Risk And Resistance ApproachAt CREA, the Resistance to Accidental
and Extreme Loads goes hand in hand with the Probability Based Risk Assessment. This
Integrated Risk and Resistance Approach adopted by CREA
differs from that normally associated with the industry in that the assessment provides a
good balance between engineering solutions and judgement based on long industry
experience, and both probabilistic and deterministic mathematical modelling. A successful
optimum solution is normally achieved by combination of both the probability of loss being
within acceptance criteria and a corresponding plant resistance.
Risk Assessment Projects to date included:
- Concept Safety Evaluation
- Formal Safety Assessment and Safety Case Preparation
- Technology Based Project Risk Assessment
- Preparation of Safety Management Systems
- HAZOP, HAZID and HAZAN
- Qualitative and Quantitative Risk Assessment
- Development of QRA Rule Sets
- Fire and Explosion Safety Studies
- Assessment of Prevention of Fire and Explosion, and
Emergency Response
- Escape, Evacuation and Rescue Analysis
- Determination of Design Accidental Loads
- Response Analysis of Equipment and Structures Using
Ultimate Strength Approach
- Establishment of Performance Standards
- Development of Risk Reducing Measures
- Risk Based Design
- Determination of Design Accidental Loads
- Analysis and Design of Resistance of Offshore and
Onshore Facilities to Accidental Loads
- Optimisation of Fire and Explosion Protection
- Plant Availability Studies and Assessment of
Regularity of Hydrocarbon Production
- Working Knowledge of Safety Legislation in the
United Kingdom, Norway, the Netherlands, Australia and Malaysia.
- Third Party Verification
- Technical Audits
RISK ASSESSMENT
CREA's engineers have carried out risk assessment
in various parts of the world responding to the requirements of the respective
legislation. Although there are some differences in the approaches used, the following is
a brief summary of the methodology involved.
Corporate Goals and Risk Criteria
The starting point of assessment of risk is to
establish an understanding of company or project overall goals and risk criteria that
concern the operating company's activities as a whole, although they may reflect country
specific legislative requirements. The design and operation should achieve the goals and
criteria by a combination of appropriate resistance of the facility to accidental events,
operational procedures and a realistic likelihood of accidents materialising based on
statistics.
Hazard Identification
The next step is identification of hazards and
potential hazardous events. Formal approaches are used to identify hazards and top events,
e.g. Hazard Identification (HAZID), Hazard and Operability Studies (HAZOP) and Failure
Mode and Effect Analysis (FMEA). HAZID and HAZOP are structured, systematic and auditable
approaches, which address process and non-process events and cover all parts of the
facility. This requires a suitable combination of involvement of operations personnel,
design engineers and safety specialists.
Qualitative versus Quantitative
Risk Assessment
Once hazards and hazardous events have been
identified, their causes, consequence and probability can be estimated and the risk
determined. Risk assessment may be on a qualitative, semi-quantitative or quantitative
basis. All involve the same steps.
A qualitative approach may be adequate for risk
assessments of simple facilities or operations where the exposure of the workforce,
public, environment or the asset is low.
A semi-quantitative method such as Risk Matrix is
useful to assess low, medium and high risks as it gives meaningful, although approximate
risk levels of fatalities, monetary values of asset impairment, loss of business or
environmental impairment. The risks may be screened and the medium and high risks carried
forward for further assessment.
Quantitative Risk Assessment provides a structured
approach to assessing the potential for incidents and expressing this potential
numerically. In QRA, statistical values are derived for potential loss of life and damage
to resources and environment. These values are used as a yardstick to measure safety, to
raise awareness for the potential of accidents and thereby developing measures to prevent
them. QRA is a tool, which assists management in deciding upon the best safety approach
and shows ways and means, (e.g. improved engineering, procedures, supervision, etc), to
prevent the potential incidents from occurring.
The application of QRA is considered to be
desirable when
- several risk reduction options have been identified
whose relative effectiveness needs to be determined;
- the exposure of the workforce, public, environment
or the strategic value of the asset is high, and reduction measures are to be evaluated;
- equipment layout allows significant risk of
escalation of the accidental event;
- novel technology is resulting in a perceived high
level of risk for which no historical data is available;
- demonstration of relative risk levels and their
causes to the workforce is needed to become more conscious of the risks;
- demonstration that risks are as low as reasonably
practicable (ALARP) is required within the operating company and by third parties,
including the regulating authorities.
