Perspective on Gas Explosions Risk Offshore; Low historic Gas Explosion Frequencies revealed in North Sea
Authors: Vinnem Jan Erik 
Published: December 2007

Abstract
Blast loads have come into the focus of offshore safety during the last years, due to high loads experienced in recent large scale tests. A comprehensive study has been carried out in order to establish historic frequencies for blast loads on offshore installations. The purpose of the work has been to enable calibration of results from quantitative risk assessments, and the consideration of what shall be the design basis with respect to blast loads. The analysis has been based on the total population of production installations in the North Sea, covering a 25 year period from 1973. 10 significant explosion incidents were identified. All the significant explosions have occurred on installations that were installed offshore before 1980. Comparisons have been made between ‘old’ and ‘new’ installations. There is a clear downwards trend in the number of explosions per year for the ‘old’ installations, as the time passes. Frequencies are presented in relation to platform years as well as explosion area years, and address also mechanisms such as impairment of explosion barrier and escalation.

Introduction
Recent research activities in the gas explosion field have indicated that blast loads on offshore installations may be higher than what has previously been known. Consequently, many offshore installations have been designed with respect to blast loading using data which may appear to be too low.
Most of the explosion research work is conducted for the most severe conditions, i.e. completely filled module, stochiometric gas concentration, etc. Location of the ignition point is nevertheless varied in the tests that have been conducted.
The impression from these tests is that quite high blast loads are created even in semi-confined spaces, loads that are somewhat higher than what is recognized as the North Sea experience.
It is on the other hand not an easy task to determine rationally what should be the approach to design of for instance explosion barriers. It will in many instances not be realistic to design according to the most extreme (conservative) conditions.
It is therefore interesting to establish a historic frequency distribution for blast loads, in relation to the extent of the overpressure or the potential impairment of explosion barriers. This may then be used in the calibration of results from quantitative risk assessments, and thus in the consideration of what shall be the design basis with respect to explosion overpressure protection.

Objectives and Scope of Work
The objectives are to assess on a statistical basis, the historic frequency of explosions on fixed offshore platforms in the North Sea, according to the extent of the blast load.
The work has been conducted in two phases, starting with a Screening phase, i.e. a coarse assessment of historic frequencies, followed by a Detailed analysis of the most severe cases.
The work has been conducted as follows:Task 1: Identify instances of an explosion on production installations in the North Sea in the periodTask 2: Screening of explosion cases for coarse categorisation with respect to blast loading. Non-trivial cases marked for further assessment.Task 3: Assessment of exposure data in the North Sea in the period 1973–97, with respect to number of production installation years exposed, type of function covered by the production installation, and number of areas between explosion barriers on each production installationTask 4: Assessment of historic frequencies of blast loading, according to coarse severity categories.Task 5: Detailed analysis of critical events and update of frequency assessment.
Contact was made with six UK and Norwegian operating companies during Task 5. All operators were quite helpful in their replies, resulting in considerable reduction of the uncertainties about circumstances of the events and their effects.