Fracture mechanics in fatigue calculations as the assistance in structural design. Case study

A case study originated from a request to perform fatigue calculations on a partial penetration weld in a steel casting on a new heavy lift ship. The ship has a special lifting system for lifting and transporting topsides of offshore oil/gas platforms. There are many sizes of offshore platforms so the lift system is designed to move along the vessel on rails, which are part of the main deck of the ship hull. The loads passing into the rails during lift operations are large, and massive steel castings of complex shapes are used to distribute these loads into the hull. The castings are very thick so it is difficult to achieve full penetration welds, and therefore the initial design proposed partial penetration welds. In order to decide if the partial penetration welds were adequate, a fatigue assessment was carried out using a fracture mechanics approach based on BS7910. Different possibilities of the bevelling of castings edges in preparation for welding were considered in the stress analysis and in the crack growth estimations. In the areas of the ship hull which experienced high dynamic stress ranges none of the different possibilities showed acceptable fatigue life, and would demand re-design. The stress intensity factors obtained through the extensive finite element analysis were compared with the analytical solutions available in literature. Both results showed good correspondence.