Analysis and Optimization of rigid pipeline with inline structures

Abstract

Advanced modern technologies and growing demand for oil and gas has led to the discovery and development of smaller and remote fields that were once considered uneconomical. They are made economically more viable by employing a subsea development scheme and directing the output to existing platform for production and processing instead of having their own platform. This has necessitated the introduction of inline structures in the pipelines with the possibility to connect these remote fields when they are developed for production. However the presence of these structures introduces many installation challenges including increased weight and additional environmental loading. In some cases this might drastically reduce the limiting sea state for installation. For the scope of the thesis work, any structure in the middle of the pipeline with stiffness and weight greater than the pipeline is considered as an inline structure. A riser and pipeline installation using J-Tube pull in method is considered as the case study for analysis in the thesis. Analysis and parametric study of the installation is made with emphasis on the initiation phase to determine the limiting sea state for the safe installation of the pipeline. The main focus of the thesis would be to analyze the possibilities to optimize the limiting sea state for the installation of the J-tube seal with the help of buoyancy units by creating a neutrally buoyant catenary during installation. An attempt to develop a generalized optimization procedure to determine the optimal buoyancy unit configuration for all inline structure installation is made although the results indicate that it might be very case specific and a general method might not exist. Analysis to understand the influence of the type of buoyancy unit, the position on the pipeline catenary, net buoyancy, number of buoyancy modules and various other parametric studies are made. In addition, challenges encountered during an inline structure installation and the modifications required to carry out the installation from the vessel is briefly discussed. The analysis reveals that geometry of the buoyancy does not have appreciable impact. A sensitivity study on the added mass of the buoyancy shows that an increased added mass reduces the buckling utilization by its out-of-phase dynamic response with that of the catenary. Sagbend buckling is the most critical concern for installation and it is at its maximum when the structure is at the sagbend. It also reveals that the best results are achieved when the net buoyancy of the module is equal to the excess weight in the catenary due to the structure. A buoyancy unit that is offset from the structure provides better result than a similar module connected over the structure and also better results than the use of multiple buoyancy modules although this might be very case specific.