Friday, July 12, 2013

New Mission-Specific Tug for Cook Inlet

Portland’s Diversified Marine’s reputation for quality construction has earned it contracts from a variety of discerning maritime companies, including Brusco Tug & Barge, Shaver Transportation and Crowley Marine Services.

The newest delivery is the Bob Franco, a new azimuthing stern drive (ASD) tugboat for Seattle-based Harley Marine Services, named after the father of Harley Marine’s president Harley Franco. The 120-foot by 35-foot boat, designed by Seattle’s Jensen Maritime, is a similar design to the successful Titan-class vessels designed for Seattle’s Western Towboat, but the new boat is adapted for ship assist and towing work in Alaska’s Cook Inlet, where a contract with Tesoro will require the tug to work in extreme conditions. To meet the challenge, the new boat is equipped with heated decks and tanks, keel coolers and an increased capacity for response operations, including a large aft work deck, 4-ton crane and plans for a crude oil skimming system.

The new boat has comfortable accommodations for 13 crew, and is powered by the newest generation of Caterpillar tier III C175-16 engines providing 2,683 HP each at 1,600 RPM. The power from these low-emission engines is transmitted through a pair of Schottel 1515 Z drives, providing a bollard pull of better than 70 tons.

The vessel was originally designed to work out of Seattle, but was requested for Alaska’s Cook Inlet by Harley Marine’s client Tesoro Corporation. The change in plans meant a change in the makeup of the tug, and required the addition of extra equipment, upgraded machinery and even a redesign of the structure of the boat.

Kurt Redd, president of Diversified Marine, says the change of mission called for some serious redesign work, including an ice-strengthened hull. Redd says his crew removed plating to go to heavier plating. “We actually removed plating and replaced it, and ice-strengthened the boat in a more original-design manner, rather than just doubling up on existing plating.” Redd says the result is a much stiffer and stronger hull.

Another change was the addition of a larger bow winch. “The boat wasn’t designed for ship assist,” Redd says. “We put large beams through the structure of the boat to make sure the winch was properly supported.” Diversified also added a bow staple for the winch that extends through two decks to the framing in the hull. “The original design didn’t have a bow staple,” Redd notes. “Now it has a really strong one.”

Shibata fendering surrounds the working surfaces of the new boat, and a 4-ton Rapp Hydra Pro crane is fitted beside the Markey aft towing winch.

On top of the heated decks are non-slip heavy-duty rubber mats generally used on Canadian arctic drill rigs. The mats, supplied by Advanced Mat Systems, were designed and cut to fit in the factory, and delivered ready to install on the working deck surface.

“This is our first installation of the mat system on a tugboat,” Redd says. “I think it came out really well. The crew says it’s nice to not have to stand on the steel deck.”

Along with the heated decks, the boat has heated windows, heated potable water and heated ballast tanks. All this heat required the fitting of larger generators and more electrical work throughout the boat. “We abandoned the two planned 100-kw generators and installed a pair of 250-kw models,” Redd says.

The mission change also called for more engineering work. “There was so much engineering we had to bring in two outside engineers,” Redd says. “The engineering changes were all a collaborative effort between our lofter and the two engineers.”

Cook Inlet averages 176 days a year of below-freezing temperatures, and 94 inches of annual snowfall. To keep the cold outside, the shipyard also beefed up the boat’s insulation. “We insulated like no boat we’ve ever built,” Redd says. “Every level in that boat has a floating floor. I’d say we went overboard on insulation.”

Redd says there’s so much insulation it’s sometimes hard to tell the boat is even running. “During sea trials, the crew had to go outside the crew space to make sure the engines were running.”

To further keep the crew comfortable, the new boat is fitted with a unique heat exchanger made by Daikin, which heats the boat in the winter and cools it in the summer. Diversified worked with York HVAC to install the system, and Diversified crews performed all the piping and insulation. “The system gives each space its own climate control, and each room has its own thermostat, so all the crew spaces in the boat are individually climate controlled,” Redd notes. The system is unique in that it uses the waste heat from the generators or the surrounding seawater to add or remove heat from the system, and uses the overhead space as the air pressure intake plenum, which allows the system to take in fresh air from the outside, no matter the weather.

Along with crew comfort and ice modifications, the mission change required the addition of a ship-assist winch and staple to the bow of the vessel. Markey Machinery stepped up to the challenge with a single-drum electric hawser winch with render/recover capabilities to match the new ship assist capabilities of the sturdy new tug.

“The Bob Franco was originally designed as a line-haul ocean towboat,” says Scott Kreis, vice president of sales at Markey Machinery. “We had gotten quite a ways into the project, and had a simple bow winch, essentially a power-in power-out line-handling unit, most of the way done, when we got a call asking for a hawser winch.”

The company went back to the drawing board, calculated the forces and line loads and sized a class-2 hawser winch for the new boat. “We’ve built about nine winches for Harley in the last year and a half,” Kreis says, “and we finished the original bow winch. It’s in stock for Harley when he needs it.”

The new bow winch is a DEPCF-52-75HP single drum electric hawser winch with a drum capacity of 750 feet of 10-inch line, a line pull of 30,800 lbs. at 378 feet per minute, and a braking force of 678,000 lbs.

The aft towing winch is a double-drum, variable frequency drive, 100-HP towing winch, with 2,500 feet of 2 1/4- inch wire rope on the starboard drum and 1,500 feet of 2 1/4- inch wire on the port drum.

“Both of the winches are electric,” says Kreis, “and there aren’t any hydraulics in the system, so they added a backup hydraulic motor at our suggestion.”

The backup motor is integrated into the deck crane’s hydraulic system, and provides the vessel with a “come-home” drive.

“With a hydraulic winch there is a secondary means of supplying hydraulic flow and pressure, but a diesel or electric system calls for a separate motor drive that can be clutched in if necessary,” says Kreis. “In this case we provide a small hydraulic motor that can be powered off another hydraulic package.” Kreis says he left it up to the yard to decide how to power the backup motor, and is pleased with the result.

The new boat, built to ABS (American Bureau of Shipping) Ice Class DO Standards, and capable of performing in the harsh Alaska marine environment, will help ensure the safety of the vessels in the Tesoro fleet, while providing a level of comfort and safety for the crew of the boat that will be the envy of the North Pacific tugboat fleet.