Every refinery, chemical, power and process facility throughout the world has a significant amount of infrastructure supporting process equipment, vessels and pipelines. Although not generating revenue in a general sense, without the load-bearing ability and the protection afforded by this infrastructure, plant processes and production would be impeded, if not halted, if interruptions occurred in the process stream. Besides the catastrophic consequences associated with manmade (i.e., fire and explosions) as well natural (i.e., tornadoes and hurricanes) disasters, long-term degradation of the infrastructure should be anticipated and scheduled maintenance planned well in advance to keep costs low and infrastructure dependability high.
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| Figure 1. Pipe rack steel beam corrosion evaluation subsequent to cementitious fireproofing removal. Corrosion often remains unseen under insulating materials. By the time the cover is displaced, the corrosion damage can be advanced and costly to repair. |
Various structural systems exist within a petrochemical facility with the most common being those structures constructed of conventionally-reinforced concrete and/or fireproofed structural steel. These structures typically provide low maintenance service and rated protection during a fire event. From an economical standpoint, structural steel compares favorably to reinforced concrete members and can be easily modified and/or strengthened at a later date should process upgrades or additional imposed loads be applied to the structure. However, long-term durability to environmental service conditions can be compromised by intimate contact with atmospheric contaminants and moisture, resulting in embedded metal corrosion of either reinforcing steel bars or structural steel embedded in cementitious fireproofing.
Essentially, concrete is a "hard sponge" with a network of small conduits or capillaries allowing directional passage of water from interior to exterior regions to cool hot contact surfaces, as during a fire event. However, these capillaries also allow external atmospheric gases and moisture to penetrate the concrete. Over years of service, these environmental conditions chemically change concrete to a point where the inherent alkalinity of the concrete materials no longer provide the passivity i.e. protection, that it once afforded the embedded steel. Unseen deterioration in the form of embedded metal corrosion can be hidden from view and not readily visible. In some cases, by the time evidence of corrosion activity surfaces, the underlying damage to the structure is advanced and very costly to repair.
During embedded steel corrosion activities, the steel metallurgy changes, with corrosion products requiring and occupying more space than the parent material. As such, significant tensile stresses are exerted on the concrete in the immediate proximity of the corroding steel member. Although inherently strong in compression concrete is relatively weak in tension. Therefore, unrestrained portions of the concrete mass (i.e., protective concrete cover overtop of the steel embedment) will crack at the corroding member interface. The progression from crack-to-delamination-to-open spall provides more access to external contaminants and moisture accelerating the processes of corrosion. Whether embedded steel bars in conventionally reinforced concrete or clips and welded-wire mesh within fireproofing concrete placed overtop structural steel members, these embedments are subject to corrosion and can initiate concrete cracking during the ongoing corrosion activity.
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| Figure 2. Corrosion of the reinforcing steel in concrete structures results in damage to the concrete and could result in diminishing of the structural capacity. The concrete in piles, such as the one shown here, draws moisture up from the immersion zone. Where it evaporates, above the tidal area, it leaves behind a high concentration of salts resulting in accelerated corrosion of the reinforcement. |
When industrial facility owners/operators wish to assess the state of their infrastructure, innovative approaches of detection, documentation and organization of collected field data need to be employed. Monetary requests for maintenance funding require significant "back-up" data to validate the true need for repair of these fixed assets. Many times during the data collection process, however, even more serious structural conditions are revealed that could directly impact the process and/or plant safety (e.g. buckling, racking or torsional failure of structural components), requiring immediate action. In the United States, many of the industrial facilities are approaching the century-mark in age and without a regular inspection/maintenance program, owner/operators can expect significant expenditures based on hidden defects detected during infrastructure condition assessments.
