Appropriate theoretical and process simulations may also be used, and subsequent iterations would lead to improving value analyses. Where no such detail is forthcoming, suitable best risk based estimates may be agreed, quantified, or if appropriate modeled. The feedback within the loop will ensure continual improvement at subsequent passes/cycles of the methodology. Alarm flags may be raised to define threshold values. The approach if applied correctly and in good faith will ensure best-latest-safe-knowledge usage, without dominance by any single group.
Failure Mechanisms: At first project contact the question of failure status and frequency must be addressed, see Figure 2. This is key to success, failure modes the end result of inadequate design, engineering performance, or low efficiency are defined by considering the combined effects of the operating environment, and circumstances upon each item, and the total integrated system. This is not easy, the corrosion related failure modes must include all possible mechanisms, 12 such as pitting, crevice, stress related - SCC/HIC/HE, corrosion fatigue, FAC/erosion, impingement/cavitation, CUI, dissimilar metal /galvanic attack, COMP-other compatibility issues, MIC, scaling/deposit cells, fretting/vibration, inter granular/selective phase, mechanical dominated (Hs, Rs, Ts, ?, etc) surface condition related (Ff, Rx, Cw, etc.) and uniform thinning and so on. A more complete listing of mechanisms and pertinent parameters characterized in the program are shown in Table 1. Modern applied techniques are now practically forensic in detail, and carefully characterizing such high risk parameters is often do-able within reasonable budget12.
The working loop resources would thus identify the most likely root cause of failure or damage, and assist suitable counter measures beyond basic corrosion allowances offered by design. This is a major challenge of persuasion, since the tendency is for designers to assume the corrosion allowance is the panacea. In reality this only solves the problem of general or uniform corrosion. The project managers, engineers and field inspectors must have full recognition of all likely corrosion phenomena through briefings or teach-in, thus ensuring appreciation, accuracy and reliability of actions. The loop will also help address any unique vulnerability identified, such as liquid metal embrittlement /caustic /sweet /sour cracking which may come into play at later stages. This would be done in accordance with pertinent standards, company specifications, and proprietary techniques per latest industrial experience accrued. Thus whilst zero failures may not strictly be possible, the approach can help eliminate the corrosion surprises, which are always a daunting prospect.
In many instances the failure mechanism is unknown/uncertain, or of a mixed nature, and not easily recognized. In such cases provided the source and site can be located (a formidable task itself) it is feasible for the strategy to specify a tactical anomaly management. Thus allowing very close monitoring of the feature perhaps using computer driven ultrasonic Cscan type mapping in order to pre-empt actual failure- the so called “just in time” approach.14 Other modern techniques such as B-scan, advanced thermal imaging and creative side stream or bypass/removable spool technologies may also be appropriate, often as screening tools used alongside daring rope access methods. The tie-in between predictive goal setting corrosion management, within reappraisal and extended verification stimulates innovation, maximizes data accuracy/repeatability/ auditability, and ensures credit and approval where due, and that recognition and responsibility, are not lost in the human equation.
Consequences, Hazards and Operability: The consequences of a corrosion related event causing failure are linked to hazards to personnel, environment, as well as the ability to operate the plant, i.e. reliability. This depends on the characteristics of the fluids released and the time-cost factors required to resolve and bring plant back on-stream. Numerical techniques rationalizing number, frequency and mean times before failure can also be successfully used to quantify such performance. The attention to the consequence detail is a powerful means of creating awareness, and much work is already in place regarding these parameters.7,11,12,15 The hazard that results from fluid release can be quantified per human proximity, duty, fluid inventory, flammability, presence of safety valves/ firewalls, and the location or impact of the failure on the production process or adjacent areas.
Following any breach of pressure containment the ability to continue safe plant workings may be affected, and limitations imposed can be quantified per extent of shutdown and time for repair. This parameter must by necessity be developed with close client cooperation, and is largely operator experience based. The essential risk parameters in the loop Figure 2, therefore comprise: Risk Analyses (steps1-3), Risk Based Inspection (steps 4&5), Risk Management (6-10), with the latter encompassing the subsets; risk based solutions and control (steps 9&10).
