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添付資料18
Comments on Robin Loscombe's Proposed Simplified Pb1 / Pb2 Equations for ISO /CD 12215-5
APPENDIX A
Justification for the simplified ncg = linear function V / (L)0.5
DNV & UNITAS Rules for HSC It is correct that DNV and UNITAS Rules for HSC give an equation to predict the vertical acceleration in terms of only V / (L)0.5 , L and a service factor.
This is a default design vertical acceleration to be used only when the designer / builder does not submit the design vertical acceleration. However the DNV Rules and UNITAS Rules for HSC also include equations to determine the maximum permissible speed and maximum permissible significant wave height for this simply predicted design vertical acceleration, in terms of vertical acceleration, speed, significant wave height and actual vessel deadrise.
In the present ISO 12215-5 the approach is simplified in that the design vertical acceleration, deadrise and maximum permissible speed and associated maximum significant wave height wave height are all given in one equation, ie equation (2) as in ABS Guide for HSC The DNV and UNITAS Rules for HSC also include several additional equations for bottom design pressures, for the reason given in my main E mail, that it is not possible to give the necessary bottom design pressures in terms of one equation, especially where that equation is based on the design parameters for bottom slamming in the planning mode.
Conclusion ;
The proposed simplified method ncg = 0.45fw.kb (V / (L)0.5 can be shown to be derived from the similar form equation in DNV Rules for High Speed Craft that is used to give a preliminary statistical estimate of the vertical design acceleration at LCG for a high speed craft in the planning mode. This DNV equation has been derived statistically and is not therefore related to any significant wave height or vessel deadrise.
Because of this derivation, this value of ncg is equated in DNV Rules to an equation that is virtually identical to equation (2) in ISO 12215-5 in order to determine for the specific vessel the maximum permissible design speed and associated maximum significant wave height.
Robin has excluded this step in his proposed method, as to equate the proposed simplified derivation of ncg to equation (2) in ISO 12215-5 to obtain the maximum allowable design speed and associated maximum significant wave height for the specific vessel would no longer qualify the method as being simpler to that already in ISO 12215-5.
The operational design criteria of maximum design speed(s) and the associated maximum significant wave height(s) are an essential part of the design and need to be clearly given in any standard for the guidance of the owner/ operator and for the protection of the standard in that the operational limits on which approval is based must be given.
Exclusion of specific operational limits indicates there are no operating speed and significant wave height limits and leaves the door wide open for implied limits to be speculated.
We consider it necessary to include in ISO 12215-5 the means of determining the design operational criteria of maximum speed and associated maximum significant wave height for the design acceleration as guidance for owners / operators and for protection of the standard. However as the simplified proposed equation to give ncg is statistically established, it is not related to any specific significant wave height or specific vessel deadrise, displacement, waterline beam etc etc and its use in place of equation (2) will make it impossible to determine accurate values of the operating design criteria of maximum design speed and associated significant wave height.
The time saved by using the proposed simplified method of determining ncg is negligible when considering the inaccuracies it introduces in the operational design criteria of maximum design speed and associated maximum significant wave height.
For the foregoing reasons the proposal in my opinion cannot be considered appropriate for inclusion in ISO 12215-5 and we see no justification in Appendix A for such a proposal. Aside from being unacceptable in our opinion from an engineering viewpoint, it is as explained above inconsistent with the requirements of all of the major classification societies, including the society requirements from which it is derived.
The present ISO 12215-5 equation (2) is simpler than the methods in the DNV and UNITAS Rules in that it includes the design vertical acceleration, maximum permissible speed and associated maximum permissible and vessel deadrise all in one equation. All craft approved to this standard are suitable for the vessel operating at maximum calm water speed in a sea state of significant wave height L / 10 (LR Rules for HSC), and may operate at lesser speeds in higher sea states. I see no reason to change this.
It is not surprising that the service reduction factors from UNITAS are not dissimilar to those in ISO 12215-5, as the UNITAS and the DNV service reduction factors, which are numerically less but similar to those in ISO 12215-5 were both referred to, at least by me, in deciding on the values to be used in ISO 12215-5.
APPENDIX B Justification for removal of Pb2 equation.
Equation Pb1 gives the design bottom slamming pressures for vessels in the planning mode. It was developed from combining the work of Heller & Jasper and Savitsky & Brown. It has been used successfully now for over 20 years for the design and approval of HSC from 10m up to 60m length in the planning mode of operation.
Equation Pb2 gives the design pressures for sailing yachts in the displacement mode, including bottom and side pitch slamming design pressures. The equation was developed initially by Ake Lindquist, a well known Finnish yacht designer, for the International Technical Committee(ITC) of the Offshore Racing Council(ORC). Unfortunately Mr. Lindquist had to retire because of an accident and ABS took over the task of developing a scantlings standard for offshore racing yachts for the ITC of the ORC.
