Session: Mooring

Paper Number: 163869

163869 - Progress in Predicting Mooring Rope Responses 

Abstract: 

The development of synthetic fibre mooring ropes was primarily driven by the oil and gas (O&G) industry out of necessity; at deeper water depths there are significant weight and cost penalties if steel components are used exclusively. Whilst synthetic ropes are potentially an enabling technology for the FOWT sector, this is a new area of application which features different loading regimes. Consequently existing design and certification practices derived for the O&G and maritime sectors may not be directly applicable. Unlike steel components, synthetic ropes typically display complex viscoelastic (recoverable) and viscoplastic (permanent) behaviour in-use, which is influenced by the base material, rope construction, as well as previous loading history. An understanding of how these components are likely to perform in service is crucial for predicting mooring system performance and asset integrity. This requires a combination of laboratory testing, field measurements and numerical analysis.

Over the last two decades considerable effort has been expended in developing techniques to predict short- and long-term rope performance, ranging from the curve fitting of test data and the development of constitutive models through to the use of highly detailed finite element models. The appropriateness of these methods depends on level of detail required and computational constraints. As yet, no universal modelling approach exists which can adequately predict the time-varying response of ropes subjected to the stochastic loading regimes which are likely to be experienced by FOWT mooring systems. Uncertainties therefore exist into how best to represent rope properties (such as axial stiffness and hysteretic damping) in mooring system models, particularly for systems comprising Polyester or Nylon ropes and the subsequent impact on other key system metrics such as platform motions and nacelle accelerations.

Leveraging a large dataset of measured rope responses, this paper will provide an overview of an alternative time domain modelling approach which aims to represent the complex behaviour of synthetic ropes in order to allow the prediction of mooring loads and device responses to an increased level of accuracy. The prediction capabilities of the approach will be assessed using measured tension-tension data featuring load cases relevant to FOWT systems. Potential methods to incorporating this approach in commercial mooring system software will also be discussed.

Presenting Author: Sam Weller Tension Technology International Ltd

Presenting Author Biography: Dr Sam Weller is a Mechanical Engineer with 17 years’ experience in the offshore renewable energy sector. He has extensive knowledge base including mechanical, data acquisition, power capture, material and hydrodynamic aspects of wave, tidal and floating wind devices. He also has expertise in experimental and numerical R&D of novel technologies. His current focus is on modelling highly dynamic offshore structures and subsea components including complex synthetic rope behaviours. He is an author of 38 peer-reviewed conference and journal papers, is an IEC TC 114 maintenance team member and technical advisory committee member for two NREL floating wind mooring projects.

Authors:

Sam D. Weller Tension Technology International Ltd
Stephen J. Banfield Tension Technology International Ltd