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10.12.1 Limiting Motion Criteria
Limiting motion criteria for landing a helicopter on a floating platform are at present usually defined in terms of maximum heave, roll and pitch motions. Large heave motions can make it difficult for the pilot to control the final stages of landing and rate of descent at touchdown, and large accelerations can cause sliding across the deck or a tendency to overturn. The motions used in this analysis must represent the motions of the helideck (rather than the motions of the vessel at its centre of gravity).
The maximum motion experienced during a given time interval depends not only on the sea state, but also on the particular sequence of waves that occurs, and on the length of the time interval. Significant variations in maximum motions often occur between one sample time interval and another. The limiting motion criteria are therefore normally interpreted as specifying ‘most probable’ or ‘expected’ maximum values occurring in a 10-minute time interval (i.e. the most likely or average value of all maxima that can occur in different randomly-sampled 10-minute intervals). Standard formulae for estimating the most probable and expected maximum motion in a given sea state are available, and are often incorporated into standard vessel motion prediction programs. Motion time series obtained from time-domain simulation programs or model tests should be processed statistically to obtain estimates of the most probable or expected maximum values.
Special care should be taken to determine whether maximum motions represent ‘single-amplitude’ (i.e. from the mean value to the maximum) or ‘double-amplitude’
(i.e. from minimum to maximum) values. Standard helicopter landing criteria are usually defined in terms of maximum double-amplitude heave motions (i.e. measured from trough to peak), but maximum single-amplitude for roll and pitch motions (i.e. measured from the true vertical).
A new approach to measuring helideck motion based on helideck accelerations is currently being developed [Ref: 65]. The measure of motion severity employed is simply the acceleration in the plane of the helideck divided by the acceleration normal to the helideck. This measure is monitored on a continuous basis over a 10-minute period and processed statistically to produce a prediction of the most likely maximum value for the next 10 minutes; the Motion Severity Index (MSI). When this is introduced the height of the helideck above the vessel centre of gravity will be of greater concern since the greater this distance, the greater the horizontal acceleration generated by a given roll motion. Maximum MSI values may be calculated and analysed using vessel motion models and procedures similar to those used to determine maximum heave, roll and pitch, together with the published MSI algorithms.
NOTE: The single-amplitude roll and pitch motions must be measured from the true vertical in order that any vessel list or trim is properly accounted for.
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离岸直升机起落甲板设计指南 OFFSHORE HELIDECK DESIGN GUIDELINES 2(32)