Envelope of operability - As a result, we can rationalize the fitness for purpose, by the bands of operability for identified critical parameters within the agreed risk driven interpretation. This may be defined on a system-by-system or component-by-component basis. In practice as plant ages these bands will be revised as new data come to light, again invoking close co-operation between RB-CIM exponents. In most cases some sort of extra condition monitoring including critical parameters such as pressure, temperature, velocity, corrosion potential, surface roughness/friction factor, pH, chlorides, sulfates, H2S, CO2 etc, will be necessitated. Thus quantifying the closeness of the operation to the design basis. Recall that the strategic aim is to maintain ongoing fitness for purpose, beyond basic code compliance. Hence the creation of an enabling methodology, which must use best available safe technology (BAST) 10 under focused knowledge management.
The risk based angle allows loop tweaking to keep costs manageable, however with the proviso of accepted downtime frequency. Otherwise the tendency to inspect all may take costly control. Thus the loop has evolved as a multi-disciplined engineering practice taking the best of marine/offshore experiences per corrosion against mechanical/metallurgical /risk/inspection and structural aspects. With knock- on effect of close synergy with appropriate NDE and accelerated laboratory tests etc. The essential steps per the loop will allow variations on a case by case basis, including overlay onto an existing system, but the core must retain design reappraisal, failure investigation, extended verification, and risk appreciation. For the RB-CIM to work however it must really be supported at the highest management level. Thereafter it can be looked upon to be self-driving, self improving, self policing, and self funding (the latter by way of enhancing production revenues).
The 10 step loop is practical, experience, and risk driven; with emphasis on safety and tendency towards being fail safe. Upon application the system enforces a stop and verify status at any time if safety were compromised in any way. Theoretical and probabilistic modeling may be referred but is not accepted as overriding. The heart of the assessment will be the plant walk round/ close visual and this will invoke updating drawings and inspection isometrics. All critical decisions will require checklist make up, review, and competent person sign off. This is sometime avoided under the unsupported guise of additional time-cost burdens; the loop process can and has, demonstrably made this a compelling overview rather than unnecessary detail which may hinder project schedules.
The program is an aiding dynamic, offering a live condition of plant at any instance, forming the backbone of the integrity decision making process. The approach allows the lessee full management capability if so desired, thus avoiding the hassles of software tie-in/upgrades/service agreements etc. The key is impartial, efficient, transparent, free flow of data to allow critical assessments, revisions, feedback, etc. However the use of an impartial third party project management would give solid edge and have advantages.
The team based group could be internal with external input or any combination thereof, however deemed appropriate. Ultimately if the plant runs at high reliability with few unscheduled shut downs, no surprise leaks, and no regulatory/safety /environmental issues, all players in the loop are into win-win. In principle the know-how generated by the process can be applied to any project whether Offshore, Marine, Pipeline or Industrial. The strategic level logic is essentially the same. The specific methods at tactical level will vary, depending on specific undertakings to assure integrity against corrosion threats. The aim always being to resolve production and shutdown conflicts within the framework of predefined performance objectives, with practical fit for purpose actions.
Performance Goals: General guidelines will be performance goal setting, and based on agreed safety critical items, however there may be instances where RB-CIM will gravitate towards a knowledge based prescriptive ruling; such as for example the selection of highly specific anodes for retrofit CP or structure survey locations. The actual level of success would be measured by any combination of predetermined objectives such as:
• Monitoring facility/unit efficiency and general condition
• Reduced downtime/increased productivity
• Reduced failures and frequencies
• Reduction in general and localized corrosion rates
• Reduction in perceived and calculated risk evaluations
Design Reappraisal and Extended Verification: The original and mainstay aspects of the RBCIM are the design reappraisal and extended verification, and the modern day prompt for this has been the critical need to maintain integrity under applications whereupon the margins for error are constrained. Up until recently most corrosionists would accept the given design landing on their desk without question, this barrier needs to be broken. If the majority of engineering problems can usually be related back to the drawing board, then it stands to reason that the first and most powerful tool in the process is the design reappraisal discipline for both new build and old build. Essentially third party verification type reviews have been ongoing for certification, classification, and good workmanship purposes for many years, but with limited predictive content in the context of corrosion/degradation phenomena. Such checking nearly always being against identified standards or codes alone. Extended verification is therefore pushing the envelope to the next level.
The loop logic is to focus the reasoning taking full cognizance of predictive through-life factors. The concept examines performance goals and prognoses, beyond minimum code including management of excursions beyond design envelope, and would draw solidly from historical risk oriented experience, and knowledge based extrapolations. Thus covering alert for known but often difficult to quantify problem areas such as waxing/hydration, sand content, stagnance, embrittlement etc often invoking consideration of the formerly secondary parameters such as Rx, Rs, ?, Ts, etc, which often play part in root cause failures at extreme design boundaries and beyond. And since the scope of design is often based on uncorroborated reservoir conditions, a means to give allowance for operations out with the design envelope must therefore be included for deepwater applications.
