Purpose-designed emergency towage vessels offer significant operational advantages for near-shore rescue towing applications
More than 30 years after the 1989 Exxon Valdez tanker oil spill in Alaska, a response organisation in the region was looking for the optimal design and technology for a new emergency towage vessel (ETV)
Prince William Sound Regional Citizens’ Advisory Council (PWSRCAC), established in the wake of the 1989 disaster, enlisted the help of Glosten to identify the best available technology (BAT) for an ETV stationed in Hinchinbrook Entrance.
When a vessel is disabled at sea, time is of the essence. Adrift and at the mercy of the elements, it cannot make headway, steer, or even remain head to weather, leaving it to turn against the wind and lay in the trough of the oncoming waves - an extremely perilous position for a ship in rough seas.
Captains and crews of responding vessels often face equally dire situations during rescue missions.
Taking a disabled vessel in tow is inherently dangerous. It requires a close approach to the ship’s bow or stern, which may be pitching and yawing violently, to establish a towing connection.
Human lives, property, marine ecosystems and local economies are utterly reliant on an effective response to prevent a catastrophic oil spill.
With so much at stake, it is important the designated rescue tug, or ETV, be designed and equipped to the highest standard to fulfil the demands of these critical missions.
PWSRCAC is looking for an ETV to safeguard tanker movements through Prince William Sound to Alyeska Pipeline Service Co’s Valdez Marine Terminal, which loads Alaskan oil onto tankers year-round.
The Exxon Valdez oil spill in this region impacted hundreds of miles of coastline and resulted in immense damage to local habitats and livelihoods.
In the aftermath, PWSRCAC set its mission to promote the environmentally safe operation of the terminal and associated seaborne transportation of oil, including through the Hinchinbrook Entrance, which connects Prince William Sound to the Gulf of Alaska.
The waters beyond the entrance are prone to actively changing and sometimes brutal weather conditions.
Ships that become disabled in this area present a risk of grounding and subsequent environmental damage.
To mitigate these risks, PWSRCAC asked Glosten to assist in finding the BAT for an ETV stationed at Hinchinbrook Entrance.
Glosten was tasked with identifying design practices and new technologies used in existing vessels for this ETV. After reviewing the available literature on ETV design from the last decade, Glosten produced a comprehensive summary of optimal ETV design features.
The summary covered principal dimensions, installed power, propulsor types, hullform, seakeeping and motions characteristics, range and endurance, free-running speed, deck machinery, towing gear, bridge equipment and installed rescue and recovery equipment.
The Glosten team then held a roundtable discussion with PWSRCAC and Prince William Sound stakeholders to ensure key design parameters for a Hinchinbrook Entrance ETV (HE-ETV) were established early on.
BAT for an HE-ETV was defined as the best achievable balance of the most important design parameters for service at Hinchinbrook Entrance. The principal parameters were a vessel delivery date of 2005 or later, overall length of between 40 and 80 m, bollard pull between 120 and 185 tonnes and a target free-running speed of 16 knots.
Glosten then compiled a database of around 4,000 towing vessels worldwide, delivered in 2005 or later, and currently in operation. Less than 10% satisfied the basic HE-ETV performance requirements related to vessel size and bollard pull.
Of these, the overwhelming majority were commercial anchor-handling tug supply (AHTS) vessels designed primarily to support the offshore oil and gas industry. Some of the best-known ETVs in the inventory were large, government-funded, multi-mission vessels designed to perform a range of duties.
However, many of these vessels exceeded the maximum length for the HE-ETV and would be extremely costly to design, build and operate.
Five stages of ship rescue
In the next phase of the study, Glosten outlined five main stages of a ship rescue evolution and underscored the corresponding design and performance traits necessary to fulfil the demands of each stage.
During the deployment stage, ETVs need to transit to the disabled vessel as quickly as possible from their starting point. Speed is essential, as is the ability to maintain that speed in higher sea states.
The quicker an ETV can arrive on-scene, the more time is ultimately available to prevent a casualty. The rescuing vessel must also have favourable seakeeping characteristics, incorporating a high bow/foc’sle, to navigate the turbulent waters of the Alaska Gulf.
