The Observer, December 2004
PRTs back in service as tether line problems fixed
Alyeska Pipeline’s Prevention/Response Tugs were cleared for a return to primary escort duty in September, after the state Department of Environmental Conservation concluded the problems causing broken tether lines on one of the vessels were being fixed.
Alyeska’s fleet includes three of the Prevention/Response Tugs, or PRTs. One of them, the Aware, twice broke its tether line during practice exercises for rescuing disabled tankers. The first break came during heavy weather in December of last year, while the second was in mild weather last June.
Alyeska’s three Prevention and Response Tugs have been approved to return to primary escort duty. Shown here is the Alert, sister ship to the Aware.
The tether lines – high-strength hawsers 10 inches in circumference – are crucial to Alyeska’s capability to rescue a disabled tanker.
Because of the two failures, the Department of Environmental Conservation banned the PRTs from serving as primary escorts for loaded tankers.
Each tanker traveling out of Prince William Sound is accompanied by two escort tugs. One – the primary tug – is actually tethered to the tanker for part of the trip to enable immediate response in case of an emergency. The PRTs were allowed to continue serving as secondary escorts, but only Alyeska’s Enhanced Tractor Tugs, the Nanuq and the Tan’erliq, were allowed to serve as primary escorts. Neither of those vessels has experienced tether line problems during exercises.
The state’s decision to reinstate the PRTs came after an investigation of the line breaks by Crowley Marine with the assistance of the Department of Environmental Conservation, the citizens’ council, and other stakeholders.
The investigation revealed the primary cause of the breaks was the line burying itself in wraps lower on the winch drum as it came under load during the exercises. The resulting compression, heat and chafing caused the breaks, according to an analysis by Samson Rope Technologies, manufacturer of the parted line.
The council and the other organizations working on the issue came to believe the fundamental issue was a failure to properly rewind the line onto the winch. Crowley – which operates the tugs for Alyeska – has since revised and strengthened its procedures for spooling the line tightly onto the winch drum.
In addition, the winch drums on the PRTs are being modified by Markey Machinery (which made the winches) to make them stronger. The modifications were expected to be complete on the Aware by mid-December 2004, with completion of the work on the other two vessels following at four- to six-week intervals. The total cost was projected at about $40,000.