27.2. Design codes of mooring systems

A design follows the design standards and can be either Working Stress Design (WSD) or Load and Resistance Factor Design (LRFD). The main limit states to consider are the

• ULS – Ultimate Limit State, which defines the extreme design conditions

• SLS – Serviceability limit state, which describes operational design conditions

• FLS – Fatigue limit state, the conditions important for lifetime

• ALS – Accidental limit state, incidental conditions as described before

The calculation to check whether a structure complies is done using safety factors on strength versus the load or action. The characteristic demand (action) based on e.g. the 100 year wave condition needs to be factored such that it is lower that the factored characteristic strength.

These factors are defined in the various design codes and standards and depend on the consequence class. The consequence class is higher when the impact involves loss of human life, impact on adjacent structures or on the environment (e.g. oil spills). The overview shows a few of the examples of factors of safety in both classes. As an example the high impact case of consequence class 2 in DNV-ST-0119 for Floating Offshore Wind states an environmental safety factor of 1,55 for ULS as compared to 1,35 for class 1. The minimum fatigue FoS is larger than 10 for all mooring components with the corrosion allowance for 50% corroded, which represents halfway the lifetime.