Determination of Wire Rope Fatigue in Riser Tensioning Systems

Chris Stewart

Senior Engineer Ensco plc Dallas, TX ©2007

Determination of Wire Rope Fatigue in Riser Tensioning Systems

The current method for calculating wire rope fatigue and the subsequent slip & cut procedure are investigated for accuracy and suitability. Wire rope types (spooled or pre-cut lengths) and terminations are also discussed and evaluated. An alternative method for measuring wire rope fatigue limits is given in this paper as well.

Introduction The current method used on some vessels for measuring wire rope fatigue in riser tensioning systems is based on “Ton-Days”. A Ton-Day is equivalent to the amount of tension in tons that the tensioner is applying to the riser system on a per day basis. This method only includes the force component applied to the system and does not allow for the work done by the system. The industry standard for tracking tensioner wire rope fatigue is based on TonCycles. This method gives a better approximation than Ton-Days, but is still lacking since most calculations do not detail the distance component accurately enough. Ton-Miles yield the best approximation of wire rope fatigue. Ton-Miles are equal to the amount of tension in the wire rope (in tons) times the distance the wire rope travels (in miles) due to vessel motion (heave). Ton-Miles are used to express the total work done by the tensioning system on the wire rope. Ton-Mile monitoring systems are currently offered by various vendors and should be considered for use on offshore drilling units that utilize wire rope tensioning systems. Fatigue Estimation If a Ton-Mile monitoring system is not in place, an approximation may be made by considering the average heave and period of the vessel as well as the total force from the tensioning system. The vessel heave and period are required in order to determine the amount of wire rope travel. The heave and period satisfies the distance component of the work done by the system. If one assumes continuous wave propagation...