Laying And On-bottom Stability Of Deepwater Pipeline

It is a brutal fact that we still have some way to go in exploitation of deepwater oil and gas compared with developed countries. Until today, we are still unable to exploit the abundant oil and gas in deepwater area of South China Sea. In deepwater, pipeline is the most favorable method in transporting crude oil and gas. The laying process, in service stability and safety in submarine hazards are the most critical key technologies.In this work, pipeline installation is investigated through numerical, analytical and experimental ways, and the pipe stability when exposed to landslide or debris flow is analyzed through analytical models, based on which the computational and theoretical analysis system for pipeline installation and its stability are established. The methods presented in this work are simple and fast, suitable for engineering, compared with traditional FEM method.The laying process, in service stability and safety in submarine hazards are the three important aspects related to pipeline installation, working behavior, and safety when exposed to extreme loading., which are exactly the focus of this work. Favorable methods for pipeline laying are brought forward based on numerical analysis, analytical analysis and indoor test. Detail investigation on pipeline behavior in service is carried out based on numerical analysis and indoor tests, and a noval way for analysing the dynamic behavior of riser is provided. A noval analytical model for pipeline reaction to submarine landslide is established. Thus, a complete analysis system for invesgating pipeline behavior has been established, which includes all the three vital aspects of the laying process, in service stability and safety in submarine hazards.First, a new numerical method for S-lay and J-lay is introduced, which can consider the influence of ocean currents and seabed stiffness. Then, an analytical model for J-lay is established to investigate the pipeline behaviors and pipe-soil interaction during installation. A series of centrifuge tests and large-scale tests are conducted to study the dynamic pipe-soil interaction during laying and service, to reveal the variation of soil strength. The safety and stability of the pipeline exposed to landslide or debris flow is analyzed at the end of this work. As the lateral soil resistance to the pipeline differs with pipe weight, two analytical models are established for two different pipe weights. From the above, the main contributions of this work include:1. Two numerical methods, as well as the calculation model, for pipeline during S-lay and J-lay are introduced. The influence of the currents on the whole pipeline is considered by aggregating the forces of ocean currents on pipe elements. The pipe-soil interaction is included by simplifying the seabed to a series of springs. Numerical iteration is used to get the final solution, which enables fast calculation. The distributions of internal forces along the pipeline and the variation of pipe embedment are both obtained.2. A novel analytical method for J-lay on plastic seabed is presented. The controlling equations of different pipeline parts are established according to their individual loading conditions, based on which the model is established. Moreover, a specific solving method for the analytical model is proposed. The contributions of different factors to pipe embedment is quantitatively analyzed to reveal to influence of pipe weight, load concentration at the TDZ, soil softening and elastic rebound of the seabed.3. The mechanism of dynamic pipe-soil interaction is analyzed through centrifuge tests. The tests have considered the variation of different horizontal movement amplitudes and vertical cyclic loads. Different combinations of horizontal and vertical movements are made to investigate the reaction of the seabed soil. Meanwhile, the variation of soil strength is detailly analyzed according to the change of soil resistance during the dynamic pipe-soil interaction process and the change of moisture content.4. A large-scale model test is designed and constructed, and a series of tests are carried out to investigate the pipe-soil interaction at the TDZ (touchdown zone). The soil strength, pipe embedment and pore pressure along the pipeline are all detailly analyzed, and the mechanism of pipe-soil interaction is revealed. Moreover, the available pipe-soil interaction models are proved to be unable to predict the real pipe embedment.5. The vector-form intrinsic finite element method is successfully used in pipeline analysis. Two pipe-soil interaction models are developed:elastic soil model and elasto-plastic soil model. The overall pipeline from water surface to the seabed is modeled to investigate the pipeline behavior during installation and work. It is found that the plastic deformation of the soil has great impact on both pipeline embedment and the internal force. Besides, the compression in pipeline at the start of dynamic cycles is a serious threat to pipeline safety.6. Two analytical models for pipeline with different unit weights are established to investigate the pipeline behaviors when exposed to submarine landslide or debris flow. The loading conditions of pipeline with different unit weights and the initial stress state can both be considered. Moreover, a specific solving method is designed. It is found that the bending strain is very important in the total strain, and the weak point along the pipeline differs with pipe unit weight.