Several Council studies have examined the technology for the escort tugs in Prince William Sound. These studies help the Council provide informed advice and recommendations about the escort system. The studies don’t just look at electronic technology, but also examine winch systems, towing gear, engine design, and much more.
October 2016 – A Summary of Current B.A.T. Requirements for Escort and Rescue Towing Tugs
This report compiles all of the research conducted by the Council on escort tug BAT and rescue towing requirements. The key findings of these studies are now found in a single document so that the performance requirements for tug used in Prince William Sound service are clearly and readily identifiable.
This report is a summary review of the findings of previous studies conducted by this firm with respect to the escort towing services and the “Sentinel” tug services at Hinchinbrook Entrance. The objective is that all the key findings of these studies should be collated into a single reference document in order that the performance requirements for each class of tug are clearly and readily identifiable.
This report compiles all of the research conducted by the Council on escort tug BAT and rescue towing requirements (the below documents on this page). The key findings of these studies are now found in a single document so that the performance requirements for tug used in Prince William Sound service are clearly and readily identifiable.
A Summary of Current B.A.T. Requirements for Escort and Rescue Towing Tugs
May 2016 – Sentinel Tug Requirements for Gulf of Alaska: Ship Drift Study
This study points out the differences between actual weather conditions and those reported by the Seal Rocks buoy at Hinchinbrook Entrance. It details the forces needed by a tugboat attempting to make a save of a disabled tanker in closure conditions at the Entrance.
Robert Allan Ltd conducted this drift study in order to close this gap in the knowledge of ship behavior and response capability. The drift study modelled both 125,000 DWT and 193,000 DWT tankers drifting from pre-determined start points in the shipping lanes, in the defined closure condition at Hinchinbrook Entrance of 45 knot winds and 15 ft. significant waves, as measured by the buoy at Seal Rocks.
Based on Council research, a single tug at the Entrance would need to develop a total bollard pull of 185T for a 193,000 DWT tanker based on closure conditions at the Entrance. Closure conditions are defined as 45 knots of wind and 15 foot seas. Wave sheltering, topographic sheltering, and buoy height make actual weather equivalent to 57 knots of wind and 20 foot waves, where a rescue tow of a disabled tanker would potentially take place.
Sentinel Tug Requirements for Gulf of Alaska: Ship Drift Study
April 2016 – Tug Bollard Pull Requirements for Rescue Towing in Prince William Sound
This memo from Robert Allan Ltd. describes the recommended tug static bollard pull requirements rescue towing of disabled tankers at Hinchinbrook Entrance and the Gulf of Alaska.
The 2016 study identifies higher bollard pull requirements for rescue towing of disabled tankers in Prince William Sound than those given in the report on the topic in 2014. As discussed above, this is due to a combination of the following two factors, both of which have the impact of increasing the required tug bollard pull.
- Increased Wave Forces
- Transverse Components of Met-Ocean Forces
Memo: Tug Bollard Pull Requirements for Rescue Towing in Prince William Sound
May 2016 – A Review of Best Available Technology for a Sentinel Tug Stationed at Hinchinbrook Entrance
This study provides recommendations on performance standards that should be seen in tugs serving in the sentinel role at Hinchinbrook Entrance.
Robert Allan Ltd. was retained by the Council to conduct a broad review of the current Best Available Technology in Escort Tug technology worldwide, and to perform a Gap Analysis of the tugs within the SERVS Fleet against that current BAT.
Sentinel Tug Recommendations:
- Single ship bollard pull of 185 tons. Two 100-ton bollard pull tugs may be more viable.
- Twin-screw, with lateral thruster or ASD propulsion.
- Have a free running speed of about 16 knots in calm water at full load displacement.
- Have an operating range not less than 2,000 nautical miles at full power.
- Have endurance at full power, at an assumed towing speed of 6 knots, of not less 15 days.
