Bow Ties And Offshore Safety Studies

Bow ties and Offshore Safety Studies

Bow Ties and Offshore Safety Case

Safety Case Bow Ties

i) Introduction: Bow Tie Assessment (BTA) technique, initiated by Shell is widely used to demonstrate Major Hazard Management (MHM) efforts of offshore operators. Moreover, bow tie is a simple graphic representation and is understood by a wide range of audience, not only by HSE managers. HEMP (Hazards & Effects Management Process) by operating companies is a UK Safety Case Regulation 2005 requirement and obviously BTA is an effective tool to demonstrate HEMP. Typically, the left side of the bow contains control / preventative barriers and the right side contains mitigation / recovery measures. It is the Shell practice to identify escalation controls for each of the barriers and recovery measures. HSE / Safety Critical Elements (SCE) are part of barriers / recovery measure whereas HSE / Safety Critical Activities are part of the escalation controls.

In this note, the possibilities of using bow ties in safety case assessments are explained.

ii) Safety Case Application of Bow Tie Assessment (BTA): If bow ties are constructed for each of the MAEs (Major Accident Events) or MAH (Major Accident Hazards) based on the HAZID (Hazard Identification), these bow ties can be modified for each of the FSA (Formal Safety Assessment) studies to demonstrate that the assessments are in compliance in UK SCR (Safety Case Regulations), 2005. BTA can be used in all offshore FSA studies and the possibilities and approach are explained in the table below:

FSA Study

Bow Tie Application

HAZID

Once the MAEs are identified, develop bow ties with all safety systems /barriers and the recommended barriers for each of the MAE.

ESSA (Emergency

The mitigation safety systems on the right side of bow tie System

Survivability

(emergency systems) will have to be assessed for survivability

Assessment)

during their designed performance objective.

ETRERA

Specific bow ties may be developed for escape, evacuation,

(Escape,

Temporary

Refuge,

Evacuation and Rescue Analysis)

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refuge and rescue emergency situations to demonstrate that all SOLAS (Safety of Life At Sea) required are in place based

Bow ties and Offshore Safety Studies

on potential impairment conditions. FEA

Specific bow ties for fire, explosion, and gas release may be

(Fire and Explosion Assessment)

developed to demonstrate that all these MAEs are controlled and mitigated using adequate barriers based on potential impairment conditions.

NHHA (Non-Hydrocarbon

Specific bow ties for dropped objects, ship collision and Hazard

Assessment)

transportation risks may be developed to demonstrate that all these MAEs are controlled and mitigated using adequate barriers based on potential impairment conditions.

QRA

The impairment frequencies of MAEs can be calculated with

(Quantitative Risk Assessment)

inputs from FEA & NHHA studies by using Fault Trees (FTA) on the left side of bow and Event Tree (ET) on the right side of the bow. One bow tie per each of the MAE could be developed as part of QRA to demonstrate that all threats and barriers

are

considered

while

calculating

impairment

frequencies. ALARP Demonstration

For a brown field QRA, the risk levels should be calculated based on actual SCE/safety barrier performance. Traffic light system could be used to represent performance of safety barriers in the bow tie diagram. If all the barriers are performing as designed / desired (Green), then the installation is regarded as ‘Safe to Operate’ and ALARP.

Verification Scheme

The Written Scheme of Examination (WSE) as part of SCE (Safety Critical Element) verification could be demonstrated by developing bow ties with specific inputs from Performance Standards (PS).

HSE Safety Case

In the FSA summary section, the bow ties developed in all assessments could be included. Finally, based on safety assessments, consolidated bow ties could be developed for all MAEs in line with potential impairment of defined sensitive receivers.

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Bow ties and Offshore Safety Studies

iii) Conclusion: Although the use of bow ties is now a common practice in safety cases, its use in all FSA studies will help to provide a consistent assessment track for all MAEs in all stages of assessment. Moreover, bow ties are graphic representations which are easy to understand by all and are capable to communicating technically complex results in a simple and straight forward manner. Author: Pillai Sreejith [email protected]

Pillai Sreejith