Direct Measurement of Large Strains in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers
Authors: Smith D. Barton , Williams Jerry G. 
Published: October 2008

Synthetic Fiber Mooring Rope constructed from high performance polyester is an important emerging technology which promises to advance the economical production of oil and gas from deepwater reservoirs in the Gulf of Mexico (GOM) and elsewhere around the world. Considerable effort has been focused in recent years to develop the technology and confidence to apply synthetic fiber rope technology in a relatively new application in which the material is expected to carry load for decades in the marine environment. Synthetic fiber mooring ropes have been deployed successfully in Brazil and have seen limited service in the Gulf of Mexico. Lightweight synthetic fiber mooring ropes have high strength and adequate stiffness, but are much more susceptible to damage than their steel counterpart. Close monitoring of their performance is, therefore, a necessary requirement to insure their continued safety and reliability. The current paper discusses ongoing research activity to explore the feasibility of using polymeric optical fibers to measure directly large axial strain in long lengths of mooring rope. The strains measured in polymeric optical fibers exhibit good one-to-one correlation with applied strains within the test range studied (10% or less, typically). Additional efforts are in progress to assess other factors including the effects of the environment, creep, and cyclic loading, and measurement of the response of polymeric optical fibers integrated into the body of a synthetic fiber mooring rope. Measurement of the accumulated strain in the rope at any point in time could provide a reliable benchmark with which to estimate the remaining mooring rope life and help establish criteria for rope re-certification or retirement. Future safe deployment of the synthetic fiber moorings will be significantly enhanced if a reliable technique can be developed to monitor their performance insitu in service.

Introduction

Rope technology in which readily available organic fiber material is spun and twisted to form a rope was developed several millennia ago. The Persians were able to spin huge ropes two feet in circumference and a mile long to build an invasion bridge across the Hellespont to Greece1. Modern synthetic fiber rope began with the development of nylon in 1938 followed later with the introduction of other fibers such as polyester. In the last two decades, with the discovery of large oil deposits in deepwater offshore reservoirs, a special application for synthetic fibers has developed as mooring rope for offshore platforms.

The traditional mooring rope system uses a catenary steel chain or rope in which the restoring force for station keeping is developed by the weight of the steel mooring. The weight of the steel lines in water must, of course, be supported by thebuoyancy of the platform. As the water depth increases, weight becomes a serious platform design issue and lightweight synthetic fiber mooring ropes have gained considerable attention as an alternative system. Alternative systems for positioning floating platforms are shown in Figure 1. Petrobras has used polyester mooring ropes for all their recent moored platforms2 and two MODU drilling rigs were recently given approval to use polyester moorings in Gulf of Mexico operations3