Engine derating, better propeller design and optimal hull designs help achieve IMO’s EEDI mandate
Following the financial crisis of 2008, as bunker fuel prices increased and freight rates dropped, improved energy efficiency came into greater focus. Higher engine efficiency was sought through derating. This trend has continued despite the fall in fuel prices in 2014-15, says engine designer MAN Energy Systems. Derating has become the de facto standard, according to the company.
Beyond the engine, efficiencies were sought through modifications to the propeller and the ship’s hull. Propeller diameters increased and significant focus was given to aft ship design. Ship designers say some designs saw efficiency improvements of 10-15% between successive design iterations, reducing the power required to propel the ship.
As fuel prices dropped, IMO’s Energy Efficiency Design Index (EEDI) came into force in 2015, which is sequentially reducing the permissible CO2 emissions per cargo-mile transported for new ships up to 2025. This has made ship designs advance on the path of overall energy and fuel efficiency. Higher propulsive efficiency has remained a key target.
The use of advanced numerical methods has led to a better understanding of hull resistance and better optimisation of propeller design, says Berg Propulsion managing director west, Jonas Nyberg, adding that, in general, a bigger propeller turning at lower speed gives a significant boost to efficiency. “Over the last five years we have seen a dramatic difference in the trends towards shipowners and designers becoming much more aware and focused on ship efficiency. This seems to be driven by upcoming regulations, EEDI and an overall greater awareness of operational cost and what influences this,” says Mr Nyberg.
MAN Energy Systems reported that in the last 10 years, maximum continuous rating (MCR) as a percentage of maximum power has dropped from 90% to 75%. When an engine is specified with a mean effective pressure (mep) below the value corresponding to the mep at maximum power, derating is achieved.
Derating an engine improves the specific fuel oil consumption of the engine as a larger part of the combustion can be performed at optimum efficiency and has been a hugely popular method for increasing efficiency, according to MAN Energy Systems naval architect Philip Holt. “This desire to derate an engine without reducing the actual specified maximum continuous rating (SMCR)/power installed, implies that a cylinder is often is added to the engine to achieve the derating effect, such going from a 6S50 to a 7S50 engine for SMCR of 8,500 kW,” he says. Alternatively, the bore size is stepped up to produce a larger displacement to deliver the same power, ie stepping from 7S50 to 6S60 to propel a handymax bulker, he adds.
An engine specified at a given MCR below maximum power cannot operate up to that power, but only to the SMCR. The torque of the engine is maintained as there is little impact on the shafting by adding an extra cylinder for the same SMCR, ie power and rpm, but a benefit is that cyclic deviations from mean torque reduces. This has a positive impact on where the barred speed range occurs, says Mr Holt.
Bigger and slower is better
Mr Nyberg has seen an increase of 10% in propeller sizes in merchant ships in the last decade. For instance, an 18,000-dwt tanker would have traditionally had a 5.4-m propeller and a 5,000-kW main engine. “Applying a 6.0-m propeller would mean you can achieve the same ship speed with a 4,500-kW engine thus building efficiency into the design on a permanent basis. This is among the least expensive choices available,” he adds.
Ships that do not require pilots, such as ferries, use controllable pitch propellers (CPP) and therefore have flexibility automatically built into the design. The main engine could be at full rpm, but the control system can apply a moderate pitch or a significantly reduced engine speed while increasing the pitch. The latter is more efficient. “The question on where the exact optimum lies still remains though, as you can operate anywhere from full engine speed and low pitch, to low engine speed and high pitch with a ship that accomplishes the same speed,” adds Mr Nyberg.
One solution is to apply a system to automatically optimise the engine speed and propeller pitch by measuring the actual fuel consumption of the engine and then minimise it. In the case of Berg Propulsion, its Dynamic Drive achieves this. “In this, the engine and propeller plant is always operating at its optimum taking into account ship speed, engine load and other limitations while optimising the fuel consumption,” he says.
Mr Nyberg talks about how the propeller should be designed based on the hull, not the engine. “It should not be like fitting an accessory to an existing engine choice,” he says. Many owners are aware of this and are working closer with ship designers and shipyards, he adds.
Having a large propeller would require a suitable aft ship hull design that provides the room. The constraints to propeller size and speed are cavitation, in which depth of immersion plays a role, and pressure pulses on the propeller, which is significantly impacted by the distance between the tip of the propeller and the hull.
Trade-offs in hull design
Safinah Group senior consultant in whole ship design and naval architecture Keith Hutchinson says typically ocean-going commercial ships, whether deadweight or capacity carriers, are still designed primarily to provide the specified transportation capability, namely cargo carrying capability, service speed and range, at minimum capex. He says ship design involves multiple and often conflicting operational performance aspects such as dimensional constraints, cargo handling, accommodation, general arrangement demands, stability, seakeeping and manoeuvring performance, even as maximum efficiency is sought.
He explains ship powering can be simplified to two components: resistance – hull, appendages, weather (waves and wind), fouling etc; and propulsion – hull efficiencies, propellers, transmission, prime-movers and so on. Mr Hutchinson says the careful and robust selection of optimal principal dimensions, which are often frozen too early in the design, is by far the most important driver on resistance and propulsion efficiency and hence fuel costs, which is a significant proportion of any ship’s opex. But, in general, dimensions, form ratios and coefficients optimised for minimal opex seldom coincide with those for minimal lightship and capex. Shipbuilders’ capabilities, production processes and infrastructural constraints may also significantly influence the design offered.
An optimal hull form has a significant influence on resistance, hence the selection and efficiency of the overall propulsion system. The bow form must consider performance at both full load and ballast draughts, and realistic sea states, not just ‘clean-calm’ conditions. The stern configuration must consider the engines, accommodate the most efficient propellers and maximise immersion when in ballast. The stern form must also afford optimal flow into the propellers while minimising resistance and facilitating good manoeuvrability and seakeeping characteristics.
Mr Hutchinson highlights the iterative nature of ship design, including the design of propellers and engine selection. “Optimising propulsion efficiency requires the close, and often creative, co-operation of the ship designer with both the propeller and engine manufacturers”.