Seabed Contact

Seabed contact has historically been a challenge for synthetic fibre ropes in mooring systems. Offshore standards, such as API RP 2SM and DNV-OS-E303, state that previously tensioned synthetic ropes should not come into contact with the seabed during operation. This can increase the likelihood of particle ingress, which can lead to a major reduction in rope life from internal abrasion.

A method of rope protection, is the use of a high performance particle filter layer, such as that found in Bekaert's MoorLine. The filter layer is wrapped around the load bearing sub-ropes and provides up to 5 micron particle ingress protection.

When the mooring analysis determines the potential of seabed contact, while in service, another approach has been to include buoyancy elements. These buoyancy modules provide the necessary uplift to keep the synthetic rope away from the seabed during operation. Therefore fulfilling the offshore standards.

Traditionally, external buoys are added to the mooring system. While this is the most common approach it can increase the complexity of the mooring line, as more components are necessary. Therefore, the cost of the mooring system, as well as a risk of mechanical failure via the additional components is increased.

Bekaert can provide a range of external buoyancy options from their network of supply-chain partners. And, to further compliment this offer, can also manufacture ropes with integrated buoyancy. Integral buoyancy eliminates the necessity of additional components and reduces the complexity and cost of the overall mooring system.

Marine Fouling

It is expected that many floating offshore wind turbines will operate in shallow water areas. As a result of being quite close to the sea surface, the mooring ropes will be likely exposed to marine fouling. This marine growth leads to additional weight and changes on the rope’s hydrodynamics, that will affect the mooring rope behaviour.

This is particularly relevant for systems with submerged buoys in equilibrium. Or TLPs that need to account for the drag and vortex induced vibrations created by the mooring lines.

Additionally, there is the concern that hard shell and other marine life, could migrate into the rope core. To the best of our knowledge internal marine fouling is unlikely due to the absence of light, nutrients and due to high internal loads.

Cut Resistance

Experience from the oil and gas industry indicate that many offshore wind farms will have strong fishing lobbies. The fishing lobbies will request that the mooring lines are trawlable for fishing vessels. Insufficient safety perimeters, combined with the potential of 'corner crossing', increase the risk of cutting damage to mooring lines.

A level of abrasion resistance is offered as part of Bekaert's MoorLine, at the rope is constructed with a braided jacket. The jacket is designed in accordance with ISO 18692.

Above and Beyond

Bekaert can also provide additional protection solutions via a portfolio of enhanced coating solutions. Perfect, for environments where seabed contact, marine fouling or cutting damage is of particular concern.

Protection for mooring lines continue to be innovated from our expertise in material science and coating technologies. These coatings offer enhanced benefits to abrasion, tear and impact resistance, compared to a braided jacket alone.

The advanced composite structure combines the known characteristics of a braided jacket with an elastomeric matrix. This additional layer provides even greater abrasion and tear resistance. As well as well as being chemically inert while having high elongation properties. It will also provide a platform to additional cutting protection and will enable for cleaning of synthetic mooring lines.