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GASCET (Guidance for the topic assessment of the major accident hazard aspects of safety cases)


GASCET is currently being updated to reflect the changes required under OSDR 2015.

The purpose of GASCET is to provide a framework of topic assessment principles and guidance for the assessment of the major accident hazard aspects of offshore safety cases.

The aims of this framework of assessment principles and guidance are:

  • to complement OSDR’s Framework for Safety Case Assessment and the Topic Assessment Templates;
  • as a component of the OSDR’s quality system for safety case assessment;
  • to provide OSDR with a basis for decisions made regarding the sufficiency of the technical content of offshore safety cases;
  • to give transparency to the assessment decisions and criteria;
  • to provide a basis for consistency in the assessment process and its outcomes;
  • to facilitate interfacing between different topic assessments;
  • to provide guidance material for specialist assessors, including those who may be new to OSDR and its component organisations (HSE & BEIS) and / or new to safety case assessment;
  • to identify, by reference, relevant technical policy and good practice;
  • to identify ‘gaps’ in the supporting topic guidance;
  • to assist assessors in the recording of judgements made during the assessment process [e.g. assessment instructions, non-acceptance issue notes, topic assessment reports, assessment recommendations etc.] by relating the judgements to the guidance;
  • to allow for peer review to be undertaken.


The document has been updated to account for the revisions to the regulatory requirements brought about by The Offshore Installations (Offshore Safety Directive) (Safety Case etc.) Regulations 2015 (SCR2015). References to the Offshore Installations (Safety Case) Regulations 2005 (SCR2005) have been removed. The document must now be read in the context of the requirements of the SCR2015 regime taking account of the revised terms and definitions, Operations Notice ON 84/2015 provides further guidance on this matter and addresses how guidance relating to SCR2015 can be applied to the SCR2005 regime where necessary.

The key terms and/or definitions that have been revised include the following:

  • Major accident – the definition outlining what constitutes a major accident has been widened and now includes those environmental impacts (as defined) that may result from a major accident. This means that any reference to MAs and derived terms e.g. major accidents hazards, needs to be considered in the new context;


  • Safety and Environment Management System (SEMS) – the definition of a management system given in the regulations remains the same but the scope is increased by the inclusion of environmental management aspects;


  • Safety and Environmental-Critical Elements (SECEs) – the term SECE replaces SCE (Safety Critical Element) and all references must now be considered in the context of both safety and environmental control


This document is intended primarily to assist topic assessors in undertaking assessment activities, but it does not purport to present definitive criteria in respect of the adequacy of the technical content of safety case submissions. Adequacy, or otherwise, is context specific and can only be finally determined by topic assessors using experience and judgement.

The document will also be valuable to Safety Case Decision Makers, Case Managers and Deputy Case Managers in understanding the basis and scope of the assessment considerations and the assessment outcomes.

An aim of the guidance is to provide the offshore industry with reasonable clarification of OSDR’s expectations of the technical content of safety cases.

The guidance should enable assessment to be undertaken in accordance with the requirements of The Offshore Installations (Offshore Safety Directive) (Safety Case etc.) Regulations 2015 (SCR2015) , through consideration of major accident hazards and the duty holder’s evaluation of risk and proposed risk control measures. Although the guidance indicates areas where measures for controlling risk are required, the document is not intended as an exhaustive statement of the available measures or of performance standards.

Whilst the guidance contains reference to specific standards, models, methodologies, etc. as representing good practice, alternative approaches proposed by a duty holder are likely to be acceptable if the duty holder can demonstrate that the alternative approaches are equivalent, or better, in terms of health, safety and the environment than those cited in the guidance. To aid the identification of information sources, references throughout this document refer to the author at the time of publication, the reader should be aware that some organisations have changed their name in recent years e.g. UKOOA became OGUK in 2007, DECC became BEIS in 2016.

The document can also be used as a reference to identify good practice, together with industry, national and international standards, which will aid OSDR’s inspection and enforcement activities.

This version of GASCET is a living document. OSDR will collect and review comments, updates and suggestions and periodically update the online version of GASCET. Please forward any comments, updates or suggestions regarding the content of GASCET to OSDR c/o HSE, Energy Division Offshore Unit, for the attention of Mr Mike Bilio, Team Leader, ED3.2 Fire and explosion risks and risk assessment. These will be reviewed by the relevant topic specialists and included when GASCET is updated: