Fuzzy fault tree analysis of oil and gas leakage in subsea production systems

This paper presents a probabilistic failure analysis of leakage of the oil and gas in a subsea production system using fault tree analysis(FTA).A fault tree was constructed by considering four major areas where the leakages can be initiated.These are:gas and oil wells,pipelines,key facilities and third party damage.Conventional FTA requires precise values for the probability of failure of the basic events.However,since the failure data are uncertain,a fuzzy approach to these data is taken which leads to the so-called fuzzy fault tree analysis(FFTA),a method that employs expert elicitation and fuzzy set theories to calculate the failure probabilities of the intermediate events and the top event through identification of the minimal cut sets of the fault tree.A number of importance measures for minimal cut sets and the basic events have been obtained which helps to identify the nature of dependence of the top event on the basic events and thereby can identify the weakest links that may cause leakage in the subsea production system.

Water entry and exit of axisymmetric bodies by CFD approach

The water impact and subsequent entry of three rigid axisymmetric bodies,a sphere and two cones,in the early phase are simulated using CFD utilizing a VOF scheme to track the free surface and the results compared with the recent experimental results available in the literature.The penetration depth,vertical velocity and vertical acceleration time histories have been reproduced well by CFD with a wide choice of mesh density,whereas the peak pressure on impact required a much finer mesh and appropriate choice of the time step.Delineating an interaction region around the trajectory of the body with fine mesh and an adaptive time stepping strategy has worked well to capture the peak impact pressure accurately with reasonable computational effort.The‘full’motion of the sphere,which is buoyant,has also been simulated using CFD allowing its 6-dof motion for several cycles of entry and exit phases.The features of the behavior,especially the loss of symmetry of the trajectory,are discussed.

CFD approach to modelling, hydrodynamic analysis and motion characteristics of a laboratory underwater glider with experimental results

Underwater gliders are buoyancy propelled vehicle which make use of buoyancy for vertical movement and wings to propel the glider in forward direction.Autonomous underwater gliders are a patented technology and are manufactured and marketed by corporations.In this study,we validate the experimental lift and drag characteristics of a glider from the literature using Computational fluid dynamics(CFD)approach.This approach is then used for the assessment of the steady state characteristics of a laboratory glider designed at Indian Institute of Technology(IIT)Madras.Flow behaviour and lift and drag force distribution at different angles of attack are studied for Reynolds numbers varying from 10^(5) to 10^(6) for NACA0012 wing configurations.The state variables of the glider are the velocity,gliding angle and angle of attack which are simulated by making use of the hydrodynamic drag and lift coefficients obtained from CFD.The effect of the variable buoyancy is examined in terms of the gliding angle,velocity and angle of attack.Laboratory model of glider is developed from the final design asserted by CFD.This model is used for determination of static and dynamic properties of an underwater glider which were validated against an equivalent CAD model and simulation results obtained from equations of motion of glider in vertical plane respectively.In the literature,only empirical approach has been adopted to estimate the hydrodynamic coefficients of the AUG that are required for its trajectory simulation.In this work,a CFD approach has been proposed to estimate the hydrodynamic coefficients and validated with experimental data.A two-mass variable buoyancy engine has been designed and implemented.The equations of motion for this two-mass engine have been obtained by modifying the single mass version of the equations described in the literature.The objectives of the present study are to understand the glider dynamics adopting a CFD approach,fabricate the glider and its variable buoyancy engine and test its trajectory in water and compare it with numerically obtained trajectory in the vertical plane.