This paper presents a study of the use of synthetic fibre mooring lines for deep-sea applications. Three materials are considered: polyester, aramid and HMPE. First, results from a test program completed recently are described. These tests on full size ropes, which include novel extensometric methods based on image analysis, have enabled the influence of mean load, load range and loading frequency on stiffness to be determined for the three materials. Ropes up to 750 tons MBL (minimum break load) have been tested. A comprehensive data set of this type has not been available previously. These data are then used in mooring system analyses, to evaluate a number of design cases for the station keeping of exploration and production platforms. This has allowed a better understanding of the domains of application of these different materials and of related issues for design. Materials research needs have also been clarified. The minimum tension criterion for HMPE and aramid fibre ropes is shown to be one of the critical technical parameters that should be further investigated in order to extend the further application of these materials for taut mooring systems design. Attention should also be given to fatigue analysis for the aramid and HMPE lines.

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.

This paper describes work performed within a Joint Industry Project aiming to evaluate the lifetime of deep sea handling ropes. Various HMPE (High Modulus Polyethylene) fiber ropes, with and without coatings, have been studied under both tensile and cyclic bend over sheave (CBOS) loading. A large test program has enabled both tension-cycle to failure relationships and empirical expressions for residual strength after cycling to be determined. A special device was then developed to apply a known couple to the sheave, allowing both dynamic friction measurements to be made and the influence of applied couple on cycles to failure to be measured. These experimental data were used in the development of a numerical model which can be used to study the influence of rope and sheave parameters.

a b s t r a c tRepeated bending over sheaves is one of the main causes of failure of synthetic fibre ropes used in marine operations. There are few published results available and even fewer models allowing lifetime to be estimated. A large new set of data from cyclic bend over sheave (CBOS) tests on 250 kN break load braided HMPE synthetic ropes is presented first, both tests to failure and interrupted tests followed by residual strength measurements. These data are analysed in order to propose an empirical lifetime model. This is identified using constant load tests, then evaluated for variable load sequences. A methodology to include rope lifetime prediction in handling system design is then discussed.

The stiffness and short term behavior of polyester mooring lines are now reasonably well understood and these materials are finding many applications offshore. Much less is known about the long term behavior of this and other synthetic materials (aramids, HMPE). This paper presents results from creep and cyclic loading tests on these three materials at different scales, from single fiber up to ropes. The behavior of aramid and polyester is compared with the predictions of a viscoelastic-viscoplastic model which includes bedding-in effects. For HMPE lines it is creep which dominates line design, and data are compared to an existing creep model. New material developments which significantly improve creep performance of HMPE-based ropes are presented. Introduction The last five years have seen much development of synthetic ropes for deep sea mooring lines. Polyester has been extensively used offshore Brazil 1. Other installations off West Africa, and more recently the first applications in the Gulf of Mexico, have confirmed the potential of this material. Permanent moorings require a guaranteed lifetime of at least 20 years, so an in-depth understanding of the long term behavior of the polymer fibers which make up the synthetic components is essential. The loading types of interest are creep (long term constant load), recovery (response after unloading), and cyclic loading at periods in the range from 5 to 500 seconds. The factors which determine the lifetime of a synthetic rope are the global response, the strain level achieved at a given time and its magnitude with respect to failure strain, and local response due to factors such as abrasion, fatigue, accidental damage and defects. This paper will concentrate on the global response, and the development of models which allow global strain to be determined at any time as a function of loading history will first be discussed. Some recent material improvements will then be presented. The paper concludes with some discussion of local failure mechanisms and work currently underway. Materials There are two materials currently attracting interest for deep sea mooring lines, polyester and HMPE (high modulus polyethylene). However, ropes based on several other fibers also offer attractive properties, including aramids (Kevlar, Twaron), improved polyesters (Pentex) and liquid crystal polymers such as Vectran and PBO. Although the latter are relatively expensive at present compared to polyester the material costs may only represent a small part of the overall costs to be considered in the installation and operation of a floating platform. It was therefore considered premature to exclude technically interesting fibers, and Table 1 shows the materials which have been examined in the current study.