Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice Rule URI3

Transcription

1 10th International Symposium on Practical Design of Ships and Other Floating Structures Houston, Texas, United States of America 2007 American Bureau of Shipping Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice Rule URI3 Sing-Kwan Lee Research and Product Development, Technology, American Bureau of Shipping Houston, USA Presented at PRADS 2007, Houston, TX, USA, 1-5 October 2007 and reprinted with the kind permission of the 10th International Symposium on Practical Design of Ships and Other Floating Structures (PRADS) Abstract The Machinery Requirements for Polar Class Ships IACS URI3 has been released and is to be uniformly applied by IACS Societies on ships contracted for construction on and after 1 March Unlike the pervious ice class rule, URI3 relies more on direct calculations for propeller strength assessment. In this paper, some concerns on general practice on finite element analysis according to the URI3 Rule will be discussed based on our current experiences. Keywords Ice propeller strength; Polar ice class; IACS URI3; Plastic FEM. Introduction In propeller strength assessment, the current IACS URI3, Machinery Requirements for Polar Class Ships (IACS), requests finite element stress analyses to be performed based on ice loads. The propeller design ice loads given in URI3 are different from the ice torque traditionally used in the past and are the results of extensive research activities. Included in the activities, there were analyses of service history of propeller damages, propeller and shaft load measurements on full- scale trials, laboratory investigations and numerical simulation of propellerice interaction. It was found through these activities the traditional ice torque was not adequate to the ice propeller strength assessment task. Rather than the inplane ice torque, the out-of-plane blade bending moments due to the backward and forward ice forces were found to be the main attribution to the causes of major blade deformation and breakage. In the development of the IACS URI3 Rule, finite element analyses based on the aforementioned out-ofplane ice load were carried out by classification societies. The results were compared with the measured stresses from the icebreakers, Polar Star and Gudingen. Based on these analyses, it was found the simplified cantilever beam method cannot predict the blade stresses with reasonable accuracy, especially for a highly skewed blade. It was then concluded FEM-based analyses are necessary to ice propeller strength assessment. To assess the blade strength based on the FEM results, a less restricted reference stress criterion is proposed in URI3. This reference stress has originally been developed to reflect the real capability of the blade to carry loads aimed to in particular for extreme ice loads that can cause plastic bending of the blade. In FEM analysis for non-ice propeller, since the stress criteria are based on elastic stress criteria such as yielding or fatigue limits, linear stress analysis is regarded to be accurate enough for blade strength review. For ice propeller, however, slight plastic deformation on blade is common and almost unavoidable. In order to consider the plastic deformation situation, the reference stress criterion is more rational. In light of the reference stress criterion, it is natural the FE stress analysis should be extended to the plastic stress analysis level so that the stress due to the plastic deformation can be taken into account more appropriately. In fact, in ABS experience, it was found that linear FE stress analysis could be too conservative for some safe designs proven by their service history. In such cases, plastic FEM analysis should be adopted for correctly assessing the blade strength. In this paper, some concerns on general practice on ice propeller strength assessment according to IACS URI3 Rule will be discussed based on our current experiences. Ice rule on propeller strength In URI3 document, the design forces on the propeller blade resulting from propeller-ice interaction, including hydrodynamic loads are provided. These forces are the expected ice loads for the whole services life of the ship under normal operational conditions, including loads resulting from Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13 219

