RISK BASED INSPECTION (AN OPTIMIZATION OF INSPECTION INTERVALS)

 

Risk based inspection ( An optimization of inspection intervals)

 

INTRODUCTION

 

In the past century, in most European countries, including Italy, the inspections for the pressure equipment, were ruled by a very sound, stable and effective system. It was based on "command and control" or deterministic approach, with inspection modes and frequencies defined by law. This approach was questioned in the early Nineties. The first idea of Risk Based Inspections (RBI) has been introduced in 1991 by American society of mechanical engineers ASME to plan equipment inspections, aiming to reduce risk without affecting costs. The risk-based inspection and maintenance strategy have been better focused by national and international standardization bodies. At now the most popular RBI codes are: the API 581 code by the American Petroleum Institute (API, 2008), the RIMAP by theEuropean Committee for Standardization (CEN, 2008) and the code by the American Society of  Mechanical Engineers (ASME, 2008). The basic idea of RBI is the optimization of inspection intervals, instead of the fixed frequencies, required by the deterministic approach. In RBI, inspection times and modes are affected by the assessment of failure risk, assumed as the product of likelihood an consequences, as well as by the tolerable risk level and by the results of previous inspections.

 

 CAUSES OF FAILURE

Root causes of failure of pressure systems, tanks and other containers include:

? Inadequate design and/or material for the loading and operating environment.

? Incorrect and/or defective manufacture.

? Unanticipated in-service deterioration such as corrosion or fatigue cracking.

? System errors in operation or maintenance or over-pressure protection.

? Malfunction of instrumentation, control systems or feed and utility supplies.

? Human factors including deliberate damage.

? External events such as fire, impacts or storms.

An integrated integrity management strategy will contain measures that address and

mitigate the possibility of these root causes of failure. Design reviews,

manufacturing quality assurance, operating training, and systems analyses are

examples of such measures. In-service inspection is a backstop to prevent failure

when a root cause has led to deterioration from the design intent or the asmanufactured

condition.

In this report, ‘deterioration’ is defined as damage, defects or degradation including:

? Macroscopic damage such as dents or gouges, bulging, deformation.

? General or localised wall thinning and pitting.

? Material flaws, cracks, and welding defects.

? Degradation of material properties due to changes in the material

microstructure.

Deterioration can result from discrete events (e.g. welding flaws, impact damage)

and the equipment may remain in that condition without further change. It

commonly relates to age and service, initiating or becoming worse with time.

Sometimes a discrete event can lead to more rapid deterioration, such as the loss of

water chemistry control.

In order for inspection to be effective, the inspection periodicity must be sufficiently

short in relation to the time between the deterioration becoming detectable and the

on-set of failure. Inspection techniques must be selected that are capable of

detecting the deterioration of concern at a sufficiently early stage with sufficient

reliability.

 

 RISK BASED INSPECTION

Tthe term ‘inspection’ refers to the planning, implementation and

evaluation of examinations to determine the physical and metallurgical condition of

equipment or a structure in terms of fitness-for-service. Examination methods

include visual surveys and the raft of NDT techniques designed to detect and size

wall thinning and defects, such as ultrasonic testing and radiography. Other

techniques might also include surface replication, material sampling and

dimensional control.

In-service inspection is most valuable where there is uncertainty about the operating

conditions, or their effect on the materials, particularly where the conditions are

such as to suggest that deterioration is taking place. Even when the service

conditions and effects are well understood, such as in high integrity plant,

inspection can provide continuing assurance of design assumptions and

manufacturing integrity. Inspection is also a priority for equipment where the

fabrication, inspection or operating history is unknown, where there is inadequate

maintenance, or where there is lack of the materials data required for assessing

fitness for service.

Risk based inspection involves the planning of an inspection on the basis of the

information obtained from a risk analysis of the equipment. The purpose of the risk

analysis is to identify the potential degradation mechanisms and threats to the

integrity of the equipment and to assess the consequences and risks of failure. The

inspection plan can then target the high risk equipment and be designed to detect

potential degradation before fitness-for-service could be threatened.

