In Depth: Minerva

One of the biggest conundrums in ship owners’ and ship designers’ offices today is which will be the fuel for ships of tomorrow. Recently, the IMO set the date for the use of lower sulphur fuels worldwide for 2020. Fuel prices have always been rather volatile, but the last few years seem like a rollercoaster and no-one dares to predict what comes next.

Sulphur emission reduction

To comply with the upcoming sulphur emission rules, there are roughly three options on the table: switch to more expensive low-sulphur fuels such as marine gas oil, switch to cleaner-burning types of fuel such as LNG and methanol or keep on sailing on high-sulphur heavy fuel oil on a vessel equipped with scrubbers.

The choice would be easy if you could know the prices of the different fuels in ten years’ time, as it’s a balance between a higher initial investment versus higher operating costs.

Belgian dredging and offshore contractor DEME Group has made a clear choice. For its newbuild vessels, the company goes all-in on LNG and sees this as the fuel of the future. In this article, we look at the trailing suction hopper dredger (TSHD) called Minerva, but DEME has three other dredgers currently being built which will also run on LNG.

Dual fuel

While designated “LNG”, most marine applications are actually dual-fuel: to ignite the LNG, a relatively small amount of marine diesel oil is injected into the cylinders. But there’s an additional advantage: these engines can also run on MDO only. This can be done if the price differential or limited availability of LNG warrant this. When the vessel sails only on diesel, the significant investment in the LNG installation is basically worthless, but the dual-fuel solution does provide DEME the necessary flexibility to operate the vessel worldwide, also in places where LNG is not yet available.

For DEME, there were several arguments to opt for LNG as their future fuel. First of all, the company has a corporate social responsibility which goes well beyond a paragraph on its web page. Anyone who has been face-to-face with the exhaust pipe of a gas-engine and that of a diesel engine can attest the difference.

The numbers back this up: LNG eliminates almost entirely the emission of particulate matter and SOx. LNG emits 65 to 90 per cent less NOx and 10 to 20 per cent less CO2. LNG is considered to be the cleanest option that is now widely available and also a very future-proof fuel, as it already complies with the strictest norms without needing any exhaust after-treatment.


While oil reserves are shrinking, the worldwide reserves of gas are abundant. A big part of the cost of LNG is in the infrastructure to bring it to the vessel, and it’s expected that economy of scale will significantly reduce the LNG price if more ships adopt it.

Finally, DEME wanted to build a harbour maintenance dredger which will work often in densely populated areas. It is expected that the cleaner emissions will score valuable points in public tenders and therefore attract more work.

These are all good reasons to consider LNG as a primary fuel, but is it also suitable for a dredger? The difficulty of dredgers, when comparing with a container vessel for example, is that during the dredging cycle, there are very different loading cases (dredging, pumping to shore, sailing, etc.) and a lot of sudden shifts in power demand. This could be a good case for a gas-electric installation, perhaps in combination with a battery bank for peak shaving, but DEME has always given preference to the simplicity and efficiency of direct drives. In this case, simplicity for the user was achieved, but it took quite a lot of programming effort to make it work.

When DEME put out a tender for a dual-fuel powered dredger, IHC had already been working on an R&D project regarding alternative fuels since 2012. The two companies matched and started working together on the design of Minerva and Scheldt River.

Integrated solution

It was clear early that LNG propulsion is not something you can easily add to an existing design of a TSHD. The requirements for the LNG storage tank, hazardous zones, bunker stations and detection systems all call for an integrated solution.

On a TSHD, there is simply too much going on on deck to just slap on a few cryogenic tanks. On Minerva, the LNG storage tank is located in a large compartment under the accommodation. The tank capacity of just over 200 m3 of LNG, stored at a temperature of -162 degrees, is enough for one week of operations before refuelling.

The tank is a double-walled stainless steel tank with polyurethane foam between both skins. As the large temperature fluctuations cause significant thermal deformation of the tank, its supports are welded on one side and mounted on a sliding track at the other side.

