ABS has issued a guidance on spectral- based fatigue analysis for vessels. This Guide provides information about the optional classification notation, SFA (years) which is available to qualifying vessels as described in the ABS Rules for Conditions of Classification.
Part 5C of the ABS Rules for Building and Classing Steel Vessels (Steel Vessel Rules) presents the simplified fatigue assessment criteria for the classification of various types of specialized vessels covered by theRules. Part 5A and 5B of the ABS Rules for Building and Classing Steel Vessels (Steel Vessel Rules) contains fatigue assessment guidance for vessels subject to the “Common Structural Rules for Bulk Carriers and Oil Tankers”. A brief description of the background and objectives of these fatigue criteria is given in Subsection 1/3.
In addition to the simplified fatigue strength criteria required for classification by ABS, the Owner may wish to apply more extensive Spectral-based Fatigue Analysis (SFA) techniques to the vessel’s structural systems. It may be an added objective of these Spectral-based Fatigue Analyses to demonstrate a longer design fatigue life than that required for classification. Spectral-based Fatigue Analysis techniques are used in addition to the Safe Hull Fatigue Assessment technique, a Permissible Stress Range method (discussed in Subsection 1/3). The fatigue life of each critical location in the structural system is assessed for adequacy. The critical locations are to be selected using the results of the Safe Hull Fatigue Assessment technique which is to be employed in the overall structural design and analysis effort.
The list of critical structural locations which are to be subjected to Spectral-based Fatigue Analysis is to be submitted to ABS for approval.Provided that Spectral-based Fatigue Analysis is conducted appropriately, ABS will grant the optional classification notation, SFA (years). The SFA (years) notation is granted if the design fatigue life is equal to 20 years or greater. The value in parentheses is the design fatigue life in years specified by the applicant in 5-year increments. The structural system is analyzed to verify that the calculated fatigue life values for the entire system meet or exceed the design fatigue life. The calculated fatigue lives are typically much higher than the design fatigue life. The actual service life of a vessel is dependent on many factors.
The SFA (years) notation denotes the design fatigue life of a vessel and is not a guarantee that the vessel or structure will achieve the design fatigue life. For vessels complying with Part 5A and 5B “Common Structural Rules for Bulk Carriers and Oil Tankers” of the Steel Vessel Rules, the design fatigue life for Spectral-based Fatigue Analysis is equal to 25 years or greater in 5-year increments.
Methodology and Assumptions
Spectral-based Fatigue Analysis is a complex and numerically-intensive technique and there are multiple valid implementations of the method. ABS does not wish to eliminate the use of any valid approach by over-specifying the SFA technique. However, it is necessary to be clear about the basic assumptions that form the basis of a valid method and highlight key details that are to be incorporated in the method to produce acceptable results. The remainder of this Guide is devoted in large part to the presentation of these topics.
A typical spectral fatigue analysis for a structural location is to evaluate its fatigue strength by comparing its stress range distribution against its fatigue capacity. The following definitions are used in the context of this Guide:
• Stress Range Distribution: Stress range probability density functions calculated per this Guide.
• Fatigue Capacity: S-N data (S-N curves) representing the number of stress cycles at fatigue failure.
• Fatigue Strength: Fatigue life (or damage) calculated per this Guide.
• Fatigue Demand: Design fatigue life.
The main underlying assumptions of the Spectral-based Fatigue Analysis method are:
i) Ocean waves are the source of the fatigue stress range acting on the structural system.
ii) The load and structural analyses are assumed to be linear as required for the frequency domain formulation and the associated probabilistic analysis to be valid. As such, scaling and the superposition of stress transfer functions from unit amplitude waves are considered valid.
iii) Non-linearities due to non-linear roll motion and intermittent loads, such as wetting of the side shell in the splash zone, can be effectively accounted for using correction factors.
iv) Due to their insignificant contributions in typical load cases it is appropriate to disregard structural dynamic amplification, transient loads and effects such as springing. This allows for the use of quasi-static finite element analysis.
For the specific SFA method presented in Appendix 2, it is assumed that the short-term stress variation for a given sea-state is a random, narrow-banded, stationary process. Therefore a Rayleigh distribution can be used to represent the short-term stress range distribution.
The Spectral-based Fatigue Analysis method is applied to each of the selected structural locations by implementing the following process:
• Determine the stress range distributions.
• Determine fatigue capacity (S-N data).
• Calculate fatigue strength (fatigue life or damage).
Section 2 provides an overview of the spectral-based fatigue analysis procedure. A schematic representation of the SFA procedure can be found in Section 2, Figure 1. An effort is made in this Guide to avoid the discussion of complicated formulae and instead to focus on the concepts being presented. It is necessary to present the complex formulae used in the calculation of fatigue damage resulting from the predicted stress range distributions, which are presented in Appendix 2. It should be noted that the contents of Appendix 2 are intended to serve as an example of a valid SFA method. It is not necessary that the process be followed exactly; however, any method used should adhere to the same principles.
Further information may be found by reading the Guide here
