Synthetic Mooring Lines for Depths to 3000 Meters

Abstract

Abstract 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. Introduction As water depth increases, synthetic fibre ropes become increasingly attractive candidates for station keeping. In spite of some initial difficulties with aramid ropes 20 years ago1, over the last five years polyester has become an accepted alternative to steel for this application. The extensive studies and pioneering applications of Petrobras have led to increased confidence 2–4 and this is the preferred option for depths down to 1500 meters. However, with the numerous discoveries of large reserves in deeper water the question of whether polyester ropes can be used at all depths has been raised. Recently polyester was used in field trials down to 1900 meters depth5 but there are few guidelines concerning alternative materials. Chaplin & Del Vecchio compared polyester ropes to steel at depths of 500, 1000 and 2000 meters6. Cloos & Bosman compared nylon, polyester, and aramid ropes at depths to 3000 meters and concluded that there is a minimum depth for each material, that for aramid being 1000 meters7. One aspect to be considered is the size and weight of large polyester ropes, and the implications for handling equipment and installation procedures. Figure 1 shows a comparison of rope sizes as a function of MBL (minimum break load). A 1000 ton MBL polyester rope has a diameter around 200 mm, and, with the several kilometer lengths to be deployed, this may exceed the storage capacity of anchor handling vessels. Figure 1 also shows that aramid and HMPE ropes are much closer to traditional steel wire in diameter. The equivalent polyester line will require two to three times the storage capacity that is needed for the other materials.