ATB Design Comes of Age

By Kathy A. Smith

The emerging trend of marrying the design of an articulated tug and barge (ATB) to a particular propulsion system to maximize efficiencies of that system has, of late, been taking hold in workboat markets. For example, one recent project involving a dynamic partnership between Bouchard Transportation Co., Inc, Guarino & Cox, LLC (GCL) naval architects, Nautican Research & Development Ltd., integrated propulsion unit manufacturers, and VT Halter Marine shipbuilders, aptly demonstrates the concept in practice. What the collaboration produced, is even more important.

Bouchard and ATBs: both top notch

New York-based Bouchard Transportation recently launched and put into service two ATBs that operate in the U.S. Gulf and East Coasts. The additions bring Bouchard’s ATB tug fleet to 20, making it one of the largest and most experienced ATB operators under U.S. flag. The M/V Kim M. Bouchard is paired with the B. No. 270 and was delivered in July of 2015. The M/V Donna J. Bouchard is paired with the B. No. 272 and was delivered in February of 2016.

Notably, the tugs are first in their class designed to be double-hulled for superior environmental protection, and both barges have cargo capacity of 255,000 barrels of petroleum products. Additionally, each tug and barge is equipped with the most up-to-date Hyde ballast water treatment system, which treats all ballast water upon taking ballast in, and discharging ballast out. “Come time, this will be an important regulated design factor for all Jones Act Tugs and Barges,” says Jon Shaw, Bouchard’s Maintenance and Repair Manager.

Both vessels are constructed with 20 cylinder EMD Tier 3 engines (producing 5,000 HP each) that meet all EPA standards. The design decision behind the hull and bulbous bow was made to help increase speed and maneuverability, while also ensuring fuel-efficiency. Extensive model tank testing with all simulated weather conditions was also an important part of the design phase to help ensure that the factors from weather or sea conditions would never affect the vessels’ maneuverability, speed, and safety. In addition, the robust Intercontinental electrohydraulic 64-inch coupling system allows the tug to stay coupled with the barge during all types of weather conditions.

Initially, Bouchard hired GCL (based in Louisiana) to develop the bid package for both tugs and barges – the two companies have a long-standing relationship going back to the 1970s. GCL has been concentrating on ATB design for the past 15 years and is also known for their OSV designs as well as yachts and standard tugs. The contract was awarded to VT Halter Marine who then subcontracted to GCL to do the detailed function design for the tug while they performed the detailed functional design for the barges in-house.

Design for Propulsion Efficiencies

GCL consulted Canadian-headquartered Nautican on their propulsion system before going ahead with the design phase. While Nautican is no stranger to ATB jobs, this was their first project with Bouchard. “We gave them (GCL) the propeller/nozzle performance curve so they could design the hull to fully take advantage of that,” says Elizabeth Reynolds Boyd, Nautican’s President. “Our propeller and nozzle system is very effective at the high speeds that an ATB can achieve, and Guarino & Cox fully utilized that capacity when they did model tests to see what to change on the hull.”

GCL had oversight of the model test program that was carried out at Oceanic Consulting Corporation in St. John’s Newfoundland. “A scale model of the tug and barge were built – the hull form – including rudders, propellers, nozzles and shafts,” explains GCL’s Greg Cox. “It was towed through the water to measure resistance at various speeds in a controlled environment in a towing tank. The data could then be scaled up to predict the resistance and power required to go at any given speed for the actual vessel.” Prior to model testing, CFD analysis was also carried out on the ATB unit to try to optimize some of the hull form and the interface between tug and barge.

The model test confirmed the efficacy of the barge’s blended skeg stern developed by GCL. The aft body of the barge differs considerably from the conventional notched stern rake design used by many. It has a much deeper notch resulting in a very shallow angle of run for the vestigial rake providing a larger ‘shadow’ for the tug and cleaner flow of water to the tug’s propellers.

Hydrodynamically, the tug and barge unit then more closely resemble a single ship-like unit with a significant reduction of resistance. The need for towing the barge is virtually non-existent, but if ever necessary, the tapered deep notch sides and the drag of the notch face will provide sufficient course stability to obviate the need for appended skegs further reducing resistance.

Below the waterline the bow of the tug is shaped to take advantage of the barge’s aft body characteristics with the minimum separation of the units required due to the tug’s motions relative to the barge. Cox states, “Fortunately, the Intercon system restrains movement of the tug to pitch about the coupler location without roll and heave. With the old wire connections the motions in rough seas would threaten to damage tug and barge, requiring the tug to leave the notch and tow with a hawser. If delayed too long, the act of disengaging could get dicey.”

