Rederij Doeksen’s newbuild ferries are ground-breaking in many ways, including being 100% fuelled by LNG
In contrast to the LNG-diesel dual-fuelled model used by many operators, Rederij Doeksen’s two newbuild ropax ferries are distinctive in being 100% LNG-fuelled.
Strategic Marine is building the two vessels and general manager commercial Mike Bell told Marine Propulsion “LNG is the unique point of [the vessels]; the fuel system is very different to the normal, traditional diesel engines. The fact that they are pressurised and temperature sensitive has to be taken into account”.
Currently being finished at Strategic Marine’s shipyard in Vietnam, Willem Barentsz and Willem de Vlamingh are due to travel to the Wadden Sea area - where they will ply the route between Harlingen and Terschelling in the Netherlands - at the end of July this year. The vessels will then have final commisioning and trails upon arrival in Europe.
At the annual Interferry conference in October last year, Rederij Doeksen managing director Paul Melles explained the key reasons behind the ferry operator's decision to commission the newbuilds. he said the company wanted to replace old tonnage (its Midsland ferry that currently plies this route), meet demand for two additional midday departures from Harlingen and Terschelling, add more car deck space, allow more flexibility (to adapt to seasonal effects) and allow more daily turnarounds and increased capacity.
To achieve these goals, Rederij Doeksen decided to use two smaller ropax ferries rather than one single, large ferry. The operator also wanted to increase the service speed from 12 knots to 14 knots to meet its new requirements.
Mr Melles commented: “We wanted to increase the speed of the ropax ferry, but to do this to a 1,000 gt vessel on a 21 nautical mile shallow route would mean using a huge amount of power and create a disturbing wash in a World Heritage area.”
Instead, the ferry operator realised that building two smaller ferries to travel at 14 knots was achievable.
The ferry operator also wanted to reduce the environmental impact by lowering NOx, CO2 and noise levels and by increasing the efficiency of the ferries, leading to lower operational costs.
A reduced ship size means less wave-making and squat resistance, leading to less propulsion power used and reduced emissions and operational costs. Therefore, Mr Melles said the decision was made to construct the ferries out of aluminium.
Water depth challenges
Water depth was an important consideration in the build as it falls to just 5 m in some areas, with some quite turbulent sea conditions.
It was decided that the most efficient hull form for the shallow waters the vessels would travel was long, slender hulls. “This offers a safe and wide stable platform with no need to carry ballast water,” commented Mr Melles.
Rederij Doeksen head of operations Richard de Vries said: “The round bilge of the two slender catamaran hulls, with the very sharp bows, give the underwater ship a nice line and, with the anodes and sea inlets recessed, it will certainly ensure a very low ship resistance.”
Mr Melles added that the ferry operator’s research showed that an LNG-fuelled ferry, with direct mechanical propulsion, using batteries for peak-shaving to power their bow thrusters was the “most practical, reliable and clean energy source” for a 21 nautical mile ferry service for a fixed A-B route. LNG tankers from terminals in Zeebrugge, Rotterdam or Eemshaven will be deployed to fuel the ferries. The plan is to bunker once a week, so the ferry operator is creating a gap in the ferries’ schedule for this.
LNG engines based on diesel models
A symmetrical propulsion system arrangement within the two hulls of each catamaran was agreed upon. Two MTU 16-cylinder V 4,000 engines were chosen – fed by two LNG tanks – with each driven by a Veth CRP azimuth propeller.
MTU director application centre marine Stefan Müller said the main challenge was that the “propellers need a propulsion system that is very focused on very good load acceptance”.
To meet this challenge, the gas supply comes through a spark-ignition system to the engines, with an injector for every cylinder. Mr Müller said it was “very important” to have “individual combustion control for each cylinder” to meet the load requirements.
He explained that double-walled fuel piping was used, which is an IGF requirement for an “inherently safe” engineroom.
Another striking feature of the engines is that, according to Mr Müller, they are the first high-speed pure gas engine to operate with diesel-like performance.
He explained: “The first parameter was to deliver a product that had a behaviour very close to that of diesel, with the same performance range and map. The engines’ dynamic acceleration behaviour meets the requirements needed for this application.”
Its engines are based on MTU’s existing S4000 marine diesel engine.
The IMO Tier III compliant engines will each deliver 1,500 kW. The power output per cylinder is 93-125 kW with an engine speed 600-1,800 rpm and fuel gas consumption of 9,561 kJ/kWh at 100% power. The engine's cooling system consists of a separate circuit for charge air cooling.
The gas gensets are Scania-based units developed and manufactured by Scania distributor Sanfirden.
Waste heat to power batteries
Waste heat recovery systems are being provided by Orcan Energy, whose ePack system will be added to the main and auxiliary engines. The waste heat recovery systems are based on the Organic Rankin Cycle (ORC). Orcan Energy explained that the ORC solution works with a refrigerant (R245fa), which “boils” at 15ºCat atmospheric pressure. Via a screw expander, the vapour will be directly converted to rotational energy. This screw expander operates at a much lower speed and has a flat efficiency curve over a wide operating range, in contrast to a steam turbine.
The outcome (active power) from the waste heat recovery system is 77 kW. This is converted into direct current by a frequency converter to charge the batteries. Both battery packs are charged during the mooring of the ferries via inverters; they each deliver 77 kW of power to the two electric bow thrusters.
The Veth propellers also boost energy efficiency. They consist of two contra-rotating screws with two rudder propellers. Rederij Doeksen said these ensure efficient and low-noise propulsion. Because the power is distributed over two smaller screws, there are fewer pulsating vibrations compared to one large screw and this improves comfort on board. Smaller screws usually have a lower propulsion efficiency compared to screws with a large diameter.
The rear propeller of each pair uses the rotating wake of the front propeller and converts it into 10% extra propulsion force.
Aside from using LNG, the ferries have many other environmentally-friendly and efficient features: solar power will be used for part of the hotel electric load; boilers are LNG-powered; low energy LED lighting is being used throughout the vessel; zinc-free anodes for cathodic protection will be deployed and silicon non-toxic anti-fouling is being used.
Mr Bell summed up: “The entire design has been done to be as energy efficient and clean as possible.”
Capacity: 64 cars and 600 passengers
Truck lanes: 130 m
Speed: 14 knots
Propulsion system: two MTU lean burn single fuel LNG engines, each driving a Veth CRP azimuth thruster
Class: Lloyd’s Register
Construction: Strategic Marine
Vessel design: BMT Nigel Gee
Interior and exterior design: Vripack Sneek
LNG fuel system: Marine Service Noord Hoogezand
Waste heat recovery systems: Orcan Energy
Gas gensets: Scania (developed and manufactured by Scania distributor Sanfirden)
Contra rotating rudder propellers: Veth
Main engines: MTU