The challenges of measuring real vessel performance
Dr Raouf Kattan, managing director of Safinah, co-wrote, with Dr R.L. Townsin (Safinah) and V.N. Armstrong (ApolloSOS), a paper on the complexity of coating performance measurement for shipowners which was presented at the recent ICMCF in Singapore. Wendy Laursen reports.
With fuel at US$600 to US$700 per tonne and ocean going vessels consuming anything in the range of 40 to 220 tonnes per day even a 1-2% fuel saving can provide rapid payback on fouling prevention investment.
The standard paint guarantee from coating suppliers allows for some fouling before a claim can be made. Typically they may allow up to 2.5%, which would already be a serious problem for vessel performance in terms of fuel consumption.The shipowner wants predictable performance to ensure that costs are controlled over the period of the charter or ownership to ensure a return on investment.
The high cost of recoating, and any subsequent economic fouling penalty, has led some paint companies to attempt to devise guarantees regarding the effectiveness of their products, based upon some measures of speed/power performance of a ship from out-docking after anti-fouling coating. Such in-service data collection and analysis is notoriously difficult.
Fuel consumption is the critical factor in determining vessel performance. The specific fuel consumption of the engine is normally determined at the shop-based engine trials carried out prior to acceptance of the engine and its installation on board the vessel. During sea trials the measured mile is used to determine if the vessel is able to achieve the contract speed and if the measured power for a range of speeds is in line with the design conditions. Though increasingly there is reference in the contract to fuel consumption, sea trial fuel consumption is measured for the specific condition of the trial and can be subject to error when being adjusted (the most accurate fuel consumption figures available at that time are those from the engine trials).
Thus when claims are made with regard to fuel savings it is very important to understand what comparison is being made. Assessing the performance of the vessel at any given time always needs to be referenced back to the results of vessel performance obtained at sea trials to allow a comparison against one set of benchmark data, as opposed to, say, data comparison before and after dry-docking.
The performance of a coating in terms of development of hull roughness, slime build up and fouling build up impact the penalty being incurred (with coating roughness being important as long as the hull is slime/foul free), thus a claim of 5% to 9% fuel saving applies to the penalty being incurred on a comparative basis to a virtual benchmark.
One purpose of in-service speed/power monitoring is to provide the ship operator with sufficient and reliable data to enable decisions to be made for the management of the underwater hull. In-service performance data collection should be of suitable quality to allow comparison with the original sea trial data and also the performance soon after dry-dock.
Today, many shipowners, paint companies and other commercial organisations offer speed/power performance monitoring systems. Monitoring systems on offer vary in their approach: for example, some collect all data and correct for weather effects, some use only fair weather data, others require the installation of a torsion meter while some do not.
The method of measuring fuel consumption has to be considered. Typically this is done by the crew reading the fuel counters once per day. However these readings for example do not take into account fuel spilt from the fuel tank and returned to the service tanks (could be up 3% to 10% depending on engine load and the use of return valves to control this). The commercial stakeholder, however, is more interested in the fuel remaining on board (ROB). The fuel quantities measured from these two sources rarely tally because of:
- The presence of water/sludge on board (1%-2%)
- Additional sources of fuel consumption e.g. incinerator.
- Actual quantity of bunker fuel received against that ordered
- Ship's 'reserves'
From a coating perspective the current focus on the analysis of fouling prevention performance is based on the speed penalty that the vessel incurs as a result of any fouling. Thus the paint companies sometimes offer speed loss as a measure of the performance of any fouling prevention measure.
The speed penalty would be critical if vessels continued to operate close to design speeds. But the average operating laden and ballast speeds are greatly influenced by market conditions. So, speed is not the critical factor by which the performance of the fouling prevention should be measured. Rather, parameters like power and fuel consumption and their incremental effect due to fouling could be better indicators.
Another important criterion that should be assessed is capacity loss. When an owner decides to acquire a ship (new or second hand) or to charter a vessel, he has in mind a route and a trade that the vessel will serve. The vessel design will have been optimised to the design parameters which would include design draught and design speed. If the vessel deviates from those design parameters by loading a part cargo or reducing/increasing speed then its fuel efficiency will reduce.
