During 2009, DNV carried out a considerable amount of work to make the Polar Class Machinery requirements more user friendly and correct misprints and errors in the first revision of the rules published in 2008.
This work has resulted in a revised draft of IACS UR I3 Rev.2, which was presented to the IACS Machinery Panel in December 2009 and obtained the Panel’s principal acceptance at the beginning of March 2010. The draft UR must be furnished with a new history file, which will include the already available technical background, before it is presented to the IACS General Policy Group (GPG) for approval.
Parallel to the above, we have prepared a Rule proposal intended to update our Polar Class requirements in Pt.5 Ch.1. Sec.8, mainly implementing the amended UR I3. The rule proposal is currently subject to an external rule hearing round . We plan to publish this revision on 1 July 2010.
We have seen that the Polar Class rules will require a lot of support even when the second revision is available. We therefore decided to introduce a new Classification Note, CN 51.1 Ice strengthening of Propulsion Machinery, addressing the application of these rules in practice. The first draft edition will be available for a limited round of consultations in April 2010. In this CN, we will present our guidelines for propulsion machinery design requirements in the Polar Class and new Finnish-Swedish Ice Class rules (FSICR). The first edition will contain among other things guidelines for:
Finite element modelling of propeller blades
The application of calculated stresses to blade designs for static and fatigue loads
The application of “blade failure loads”
The simulation of dynamic ice load introduced responses in shafting
The calculation of shafting component scantlings
Ice loads on the azimuth thruster/pod structure
The Machinery Section at Høvik has also organised several “mini-workshops”, where we have shared our interpretations of the Polar Class rules and new FSICR and tools to calculate the loads, etc. These workshops have given us valuable feedback from users, resulting in several amendments to the abovementioned draft UR I3 and our Polar Class rule proposal. Among other things, we have proposed reducing a spindle torque caused by blade failure load to half of its current value. A number of finite element analyses showed that, if the force is acting on its defined location, the calculated blades were deformed locally and not, as intended, close to the root fillets. The new spindle moment arm, 50% of its original value, was proven to produce the expected results.
Another change is the deletion of the blade edge and tip thickness requirements. Despite the correction of an error in the current formula, the requirement seemed to be decisive for the blade profile, which was not the intended result. The deletion of this requirement does not, however, mean that propeller designers should not care to make blade edges and tips sufficiently strong to withstand contact with multi-year hard blue ice with pressures that may reach 30 to40 MPa very locally. For example, FEA can be used to determine local stresses applying local ice loads determined as per item 6 to the relevant blade edge and tip areas. (Fig.1)
Ice loads acting on “appendices”, i.e. on parts sticking out of the ship hull, such as a thruster/pod, that are thus subject to contact with ice moving along the shell, turned out to be a major challenge, when applying the PC rules.
The way of calculating ice loads used since the early 1990s was based on ice pressure definitions for ice class notations ICE-05 to15 and POLAR-10 to30 and did not fit the PC hull ice load model. The author saw it as quite obvious that the ice pressure/load on an appendix cannot increase above a certain level even if the ship’s displacement (mass) increases.
We had also experienced that the “1990s load model” was too conservative and had room for relaxation. A new load model made by the author based on the propeller blade “backward bending load” showed quite promising results. This method was further developed and calibrated with measurement results in close cooperation with ABB Marine’s Ms Pirjo Määttänen and Messrs Samuli Hänninen and Torstein Heideman.
Mr Geir Dahler, head of the Rotating Machinery Section in DNV’s Technical Advisory Ship and Offshore, carried out several numerical simulations of ice load responses providing increased understanding of the dynamic versus static share of measured ice loads. According to this model ice load, “F” on a defined area can simply be expressed by:
F = po0.8 (A C0.3)exp C1 C2 C3 C4 [MN];
Where po is reference pressure on 1 m2 (range 1…6 MPa); A is considered area [m2]; C is ice class parameter f(A; Hice); exp is 0.3 for A less than1 m2, otherwise 0.85; C1 to C4 consider load location and type of unit, location on board, ship type and finally statistical uncertainty.
The following load cases must typically be considered for an azimuth thruster or pod installation:
Transversal force on strut
Transversal force on pod
Axial force on strut
Axial force on pod, or propeller hub for pulling propeller
Transversal force on nozzle if any
Axial force on nozzle if any
During the mini-workshops with relevant manufacturers, this new calculation method has been presented and discussed. The feedback from the makers has been far positive. Now this method will be published in the Classification Note and is referred to in the new revision of PC rules.
Text: Lasse Norhamo
Date: 05 July 2010