Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule

Transcription

1 Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule Sing-Kwan Lee Research & Product Development, Technology, American Bureau of Shipping Houston, Texas, USA Originally presented at the IceTech Conference held July 0-3, 008, in Banff, Alberta, Canada ABSTRACT While there are power saving and high thrust capability advantages of CPP (Controllable Pitch Propeller) for ice propulsion, the introduction of CP mechanism makes the propulsion system more vulnerable when operating in heavy ice condition. In CPP designs, the recently released IACS URI3 Machinery Requirements for Polar Class Ships provides the design forces on the blade resulting from -ice interaction. These design forces are also the source of the spindle torques, which can be used for the determination of CPP mechanism scantlings. In this paper, s formulae in URI3 for open and ducted CPP will be adopted for an existing ice CPP for its mechanism check to evaluate the rationality of the Rule. To further explore the influence from different design principles on CPP scantlings, both the maximum life time ice load and the blade failure load provided in URI3 will be applied on the existing CPP to check its mechanism strength. The detailed comparison of the strength check outcomes provides very useful information for ice CPP design. KEY WORDS: Controllable Pitch Propeller, IACS, URI3, Blade Strength, CPP Mechanism Scantlings. INTRODUCTION From a propulsion point of view CPP (Controllable Pitch Propeller) is a better design option compared to FPP (Fixed Pitch Propeller) for ice going ships. A current comparison (Lee, 008) of ice propulsion capability for open CPP, open FPP and ducted FPP showed that CPP design not only is the best energy-saving propulsor among the designs but also has the capability to continuously generate enough thrusts for severe ice conditions even when the FPP designs are broken down. While there are aforementioned energy saving and high thrust capability advantages of CPP (Controllable Pitch Propeller) for ice propulsion, the introduction of CP mechanism makes the propulsion system more vulnerable when operating in heavy ice condition. The CPP design can only be safely applied if the propulsor strength, including blade and CP mechanism, for resisting extreme s can be sufficiently assessed. In strength assessment, the currently released IACS URI3 Machinery Requirements for Polar Class Ships (008 IACS) provides the design forces on the blade resulting from -ice interaction. These forces include the backward and forward ice forces which were found to be the main attribution to the cause of major blade deformation and breakage. In CPP design, these forces can be used as the original source of the spindle torque for determining CPP mechanism scantlings. Although IACS URI3 Rule defines the design loads on ice, the rule does not detail the scantling for CPP mechanism design. The design is relied upon more in engineering practices in industry. As IACS Polar Rule becomes effective as of March 008 accompanied with the expectation of more and more application of CPP to be applied in ice propulsion, comprehensive studies based on the URI3 for CPP mechanism could be illustrative and useful. In this paper, ice loads formulae in URI3 for open and ducted CPP will be adopted for an existing ice CPP for its mechanism check to evaluate the rationality of the Rule. To further explore the influence from different design principles on CPP scantlings, both the maximum life time and the blade failure loads based on the rule formulae will be loaded on the existing CPP to check its mechanism strength. The detailed comparison of the strength check outcomes provides very useful information for ice CPP design. ICE RULE SUMMARY The design s 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 were analyses of service history of damages, and shaft load measurements on full- scale trials, laboratory investigations and numerical simulation of -ice interaction. Through these activities it has been shown that the traditional ice torque was not adequate to the ice strength assessment task. Rather than the in-plane 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-of-plane were carried out by classification societies. The results were Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule 119

