ABS - 115
GUIDE FOR FATIGUE ASSESSMENT OF OFFSHORE STRUCTURES
|Publication Date:||1 June 2020|
General (1 June 2020)
Fatigue assessment* is a process where the fatigue demand on a structural element is established and compared to the predicted fatigue strength of that element. One way to categorize a fatigue assessment technique is to say that it is based on a direct calculation of fatigue damage or expected fatigue life. Three important methods of assessment are the Simplified Method, the Spectral Method and the Deterministic Method. Alternatively, an indirect fatigue assessment may be performed by the Simplified Method, based on limiting a predicted (probabilistically defined) stress range to be at or below a permissible stress range. There are also assessment techniques that are based on Time Domain analysis methods that are especially useful for structural systems that are subjected to non-linear structural response or non-linear loading.
Fatigue Demand is stated in terms of stress ranges that are produced by the variable loads imposed on the structure. (A stress range is the absolute sum of stress amplitudes on either side of a 'steady state' mean stress. The term 'variable load' may be used in place of 'cyclic load' since the latter may be taken to imply a uniform frequency content of the load, which may not be the case.) Fatigue-inducing loads are the result of actions producing variable load effects. Most commonly for ocean-based structures, the biggest influences producing the higher magnitude variable loadings are waves and combinations of waves with other variables such as ocean current, and equipment-induced variable loads. Since the loads considered vary over time, it is possible that they could excite dynamic responses in the structure; this will amplify the acting fatigue inducing stresses.
Fatigue demand is to be determined using an appropriate structural analysis. The level of sophistication required in the analysis in terms of structural modeling and boundary conditions (i.e., soil-structure interaction or mooring system restraint), and the considered loads and load combinations are typically specified in the individual Rules and Guides for Classification of particular types of Mobile Units and offshore structures. A coarse mesh finite element model is typically employed in the screening process to identify fatigue sensitive areas. For the fatigue assessment of each identified area, a local detail model with a finer mesh should be used.
When considering fatigue inducing stress ranges, consideration is to be given to the possible influences of stress concentrations and how these alter the predicted values of the acting stress. The model used to analyze the structure may not adequately account for local conditions that will modify the stress range near the location of the structural detail subject to the fatigue assessment. In practice, this issue is resolved by modifying the results of the stress analysis by the application of a Stress Concentration Factor (SCF). The selection of an appropriate 'geometric' SCF may be obtained from standard references, or by the performance of Finite Element Analysis that will explicitly compute the geometric SCF. Two common examples of geometric SCFs are a circular hole in a flat plate structure, which nominally has the effect of introducing an SCF of 3.0 at the location on the circle where the direction of acting longitudinal membrane stress is tangent to the circular hole. The other example is the case of a transverse ring stiffener on a tubular member where the SCF to be applied to the tube's axial stress can be less than 1.0.
* Note: ITALICS are used throughout the text to highlight some words and phrases. This is done solely to emphasize or define terminology that is used in the presentation.