For a number of years, the creep performance of standard High Modulus Polyethylene (HMPE) fiber types has limited their use in synthetic offshore mooring systems. In 2003, a low creep HMPE fiber was introduced and qualified for semi-permanent MODU moorings. This paper reports on the introduction of a new High Modulus Polyethylene fiber type with significantly improved creep properties compared to other HMPE fiber types, which, for the first time, allows its use in permanent offshore mooring systems, for example for deep water FPSO moorings. Industry guidelines and standards mentioning HMPE creep are briefly discussed, and results on fiber and rope creep experiments reported. Laboratory testing has shown that ropes made with the new fiber type retain the properties characteristic of HMPE such as high static strength and stiffness and yarn-on-yarn abrasion resistance.

Polyester synthetic mooring lines are used widely in deepwater moorings for MODU (Mobile Offshore Drilling Units). However, practical and technical limitations in the deployment of polyester tethers beyond water depths of 2,000m for short-term MODU moorings has prompted the development of alternative synthetic mooring materials. This paper describes the application of Low Creep, High Modulus Polyethylene (HMPE) mooring systems for deepwater MODUs. HMPE has many practical offshore handling benefits compared with polyester. The ropes are lighter, easier to handle and have a smaller diameter than comparable polyester ropes for the same minimum breaking load (MBL). From a technical perspective, the neutral buoyancy of HMPE, low creep rate and stiffer rope properties are ideal for ultra deepwater mooring systems. In addition, HMPE ropes are easier and quicker to deploy than polyester, leading to operational cost savings when moving MODU rigs between locations for drilling and work over. Mooring system storm survivability is of increasing concern to operators. Hybrid mooring lines containing individual sections of rope made with HMPE and polyester yarns have been developed and shown to comply with the latest American Petroleum Institute (API) requirements. These hybrid lines combine the attributes of polyester and HMPE enabling naval architects to more accurately engineer the mooring systems to better suit prevailing offshore and storm conditions.

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The Thunder Hawk semi-submersible platform is located in 6060 ft. water depth, Mississippi Canyon, Block 736, Gulf of Mexico. The mooring system is composed of chain-polyester rope-chain. On April 14, 2009 line #10 was installed on the platform. On May 15th, 2009 while attempting to complete construction stretch removal operations of the polyester mooring ropes, an equipment failure caused line #10 to fall completely to the seabed. The mooring line was subsequently recovered on June 11, 2009 and a spare polyester mooring line was run in its place. The mooring line was fitted with two long segments and a test insert, so all testing and evaluation was done on the recovered #10 test insert. In order to maintain the " used" rope as a spare, a qualification program was proposed to BOEMRE (now BSEE). The evaluation first required a complete visual inspection of the mooring line and components and then a detailed two phase evaluation of the test insert. Fatigue testing was completed June 14, 2012. The results and conclusions of this testing provide insight into the expected behavior of polyester mooring ropes and potential testing problems. Microscopic inspections using SEM were done on the rope fibers. The results show significant formation of salt crystals on the rope fibers. EDX examination was not possible on dissolvable solids as it was not possible to transfer the material to the EDX slide. Therefore the results were not conclusive. Break testing qualification consisted of three (3) subrope breaks and one break after a fatigue test representing 25% more than the API spiral strand equivalency. Regulatory approval required the mechanical testing. Test results indicate the polyester rope met the required break strength, was not affected by soils ingress, and had additional improvement in strength after the fatigue test. Installation of polyester moorings using a pre-lay approach can significantly reduce the mooring installation cost for deepwater floating platforms. This data presents real world test results and testing issues from a typical polyester mooring rope in a deepwater Gulf of Mexico application. Introduction The Thunder Hawk facility is operated by Murphy Exploration & Production Company and leased from SBM. The semi-submersible has twelve (12) mooring lines consisting of four (4) groups of three (3) lines each. The mooring lines consist of a chain, two (2) long segments of polyester, one or two test inserts depending on the corner, platform chain and messenger chain. Connections between the polyester and between the polyester and chain are made with plate style H-links. A fairlead is located on the hull and a chain stopper with load monitoring is located at the the top of column. Dedicated windlass units on each column provide pay in and pay out capability and mooring line tensioning. Each mooring line has a chain locker to store the messenger chain and extra length of the platform chain. The polester ropes used are 254mm diameter with a qualified minimum breaking strength of 1930 te. The long segments are 1261 m each, and test inserts are 15.74 m long. One complete spare rope was purchased with the original order, consisting of two (2) long segments, two (2) test inserts, and connectors. On April 14, 2009 the mooring line was installed and tensioned to approximately 204 te (450 kips). This tension is slightly above the target tension for this line. On May 12, 2009 construction stretch operations were undertaken for the polyester. On May15, 2009 construction stretch for line #4, the Southeast corner of the platform, had line tensions of about 408 te on the Northwest corner of the platform. While adjusting line #10 the load released suddenly and the complete mooring line fell to the seabed. The top end of the mooring line was carried deep into the mud due to the weight of the platform chain.

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. Table 1: Preliminary Rope Property Comparison MBL (tonne) Diameter (inch) Dry Weight (lb/ft) Mean EA (lb) Low Freq. EA (lb) Wave Freq. EA (lb) Typical Polyester Rope 800 6.33 12.7 2.29E+07 3.88E+07 5.47E+07 MODUline® Polyester Rope 800 6.54 12.97 2.25E+07 3.33E+07 4.02E+07This study will examine the benefits of various lengths of MODUline® Polyester inserts into taut mooring lines to determine the overall impact on survivability and operability of a moored MODU in a wave-dominant environment. This will be accomplished through the utilization of the ANSYS AQWA software suite to examine a generic MODU hull at 1,000-ft an...