2 compared with the measured stresses from the icebreakers, Polar Star and Gudingen. Based on these analyses, it was found that 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 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 was originally developed to reflect the real capability of the blade to carry loads aimed particularly towards extreme s that can cause plastic bending of the blade. Ice Rule for Propeller In URI3 Rule, the design forces on the blade resulting from -ice interaction, including hydrodynamic loads are provided. These forces are the expected s for the whole services life of the ship under normal operational conditions, including loads resulting from the changing rotational direction of fixed pitch s. The Rules cover open- and ducted-type s with fixed or controllable pitch designs for the following Polar ice classes defined in URI1 (IACS). Table 1: Ice class defined in URI1 Polar Class PC 1 PC PC 3 PC 4 PC 5 PC 6 PC 7 Ice Description (based on WMO Sea Ice Nomenclature) Year-round operation in all Polar waters Year-round operation in moderate multi-year ice conditions Year-round operation in second-year ice which may include multi-year ice inclusions. Year-round operation in thick first-year ice which may include old ice inclusions Year-round operation in medium first-year ice which may include old ice inclusions Summer/autumn operation in medium first-year ice which may include old ice inclusions Summer/autumn operation in thin first-year ice which may include old ice inclusions Design ice forces For the sake of briefness, only the s formulae for open are noted down here. For ducted, the details can be referred to the original URI3. Maximum backward blade force F b in [kn] unit 0.7 EAR Fb = 7 Sice[ nd] D when D D Z limit EAR 3 Sice ice [ nd] DH Fb = when D > D Z limit 1.4 where D =.85 H [m] limit 0 ice H ice = design ice thickness (see Table ) S ice = ice strength index (see Table ) D = diameter in m EAR = expanded blade area ratio Z = blade numbers n = 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 EAR Z EAR 1 D H Z d 1 D D D f = 50 Sice D when limit f where = 500 S ice ice [m] Dlimit = H ice d 1 D d = hub diameter [m] when D > Dlimit Table : Values of H ice and S ice for different PC ice class Ice class PC1 PC PC3 PC4 PC5 PC6 PC7 H ice [m] S ice Load s In ice blade strength assessment, according to URI3 load s 1-4 have to be covered, as given in Table 3 below, for CP and FP open s. In order to obtain blade s for a reverse rotating, load 5 also needs to be considered for FP s. Table 3: Load s defined in URI Force Loaded area (refer to Figure 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. times the chord length. 0.5 F b Uniform pressure applied on the back of the blade (suction side) on the tip area outside 0.9R radius. 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. times the chord length. 0.5 F f Uniform pressure applied on face (pressure side) on the tip area outside 0.9R radius. 0.6 min{f b Uniform pressure applied on face,f f } (pressure side) to an area from 0.6R to the tip and from the trailing edge to 0. times the chord length Stress criterion For strength, URI3 uses the following plastic stress criterion. σ ref 1.5 σ where σ = calculated stress for the design loads; if FE analysis is used in estimating the stresses, von Mises stress shall be used σ ref = reference stress defined as min{0.7σ u, 0.6σ +0.4σ 0. }; σ u is ultimate tensile strength, σ 0. is proof strength 10 Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule

3 For the design based on the blade failure force, the spindle torque is calculated as follows. Q ex = /3 F ex max{l l, l t } where l l and l t are the distances of axis of blade rotation of leading and trailing edges. CPP MECHANISM SCANTLING Although design loads for CP mechanism design are defined in URI3, there is no scantling formula provided. In industry the design is relied upon more in engineering practices. In CPP mechanism design, due to the complexity of the mechanical system, the direct calculation method is not practical. In general, the design and analysis of the system are based on appropriate simplification of the pitch control and actuation mechanism elements in the system Scantling formula for the mechanical elements are then developed based on common mechanical design principle. In the following sections, the loads on the critical elements, the stresses due to the loads, and design failure criteria are addressed briefly. For greater details of the design method the reader can refer to ABS (005). Fig. 1 Loaded area for different s Blade failure load Instead of the design using the life time