Response and energy dissipation of rock under stochastic stress waves

The response and energy dissipation of rock under stochastic stress waves were analyzed based on dynamic fracture criterion of brittle materials integrating with Fourier transform methods of spectral analysis. When the stochastic stress waves transmit through rocks, the frequency and energy ratio of harmonic components were calculated by analytical and discrete analysis methods. The stress waves in shale, malmstone and liparite were taken as examples to illustrate the proposed analysis methods. The results show the harder the rock, the less absorption of energy, the more the useless elastic waves transmitting through rock, and the narrower the cutoff frequency to fracture rock. When the whole stress energy doubles either by doubling the duration time or by increasing the amplitude of stress wave, ratio of the energy of elastic waves transmitting through rock to the whole stress energy (i.e. energy dissipation ratio) is decreased to 10%-15%. When doubling the duration time, the cutoff frequency to fracture rock remains constant. However, with the increase of the amplitude of stress wave, the cutoff frequency increases accordingly.

Stress Analysis of Top Tensioned Riser Under Random Waves and Vessel Motions

The bending stresses of top tensioned riser(TTR) under combined excitations of currents,random waves and vessel motions are presented in this paper,and the effect of the internal flowing fluid on the riser stresses is also considered.The computation programs which are used to solve the differential equations in the time domain are compiled and the principal factors of concern including the angular movements at the upper and lower ends of the riser,lateral displacements and bending stresses are presented.Then the effects of current velocity,random wave,top tension,vessel mean offset,low frequency motion and internal flow velocity on the bending stresses of the riser are analyzed in detail.

Dynamic Analysis Method of Offshore Jack-up Platforms in Regular and Random Waves

A jack-up platform, with its particular structure, showed obvious dynamic characteristics under complex environmental loads in extreme conditions. In this paper, taking a simplified 3-D finite element dynamic model in extreme storm conditions as research object, a transient dynamic analysis method was proposed, which was under both regular and irregular wave loads. The steps of dynamic analysis under extreme conditions were illustrated with an applied case, and the dynamic amplification factor （DAF） was calculated for each response parameter of base shear, overturning moment and hull sway. Finally, the structural response results of dynamic and static were compared and analyzed. The results indicated that the static strength analysis of the Jack-up Platforms was not enough under the dynamic loads including wave and current, further dynamic response analysis considering both computational efficiency and accuracy was necessary.