Small-Scale Testing of Damaged Synthetic Fiber Mooring Ropes

Abstract Synthetic fiber mooring rope is an important emerging technology helping enable the economical exploration and production of petroleum from offshore deepwater reservoirs. Polyester moorings have been used successfully for years in Brazil and were recently approved by the MMS for use by two MODU drilling operations in the Gulf of Mexico. One of the unknown issues associated with the use of synthetic fiber mooring ropes is how to account for damage and the associated lifetime and reliability predictions. The MMS has sponsored research to address the damage tolerance issue including the activity reported in the current paper. This paper describes the results of controlled, damage-tolerance, static tension tests on elements and subrope components taken directly from representative polyester mooring rope products. The results should serve as a foundation for further research into the behavior of ropes with damage including more sophisticated analytical modeling. Introduction Considerable emphasis is being placed worldwide on the economical recovery of petroleum resources from deepwater. The definition of deepwater has expanded in the last decade to now include depths as great as 10,000 feet and deeper. Many different concepts have been proposed for deepwater platforms including FPSO's SPAR's, and TLP's. Each concept depends on the use of a mooring line or tether to keep on station. The taut leg mooring line concept in which multiple light-weight synthetic fibers such as polyester are used as a rope suspended from the platform to the seabed at an angle of approximately 45-degrees from vertical is a very attractive candidate and the concept is growing in popularity. Petrobras has made a major commitment to this concept1 with over 1½ dozen platforms so anchored and there is considerable interest emerging in using the synthetic fiber mooring rope in other parts of the world including the Gulf of Mexico. The American Petroleum Institute has prepared guidelines for the design, manufacture, installation and maintenance of synthetic fiber mooring ropes2. The Minerals Management Service (MMS) recently approved the use of FPSO systems in central and western Gulf of Mexico regions3 and two MODU drilling platform have recently been given approval to use polyester mooring rope in Gulf of Mexico deepwater drilling programs. The MMS is interested in the durability of synthetic fiber mooring ropes and has been proactive in sponsoring research and testing to characterize handling and installation damage4 and to assess the effects of damage. Damage can result from handling the rope during installation, be the consequence of wear experienced during service, or be caused by ingress of sand or marine growth. The integrated program involves testing small-scale rope components (the focus of the current paper), analytical modeling, and a largescale test program. Results from the current study will be used in subsequent validation of an analytical model. The ultimate goal of the program is to develop guidelines for addressing safety and reliability issues associated with damage.