Understanding barred speed range during simulations and sea trials
Spending too long in the barred speed range (BSR) can have a detrimental effect on shaftline fatigue lifetimes. And this issue is increasingly seen on the latest digital two-stroke eco-efficient prime movers. To combat this problem, owners should assess a vessel’s power margin, rather than relying on industry standards, says ABS’ manager of corporate technology Chris Leontopoulos
Following the introduction of the IMO Energy Efficiency Design Index (EEDI) in 2013, a number of trends have affected the design of vessel powertrains. These include more efficient, larger diameter propellers, more efficient, de-rated engines with lower RPM, shorter shaftlines and reduced engineroom space.
At the same time, vessel operators have reported what they consider to be an unacceptable length of passage time through the Barred Speed Range (BSR) during sea trials. This has led to requests from operators to assess shaftline fatigue and to make predictions on the time required to pass through BSR as early as at the plan approval stage; such requests are proving challenging for class societies, shipyards and engine makers alike.
Traditionally, the International Association of Classification Societies (IACS) has approved the torsional vibration characteristics of a vessel’s powertrain, based on IACS UR M68, which defines two limits, both of which stem from a fatigue approach combined with actual experience and are characterised as ‘semi-empirical’.
The BSR is a ‘barred’ or a ‘forbidden’ speed range of operation; most classification rules, in addition to applying UR M68, would include a qualitative clause, such as “to be passed through as quickly as possible”. This is done to avoid accumulating fatigue cycles, which stem from operating in the zone of a major torsional resonance of the powertrain.
In pre-EEDI days, BSR passage was limited to a few seconds; therefore, potential accumulation of torsional cycles, leading to fatigue and failure, was insignificant and created little cause for concern. Post-EEDI, the small power margin (PM) attained by the new Eco-efficient engines around the upper and lower limit of the BSR can result in torque being insufficient to pass quickly enough through the BSR.
A prolonged stay inside the BSR would not only trigger engine alarms in the engine control room, but also create cause for concern among operators, from the twin perspectives of fatigue and vessel manoeuvrability, particularly in bad weather.
The substantial increase in the time needed to pass through the BSR has caused operators to question the validity of IACS UR M68, since the rule does not explore the effect of time duration, and is constrained in the determination of the upper and lower limits of the BSR.
To assess the effect of the time inside the BSR, certain data and parameters must be considered in a simulation, such as the prediction of shaftline acceleration times throughout the operating range, including the BSR. This simulation requires data from the engine control system, including speed governor functions and fuel index, to establish the actual torque versus speed.
Additional data is required to define the dynamic torque capacity, or acceleration performance of the engine under load. This would also involve the torque limiting function of the engine control system and possible effects from turbochargers, or other details of the combustion process.
Data regarding the vessel dynamics, including but not limited to, torque and thrust coefficients and the advance coefficient of the propeller, would also be required for this simulation. An assumption regarding the propeller light running margin (LRM) has to be made, and the initial and final vessel speed in knots, plus the ship resistance, are also required.
Clearly, the amount of data needed for such predictive simulations is substantial and complex.
It is unlikely that the necessary data would be available to make such calculations during the plan approval process at the design stage of every new building. As such, a substantial number of engineering assumptions need to be made, affecting the accuracy of the review calculation.
A way forward
Acceleration problems on existing vessels are typically solved by changing engine parameters to increase the dynamic torque, although propeller modifications may also be applied. Increasing the engine torque in the BSR can be achieved by increasing the index limiters, thus allowing more fuel to be injected, while ensuring that the EEDI value is not affected.
Experience from actual sea trial measurements shows that by ‘over-boosting’ the torque inside the BSR, the actual shaftline stress level is slightly reduced during acceleration within the BSR. The consequent reduction in stress is mainly explained by the shorter time spent on the resonance – resulting in less time for the vibration amplitude to build to its maximum level – coupled with increased propeller damping, due to the heavier propeller curve during acceleration.
In terms of fatigue lifetime, there may exist an optimum level in which to pass through the BSR using torque ‘over-boost’; reducing this level further may ultimately result in shorter lifetimes, due to potentially increased stress/torque amplitude responses.
The issue of BSR transit time quantification does not only affect shaftline fatigue life; it also affects a vessel’s manoeuvrability. The PM can vary depending upon the selected propeller curve for operation (eg scantling trial, design trial, ballast trial and bollard pull (BP) curve). A small PM and/or a small LRM indicates both reduced acceleration capability and manoeuvrability.
According to the engine makers, a minimum value of at least 10% of PM at the upper limit of the BSR, using the BP curve of the propeller under consideration, should be used to ensure sufficient acceleration capability to pass through the BSR.
The engine makers say that a satisfactory BSR transit time should never exceed 30 seconds during sea trials. If the 10% limit cannot be achieved, additional specialised fatigue calculations may be required to demonstrate acceptable levels of powertrain fatigue lifetime and manoeuvrability.
For cases where the BSR transit time is unacceptably long, engine makers have resorted to applying increased torque, using Dynamic Limiter Functions (DLF) via intervening engine control software systems. However, measurements demonstrate that this approach does not increase the actual vibratory stresses, despite the higher gas torque excitations, unless excessive power is applied.
As BSR transit time predictions require data that is generally unavailable at the plan approval stage, hence more assumptions must be made regarding transient analysis calculations; this may have a negative impact on the accuracy and sensitivity of the results in terms of fatigue lifetime estimations. It may be more advantageous to assess BSR transit times via the PM concept, rather than modifying the IACS UR M68 methodology, as the main engine makers are proposing to IACS.
The Wärtsilä Shaft Alignment Breakfast Briefing
Join a live one-hour seminar where you will be able to learn about the issues and technology affecting and improving propeller shafts.
The event discussion will be led by Wärtsilä Alignment and Measurement Products and Services, and hosted by Riviera Maritime Media.
The seminar will discuss understanding Barred Speed Range during simulations and sea trials, the link between propeller immersion and the risk of propeller shaft-bearing damage, avoiding stern-tube bearing failure, and new insights into shaft alignment during sea trials.
Attendance is for shipowners, operators and managers. Your application to join the seminar will be confirmed within 48 hours of application. Confirmed shipowner attendees will also receive a VIP invitation to the Marine Intelligence conference following the breakfast briefing, where Marco Ryan, Chief Digital Officer will be giving the keynote address.
Date: Wednesday 23 May 2018
Time: 07:30 – 08:30 CEST
Hotel Atlantic Kempinski Hamburg
An der Alster 72-79