Blast Assessment For A Jack-Up Modu Drill Floor
Authors: Wolford A.J. , Phelps P.J. , Halbleib B.L. 
Published: December 2007

A Computational Fluid Dynamics (CFD) model was developed to assess the blast loading consequences for the BOP control unit on the Glomar Moray Firth I, a three-leg, harsh environment, cantilever jack-up. The BOP control unit (BOPCU) is partially shielded by the draw works and other drill floor equipment. The scenarios analyzed were all variations of explosions of a stoichiometric methane cloud resulting from a blowout. In all cases, the principal venting direction was upwards, and the degree of confinement due to the drill floor obstructions was minimal. There was little evidence of turbulence-enhanced flame acceleration. Based on a conservative estimate for the laminar flame speed, the maximum over-pressure at the BOPCU was predicted to be 2240 Pascals (0.32 psi) for the case of a 20 meter elevated ignition location. The comparable prediction for deck level ignition was 1600 Pascals (0.23 psi). The largest overpressure, 19,020 Pascal (2.89 psi), was generated by using the higher flame speed. Overpressure was found to be most significantly affected by flame speed of all the variables measured. General insights and understanding gained are applicable to other MODUs with questions relating to semi-confined explosions in critical drilling areas.

Background

Global Marine Drilling Company conducted an evaluation of the Glomar Moray Firth I (OMFI) to assess its compliance with Norwegian Petroleum Directorate requirements. As part of this evaluation, DNV was commissioned to perform a quantitative risk assessment of the location of the BOPCU.

The approach taken to model the blast effects needed to account for the spatial variation in pressures, for the possible enhancement of flame speed due to the presence of the obstacles on the drilling deck, and for the "shielding" effect expected to be provided by the draw works. To meet these requirements, CFD technology was chosen over simpler analytical methods (e.g. TNT, multienergy) for simulating the blast loading characteristics.

Objectives

The objectives of the study were:–To formulate, build, test, and apply a time-dependent three dimensional (3-D) CFD simulation model, and–To use the model to predict the propagation of blast waves from selected worst case release scenarios into the regions between release point and BOP control unit.

Physical Problem Considered

The physical problem addressed in this paper is the propagation of pressure wave phenomena such as that resulting from accidental releases of flammable hydrocarbon gas and subsequent ignition, and the damage that could result to equipment on the MODU. The ignition of clouds of flammable gas is termed vapor cloud explosions. It is useful at this point to review the basic mechanism that can give rise to vapor cloud explosions.

Vapor Cloud Explosions. Vapor cloud explosions can be classified into two categories based on the rate of flame front acceleration; either subsonic (deflagration) or supersonic (detonation) speeds. For a semi-confined ignited vapor cloud, disturbances in the flow field ahead of the flame front cause the flame to accelerate. This occurs because obstacles in the flow field generate flow velocity gradients. In a vapor cloud with sufficient fuel and proper air-fuel misture, upon encountering an obstacle, the flame front is stretched and bent, resulting in an increase in flame surface area. The fuel consumption rate therefore increases and causes an increase in heat flux. As the flame front accelerates around the obstacle and results in a self- propagating acceleration of the flame front. If the velocity of the flame front reaches sonic velocity, then the deflagration transforms into a detonation.