Thus a key measure of vessel performance is its ability to deliver the required cargo tonne-miles per voyage as cost effectively as possible. A vessel that has a design cargo capacity of 105,000 tonnes engaged on a trade-route of 5,000 nautical miles will have a capacity of 525,000,000 cargo tonne miles. The time (speed) in which it can deliver this capacity is dependent on the speed of the vessel and dictates the earning capacity of the vessel.
In this example, a vessel with a design speed of 14.9 knots, which is chartered at 14 knots and carries only 80% of its designed cargo capacity, will make approximately 21.46 voyages per annum of which half would be in ballast, based on 300 sailing days per year. Thus the vessel would deliver about 4.03 billion tonne miles of capacity per annum as compared to a designed 5.63 billion tonne miles per annum.
This implies a lost capacity to an owner of about 28% in tonne miles per ship per annum. A further 8% could result from any moderate slime/fouling. If the trade requires seven ships to meet capacity, then the implication is that up to three more ships may be required to deliver the same tonne mile capacity. While fouling plays an important role in the management of tonne mile capacity, the disjoint between vessel design and operational conditions can also play a critical role.
As an example, the additional fuel consumed to maintain 14 knots speed with a 20% increase in power consumption will be about 7.4 MT / day.
Thus the key issue for various stakeholders can be addressed in different ways based on the above scenario:
- Vessel availability – up to three more vessels required to deliver the same capacity.
- Shipowners – cargo capacity loss of 28% or speed loss of 0.9 knots or increased fuel consumption of 7.4 MT/day in laden passage.
- Global society – increased emissions from three more vessels and from inefficient operations and the cost and environmental impact of building/operating those vessels.
What is also interesting to observe is that despite all the emphasis being placed on hull roughness and the merits of one type of coating over another, how few paint specifications actually specify a hull roughness value for the newly delivered ship and if specified how few times it is actually measured.
This is the same for dry-docking, where these days hull roughness is rarely specified and rarely measured (usually as a result of time constraints). Perhaps the introduction of standards for this will increase the demand for better designed and built vessels and enhance the operational performance in service.
Many factors influence the achieved surface roughness of the hull; perhaps the most critical is not the type of coating chosen but the quality of the surface preparation and spray application. There can be a considerable difference in surface roughness between airless spray applications made at 60cm distance from the surface to that made at 40cm. In addition overspray can also increase roughness by up to 50µm.
There is a considerable body of knowledge on hull vessel performance for a roughened hull, but there is considerably less on slimed and fouled hulls. Slime is very difficult to assess but can have considerable impact on vessel performance, and submissions to the IMO would indicate that the contribution of slime to the added resistance of the vessel is much greater with some coating types.
Diver reported, visual description of fouling, is the predominant way of assessing the severity of slime and/or fouling. While videos are taken to record the findings of divers and the results of subsequent cleaning it is the author’s experience that the quality and veracity of some of the videos can be questioned as well as the assessment of the extent of fouling or the quality of the cleaning process that is achieved.
The focus has generally been on how to manage the hull once the vessel is in service, but decisions and work carried out at the design and newbuilding stages can adversely impact vessel performance. Data is needed in key areas such as the impact of spot blasting versus a full blast and the real cost in terms of capacity of practices such as underwater hull cleaning or grooming on a regular basis.
What is remarkable is how little information is readily available in terms of through life performance of the underwater hull. There are many case studies that compare a vessel before and after dry-docking, or sister vessels over a short period of time, but no systematic analysis over the long term, say 10 to 15 years, is readily available in the public domain to guide further study. It is hoped that as a result of increased application of hull performance monitoring software that over time such data may become available.
In the end it is neither speed nor fuel consumption but capacity that an owner buys when he buys a ship. The need is to deliver that capacity as cost-effectively as possible, and fuel consumption and consequently the performance of the underwater hull is critical to the cost-effectiveness of the ship.
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