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Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

Consideration on DLC Used for Mooring System Design of Floating Wind Turbine
Masakatsu Saito, Toshiki Chujo
National Maritime Research Institute, JapanTokyo, JapanAt this stage, it would be useful to sort out the relationship between
IEC61400-3-2 and ISO19901-7, and to clarify the DLC concept thatmatches the design of the mooring system.
As mentioned above, IEC61400-3-2 requires designers to perform
safety assessment of mooring systems in accordance with ISO 19901-7.
Therefore, it is necessary to perform the safety assessment shown in
Table 1 specified in ISO19901-7. Here, Intact, Damaged, and Transient
indicate the conditions of the mooring line. The lower limit of thesafety factors depends on the line condition (undamaged, damaged, ortransient) and the analysis method (static or dynamic).

ABSTRACT
IEC61400-3-2 shows the conditions for safety assessment of FOWT
(Floating Offshore Wind Turbines), DLC (Design Load Case). DLC iscomprehensively defined for FOWTs, and designers are required toperform 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 thedesign of the mooring system.
In this paper, the authors designed a mooring system for FOWT andanalyzed the strength of the mooring system. From the results, weinvestigated the minimal configuration of DLC for mooring systems.
We also proposed an efficient procedure for analyzing the strength ofmooring.
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. Inaddition, if the Fatigue Limit State is analyzed before the UltimateLimit State is analyzed, the sea condition that maximizes the maximum
tension of the mooring can be found which is efficient for design spiral.

Table 1 Analysis required by ISO19901-7.
Limit State
Ultimate Limit State
Fatigue Limit State

Physical Quantity
Max tension in intact condition
Max tension in damaged condition
Max tension in transient condition
Fatigue life

The DLC defines the environmental conditions and turbine operating
conditions for performing safety assessment of these offshore windturbines. Table 2 shows all the DLCs currently shown in the IEC
documentation.

KEY WORDS: Floating; Wind; Turbine; DLC; Mooring; Line;
Ultimate Limit State.

Table 2 All DLCs shown in the IEC documentations.

INTRODUCTION
Design Situation
Power production
Power production plus occurrence of fault
Start up
Normal shut down
Emergency stop
Parked (standing still or idling)
Parked and fault conditions
Transport, assembly, maintenance and repair
Power production
Parked (standing still or idling)

Designers perform design and safety assessment of FOWTs (FloatingOffshore 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, anddesigners are required to perform safety assessment for all DLC. On theother hand, mooring designers are required to design the mooring
system in accordance with ISO19901-7 in IEC document. However, thedesign requirements shown in ISO19901-7 do not cover FOWTs, andthere 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 systemby 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.

DLC
1.1 ~ 1.6
2.1 ~ 2.6
3.1 ~ 3.3
4.1 ~ 4.3
5.1
6.1 ~ 6.4
7.1 ~ 7.2
8.1 ~ 8.49.1 ~ 9.3
10.1 ~ 10.3

DLC2.6, DLC4.3, DLC9.1-9.3, DLC10.1-10.3 are shown in IEC614003-2 for Floating Offshore Wind Turbine. Other DLCs are shown inIEC61400-3-1 as Offshore Wind Turbine general purpose. As262