Prospero: a future-proof design for the small, clean tanker sector

The 14 cargo tanks are coated with Jotun Tankguard Special. The cargo pipes self-drain down to the manifold and from the manifold to the cargo tanks using a crossover system. The inert gas is supplied via a combined dual-fuel thermal oil heater and an inert-gas producer supplied by Göteborgs Energy Systems AB (GESAB). This is a boiler with heat recovery that uses the exhaust gases from the oil-fired heater to produce inert gases to the cargo tanks, via scrubber/cooling unit. It is fitted with a Catamiser system connected to diesel generators and main engine onboard. Low NOx gases are used as preheated combustion air for the thermal oil heater/boiler and by this, a higher efficiency grade of the burner/heater will be achieved, lowering the fuel consumption to the burner.

Other pumps include scrubber, fire, standby, bilge, greywater, and fuel, while various portable pumps have been supplied by DESMI.

Electric cargo pumps are considered ‘good neighbours’ in small ports, where the whine of hydraulic pumps all through the night is no longer regarded as acceptable, especially when alternatives are available.

On the deck, the mooring winch and the anchor windlass are also electric powered, supplied by THR Marine of Groningen in the Netherlands. As well as being electric, THR Marine winches and windlasses are up to 25% lighter than conventional hydraulic models – meaning less mass and less fuel required to move it.

Two other main pieces of equipment have been chosen on the basis that they do not require oil: the stern tube seal and bow thruster. The stern tube seal is a Wärtsilä water-lubricated stern tube seal and the electrically controlled and driven bow thruster (model WTT-14 FP) is also supplied by Wärtsilä.

The ballast-water treatment system is supplied by Headway of China. Headway’s OceanGuard HMT-1500 ballast-water management system (BWMS) uses filtration, advanced oxidation and electrocatalysis to deal with organisms. Headway BWMS units have been fitted by Shanghai Waigaoqiao, Zhoushan Yangfan and Jiangsu Hantong shipyards since 2014 and received US Coast Guard type-approval in January 2020.

“The hull shape and antifouling coating contribute significantly to lowering emissions”

It is believed that Prospero and Pacifico will be the first tankers to be fitted with the Orcan Energy’s efficiency PACK, a waste-heat recovery system that is based on an Organic Rankine Cycle (ORC). The efficiency PACK uses waste heat from the thermal oil system and the jacket cooling water to produce electricity, thereby recovering waste heat from both the main engine and the auxiliary engines. In order to do that, a heat exchanger transfers waste heat from the thermal oil circuit to the ORC. The HT water is routed through the efficiency PACKs directly. Here, the refrigerant is evaporated – a non-toxic, non-flammable hydrocarbon – and routed to the expansion machine as superheated vapour. The highly pressurised refrigerant is expanded, thereby driving the expansion machine. The rotational energy is used to drive a generator that produces electricity.

One of the main advantages of the expansion machine inside the system is that it is very well suited to fluctuating amounts of waste heat, meaning that it retains its efficiency. The efficiency PACK will provide well over 80 kW of additional electrical net power to the ship’s grid while discharging, because of the energy-intensive inerting operations. At sea, the waste-heat recovery system contributes significantly to the onboard electricity demand, by feeding back more than 70 kW into the grid.

The efficiency PACKs result in a CO₂ reduction of 295 tonnes and fuel savings of around 100 tonnes per vessel. In other words, for both tankers combined, the efficiency PACKs will save the equivalent of approximately 5,000 gas tanks for cars, or the electricity consumption of over 300 German three-person households every year.

Donsötank managing director Ingvar Lorensson, said: “This generation of tanker is designed with a focus on energy efficiency from the beginning. To recover the waste heat and convert it to electricity is one step to minimise the emissions. We came in contact with Orcan Energy at an early stage and their solution fits perfectly into our vessels.”

The installation of the efficiency PACKs was supported by the Swedish Environmental Protection Agency. The Swedish state Klimatklivet (Climate Leap) initiative aims to reduce emissions that affect the climate by making local investments for the greatest possible climate gain.

Also contributing significantly to the lowering of carbon emissions by reducing fuel consumption is the hull shape and antifouling coating. The vessel series has been designed by FKAB of Sweden and the hull is optimised to achieve maximum cargo intake, with low fuel consumption, and excellent performance in calm/ harsh weather conditions.

The hull is coated in Jotun’s SeaQuantum, which is the third generation of its silyl acrylate technology. SeaQuantum ensures a lower volatile organic compound (VOC) content and higher volume of solids, ensuring compliance with strict global and regional VOC regulations. It has a long-term hull performance up to 90 months, with Micro Zone technology that prolongs the time between the activation of coating biocides and their dispersal, keeping them closer to the hull for longer, creating a protective shield against fouling.

These features are good example of marginal gains adding up. In a reference to the Team Sky cycling equipe, a company spokesman said that amassing small marginal gains is as important as one large gain. In this case, the large gain is from the propulsion system, which combines the use of LNG/LBG as a marine fuel with a hybrid system.

The main engine is a Wärtsilä W10V31DF, which is four-stroke dual-fuel unit, capable of operating on LNG and/or LBG. This engine has been awarded the Guinness Book of Records ‘Most Efficient 4-Stroke Diesel Engine’ award.

The main engine forms part of a propulsion system that includes WE-Tech of Finland’s hybrid technology.

