Experimental Study On Pipe/soil Interaction Of SCR At TDZ In Deep Water

Nowadays, 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 dynamic laying safety and as-laid service stability are the most critical key technologies.With the depletion of relatively shallow water fossil fuel reserves, more and more oil and gas reserves in ever deeper water have been exploited significantly in recent years, where the floating production vessels or platforms such as semi-submersible or SPAR production platforms, floating production storage and offloading vessels (FPSO) are often utilized instead of fixed production structures. In addition, a floating production system (FPS) usually consists of a mooring system and risers, which transport the hydrocarbon production or export the processed fluid between the subsea wells and the floating platforms. The introduction of these compliant floating systems for offshore hydrocarbon production has led to the development of new designs for the risers. Nowadays, SCR (Steel catenary riser) has become a more cost effective option than traditional vertical or flexible risers for oil and gas transportation in deep water. Accurate analysis of riser fatigue is heavily dependent on the interaction between riser and the seabed soil, which is a research focus in recent years.Firstly, this paper numerically studies the nonlinear dynamic response of SCR, especially the TDZ dynamic response. And then it simulates the3D interaction between SCR and typical clay seabed through large scale laboratory test, which is used to simulate the dynamic pipe/soil interaction by appling cyclic motion at the end of the pipeline. Based on numerical and experimental results, a detailed analysis of the trench forming process, the pipeline internal force variation and the development of excess pore water pressure at pipe bottom is made. It explains the mechanism of pipe embedment depth increasing from the point of the excess pore pressure accumulation. And it proposes some recommendations for the design of SCR from the point of limit damage and fatigue failure. In addition, this paper also studies the soil strength change below the pipe invert with dynamic pipe/soil interaction, which helps to the understanding of soil softening. Specific work includes the following content:1.Based on a large semi-submersible platform of the South China Sea, it establishs the dynamic interaction numerical model of "semi-submersible platform-SCR-seabed" through the finite element software Orcaflex, combining with the hydrodynamic analysis software AQWA. The numerical model can consider multiple effects of wave, current and other environmental factors to investigate the nonlinear dynamic response of SCR, especially the dynamic response of the TDZ. It can provide some references for the indoor large scale test.2. Considering the silty clay seabed, it designs and builds the large scale laboratory model test of offshore pipeline and soft soil for the first time. And it introduces the composition and functions of test system and some test preparation work in detail.3. Based on this large scale three-dimensional model test system of soil/structure interaction, we carry out a series of tests. And the main research aspect is the vertical pipe/soil interaction at TDZ, including static test, dynamic cycling test and T-bar test after the dynamic cycling test. Through the analysis of the monitoring displacement, excess pore pressure and pipe bending moment, we study in depth the mechanism of vertical pipe/soil interaction about soft seabed. And it reveals the rule of pipe embedment increasing and investigates the effect of soil suction on the pipeline internal force variation to provide support for the SCR fatigue analysis at TDZ.