"How accurately do the 'shop test' and 'sea trial' results of the main engine and auxiliary diesel engines reflect reality?"
Abstract
This study demonstrates how, if ship main and auxiliary engine shop tests are not conducted in a test cell that does not comply with ISO standards, the normally higher specific fuel consumption can be underestimated in accordance with contracts, and how this impacts on CII and CII rate values. The maximum error specified in contracts as 6% can actually reach 10%. This difference can create a difference of nearly daily 0.5 to 1 MT in ships' daily fuel consumption. While the total reduction factor (Z) is 18.5% by 2030, the fact that the measurement error alone will reach 10% is undeniable. For accurate ship CII rate calculations, it is critical that shop tests be conducted in a test cell that allows measurements in accordance with ISO requirements, that the measurement devices used in sea trail tests be standardized, and that measurements are performed with standard fuels.
Introduction
As part of the 2050 net carbon zero target, the IMO and the European Parliament are continually expanding structural and operational regulations. The EEDI and EEXI metrics cover structural efficiency regulations for newbuilds and existing ships, respectively, while the EEOI and CII and CII Rate metrics regulate operational efficiency. The EU ETS, FuelEU, and MEPC 83 also aim to accelerate this process through economic measures. The Carbon Intensity Indicator (CII), calculated according to the AER (Annual Efficiency Ratio) metric, is an IMO regulation that aims to gradually reduce carbon emissions from ships, lowering the marine industry's overall environmental footprint. This mandatory measure falls under Annex VI of the International Convention for the Prevention of Pollution from Ships (MARPOL) and targets emissions from vessel operations. The carbon reduction factor required to achieve the "C" rate value has been determined to be 2% lower each year until 2027, and 2.625% lower annually between 2027 and 2030. In this case, the total reduction target to achieve the "C rate" between 2023 and 2030 is 18.5%, totally.
Table-1. Reduction factors (Z) and cumulative reduction depending on the years
The IMO sets concrete targets for ships to achieve the required CII value. However, one of the most important points overlooked by the IMO regarding reducing carbon emissions is its lack of a mandate for calculating the total uncertainty in shop test fuel consumption measurements for main and auxiliary engines. Standards such as ISO 3046-1:2002 exist to standardize existing measurements. However, it is even more crucial that measurements be conducted in a test cell capable of conditioning in accordance with ISO standards. A standard should be established for environmental conditioning within the test cell according to ISO standards and for fuels with a constant calorific value (LCV = 42.7 MJ/kg). Furthermore, upper limits for uncertainties arising from measurement devices (water brake load cell uncertainty, flow meter uncertainty, etc.) should be specified, and the total measurement uncertainty arising from all devices used in the measurement should not exceed ±2%. Otherwise, speculative corrections may be made to prevent discrepancies between sea test results and shop tests.
As a result, the shipowner pays the price for the high fuel consumption resulting from high measurement uncertainty in the form of lower letter values and increased fuel consumption.
Table 2 shows shop test fuel consumption data, corrected according to ISO requirements, for two four-stroke main engines of the same model and manufactured in the same years, on two sister ships currently in commercial operation. The only difference between the two engines is that the tests were conducted in two different countries.
Table-2. Sister ship same model four stroke Diesel engne ISO corrected SFOC values
An examination of the shop test data for the two main engines presented in Table 2 reveals that one vessel consumes 100 MT more per year at 75% load than the other (based on a 250-day annual voyage). However, the measurement results for both main engines have been adjusted for standard environmental conditions and standard fuel LCV values.
Why did this difference occur when the measurement results should have yielded the same results under standard environmental conditions?
Let's explain this question step by step and explain why the main engine and auxiliary diesels should be tested in test cells conditioned to standard atmospheric conditions. If there are any discrepancies in the shop tests that violate the contract, we will show which parameters can be adjusted to ensure that the sea trails power and specific fuel consumption are within the contract. Figure 1 shows a typical shop test.
Figure-1. A typical main engine test bench (test cell)
Specific Fuel Consumption in Shipbuilding Contracts
One of the most important issues companies should consider in new shipbuilding contracts is the detailed calculation of the total guaranteed error in specific fuel consumption at the measured loads (25%, 50%, 75%, 85%, and 100%) of the main engine shop test data. It is also important to ensure that the sea trail results of the guaranteed fuel consumption are consistent with the shop test results.
For example, suppose the contract guarantees a specific fuel consumption of 156 +6% g/kWh at 75% engine load.
Now, let's move on to an example of how this value can be manipulated in sea trail tests.