Hazardous events do not necessarily cause loss of
life or damage. The development of a hazardous event into a serious incident depends on
the effects of mitigating factors; e.g. gas detectors activating a shutdown system can
sense un-ignited hydrocarbon release. If immediate ignition occurs, the signal from fire
detectors must be received by operating personnel and the signal can activate shutdown and
deluge systems prior to further escalation.
Estimation of Likelihood of Events
The formal technique used to project the
development of events into incidents is Event Tree Analysis. The estimation of frequencies
and probabilities of events in Event Trees is based either directly on statistical
analysis of historical data, or derived by using Fault Trees. Historical data should
ideally include the number of successful events recorded with the number of failures.
Assessment of Consequences of the
Incident Scenarios
An assessment of the consequence of incident is
required for those scenarios in which the failure of safety systems and the absence of
mitigating factors lead to potential escalation (e.g. escalation of initially controllable
releases of hydrocarbons into major fires and explosions). Physical effects from releases
of hydrocarbons or toxic material such as dispersion, explosion overpressures and heat
radiation from fire are evaluated. This includes the response of equipment and structures
to the overpressure, fire, impact, etc. which is simulated to assess whether escalation is
a realistic possibility and the extent of damage following the escalation.
Physical Effects Modelling is key to all safety
critical systems that provide
- detection of the hazardous event developing,
- communication of the detected event,
- control of the event,
- prevention of escalation,
- suppression of the incident, and
- escape, evacuation and rescue of personnel,
since the systems must be designed to survive the
incident and remain functional for a specific period of time. An important input to these
calculations is the leakage rate and its time dependence, the time dependent behaviour of
fire and its effects on equipment and structures. A safety critical system may be damaged
and render inoperative once it has fulfilled it's desired function. This is taken into
account in the simulations of the thermal or structural response.
Calculation of the Potential Loss
from Incident Scenarios
Having assessed the frequency and consequence for
each of the incident scenarios of the Event Tree, it is possible to calculate the
statistically expected loss for each scenario. The total statistically expected loss could
then be calculated by summation of the loss over all scenarios.
Several forms are available to express risks. Fatal
Accident Rate (FAR); which is often used in the earlier phases of a project; gives the
number of expected fatalities during 100 million working hours. FAR is used to describe
the work-related risk that personnel are exposed to. It does not distinguish, however,
between the nature of incidents or their magnitude.
Risks for entire operation or development
considering all major incidents are normally described by means of Potential Loss of Life
(PLL) which gives the expected number of fatalities in one year.
Individual risk is usually expressed as risk of
fatality per annum (IRPA) for a named type of worker.
A commonly used presentation form for risk to the
public is the so-called risk contour. The number at a contour represents the frequency at
which a person, assumed to be permanently present at the location of the contour, sustains
a given level of harm.
Another frequently used method to represent risks
to workforce personnel or the surrounding community is a probability/consequence diagram
also called an F/N plot where 'F' denotes the frequency of a potential event and 'N' the
number of associated fatalities.
RISK BASED DESIGN
In today's economic climate designers are required
to develop facilities on minimum budgets and on fast track development programmes. Added
to this, the facility often has to operate unattended and achieve an overall availability
in the region of 99% and, for good measure the risk to life, the environment and business
interruption shall be ALARP. Traditionally, major hazards and risks have generally been
addressed once a firm concept and design has been established. Typically 80 to 90% of a
project costs are fixed at the feasibility stage and unless risk assessment is brought
into the project at an early stage, the project development may be disastrous. A
risk-based design facilitates minimisation of risk from major hazards, whereby the design
is developed based on an understanding of the potential risks posed by the development.
The overall risk based design approach requires
early studies and activities to be performed with the general objective being
- the appropriate concept options are reviewed to
ensure that the requirements for the management of the major hazards have been included,
and
- the preferred option is designed to prevent, control
and mitigate major hazards.
