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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Sources of risk to marine systems include equipment failure, external events, human error, and institutional error. Equipment failure, the most readily recognized hazard on ships, may be categorized as either independent failure, such as the loss of steering because of the failure of a power steering pump or common‐cause failure, such as the loss of propulsion and steering resulting from a total loss of electrical power to the ship. Risk from external events arises from hazards such as collision with other ships, sea state; wind, and ice or other weather factors. Humans provide another source of risk to marine systems when they lack skill, are excessively fatigued, or commit sabotage. Institutional failure creates risks from poor management including inadequate training, poor communications, and low morale. Risk studies may be classified according to whether they focus primarily on assessment, management, or communication; these aspects of risk studies are described to prepare users and readers of this paper for performing risk‐based analysis of marine systems. Methods are provided in the paper that can be used to develop risk‐based standards for system safety. The relationship between risk and standards is studied from a historical perspective. Great successes in controlling risk to health and safety are exemplified by the development of design methods for buildings, bridges, or super tankers that render them capable of withstanding extreme storms. Yet, familiar risks persist while less familiar ones escape attention and new ones appear. Ironically, management of some of the most difficult risks has led to improved standards of living. This paper provides background information, introduces fundamental concepts, and offers examples of risk methods applied to marine systems.

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