Today’s blog post will cover the basics of propulsion shaft design and analysis on ice-classed vessels.
It is written by my colleague Qifeng Wang who is our Product Responsible for the Nauticus Machinery calculation package. During the past years our team in Shanghai have built up considerable competence in the new polar ice-class rules as a result of close cooperation with DNV Maritime.
Due to more exploration activities in arctic areas, we have seen many projects for ice-classed vessels during the past years. However, the revised ice-class rules are quiet new for the industry and maybe you will get lost and confused about the rule requirements when you are designing the shaft-system for ice-classed vessels. Here I will briefly talk about how to consider ice-class requirements in the shaft design according to my experience as a software developer working on one of software products.
IACS UR 13 provides the requirements for propulsion machinery in ships navigating in polar waters. It applies for ships contracted on or after 1 March 2008 and from “strong to weak” it is divided into class notations PC1 to PC7 as you can see in the figure below. Northern Baltic Ice Rules is based on Finish Maritime Administration Reg. 2008. It applies for ships contracted on or after 1 January 2010 and it is divided into four class notations ICE-1A* to ICE-1C. DNV Rule Part5 Chapter1 inherits these common ice-class rules in section 3 (equivalent to Baltic Ice Rules) and section 8 (equivalent to Polar Ice Rules).
Designing propulsion shafts according to ice class requirements can be split into two parts:
- Calculating the ice class impact and determining the response torque spectrum
- Code check
Calculating the ice class impact and determining the response torque spectrum
Calculating the ice class impacting and determining the response torque spectrum on shaft according to two-parameter Weibull distribution. The main objective in this part is to calculate the maximum response torque amplitude on the shaft due to ice impacting and the total number of load cycles. The Weibull distribution can be determined by these two values.
Example of an ice load spectrum (cumulative distribution) with total number of ice loads in a semi-log scale
This part is well defined in the ice-class rules and the ice impact calculation normally requires simulations in the time-domain. However Baltic Ice-class rule also provides a simple approach to calculate the maximum response torque when the design meets certain conditions.
Code check part
Static strength check is to prevent shaft yielding due to peak ice torque and the calculation is transparent and easy. Calculate the safety factor by comparing the maximum torsional stress with the shaft yield strength then compare it with required safety factor. Of course the concentration factor should be considered in the stress calculation.
The blade failure load criteria is not that complicated either. The ice-class rules a clear definition on how to calculate blade failure load by empirical methods. The rule requires that the loss of the propeller blade shall not cause any significant damage to other propeller shaft line components.
The fatigue strength calculation is perhaps the most complicated part. Different from the “open water” loads, there are no dominating load cases in ice-class load. So Palmgren-Miner’s approach is needed to calculate cumulative damage ratio. The Weibull distribution spectrum needs to be included into S-N curve to calculate the cumulative damage ratio. However the common ice-class rules do not describe which allowable stresses should be used for shaft with different type of materials and sizes. However, DNV’s Classification Note 51.1 describes in detail about the fatigue calculations of shafting, blades and CP-mechanism. The calculation procedure is complicated and difficult to do by hand calculation.
In the real case calculation, there are more calculation details and requirements needed to be considered. DNV Software’s Nauticus Shaft Fatigue tool can be used for the code checking on shaft according to the new ice-class rules and the simple approach to calculate the maximum response torque (described in Baltic Ice-class rule) is also implemented in the software.
Author: QiFeng Wang