Fractures on a drillship derrick, evaluation and improvement

Abstract

During the service life, the drillship derrick structure is exposed to wave and wind actions and drilling operations. Due to these actions, the derrick structural members are subjected to the variable amplitude stresses and very high number of load cycles that may generate the fatigue cracks, leading to the total structural fracture eventually. The fractures found on the footing of a drillship derrick are analysed, and based on the structural durability, the improvements are determined for the specific case. To perform the structural durability evaluation and to propose the necessary improvements, at the beginning of the analysis, it is important to define the loadings (corresponding loads due to waves, wind and drilling acting on the derrick structure) taking into account the drillship service. After that, in order to validate the fatigue life of the structure, the stresses caused by these loads must be determined, their representative spectra (cumulative frequency distributions) have to be derived and the data regarding corresponding allowable stresses should be defined. Using the methods defined in the classification societies' rules and the finite element analysis, the procedure for determination of the operational stress spectra and evaluation of the fatigue life utilising the hot-spot stress approach for highly stressed areas of a drillship derrick, cracked at usage, is presented.