expected maximum forces, blade failure force is another alternative for CPP mechanism scantling design if the so-called selective strength design principle is used. Under the principle, the design of CPP mechanism is based on the blade failure force so that the blade will be failed first to protect the CP mechanism. According to URI3, the blade failure force is acting at 0.8R in the weakest direction of the blade and at a spindle arm of /3 of the distance of axis of blade rotation of leading and trialing edge which ever is the greatest. The blade failure load is: Load on Critical Elements It is obvious that the source of loads on mechanical elements in CP mechanism is from the blades. The forces on blades will transfer to the mechanical elements through a load path. It should be noted that in addition to the external forces from blades, there are internal forces generated by friction in the connected elements. The friction will increase hydraulic force required to change pitch but could reduce hydraulic force required for holding pitch. In our analysis for CP mechanism, this aspect is also considered for collar bearing, trunnion bearing and sliding block bearings. Loads and coordinate system Loads on a blade consist of hydrodynamic loads, centrifugal loads, and s. Figure shows the coordinate system used in our calculations. In the most general, three forces and moments along x, y, and z axis should be considered. 0.3 c t σ ref 3 Fex = 10 kn 0.8 D r where c, t, and r are the length, thickness and radius of the cylindrical root section of the blade at the weakest section outside the root fillet. Spindle torque for CP mechanism In determining CPP mechanism scantling, spindle torque is an essential load to be considered. According to URI3, the spindle torque Q smax around the spindle axis of the blade fitting shall be calculated both for the load s aforementioned for F b and F f. If these spindle torque values are less than the default value given below, the default minimum value to be used. Default value: where value. Q smax = 0.5 F c 0.7 kn-m c 0.7 = length of the blade chord at 0.7R radius, in m F = either F b or F f, whichever has the greater absolute Fig. Loads and coordinate system Load path As mentioned earlier, the forces exerted on a blade will transfer to CP elements through a load path. Basically, the critical Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule 11

4 elements include blade, blade flange, blade attachment bolts, blade attachment dowels, crank disk, sliding shoe, crosshead, actuator rod and piston. A schematic sketch for these elements is plotted in Figure 3. However, it should be noted that there are many different designs in industry for CP mechanism. It is impossible to list all of them here. This one shown in Figure 3 is only for illustration purposes. Fig. 4 Load path from blade to bolts and dowels Fig. 3 Critical elements in CP mechanism A summary of the load path for CP mechanism is listed in Table 4. The table provides the information of loads transmission among the mechanical elements. Stress on Mechanical Elements After the loads on each mechanical element have been determined, the stresses on the mechanical elements can be evaluated based on elasticity mechanics. For example, if the bearing force, F pl, on pilot is known, the stress, σ pl, on pilot can be calculated as σ pl = F pl /(DPL XLPL) (see Figure 5). Similarly, for the dowel, if friction in the joint can be ignored for conservation, due to the small engagement and close fitting into the hole, bending is neglected. Therefore, only simple shear is considered. The shear stress, σ ds, at the dowel can be calculated as σ ds = 4F d /π DDL under the load F d. For other stress formulae, details can be referred to ABS (005) Table 4 Load path of mechanical elements in CP mechanism Figure 5 dimensions of pilot and dowel To expand upon the information in this table, a load transmission path from a blade to bolts and dowels is further explained by Figure 4 along with the third row of Table 4. As seen, the blade loads, Fx, Fy, Fz, Mx, My, Mz are transferred from the blade to blade flange. Once the blade flange receives the loads, it will pass vertical force, Fy, and horizontal moments, Mx and Mz, to blade attachment blots; while the horizontal forces, Fx and Fz, and spindle moment, Fy, will be transferred to the dowel. Due to the limitations of the length of the paper, the detailed load formulae for each critical element will not be provided in this paper. Readers can refer to ABS (005). Stress criteria According to ABS Rule, the strength of the pitch changing mechanism of controllable-pitch s should be at least 1.5 times that of the blade. Table 5 summarizes the criteria for pitch changing mechanism based on ABS Rule requirement. It should be noted that for some mechanical elements such as blade pilot and blade bolt seat since they are only subjected to compression, material ultimate strength is used instead of yielding strength. For the mechanical elements under maximum hydraulic load, the safety factor 1.5 is used instead of 1.5 time blade strength safety factor (Table 6). 1 Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule

