scope:

Introduction

Background and Applicability

This Guide provides a detailed description of the method to perform spectral fatigue analysis for ship-type offshore installations in order to obtain the optional classification notation SFA. Spectral fatigue analysis relies on the presumed linearity of wave-induced loads with respect to waves. This condition is sufficiently satisfied for ship-type offshore installations; such as Floating Production, Storage and Offloading (FPSO) and Floating Storage and Offloading (FSO) systems. For a ship-type hull, linear diffraction forces are the dominant component of wave load. The application of the spectral fatigue analysis method to a ship-type hull is presented in this Guide. For other hull configurations composed of members with relatively large cross sectional dimensions; such as a Tension Leg Platform, Spar, or Column Stabilized units, a spectral approach is often employed; but with appropriate modifications to account for the influence of nonlinear drag forces that tend to become more important as cross sectional member dimensions decrease. This Guide assumes that any required modification to linearize the wave-induced load effects has been satisfactorily accomplished.

This Guide employs basic concepts and terminology that were defined in the ABS Guide for the Fatigue Assessment of Offshore Structures (2003). In that reference it is stated that:

"Fatigue assessment denotes a process where the fatigue demand on a structural element (e.g., a connection detail) 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 called 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 nonlinear structural response or nonlinear loading."

In this Guide, the fatigue assessment technique that is presented is a direct calculation method based on the spectral analysis method, which can produce a fatigue assessment result in terms of either expected damage or life. The fatigue strength of structural details is established using the S-N curve approach that is specified in the referenced Guide.

It should be borne in mind that for the hull structure of an Offshore Installation, wave-induced loading is usually the dominant source of fatigue damage. However some types of floating offshore structures may also be subjected to significant fatigue loading from other loading sources. This can be true for hull types that undergo frequent loading and discharge of produced fluids. For example, in FPSO and FSO systems, such load changes can induce large ranges of hull girder stress and secondary stress (albeit at lower cycles than direct wave loads). Such load cycle fatigue is also addressed in this Guide. In addition, fatigue loading may also be induced by the operation of equipment associated with the function of the Offshore Installation. The extent to which the spectral-based fatigue method can or will be adapted to take into account these "non-wave" sources of fatigue damage must be further considered by the designer.