Equation Pb2 was reviewed by the ITC and then assessed by all of the ORC National Authorities, and their yacht designer advisers. It has been applied successfully now for over 20 years to ocean racing yachts and cruising yachts, from about 10m to 40m length.
Because Pb1 and Pb2 give design pressures for two entirely different modes of operation they cannot be replaced by one single design pressure equation, which in this case that has been developed exclusively for vessels in the planning mode. It is for this reason that the Rules of all major classification societies for high speed craft that both operate in the planning mode and in the displacement mode have at least two different design pressure equations. One for the planning mode and at least one for the displacement mode.
Regarding reference to Paul's premise that Pb1 with ncg = 1.0 gives very similar values to Pb2, we would remind Robin and other WG members that because the ISO standard values of Kar are increasingly less (with reduction in boat length) than the F factor in the ABS Guide for ORY, equation Pb2 (even now again in the ABS format) gives scantlings substantially less than required by the ABS Guide where Kar and F are less than 1, which is the general case.
A number of years ago we compared the fore end design pressures given in the ABS Guide for ORY with those given in the ABS Guide for HSC (essentially equation Pb1 and equation (2) for ncg). Assessment of 29 actual yacht designs from about 10m to 24m length showed that the design pressures given in the Guide for ORY corresponded to vertical accelerations at LCG of 3g for the small (10m) yachts and 2g for the large (20m) yachts when equated to the equation for Pb1. This work is described in "Ocean Racing Yachts - Structural Criteria" given at RINA International Conference on The Modern Yacht, 1998.
Regarding material presented in Appendix B, we have following specific comments.
1) HOW can the vertical acceleration at LCG, predicted by the revised ncg equation to be 1.062g, be considered to accurately give the design vertical acceleration when the equation does not include the recognized parameters for LCG vertical acceleration, of vessel speed2 actual vessel deadrise at LCG and significant wave height?
2) We note from the figure of comparison of Pb1 and Pb2 that it is based on Kar = 1.0.This is not a true comparison. As noted above the ISO standard values of Kar less than 1.0, which is the general case, are substantially less than the corresponding ORY Guide values of F for large shell panels, or for members supporting large shell panels.
3) For sailboats the maximum attainable speed is extremely difficult to determine and even when estimated, the maximum slamming loads generally occur at lesser speeds when beating in substantial seas. ABS computer simulation based on actual sailing yacht sea data determined the maximum slamming pressures to have occurred when beating at about 7 to 8 knots in seas of significant wave height of about 3 m. The resulting pressures compared with those given in the Guide for ORY.
We therefore cannot see how equations developed from and for high speed craft in the planning mode, using as input the maximum planning speed, can be considered to accurately give the maximum slamming pressures on a sailing yacht beating in the displacement mode.
4) Referring to the first series tables of comparison.
a) It appears that the new simplified ncg equation gives values of vertical acceleration about 2.5 times greater than those given by ISO equation (2). No explanation or justification is given for this. Perhaps Robin can explain how vertical accelerations can be predicted without a given significant wave height?
We note for No 2 and 3 sail boats that the new simplified method gives vertical accelerations of 1.28g and 1.35g, for speeds of 10.39 and 12.73 knots (for unspecified sea states).
However the results given in the paper "An Experimental Investigation of Slamming on Ocean Racing Yachts" would seem to substantiate our conclusion that the new simplified method predicts excessively large vertical accelerations. From this paper for an 18 m racing yacht, a slamming pressure of 1.72g at LCG was recorded when beating at 8 knots in significant wave heights of 5.1m, and 1.22g was recorded at LCG while sailing at 22 knots in seas of significant wave heights of 4 m.
b) We have determined the design pressures from the ABS Guide for ORY. For boat No 2 the ABS Guide design pressure is 52.8 kN/m2 and for boat No 3 it is 57.6 kN/m2.
c) Robin's comment that for sail boats the difference is due solely to reduction factors excludes the fact that the vertical accelerations given by the new simplified ncg equation have been increased by a factor of 2.5 / 2.8. It appears that these increased vertical accelerations were necessary to bring the design pressures given by the Pb1 equation up to the design pressures given by the ABS ORY Guide.
5) As far as we are concerned Bippe's validation of the ISO standard by comparison with existing motor yachts showed reasonably good correlation, for which only some minor adjustment was required.
Conclusion:
Because Pb1 and Pb2 give the bottom design pressures for two entirely different modes of operation, it is impossible to replace them by one single design pressure equation.
For this reason the Rules of all class societies for high speed craft that operate in both the planning mode and displacement mode have at least two different bottom design pressures based on the different relevant parameters for the two different modes of operation. To replace both by one equation because for certain values of the different parameters they give similar design pressures cannot be considered to be a valid standard as it will not give the correct design pressures throughout the parametric range of the standard. For this reason we must keep the equations for Pb1 and Pb2.
R.Curry,
6 June 2002
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