2 the changing rotational direction of fixed pitch propellers. The Rules cover open- and ducted-type propellers with fixed or controllable pitch designs for the following Polar ice classes defined in URI1 (IACS). where 2 [m] Dlimit = H ice d 1 D d = propeller hub diameter [m] Table 1: Ice class defined in URI1 Polar Class Ice Description (based on WMO Sea Ice Nomenclature) PC 1 Year-round operation in all Polar waters PC 2 Year-round operation in moderate multi-year ice conditions PC 3 Year-round operation in second-year ice, which may include multi-year ice inclusions. PC 4 Year-round operation in thick first-year ice which may include old ice inclusions PC 5 Year-round operation in medium first-year ice which may include old ice inclusions PC 6 Summer/autumn operation in medium first-year ice which may include old ice inclusions PC 7 Summer/autumn operation in thin first-year ice which may include old ice inclusions Design ice loads For the sake of briefness, only the ice loads formulae for open propeller are briefed here. For ducted propeller, the details can be referred to the original URI3. Maximum backward blade force F b in [kn] unit 0.7 EAR 2 Fb = 27 Sice[ nd] D when D D Z limit F b EAR 23 Sice ice [ nd] DH = when D > D Z limit Table 2: Values of H ice and S ice for different PC ice class Ice PC1 PC2 PC3 PC4 PC5 PC6 PC7 class H ice [m] S ice Load cases In ice blade strength assessment, according to URI3 load cases 1-4 have to be covered, as given in Table 3 below, for CP and FP open propellers. In order to obtain blade ice loads for a reverse rotating propeller, load case 5 also needs to be considered for FP propellers. Table 3: Load cases defined in URI3 Force Loaded area (refer to Figure 1) case 1 F b Uniform pressure applied on the back of the blade (suction side) to an area from 0.6R to the tip and from the leading edge to 0.2 times the chord length. case F b Uniform pressure applied on the back of the blade (suction side) on the propeller tip area outside 0.9R radius. case 3 F f Uniform pressure applied on the blade face (pressure side) to an area from 0.6R to the tip and from the leading edge to 0.2 times the chord length. case F f Uniform pressure applied on propeller face (pressure side) on the propeller tip area outside 0.9R radius. case min{f b,f f } Uniform pressure applied on propeller face (pressure side) to an area from 0.6R to the tip and from the trailing edge to 0.2 times the chord length where D limit Hice = [m] H ice = design ice thickness (see Table 2) S ice = ice strength index (see Table 2) D = propeller diameter [m] EAR = expanded blade area ratio Z = blade numbers n = propeller rps [1/s] For CPP, n = nominal rotational speed at MCR in free running condition For FPP, n = 85% of the nominal rotational speed at MCR in free running condition Maximum forward blade force F f in [kn] unit F F f f EAR 2 = 250 Sice D when D Dlimit Z EAR 1 when = 500 Sice D H D > Dlimit ice Z d 1 D 220 Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13

3 Fig. 1: Loaded area for different cases Stress criterion For propeller strength, URI3 uses the following plastic stress criterion. ref 1.5 where = calculated stress for the design loads; if FE analysis is used in estimating the stresses, von Mises stresses shall be used ref = reference stress defined as min{0.7 u, }; u is ultimate tensile strength, 0.2 is proof strength Loads on propeller As known, there are contact (ice) and non-contact (hydrodynamic) loads on ice propellers. Non-contact load considered here is the hydrodynamic load on blade for propulsion under open water condition. For contact load, it is mainly the ice load due to the ice milling and impact processes. The difference between these contact and non-contact loads is significant. To illustrate this, a four blade PC7 polar ice class CP propeller is selected, and its hydrodynamic loads on the blade at MCR conditions (ahead and crash-stop operations) are compared to the different ice load scenarios defined in IACS URI3. This PC7 propeller is a highly skewed design with a diameter 5400 mm. The details of the propeller geometry are given in Table 4 and Figure 2 as follows. Fig. 2: Blade expanded outline and section profile Hydrodynamic load The hydrodynamic loads are calculated by ABS PropS2 software, which can handle ahead and crash-stop for CP propeller (ABS, 2005). Calculated thrust, torque, power, and pressure difference are summarized in Table 5 for the ahead condition. The calculated propeller absorbed power shown in the table is matched with the engine MCR power. The calculated hydrodynamic pressure difference (net pressure) distribution on blade is plotted in Figure 3. As seen, high net pressure occurs near the leading edge area. This net pressure distribution can generate not only the moment to bend the blade forward but also the spindle torque to twist the blade. Table 5: Propeller performance and load at ahead MCR Thrust Torque Absorbed Max. pressure Power difference kn kn-m kw N/mm Table 4: Propeller particulars Fig. 3: Hydrodynamic pressure difference at ahead MCR Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13 221

4 For crash-stop operation of CP propeller, propeller is turned along the spindle axis with Δθ to a position with negative pitches (Figure 4). Usually, Δθ should be selected to absorb the engine power as much as possible to generate maximum backward thrust. It should be noted after Δθ turning, the original pressure side in ahead operation becomes the suction side in crash-stop operation. Also as is well-known, the section profiles are distorted (Figure 5). This geometry changing affects a lot the propeller hydrodynamics. As shown in Table 6, same absorbed power (6300 kw) cannot generate same thrust force ( kn) as the ahead condition in crash stop operation. Only 47% of the ahead thrust can be generated in crash stop operation. Table 6: Propeller performance and load under crash stop Thrust Torque Absorbed Max. pressure Power difference kn kn-m kw N/mm Fig. 6: Hydrodynamic pressure difference under crash-stop Fig. 4: Ahead vs. crash stop position Ice load Backward and forward ice forces, F b and F f, are calculated based on IACS URI3 formulae mentioned earlier. Table 7 below summarizes the results of ice and hydrodynamic loads resulting in backward and forward blade bending. Also included in the table are the ice and hydrodynamic torques. For hydrodynamic loads, it should be noted that the values in the table are for the whole propeller. For each blade, dividing the loads by 4 should be done. It is found that the ice loads per blade are much higher than the hydrodynamic loads. Table 7: Comparison of ice and hydrodynamic loads Fig 5: Blade expanded outline and distorted section profile The pressure difference for the crash stop is plotted in Figure 6. As seen in the figure, quite a different pattern occurs as compared to the ahead condition. Basically, the highest net pressure is localized around the trailing edge at outer radius area (>0.7R). As the total backward thrust reduces compared to the ahead thrust, the maximum net pressure also shows smaller value compared to ahead condition (see the values in Table 5 and 6). Ice load per blade Hydrodynami c load whole propeller Forward force Backward force Torque kn kn kn-m Ahead Crash stop Ahead Crash stop Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13