Getting the Funds
Many tools have been developed over the years to assist owner/operators with maintenance funding allocation, the most successful and practical application of which is likely the LQQ method. The LQQ assessment method consists of an evaluator who first locates (L) the areas of distress within a member or structure. Next, the distress is qualified (Q) following industry guidelines and then prioritized according to safety, structural integrity and aesthetics. Finally, distressed areas are quantified (Q) for repair construction cost-estimating purposes. This important process provides an initial baseline of existing conditions at the time of inspection. As a result, Condition Survey Documents are developed with corresponding notations as to the Distress Priority. It has to be recognized that with time, distress advances and over extended periods, Distress Priority designations will change to reflect additional deterioration. Owners/operators have found Condition Survey Documentation to be a useful tool when applying for maintenance funding. Additionally, Condition Survey Documentation provides a tangible work-product for repair construction cost-estimating purposes. Estimated costs, based on accurate documentation, provide owners/operators with a clear picture of their infrastructure needs so they can develop factually based maintenance budgets.
As with fixed equipment, many owner/operators have turned to Risk Based Inspection (RBI) Programs following industry best practice guidelines, which provide a "tool" to determine their infrastructure needs and provide a systematic approach to addressing inconsistencies and below-standard conditions. One such approach that has been adopted by industry is the Plant Condition Management System (PCMS). The PCMS allows owners/operators to assess their infrastructure by unit or plant-wide. Typically, this evaluative tool employs Risk-Based Inspection Techniques and effectively identifies:
Type of Construction, Item
Environment
Other Design and Operating Conditions
Deterioration/Damage Description, Characterization
Possible Causes
Condition Assessment
Safeguards
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| Figure 3. Brazing wire to establish continuity prior to Lifejacket® installation. Galvanic cathodic protection requires steel in the concrete to be electronically continuous. The structure is tested for electrical continuity and corrections are made as necessary by brazing, welding or establishing mechanical connections between the bars. |
Advances in non-destructive corrosion tests of concrete embedded metallic structures now permit the assessment of fire-proofed steel and steel reinforced concrete components before damage is apparent. These techniques include electrochemical potential mapping, corrosion rate analysis and concrete and cementitious fireproofing resistivity measurements. When used in combination with exploratory excavations, laboratory quantification of contaminants in the concrete (i.e. chlorides, sulfates) and a thorough inspection of the structures, as detailed above, these techniques can have a significant impact in the timing and prioritizing of repairs and rehabilitation schemes. In addition, owners/operators can use this information to develop degradation mitigation strategies, such as cathodic protection, well in advance of damage requiring costly repairs and unscheduled outages.
As seen above, a comprehensive understanding of deterioration and its various manifestations can assist plant maintenance departments in developing effective maintenance budgets for process support infrastructure. In many cases, this information is critical for continued operation of a facility. Unexpected business challenges continue to arise in this industry. However, with evaluative tools now available, owners/operators can at least determine their infrastructure needs to a reasonable level of certainty without sacrificing safety.
Case History
Case in point is an effluent intake bridge structure spanning a raw water inlet from the Gulf of Mexico for a major petrochemical manufacturer in the southwest region of the United States. The structure, constructed of reinforced concrete with a cast-in-place deck slab founded on a series of integrally placed "pile cap beams" that were then founded on 65 precast/prestressed reinforced concrete driven piles, was constructed in the mid-1950s. Approximately 300-feet long and 22-feet wide, visual observations made by plant personnel revealed extensive deterioration in the form of cracks, delaminations, open spalls and corroding reinforcing steel bars. In order to adequately define the condition of the structure globally, a Condition Assessment of the bridge structure was initiated that employed a tactile investigation. The investigation included Non-Destructive (NDT) and Semi-Destructive Testing (SDT) techniques in an attempt to collect enough data to adequately characterize the structure from a materials standpoint, both chemically and physically, as well as from a structural standpoint.
The test results showed that a chloride-laden environment conducive to embedded metal corrosion existed within evaluated concrete members and had resulted in large areas of concrete distress of varying severity. The source of chlorides was determined to be from brackish water contact and the severity of the environmental exposure conditions diminished with elevation and distance from the chloride source. Deterioration ranged from several inches to full-section structural losses in pile members. Based on the results of the investigation, a total of 40 of the 65 piles required varying amounts of repair. Additionally, the bridge structure was immediately barricaded to all plant pedestrian and vehicular access until a repair program was implemented.