Guarino adds, “The design of the barge’s bow has been developed to not only reduce resistance but also to minimize degradation of speed in rough weather. In lieu of the blunt barrel stems or scow type rakes these barges are more ship-shaped with finer waterlines and ‘U’shape fore body sections. A bow has a relatively large bulb intended to improve seakeeping and add volume lost using the finer fore body sections.”

Nautican Efficiency: at any speed

The tugs are fitted with two 148-inch stainless steel wheels in Nautican’s triple flap on kort nozzles for more efficient propulsion and more efficient maneuverability. “One of the reasons we like the Nautican nozzles is historically other systems were only good up to a speed of eight or nine knots,” says Guarino. “After that, they would start contributing to the resistance instead of helping. With the Nautican design nozzle, they’re effective at much higher speed.”

During the sea trials carried out last June on the Kim J. Bouchard without the barge, the tug reached just over 17 knots, reports Wayne C. Wingate, Nautican’s Propulsion Technical Director, who was aboard at the time. He says several of the captains – a Halter captain and three captains from Bouchard – were impressed with how the tug steered and accomplished different manoeuvers. “They found that the vessel was tracking easily, with less effort required to keep a straight line course.” As the team progressed throughout the day, they began doing turning circle and zig zag maneuvers.

“I had not been on a vessel of this type on a zig zag maneuver,” adds Wingate. “The buzz around the decks was that most of these guys had typically see rather harsh heel conditions when zig zagging. Some people found a secure spot to hang on or went inside to prepare for a severe heel or rollover but everyone was shocked when that didn’t happen. There was an initial heel to about 17 degrees, then the vessel quickly steadied out at a lesser heel angle (approximately eight to 10 degrees). The actual radiuses and maneuverability was super tight but it was still easy to steer the boat.”

Pre-Swirl Stators reduce the rotational losses incurred by the propeller and further increase the performance of the nozzle system. By directing the flow to swirl in the opposite direction of propeller rotation, the energy that is normally lost in wake rotation is recovered. This increases thrust by an additional four to six percent.

“The stators are integrated into a hub which is the structural support for the shaft which replaces typical struts,” says Reynolds Boyd. “The stators have both structural and hydrodynamic benefits. And behind each propeller there is a set of three high-aspect ratio rudders that are very tall and skinny, which provide a tremendous turning force so the maneuverability of these things is remarkably different than a traditional arrangement. They’re also built into a head box, combining the nozzles and rudders, to make shipyard installation easy.”

Full Steam Ahead

The tugs meet SOLAS requirements and have a fuel capacity of 600 cubic meters which requires the hull to be double hulled. “Most have a fuel capacity of less than 600 cubic meters, but these particular tugs have a higher fuel capacity for a greater range,” says Cox. “They also have an ABS notation called ACCU, which makes for a higher level of automation and monitoring, with a central control room to monitor the engine room and machinery from.”

Typically when ATBs are going at full speed in open water, it’s relatively easy to have a rudder that does the job. “But when you’re coming in and out of a port, you’re using significantly lower speed and yet you need to do much more turning and handling and maneuvering than in ocean transit,” says Reynolds Boyd. “That’s where the triple rudders are really outstanding. According to our testing, they provide 70 percent of bollard in side force. These boats were a significant step for us in terms of actually getting the speed out of the system that it’s capable of getting. Both the hull and the propulsion system have to be designed for it. We’ve seen a tremendous surge in interest for our system on ATBs because those boats are performing so well.”

The ability to stay connected to the barge at all times continues to prove to be the safest, most reliable and efficient way for Bouchard to transport petroleum products. “These are high end rudders that prove to produce the most efficient control for vessels of this capacity,” says Bouchard’s Shaw. “Both ATB units have been exceeding all performance expectations, and consistently deliver at maximum efficiencies. Their design continues to prove to be a reliable and safe transporter for both our customers and their crew.”

It shouldn’t be a surprise that one of the nation’s biggest and most experienced operators of ATB’s, also has become one of the most efficient, and environmentally correct, as well. But, any operator can put together this enviable team to create similar performing vessels. That said; Bouchard did it first.

The Author
Kathy A. Smith is a Victoria, BC-based maritime writer who has penned more than 100 published trade articles.

(As published in the July 2016 edition of Marine News)

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