5 Table 8: Calculated ice loads based on URI3 ice/hydro force ice/hydro pressure per blade max. value kn N/mm2 Case Case Case Case Ahead Crash Strength assessment Fig. 7: Ice pressure distribution URI3 case 1 & 3 Based on the calculated ice loads, FE analyses were performed for the propeller. In this section, some experiences of propeller strength assessment are reported. Elastic vs. plastic stress analysis As mentioned earlier, stress criterion for blade strength is based on reference stress ref that is beyond yielding strength. It is natural to ask what the difference between elastic and plastic FE analyses will be when ice load causes the blade stress beyond the yielding limit. To investigate this, both the elastic and plastic stress analyses are performed for the PC7 ice class propeller. The results are summarized in Table 9 and 10. Since the propeller is a CPP, load case 5 is not requested according to URI3 rule. Table 9: Elastic stress analysis results Ultimate tensile strength: 590. N/mm2 0.2% proof strength: 245. N/mm2 Fig. 8: Ice pressure distribution URI3 case 2 & 4 For ice load cases 1 and 3 defined in URI3, the ice pressure distribution on blade (Figure 7) is more or less similar to the hydrodynamic pressure in open water ahead condition. They are both concentrated in the leading edge area, although ice pressure is much higher than hydrodynamic pressure. For ice load case 2 and 4, the ice pressures (Figure 8) are totally different from the hydrodynamic pressure distribution and will cause totally different stress distribution on blade. However, in any cases the ice load induced stresses are expected to be much higher than the hydrodynamic load induced stresses, as ice loads are much higher than hydrodynamic loads (see Table 8). Reference Calculated S.F. stress (Von Mises)stress N/mm2 N/mm2 >=1.5 Case Case Case Case Table 10: Plastic stress analysis results Ultimate tensile strength: 590. N/mm2 0.2% proof strength: 245. N/mm2 Reference Calculated S.F. stress (Von Mises)stress N/mm2 N/mm2 >=1.5 Case Case Case Case Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13 223

6 In elastic FE analyses, it is found the blade strength cannot pass the safety factor > 1.5 reference stress criterion for case 2, 3, and 4. Also, it is noted that although the case 1 pass the safety criterion, the calculated stress is already beyond the yielding limit (yielding strength = 245 N/mm 2 ). In plastic FE analyses (Table 10), the results show that once the stress excesses yielding limit, the increasing rate of the stress to the ice loads is largely reduced compared to elastic FE analyses. Accordingly, the final stresses for case 2, 3, and 4 are still within the reference stress criterion safety limit. slightly expanded compared to the elastic FE analyses due to the material yielding. For ice load case 2, again the stress contours are plotted in Figure 11 and 12. In this case, since the ice load causes the blade stress ( N/mm 2 in elastic analysis) far beyond the blade yielding strength, the plastic FE analysis shows obvious difference from the elastic FE result. Basically, the high stress in plastic FE analysis is extended greatly indicating that large area is under yielding. As is well-known, once the stress passes the yielding point, the stress-strain relation will follow a material curve with less slope, and this causes the plastic stress becomes lower than the elastic stress. This is the main reason behind for plastic FE analyses come up lower stress values. Fig. 9: Stress contour elastic FEM, case 1 F b load Fig. 11: Stress contour elastic FEM, case 2 0.5F b load Fig. 10: Stress contour plastic FEM, case 1 F b load To have better idea of the stress distributions on elastic and plastic FE results, stress contours of ice load case 1 are plotted in Figure 9 and 10 for elastic and plastic FE analyses. For this case is not much far off the stress beyond the yielding limit, the stress contours in elastic and plastic analyses does not appear too much different, although it does find in plastic FE result (Figure 10) the high stress area is Fig. 12: Stress contour plastic FEM, case 2 0.5F b load Blade edge strength In ice operation, experience shows edge area damage is the most frequent damage on the blade. Unlike the whole blade damage caused by the pervious forward and backward ice forces, edge damage is the result of highly concentrated ice 224 Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13

