The main engines of almost all existing vessels are both designed and optimized for one specific vessel speed and engine load. The introduction of slow steaming in many ship segments has drastically lowered the actual transit speed from design levels, thus leaving the vessel and its engines operating at none-optimized load levels. De-rating the engine offers the possibility to lower the vessel’s maximum speed, specified maximum continuous rating (MCR), and thereby optimize actual load point with design load point. This results in higher efficiency with reduced specific fuel oil consumption (SFOC) at the new optimum design point.
Applicability and assumptions
This measure is suitable for all ship types and ages where a top speed reduction of 10% to 15% can be expected, and the principle of de-rating an engine for vessels is equally important for when in operation as for choosing the engine type and propeller design for new ships.
De-rating of the main engine, be it permanent or temporary, can be done by different methods varying in cost, flexibility and effort needed. The measure is especially relevant in today’s slow-steaming markets. However, many ship owners are hesitant to reduce the vessels’ top speed. Flexible and reversible de-ratings already exist and can be very attractive, keeping the option easy, and with low cost, speed up again if the market changes. Measures to achieve this include, but are not limited to:
- installing shims between the crosshead and piston rod to reduce stroke length
- cutting out one or several turbochargers, either with permanent or flexible flanges
- cutting out/deactivating cylinders
- various tuning methods/settings of the engine, incl. slow steaming kits (also for retrofit)
The main principle behind the fuel saving benefits from de-rating an engine is derived from maximizing the engine’s maximum cylinder pressure (Pmax) ratio to their cylinders’ mean effective pressure (MEP). A de-rated engine can also be further tuned to optimize the efficiency at the lower operating points. This may be complemented by reduction in cooling capacity of auxiliary systems, or by installing variable frequency drives on pumps, etc.
De-rating an engine will have an impact on the turbochargers and the engine’s NOx-emissions, and requires an evaluation by engine makers and regulatory bodies. New torsional vibration assessments are also typically needed for engine de-rating studies. Some de-rating measures, especially for mechanically controlled engines, may require additional NOx reduction measures that increase the SFOC.
De-rating the engine also opens up for a beneficial propeller exchange, as optimizing the propeller characteristics for better performance at lower engine speeds can shorten the payback time of the de-rating. This however typically also increases the project’s capital investment substantially.
It is possible to achieve a reduced RPM with the same power output for certain load ranges when de-rating, enabling a larger and slower propeller, which typically increases the propulsion efficiency.
De-rating is usually performed during docking and is expected to take approx. 7 to 10 days in dock, dependent on de-rating option. Main challenges are machining of liner and delivery of necessary equipment in time for docking.
Before de-rating it is normal that either makers or other consultancy firms perform engine specific studies to evaluate the potential for de-rating, and the most optimal point, including new turbocharger matching, torsional vibration calculation, and a new propeller design performance evaluation.
Cost of implementation
The cost of implementation is $60,000 to $3,000,000 (USD), dependent on starting point and method and extent of de-rating.
For reference, turbocharger cut out with permanent blinds represents the lower end of the cost implementation scale, while de-rating via more complex and universal measures is estimated at around $1,000,000 (USD) for 5 to 7 cylinder engines. For the largest engines around $2,000,000 (USD) is estimated, and taking into account fitting of a new propeller the high end of the cost scale is reached, i.e. $3,000,000 (USD).
The reduction potential is estimated at 2% to 10% of main engine total fuel consumption.
Whichever de-rating method is most cost-beneficial typically depends on the vessel’s current operational speed compared to optimum design speed and the engine type/size.
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