As with many other coastal regions of the world, Prince William Sound in Alaska is utilized for the marine transportation of crude oil. Crude is delivered to Port Valdez via the Trans-Alaska Pipeline and shipped by ocean going tankers to ports along the west coast of the United States.
After the disastrous grounding of the tanker Exxon Valdez on Bligh Reef in 1989, a number of prevention and response mandates were imposed on the oil industry by the Oil Pollution Act of 1990. Among these requirements was the establishment of a dual tugboat escort system that requires loaded tankers to actually be tethered to a highly capable tug prior to leaving Port Valdez and transiting the confined waters of Valdez Narrows. Escort tugs are considered to be one of the most effective tools available in the prevention of accidental oil spills.
Escort tugs operating in Prince William Sound are contracted for their service by an organization known as SERVS, the Ship Escort Response Vessel Service. This organization is a subsidiary of Alyeska Pipeline Service Company, operator of the Trans-Alaska pipeline.
There are two types of tugboats used in primary tanker escort service by SERVS. Owned by Crowley Maritime, they are the Prevention and Response Tugs (PRTs) and Enhanced Tractor Tugs (ETTs). These tugs use deck mounted winches and high-strength towlines to stop and turn fully loaded oil tankers that can weigh 193,000 tons, in a matter of minutes.
The firm of Robert Allan Limited was retained by the Prince William Sound Regional Citizens’ Advisory Council (PWSRCAC) to investigate the towing systems in use aboard these escort tugboats and then look into how these towing systems compare to what is considered the current Best Available Technology (BAT) in use by escort towing systems worldwide. PWSRCAC is an independent, non-profit organization that seeks to promote the safe shipping of crude oil in Prince William Sound, Alaska.
Robert Allan of Vancouver, British Columbia, is a world-renowned subject matter expert in the field of escort tugboat design and operation. In its final report accepted by the PWSRCAC Board of Directors in September of 2012, the naval architectural firm made a number of recommendations to improve the escort tug towing gear.
Winches installed aboard the Prevention and Response Tugs and Enhanced Tractor Tugs were found to be well maintained and in good working condition. Tow winch technology has evolved dramatically since these vessels were constructed some ten to twelve years ago.
It is noted that the absence of load render-recover capability on these tugs is considered a deficiency in the world of modern escort capable tugboats. Render-recover winches offer the ability to reduce tension on a towline under significant load when the line reaches 50 percent of its breaking strength. The ability to reduce a high load in rough weather is a major safety advantage.
This feature is especially valuable during towing operations in high sea states during indirect tows. An indirect tow is a situation where a tugboat is required to tow another vessel at an angle off its direct line of travel. It is considered a more difficult towing maneuver since the forces involved can easily overturn a tugboat. A high wave can greatly magnify the load seen on a towing winch and break a towline or damage a tug. Indirect towing is routinely used in tanker escort operations.
Additionally the study notes that the lack of a level wind device installed on the deck winches is seen as a deficiency that can contribute to increased line wear and line failures. A winch level wind mechanism is similar to the same device used on a fishing reel to keep line level on its spool. A major problem that can and has occurred in escort winch operations is the tendency for the towlines to dive into other wraps of the towing line on the winch drum. There have been winches developed that spool line onto the drum at a coarse pitch that lays alternating layers across each other diagonally. This type of system prevents line diving, even if spooled line tension is relatively low.
Only the Enhanced Tractor Tugs are designed for use in the indirect towing maneuvers required to execute a tanker save in the Sound. Actual indirect steering and braking force values for the ETTs are not available, making an accurate evaluation of escort towing gear strength in comparison to the forces generated by the tugs impossible. Robert Allan has recommended full scale testing of towboat performance and stability be conducted to determine indirect steering and maximum towline force of the escorts.
The majority of the escort towing industry has adopted the use of High Modulus Polyethylene (HMPE) fiber towlines. Robert Allan Ltd. noted the use of this line within SERVS and that they had an excellent management program in place to care for the lines.
The study found that the ETT and PRT vessels are powerful tugs and are well equipped. Their towing systems however fail to reach today’s world standards for the definition of Best Available Technology.
A number of actions would need to be taken to bring the primary escort tugs used in Prince William Sound up to current world standards for Best Available Technology:
ETT Recommendations to Reach BAT
- Conduct full-scale indirect towing tests or perform computer analysis to verify capability of steering and braking forces generated by the tugs
- Perform an analysis of vessel capability using Det Norske Veritas’ escort tugboat stability criteria
- Based on adequate testing, specify and install a render-recover winch with spooling gear suitable for these tugs
- Maintain current towline systems and towline maintenance practices and inspections currently in use
PRT Recommendations to Reach BAT
- Conduct full-scale direct and transverse arrest towing tests or perform computer analysis to verify forces generated by these tugs
- Perform an analysis of vessel capability using Det Norske Veritas’ escort tugboat criteria
- Based on adequate testing, specify and install a render-recover winch suitable for the tug
- Maintain current towline systems and towline maintenance practices and inspections currently in use
A copy of the study’s executive summary and final report are available at the PWSRCAC website.
Copyright – 2013 by Alan Sorum.