Offshore fixed platforms built in the 1960s–1970s, and sometimes even in the 1940s, are in operation worldwide. The operation of oil and gas hydraulic structures is carried out in the difficult marine conditions.
Throughout the entire period of operation, the offshore oil and gas platforms are exposed to the effect of loads (from wind, waves, seismic, ice, from the bulk of vessels and others). Corrosion, temperature, vibrational and other effects, which also affect the change in the values of amplitudes of the alternating stresses, should be taken into account. The combined action of the loads and effects leads to the occurrence of the alternating stresses in the structural elements of the offshore platforms that cause fatigue damage. Gradually developing from the microdefects, these damages grow into a main crack, which can lead to the destruction of the offshore platform. Therefore, study of the fatigue processes occurring in the elements of offshore platforms is an important and urgent task. The main characteristic, which is a measure of the fatigue damage, is the endurance limit. The comparative analysis of various methods for determining the endurance limit in relation to the elements of the offshore platform with accumulated damage is provided in the article. National and foreign regulatory documents, experimental and theoretical methods for determining endurance limits were analyzed. As a result, it was found that all the values of the endurance limit lie in the same area both in terms of the cycle base (the range of values is about 108 cycles), and in the value of the endurance limit itself (from 14 to 31.25 MPa for steel grade ВСт3Сп5 and from 14 to 41, 5 MPa for 09Г2С steel). The values obtained by Henry's method are identical to the results obtained by the experimental method.
The results of the analysis make it possible to clarify the value of the residual life of platform elements with accumulated damage.
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