Validation of Blowout-Rate Calculations for Subsea Wells

Abstract

Summary Calculating the blowout rate of oil and gas wells is commonly one of the first steps in environmental-impact assessment, contingency planning, and emergency response. The blowout rate is a direct measure for the economic and environmental damage caused by a blowout and an indicator for the effort required to regain control over the well. Hence, a simulator was developed to estimate blowout rates. This simulator was validated for field cases by comparing calculated blowout rates with estimates based on observable phenomena such as flame length and heat release rates. This limited validation to onshore- and platform-well blowouts, which are usually governed by critical outflow conditions at surface, because ambient pressure is usually considerably less than the wellbore pressure just upstream of the outflow. For subsea wells, blowing out against the substantially higher pressures at seabed, this does not apply. The blowout rate is determined by the total system performance from inflow at sandface to outflow at seabed. To validate the blowout-rate calculations under these conditions, data were collected on high-rate well flow through an annulus against elevated surface pressures, resembling subsea blowout conditions. A comparison of the measured rates with the calculated rates demonstrated that the rates could be predicted with a high accuracy, provided that the mechanical condition of the tubulars is taken into account properly. Default assumptions for the hydraulic roughness of the tubulars lead to overestimates of the blowout rates and consequently to worst-case estimates for the environmental and economic damage caused by a blowout.