ABS has released its updated Fatigue Assessment of Offshore Structures guide, which incorporates updated approaches for addressing fatigue including new S-N curves and guidance regarding fatigue strength based on fracture mechanics.
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.
The main purpose of the Fatigue Assessment of Offshore Structures Guide is to supplement the Rules and the other design and analysis criteria that ABS has issued for the Classification of some types of offshore structures. The specific Rules and other. Classification criteria that are being supplemented by this Guide include the latest versions of the following
documents:
• Rules for Building and Classing Offshore Installations
• Rules for Building and Classing Mobile Offshore Units
• Rules for Building and Classing Single Point Moorings
• Rules for Building and Classing Floating Production Installations
While some of the criteria contained herein may be applicable to ship structures, it is not intended that this
Guide be used in the Classification of ships.
The June 2020 update of the Fatigue Assessment of Offshore Structures guide includes:
• Updated ABS S-N curves for tubular joints
• Updated FEA stress extrapolation procedure
• New section for post-weld improvement
• New section for new design S-N curves based on fatigue test data
• Updated time domain analysis method
• Updated fatigue strength based on fracture mechanics
• Updated formula for eccentricity SCF on double-sided plate butt welds
• Updated the section of existing structures
The revised Fatigue Assessment of Offshore Structures Guide became effective from 1 June 2020.
Download the 61 page pdf: Fatigue Assessment of Offshore Structures
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