7 impact pressure. In IACS URI3, this is treated with a simplified model a cantilever beam subjected to a uniform pressure load. For the convenience of later discussions and FEM comparisons, the blade edge rule formula is re-derived here. According to URI3 method, when the propeller blade hits ice block, ice impact pressure will concentrate in a small area (typical area used in URI3 is an area of 2.5% chord length by 2.5% chord length around edge). Uniform pressure p ice 16 MPa is taken as the ice impact pressure in URI3. As shown in Figure 13, a cantilever beam model (with length x, thickness t and width B) is used to evaluate the stress in a narrow strip of the ice impact area. This results in the stress at x as follows: = 3 ( x / t) p ice Fig. 13: Simple beam model for edge strength URI3 rule Comparing the above stress formula with URI3 blade edge thickness t edge formula (section I in URI3), the actual safety factor SF based on stress value used in URI3 is obtained as follows: ref SF = = ( SS 3p ( x / t ) ice edge 2 2 ice ) 2 / 3 where S = safety factor defined in URI3 = 2.5 for trailing edges = 3.5 for leading edges = 5 for tip S ice = ice strength index; 1. for PC6 PC7; 1.1 for PC2-PC5; 1.2 for PC1 t edge = thickness at x p ice = ice pressure; 16MPa x = min{2.5% chord, 45mm}; for radius < 0.975R propeller radius, it is measured from edge along cylindrical sections; for radius > 0.975R propeller radius (tip area), it is measured perpendicularly from the edge For the seven polar ice classes, the actual edge safety factors based on stress values are calculated using the above SF formula and summarized as follows: Table 11: Stress based safety factors for Polar ice class Ice class S ice Leading edge S = 3.5 Trailing edge S = 2.5 Tip S = 5.0 SF PC PC PC5 PC6- PC To evaluate the validity of simplified beam approach, FE analyses are performed for the previously selected skewed propeller. In the FE analyses a 16MPa uniform pressure with 2.5% chord by 2.5% chord loaded area is applied in a different radius from 0.5R to 0.9R around edge and mid-chord at tip. The comparisons of FE analyses and Rule calculations for edge/tip stress are summarized in the following table. Table 12: Comparison of Rule and FE results for edge stress Location FEM calculated N/mm2 Leading edge URI3 calculated N/mm2 FEM calculated N/mm2 Trailing edge URI3 calculated N/mm2 0.5R R R R R Tip* * Tip at mid-chord As seen in the table, simplified beam approach tends to overpredict a lot the edge/tip stresses. The over-prediction of the stress in URI3 simplified model becomes more serious at outer radius and also the trailing edge area. It should be noted that for this CP propeller, there is no need to have this safety check for the trailing edge, although this is done here for completeness. Safety factors ( ref /, here ref = 383 N/mm 2 ) based on FEM and simplified beam approach are calculated. Summarized in Table 13, the URI3 required safety factor for edge strength from Table 11 are also listed. Table 13: Safety factor comparison of Rule and FE results Location Leading edge S.F. based on FEM S.F. based on URI3 > 4.08 Trailing edge S.F. based on FEM S.F. based on URI3 > R R R R R Tip* * S.F. > 8.33 for tip For this PC7 ice class propeller, URI3 simplified model concludes the propeller has inadequate edge strength to sustain ice impact load. However, if the same required stress based safety factors derived from URI3 (Table 11) are used, the FEM analyses conclude the leading edge and tip are safe for 16 MPa Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13 225

8 ice impact load. For trailing edge area, FEM results predict the trailing edge could be a failure at 0.9R. This is consistent with the fact that CP propeller can be designed with weaker trailing edge strength, as no reverse rotation is needed in CPP operation. Trailing edge safety check is only requested for FP propeller in URI3. To obtain a general picture of the ice impact induced stress, stress contours are plotted for the cases with ice load at 0.5R areas for leading and trailing edges (Figure 14 and 15). As seen, since the ice pressure is highly concentrated in a small area, the stress also shows a highly localized pattern. However, for tip load case, the highly localized stress pattern doesn t appear at tip. Instead, a relatively large area with high stress is found in the inner middle area around 0.8R (see Figure 16). Fig. 16: Stress contour 16MPa ice pressure at tip To further investigate what causes stress pattern to diverge from the tip concentrated pattern for the tip concentrated load case, the original skewed blade is changed to a zero skewed blade. With the same ice load applied as before on the blade, FEM analysis is performed. The stress contours obtained are plotted in Figures 17, 18, and 19. As seen, all stress patterns are highly concentrated pattern. This concludes blade skewness is the main cause for the non-concentrated stress pattern before and the highest stress shifting. This fact also implies simplified URI3 approach for edge strength assessment is not appropriate for highly skewed propeller even for the approximate location of the highest stress. Fig. 14: Stress contour 16MPa ice pressure at 0.5 R L.E. Fig. 17: Stress contour 16MPa ice pressure at 0.5 R L.E. Fig. 15: Stress contour 16MPa ice pressure at 0.5 R T.E. 226 Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13

