First Class Certification on HMPE Fiber Ropes for Permanent Floating Wind Turbine Mooring System

Abstract

High Modulus Polyethylene (HMPE) fiber ropes have been widely employed in a large variety of marine and onshore load bearing applications, but not yet in permanent offshore mooring systems due to their well-known high creep behavior. The Lankhorst Gama98® HMPE mooring ropes based on low creep Dyneema® DM20 fiber were recently introduced and certified by ABS for the offshore WindFloat Atlantic project's mooring system at a depth of around 100m, which is the first time such HMPE fiber ropes have been used for permanent mooring. Compared with the conventional application of fiber ropes, usually employed as middle segments of mooring lines, the application of HMPE ropes as top segments directly connected with the platform in shallow water creates more challenges from the impact of temperature, UV and marine growth. For the permanent mooring of WindFloat Atlantic, the HMPE mooring rope was incorporated in the catenary mooring system by Principle Power to transfer the restoring force generated by the bottom chain weight in an efficient and direct manner to the floating platform. This lightweight mooring line enables lower pre-tensions while maintaining the same station keeping performance and excellent fatigue life. This ultimately drives down the pulling capacity requirements for installation of the mooring system and hook-up of the platform. Lessons learned from this application could result in a more efficient approach to HMPE mooring line installation. With such groundbreaking application, the HMPE rope is being demonstrated as a permanent mooring component and its performance assessed in this wind energy project, which may benefit the offshore wind industry. It may also be useful to oil and gas in a permanent use application context. This paper presents (i) the ABS process details in assessing the proposed HMPE ropes with DM20 low creep fiber for meeting class requirements and WindFloat Atlantic project specific needs, and (ii) address the offshore industry concerns with HMPE ropes such as creep, creep rupture, UV effect, and marine growth.