With 18 cargo tanks of 3,000m3, the 50,000 dwt chemical/product carrier Stena Impression can carry the maximum volume of IMO II cargo permitted per cargo tank. The vessel is the lead one in a series of 10 on order at China’s Guangzhou Shipyard International (GSI). By utilising every cubic metre on the vessel (in tandem with the company’s logistic capability), operator Stena Weco says it can offer lower unit costs on freight and fill up its ships. In other words: clients pay less and Stena Weco earns more. Competitor vessels, by contrast, tend to have larger cargo tanks, but must run part loaded because of the IMO limit on 3,000m3 of IMO II product per cargo tank.
“We recognised that instead of running ‘regular’ MR tankers with IMO II and IMO III capacity, we would get greater benefits from having pure IMO II tankers,” explains Stena Bulk chief executive officer and Stena Weco managing director Erik Hånell. “This approach also means we can be more specific when it comes to specifying equipment and plant. The vessel is also operationally more efficient. A conventional MR tanker with its larger tanks and activity across both IMO II and IMO III cargoes might require a week to 10 days to clean its cargo tanks in preparation for the next cargo. Our arrangement allows us to outperform that benchmark. This vessel is a one-of-a-kind and the first of its type in the world,” says Mr Hånell.
“All of these vessels have been built for operation rather than asset plays. We calculate that at the time we made the order, we have spent around US$30 million more on this series of vessels than we would have for 10 conventional MR tankers. But the designs will allow us to make daily operational savings of at least US$5,000, meaning we recover this outlay in about a year,” he says.
Stena’s MR vessels ply a triangulated route. Mr Hånell explains that these vessels will carry a lot of palm oil from Singapore to Europe and the US, with some side trips to China, India and Pakistan. He says “Soya bean oil from South America to China is another trade, and caustic soda from Korea to Australia and clean products from Europe to the US are other ones. We will of course try to get more into easy chemical trades, an area we have not traded in before.”
To support rapid loading and switching between different specialised cargo parcels across a range of temperatures, the cargo piping is of welded stainless steel. This is in line with FOSFA recommendations and the chemical trade. The deep well hydraulic Framo cargo pump system provides full flexibility, so that all cargo pumps can run simultaneously while controlled individually.
“Historically, we have used electrical pumps to a large extent. But for the IMOIIMAX vessels we favoured the hydraulic system due to its flexibility and stripping capacity,” says Jacob Norrby, Stena’s tanker newbuilding manager at Stena Rederi.
“We have a requirement to be able to circulate cargo in all tanks simultaneously. To do this with an electric system you have to add a number of converters, which means more complexity and a higher cost. When it comes to cargo heating, steam coils have been selected. In our experience, those together with the cargo circulation allow for a better solution than deck heaters,” he says.
When it comes to cargo tank inerting, an Air Products nitrogen system, rather than traditional inert gas generator, has been preferred. This, says Mr Norrby, not only allows for an extended cargo list but also decreases the time between discharge and loading due to faster tank cleaning. There is no need for portable nitrogen bottles for purging and/or cargo tank padding. It is also more fuel efficient.
The tank cleaning system provides for four tanks to be washed simultaneously with heated as well as cold sea and fresh water. The three fixed machines per tank have been laid out according to the Lloyd’s Register voluntary Effective Tank Cleaning class notation, to minimise shadow areas.
Not only has the IMOIIMAX been streamlined for commercial operation, it has literally been streamlined to support fuel-efficient operations. Mr Norrby says that the enhancements factored into the design by Stena’s innovative design department, Stena Teknik, mean that efficiency gains of 15–20 per cent are realised over the previous MR generation on the hull resistance and propulsion efficiency alone. The design features what Stena says are new and revolutionary tanker hull lines, which have been made possible through a very fruitful and close cooperation with GSI, as well as by drawing on Stena’s experience with roros and ropax vessels. The lines around the accommodation area and bridge have been rounded so that the vessel is more aerodynamic.
A number of other features have been specified by Stena Teknik to keep overall energy consumption to a minimum. It must also be said that the professional and cooperative approach of GSI has been instrumental in realising this unique design.
The MAN6S50ME-B9.3 type main engine is the latest version. It has higher energy efficiency compared with earlier versions. Together with the de-rating, the low load optimisation of the main engine – by means of an exhaust gas by-pass valve – makes the thermal efficiency extremely high. “For this size of engine we have never seen anything like it before,” says Mr Norrby. Additionally, an auto-tuning system ensures the combustion process in each cylinder continuously runs optimally.
