Session: 09-01-10 Wind Energy: Moorings 2

Paper Number: 130846

130846 - Nylon Ropes for Mooring of Floating Offshore Wind Turbines – the Nymoor Project 

Mooring of floating offshore wind turbines (FOWTs) is challenging with the designs showing line components with quite large diameters. The relatively small water depth catenary (say 60 - 200 m water depth) combined with large mean wind thrust loads stretch the windward lines. This increases the horizontal restoring stiffness significantly, and the mooring system loses compliance with the wave frequency motions as lines become almost taut. Consequently, the lines are subjected to large dynamic load cycles under wave frequency excitation, and large diameter chain segments are required to accommodate fatigue damage. The high horizontal restoring stiffness also gives large extreme line loads.

Nylon ropes have the potential to alleviate the above problem due to its axial flexibility. However, there is very little experience with use of nylon for permanent mooring of floating structures. Of particular concern is the long-term endurance and behaviour of nylon ropes in the ocean environment. Furthermore, new models are required for the mechanical properties of the nylon ropes, and design analysis tools must be updated to accommodate the new models.

The paper starts by presenting the state-of-the-art in the topic of nylon ropes with the focus on mooring applications. It reviews and documents some of advantages of nylon ropes for mooring of FOWTs with results from two case studies. It also identifies knowledge gaps. Then it describes the NYMOOR research project, which addresses some of the knowledge and technological gaps. We start with the project objectives. The assumed hypotheses are formulated based on the best knowledge in the topic. Then the project methodology and the research activities are described in detail. The methodology follows three main steps: a) perform extensive testing of nylon yarns and sub-ropes and combine with mathematical/numerical material and structural modelling to identify mechanical properties, degradation mechanisms, failure modes, and long-term performance of nylon ropes; b) based on the test results, develop and implement a numerical model for creep and stiffness behaviour of nylon ropes appropriate for design analysis tools; c) Propose a procedure based on structural reliability to quantify the life-time probability of failure of nylon mooring ropes and the factors contribution most to it.

Presenting Author: Nuno Fonseca Sintef Norway

Presenting Author Biography: Naval Architecture degree from the University of Lisbon, 1991
MSc from the University of Glasgow, 1993
PhD from the University of Lisbon, 2001
Assistant Professor at the University of Lisbon until 2013.
Senior Scientist at SINTEF Ocean 2014-2022
Chief Scientist at SINTEF Ocean from 2022

Authors:

Nuno Fonseca Sintef Norway
Heidi Moe Føre SINTEF Ocean
Stian Høegh Sørum SINTEF Ocean
Rune Gaarder SINTEF Industry
Bernt Johan Leira NTNU
Rui Pedro Faria Bridon-Bekaert
Michael Kent Bridon-Bekaert