Compact Azipod units promise the means of producing more efficient thrust for drilling vessels, saving fuel cost and cutting emissions. OE hears from developer ABB Marine.
Bunker prices for the IFO380 grade fuel used by the majority of ships were on the rise at time of writing, reaching around $465 per tonne. This is by no means a record level, but marine fuel costs can represent up to 60% of a ship’s overall operating cost*.
Meanwhile, increasing pressure is being exerted by legislators to limit emissions of the NOx, SOx and particulates that go hand in hand with burning such low grade fuel. In total, shipping consumes around 300 million tonnes of bunker fuel per year.
Accordingly, the Second International Maritime Organization Study on Greenhouse Gas Emissions (IMO GHG) concluded that shipping was responsible for 2.7% of global CO2 emissions. The IMO has furthermore suggested that, if unrestricted, emissions may increase by 150-250% by the year 2050 due to expected growth in international seaborne trade.
Again, according to recent estimates from Helcom, the total NOx emissions from ships in the Baltic alone amounted to more than 393,000t in 2008.
While a precise framework covering how ship-specific emissions legislation would be developed eluded the great minds visiting the UN Framework Convention on Climate Change (UNFCCC) summit in Copenhagen (COP15) last December, the IMO has already debated an Energy Efficiency Index that points towards a world where individual ships are assessed in terms of their environmental credentials. Careful fuel management and increased efficiency have therefore become vital for both financial and environmental reasons.
In September 2009, the UK Chamber of Shipping said that it expected new technologies and designs to deliver energy efficiency savings of up to 40% on new ships relative to typical ships delivered in the 1990s.
Beyond smoother foul release hull coatings and more efficient engine ignition, one such technology that can be shown to deliver tangible energy savings is the Azipod CZ thruster.
ABB Marine recently raised the capacities it offers of its Azipod CZ thrusters to 4.5MW, specifically targeting the drillship and rig markets, where thrusters are used to hold station as part of dynamic positioning. The company says that using a podded-type solution in a drilling rig application contributes to an increase in overall propulsion efficiency in a range between 10% and 15% when compared to conventional technology.
‘Designs for any drilling rig with electric propulsion and dynamic positioning have to address two key issues,’ says Jorulf Nergård, ABB AS Marine & Cranes’ VP sales, floaters. ‘The first is the need for continuous availability of the electric power plant, which is essential for safe and reliable operations. The second is a desire to reduce fuel consumption and emissions, which is driven by the high cost of oil and demands to improve environmental performance. The podded solution has a significant impact on the latter issue.’
Alf Kare Adnanes, ABB Marine technology manager, outlined what was at stake. ‘Let us consider a sixth generation semisubmersible with eight thrusters, each offering 3.8MW, for example. Assume that thruster capacity is utilized at around 25% on average over the course of the year. As a year consists of 8760 hours, we can say that the thrusters need to supply 8 x 3800 x 8760 x 0.25, or 66,576,000kWh of energy to the thruster shaft. With 10% transmission losses assumed, this means 73,233,600kWh of energy needs to be produced by the diesel engines.
‘The specific fuel oil consumption is in the range of 190g/kWh at the optimum loading point. This is not achieved in practice, so 200g/kWh is more realistic. Therefore, the total fuel needed to feed the thrusters (excluding other loads) amounts to 14,646,720kg of diesel oil per year. If we can save 10% of this by better efficiency of the thrusters, that means 1465t of saved diesel oil on an annual basis.
‘If the MDO price was, say, $600/t, that would add up to $880,000 saved each year. That may not sound like much when compared to a total rig rate, but it is still money in somebody’s pocket, and also means lower lubrication consumption, and lower demand for installed engine power. It would also correspond to 4687t of CO2 reduction.’
Nergård compares and contrasts the Azipod CZ with a conventional thruster solution as installed on a drilling rig. ‘A podded solution does not require additional reduction gears and shafts in between the electrical motor and the propeller that suffer from power losses,’ he notes. ‘Instead, the pod’s motor is connected directly to the propeller. With this solution we can also use far fewer bearings compared with a conventional solution.’
He adds that a 4.5MW Azipod CZ thruster competes realistically with a traditional L-drive thruster ‘rated 5MW or more’ because the podded solution suffers no losses in the gears and bearings in the transmission of thrust from the motor to the propeller, and no loss in the nozzle and towards the hull. ‘In the traditional solution there are six shaft bearings compared with only two in the Azipod unit solution. In practice, this yields a saving equivalent to around 3.5% in terms of energy used.’
The motor used in the podded solution is directly connected to a triple sealed shaft, and the housing is directly cooled by seawater. ‘The permanent magnet motor type used in the Azipod design offers about 2% better efficiency than typical induction motors,’ says Nergård. ‘Because the motor is cooled directly by seawater and is not located inside the pontoon or hull, there is no heat dissipation from the motor inside the hull that needs to be handled by the vessel’s cooling system. Therefore we save the vessel cooling systems around 4% of their heating capacity. For a single thruster, that means a heat saving of around 130kW, while for a complete propulsion system that adds up to a 1MW saving.’
Further savings are conferred on the podded solution by virtue of the configuration of the controls. Propeller speeds are controlled by a frequency converter, where the voltage source is controlled by ABB’s direct torque control (DTC) software. The frequency drive is fed from the electrical 11kV system with a quasi 12-pulse transformer (all are also water-cooled).
‘The steering gear is also controlled by electrical motors, where conventional solutions tend to feature a hydraulic power pack. ‘This means that we do not need to run the electrical steering motors unless we are rotating the thrusters,’ explains Nergård, ‘whereas in the hydraulic power pack solution the electrical motors need to run continuously.’
He says the largest impact is felt when the propeller shaft is tilted 7° in relation to the pontoons or the drillship hull. ‘In this case, we produce a water jet that interferes significantly less with the other pontoon or the other thrusters, when compared with the most common solution, where only the nozzle is tilted. Depending on the position and location of the Azipod unit, interference can be cut by anything between 4% and 8%. The DP system can thus configure the direction of the thrusters more efficiently.’
While the weight of the Azipod thrusters is clearly greater than is the case for conventional thrusters, because the motor is located in the propeller housing, maintenance, dismantling and re-installing is far less complex and can be achieved without interrupting drilling operations.
‘One of the keys to the design of the Azipod thrusters is its simplicity,’ says Nergård. ‘In order to achieve the greatest degrees of reliability and redundancy, the Azipod CZ has as few parts as possible.’
Summing up, Nergård says: ‘A podded solution offers the best ratio between electrical power produced and resulting thrust. The podded solution yields higher electrical power redundancy, meaning that the owner has a choice as to whether to keep more power in reserve, or to make the design more cost and weight effective by reducing the size of the diesel generator sets. In some cases, this can mean a reduction in the number of cylinders required – say from a 16-cylinder engine to a 12-cylinder engine. This, in turn, can mean a saving in both fuel consumed and CO2 emitted.’ OE
* Container ships fuel as a proportion of overall operating costs. Vessel Usage Study. Helsinki Commission.