Timing and methods guidance. Design Reappraisal at the beginning and Extended Verification towards the end of the loop when latest corrosion and excursion profiling data are describable seems logical. But it is accepted that case specificity can override, and so the loop is flexible, and some re-arrangement regarding where and when to do these critical steps is possible. The extent of defining excursions is difficult and may be quite subjective, however a good basis can be found from previous in-house or cross operator experiences. The caveat being that the loop insists on all such profiles to be operator defined and agreed. The important point is not to ignore them, reappraisal as defined will ensure that the necessary attention albeit demanding, is given. Close adherence to the loop is expected to ensure that such decisions are ordered and self improving upon iteration. The asset-holder or lessee will recognize the advantages for maintaining integrity and performance. And as such since design documentation must exist by law in most jurisdictions, the intent is to review selective/available plans and not necessarily detailed P&ID's etc, for the risk based inspection monitoring. The added value is in the objectives to minimize or eliminate corrosion problems and not to censure or reprove the design, which seems on occasion to be a design house concern. The end result will invariably increase stature of the design, and help maintain critical integrity for production continuity. The campaign should be directed where possible at existing client procedures and specifications, and may thus be developed as a powerful superimposing rather than replacing predictive tool, aimed at giving all parties greater peace of mind for continued life cycle operation.
Where insufficient information exists records should be examined and system walk through used to generate "as-installed/existing drawings", at the same time an NDE orientated baseline survey must be carried out to augment the data available, and to facilitate comparative data. Specific regulatory requirements and commercial standards must in all cases be unambiguously defined with client participation in the review. It can be argued that most materials and corrosion related problems could be addressed using published data alone. However the advantages of reviewing company specific design/operations data via independent route, and engaging the company field engineers can be an order of magnitude greater; often giving the opportunity to identify events in a chain of actions which could be precursors to failure. Thus if we accept this then the only real hindrance to resolution is access and motivation. This is true knowledge management, and the application of RBCIM can provide the meaningful purveyor.
Personnel and Resources: All RB-CIM plays must buy into the defined scheme objectives. To effect this, some team cross training will be necessitated and the immediate challenge is to select either engineers for corrosion training or to pick corrosion specialists for suitable structures/mechanical/ process training. The answer may be critical since the results at the end of the day must guarantee true multi-disciplinary objectivity. This is an important issue since experience has shown time and again that resolution to problem areas is often governed by the specialism of the incumbent. Not always a healthy sign. It would seem logical that since corrosion is by far the largest component of integrity management then it stands to reason that the project would be best fielded by trained corrosion personnel. Generally each discrete step in the process will where appropriate be backed up with staff cognizant with company reports and relevant practices such as: API, ASME, NACE, BSI, etc. The findings issued can be used by the client to enhance any compliance requirements, verification, productivity, and indeed the commercial decision making process. This added value aspect also provides a tangible means to preserve people worth, promote innovation, and engage the better use of existing technology, the last of which is often cited as being a major weakness.
Typically the prioritization, inspection, monitoring, and control criteria defined within the strategy are implemented through the development of an acceptable schedule as work planning.12,15,17,18 These should be prepared via a written scheme of detailed corrosion/ inspection, and reflect inasmuch possible per existing company protocols to minimize costs. The inspection work scopes (reporting templates and equipment) having been compiled using the risk based corrosion threats identified, should also provide checklists and foreseeable apparatus requirements.12 A good inspection baseline will ensure sound and meaningful subsequent surveys, and so effort/resources expended at this stage will be made worthwhile. Additionally modern communication paths such as the internet offer opportunities to efficiently enhance data review almost instantaneously. Thus previously identified problems with time lag should be virtually eliminated.
Numerous attempts at inspection priority rankings have been successfully made over the years for structures, pressure plant, and pipelines each with merit.7,12,15,17 The method under discussion herein may use similar means to quantify integrity, the option would be left to the duty holder/lessee, and tend to evolve on a project specific basis. In this case at first pass, Figure 4, there will be outputs from each of the 10 steps, however we would expect more critical outputs after failure analyses, re-appraisal, and inspection prioritization (urgent, high, medium,low, inert - UHMLI) risk categorization, with subset risk codings also being feasible. The relative proportions of likely efforts are best exemplified in schematic form by Figure 5. Documenting this will provide invaluable company specific data for subsequent passes, and indeed future projects targeting both better predictive prevention, and less reactive cures.