Hydrodynamic performance of multi-component structures in oscillatory flow, from blow-out preventer to dual Cylinder interference

Abstract

As one of the key components for the wellhead integrity, the Blow-out Preventer (BOP) is designed and constructed to prevent abnormal pressure change in the well and keep the blow-out from happening, and therefore is essential for the whole well- being of the offshore drilling system, and this calls for a careful investigation on the understanding of the BOP dynamics and its effect on the whole system. However, due to the complexity of the structure itself, the hydrodynamics of the BOP are difficult to model and therefore is the main focus on this thesis. First a general overview will be given on the challenges of offshore systems during the drilling phase when the BOP is installed directly above the wellhead. The current industrial standard suggested by DNV on the modelling of the BOP will be given. In order to re-evaluate the problem, the non-dimensional analysis will be carried out and the key hydrodynamic effect parameters of the KC number, [beta] number and the angle of attack [alpha] will be identified. First sets of the experiments on scale-down BOP model conducted in the MIT Towing Tank show that the experimental measured hydrodynamic coefficients are drastically different from the industrial recommended modeling coefficients that the added mass coefficient Cm and the drag coefficient Cd both have a much larger value than the industrial model provided, and they vary significantly as the function of the key parameters. An equivalent box model was built and tested to capture the external shape of the BOP and used to address unusual hydrodynamic behavior. The box experiments successfully captured some major trends of the BOP model. It revealed that, first, the externally rectangle shape of the BOP will have a major impact on the variation of the added mass coefficient; second, the BOP model works in the range of overall laminar flow regime and thus, results in an inversely proportional relationship between the drag coefficient and KC number. However, the box model does not exhibit the large values of drag and added mass coefficient found in the BOP, which must be attributed to the multi-component structure of the BOP and the hydrodynamic interaction of the components. This was later confirmed through numerical and experimental visualization. Experiments on a model consisting of multiple cylinders exposed to the oscillatory flow are carried out in the MIT towing tank with varying parameters on KC number, [beta] number, Gap ratio and angle of attack [alpha]. Experimental results show that for side-by-side, at certain gap ratio, the drag coefficient of each cylinder will experience an increase, compared to the hydrodynamic of the single cylinder. This confirmed the BOP multi-structure hydrodynamic interaction effect. Also numerical work has been carried out through a 2D BDIM code, named Lilypad. The result confirms the experimental work, revealing that the increase in the drag coefficient is due to the formation of a jet between the Karman streets of the two adjacent cylinders.