Development of a Method for Assessment of the Remaining Fatigue Life of Steel Structures of Existing STS Cranes

Abstract

STS cranes (Ship-to-Shore cranes) are designed for a finite fatigue life, but it is often preferred by the terminal operators to keep the crane in operation once the design fatigue life has expired. When the crane is operated after it reached its expected service life, the risk of fatigue failure increases. Therefore the structural lifetime of the crane needs to be assessed in order to enable operation of the crane after it reaches its design fatigue life without compromising safety. Methods to assess the remaining fatigue life in industries like offshore and aviation are stan- dardized, but for cranes such methods do not exist. Therefore an assessment method for the structure of the crane needs to be put forward. The assessment method should be able to calculate the remaining lifetime of the crane and determine whether inspection of the crane is necessary (and if so, how long the intervals should be). After inspections are completed, the method should determine whether repair works need to be performed to ensure the structural integrity of the crane. The main research question of this thesis is formulated as follows: What is the most appropriate method to assess the remaining fatigue life of steel structures of existing STS cranes? To determine which type of fatigue assessment methods can be applied to STS cranes, a review of assessment methods for STS cranes, bridges, aircraft and offshore structures is performed. The method that is selected when the crane is operated within its design fatigue life is the calculation procedure for fatigue used in the design stage, because this means that the remaining fatigue life can be determined using information that is already available. When the crane is operated outside its design limits, a crack growth model is used to calculate the remaining fatigue life. The steel structure of a general STS crane consists of four main components; bolted con- nections, pinned connections, welded connections and base material. It is determined that the majority of fatigue failures in steel structures occurs at welds, fatigue cracks in the base material contribute slightly and the amount of fatigue failures of bolted and pinned connec- tions is negligible. The fatigue assessment model therefore only considers cracks in the base material and in welds. The components at the connections at the forestay and backstay, the connections between the crane boom and the portal beams as well as the connections at the legs are even more susceptible to fatigue failure because these areas are subjected to relatively large fluctuating loads. In order to determine the remaining fatigue life of the crane, the crack size which will cause failure needs to be known. This critical crack size depends on the geometry of the detail under consideration, the used material and the stress at that detail. This means that the critical crack size is not constant across the steel structure of a general STS crane. When the critical crack size is known, the crack growth rate needs to be calculated in order to determine the time between crack initiation and failure. The structure needs to be checked for cracks before a member is expected to fail. Therefore the value for the remaining fatigue life is used to schedule inspections as well. The inspection interval depends on the crack size which can be determined with sufficient reliability, which in turn depends on the type of inspection method that is deployed. In case cracks are found during inspections, repair methods are available to extend the re- maining fatigue life of the crane. The methods that can be used to repair cracks in steel structures of STS cranes are the gouge-and-weld method, mounting doubler plates, drilling crack arrest holes and modifying the structure. The selection of the repair method is based on the remaining fatigue life of the structure, the location of the crack and potential earlier fatigue crack repair works. The most appropriate method to assess the remaining fatigue life of the crane therefore consists of an inspection schedule, where the inspection intervals are determined based on the remaining fatigue life. When the crane is operated within its design limits, the inspection intervals are based on the safety factors for fatigue as defined in the EN 13001 design standard for cranes. When the crane is operated after its design fatigue life is expired, the remaining fatigue life is calculated using a crack growth model. The inspection methods that are used to inspect the crane are determined based on the value for the critical crack size and the crack size which can be accurately determined by the inspection methods. When the remaining fatigue life is insufficient, repair works are scheduled to repair the crack and thus extend the fatigue life of the crane. The repair methods are selected based on the type of crack (cracks at the surface or internal cracks) and its location. This method is repeated until it is not economically feasible to further extend the fatigue life of the crane.