5 Table 5 stress criteria for elements subjected to s and forces normal to the spindle axis are taken on the inner diameter of the bearing ring. There is one crank pin near the leading edge of the blade. The crosshead is integral with the piston actuating rod (Figure 8). The piston is fastened to the actuating rod with bolts (Figure 9). Table 6 Stress criteria based on maximum hydraulic load. Fig. 7 Integral Bearing ring CASE STUDY In this section, the aforementioned URI3 s will be applied to a selected to check the safety of the design. The design s for open and ducted subjected to -ice interaction loads and blade failure loads are considered in this study. For the blade strength assessment, FEM analyses are preformed while the safety checks for the CP mechanism components are based on the scantling formulae in ABS (005). Fig. 8 Crosshead integrated with piston actuating rod CP Propeller Selected for Study The selected CPP is a typical hub design used in industry. The has four highly skewed blades (Figure 6) with a diameter 5. m. It is a twin screw design for a fast ferry and classed as Baltic ice class IAA (equivalent to IACS PC6). Fig. 9 Piston fastened to the actuating rod with bolts Fig. 6 Highly skewed blade profile of the CPP The blades are secured to the hub with 8 blade bolts. There is a pilot on the blade and one dowel between the blade and the crank ring. The bearing ring is integral with the hub (Figure 7) Design based on Life Time Maximum Loads Ice loads including backward force, F b, forward force, F f, and spindle torque Q smax, are calculated based on the URI3 formulae mentioned earlier. In addition to an open, a ducted CPP with the exact same geometry and operating rpm is considered. This ducted CPP is used to calculate how much the duct can protect CP mechanism. The Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule 13

6 following sections summarize the calculations of the s, their induced stresses on blade and the stresses in the CP critical elements. paper length requirements. Ice Loads of Open and Ducted CPP Cases Ice loads for open and ducted are provided in the following table. Table 7 Backward force, forward force and spindle torque Forward force Backward force Max. Spindle torque kn kn kn-m Open a Duct b a Spindle torque is due to 3 load (F f ) b Spindle torque is due to 1 load (F b ) As seen in the table, the forward forces for open and ducted are the same. This condition is mainly due to the ice load mechanism being the same in open and ducted for the scenario of ice hitting the blade from face side. In fact, the forward formula for open and ducted is the same as found in the Rules. However, the calculation result for the backward ice force and spindle torque for ducted does reduce, compared to the open, due to the duct protection effect. For the spindle torques of open and ducted s, it has been found that the default value is smaller than the actual values calculated by F f and F b. Stress and Safety Factor of Propeller Blades Using the pervious calculated F f and F b forces, FEM analyses are performed for the load s 1,, 3, and 4 for the open and for the load 1 and 3 for the ducted. For the ducted, the load and 4 are excluded as the blade tip ice impact/milling scenarios ( and 4) seldom occur in ducted due to the duct protection FEM stress analysis results are summarized in the following table. Safety criterion based on the reference stress is used to evaluate the structural safety of the. It is noted that according to the Rules criterion (S.F. > 1.5) the s (open and duct) are strengthened sufficiently to sustain the ice loads. Table 8 FEM stress and safety factor for open and ducted σ ref = 45.9 N/mm Load FEM stress Safety factor N/mm σ ref σ 1.5 σ Open Case Case Case Case Duct Case Case To have a general idea of the stress pattern on the blade, stress