The basis of the approach is to use hazard
identification and other risk assessment techniques to ensure that the best facility
option is chosen and further that the preferred option is developed using the risk
assessment techniques to support design decisions. Typically risk based design is carried
out in the following steps:
Step 1 - Determination
of Acceptance Criteria
These are facility-specific criteria for the design
based on the corporate risk criteria of the operating company.
Step 2 - Determination
of Accidental Loads
Based on process flow diagrams, process and
instrument diagrams and generic hazard lists, preliminary consequence modelling is
performed to determine the most important accident loads, e.g. fire and explosion loads.
For fires it is possible to determine the sizes and
duration which are used for, for example, for riser layouts, number and location of
emergency shutdown valves, determination of separation distances, fire protection, etc.
For explosions, it is possible to carry out
modelling for a range of layout options, equipment configurations, decks and walls.
Step 3 - Development
of General Facility Arrangements
Once the preferred option has been chosen, the risk
and safety discipline works with the layout designers to identify that various layout
options available for locations of pipelines, risers, high pressure systems, muster areas,
etc. The input from fire and explosion modelling provides a valuable input. As this stage,
a great deal of inherent safety is achieved. The design is not fixed yet so that moving
equipment around does not have much effect on the design process as no drawings have been
issued.
Step 4 - Hazard
Identification and Design Development
Once a general arrangement has been made with
respect to the general design philosophy and the first layouts have been produced, a HAZID
workshop should be undertaken with the contribution by designers, discipline engineers and
operations personnel.
This exercise requires an assessment of the
potential major hazards associated with the facility and requires the workshop group to
identify
- the consequences of the major hazard on the
facility,
- the credibility of such an event,
- the design (hazard management) measures in place to
prevent, control and mitigate the hazard,
- where no design measure has been provided, the
hazard management options available, and
- the functional design specification performance
standard for the design measure.
This stage goes a long way towards building a
design with a deep foundation of inherent safety. System based performance standards are
established related to the principal safety measures and safety critical systems for
- detection,
- communication of the detected event,
- control,
- prevention of escalation,
- suppression, and
- escape, evacuation and rescue.
The performance standards address the aspects of
- system functionality,
- reliability and availability,
- survivability, and
- dependency/interactions between systems.
Step 5 - Risk
Quantification
Some decisions require a higher degree of support,
therefore the use of Quantitative Risk Assessment and Cost Benefit Analysis is
recommended. At this stage, QRA helps to support decisions in making comparisons between
either total facility options or whether the selection of one type of equipment against
another has benefits.
Step 6 - Safety
Discipline Support to Detailed Design
Further detailed design development is supported by
safety studies so that a final detailed QRA is confirmatory rather than time consuming
design iteration.
Resistance to Accidental Loads
The resistance to accidental and extreme loads is
analysed in Steps 3, 4, 5 and 6. The high level nature of data available in Steps 3 and 4
justifies the used of approximate models whilst Steps 5 and 6 progressively involve more
complex models as the requirement for accuracy increases. This includes temperature time
histories of equipment and structures, deterioration of strength due to elevated
temperatures, resistance against impact, etc. QRA normally has simple rule sets that are
often over-conservative. Realistic rule sets can be developed based on the analysis of
response to accidental loads.
As for example, requirements for fire protection of
equipment are conceptually determined based on simplified heat-up calculations in Steps 3,
4 and 5. The fire protection should be optimised in Step 6 based on
- effects of fire water on reduction of heat flux and
cooling of pressure vessels,
- depressurising/rupture calculations,
- evaluation of failure modes,
- consideration of passive versus active fire
protection, and
- interaction between fire and explosion protection.
System resistance should be taken into account
rather than making decisions on the basis of failures at component level. Supporting
structures, for example, are redundant systems with a considerable resistance beyond first
yield. They normally provide adequate support even if some individual members attain very
high temperatures. The behaviour of equipment and structures beyond first yield can be
simulated with confidence using ultimate strength techniques.
COMBINED EXPERIENCE
In addition to the Risk Assessment experience, the
advantage of CREA is that its engineers are experts in the quantification of ultimate
strength capacities of plant and structures subjected to the following hazards:
|