Repair Scenarios
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| Figure 4. Lifejacket® is delivered to the jobsite preassembled in two halves. Here, the zinc anode and mesh can be seen installed on the interior of the stay behind fiberglass form. Also, the pumping port seen here is used to fill the annulus between the Lifejacket and the concrete pile with an approved mortar. |
The immediate need for the rehabilitation of the structure was indeed structural. However, it was noted that the source of the damage was corrosion of the reinforcing steel within the piles. As such, the repair schemes included evaluation of structural repairs and mitigation of continuing and future corrosion activity. The three repair options thoroughly explored were:
Conventional repairs with localized galvanic anodes
Fiberglass jacket with Cathodic Protection System Repairs (i.e., Lifejacket®)
A hybrid combination of conventional repairs with localized galvanic anode and fiberglass jacket with galvanic Cathodic Protection System
Embedded galvanic anodes were proposed as a means for controlling corrosion in areas surrounding new cementitious patches. However, this approach was deemed inadequate for areas more than a few inches away from the patch and insufficient for overall cathodic protection of the structure. In contrast, the Lifejacket® system is a proprietary technology that encompasses repair and protection strategies in one single installation procedure. This system incorporates a fiberglass external jacket with an integral anode mesh affixed to the inside of the jacket. The jacket is supplied in two half sections, which are assembled together on the prepared pile. Once installed, the jacket is left permanently on the pile functioning as a permanent form and as galvanic (sacrificial) corrosion protection of the entire concrete pile.
For this application, the hybrid system that was evaluated consisted of installing embedded galvanic anodes as above for those piles that exhibited small, localized damage, and using a reduced number of Lifejackets® for those piles exhibiting larger areas of damage requiring form and place techniques. This approach, as well as the first option of using embedded galvanic anodes, was abandoned, because it does not guarantee a long life expectancy for the corrosion mitigation strategy.
One of the biggest advantages in using the Lifejacket® system is that it restores concrete section loss and provides structural strengthening. Also, there is no need for wiring and complex conduit systems. The system operates maintenance-free over its design life with no additional utility bills, consultant fees or reapplication costs. Sacrificial cathodic protection offers a noticeable advantage for structures where routine monitoring and maintenance is impractical, the structure is difficult to access, or there may be other restrictions such as unavailability of electric power. Also, owners/operators can dedicate plant staff resources to other important maintenance activities without adding to their already stretched capacity.
Based on a close review of the various repair scenarios, the "Lifejacket®" option was selected by the owner to structurally repair the concrete piles and provide extended protection against the aggressive brackish water service environment. The repairs were staged once the bridge was shored and the inlet was bulkheaded to control tidal water level fluctuations. Additionally, the underside of the bridge was fully scaffolded to eliminate the need for divers during repairs.
Successful Repair
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| Figure 5. The Lifejacket® system incorporates repairs and corrosion protection in one operation. Here, a completed installation is shown in the tidal area. |
Over a period of several weeks in 2001, the 40 Lifejackets® were installed in accordance with manufacturer's recommendations. The owner will benefit from selecting the Lifejacket® system as it will provide a "stay-in-place form" that accommodates concrete repair material placement. Further, it provides the owner with the protection afforded by the integral Galvanic Cathodic Protection System. The lessons learned through this case study are a good reminder to all refinery, chemical, power and process facility owners and operators about the importance of scheduled maintenance. The state of the technology today allows qualified and experienced personnel to conduct evaluations that can help optimize and prioritize maintenance and repairs throughout their facilities. In addition, these activities can shift maintenance from a reactive and costly process to a truly preventive activity. Experienced professional firms now have powerful toolboxes at their disposal including evaluation tools, advanced testing techniques and a variety of countermeasures to help owners/operators achieve optimum availability from their facilities and operate at maximum efficiency. By contacting and working with a qualified repair contractor, facility owners/operators may avoid costly, long-term repair projects and shutdowns through regular inspections performed as part of the Plant Condition Management System. With the evaluative tools and processes now available, owners/operators can more easily determine their infrastructure needs in order to decrease down-time as well as increase safety.