Normally auxiliary engines are fuel optimised for high-load operation rather than part-load operation despite part-load operations being predominant. For IMOIIMAX, the new auxiliary engines have been part-load optimised, improving the overall performance and reducing fuel consumption.
Mr Norrby explains that one dilemma with having a main engine “that is extremely energy efficient is that the exhaust gas temperature falls, so we have less energy in the exhaust gases to produce the steam needed for onboard services and purposes. What we gain in energy efficiency on the main engine we would lose when firing the composite boiler. For this reason we opted for a new Alfa Laval-Aalborg multi-inlet exhaust gas boiler, a first for us. This boiler not only recovers energy from the main engine’s exhaust gas but also recovers the exhaust gas energy from two of the auxiliary engines. In addition, the boiler has an oil-fired section providing for increased steam production to suit the vessel’s domestic steam demand – should that be needed – without the need to run the bigger oil-fired boilers.”
Two oil-fired boilers provide for flexibility and redundancy, which is of particular importance when carrying heated cargoes such as palm stearin oil. In many instances only part-steam heating capacity is required. In such cases, one boiler can be run more efficiently than having only one big boiler operating at part load.
The company selected an Alfa Laval Pure Dry sludge/waste oil treatment system that separates the sludge into clean fuel, water and solid sludge. “We can actually reuse fuel that is normally left in the sludge and put it back into the system,” explains Mr Norrby. “We estimate that we will regain 1 per cent of lost fuel using this technology.”
A propeller with a boss cap fin has been installed. “This makes sense. Any propeller generates a hub vortex that represents energy losses. With the cap fin the vortex energy is regained” says Mr Norrby.
“We discussed having a shaft generator for power take off, but it is relatively expensive: it takes a long time to achieve a return on the investment,” he adds. “The propulsion train follows a shaft line with Kobelco air seal type stern tube sealing and stern tube bearings and lube oils that are complaint with US EAL requirements.” A Kyma shaft power meter and ship performance monitoring system has also been selected.
Stena Impression is 183m long, has a 32m breadth and a design draught of 11m. It has a total cargo tank capacity of 54,000m3 and segregated ballast tanks of 20,000m3. The 18 cargo pumps can run simultaneously and provide a total maximum discharge rate of 3,000 m3/h.
The vessel has an integrated Furuno bridge system that allows access to all essential navigation information from the centralized workstations. System components include two radars, three ecdis, conning display, two gyro compass, autopilot, two DGPS, Navtex, speed log, AIS and echo sounder.
The deck area is neatly laid out. For mooring equipment, the company has opted for Hatlapa mooring winches and windlasses. Rolls-Royce Marine supplied the steering gear. The provision and hose-handling cranes have been supplied by TTS. Xinjiang supplied the free-fall lifeboat and rescue boat, and Viking supplied the liferafts.
Each tank has a Framo cargo pump. Each cargo tank has double valve segregation with valves from Somas and P/V valves from PresVac.
An Emerson Rosemont cargo tank level gauge (radar beam type) system monitors the tank levels. Scanjet tank cleaning machines have been installed.
In the engineroom, an MAN B&W 6S50ME-B9.3, developing a total of 7,200kW, drives the propeller. A Unitor sprinkler and CO2 system protects the engineroom. Consilium supplied the fire detection and alarm system as well as the voyage data recorder. Kongsberg delivered the engineroom alarm and monitoring system. Other auxiliary equipment includes Wilhelmsen-supplied air conditioning and refrigeration equipment, a Brännströms Elektronik oil discharge monitor approved for the mixture of biofuel with petroleum of mix greater than 75 per cent, and a Tank System oil/water interface detector and dip valve. The starting air compressor was supplied by Sperre, the screwtype working air compressor by TMC and the purifiers by Alfa Laval.
The hull is protected by a Cathelco impressed current cathodic protection system, and coatings have been supplied by Jotun. A Headway Technologies ballast water treatment plant has been installed. “We needed a system that could treat SW, FW and brackish waters as well as water with high turbidity. Of the systems on the market today, we found this one met our requirements. Crucially it does not involve the use of chemicals, and we do not foresee excessive maintenance. Also, we wanted a system that we would operate only when ballasting, Mr Norrby explains.
The vessel will be operated by a crew of mixed nationalities. Extra cabins have been built to allow for a few cadets to sail on board from time to time. Social amenities have been prioritised. These include state-of-the-art recreational areas, accommodation and complimentary Wi-Fi with sufficient bandwidth to allow for Skype calls home. TST
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