9 Table 14: Edge stress comparison between skewed and nonskewed propellers Leading edge Trailing edge Location FEM calculated N/mm2 FEM calculated N/mm2 Nonskewed Skewed Nonskewed Skewed 0.5R R R R R Fig. 18: Stress contour 16MPa ice pressure at 0.5 R T.E. To further illustrate the stress distribution difference of the skewed and non-skewed propellers from the ice impact load, the stress contours for the case with the load applied at 0.9R are plotted in Figure 20 and 21. As seen, there is an obvious difference of the stress pattern due to the propeller skewness. For skewed propeller, the localized impact pressure has larger influence area than the non-skewed propeller. Finally, to further investigate the propeller skew geometry effect on the blade edge stress due to ice impact loads, FEM analyses are performed for the zero skew propeller blades under the ice impact load (16 MPa) at different radius at leading and trailing edges. The calculated stresses are summarized in Table 14. As shown in the table, the skewness of the propeller does have significant effect on the blade edge stress. This implies although impact ice pressure is localized in a small area, its induced stress is not only affected by the local propeller geometry but also by the propeller global geometry parameter. Fig. 20: Ice impact loads at 0.9R leading edge for skewed propeller Fig. 19: Stress contour 16MPa ice pressure at Tip Fig. 21: Ice impact loads at 0.9R leading edge for non-skewed propeller Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13 227

10 Concluding Remarks In this paper, the general practice on ice propeller strength assessment based on the IACS polar ice rule URI3 are discussed. The findings of our FE analysis practices through a PC7 ice class highly skewed CP propeller are summarized as follows: Compared to ice load hydrodynamic load is small. For an ice class propeller, ultimate strength of blade should be controlled by ice force. However, for fatigue failure, hydrodynamic load could still be the dominated source (although the fatigue analyses are not presented here because of the length limit of the paper, the results do show the ice induced fatigue stress is much lower than the hydrodynamic stress). In the viewpoint of the blade strength safety, crash-stop for the propeller concerned is not a critical operation condition. This is mainly because under the crash stop operation the propeller cannot generate a large backward force (restricted by the engine power) to endanger the blade. It is found the net pressure distribution on blade under normal MCR ahead operation is similar to ice load case 1 and 3 in URI3. Basically, the high pressures are localized around the leading edge area. These loads can generate not only the moment to bend the blade forward (backward also for ice load) but also the spindle torque to twist the blade. Due to the twisted torque, the maximum stress is found to be shifted to near the leading edge at the root area (see Figure 9), instead of at the mid-chord at root section as expected in conventional blade propeller. For ice loads, both elastic and plastic FE analyses were performed. In elastic FE analysis, it is found that the calculated stresses are beyond the yielding limit and cannot pass the reference stress criterion in URI3. As the stresses on the blade already are in plastic stress region, the stress analysis based on elastic FE may not be appropriate for this ice propeller strength assessment. By using plastic FE analyses, it is found the calculated stresses are reduced and satisfy the reference stress criterion. This result is consistent with the service experience of this propeller. In general, we tend to agree if the calculated stress is beyond yielding limit, plastic stress analysis needs to perform. Blade edge strength is studied by using URI3 Rule simplified calculation and FEM analysis. 16 MPa ice pressure proposed in URI3 is used in the analyses. It is found that the simplified URI3 rule calculation tends to over-predict the stress a lot compared to FEM analysis. In the blade edge strength study, it also finds although the ice impact pressure is localized in a small area around the load applied location, the stress is influenced by the global geometry. This is proven by the large change of stress due to the propeller skewness changing. In other words, in the edge strength assessment, the 3D effect of the blade geometry should be considered. References ABS (2005) PropS2 User s Guide & Manual IACS (2006) URI1 Polar Class Descriptions and Application IACS (2006) URI3 Machinery Requirements for Polar Class Ships 228 Engineering Practice on Ice Propeller Strength Assessment Based on IACS Polar Ice - Rule UR13