During installation of polyester mooring lines for the Red Hawk cell spar in the Gulf of Mexico, one segment accidentally contacted the sea bed. Kerr-McGee Oil & Gas Corporation (Kerr-McGee) is the operator and Devon Energy Corporation (Devon) is a 50% partner in the facility. The operator and the regulatory agencies raised concerns regarding the effect of possible soil ingress on the durability and performance of the polyester rope.
A two-phase test plan was established and conducted to demonstrate that Whitehill VETS 379 polyester rope retained 100% of the new rope break strength, modulus, and fatigue life after contact with the sea bed. Microscopic imaging and chemical analysis indicated that individual braided strand jackets on each sub-rope limited the particle size in contact with the strength member fiber. This actual case history clearly demonstrates that the strand jacket provided sufficient protection from the adverse effects caused by sea bed contact.
The results of the two-phase test plan are intended to provide technical justification for relaxing current philosophies requiring line replacement when touchdown occurs.
The Red Hawk Cell Spar is the first application of a cell spar as a host for oil production operations. The cell spar installed in Garden Banks 876/877 in the Gulf of Mexico. The Spar is located in approximately 5,300 ft of water with a chain-polyester-chain mooring system. More details of the overall project and mooring design criteria are detailed in Ref. /1/ and Ref. /2/.
Soil ingress is an important factor in the use of polyester mooring lines. It has been observed that small particles allowed to imbed into the inner load bearing fibers of some synthetic ropes can reduce the mean break load (MBL) and fatigue life over time. This issue has prompted regulatory agencies in the Gulf of Mexico to require that polyester mooring lines never contact the sea bed. To prevent such an occurrence, mooring designers and installation contractors take this requirement into account when developing and installing the mooring system.
To limit particle ingress, the polyester rope manufacturers developed a rope that consists of an outer jacket on the overall rope and individual jackets on each strand of each sub-rope. This impedes foreign particles from contacting the load bearing fibers. As part of the installation phase, the installation contractor utilized a two boat operation to maintain a clear distance above the sea bed, Ref. /3/.
Despite all attempts to keep the polyester clear of the sea bed, during hook-up operations, one of the spar mooring lines briefly contacted the sea bed. ROV surveys of the mooring line did reveal faint traces of soil on the exterior jacket of the rope. Rather than immediately replace the section of polyester that had touched the sea bed, a proposal to perform a series of tests on soiled rope removed from service was accepted by the regulatory agencies. The purpose of these tests would be to prove that soil particles did not penetrate the double jacket polyester construction and full break strength and fatigue could be achieved for the twenty-year design life.
