Integrity Assessment On K/T/Y Nodes Of Jacket Repairing Underwater

The jacket platform is one of the widely used offshore oil platforms. Fatiguefracture is apt to occur on the stress concentration site of welded joints (such as K/T/Ypipe nodes) due to the complex stress. It is very difficult and expensive to detect andmaintain the jackets as the main mechanical structure is located underwater. Therefore,it is very significant to assessment the welded join with flaw by ECA to determinewhether to repair it.In this paper, the fracture toughness and tensile property of E36steel used inmarine structures and the welded joints were studied. Results indicate that yield andtensile strength of four groups welded joints are all higher than that of E36, which allbelong to high strength match, but the elongation of four groups welded joints islower than the parent metal E36and the elongation of the underwater wet weldingjoint is only about8.0%. In addition, the welded joints of FCAW and locally dryunderwater welding have excellent fracture toughness and the CTOD characteristicvalues are0.5265mm and0.3368mm separately. While, the joint fracture toughness ofunderwater wet welding is poor and the CTOD characteristic value is only0.0440mm.In this paper, fitness for use as the principle, fatigue and fracture assessmenttechnical route of the K/T/Y pipe nodes of jackets is made according to BS7910standard. Before and after repair the K/T/Y nodes underwater, fatigue and fractureassessment are carried out for HZ25-8DPP jacket by the CRACKWISE developed byTWI and based on the BS7910standard. Results show that the residual life ofstructure can’t meet the design requirement when crack in weld toe in K/T/Y jointreaches a certain size and need to be repaired. When repair it by the locally dryunderwater welding, the residual life of structure can not only meet the designrequirement, but also increased significantly. In addition, the limit flaw size of weldedjoints of locally dry underwater welding is close to that of FCAW and the differencein depth is about0.5mm under the same crack length.