Fitness For Service Assessment of Bulged Sewage Tank

Abstract

Based on the visual inspection and measurement, bulging was found on the Sewage Tank. There are cracks found in some areas of welded joint on the tank wall. The suitable recommendation needs to be found in order the equipment to be operated safely and reliably. The finite element simulation is aimed at estimating the loading experienced by the tank prior to the damage, and providing recommendation for repair,  replacement or other suitable options. In order to estimate the cause of bulging, several  steps have been carried out. The sewage tank is modelled using a finite-element based software, under a normal up to the maximum allowable loading condition, using a linear-static analysis. Based on finite element simulation, under normal loading  of 0.15 bar with 6 mm thickness, the tank is considered safe with a safety factor is 2.87, compared with a minimum specified Yield Strength of material of 267 MPa. Whereas, under the high loading of 0.4 bar, the safety factor drops to 1.50. Due to estimated loading case that caused the permanent deformation, some of the tank walls experience tensile residual stress. This condition is considered unsafe. Therefore, removal of the residual stress is required.