Hybrid propulsion offers several benefits to tug owners operating in emission control areas, as naval architects at OSD discovered when designing the Azistern-e tug
Naval architects at Offshore Ship Designers (OSD) have developed a series of hybrid tugs after years of research into different propulsion combinations and hull types. This Dutch group developed a series of Azistern-e tugs with overall lengths ranging from 20 m up to 26 m and bollard pull capabilities between 35 tonnes and 75 tonnes.
Azistern 2035e is the smallest of three hybrid propulsion tug designs at 20 m long with a design draught of 3.75 m. It would have a tonnage of less than 200 gt, but bollard pull of around 35 tonnes. This would be suitable for smaller harbours and inland waterways.
OSD’s Azistern 2250e has a length of 21.45 m, draught of 4.6 m and bollard pull of 50 tonnes for operations in terminals and harbours. If an operator needs a terminal tug with more power then it could use an Azistern 2575e design. This has a design capability of 75 tonnes of bollard pull from a 25 m tug (see Table 1). These designs have been approved by Bureau Veritas as electric propulsion tugs with unrestricted navigation.
OSD started this design project with the aim of creating a 22 m tug design that was compliant with IMO Tier III. This would have hybrid propulsion, combining batteries with smaller diesel-electric arrangements for lower fuel consumption and emissions and fewer running hours when compared with diesel-only driven tugs.
As OSD went through this research it identified that hybrid tugs would have 42% less fuel consumption with a diesel-electric and battery configuration in the engineroom compared with a diesel-direct one.
OSD technical manager Herm Jan de Vries said batteries reduce the load variations on engines and can provide enough power for most of the tug’s movements. Naval architects found there was a 75% reduction in engine running time when batteries are operating in parallel with diesel engines (see Table 2). These calculations are based on an average sailing profile of a tug operating in a harbour in the Netherlands.
With a hybrid propulsion, transit sailing and manoeuvring can be performed using the batteries with zero emissions. When maximum bollard pull is required for towage work, diesel-electric propulsion is used with a boost in power from the batteries. “A hybrid system employed on a tug is the most fuel-efficient in terms of mobilisation and towing capability,” Mr de Vries said.
“A hybrid system employed on a tug is the most fuel-efficient in terms of mobilisation and towing capability”
OSD created its first hybrid tug design, Azistern 2250e, following interest from owners and shipyards. This 21.45 m harbour tug design would come with two 970 kW variable speed diesel generator sets and two 400 kWh lithium battery packs. Mr de Vries said the first Azistern-e was designed to generate 50 tonnes of bollard pull.
Its generators produce 1,350 kVA at 690 V and 40-60 Hz, with revolutions varying between 1,200-1,800 rpm. Each battery pack would have a design lifetime of 10 years and would weigh around 17,000 kg. They would each take up a rack floor area of 17 m2 and would be directly coupled to a common DC-Bus on each switchboard.
More details of this series will be in the upcoming issue of Tug Technology & Business.