Laboratory Study of Used HMPE MODU Mooring Lines

Abstract

Abstract Test results of residual properties of two 12-m HMPE synthetic fibre tether inserts individually used at four and eight deepwater wells respectively in up to 2,049 m water depths are presented against new tether test data and operational loading history. The paper concludes by demonstrating negligible changes in performance from new unused synthetic fibre rope and provides design engineers with new performance data. Introduction In May 2000 Unocal Indonesia, Transocean Sedco Forex, Marlow Ropes and DSM High Performance Fibres set an industry first by successfully utilising High Modulus Polyethylene (HMPE) synthetic fibre rope to moor a MODU (Mobile Offshore Drilling Unit) in 2,049 m. The HMPE fibre rope moorings were used at eight well locations in water depths ranging from 1,219 m to 2,049 m. After completion of the eight-well drilling program two 12-m in-series inserts were recovered for laboratory evaluation. This aimed to assess the residual properties and remaining service life of the HMPE ropes and compare the data to new tether data and prediction tools used in the design of new mooring tethers. The evaluation included residual strength, residual tension-torque fatigue performance of the HMPE tether loaded in line with a conventional 6-strand wire, microscopic analysis, residual creep characteristics and visual inspection. Properties such as elongation, stiffness, rotation, internal condition and the residual strength of both the HMPE tether and the wire after completion of the fatigue test were evaluated. History of Use In the spring of 1999 Unocal Indonesia began planning an eight to ten well deepwater exploration drilling program in 1,676 m to 1,981 m water depths offshore East Kalimantan in the Makassar Strait. At that time Unocal's surface stack drilling capability from the Sedco 601 semi-submersible drilling rig was limited to water depths of about 1,524 m with a conventional wire-chain mooring system. To drill in deeper water required introducing a component that reduced the dead weight of the mooring system, yet was easily installed with local anchor handling vessels (AHVs). After evaluating several options, Unocal Indonesia contracted with Marlow Ropes and DSM High Performance Fibres for constructing ten 914-m long Superline Steelite Xtra Dyneema fibre ropes with a 3502 kN minimum spliced breaking load. The fibre ropes were used at eight well locations between November 1999 and June 2000. Given that the moorings would be non-torque balanced, it was decided to limit the fibre rope use to an eight to ten well drilling program, or not exceeding a period of one year's use. Two 12-m long fibre rope inserts were placed into the mooring systems for testing at the end of the drilling program. Figure 1 shows a general schematic of a mooring leg and the positions of the HMPE rope and test insert. The HMPE ropes were inserted between the rig's 6-strand wire and a pre-installed (pre-laid) wire-chain mooring system consisting of 6-strand steel wire rope and chain.