 

Sometimes the term risk informed inspection is used. This was first introduced by

the US Nuclear Regulatory Commission in order to emphasise the link but not a

direct correlation between risk and inspection. If risk based inspection is understood

to be inspection planned on the basis of information obtained about the risk, then

the two terms are synonymous.

Inspection provides new information about the condition of the equipment. This

may be better or worse or the same as previously estimated, but the effect is to

reduce the prior uncertainty. New information can therefore change the estimated

probability of failure.

An impending failure and its consequences are not prevented or changed by risk

based inspection unless additional mitigating actions are taken. Inspection is an

initiator for actions such as the repair or replacement of deteriorating equipment, or

a change to the operating conditions. By identifying potential problems, risk based

inspection increases the chances that mitigating actions will be taken, and thereby

reduces the frequency of failure.

 

In other words : risk-based inspection (RBI) is a risk assessment and management process that is focused on risks associated with failure modes initiated by material deterioration, and controlled primarily through equipment and structural inspection. As a risk-based approach, RBI provides an excellent means to evaluate the consequences and likelihood of component failure from specific degradation mechanisms and develop inspection approaches that will effectively reduce the associated risk of failure.

RBI combines risk assessment and risk management techniques with all inspection activities, such as planning, inspecting, documentation and data analysis, to develop inspection plans that direct inspections towards the areas of highest risk.

Considering the built-upon knowledge gained from baseline surveys and reassessments, the risk-based inspection plan is developed to monitor critical functions and structures of an offshore installation. It may include the following elements:

? Conduct hazard identification and assessment.

? Apply likelihood quantification (such as via structural reliability calculations) and consequence modeling techniques and related assumptions.

? Apply a variety of qualitative and quantitative risk assessment techniques and risk evaluation criteria.

? Develop an optimized inspection plan based on the above combination of risk analyses. The RBI program must address all deterioration mechanisms present in the structure or system to be covered by the RBI plan. Both the likelihood and the consequence of the damage are to be considered under all applicable operating conditions. The extent of damage must be anticipated under all operating conditions, including operational transients, severe weather conditions or other extreme loading.

                         

When implemented, RBI programs involve the actions to:

? Perform inspections defined in RBI plan.

? Perform inspection data review/evaluation.

? Identify any anomalous conditions.

? Develop and implement corrective action such as enhanced monitoring and initiation of repairs.

? Revise and update the RBI programs, as necessary based on knowledge gained during the execution of the RBI plan.   

For implementing RBI you can contact RINA Consulting.(www.rinaconsulting.org)

 

 SERVICES FOR PRODUCTION

1. O&M Manuals
2. Training Design, e-learning and 3D modeling
3. Maintenance and Inspection Engineering:
4. RBI (Risk Based Inspection)
5. RCM (Reliability Centered Maintenance)
6. SPIR (Spare Parts Interchangeability Records)
7. CMMS implementation & management
8. Condition Based Maintenance
9. RFTS - Right First Time Startup
10. Structural Health Monitoring Systems
11. Material/Mechanical handling studies
12. Corrosion management
13. Failure Analysis & Fitness for Service

 

Reference :

 

GABES-NAWARA PROJECT

Rina consulting was awarded by the engineering contractors for:

1. Fire and Explosion Risk Analysis (FERA), QRA and ALARP Demonstration
2. EERA (Evacaution Analysis) and ESSA (Emergency Systems Survivability)
3. Hazard Identification Workshops (HAZID, ENVID, HAZOP, etc.)
4. Bow-Tie analysis and Safety Critical Elements identification
5. FMECA and RAM Analysis
6. HSSE Case, Design HSSE philosophies and related design reviews
7. Safety Procedures for construction, Installation and commissioning
8. Ergonomic & Working Environment studies
9. EIA and Environmental engineering
10. F&G, Noise Study, Dropped Object and Safe Handling study, etc.
11. Operation Readiness & Assurance, Operating envelope Report
12. RBI and RCM
13. Technical Specification for SAP Activities and Templates Preparation
14. O&M Philosophy and Plans; Site Preservation Procedure
15. Spare Parts and Training Materials