Increased offshore MODU mooring performance through MODUline® polyester rope

Abstract

Shallow water (~1,000 - 3,000-ft) mooring system configurations are often driven by two factors: mitigation of subsea infrastructure and cost vs. performance. Typically, shallower water fields are highly congested with manmade infrastructure, as well as a significant marine life presence, which can pose a problem for drilling operations in harsh environments. Conventional catenary rig systems, composed of wire and/or chain fair well in these water depths, but require significant grounded length. However, taut systems (synthetic or wire rope) require little contact with the seafloor, but often do not produce the desired survivability. The increased survivability of conventional catenaries in shallower waters is attributed to the stiffness of the mooring system. Taut systems are stiffer than catenary systems that contain similar components (ie. all wire and chain, or steel and polyester). However, the deeper the water, the longer the overall mooring line lengths, which causes the mooring system stiffnesses to converge as mooring line weight becomes the dominant factor. Similarly, the less total mooring component length in the system, the closer the systems come in total weight, causing the difference in system stiffnesses to be the dominant factor. The ideal mooring system for shallower water depths should include the positives of using a steel catenary and a polyester taut system, while reducing the negatives of each system. MODUline® Polyester Rope has similar strength to weight ratios as conventional polyester mooring rope, but provides significant increases in elasticity. Preliminary data for linear mooring stiffness comparisons of MODUline® Polyester Rope with typical polyester rope of similar break strength is shown in Table 1. Typically, MODU offsets from environmental loading are dominated by mean loads. As shown in Table 1, the mean stiffness of MODUline® Polyester Rope is comparable to that of traditional polyester rope, which indicates comparable vessel offsets under the same environmental loading conditions. However, wave frequency stiffnesses are significantly different between the two ropes, signifying a drastic decrease in dynamic tensions on mooring equipment attached to MODUline® Polyester Rope over conventional polyester rope. This elasticity, when introduced to a taut polyester mooring system, can dramatically improve system survivability, while maintaining a smaller subsea footprint.