Prediction of the long term behavior of synthetic mooring lines

Abstract

Abstract 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.