Blast Loads for Offshore Modules
Authors: Corr R.B. , Snell R. , Tam V. , Simmonds S. 
Published: October 2008

This paper discusses the methods by which the overpressures created by a hydrocarbon explosion within a congested offshore module may be calculated. The nature of the results obtained from numerical simulation are discussed leading to an approach by which a structural loading suitable for use in a linear elastic analysis package for code verification.

Strain rate effects, load combinations and deformation criteria are also discussed.

INTRODUCTION

The calculation of the effects of blast loading on offshore modules requires an interface between structural engineers and specialists in combustion modeling. The data produced horn a combustion simulation requires careful interpretation if costly over-design or unnecessary strengthening of an existing structures is to be avoided. As with any interface between disciplines a common understanding has to be attained to produce optimum results.

Blast loading is significantly different from the other forms of loading normally considered in the design of offshore topside structures because of its dynamic nature, the response of the structural system, and the fundamental uncertainties associated with the design event.

Acceptance of a degree of permanent damage or deformation of the structure may not only be tolerated but may be an essential feature of the design. Under these circumstances the normal code checks which are stress related may no longer be appropriate.

If the dynamic response of the structural system, plastic deformation and strain rate effects are taken into account and if safety factors are removed from the structural code checks then structures may be designed to tolerate a high blast loading for comparatively little incremental cost.

HYDROCARBON EXPLOSIONS IN CONGESTED AND PARTIALLY CONFINED MODULES

When a hydrocarbon/air mixture is ignited it burns in a flame front radiating from the point of ignition. This process is termed a deflagration. The temperature could rise to 2200K following the ignition of an (optimum) stoichioiometric hydrocarbon/air mixture. If the gas is allowed to expand freely this would lead to an eightfold increase in volume. Within the confines of an offshore module the gas is prevented from expanding freely and hence the pressure rises.

The processes involved in a gas explosion are complex and are the subject of current research. The expanding gas from combustion produces a flow ahead of a moving combustion front. When this flow encounters equipment and module structures, turbulence is generated. As turbulence has the effect of increasing combustion rate, the combustion front accelerates. The pressure rise experienced inside the module depends upon the rate of combustion and rate of removal of gas from the module. Thus overpressure created by gas module explosions within a module vary considerably from case to case, and are highly dependant on module configuration and internal layout.

BP Research have carried out a large number of simulations of blasts within modules. In some cases peak pressures in excess of two bar (200 kN/sq m) have been calculated with average values as high as one bar (100 kN/sq m). For comparison these pressures are an order of magnitude greater that those used for live load.