Reliability Assessment For Reeling Installation And Multiaxial Fatigue Analysis On Reeled Deepwater Pipelines

As the oil and gas exploration goes into deepwater acceleratedly, the production system of oil and gas exploration is being built in deeper waters. Deepwater pipeline is an important part of the oil and gas development system and its security is critical, which once damaged will bring huge economic losses, even the environmental pollution. The rapid development of deepwater oil and gas exploration also brings lots of challenges to pipeline installation technology, and at present the most common installation methods include S-lay, reel-lay, J-lay and drag-lay.Reel-lay method, in which pipe segments are welded onshore and then transported to the sea for installation, is one of the most e?cient and economical methods of pipeline installation, suitable for a variety of deepwater pipelines (steel catenary risers - SCRs, umbilicals, etc.). However, changes of material properties may turn up due to the plasticity during installation, leading to changes in mechanical properties or even structural damage, especially for such influencing factors: residual stress and mechanical properties inconsistency in the weld of SCR, weld defects, and interaction among tubes in the umbilical, which are the key factors should be considered during the reel-lay installation.The reel-lay installation's affection on the fatigue performance of deepwater pipelines is also an important aspect to consider. Fatigue is a key factor in the pipeline design. In the previous studies, most of the fatigue life predictions are based on the uniaxial method. However, as the water goes deeper, the work environment is becoming more complex, inducing the stress-strain state no longer simply uniaxial, so it is more accurate to use multiaxial approach for the fatigue life prediction of pipelines. Besides, the reel-lay installation process has stochastic or uncertain characteristics in several aspects. A large part of these stochastic factors are from the uncertainty of material properties and geometry size, which may result in the mechanical performance that not the same as predicted during installation, even destroyed, so it is necessary to do the reliability assessment considering uncertainties on the reel-lay installation process, to ensure the security of deepwater pipelines.The influence of reel-lay method on fatigue life of deepwater pipelines is studied, and the reliability assessment on the reel-lay installation process is carried out. The main work is as follows:1. Numerical simulation of reel-lay installationFinite element analysis is applied with contact and plasticity formulation, and the load history during installation is simulated, focusing on the mechanical performance of the weld in SCRs and tube interaction in umbilicals. The impact of the weld defect (lack of penetration and lack of fusion) during the installation process is also discussed.2. Multiaxial Fatigue Analysis on reeled pipelinesBased on FEA results, the same loads are applied to the reeled and non-reeled pipelines respectively, and then multiaxial method is used to do the fatigue prediction. The influence of reel-lay method on fatigue life of deepwater pipelines is discussed. 3. The reliability assessment on the reel-lay installation process Material parameters and geometry size are chosen to be stochastic variables, and design of experiments is conducted to choose sample points. Then non-linear finite element analyses of these sample points are executed, and the approximation model based on Response Surface Model (RSM) is constructed to describe the variation in the response with respect to variables from the sampled data. With the approximation model, first-order second moment and Monte Carlo method are used to do the reliability assessment on SCR reel-lay installation process.Finite element method is used to simulate the reel-lay installation of pipelines, and the mechanical performance of the weld in SCRs and tube interaction in umbilicals are discussed. Based on the finite element analysis, multiaxial method is used to do the fatigue prediction, and results indicate that the reel-lay method can reduce the fatigue life of pipelines significantly. Stochastic analysis is used to do the reliability assessment on the reel-lay installation process, and the method is helpful to practical engineering design.?...