Assessment of the Blast Resistance of Offshore Topsides Structures
Authors: Walker S. , Hu Chunsheng , Williams J.R. 
Published: October 2008

This paper describes the methodology used to assess the blast resistance capacity of compartments of existing and proposed offshore structures. Unlike previous work published in this area the emphasis is on structural assessment of whole compartments including load bearing walls, floors and ceilings. Methods of determining reaction and dynamic pressure loads on equipment and primary steelwork are described.

Practical guidance is given on the interpretation of existing structural codes for the assessment of existing structures and the design of new structures.

Design charts are given which enable the dynamic response of structural components to be calculated for selected pressure time histories. The response may involve deflections in the elastic, elastic/plastic and purely plastic regimes. The components may be under an imposed static loading (either in-plane or out-of-plane) during the explosion.

INTRODUCTION

Blast walls are usually designed to deform plastically and act predominantly in bending to minimise the effect on the primary structural members of the platform. A great deal of effort is usually expended in designing the edge connections of the blast wall so that the reaction loads are transmitted to the supports without damage to the supporting steelwork. Because of the inertia of the wall it is possible to design these connections such that the transmitted shear forces and moments are much less than the peak overpressure force times the area of the wall. Purpose-built blast walls are usually free standing and are not an integral part of the supporting structure. There are a number of references [1,2,3] which deal with the dynamic response of free standing blast walls.

Many older modules were not in fact designed against blast loading at all, or rather optimistic peak pressure levels were assumed (0.1 bar or 10 kN/m2). Outmoded venting formulae were often used [4.5J which have been shown to grossly underestimate the expected peak overpressures as they do not take account the effect of obstacles and flame acceleration effects in large enclosures. It may also have been assumed that immediate pressure release would occur due to cladding connection failure. In many cases the proximity of adjacent compartments, the inertia of the cladding, blockage by piping and equipment and the high rate of volume generation make cladding failure ineffective. In fact some cladding attachment systems have been found to have a higher capacity than the design pressure used for blast walls in the same compartment, although plastic design methods often indicate a higher blast resistance for these walls than was previously expected.

It is useful at this stage to briefly review the conventional method of free standing blast wall evaluation in order to discuss the extensions of the method necessary to evaluate compartment resistance. This is covered in the next section.

BLAST WALL ASSESSMENT

There are a number of simplified methods available for the evaluation of blast resistance in the literature [1,6,7] as well as more direct methods of calculation using complex finite element codes. Preferred methods incorporate non-linear, finite deflection and dynamic effects.