Not every tug claiming to be escort rated can perform the required duties safely and effectively. That was one of the messages delivered by world-renowned tug designer Robert Allan of Robert Allan Ltd. at the Tugnology ’19 conference in Liverpool, England earlier this year. Tugnology is a two-day technical conference on the design, construction, operation and economics of tugs.
Allan, the executive chairman of the Vancouver, British Columbia-based naval architect and marine engineering firm, provided some background on the history of tug escorts before co-worker Brendan Smoker presented RAL’s paper on escort tug safety.
“Thirty years ago, the world of tug design really got turned on its head,” Allan said. “The Exxon Valdez incident had regulatory repercussions. What arose from that in the United States was the regulatory act known as the Oil Pollution Act of 1990. That act invoked tug escorts. But the reality was that the world of tug operators, tug owners, and tanker owners and operators didn’t really understand what they were asking for.”
No one really understood what was about to happen. “But the world that is represented by the community in this room really responded to that challenge and moved ahead,” said Allan.
In the early 1990s, Foss Maritime designed a pair of big tractor tugs, the 137'x46' Lindsey Foss and Garth Foss. The 8,000-hp tugs were built by Halter Marine and delivered in 1993 and 1994. “They were really the first true purpose-designed tanker escort tugs in the world.” The tugs deliver about 80 tonnes of bollard pull.
In Europe, things developed differently. “Voith had done a really wonderful job of convincing oil companies that the cycloidal propeller was the best available technology for escort,” Allan said. “For many years it was considered as the only solution.”
European tug owners took the direction of very large and very powerful ASD tugs. Though these were “magnificent” deepwater towing tugs, they were not really configured well for tanker escort. “Most of them had very high fo'c'sles, relatively low freeboard aft, so they really set the use of ASD tugs in tanker escort back a generation. We had a lot of demand from our clients that ‘there’s got to be a better way’ so that we could do tanker escort with an ASD, so that set us on a course of research that began in the early and mid ’90s.”
What RAL was able to demonstrate through model testing and analysis was that regardless of the type of propulsion “there was a configuration that could do the job.”
“The important proviso behind all of that is that not every tug claiming to be escort rated — and just about any tug can be escort rated is an important point to note — doesn’t necessarily mean that it can perform those duties and [do it] safely or effectively,” Allan said.
Through the 1990s, a whole new generation of escort tugs emerged, with some very unique hull forms, and the knowledge base grew.
MECHANICAL ELECTRIC HYBRID
In his paper “Development of a Mechanical-Electric Hybrid ASD Tug” presented with Kristian Eikeland Holmefjord of Kongsberg Maritime, Jonathan Parrott, senior naval architect at Jensen Maritime Consultants, discussed the tug Delta Teresa. Nichols Brothers Boat Builders delivered the tug in April to Baydelta Maritime. The 100'x40', 5,350-hp hybrid tug works in and around San Francisco Bay. It was designed to reduce air emissions, lower fuel and maintenance costs, and boost bollard pull on demand.
For several years, JMC worked with Rolls-Royce (Rolls-Royce commercial marine is part of Kongsberg) on a mechanical-electric hybrid propulsion system for ASD tugs to boost available power to the thrusters. The Delta Teresa utilizes lower horsepower EPA Tier 3 main engines and an electric motor on each drive to maintain the same bollard pull rating as its sisterships. The propulsion system is designed so that the vessel is able to transit and maintain stationkeeping using small auxiliary diesel generators and electric motors without the main engines running, reducing emissions as well as maintenance and operating costs.
Baydelta has been a Jensen customer since 2006 and has built six of JMC’s 100' Valor-class ASD tugs at Nichols Brothers. The main goal was to implement the hybrid system into the existing Valor-class design with minimum modifications. The original design had Rolls-Royce US 255 FP top-mounted drive units, powered by Caterpillar 3516 engines rated at 3,386 hp each.
To accommodate the change to the hybrid system, modifications to the existing Valor-class design included the following:
- Modifying the locations of the main engines and Z-drives.
- Replacing two 175-kW diesel generators with three 300-kW gensets.
- Revising the exhaust for the new gensets.
- Replacing the existing service switchboard with a combination RR switchboard/control center.
- Adding an electrical cabinet for the drive motor variable frequency drives (VFDs) and cooling.
- Revising the pilothouse consoles for upgraded control heads and panels.
- Upgrading the machinery space ventilation supply to account for additional cooling requirements.
The concept of the mechanical-hybrid tug is not new, but acceptance of this type of tug has been slow, mainly due to the initial cost of the installation, Parrott said. However, as more examples of this type of hybrid tug are put in service, the long-term cost benefits will be better documented.
In addition, Parrott concluded, with the rapid advances in battery technology, the installation of battery-assisted drive trains will increase, where space allows.