Synthetic “mud ropes” for offshore mooring applications - field history and testing data

With the continuing development of synthetic ropes and real-world experience gained through their use in offshore mooring operations, it is evident that previous reservations about the use of synthetics in soils are no longer warranted. This is proven through the extensive documentation of field service history, coupled with rope and fiber testing of a jointly developed High Modulus Polyethylene (HMPE) “mud rope,” which is presented to industry in this paper. The "mud rope" examined in this paper is of 8×3 open-braid HMPE construction. This open-braid construction facilitates visual inspections of each rope, allowing for significant time savings during expensive offshore operations. Each of the subropes within the main rope has its own separate filter barriers and jackets. These individually jacketed subropes allow for more residual strength in the rope should a breach in the jacket occur and cause premature fiber wear and internal abrasion. As the ropes are comprised of lighter HMPE material, they are easier to handle offshore on the deck of an Anchor Handling Vessel (AHV), reducing the likelihood of accidents and injuries. This low density also allows the ropes minimal interference in the freefall trajectory of the gravity-installed anchor to which they are directly connected. The ropes have initial lengths of ~335-ft, and due to their direct connection to the gravity-installed anchor, much of the rope is under cyclic tension under the mudline. Given the excellent fatigue and abrasion resistance of HMPE, in addition to all of the aforementioned benefits, it is an excellent choice for ropes that are under continual use under harsh conditions. The service life of this rope spans from its first installation into Gulf of Mexico soils on 3/12/08 to the final retrieval date of 1/20/10 (~679 days). The actual total time in the mud is documented to be ~544 days. During that service life, the rope was placed on two semi-submersible MODUs (Mobile Of- shore Drilling Units) and in six different mooring system configurations. In late August of 2008, the rope was connected to a MODU during hurricane Gustav, experiencing storm loads calculated to be in excess of ~1,050-kips (~1,050,000-lbs). The rope was inspected and measured to determine if any permanent damage was done to it. As the rope did not experience loading near its MBL (Minimum Break Load) of 800 metric tones (~1764-kips), the rope was deemed safe for further use. After minor jacket repairs, the rope was placed back into service, where it remained until January of 2010. After being retired from service, the rope underwent several destructive tests. Testing was performed on the rope fibers to confirm fiber creep, static stiffness, and post-service MBL (Minimum Break Load). A similar testing scope was performed on the rope to examine overall rope creep, rope static stiffness, and post-service MBL. This paper documents the detailed records of an HMPE "mud rope" throughout its service life, along with the rope and fiber testing techniques and results that were used to verify its integrity after the rope was retired.