Basic Research Of The Local Structure Of Fatigue Behavior Of Marine Engineering Structures

With the development of large-scaled exploitation of ocean resources, diverse offshore structures have been being developed. Offshore structures are constantly subjected to the varied wave-induced loads, which accounts for the changing stress of the internal members and due to which structural fatigue damage would surely occur. Therefore, it is always considered as a main form of damage in ocean platforms.Presently, most structures of platforms are fabricated by steel tubes. In the connection of these tubes, tubular joints, high stress concentration is easily emerged in the vicinity of the intersection owing to the influence of structure and welds. Hence the tubular joints are the most critical but the weakest members of platforms. It is obviously necessary to study the fatigue behavior caused by detailed structures of the joints, and to control the occurrence of fatigue cracks through determination of the hot spot stress location in the joint. Meanwhile, grinding the crack tip to hold up the propagation speed makes it possible to extend the maintenance period, to decrease the maintenance difficulty and to save the maintenance expenses.Regarding the fatigue behavior resulted from the local structure, the above mentioned aspects are dealt with in the thesis as follows.Firstly, based on the literature review in this research field, the significance of the study is presented. Then a brief introduction to the general situation of offshore structures is outlined in Chapter 1, including the ocean platform structure system, designed loads, work principle and structural fatigue characteristics. Chapter 2 and Chapter 3 mainly concern the structural fatigue damage mechanism and fatigue analyses for the joint, which covers the basic concept of fatigue and analyses methods, choice of stress and stress concentration factor (SOF). Chapter 4 emphasizes the whole procedure of fatigue analyses for the typical tubular K-joint, from modeling, meshing, choice of loads and boundary conditions to definition of hot spot stress and SCF, and final calculation of fatigue life. In the end, numerical simulation analyses are carried out and the fatigue lifetime does extend through grinding of the crack tip.