A novel method based upon damage mechanics is suggested to estimate the fatigue life of offshore structure members for great water depths. A new mathematical model of fatigue damage is firstly developed at University of Petroleum. China and termed Fang-Jia" s model. A macroscopic method of measuring the damage factor is put forward By processing data obtained from fatigue experiments with surface crack propagation. The determination of steel damage parameters for offshore structure member's is provided. This new mathematical model is verified by the fatigue experiments of rigid steel test pieces and compound material flexible structure members used for great water depths. Through experimental examination the total fatigue life estimation formula by means of damage mechanics under constant or random loads is given.
The offshore structure members used for deep water can be divided into two kinds: one is called rigid structure members. the other flexible. Because of some complex reasons. the compound material flexible structure members are used for deep waters usually more than 100m. However. to either rigid or flexible structure members. the fatigue is one of the important problems of ocean engineering studies for deep waters. Because not only the examination of crack generation. but also the repair operation are all very difficult in deep waters. more attention IS paid to improving the accuracy of fatigue life estimation of deep water structure members by ocean petroleum engineering scholars at present. It is well--known that the traditional fatigue life estimation has its basis on S--N curves. This method is simply audio visually but the expenses of experiments are very high. So fracture mechanics method has been recommended by DNV to estimate fatigue life recently. Therefore we try to use a new damage mechanics method to estimate the fatigue life of structure members for great water depths.