contour plots for the open subjected to 1 and are drawn in the Figure 10. For ducted, since the loaded areas on blade are the same as the open s, the stress contour patterns are similar to the open s with the exception that the stress magnitude is smaller due to the smaller F b. The plots of them are not drawn due to 10a. Blade stress due to load 1 (edge load) 10b. Blade stress due to (tip load) Fig. 10 Stress caused by edge and tip loads for open Stress and Safety Factor of CPP Mechanisms Table 9 CPP mechanism check for open CPP Forward F f = 1,081.6 kn Q smax = kn-m Backward F b = kn Q smax = kn-m S.F. S.F. STRESS PART STRESS N/mm N/mm Blade Flg. Bend Pilot Brg Blade Bolt Shank tens Threads tens Seat compress Shear under HD Thread shr BLD DWL Crank Ring Pin bend Hyd. Load bend Flange PC+PM bend Brg Ring/Hub Shear CrossHD bend Hyd Load Bend A Rod Tens AR at XHD Tens PST bolt Tens PST Bend Table 10 CPP mechanism check for ducted 14 Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule

7 Forward F f = 1,081.6 kn Q smax = kn-m Backward F b = kn Q smax = knm S.F. N/mm S.F. STRESS PART STRESS N/mm Blade Flg. Bend Pilot Brg Blade Bolt Shank tens Threads tens Seat compress Shear under HD Thread shr BLD DWL Crank Ring Pin bend Hyd. Load bend Flange PC+PM bend Brg Ring/Hub Shear CrossHD bend Hyd Load Bend A Rod Tens AR at XHD Tens PST bolt Tens PST Bend Tables 9 and 10 summarize the safety check results for this CP mechanism design. At least two loading conditions, namely forward and backward loads, should be considered in each design (open and ducted ). In the two loading conditions, the actual spindle torques that are applied on the CPP mechanism are based on the following values: Forward : Spindle torque = max{torque due to F f, 0.5 max{f f, F b } c 0.7 } Backward : Spindle torque = max{torque due to F b, 0.5 max{f f, F b } c 0.7 } Redesign of the failed components According to the calculations, it has been shown that the CP mechanism parts, pilot and dowel (see the red texts in Table 9 and 10), cannot fulfill the safety criteria mentioned in Table 5. It is worth investigating whether or not the dimensions of the parts can be slightly changed to sustain these s. As it is known, the stress is inversely proportional to the areas subjected to the loads. Mathematically, these can be expressed (see also Figure 5 for the meanings of the symbols) as follows: Letting that i) f p and f d are the area increasing factors of the pilot and the dowel for the compliance of the required safety criteria ii) s p and s d are material strength increasing factors for the pilot and the dowel iii) sf p and sf d are original safety factors for the pilot and the dowel (S.F. in Table 9 and 10), and using the pervious equations for stress and safety factor, the area increasing factors can be calculated as follows: Pilot: Dowel: f f p d.5 = sf s p.5 = sf s To decide how to increase the dimensions for those parts, the original dimensions for the pilot and the dowel are plotted in Figure 11 for reference. As the dowel diameters (DDL) need to increase a lot, it follows that the pilot diameter needs to appropriately reduce. Eventually, the final design of the pilot diameter DPL is reduced to 00 mm to provide more spacing for the dowel diameter increase. However, it has been noted in our several trials that if the dowel material is not allowed to enhance then the increase of dowel diameter will be too large to get an acceptable design for this CP mechanism. In addition to the increase of the dowel diameter, the strength of the dowel material also needs to increase so that the dowel diameter can maintain in a reasonable range. In this design, the original dowel material is ABS material type 4 with yielding strength 45 N/mm. The final dowel material will be SST 1341 with yielding strength 39 N/mm. This means s d is 1.6. Here, we maintain the pilot as the original material, i.e. s p = 1.0, in our new design. d p d Stress on pilot 1 1 = area DPL XLPL Stress on dowel 1 4 = area π DDL Also, it is noted that the safety criteria (Table 5) for the parts are as follows: Safety criterion for pilot: ultimate stress 1.5 blade safety facor =.5 actual compression stress Safety criterion for dowel: yielding stress 1.5 blade safety facor =.5 actual stress Fig. 11 Detailed dimensions of pilot, dowel, and crank ring Based on the formulae developed earlier, the increasing area factors are calculated and summarized in the following table for the pilot and the dowel. Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule 15

