SYNTHETIC MOORING LINE FAILURE ANALYSIS

Summary

A series of mooring line failures occurred on two identical oil tankers over a five-year period. All failed lines had been used on stern mooring winches. The residual conditions of the broken lines and other lines on the vessels were tested to identify failure modes. A separate study on HMPE mooring line failures on another fleet of tankers also recorded that most line failures occurred at stern winch positions.

Stern mooring line failures are most likely caused by long-term exposure to moderately elevated temperature due to proximity to the engine room and exhaust stack. This is supported by peak temperature measurements recorded in service. The stern lines were exposed to a peak temperature approximately 16.5°C higher than that recorded on the bow lines. It is also supported by known elevated temperature characteristics of HMPE fiber.

Residual strength tests showed that the greatest strength loss was usually not located near the end of the rope.  The lowest recorded strength was typically found in the section of rope which contacted the fairlead during vessel moorings.  The practice of only testing a short segment from the service end of a line is not a reliable way of determining rope condition.

End-for-ending is commonly performed to extend the service life of mooring lines. Caution is warranted regarding this practice due to the potential change in rope properties that can occur on the storage side of a winch. Rope which remains unused on the storage side should not be assumed to be in “new” condition.

One possible mitigation is the practice of covering winches with tarps to protect the lines.  The installation of thermal barriers may also prevent localized heat degradation on stern winches.

Realistic service life expectations must consider the complex environment of each mooring line position. Testing-based programs should include full lengths of representative mooring lines from various winch positions to characterize changes in HMPE fiber and rope performance properties over time.  More study is needed on the influence of storage and ambient temperatures to further reduce the likelihood of rope failures.

Author(s): E. Huntley, M. Huntley