scope:

Purpose and Applicability

Part 5C of the ABS Rules for Building and Classing Marine Vessels (Marine Vessel Rules) includes the fatigue assessment criteria for the classification of various types of specialized vessels covered by the Rules. Part 5A and 5B of the ABS Rules for Building and Classing Marine Vessels (Marine 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 SafeHull 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 SafeHull 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 in accordance with the procedures included in this Guide or equivalent, ABS will grant the optional classification notation, SFA (years) or SFA (years, WWT). In either case, as a minimum, a vessel is to meet the fatigue strength criteria in the Marine Vessel Rules as described above. The SFA (years) notation is granted if the design fatigue life value is equal to 20 years or greater under the wave conditions of unrestricted service defined in 5/5 TABLE 1. The SFA (years, WWT) notation is granted if the design fatigue life value is equal to 40 years or greater under the wave conditions of worldwide trading service defined in 5/5 TABLE 2.

If a vessel is primarily operating in seas that exceed the worldwide trading pattern (i.e., a vessel operating half or more than half of its life in onerous seas including North Atlantic or North Pacific), SFA (years, WWT) is not applicable. It should be noted that the calculated fatigue lives for different design wave conditions may produce significantly different fatigue lives.

The value in parentheses is the design fatigue life in years specified by the applicant in 5-year increments starting at 20 for SFA (years) and 40 for SFA (years, WWT). The structural system as a whole is analyzed to verify that the calculated fatigue life values for the entire system meet or exceed the design fatigue life. The actual service life of a vessel is dependent on many factors such as initial design, operational and maintenance schemes. The SFA (years) or SFA (years, WWT) 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.

In order to obtain the SFA (years) or SFA (years, WWT) notations, SFA methods must be applied to the entire vessel.

For vessels complying with Part 5A and 5B "Common Structural Rules for Bulk Carriers and Oil Tankers" of the Marine Vessel Rules, the design fatigue life for Spectral-based Fatigue Analysis is equal to 25 years or greater in 5-year increments.