8 Table 11 Increasing area factors for pilot and dowel Pilot: Original S.F. Open forward Open backward Ducted forward Ducted backward f p Dowel Original S.F. s d 1.6 f d For CP mechanism design, if blade failure force is applied for this design, no CP mechanical element will pass the failure criteria. From this exercise, it may conclude that the selective strength design principle based on F ex force probably is not practical for CPP design. However, if the life time maximum load is used for selective strength design, in our study the CPP design is still in a reasonable scantling range. Table 1 summarizes the final dimensions of the dowels and pilots which comply with the safety criteria in Table 5. As seen, the increasing dimensions are in a reasonable range for this CP mechanism design. Table 1 Dimensions for safe design for pilot and dowel Pilot: Original dimension XLPL - safe dimension Dowel Original DDL DDL safe dimension % of DDL increase Open forward mm Open backward Ducted forward DPL = 350 mm XLPL = 18 mm mm 4.1 mm DPL = 00 mm 180 mm 09 mm 17.5 mm mm Ducted backward mm 15 mm 16 % 0 % - 5 % 19.5 % Design based on Blade Failure Load Instead of using the life time s as the design loads, the blade failure load may be another option for design based on a so-called selective strength design principle, which allows the blade to break first in order to protect other CP mechanism. In this section, simple calculations will be performed to check the rationality of this design principle for CP mechanism. In principle, the blade failure force F ex formula is only valid for a conventional design. For a highly skewed, the blade failure force is much smaller than the value calculated from the F ex formula. In our pervious FEM analysis for the same open under lifetime maximum load, it was found that the 4 load caused the blade near to failure criterion limit (S.F > 1.5). The safety factor for the is The for the actually is just 0.5F f (540.8 kn, see Figure 1 and Table 7) acting on tip area. However, if the F ex formula is applied for the, it has been found that the blade failure force is 6,19.15 kn (see Table 13), which is huge compared to 0.5F f. Also, it should be noted that the failure of the blade is not at the root area as assumed behind the F ex formula, but is located at outer radius, ~ 0.7R (see Figure 1). Fig. 1 Blade stress pattern under 4 load scenario Table 13 Blade failure force and spindle torque D 500 r 1788 c 191 t 90 σ ref 45.9 F ex = 0.3ct σ ref / [0.8D r] = 6,19.15 kn Q smax = / = kn-m CONCLUDING REMARKS This paper applies the load formulae in IACS polar class rule URI3 as the design load for a CP mechanism design. The Rule load formulae include the life time maximum s and the blade failure load. An existing CPP classed as Baltic ice class IAA was used in this study. Through the study results, our concluding remarks are drawn as follows: URI3 life time maximum s (F f and F b ) are higher than the Baltic ice class load (mainly the ice torque defined in Finnish Sweetish Ice Rule) for the equivalent ice classes PC6 and IAA. This results in the existing Baltic IAA ice CPP design failure in its pilot and dowel parts. URI3 life time maximum s are in a reasonable range for CP mechanism in this low ice class PC6. The redesign study preformed in this paper shows that slightly changing the dimensions and the strength of the pilot and the dowel can fulfill the safety criteria required in general practices. For higher ice class, as the s will increase a lot, the CP mechanism design will be quite a challenging task. The following table summarizes the s for different PC classes of the studied open. Ice class PC6 PC5 PC4 PC3 PC PC1 F b kn F f kn Q smax knm Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule

9 The blade failure load F ex as the design load for CPP mechanism design is also studied in this paper. It has been found that the selective strength design principle based on F ex may be too conservative causing the difficulty of CP mechanism design. Also, the simple formula for F ex does not reflect the blade failure mechanism for highly skewed. REFERENCES ABS PropS User s Guide & Manual, 005 IACS URI1 Polar Class Descriptions and Application, 008 IACS URI3 Machinery Requirements fro Polar Class Ships, 008 Lee, S.K., Combine Ice Class Rules with Direct Calculations for Design of Arctic LNG Vessel Propulsion, CasTech08, Bangkok, Thailand, March, 008. Ice Controllable Pitch Propeller Strength Check based on IACS Polar Class Rule 17