Risk Analysis of MODU'S Following Lord Cullen's Report
Authors: Stiff J.J. , Kucharski Mike , Jack R.L. , Trickey J.C. 
Published: October 2008

Early in 1991 Santa Fe Drilling Company (North Sea) Ltd. decided to take a lead in developing their offshore Safety Cases particularly with respect to their 'forthwith"items. To achieve this aim they contracted Noble Denton to assist them in carrying out the Cullen "forttmvith" studies. This paper describes the methods and assumptions made in undertaking these analyses.

INTRODUCTION

Lord Cullen published his report on the Piper Alpha Disaster in November 1990(1)and by so doing set the scene for major changes in the way offshore structures, and the companies that operate them, areassessed, The report contains 106 recommendations that are likely to become part of the legal frameworkcovering the U.K. offshore continental shelf. In producing these recommendations Cullen drew heavily the results of similar enquiries, the Control of Industrial Major Accident Hazard (CIMAH) regulations and the Norwegian regulations. The intent is that, by taking on his recommendations, offshore safety will be improved and that the propensity for catastrophic events will be significantly reduced. In order to achieve this he concluded that:Companies should be required to submit a safety case demonstrating that their installation are safe and that the management structure issuch as to be responsive to safety issuesThat the current prescriptive regulations shouldbe changed out with regulations based on the precept of setting goals.The regulatory bodies concerned with safetyoffshore should be rationalized to be more in line with other onshore industries

When the report was issued, it was realized that there could be some considerable delay before theregulatory regime could be changed to reflect the recommendations. In order to circumvent some of this delay, certain recommendations were made with the statement that the relevant study be performed "forthwith.., without waiting for legislation". These studies are?available in full paper There is little dispute that Cullen's report contains many very important and valuable proposals, some of which have been around since the turn of the century(2). However the report also has some limitations, particularly when applied to Mobile Offshore Drilling Units (MODUs). In many cases these are only problems of detail, but they are indicative of a deeper problem. Take, for example, the emergency systems review: Cullen specifies that the severe accidents that he had in mind when producing recommendation 69 were fire, explosion and strong vibration. The emergency systems were to include the emergency power supply, communications and the Emergency Shutdown (ESD).

While these systems and accidents may be sensiblechoices on a fried production platiorm, they are less than ideal for a MODU. On a MODU, the ESD system is basically synonymous with the BOP system, However in no way can the simplicity of this system be compared with the complexity of a production platform ESD.

The potential accidents on a MODU, and the vital/emergency systems need to be considered in a different light from those on a production platform. Indeed this difference applies to much of what is said in Volume Two of Cullen's report.