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- LEC Sustainable Shipping Technologies Forum 2021
The purpose of the Fuel Consumption Tool is to make it possible for ship operators to estimate the fuel consumption of a given ship with a selected hull surface condition. The fuel consumption is always expressed relatively, i.e. the difference in fuel consumption between two conditions.
The tool will be executed from the Skin Friction Database webpage after that the user has entered data for a specific ship. Therefore, the tool has to be rather simple, quick, and require only input data that is easily available to the ship operator.
The following input is required:
· Length of ship (m)
· Operational speed (knots)
· Operational power (kw)
· Number of propellers
· Two deltaCf
· Specific fuel oil consumption (g/kWh) - optional
The output is given as:
· Change in power in kW
· Change in power in %
· Change in fuel oil consumption in t/day
The target is to extract a DP=P1-P2 corresponding to two different hull surface roughness.
The power difference should be valid roughly around a given operational power, which we call P0.
We use the ITTC method for sea trial evaluation (ITTC 7.5-04-01-01.1) to derive the change in power due to a change in resistance:
V ship’s speed through water
ηD propulsion efficiency coefficient in ideal condition
xP: overload factor derived from load variation model test
ΔR: resistance increase.
In a sea trial evaluation, P1 is the ideal power and P2 is the power affected by an added resistance. In the roughness evaluation tool we assume instead that P1 and P2 are the powers corresponding to two different hull roughness.
The first equation can be rewritten as:
The added resistance DR is the difference in resistance between the two hull surfaces:
Hence, B is
A can be extracted from:
where P0 is the actual power that the user wants to analyse the difference around. CF0 is the ITTC-57 plate friction. CR , k and x are taken from statistics.
Theoretically, P0, V0, hD, CR and CFO should be valid for the same condition, and P0 should be equal to P1. However, sensitivity analysis have shown that deviating from this is not at all important for derived value of DP.
Difference in fuel oil consumption
The change in fuel consumption can be computed from the power difference DP, in the following way:
DFOC is the difference in fuel oil consumption in t/day
DP is the difference in fuel oil consumption in kw
SFOC is the specific fuel oil consumption in g/kWh.
It is common that the ship operator knows the SFOC from performance logging or engine control panel onboard. If the user does not enter SFOC, a standard value of 170 g/kWh will be used.
The method described here is a simplified procedure instead of evaluating the power for the two conditions with the full ITTC-78 methods, which would require more ship specific input.
To verify that this simplified approach is close enough, a comparison has been done for one case. The power is extracted using the full ITTC-78 method for various ks. This is then compared with the simplified approach. DCF is computed with Bowdens formula in both cases. Figure 20 shows that the resulting power difference is very close with both methods.
Verification of simplified tool method against ITTC-78
Three hydrodynamic factors are required: the load variation factor xP is, the residuary resistance coefficient CR and the form factor k. These can only be derived from model test. In practice, it is very unlikely that the intended user of the tool has any information on these factors. A majority of the vessels are chartered, and the ship operator has no access to the ships model test reports. Even for owned vessels, it is not likely that the superintendent can find the ships model test report. Even if they would find a model test report, the factors can well be extracted with using a different definition that SSPA standard. Every towing tank use their own version of ITTC-78 method. Therefore, a better option is to use empirical values instead of ship specific.
Statistics of xP , CR and k is extracted from the most recent, normal hulls in design condition tested at SSPA, in total 298 single skeg and 125 twin skeg. The median values are used in the tool in the webpage.
The vessels are within the following limits:
· 11<L<380 m
What is the effect of using empirical values instead of using the actual values for each ship? Table and figures below show the error for the 424 vessels from the database, expressed in %-units.
It is clear that assuming CR is the largest error source. Instead of using a fixed empirical value, an option could be to use regression formulas to get CR. This was tried here but gave no improvement. CR is simply too ship specific. However, the resulting error when assuming a standard value for CR is totally acceptable considering the context where the method will be used.
With these assumptions, for 95% of the ships the expected error is smaller than ~1 percent-unit. This means that if the power increase DP is predicted to be 10% with the computation tool, it means DP is between 9%-11%.
Errors from assuming empirical values
For 95% of the ships the
error is smaller than:
From assuming k, xP and CR
From assuming k
From assuming xP
From assuming CR
Error from assuming k, xP and CR from statistics