• The most important engineering parameter is fiber rope modulus. Aramids and HMPEs have approximately 3 times the stiffness of polyester, and the failure elongations at break in % is 1/3 to 1/4 that of polyester. The new material PEN is ~ twice as stiff as polyester (PET). PEN otherwise has similar properties to polyester, and is a good candidate if stiffer moorings are needed.
• Aramids (Kevlar, Twaron, Technora) when used for 20 year life production moorings have poorer tension/tension fatigue than polyester and struggle with compression fatigue (z-kinking) when cyclic loads are very low. Avoiding low tension cycling by mooring design (proper pretensions) can minimize the compressive fatigue issues. Creep is not an issue with aramids.
• HMPE (Dyneema, Spectra) when used for a 20 year life production system has a serious problem with creep, and cannot be used for that application. But for mooring drilling rigs, HMPE could work well, because of the shorter times under load. Aramids and HMPEs have a diameter like that of steel rope for the same breaking load, and thus would have a diameter 30 % smaller than polyester. HMPE would be tough, compact, and it floats. Creep can be monitored by checking rope change in length (between drilling jobs) and discard it if the creep elongation reaches say 7%
• LCP (Vectran) and plasma (spectra), along with HMPE, are more useful for synthetic lifting lines than for long-term mooring systems.
• Polyester cost ~ $3/kg while the aramids (HMPEs and LCPs/plasmas) have a fiber cost ranging from $25 to $70 per kg.

Fully coupled analysis of the floater, mooring lines, SCR risers, and Top Tensioned Risers, has been completed. For the floater, the contractor analyzed in water depths from 5,000 feet to 15,000 feet:

• Polyester moorings (the cheapest fiber) can be easily designed for the full range of water depths and high modulus fibers are not necessarily needed.
• High modulus fiber ropes can be used as well as polyester, provided that there are no other structural weaknesses in the fiber rope.

  A workshop is scheduled for September 29th in Houston to:

1. Present findings and collect feedback.
2. Show the results of data collection and compiling of advanced fibers and ropes technical parameters and collect feedback
3. Brainstorm failure modes of advanced fibers as compared with of polyester ropes.

Contractor's aim is to collect specific technical data to support a claim, that there are advanced fiber rope mooring systems that can perform at least as good, if not better than current technology, which is a polyester mooring system for deep water

Last Updated: 10/29/2009, 09:18 AM