A Method for Estimating Lifetime Loads and Fatigue Lives for Swath and Conventional Monohull Ships

Abstract

The potential they offer for enhanced seakeeping ability and increased operational effectiveness has generated an interest in Small Waterplane Area Twin Hull (SWATH) Ships. The novelty of this concept requires new design load criteria since the existing methods for conventional monohull displacement ships do not apply to the primary wave loads acting on the submerged hulls and water piercing struts of SWATH ships. A method and associated computer program are developed for predicting primary load fatigue spectra for SWATH ships. The minimum input data required include a response amplitude operator for side load in beam seas, and operating speed and heading probabilities for the ship in question. The output data includes a listing of response (side load) magnitudes and the number of cycles exceeding each of these magnitudes. This fatigue load prediction method was applied to response data from several SWATH prototypes and wave‐tank models, and to variations derived from an existing computer program for obtaining SWATH side loads in waves. From these studies, an algorithm was derived for predicting the single lifetime maximum primary side load for SWATH ships. This algorithm requires only a knowledge of the ship's displacement and principal dimensions. Lifetime stress exceedance curves were developed for several nominal design stresses. The curves were used with linear cumulative damage theory to predict expected fatigue life as a function of design stress. Existing small specimen fatigue data for HTS, HY‐80, and HY‐100 were used as a basis for fatigue life calculations. Analyses indicate that design stresses of 9 tsi (140 MPa) to 18 tsi (280 MPa), depending upon material and conditions, result in acceptable expected fatigue lives. The fatigue load prediction method developed was also applied to obtain lifetime maximum primary load and stress predictions for several conventional monohull ships. The load and stress levels obtained, together with the SWATH side load algorithm, provide a basis for designing SWATHs with static strengths equivalent to current monohull displacement ships.

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