The ASD tractor becomes the all-American tugboat

The ASD tug Lynne Moran working with an LNG tanker at an LNG terminal in Freeport, Texas, that opened earlier this year [Brian Gauvin photos]

Were it not for Edison Chouest Offshore, American tugboat companies might have turned in a perfect record in the past few years: Americans just don’t go for the “true” tractor-tug design.

Whether it’s a tendency toward irascibility, stubbornness, national pride, practicality or a natural inclination toward versatility, almost all tugboat operators in this country go for the azimuthing stern drive (ASD) form of modern harbor tug. They usually do not even consider other forms of tug designs that are more popular in Europe and Asia.

Of the dozen or so new tractor-style tugs built each year in the United States, almost all are of the ASD design, with power ratings ranging (so far) up to 10,000 hp (Crowley’s Nanuq, Tan’erliq and Alert).

But this year the folks at Edison Chouest have been introducing the first of eight new tugs for LNG work that are of the so-called “true tractor” design. These are big tugs, measuring 110 feet overall, and putting out about 80 tons of bollard pull. As with three others in the Chouest fleet built more than a decade ago, they are powered by forward-mounted azimuthing propellers in nozzles. The drives are located side-by-side, just forward of amidships.

Chouest has built and deployed four of these for ship-assist work at Cheniere Energy’s new LNG importation terminal on the Louisiana side of the Sabine River. The company is presently constructing four more for assignment at the new ExxonMobil Golden Pass LNG terminal a few miles upriver. That will be a total of eight new tugs with forward-mounted z-drives — more than enough to spoil any trend that might have been building up in the record books.

For years it was said, mostly by those with proprietary interests, including members of the Chouest family, that the only true tractor tugs were those with propulsion systems attached to the forward end of a tug. This included both the Voith Schneider “water tractors” and others with forward-mounted azimuthing propulsion gear. In part because of the casual attitude of Americans, this distinction about what constitutes a true tractor tug has gradually been eroded or expanded to include tugs equipped with stern-mounted azimuthing thrusters.

ASD tugs are chosen so often because they have passed virtually all performance tests to everyone’s satisfaction, and because they tend to be somewhat less expensive, and because there is a large and expanding pool of technology, training, experience and expertise available right here, often within the same port or just down the coast. Oh, and don’t forget that they can tow things like barges and ships.

Moran Towing Corp., for example, a company that holds the most LNG contracts for tug service in the United States, reports that it does not consider any other type of

Two classes of Edison Chouest tugs working with an LNG tanker at Cheniere Energy’s LNG terminal on the Sabine River in Louisiana. The newest and biggest of these tugs, such as SP Amber in the foreground, are true tractors powered by twin forward-mounted azimuthing propellers in nozzles. [photo courtesy Mary Meaux]

tug propulsion and is happy with the ASD design. Moran, which currently operates an expanding fleet of 21 tractor-style boats (including four of the so-called combi-tugs), makes all proposals for LNG business using performance criteria based on the ASD design with which it is experienced. Obviously that formula works for Moran and its clients.

Moran, with industry partners, has recently won new contracts to provide terminal ship-assist and escort service for LNG facilities in Freeport, Texas, Hackberry, La., and at Costa Azul, Mexico. All three contracts involve ASD tugs. The Costa Azul terminal, near Ensenada on the Baja Peninsula, is particularly demanding as it requires tugs to bring LNG tankers in from sea in up to 10-foot seas to a new receiving terminal located behind a breakwater. Moran, with its industry partner, is building four new 6,575-hp ASD tugs in Spain for this service.

Boston Towing & Transportation has two tractor-style tugs under construction for use at an offshore LNG terminal off the Massachusetts coast. Both are of ASD propulsion design.

Bill Skinner, operations manager, said the company submitted its proposal to Suez Energy N.A. with ASD stern drives specified for reasons of economy, versatility, experience and performance capability. The request for proposals (RFP) for those tugs originally specified that proposals could include Voith Schneider water tractors or azimuthing stern drives with the drives located as far forward as amidships, he said.

“To some extent the choice depends on the mission,” he said. “In this case it might not have been practical to have the Voith water tractors because we’ve got to be able to tow and the Voith boats are, in my opinion, more for maneuvering.”

Boston Towing reported it had signed a 20-year contract for service at the Neptune Offshore LNG Terminal. Both tugs required for the contract were designed by Robert Allan Ltd. of Vancouver, British Columbia. They are a 128-foot 5,400-hp tug and a 101-foot 5,400-hp tug, both with controllable-pitch z-drive propellers. Delivery of those two tugs in 2009 will bring Boston Towing’s fleet of tractors to four in addition to eight other conventional tugs.

Technological advances with ASD tugs over the years has helped the design to meet and surpass almost all performance criteria demanded of them, according to Greg Brooks, a marine consultant who specializes in tractor-tug technology and training.

