The goal was to develop a feasible in situ method for determining the structural health of synthetic fiber mooring ropes (SFMRs) used to anchor floating oil and gas production platforms in deepwater operations. This method would form the basis of a monitoring system that provides increased safety and reliability during the performance lifetimes of installed SFMR systems (see schematic).
This project was funded through DOE's Natural Gas and Oil Technology Partnership (NGOTP) Program. The Partnership Program establishes alliances that combine the resources and experience of the nation's petroleum industry with the capabilities of the national laboratories to expedite research, development, and demonstration of advanced technologies for improved natural gas and oil recovery.
Oak Ridge National Laboratory (ONRL)
Oak Ridge, TN
The Woodlands, TX
The ultimate strain of SFMRs is 3-8%. Test data indicate that the strain in SFMR at failure is essentially independent of load path or history. Measurement of the accumulated strain in the rope thus should provide a reliable benchmark with which to establish criteria for rope re-certification or retirement. Environmental conditions associated with hurricanes and loop currents are more severe in the GOM than elsewhere, and the Minerals Management Service is requiring complicated inspection procedures to ensure their reliability. These inspection procedures involve inserting short test sections in the upper portion of the mooring rope, with periodic retrievals of the test sections for ultimate strength testing. The removal procedure is expensive and introduces undesirable operational risks, including damage to the rope. Furthermore, claims that terminations of the short test sections yield the weakest links in the rope assembly have technically valid justifications.
A method for direct measurement of the large strains typically experienced by SFMRs in offshore operations using large-strain capability plastic optical fibers (POFs) and Optical Time-Domain Reflectometry (OTDR) methodology was developed and demonstrated. The technique is a very practical method for accurately measuring the strain and accumulated strain growth in an SFMR over time, and it can provide the data necessary to apply maximum strain criteria to assess the structural health of the rope periodically and following events such as storms or large loop currents.
The method incorporates the use of POFs as strain sensors. Several formulations of POF were investigated to determine the one most suitable for the application. A POF made from perfluorocarbons was found to provide the most merit for the application, having the properties of low attenuation (light intensity loss), large strain capability (10% or greater), and elastic properties similar to the polyester rope during cyclical loading. A method for inserting reflective interfaces in the fibers also was demonstrated. The insertion of these interfaces enables gauge lengths of 1 to 10s of meters to be created at arbitrarily selected positions along the fiber. POFs have large strain capability, but they are susceptible to local bending and environmental degradation. To compensate for this susceptibility, a method for encapsulating the fiber in a subrope within the mooring rope was demonstrated, and methods for obtaining fiber ingress/egress from the mooring rope were evaluated. The method for protecting the fiber is being refined into a practical system, including integration of protective shielding during manufacturing.
Results of laboratory tests comparing the applied strain in a small rope element with the strain measured using an embedded POF for the first five cycles of loading are presented (see chart). Loading was applied in a stroke control mode, and the maximum strain was about 3% strain in the first cycle but reduced to about 1.8% in subsequent cycles due to termination seating and relaxation of the polyester. Excellent correlation was recorded between the applied strain and the strain measured using the POF and OTDR technique.
Whitehill Manufacturing Corporation, a partner in this NGOTP project, supplied the polyester mooring rope.
The use of SFMRs on offshore drilling and production platforms in ultradeep water is an important technological advancement. SFMR makes possible significant weight savings, lower cost, and reduced interference with adjacent property development via deployment in the taut-leg configuration. The initial use of SFMR was by Petrobras in Brazil, and polyester mooring ropes are now planned for deployment in the Gulf of Mexico (GOM). Kerr-McGee soon will be the first oil company to use polyester mooring ropes for station-keeping on a GOM production platform (Red Hawk, located in 5,300 feet of water). Numerous discoveries have been made in even deeper water, and many more synthetic fiber mooring rope installations are expected for both drilling and production.
Among the major project accomplishments, researchers:
DOE funding terminates in September 2005. Additional R&D effort is needed to leverage the promising laboratory results into a practical offshore strain monitoring system. For example, effort is needed to address related factors, including the long-term effects of the environment and extended cyclical loading, and laboratory and field tests are needed to measure the response of polymeric optical fibers integrated into the body of representative synthetic fiber mooring ropes.
Recent hurricanes and resulting damage to GOM platform moorings have intensified interest in providing improved structural-health monitoring methods. The investigators are working with SFMR manufacturers and oil companies on ways to introduce the technology into offshore operations.
Smith, D. Barton, and Williams, Jerry G., Direct Measurement of Large Strains in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers, paper 14242, Offshore Technology Conference, Houston, May 6-9, 2002.
Smith, D. Barton, and Williams, Jerry G., Monitoring Axial Strain In Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers, Paper OMAE2003-37402, Proceedings of the 22nd International Conference on Offshore Mechanics and Arctic Engineering, Cancun, Mexico, June 8-13, 2003.
Williams, Jerry G., and Smith, Barton D., Direct Measurement of Axial Strain in Synthetic Fiber Mooring Ropes Using Polymeric Optical Fibers, 4th International Conference on Composite Materials and Structures for Offshore Operations, October 4-6, 2005 (in preparation).
U.S. Patent (publication pending), Williams, Jerry Gene, Smith, David Barton, and Muhs, Jeffrey David, Measurement of Large Strains In Ropes Using Plastic Optical Fibers.
Industry Contributions: $42,000 (8.4% of total)