- Fully satisfy the requirements of the US Coast Guard Towing Stability Criteria (46 CFR 173.095).
- Be fully operational in actual weather experienced during closure conditions.
- Meet Class Society salvage and firefighting standards.
- Have render-recovery towing winch systems.
- Have main towing gear with components rated with a design load of at least 3 x Bollard Pull.
A Review Of Best Available Technology For A Sentinel Tug Stationed At Hinchinbrook Entrance
2013 – A Review Of Best Available Technology In Tanker Escort Tugs
This council study found that the escort tugs being used in Prince William Sound didn’t currently represent the best technology for these types of boats worldwide. The council hired Robert Allan Limited of Vancouver, Canada to compare performance of a worldwide fleet of similar escort-rated tugboats, over 35 meters in length, to the escort tugs used in Prince William Sound.
The goal of this study was to determine if the tugs used by SERVS are using the best technology currently available in escort tug design today. Another question focused on the SERVS practice of using the ETT and PRT tugs interchangeably as primary or secondary escort tugs.
The most capable tugboats used by SERVS for tanker escorts are found in two classes of vessel, the “Enhanced Tractor Tugs”, or ETT, and “Prevention and Response Tugs”, or PRT.
The report concluded that neither the ETT or PRT tugs represent the best technology in use by escort tugs today.
For more details, download the full report:
A Review Of Best Available Technology In Tanker Escort Tugs
2012 – Escort Winch, Towline, and Tether System Analysis
In 2012, the council contracted with Robert Allan Ltd. to conduct an investigation into the nature of the towing systems in use aboard the existing escort tugboats in the Ship Escort Response Vessel System (SERVS) in Valdez, Alaska. The project also attempted to determine how those systems compare to the current Best Available Technology (BAT) in escort towing systems worldwide.
The contractors found that the winches on both tugs are well-maintained and appeared to be in good working order, although in neither case were the winches operated during the onboard inspection.
The second stage of this study involved a survey of the latest technologies used in escort towing systems in use in various jurisdictions worldwide. In order to complete this work, companies known to be actively involved in escort towing operations or involved in supplying towing system equipment to operators engaged in escort operations were contacted directly.
The research showed that the SERVS tugs are at the larger, more powerful range of vessels engaged in similar service worldwide. The SERVS tugs satisfy the ABS Class requirements for escort tugs, however the trend in the escort industry worldwide is to look to the more stringent DNV escort criteria as the generally accepted standard.
The SERVS tugs are well-equipped vessels. The research determined, however, that the towing systems fail to reach today’s BAT definition in the type of escort winches used. Escort winch characteristics have changed more than anything else in escort technology in the past 10–15 years. Due to the demands of various projects and ongoing research into the problems encountered, winches have been built recently which could never have been conceived of at the time of building the SERVS tugs.
For more details, including recommendations to improve the current system, download the full report:
Escort Winch Towline And Tether System Analysis
2012 – Classification Society Tug Review
In 2012, the council requested Det Norske Veritas (DNV) to review specifications and performance testing data for Prince William Sound escort tugs with appropriate Class society standards for escort tugs.
DNV, headquartered in Norway, identifies, assesses, and advises on how to manage risk, specializing in various industries, including maritime and oil and gas industries.
DNV’s used their own widely accepted Classification Society Standards for escort service to assess the performance of the Ship Escort Response System’s (SERVS) Prevention and Response Tugs (PRT) and Enhanced Tractor Tugs (EET) in Prince William Sound. This assessment determined the extent to which these tugs were in compliance with the Classification Rules for such escort tugs.
One of the main objectives of the project was to safeguard the Prince William Sound from potential damaging consequences from oil spill and terminal and tanker operations by advising stakeholders (like the Alyeska Pipeline Service Company), verifying practices and identifying improvement opportunities. The verification to Class standards of the available escort tugs in Prince William Sound fits directly into this objective as it verifies whether the tugs are fit for service.