“People look at the Voith Schneider tractors and they say that boat really performs and turns in some really big numbers,” said Brooks. “Well, it makes those big numbers mostly because the hull is so huge. You know that Crowley’s 7,000-horsepower tug Response can produce 7,000 pounds of steering force and everyone says that’s terrific. But they’ve lost sight of the fact that the boat is 129 feet long. It’s all a matter of how big is the hull that you have in the water, just as the larger wing on an airplane will give you more lift.”

Brooks, who is sought-after as a consultant for tractor tugs in most U.S. ports, said z-drive boats like Robert Allan’s Z-tech design reflect the latest design trends and can easily outperform most of the earlier z-drive tractors.

At the same time, history has shown that a Voith Schneider water tractor with cycloidal drive comes with a price tag almost twice that of a similarly powered ASD tractor.

So infrequently are forward-mounted z-drive tugs built in this country that MarineSafety International, a major company in the simulated training field, does not even provide training for that type of vessel. Eugene Guest, director of the company with operations in Newport, R.I., and several other locations, said the company can train crews of conventional tugs, ASD and cycloidal drive units, but is not currently set up to provide training with new tugs like those just introduced by Edison Chouest.

“We don’t get much call for that,” said Guest. “The emphasis is on the azimuthing stern drive. There’s a lot of people around who have expertise with that type of tug and they’ve demonstrated to the LNG energy carriers that these tugs are fully capable. In these cases they are just focused on the needs of LNG ships. It appears as though the ASD design has gained ascendancy.”

As of June 2008, a total of 224 tractor-style tugs of all designs were operating in mainland U.S. ports plus Hawaiian waters, according to the annual compilation of tractor tugs in American Tugboat Review, an annual special issue of Professional Mariner. After eliminating nine of the combi-tugs (single-screw aft with z-drive added forward) and two tugs that have single z-drive propulsion, that leaves 215 tugs operating with what might properly be called tractor-style design. These include 20 tugs with Voith Schneider cycloidal drive, 11 with forward-mounted z-drives, and six with in-line fore and aft z-drives (ship docking modules or SDMs). The rest — all 178 of them — are all azimuthing stern drives with twin z-drives mounted side-by-side near the stern. That makes 83 percent of U.S. “tractor” tugs being ASDs.

This comes as no surprise to the owners and operators of Baydelta Maritime in San Francisco.

Operations manager Fred Henning said the company did not seriously consider any other propulsion design as it prepared recently for construction of a pair of 6,800-hp tractor-style tugs (Valor and Vigilant) for ship-assist work and occasional towing assignment.

“I think the z-drive aft configuration gives you the most opportunity for both working in the harbor and for having the ability to tow offshore,” he said. “Having the z-drives forward pretty much limits you to harbor work because they are not a good offshore-towing boat.

“In our case, we wanted a good multi-purpose boat. By putting the z-drives aft we are able to get a bigger and longer skeg under the bow end. We wanted a good multi-purpose tanker-escort boat and by putting the z-drives aft you get the longest skeg for conducting indirect maneuvers.”

Henning added that his company has experience with the use of forward-mounted z-drive propulsion since it once chartered two 3,200-hp Kinsman-class tractors. “We had them here and we towed with them a bit and they weren’t very good for towing because of what we considered to be poor directional stability.”

Tug operators in Europe and Asia tend to build new vessels in roughly the same proportions as their counterparts in America. Europeans, however, seem more willing to build alternative designs including the well-regarded Rototug that employs three azimuthing z-drives, one mounted aft, and two mounted near amidships. European operators, unlike Americans, also continue to build new Voith Schneider water tractors.

In small numbers, overseas operators also continue to build tugs with forward-mounted z-drives. The practice is to call these ATDs — the acronym standing for azimuthing tractor drive. Damen Shipyards Group, for example, is a Netherlands-based conglomerate operating more than 30 shipyards and specializing in producing vessels to standardized designs. The company today offers two ATD tugs, one of 81 feet LOA and the other being 96 feet in length.

Another European shipyard, Union Naval Valencia SA, of Spain, recently completed a 97-foot tug of 3,100 hp with forwarded mounted z-drives for the Port Authority of Bejaia, Algeria. The tug, with MAK diesels and Rolls-Royce drives, is described as being the first of a new line of tugs designed in-house.

Crowley Maritime is another major U.S. tugboat operator that is happy with the ASD tug design and is on the verge of building more, according to Jonathan Parrott, president of Crowley’s naval architecture firm, Jensen Maritime Consultants of Seattle. As an example Parrott mentioned that Crowley, facing eventual need to replace its fleet of 25 Invader-class offshore tugboats, will likely replace those classic tugs with both a new class of offshore conventional tug and by another new class of ASD tugs capable of both harbor work and general towing assignments.

Parrott mentioned that while the forward-mounted z-drive tugs are perfectly capable of ship-assist work, they tend to be deeper draft vessels and he also stressed that they do not make good towing vessels.

“Any time you have a z-drive in the bow and your tow winch on the stern I don’t think you are going to have a captain being very comfortable towing anything for a very long distance,” he said.

By Professional Mariner Staff