While there has been considerable research effort over the last decade aimed at refining risk based fatigue inspection planning techniques, there has been relatively less effort aimed at using the same techniques to optimise the design of welded joints. This paper discusses the use of a simplified, semi-probabilistic technique for optimising the design of tubular joints on the basis of their total through-life costs. A model is developed which combines the probability of fatigue failure with initial construction costs, inspection costs, and repair and failure costs. An example study of a critical joint in a minimum structure is provided. In this example, where costs associated with joint failure were assumed to be purely financial, an optimum target probability of failure (threshold for inspection) was found to be 10−4 to 10−3. Provided the target probability of fatigue failure used in inspection planning is kept below this level, design in accordance with API RP2A appears to provide a reasonably optimised joint. The sensitivity of the total cost to this target failure probability, and a range of other important cost variables, was also investigated.

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