Development of HMPE Fiber for Permanent Deepwater Offshore Mooring

Abstract

Abstract For a number of years, the creep performance of standard High ModulusPolyethylene (HMPE) fiber types has limited their use in synthetic offshoremooring systems. In 2003, a low creep HMPE fiber was introduced and qualifiedfor semi-permanent MODU moorings. This paper reports on the introduction of anew High Modulus Polyethylene fiber type with significantly improved creepproperties compared to other HMPE fiber types, which, for the first time, allows its use in permanent offshore mooring systems, for example for deepwaterFPSO moorings. Industry guidelines and standards mentioning HMPE creep are briefly discussed, and results on fiber and rope creep experiments reported. Laboratory testinghas shown that ropes made with the new fiber type retain the propertiescharacteristic of HMPE such as high static strength and stiffness andyarn-on-yarn abrasion resistance. Introduction With oil and gas field exploration going deeper and further offshore, mooringsystem designers are faced with engineering mooring systems that balance thedemands of maximum platform offsets, wind and wave peak loads, and long-termhigh tensions in loop currents. Polyester ropes are commonly used for deepwater moorings. Beyond 2,000m waterdepth, however, the high stretch of the polyester rope becomes a problem as thelonger mooring lines allow greater horizontal offsets. A 2,000m polyester linemay have 40m elongation, while a 3,000m line would allow 60m elongation underthe same environmental conditions, creating greater horizontal offsets whichmay exceed the limits of risers. Using High Modulus Polyethylene with similarbreak load these offsets would be only 12m for a 3,000m line. In addition, High Modulus Polyethylene is now widely considered to be the mostsuitable material for these longer deepwater mooring line lengths. The fibersare characterized by high strength and high modulus, producing lighter andsmaller diameter high stiffness ropes, providing both technical and operationaladvantages over traditional polyester mooring lines. A stiffer HMPE mooringsystem is potentially more riser friendly than polyester. HMPE ropes typicallyhave an extension at break of 2%−2.5% for a worked rope. During station-keeping, wave movements impose cyclic loadings on mooring lines, causing fluctuating fiber elongation. The mooring lines are subject totension-tension fatigue loads. HMPE fiber ropes have shown a longer fatiguelife compared to polyester ropes for the same rope construction and are notvulnerable to axial compression fatigue compared to aramid fiber (1),(2). However, the high stiffness of HMPE can also be a limiting factor. In highstorm and hurricane risk areas the mooring system needs stretch to maintainstorm survivability. Hybrid mooring lines combining HMPE rope segments withpolyester rope segments provide the stiffness needed to handle maximum loadsduring station-keeping in storm, while ensuring sufficient elasticity to damppeak loads induced by waves (3), (4), (5). The pretension is responsible for the long-term loading of the mooring lines.HMPE fibers are sensitive to these long-term static loads, and willirreversibly elongate proportionally with time. This phenomenon is known ascreep. Excessive creep causes an increased offset of the moored vessel. Thedegree of creep is dependent on HMPE type, operating temperature, mean load andloading time.