Modelling fiber rope load-elongation properties - Polyester and other fibers
Authors: Francois Michel , Davies Peter , Grosjean Francois , Legerstee Franck 
Published: July 2010

Polyester fibre ropes are today widely used as mooring lines for floating offshore platforms, but other materials are now also proposed. Fibre ropes are also extensively used in other marine applications, such as SPM hawsers, where Polyamide (Nylon) is the preferred material.

Evaluating the response of the system requires a description of the load-elongation properties of the rope. This has been the subject of extensive investigations over the last 12 years, during which testing methodologies were developed, and applied to a wide range of products. The primary aims were to understand the response of these fibre ropes in the loading regime specific to these applications, and to provide pertinent data for design of the systems.

This paper presents an overview of the testing practices and describes the practical model that has been developed, initially for polyester ropes. Indeed this model was found well suited to describe the behaviour of ropes made from most fibres, based on both the traditional multi-filament yarns and more recently for new products using monofilaments, with different rope constructions. As to Nylon ropes, recent work has highlighted a somewhat more complex behaviour than other materials.

Context

Fibre ropes are extensively used in many marine applications. One critical area of interest is their application as mooring (anchoring) lines, for the station-keeping of deep-water floating offshore platforms. Following a development period, and more than twelve years after the first installations of floating production systems by Petrobras in Brazil, this technology has reached a stage of maturity: fibre rope station keeping systems are now employed commonly offshore Brazil, and also in other regions around the world (in the Gulf of Mexico, off West Africa, …). As to materials, polyester is primarily used in this application. Evaluating the response of these systems, and then the adequacy of maximum offset and line tensions with the relevant acceptance criteria, requires a description of the load-elongation properties of the rope. However, these properties are rather complex to evaluate and specify, in comparison with the linear elastic behaviour of equivalent steel components, as they are both non-linear and time dependent. Besides, the working conditions and loading regimes of a rope in an anchoring line are quite specific with respect to those in more common applications of fibre ropes.

From 1996 to 2007, a group of French research institutes and companies worked on synthetic fibre ropes for offshore applications. Some of this work has been published at previous OTC conferences (see references at end), and the extensive testing performed by IFREMER and IFP within those projects resulted in the development of a polyester rope model. This will be described below.

In parallel various other materials were studied, including ropes based on high performance fibres such as High Modulus Polyethylene (HMPE), Aramids, Liquid Crystal Polymer (LCP) and mixtures of these, and stiffness models were also developed for these materials. Then, very recently, monofilament polyester ropes became available. These may offer an alternative to multi-filament polyester in some applications, and they have also been characterised. Finally, a Joint Industry Project OHP (Offloading Hawser Properties) performed between 2007 and 2009 enabled the model to be extended to nylon ropes used in hawsers for single point moorings. The present paper describes these studies of different ropes and shows the applicability and limitations of a practical stiffness model by which the rather complex - non-linear and time dependent- behaviour of polymer fibres can be addressed in mooring analyses.