When an ETV arrives on-scene, it may not be possible or advisable to attempt to connect to the disabled vessel right away. But when the opportunity presents itself, it should be able to do so reliably, safely and efficiently.
The second and third stages, standby and connection, require a vessel primed for manoeuvrability, agile and responsive enough to maintain position in front of a disabled vessel in heavy seas and pass towing gear.
Once a connection is successfully established, the ETV must have sufficient propulsive power and bollard pull for stage four: towing the vessel.
It must be able to maintain positive control of the ship in all conditions and prevent it from drifting towards lee shores or other hazards.
Once the ETV is relieved by a commercial salvage tug, or the vessel has been safely anchored or secured in a port of refuge, the fifth and final stage of the rescue evolution is complete.
Based on the demands of these operational stages, Glosten concluded a state-of-the-art ETV must be designed to perform exceptionally well in three critical areas:
From a design standpoint, however, these performance demands are somewhat in conflict; optimising a vessel design for one stage of the operation can penalise its performance in another.
The most apparent example is that of vessel size in relation to manoeuvrability and agility. As vessel size (namely length) is increased to enhance seakeeping and achieve higher free running speeds, this can affect vessel agility which cannot always be overcome by adding lateral or omni-directional thrust.
Top-scoring rescue vessels
Glosten applied the four principal parameters established during the roundtable to the 4,000-vessel database to isolate the 17 top-scoring, rescue-capable vessels, ranked according to 19 different performance criteria.
Only eight purpose-designed ETVs were identified in the global inventory that both satisfied the principal requirements and were serving in a dedicated standby ETV role.
Based purely on raw score, the top finishers were the French ETV Abeille Bourbon, followed closely by the renowned German ETV Nordic.
Because cost was a key evaluation criterion, the winner was selected based on performance and capability in relation to cost. The 17 vessels were graphed accordingly.
Vessel size (length, beam, draught) and power (installed power, propulsion equipment) parameters were used as proxies for capital and operating cost due to the limited availability of expenditure information and difficulty of comparing costs across build years, shipyard locations, currency exchange rates and flag states.
Glosten determined that the vessel most representative of BAT for the subject application was Spanish vessel Luz de Mar: a purpose-designed ETV operated by the Sociedad de Salvamento y Seguridad Marítima (Sasmar).
Unlike modern AHTS and national multi-mission vessels designed to support a range of government or commercial offshore functions, the Luz de Mar design spiral was focused specifically on optimisation for offshore ship rescue and response.
It is the only vessel in the subset that achieves an optimal balance of speed and manoeuvrability, which sets it apart from the rest of the field.
Thanks to its intentional design, Luz de Mar is capable of a high free-running speed of 16.4 knots.
It has high manoeuvrability and agility afforded by a powerful azimuth stern Z-drive propulsion system, adequate bollard pull and the ability to respond and aid vessels in distress in a variety of ways.
With the winner identified, Glosten then performed a gap analysis between Luz de Mar (as BAT) and the current standby tug serving Hinchinbrook Entrance.
The analysis showed that implementing BAT at Hinchinbrook Entrance could introduce new rescue and response methods and capabilities; enhance the reliability, safety and efficiency of the overall rescue evolution; and improve nearshore response times.
Applied together, these could substantively reduce the probability of a drifting tank vessel grounding or other casualty in the area.
The study also illuminated an important truth: purpose-designed ETVs offer significant operational advantages for nearshore rescue towing applications.
There is a widely held conviction that large AHTS vessels, by virtue of their sheer size and power, are optimised rescue tugs.
But the design of these vessels simply does not prioritise the features most important to a successful emergency response.
As a result, there are inherent differences in hull form, location of primary propulsors and general arrangement that result in a suboptimal balance of free-running speed with agility, responsiveness and bollard pull.
Sensible proactive investment in a next-generation coastal ETV for Hinchinbrook Entrance would signify a commitment to the protection of vessels and crews, the local environment and Prince William Sound stakeholders. Using state-of-the-art equipment is arguably the best tool that we have for reducing the probability of future disasters.
Authors: PWSRCAC project manager Alan Sorum and project manager Peter Soles