, Sales Manager at 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 350 mT, the FPSO Cidade de Santos MV20 would need a wire rope with a diameter of approximately 96 mm and a minimum break load of 796 mT. With a minimum D/d ratio of 18 for low-rotation wire rope, the required bending diameter would be 1,728 mm. But there was a problem: the diameter of the FPSO’s underwater fairlead was only 1,500 mm.

The only solution: Dyneema®-based rope

Luckily, the PH team had an answer. What MODEC needed was a pull-in rope as strong as a steel wire rope, but with a bending diameter of under 1,500 mm, so it could be used with the existing underwater fairlead. And for that, it needed a synthetic rope made with Dyneema®, the world’s strongest fiber™.

Lighter and stronger than their steel wire rope equivalents, ropes made with our ultra-high-molecular-weight polyethylene (UHMWPE) can endure a much lower bending radius than the equivalent steel wire rope – making them the perfect solution for MODEC and the FPSO Cidade de Santos MV20.

The order went to Gaylin, who supplied PH and MODEC with a Samson 120 mm Quantum™ 12 rope made with DPX™ fiber technology. With an MBL of 887 mT, it shows much lower strength loss when bent than a comparable steel wire rope under the same conditions. It also passes winch rollers easily and is seven times lighter than its steel wire rope equivalent.

Checking all the boxes

Not only did the Dyneema®-based rope fulfil the fairlead and bending diameter specifications, but it also met all MODEC’s other criteria. It took up less deck space, needed just one winch at each end, and was much easier to handle and position on the winch’s rollers. What’s more, the braided rope is intrinsically torque neutral, meaning no twist in the mooring chains – an added extra greatly appreciated by the installation teams!

Standing up to the strain: Samson 120 mm Quantum™ 12 rope

MBL 887 mT
70 meters long
Made with Dyneema®
Includes Samson’s patented DPX™ technology
Hybrid PET/Dyneema® at rope strand surface
Greater friction helps hold rope in place and increase abrasion resistance

Why fiber mooring lines outperform steel

Steel wire rope is too rigid to handle and position easily on rollers when changing the line pass. Soft ropes – like those made with Dyneema® – are easy to guide wherever needed.

With steel wire rope, the anchor chain installation must be done with zero or near-zero twist induced in the chain. Samson Quantum™ 12 rope with Dyneema® is torque free.

In the case of the Cidade de Santos MV20, the minimum bending diameter for low-rotation wire rope under full workload (350 t) is 18 times the diameter of the wire rope. The size of the sheave and fairlead on rope travel pass is too small to accommodate a 96 mm wire rope.

When wire rope goes through the sheave and fairlead, the socket or bending tensioner at the termination point will suffer greater bending. This reduces safety and increases the risk that the mooring chain will drop to the seabed.