Permanent mooring systems can have a lifetime span of 20 to 30 years and Floating Production System (FPS) operators often face mooring system integrity issues including mooring line failure.
Maritime history show that mooring systems fail in a wide range of water depths, regions and environmental conditions (mild to severe sea states). Consequences of mooring failure can be very serious from asset damage and production interruption to environmental issues and even personnel loss. When it comes to mooring line failure, several elements can be responsible, from basic wear-and-tear and corrosion to integral production defects. These include, but are not limited to:
1 – Wear: Wear can occur when the mooring line rubs on adjacent line components at connecting links, fairleaders, bending shoes, etc.
2 – Fatigue damage: Fatigue damage can be due to crack initiation and propagation from repetitive axial and bending stress.
3 – Abrasion: When chain gets in contact with the seabed, sediments can be abrasive and friction can erode the chain.
4 – Corrosion: Rust and corrosion can result from chemical reactions between the material and the surrounding environment. Growth of marine life (algae, barnacles…) can further contribute to the need to replace the mooring line in order to avoid failure.
5 – Damage: Chain, wire rope or polyester rope can get damaged during installation operations or inspection operations, as well as from dropped objects, and other external events.
6 – Flawed materials: Impurities in the materials, improper heat treatment, improper or non-compliant assembly, or poor coating or lubrication are all reasons that can lead to a weaker material, and consequently to failure.
7 – Excessive tension: Mooring lines should be inspected after exposure to severe environmental conditions (storms, hurricanes…) or to extreme loads.