Numerical study on heated pipe-saturated soil foundation interaction considering thermo-osmosis effect

Heated pipe transportation is one of the most common used ways of offshore oil and gas transportation, and the mechanism of pipe-soil foundation interaction is the key to the controllable thermal buckling design of heated pipes. The change of pipe temperature not only causes the strong thermo-hydro-mechanical coupling response of the surrounding seabed saturated soil foundation, but also induces the migration of pore water under the temperature gradient, showing a significant thermo-osmosis effect. In this paper, a numerical study is presented to study the heated pipe-saturated soil foundation interaction considering thermo-osmosis (T-O) effect based on the OpenGeoSys finite element platform. Based on the mass equation and energy equation, the theoretical expression of the influence factors of thermo-osmosis effect was derived and the critical influence factors were proposed, which determined the critical values of T-O coefficient for some common soil and water parameters. Through the secondary development in OpenGeoSys embedding the T-O effect, a heated pipe-saturated soil interaction model, which can consider the thermohydro-mechanical coupling behavior and the T-O effect of soils surrounding the pipe, was established. The validity of the model was verified by comparing with the analytical results, and then the effect of T-O on the pipe-soil interaction was discussed based on the influence factors of T-O effect and numerical results, including the evolutions of pore water pressure and vertical displacement of seabed soils around the pipe with different oil transportation temperature and different initial buried depth during the operation period. Results showed that the critical value of the T-O cofficient is 43x10(-12) m(2)/(s . K), which provides a basis for evaluating whether the T-O effect is considered in practical engineering. The T-O effect has little influence on the peak value of pore pressure of soil, but it can cause significant stable-state negative pore pressure. The proportional relationship between the amplitude and the peak value of the negative pore pressure is determined by the proposed influence factor of T-O effect. During pipeline operation, temperature often induces soil weakening and negative pore pressure, which have adverse effects on pipe stability. This adverse effect is enhanced with the increase of oil transportation temperature and depth within 3 D (D is the diameter of the pipe), but there is no significant difference in this adverse effect caused by T-O with depth above 3 D.