Oil & Natural Gas Projects
Exploration and Production Technologies
Life Performance Monitoring of Synthetic Fiber Mooring Ropes Using Polymeric
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.
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).
Oak Ridge National Laboratory (ONRL)
Oak Ridge, TN
The Woodlands, TX
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
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.
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
Among the major project accomplishments, researchers:
- Developed a method for direct measurement of large strains using POFs
- Identified affordable POFs with low attenuation, large strain capability,
and constitutive properties similar to the polyester fiber during cyclical loading.
- Developed means to impose the same strain in the POF as experienced by
the synthetic fiber rope into which it is placed, as well as methods to protect
the optical fiber against damage.
- Developed a method to place reflective interfaces at intermediate locations
along the POF and thus measure strain in discrete sections of the rope.
- Obtained a U.S. patent for this measurement technique.
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
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
Project Start: June 12, 2001
Project End: June 11, 2005
DOE Contribution:: $460,000
Industry Contributions: $42,000 (8.4% of total)
NETL - Rhonda Jacobs (firstname.lastname@example.org or 918-699-2037)
ORNL - Barton Smith (email@example.com or 865-946-1290)