The IMO introduced the EEDI to reduce CO2 emissions per ton-mile of cargo. An unintended consequence: ships with low engine power causing poor manoeuverability and, in some cases, a possible breakdown of the propulsion line due to dynamic stresses above the critical value while passing the BSR. DNV GL has, as the first classification society, introduced class requirements to avoid shaft fatigue failure during service life caused by long passing times. This technical news provides information on how to mitigate challenges associated with high dynamic stress.
Relevant for ship owners and managers of tankers or bulk carriers with 2-stroke engines as well as for yards, suppliers, design offices and flag states.
|Fig. 1a: Typical direct coupled propulsion system||Fig. 1b: Typical shaft fracture from torque|
All propulsion systems with direct coupled, two-stroke engines have resonant frequencies in the operating speed range where torsional vibrating stress may be critical. Dynamic stress above a defined critical value requires a barred speed range (BSR), which must be passed rapidly. Extra power must be provided by the engine to overcome increased resistance from acceleration, weather, fouling, etc. Passing the BSR is more demanding if takes place at a high rpm range, as the vessel then needs more time to accelerate from dead slow or standstill. Short stiff shafting and engines with few cylinders will result in a BSR at a high rpm. Typical designs with such challenges are bulkers and tankers with five-cylinder engines, as illustrated in Figure 1a. Figure 1b illustrates typical damage.
Propulsion designers, some owners, and class are aware of this challenge. Engine designers have handled this within CIMAC (The International Council on Combustion Engines). For instance, MAN Energy Solutions has devolved a dynamic limiter function (DLF), while WinGD suggests other actions. Class has highlighted the topic with a revision of IACS UR M51, but without requirements. DNV GL has seen the need for immediate action and has initiated R&D in dialogue with engine designers and yards, which resulted in new rule requirements.
New DNV GL rules
The DNV GL rule revision published in July 2018 (RU-SHIP Pt. 4 Ch. 2 Sec. 2 Torsional vibrations) presents two main requirements:
- Maximum time passing the BSR (design and trial verification)
- Power margin at the upper limit of the BSR (design)
The maximum time passing the BSR is calculated using a formula in which time is reduced exponentially by the stress ratio. The formula is derived from the fatigue theory by use of Miner’s damage ratio, and is qualified by case studies. The number of cycles depends on the operational profile through service life, which is unknown at the design stage. DNV GL has used 25 years’ service life and ten passings per week as a basis for the development. Shafting with shrink fit coupling or multi-fillet flange and a stress ratio of 85% will result in 26 seconds of allowed passing time.
It is required to have a 10% power margin at the upper limit of the BSR, which is in accordance with CIMAC’s recommendation and definition. The main motivation behind this is to avoid problems with verification of passing time at trial.
Trial and measurements
Passing the BSR shall be demonstrated accelerating and decelerating both going ahead and astern as required by IACS UR M51. Time passing the BSR accelerating ahead shall be measured and within an approved value. Measurements of torsional stress shall be carried out on the first vessel delivery in a series. Calculated stress above 85% of maximum allowed transient stress requires instrumentation with strain gauges instead of the less accurate encoder.
There are basically three solutions to this challenge:
- Increase the power margin (see Fig. 2 below)
- Move the BSR to a lower rpm: Lower natural frequency by making shafting more flexible (e.g. use high tensile steel or increase shaft length) Increase excitation frequency by increasing number of engine cylinders
- Increase distance between engine and propeller bollard pull curve: Both engine and propeller can be re-designed, but increased engine power will affect EEDI. A lighter propeller design will increase the light running margin and, therefore, the power margin as well.
- Highlight the challenge at the design stage and avoid problems during sea trial
- Take the propeller design into account before the final selection of engine
- Be aware of short stiff shafting in combination with 5-cylinder engines
- Ensure the power margin is at the upper end of the BSR
- Comply with DNV GL rules, July 2018 edition or later
|Fig. 2a: Power vs rpm characteristics||Fig. 2b: Frequency response from torsional vibration analysis|
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Technical background - see page 3-9 on PDF document available below.