Figure 1. Catenary (left) and Taut Leg (right) moorings.
ELEVATION VIEW OF MODU MOORING TYPES (CURRENT AND PROJECTED)
Wire rope standards issued by API include Spec 9A
Specification for Wire Rope (11th edition as of 2012) and RP 9B Application, Care, and Use of Wire Rope for Oil Field Service (13th edition as of 2012). Tese standards refer primarily to wire rope use in land based, dry applications. Remarkably, there is not currently an API Standard that addresses the large sizes and constructions of wire rope used for offshore mooring systems. Te development of such a standard is within the scope of Resource Group 2 (the body within API that is responsible for development of standards relating to offshore moorings) in the near future. Over the years, raw materials for wire rope and chain grew
in quality. With the development of more accurate and efficient heat treatment processes, chain links and wires could be produced with higher grades and strengths (i.e., R-4 and R-5 Chain: EIPS, EEIPS and EEEIPS wire ropes). Low torque jacketed spiral strand wire was also developed which had better performance long-term than standard 6x36 IWRC wires. All of these higher strength components made it possible for many rigs to meet the newer, stricter, design requirements without a total refurbishment of the mooring fairleads and chain handling equipment. Chain use in offshore applications is guided by API Spec
2F Specification for Mooring Chain (6th edition in 2010). Te first edition of this document came out in 1974. Inspection of chain quickly emerged as a critical process in controlling the quality and safety of offshore mooring installations. API RP 2I Recommended Practice for In-Service Inspection of Mooring Hardware for Floating Drilling Units (1st edition 1987, 3rd edition 2008) gave additional guidance on how to inspect the quality of chain, connectors, wires and synthetic ropes in a mooring system to ensure system integrity.
26 MAY-JUNE 2014 WIRE ROPE EXCHANGE
THEN CAME DISCO AND POLYESTER… In the mid-to-late 1970s, there was a flurry of activity in the mooring community as attempts to reach deeper and deeper depths became more financially viable. Synthetic fiber ropes manufactured by a variety of rope companies were being evaluated and assessed. Fibers considered in the initial evaluation included nylon, polyester, and aramid fibers because of the high global production capacity. Other fibers such as polypropylene, high modulus polyethylene (HMPE such as Spectra or Dyneema), and liquid crystal polymer (LCP such as Vectran), were not considered because of performance or production limitations. In the ensuing years, there were many experiences that led to today’s understanding of good mooring practices. Nylon ropes were among the first man-made fiber ropes used
offshore entering in towing applications in the 1950s. Because of their low stiffness and high elongation characteristics, they made a great product for applications where loads would vary widely at very rapid rates. Unfortunately, because of the creep and wet characteristics of the fiber (namely strength loss when wet), they were not accepted for deepwater moorings. Polyester fibers quickly emerged on the offshore mooring
front because of their better strength performance and lower elongation than nylon. Polyester fibers are readily available thanks to the large capacities for production that resulted from the leisure suit fashion of the disco era (actually, airplane tires and other industries pushed the development of polyester fiber manufacturing capacities). Polyester had excellent fatigue properties and after several years of scepticism, has gained wide acceptance and is currently the most commonly used man- made fiber for offshore mooring ropes. During the 1980s many joint industry projects (JIPs) were formed where polyester was
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