Twisting Effect on Deepwater Moorings

It is a golden rule of deepwater mooring that twisting the mooring line during installation should be avoided. The reason is simple; twisting increases the risk of abrasion between sub-ropes leading to premature failure of the mooring line. In reality, some degree of twist is likely to occur during rope manufacture, pre-lay and hook-up. The question is: how much twist is acceptable? Until now this remained an open question. What is the short and long-term impact of twisting the rope? Researchers at Lankhorst Euronete decided to investigate. At last month’s OTC they reported their findings in a paper entitled: The Twisting Effect on the Polyester Ropes during the Deep-Water Mooring Installation (OTC-35343-MS). Special thanks to Leandro Haach, Synthetics Innovation Director, Lankhorst Euronete Portugal for presenting the paper, and co-authors José Canedo, Alexandre Tomazi and Eric Duarte.

The primary recommendation remains - avoid rope twisting during installation, pre-laying and hook-up. In addition, the paper identifies some potential causes of twisting, and suggests improvements in installation procedures. These include preventing twisting or damage to pre-laid tethers on the seabed that could potentially allow particles to penetrate the rope core, and considering the use of a torque-neutral rope for the rope installation winches.

The benefit of including an indelible stripe along the full length of the tether rope is also identified as providing a useful visual guide for ROV inspection after the mooring system installation is completed.

The value of involving the rope manufacturer in planning and implementing mooring installations is not to be under-rated. Most manufacturers have specialised installation teams that take precautions to prevent twists and damages during rope handling. If necessary, these teams can also conduct repairs using the correct materials and procedures.

Which brings us to the final point, if in doubt about any aspect of deepwater mooring be sure to consult the rope manufacturer.

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