Water Journal : Water Journal May 2015
water MAY 2015 34 Feature article To complicate matters further, the as-designed structural behaviour was found on many occasions to differ from that in- service. For example, while the original design intent was to have a “sliding wall base”, often the base joint had deteriorated to such an extent that a portion, and in a worst-case instance, the entire wall circumference, was now essentially “pinned”. This variance in support conditions resulted in wall stresses vastly different to as- designed, with potentially harmful results. This information, combined with the FEA, enabled a scenario analysis to be developed linking service life, structural capability, reservoir capacity and options for remediation. This in turn enabled a risk profile to be formulated with asset owner involvement, based on prioritising likelihood and consequence. The assessment of risk, although often subjective, follows key parameters and is prioritised dependent on the functions of: • Likelihood: How likely is it that the adverse outcome could happen? Categorised typically from “Rare” to “Almost Certain” • Consequence: How severe could the impact of each risk be on the owner? Categorised typically from “Negligible” to “Extreme” The risks associated with any potential failure of a reservoir are many and varied and may include: • Safety; • Financial; • Reputation; • Environmental; • Legal; • Business disruption. Residual risk and condition ratings were established using pre- defined and agreed criteria, combining hard engineering data with client liabilities. The clients now had a prudent and informed decision-making process regarding future reservoir usage. engineering remediaTion At Vinsi, often the first consulting engineering option considered is “do nothing”. This can often be a perfectly manageable strategy when appropriate operation management, maintenance and engineering nous are combined. In these cases, “do nothing” was not a viable solution. A number of process controls incorporating reservoir level monitoring and an inspection regime to observe bar failures were implemented to enable assets to remain on-line. This was critical in enabling the asset owners to continue to service their customers. Measures, including controlled induced failure, grout injection and structural strengthening were used in remediation works. Figures 2, 3 and 4 illustrate appropriate remediation measures. Figure 2 shows those bars that were considered in the “line of fire” to personnel when accessing the reservoir roof, or equipment were failed and removed in a documented, controlled procedure. Figure 3 shows existing vertical stressing bars, which were protected by grout injection. Introducing a passive film around the bar limits the rate of corrosion to the remaining sound bars and significantly reduces the risk of an unpredicted failure. These works were subject to a number of trials to resolve constructability issues prior to site mobilisation. As Figure 4 shows, areas where bars were failed, as well as areas defined through the analysis as being structurally inadequate due to altered site restraints, were typically strengthened through the use of carbon fibre laminates. This process is relatively simple, with quick turnaround times. ConClusion These case studies show that for many reservoirs of this type it is more likely to be a case of when, rather than if, a failure will occur. They also show that, using a co-operative partnership with asset owners, considered and proactive inspection and monitoring/repair regimes combined with sound engineering judgement, the risk of failure can be minimised to acceptable levels. Figure 2. Controlled induced failure of vertical bars. Figure 3. Grout injection to existing bars.
Water and CSG
Water Journal June 2015