which an error band of 2-3% likely applies in the translation of results to full scale. Yet we rely on model test results even when the device has a similar performance enve- lope to that of the error band. And can we accurately measure the performance of a device added onto a ship when its perfor- mance is 1-2%?”
in sea going conditions, application of more extensive weather routing systems, conduct of energy audits and deployment of perfor- mance monitoring tools. Similar activities are in hand in respect to its tanker fleet(s).
One of the biggest problems is that often the media quote the benefits of a device at a single measurement point, they don’t take overall performance of the vessel into ac- count, he says. “And then this single point performance figure becomes enshrined and relied upon within the industry. However, it is rare that vessels sail in calm conditions, at their design draught and at their design speed. It could be possible that a device that appears beneficial at this ideal condition offers additional drag when considered over the operational profile of the vessel.”
Mr Connolly adds that there is a lot of con- fusion about what actually works on ships. “We need to get clarity about what you should apply and how you should analyse it.” He calls on MARIN and all the hydro- dynamic experts to guide the industry on how to deal with the new energy saving technologies, explain where the limitations are and provide a roadmap that the industry can follow. And additionally, put to bed some of the misconceptions.
Favours numerical analysis (CFD) Mr Connolly says he is not convinced the answers to the real value of energy saving technologies and devices will come from model testing. “Model testing has its diffi- culties given that the majority of devices operate within the ship’s boundary layer to
He actually sets more store on numerical analysis - specifically CFD. “Our approach has been very much geared to assuring ourselves that the fundamentals of improved performance associated with a technology hold true when deployed to a chosen target vessel”. This in the majority of cases involves the application of sophisticated numerical modelling (CFD). The appropriate modelling method however, needs to be deployed to the target ESD under review. Incorrect choice can lead to over simplification of hydro- dynamic interactions and correspondingly, over estimation of energy savings. Again the appropriate measurement of performance also needs to be considered, simply consid- ering drag reduction is not appropriate and one must consider the impact upon propul- sive power with specific emphasis given to the change in propulsive characteristics of the propeller.
“We have tended to utilise model testing (both aerodynamic and hydrodynamic) to validate our numerical modelling. Our numer- ical modelling is conducted at full scale and this, coupled to Sea Trial data provides a sound approach. Against this backdrop we identify from operational data for the target vessel her speed, draught and environmen- tal (waves) profiles. The performance of the ESD is then assessed against these derived profiles in order to arrive at the likely impact that the device will have in operation. If from the numerical modelling we can determine a business case for pursuit we seek to under- take a further round of analysis, normally conducted independently, to act within an assurance role. This might take the form of model testing. Deployment at ship in a trial would then follow if the business case holds true.” Obviously measurement at ship over an extended period is a pre-requisite for devices where the savings are likely to be <5% and questionable in terms of prac- ticalities where impact of a device is likely less than 2-3%, he points out.
For some ESDs it is questionable as to whether you will truly be able to measure
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performance at ship. That in itself does not preclude adoption. Under these circum- stances the business case will need to rely wholly upon the analysis and one must have faith that the methodology adopted is the best that can be deployed.
For this reason Mr Connolly welcomes the MARIN-led REFITSave2 JIP and thinks it is long overdue. The REFIT JIP aims to identify the correlation between numerical modelling, model testing and performance at ship of ESDs. This JIP should provide better insight into the methods and tolerances that should be applied when selecting a device. “This is thwart with difficulties but it is required to gain a true understanding of these devices. We have learnt a lot from REFIT and our own research. But I do see model testing and numerical modelling in reverse.”
Well-designed ships rather than ESDs And in some ways, the industry is taking the wrong approach to ESDs in general, he suggests. “ESDs are in more and more cases viewed as the answer to improving a design’s performance when in reality the greatest gains in performance are enshrined in optimising the hull dimen- sions, improving the lines and flow into the propeller and better matching the design to the operational conditions. One could be rather cynical and suggest that adoption of ESDs works when the design of the hull and propulsion system is not correctly executed. The future lies in designing holis- tically better ships.”
In a more pragmatic stance “recognising that there is a diminishing return as the design cycle moves to ESD does not suggest that they should be ignored. Rather that they should be correctly matched”. For MARIN, it is important to make a stand on how you should measure the performance of these devices and how to apply them. “MARIN
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