A new study seeks to reduce stress created by operation in the barred speed range or the impact of ice and so increase the life of propulsion systems
Torsional dampers are often selected and dimensioned so they perform effectively long-term in steady state operations. But in some cases, they may need to be designed for transient conditions when vibrations and torsional stresses may be abnormally high. For instance, in some ships, the upper limit of the barred speed range (BSR) may be too close to the MCR, so passing the BSR quickly may not be feasible. In such cases, the high torsional oscillations during the BSR may need to be accounted for during design, as otherwise the high vibrations experienced for a longer period may reduce the life of a propulsion system.
Further, in ice applications the impact of ice on the shaft system and the resulting high vibrations may need to be taken into account. Simulation can help design dampers for safe operation in such conditions, according to a CIMAC 2019 paper titled, “Capabilities of transient simulations in torsional vibration damper selection”, by Klaus Prenninger, Andreas Thalhammer and Matthias Geislinger of Geislinger GmbH. The authors say that in ice-class design, viscous and spring dampers are superior choices to tuning wheels, although viscous dampers may be limited by space and weight constraints.
Typically, for two-stroke engines with five-, six- and seven-cylinders, a strong first order resonance below 60% of the nominal speed is present. Taking the case study of a slow-speed, two-stroke engine with six cylinders and a medium bore, a tuning wheel would have a higher vibratory torque and stress response than spring dampers. As the diameter of the spring dampers increases, the vibratory stresses reduce.
Five seconds is the rule of thumb for BSR passage. The possibility of a five-second passage is, however, restricted to systems with a comparably small resonance at a low speed range and a sufficiently large power margin at the upper end of the BSR.
A DNV GL formula lays out the time that can be allowed in the BSR. The engine with a tuning wheel, in the above case study, would be allowed a BSR time of 6.3 seconds, while a spring damper of 260 cm (D260) would increase the maximum permissible time to 22.1 seconds. A D280 (280 cm diameter) dramatically increases the maximum permissible time in BSR to 87.5 seconds in this case study.
Simulations show that if the time that can be spent in the BSR is longer, the rate of change of vibratory stress is smoother, too. The paper notes that a safe passage through a BSR can be guaranteed by an appropriate spring or viscous damper. “There is also the possibility of avoiding altogether a BSR by applying specific torsional vibration (TV) damper solutions, with which there are no further operational restrictions due to TV,” the paper says.
For ice-going vessels, a special ice classification is necessary to prove there is sufficient protection against failures due to additional (torsional) loads caused by rough ice-covered sea conditions. Besides steady-state TV analysis, national agencies and many classification societies have agreed upon the requirement of transient ice-impact simulations in order to receive a corresponding classification. There are basically four ice classes based on Finnish ice-class rules, as per the Finnish Transport Safety Agency (denoted by IC, IB, IA and IA+) and seven polar ice-class rules based on IACS Polar class rules (denoted by PC1 – PC7). For the two-stroke engine market the smaller ice classes (IC–IA+) represent the majority of ice-class projects.
In ice-class vessels, the speed can drop massively after ice impact. In some cases, the engine can even stall. This typically causes a speed drop into or below the BSR, resulting in high torsional loads occurring along the propulsion line by accelerating additionally in the main resonant speed range.
The paper considers the worst-case ice-torque excitation response, as per ice-class IC, at the intermediate shaft for a propulsion system equipped with a fixed-pitch propeller and a six- cylinder two-stroke engine with a small bore. The worst-case scenario is calculated with respect to the phase angle, where the initial speed is taken into consideration in the range of 20 rpm to 117 rpm (nominal speed). For this case study, two different types of torsional countermeasures are compared: the largest possible tuning wheel; and a spring damper (D 220). A reduction of at least 25% of the peak torque is achieved by applying the spring damper D 220 compared to any tuning wheel, according to the paper.
An essential parameter for the ice torque excitation definition of different ice classes is the ice block thickness Hice, which is different for each ice class. Increasing the parameter Hice has two effects: firstly, the scaling of the ice-impact torque excitation is dependent on the ice block thickness; and secondly, the number of propeller-ice block interactions depends not only on the number of propeller blades but also on the ice-block thickness.
Simulation with viscous dampers with different outer diameters but similar stiffness and damping characteristics shows that Hice has a significant impact on the worst-case scenario – the greater the Hice, the higher the peak torque responses.
Within each ice class, the peak torque could be reduced by applying a larger viscous damper. But there are practical limits on how much ice-induced torques can be reduced, given practical constraints on the installation space and weight of the viscous dampers. Viscous dampers are likely to be much larger than steel spring dampers, giving rise to practical considerations.
The authors say it is possible to find spring damper solutions for all kinds of ice classes – ranging from small ice-classed vessels up to ice breakers with medium- or high-polar classes. A study on a PC4-classified vessel with a seven-cylinder, medium-bore two-stroke engine was made with a spring damper of D 340, initial stiffness of 44.5 MNm/rad and a relative damping of 500 kNms/rad. The spring damper’s stiffness was varied up to 8% and the relative damping coefficient varied by up to 12% in a simulation.
The study revealed that the simulated torsional responses along the different components respond in a different way to the stiffness-damping-variation of the damper. Whereas on some components the worst-case, ice-torque excitation behaves almost linearly (e.g. for the propeller shaft and the damper), other components show a non-linear behavior with respect to a variation of the damper characteristics.
This means that variation of the parameters reduces torsional loads on some propulsion parts, but increases them on others. Finding an optimum solution will involve a thorough analysis to evaluate all the effects, the authors say. They conclude that optimum dimensioning of spring dampers can provide solutions to all ice-class requirements where other mechanisms, such as tuning wheels, may be insufficient.