LNG piping

Bunkering of LNG is done with either 2-inch hoses from a truck or with 4-inch hoses from a barge or bunker station, such as the ones in Rotterdam, Zwijndrecht (NL) and Zeebrugge. The onboard filling station has stainless steel drainage provisions to avoid LNG spillage on deck, as such a spill would make the steel brittle, which can cause cracking.

All piping where the gas circulates in nonliquid form is executed double-walled. The space between the pipes is ventilated, along with the space around the tank, the carters of the engines and the gas street.

These vent pipes are led to masts which bring the ventilation point to seven to ten metres above deck. The jacket pipe system was prefabricated of 316L-grade stainless steel and the sections were joined together onboard by welding.


A nitrogen generator type NGP10+ from Atlas Copco is installed on board for two purposes. The nitrogen gas produced is used to purge the hose couplings after bunkering and it is also used to regulate the pressure in the tank, as adding air to methane vapours could result in an explosive mixture.

Instead of using the boil-off of the tank in the engines, the system uses nitrogen to cool the tank to avoid the generation of boil-off altogether. The complete LNG system and propulsion system was supplied by Wärtsilä as an integrated package.

For the main engines, DEME specified dual-fuel engines from ABC (Anglo-Belgian Corporation). Wärtsilä’s daughter company SAM Electronics supplied its NACOS integrated navigation, automation and control system. As all recent TSHD’s for DEME, Minerva can be operated by only one person on the bridge.

Two-speed gearbox

Even with a controllable pitch propeller, the difference in required propeller characteristics during low-speed dredging at 1-2 knots and sailing at 12 knots is so big, that propeller design is a compromise between extremes. On Minerva, IHC introduced an innovation to improve the propeller efficiency: the two-speed propulsion gearbox. As the propeller rotation speed in both cases is now less far apart, it allowed designing a more efficient propeller.

Efficiency was sought throughout the project and for instance, the working air compressor is a VSD type from Atlas Copco which uses a permanent magnet motor making it 50 per cent more efficient than conventional designs, without belt drives.


The main engines are each coupled to a gearbox driving a propeller shaft with controllable pitch propellers in nozzles. On its forward side, the port side main engine has a shaft passing through the engine room bulkhead to drive the dredge pump. The starboard main engine drives a generator on its forward side, which provides the power for the jet pump and the hotel load. Fluctuations in load caused by the dredge pump are very common, and it was a challenge to isolate these fluctuations from the direct-driven propeller (which would cause thrust fluctuations). In fact, what looks like a simple throttle on the wheelhouse, is actually a “fly-by-wire” speed controller for the propeller.

The software behind the lever governs the pitch of the propeller as the rotation speed of the main engine fluctuates due to the dredge pump. The software keeps the provided thrust from the propeller constant at varying engine speed. The starboard main engine drives a generator, and here also, flexibility is ensured. To allow for variations in engine speed, the ship’s onboard power grid has a frequency which can vary from 50 to 60 Hertz.

Docking interval

Another innovation is found in the hopper. Minerva is the first vessel to feature bottom door seals which can be replaced without having to drydock. This, along with a thicker anti-fouling coating and the In-Water-Survey notation allows increasing the interval between drydockings from 5 to 7.5 years.

This results in a greater flexibility in planning long-term contracts and therefore in a higher profitability.


With Minerva and her larger TSHD sister Scheldt River which will be delivered later this year, DEME shows that striving for efficiency and profitability goes hand in hand with corporate responsibility.

In Royal IHC they have found a technology partner which is willing to go the extra mile and develop these new ships rather than building a safe copy of an existing vessel.

While the LNG option reportedly adds about 10 to 15 percent to the build cost, it’s an investment that will surely be recovered when MGO prices get their “IMO-2020 hike”. But perhaps more importantly, at near equal costs, they are the cleanest ships which should be the first ones to be contracted, anywhere in the world.

Minerva is without a doubt the cleanest TSHD on the planet right now. Having the guts to innovate and invest in energy-efficiency will be compensated in hard-earned cash. Green is obviously more than the colour of their ships for DEME.

Bruno Bouckaert

This article was previously published in Maritime Holland edition #3 – 2017.

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