Offshore Structures—Statistical Strain Recording

A survey of structural problems on offshore rigs indicates a need for a better understanding of fatigue loading produced by dynamic effects initiated by the oceanic environment. For general design purposes a valid loading spectra might be produced by measuring elastic strain cycles in selected oceanic areas, over extended periods. Examination of a typical strain sensor output shows that the record can be handled as counts of cyclic loading at preselected magnitudes. These indicate the loading spectrum for that particular area, and also can be used, in association with Miner's cumulative damage hypothesis, to provide an estimate of the damage produced by the loading history. A simple instrument is described which will count the number of times particular strain levels have been exceeded. Numbers of counts can be printed out automatically for periods of up to 8 weeks, and can also be read manually at any time. Special provision is required if long term variation in mean strain is to be corrected.