Fatigue assessment of fixed offshore platforms is essentially important to ensure their structural safety. Because of the small-scale leg diameter, these structures are often drag dominated, and linearized wave force is often utilized for the fatigue estimation. To evaluate the influence of nonlinear effect on fatigue damage, the nonlinear drag force is directly incorporated into the computation of the wave force spectrum based on autocorrelation function. The features and the contributions of nonlinear wave force are first investigated in the frequency domain. Then, structural fatigue damage associated with the linear and nonlinear wave forces is compared using a time-domain fatigue assessment method. Investigation indicates that the influence of nonlinear drag force is closely related to the characteristic diameter of the structure. The nonlinear drag force can contribute up to 10% of the total wave force and lead to about 10% structural fatigue life reduction when the characteristic diameter is small.

Fatigue assessment for jacket platforms is an indispensable practical issue. Because of the small-scale leg diameter, these structures are often drag dominated and wave-induced force in these structures can be tackled by using either linear or nonlinear form of Spectral Morison Equation. However, it is really complicated and difficult to incorporate nonlinear form of the Morison Equation to acquire the spectral density of the wave force, which is an important step of fatigue estimation. In this paper, in order to estimate the influence of nonlinear effect in wave force, fatigue assessments containing nonlinear effect for the fixed offshore structure are presented. Firstly, shallow-water jacket model locating at a water depth of 20 m is established and involved in calculation. Besides, for the sake of validating the effectiveness of the nonlinear term, the linear and nonlinear form of wave-induced force spectral densities are calculated by the Morison Equation in frequency domain. Secondly, the fatigue life of the jacket platform is assessed in time domain, where time-history of wave force can be obtained by transforming the linear or nonlinear wave force spectral densities from frequency domain to time domain. After the contrast of acquired fatigue life, the comparative results can indicate that the nonlinear drag force contributes a 14% fatigue damage to the total and the influences of the nonlinear term cannot be ignored for the jacket model.

Abundant oil and gas resources are stored in the continental slope region of northern South China Sea. However, due to the submarine landslide disasters, submarine pipelines and cables and other submarine facilities were damaged, which seriously affected the development of oil and gas resources, leading to a huge economic losses. In order to ensure the safe operation of the submarine gas pipeline, it is necessary to carry out some researches in the submarine slope stability. In this paper, an uncertainty analysis method is applied and a distribution model which is suitable for slope stability influencing factors of this region is put forward. In addition, the risk of submarine slope is analyzed, plotting submarine landslide risk evaluation maps. Different stochastic characteristics of variables such as water depth, angle of slope, shear strength and earthquake acceleration etc can be reflected reasonably, which is helpful for the safety of oil and gas transportation.