IEC61400-3-2 shows the conditions for safety assessment of FOWT (Floating Offshore Wind Turbines), DLC (Design Load Case). DLC is comprehensively defined for FOWTs, and designers are required to perform safety assessment for all DLC. However, mooring designers are required to design the mooring system in accordance with ISO19901-7, so it might be thought that the DLC does not match the design of the mooring system.
In this paper, the authors designed a mooring system for FOWT and analyzed the strength of the mooring system. From the results, we investigated the minimal configuration of DLC for mooring systems. We also proposed an efficient procedure for analyzing the strength of mooring.
The minimal configuration of DLC for mooring are DLC1.3, DLC6.1, DLC6.3, DLC7.1, DLC9.1, DLC9.2, DLC10.1 and DLC10.2. In addition, if the Fatigue Limit State is analyzed before the Ultimate Limit State is analyzed, the sea condition that maximizes the maximum tension of the mooring can be found which is efficient for design spiral.
Designers perform design and safety assessment of FOWTs (Floating Offshore Wind Turbine) based on IEC61400-3-2. IEC61400-3-2 provides the conditions for safety assessment of FOWTs, i.e., DLC (Design Load Case). DLC is comprehensively defined for FOWTs, and designers are required to perform safety assessment for all DLC. On the other hand, mooring designers are required to design the mooring system in accordance with ISO19901-7 in IEC document. However, the design requirements shown in ISO19901-7 do not cover FOWTs, and there is no concept of DLC. Considering the DLC described in the IEC document, the mooring designers are thought to be over-requested.
Demonstration and commercial FOWTs have been built in Japan. Then it is thought that the mooring designers designed the mooring system by the method described in ISO19901-7 based on all the DLC shown in IEC documents. It is said that the IEC is in the process of revising IEC61400-3-2 to correct this situation, but it has not been published yet.
At this stage, it would be useful to sort out the relationship between IEC61400-3-2 and ISO19901-7, and to clarify the DLC concept that matches the design of the mooring system.