Both Prospero and Pacifico are shore-power-ready, but questions remain over where to place the connections on the vessel. Some ports are introducing shore-power amidships at the berth, and others at the stern. The mid-ship option is the likely choice for Q1 2023, said Donsötank technical manager Henrik Lorensson. The vessels are ready to receive 6.6 kV from shore – enough for cargo-discharging operations and charging the batteries.

The utilisation of cleaner shore power can reduce emissions in harbour and create major savings. The battery package (ESS – supplied by Corvus) is connected to the common DC-link of the main propulsion drives and provides power for peak-shaving and backup functionality.

“This generation of tanker is designed with a focus on energy efficiency from the beginning”

The Corvus battery pack forms an integral part of the propulsion solution, with 500 kWh connected to the switchboard to aid with peak shaving and as an alternative to running auxiliary engines in narrow passages. There is no lag when demanding power, whereas an auxiliary engine would require time to spool up to produce the required power.

This balances the loads between the main engine and the auxiliary engines (two Wärtsilä 1400W8L20s) during voyages or cargo operations and is estimated to save 284 tonnes of CO₂ in one year in open sea and 269 tonnes of CO₂ in one year in port.

The oily water separators have been supplied by RWO and are of the classic OWS-COM 0.5 design, with discharges of bilge water monitored by an advanced Clean Bilge Monitoring system. On the liquids management side, other energy saving/waste-heat utilising features include the HYDRONIQ 600CX1200 rackcooler. This system is used to cool the engine and other components instead of the traditional open-to-sea box-cooling system. In the rackcooler, a closed-loop, high-pressure flow of seawater runs around CuNi pipes (which prevent marine growth). Individual removal for servicing is possible due to the ‘rack’ arrangement of the heat exchangers.

Driven by frequency-controlled pumps, the flow of water is agnostic to the speed of the vessel. The loss of cooler boxes also achieves a better hydrodynamic profile. The HYDRONIQ rackcooler uses less energy, has fewer servicing parts and does not require drydocking as part of the maintenance programme. Being separated from the hull also reduces bi-metallic corrosion.

The innovations continue with the mooring arrangements. The crew of Prospero are mainly Swedish and Filipino, and the deck equipment and safety areas have been designed with typical Swedish dimensions in mind, offering a larger working area. The choice again is for electric power over hydraulic when it comes to the winches and windlasses. The vessel uses Wilhelmsen’s anti-snapback mooring rope which fall to the deck should the mooring rope part, avoiding the dangerous whiplash associated with traditional ropes. There are eight drums forward and eight drums aft and the layout is designed for safe operation in accordance with MEG 4.

Complementing the 6.2 m controllable pitch Wärtsilä propeller is a specially designed full-twist leading-edge rudder. The rudder is optimised for the propeller wake field and slipstream analysis points to significant savings.

Less of an innovation, more of an evolution, is the quality of the accommodation and furnishings, which employ the latest ideas on crew wellbeing. Again, the cabins and restrooms are immediately recognisable as Swedish in style. Crew wellbeing is supported by a full communication package, with the bandwidth to stream films and games.

In the bridge, the majority of equipment has been provided by local supplier Simbo Marine Systems and consists of Furuno navigation and radio equipment (X- and S-band radio, AIS, ECDIS, alert system and so on). Simbo also supplied the Anschuetz gyro compass and rate-of-turn indicator. The ship’s monitoring and control system is supplied by Smartautomation.

The Seematz window wiper ensures a view forward from the bridge under all conditions, and that view is of a tanker that already achieves the 2030 and 2050 IMO GHG requirements.

Prospero dimensions

Type: IMO 2 oil products and chemicals tanker

Builder: Wuhu Shipyard, China

Delivery: 2021

Capacity: 22,543 dwt

Capacity: 28,000 m³

LOA: 167.3 m

Beam: 26.5 m

Draft: 9.4 m

Speed: Economical service speed 13.5 knots at design draught, 86% MCR with 15% sea margin and shaft generator output 250kW, FOC LNG 15.0 tonnes per day and FOC MGO 17.8 tonnes per day.

From the sea trail result, the consumption of LNG is 10.0 tonnes per day at 12 knots.

Class Notation: DNV, + 1A1 Tanker for Oil Chemical, ESP, ICE (1A), CSR, EO, HL(1,54), TMON, BIS, NAUT(OC), CLEAN (Tier III), Recyclable, INERT, VCS(2), RP(1,22), BWM(T), COW, SPM, LCS, Gas fuelled.

How WE Tech systems are used on the vessels

WE Tech’s Solution package is used on both vessels and is based on WE Drive (variable frequency drive technology) and permanent magnet shaft generator technology. It provides economical operations, take-me-home mode, boost mode, efficient power distribution and an energy storage system (ESS).

In Power Take Out (PTO) mode, there is no need to run auxiliary engines and generators when sailing, reducing fuel costs, emissions and saving on maintenance requirements. Switching to the Power Take In (PTI) Boost Mode allows the vessel to adapt to the most demanding conditions, such as those in ice-covered water, using the shaft generator as a reliable and economical auxiliary propulsion drive.

In addition to offering lower fuel and operational costs while sailing, the solution offers an efficient method to distribute power to bow thrusters and the cargo-handling system of the tankers. The WE Drive can condition any available voltage and frequency, ensuring the tankers’ electrical system is unaffected by various national power grids. The utilisation of cleaner shore power can reduce emissions in harbour and create savings. The ESS, which is connected to the common DC-link of the main propulsion drives, provides power for peak-shaving and back-up functionality.