Bow ties and Offshore Safety Studies

Pillai Sreejith

Bow Ties and Offshore Safety Case 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 System Survivability

Assessment)

The mitigation safety systems on the right side of bow tie

(emergency systems) will have to be assessed for survivability

during their designed performance objective.

ETRERA

(Escape, Temporary Refuge,

Evacuation and Rescue Analysis)

Specific bow ties may be developed for escape, evacuation,

refuge and rescue emergency situations to demonstrate that

all SOLAS (Safety of Life At Sea) required are in place based

Safety Case BowSafety Case BowSafety Case BowSafety Case Bow TiesTiesTiesTies

Bow ties and Offshore Safety Studies

Pillai Sreejith

on potential impairment conditions.

FEA

(Fire and Explosion Assessment)

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

developed to demonstrate that all these MAEs are controlled

and mitigated using adequate barriers based on potential

impairment conditions.

NHHA

(Non-Hydrocarbon Hazard

Assessment)

Specific bow ties for dropped objects, ship collision and

transportation risks may be developed to demonstrate that all

these MAEs are controlled and mitigated using adequate

barriers based on potential impairment conditions.

QRA

(Quantitative Risk Assessment)

The impairment frequencies of MAEs can be calculated with

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.

Bow ties and Offshore Safety Studies

Pillai Sreejith

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

pillai_sreejith@hotmail.com