Finnpusku is an integrated tug and barge system owned and operated by ESL Shipping, a Finnish shipping company that specializes in bulk cargo transports in the Baltic Sea. The system was developed in the 1980s by Finnlines, another Finnish shipping company that also managed the vessels until 2003, in co-operation with Rautaruukki to transport raw materials to the Raahe Steel Works. Two pushers and five barges, four of which remain in service, were delivered by Hollming in 1986–1987.
Today the Finnpusku system consists of pushers Rautaruukki and Steel, and barges Board, Botnia, Kalla and Tasku.
Concept
Separating the expensive machinery section from the cargo space offers several advantages in comparison to conventional vessels, one of the most important being the ability to operate on the "drop and swap" principle which minimizes the turnaround time in port for the pusher and its crew. When a pusher-barge combination arrives at a port, the fully laden barge is left for unloading while the pusher picks up an empty one and leaves again. In theory the system works on optimum efficiency when there are as many barges as there are pushers and ports of call – the barges are always either being loaded or unloaded, or underway with a pusher. In addition to reducing unprofitable waiting time such operation principle allows more time for the unloading of the barge, removing the need for expensive cargo handling equipment in the unloading port.
One of the major arguments in favor of integrated tug barge systems is the small number of crew required to operate such vessel — whereas a conventional vessel with capacity in par with the Finnpusku pusher-barge combination would require a crew of 16–17, the integrated tug-barge unit can be operated by a crew of only 9. In some cases an integrated system is adopted only for this reason and the pusher is rarely, if ever, decoupled from the barge.
An integrated system has several technical advantages in comparison to towed barges. Traditionally tugs, being short in length, have to operate at relatively high Froude numbers resulting in high wave making resistance and the barges, being towed in the tugs' wake, have skegs that improve directional stability but increase drag. Positioning the tug behind the barge in a stern notch improves the hydrodynamic efficiency of the combination, resulting in significant reductions in the total resistance. In addition the tug, operating in the wake of the barge, has better control over the combination and thus improves seaworthiness and maneuverability in comparison to the traditional towing arrangement.
Integrated tug barge systems are generally divided into three generations by the type of coupling. In the first generation systems the tug is connected to the barge by wires or chains. However, because the two vessels are subject to different motion responses due to their shape, displacement and position on a wave, such operation is only possible in calm sea conditions — in rough weather the tug has to disconnect from the barge and continue the journey by towing it in a traditional way. The second generation systems are designed with a deeper stern notch and improved coupling devices to allow operation in heavier seas while still permitting relative motion between the two vessels. The third generation systems such as the Finnpusku system, equipped with rigid or articulated mechanical coupling, allow operation in all sea states and even in ice conditions as the pusher-barge combination behaves hydrodynamically like a conventional ship. However, in some cases the specialized hull form of the pusher, designed to form a streamlined hull when coupled to the barge, might lead to problems with stability and seakeeping when the pusher is operated independently.
Development and construction
The history of the Finnpusku integrated tug barge system dates back to 1964 when Rautaruukki started steel production in the new steel mill in Raahe, Finland, and began supplying the factory with ore concentrate, coal, coke and other raw materials. Around the same time the Finnish shipping company Finnlines researched transporting cargo by barges and developed the first version of the Finnpusku system, but due to problems in funding and the devaluation of the Finnish markka in 1967 the idea was not pursued further. However, an upgraded concept was developed in the 1970s.