First, let's discuss the environmental conditions and measuring devices that affect main engine specific fuel consumption. Factors affecting fuel consumption of main and auxiliary diesel engines can be listed as follows:
• Environmental conditions (humidity, air temperature, barometric pressure, coolant temperature)
• Fuel calorific value (LCV)
• Uncertainties of measuring devices (including the timer uncertainty used in fuel consumption measurements, fuel flow meter uncertainty, speed measurement uncertainty, thermometer uncertainty, density correction based on fuel temperature, water brake uncertainty (load cell uncertainty if using a load cell), moment arm length measurement uncertainty, etc.)
Under current conditions, measurements are performed at normal ambient temperatures and based on the fuel being measured. Fuel consumption measured under ambient conditions is reported as Measured SFOC (Specific Fuel Consumption). Table 3 shows the specific fuel consumption values over a 10-minute period during the shop test.
Table-3. Two-stroke main engine shop test data
When we calculate the 10-minute consumption times in this table, we see that the measurement times are actually different. This difference indicates that the actual measured SFOC value is higher. This is an intentional data distortion. Table 4 shows the actual measured SFOC value for the sea trail data.
Table-4. Measured SFOC values when correct measurement times are entered
Now, let's examine the SFOC value when the measured conditions and fuel used during the sea trail are corrected to ISO conditions. Table 5 shows the ISO Corrected SFOC values when the ambient and LCV values of the measured fuel consumption are corrected to the standard environment and standard LCV values. Table 5. ISO Corrected SFOC values at different loads calculated by the manufacturer.
Table-5. Sea trail data corrected to ISO conditions
Interestingly, at 78% load, the measured SFOC value is 178.82 g/kWh, but when corrected for ISO requirements, it becomes 165.13 g/kWh.
So, when the environmental conditions and fuel LCV are corrected, does the SFOC value actually decrease by 13.69 g/kWh? What parameters are manipulated here to cause such a significant difference?
When the environmental conditions are reduced to 25°C ambient temperature, 25°C air cooler cooling water inlet temperature, and 1000 mbar barometric pressure, the SFOC value corrected according to ambient conditons changes to 2.148 g/kWh. The main speculative parameter here is the LCV of the fuel used in the sea trail. Note that the LCV of the fuel used in the tests was 39.91 MJ/kg. Having conducted nearly 1,000 fuel analysis evaluations to date, I've rarely encountered a HFO LCV value below 41 MJ/kg, let alone below 40 MJ/kg.
Now, let's take the calorific value of the fuel used during the test as 41.2 MJ/kg and recalculate the ISO Corrected SFOC value. Table 6 shows the SFOC values reduced to ISO specifications if the fuel's LCV were set at 41.2 MJ/kg instead of 39.91 MJ/kg, while all other values remain the same.
Table-6. ISO Corrected SFOC value when the LCV value is entered as 41.2 MJ/kg
When the ISO Corrected data is examined using the LCV value of 41.2 MJ/kg, the ISO Corrected SFOC value at 78% load increases from 165.13 g/kWh to 170.88 g/kWh. Calculations using two different LCVs result in a 5.75 g/kWh difference in SFOC values.
Table 7 shows the annual cost of this value, including taxes, for a ship cruising 250 days a year due to excess fuel consumption, and what the CII rate values would have been over the years if the speculative value had not existed.
Table-7. Annual costs caused by measurement errors and speculative parameter changes and their effects on the CII rate value
Examining Table 7, assuming the ship consumes this fuel between EU ports, the annual additional cost, including taxes, is $176,808. Similarly, if it had consumed fuel according to the contractual values, it would have remained at the C-rated fuel rating for another two years, for the same consumption figures and voyage distance.
Now, let's check whether the contractually guaranteed SFOC value has been exceeded. At the beginning of this article, we stated that the contractually guaranteed SFOC value is 156 g/kWh + 6% at 78% engine load. Interestingly, a 6% increase from 156 g/kWh is 165.36 g/kWh. However, we calculated the ISO Corrected SFOC value for 41.2 MJ/kg as 170.88 g/kWh. In this case, the ISO Corrected SFOC value is 3.5% higher than the contractual value. The total error is 9.5%. In this case, the manufacturer must pay an additional penalty for not complying with the contractual commitment.
To summarize,
* The IMO should require a test cell capable of conditioning shop test measurements according to ISO requirements.
* The use of standard fuel in both shop tests and sea trails should be mandated, and measurements should be made with standard MGO.
* The measurement uncertainty of all measurement devices should be standardized, with the total uncertainty not exceeding 2%.
* The total uncertainty should be stated for each device, and the calculations should be clearly and transparently included in the shop test and sea trail result documents.
In contrast, the letter values of ships that consume more fuel than normal put ship owners in a difficult position.