Koninklijke Vereniging - Société Royale



Benchmarks in technical and commercial perfomance

As fuel performance monitoring becomes more widespread in the tanker industry, the gathering of quality data concerning at sea consumption is only the beginning.

Through analysis joined with historic data, it is possible to establish realistic technical baselines, as well as commercial benchmarks, thus enabling a closer communication between technical and commercial departments.

This standard is often denominated ‘baseline’ or ‘benchmark curve’. It is, however difficult to find a definition for these concepts, and even more difficult to find nominal values for such standards.

The purpose of this article is to clarify matters and establish some principles for a numerical determination of benchmarks to be used by PDI in Propulsion Dynamics’ CASPER reports.

Baseline - In the patented CASPER performance analysis reports, the baseline reflects simply the resistance of the new, clean ship.

This resistance is relatively well-defined, and the ‘base line’ is therefore a value, which may be regarded as constant over time. If this resistance is given the value 1 at design draft and at design speed, the actual ‘added resistance’ is simply the factor, which shows the actual increase of resistance (of hull and propeller) in relation to the baseline resistance, when all other parameters, such as wind, waves, sea current, draft/trim and speed, have been corrected for.

The baseline may of course be changed over time, either if it is found that the initial clean ship resistance was erroneous, or if the ship has been modified (such as installation of energy saving devices).

A sometimes called ‘Dynamic Baseline’ behaviour expresses that the ‘new ship performance’ can not be maintained over time, even with the best possible maintenance husbandry. This can, however also, and more correctly, be reflected by a suitable benchmark curve, as shown in the following.

Benchmarks - The definition of a benchmark is that it represents some kind of a standard, to which something could be compared. A technical standard for comparison could be based on the PDI experience with ships of similar type. This is reflected in the following diagrams.

Benchmarks may of course also be established based on commercial standards, such as common charterparty requirements, or possible an operational benchmark whereby (anonymous) competitors’ best practice for shipping companies that wish to participate in anonymous benchmarks is compared.

In the following it is mainly the technical benchmarking, which is described.

Figure 1 shows the different kinds of CASPER added resistance.

It is seen that there is the so-called permanent added resistance, which covers the permanent degradation over time of the wetted surfaces. The permanent added resistance may be removed, but that will normally require large investments in drydocking time in order to replace of a large part of the shell plating, smoothing of weld beads, etc.

There will also be a kind of semi-permanent added resistance (or basic roughness), which may mostly be removed by extensive blasting in drydock. Full ‘white metal’ sand blasting will, however normally only take place every 10 years.

Finally, there is the added resistance caused by marine growth. It will increase over a docking period, unless some kind of in-water cleaning of hull and/or propeller is carried out.

Figure 2 shows how the marine fouling added resistance develops over a docking period, if nothing is done. It also shows how the added resistance is influenced by hull/propeller cleaning.
Finally, Figure 3 shows how the development over a 25 years lifetime could be for a typical tanker.

A mean benchmark curve is shown, and proposed validation areas are indicated by colours, amber for ‘normal’, green for ‘good’ and red for ‘bad’, respectively.

The curves and the colour zones reflect only some of the principles. The real curves and zones to be utilised should be determined in co-operation with the tanker owner, when a strategy for the use and the maintenance of their ships has been determined.

Benchmark curves drawn for the purpose of fuel saving and maintenance guidelines should normally reflect general experience, however, it is obvious to rely especially on detailed experience with similar ships, similar hull surface treatment and similar procedures for maintenance.
The first thing to do, therefore is to decide on a strategy for hull/propeller maintenance, including the choice of hull coating types and intervals for drydocking and underwater cleanings and then determine the benchmarks in such a way that it can be checked whether the goals of the strategy is reached or if further action is required.
In CASPER, the results of over 4,000 ship-years; tracking daily or weekly hull performance and 1,000 drydockings together with hull cleanings and propeller polishes, have been analysed, and therefore it is easy to evaluate the efficiency of the hull coatings and aforementioned maintenance activities.
Benchmarks can be drawn that are purely technical (with sea trial performance as a basis) or benchmarks that are operational (using values for similar ships of similar age and time-out-of-dock). Further, benchmarks can also reflect commercial goals (using charterparty figures for speed, fuel consumption, weather, etc) and can be defined accordingly.
It is therefore likely that different departments within a tanker operation will need different kinds of benchmarks. This is possible and will be reflected by the CASPER presentation of analysis results.

*This article was supplied by Propulsion Dynamics.





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