The articulated tug and barge expands into offshore waters
Proposed ATB container unit – LNG as fuel, with gas carried in tanks on the barge, is being considered for this type of application
Dag Pike examines how the tug and barge concept, so popular on North American inland waterways, has grown into a viable proposition for deep water transport to and from the US and elsewhere.
Transportation of goods by towed barge has been a feature of the American coastal trade for a long time. Barge transportation offers lower costs, and despite its slow speed and possible weather it has grown into a major industry that accounts for much of the fuel transportation around the US coastline. Now tug/barge systems are expanding into international waters and challenging conventional shipping.
The basic tug/barge concept is simple - a tug tows the barge astern on a wire towline. Such a system has obvious disadvantages including weather limitations and risks, slow speeds and the difficulty in maintaining time schedules. Push towing has been a feature of the US inland waterways for years. An advantage of using tug/barge systems was that the regulations allowed the vessels to be classed as tugs, with reduced manning and construction requirements compared with comparable ships. There was also the possibility that the tug could transfer to an empty barge for the return voyage, so that the tug would remain fully employed without waiting, but in practice tugs tended to stay with their allocated barge.
The tug/barge system is now developing in offshore waters, with two separate tracks of development. One was simply the notch tug - the tug slotted into a notch at the stern of the barge, which was held in place by wires, with heavy fendering. This allowed higher speeds and safer operation in adverse conditions with the possibility of the tug disengaging to act as a normal towing tug if conditions deteriorated.
As the requirement developed for larger capacities some operators adopted a form of tug that integrated totally with its ‘barge’ and the two never parted company, forming in effect a fully integrated ship. It was claimed that the tugs involved were not seaworthy on their own account and that these vessels were simply ‘rule beaters’. Eventually the US Coast Guard started to clamp down on this approach by tighter regulation.
Typical of these was the OSG Vision - a 997grt tug powered by twin diesels of 12,000bhp. This 153ft (47m) long tug had a beam of 51ft (15.5m) and a draught of 26ft (7.9m) but its hull shape come under criticism as being unseaworthy without its attached barge and it lacked aft towing gear and was unable to undertake conventional towing.
Alongside these developments emerged the Articulated Tug/Barge (ATB) that adopted a new approach to the link between the tug and the barge mainly prompted by the Coast Guard regulations. Various patented systems were developed such as Itercon, Articouple, Jcomarin and Artubar which in general made the connection between the tug and the barge through massive side pins that were extended horizontally once the tug was in the notch. These pins engaged with a vertical slot on the barge to create a semi-rigid connection that was to prove much more seaworthy than previous systems.
The big advantage was that both tug and barge could be designed to be fully seaworthy units with the tug capable of operating independently at sea. With careful hydrodynamic design the efficiency of the units was improved to allow higher speeds. ATBs could operate at speeds of 12 to 15 knots, comparable with ship operations.
ATB operations have expanded over recent years with oil barges close to 50,000dwt capacity, and the concept adopted for ro-ro and container transport between the US mainland US and Caribbean islands. Trials have shown the ATB is fully controllable in challenging river entrances. The upsurge in demand has come partly from the rapid expansion of US shale oil and gas, while ATBs are used for tanker lightering operations. One operator uses a 655ft (200m) barge for lightering tankers before they navigate the Delaware River to the terminal.
With ATB designs now having reached maturity, reliable ETAs and higher speeds are possible, along with greater crew comfort and better awareness from the crew of rough sea states. Compared with towed barges, operators claim a 25% fuel saving and comparable seakeeping to a ship. Tugs and barges can operate independently and are thus interchangeable.
Pascagoula shipyard VT Halter, an ATB specialist, has built a series for New York operator Bouchard Transportation. The latest Bouchard contract is for a pair of 145ft (44m) 6,000 hp seagoing tugs equipped with the Intercom connection system, to be used with existing barges. The hydraulic pin link between tug and barge is 64in (1.6m) diameter, with a cog end that engages into a rack on the barge to account for variations in draught.
What is claimed to be the most powerful ATBs ever built was for Crowley Maritime, which operates a large ATB fleet. Its Legend was one of three tugs powered by a pair of Wärtsilä diesels coupled to CP propellers for a total of 16,320hp, for speeds up to 15 knots. Crowley says it has invested over $1 billion over the past 10 years in upgrading its ATB fleet with 17 tugs and barges up to 45,000dwt.
The Jones Act has helped development of these larger ATBs, which have established a good safety record around the US carrying oil and chemical products. The proven safety of these is likely to lead to the concept being adopted elsewhere, such as Mexican and South American waters. ATBs could even offer viable container feeder and bulk transport services in Europe – trials have taken place with ATBs carrying coal around Denmark - and the Mediterranean. A container service is to be established between Portland in Maine and New York with an ATB container barge, under a joint venture between McAllister Towing and the Maine Port Authority. LNG as fuel is being considered for this 894teu ATB unit.
OSG Ship Management has contacted us, and points out that: "the OSG Vision, built 2010, is a modern AT/B tug boat that is every bit as seaworthy on its own as any other modern AT/B tug out there. It has passed all stability requirements as a stand-alone vessel as required by the United States Coast Guard, including the 'towline criterion'. And while neither the OSG Vision nor most other modern AT/B tugs are fitted with a tow winch, the OSG Vision does have a 150-ton tow bitt and a capstan on the aft deck for conventional towing in the highly unlikely event the need arises. This is no different from the Crowley AT/B tug Legend mentioned later in the article."
OSG Ship Management additionally adds that it is one of its vessels, the 58,000dwt tank barge OSG 350, which is employed in lightering tankers on entry to the Delaware River, and it is actually the OSG Vision that mates to this barge. This AT/B has a sister AT/B, the OSG Horizon / OSG 351 (built 2011). [Added by Editor, 7 November 2014]