Inspection is required to determine the physical condition of a structure. Effective inspection programs play a significant role in minimising structural failures and extending the life of the structure. Grace Go, Richard Morgan and Frank Gatto from Aspec Engineering explain.
Industrial structures for mines, ports and heavy industry require significant capital investment in physical assets as a necessary part of their operation. Some facilities are well advanced in their design lives and the intention is to continue to use them in excess of this period. There is also a need for ongoing upgrades to these facilities to improve occupational health and safety standards and to improve productivity and throughput.
Operators and owners need to manage their assets effectively and should aim for an optimum risk profile which matches the risk for the business. Too high a risk can lead to unacceptable costs due to unexpected failures and shortened useful life. Too low a risk can mean high costs due to over-design and excessive maintenance.
Effective inspection programs can play a significant role in correctly assessing the risk level of structures, minimising the risk of structural failures, focusing on the required type of maintenance and extending the life of the structure.
Degradation will occur over time on industrial structures due to factors such as impact damage, corrosion and metal fatigue from repetitive and dynamic loading. These can lead to a reduction in structural reliability during an asset’s design life.
To understand the rate at which degradation is occurring, traceability is required so that a recording can be made at the same locations on the structure during subsequent inspections. A condition rating system is then utilised to track the level of degradation over time. As part of this, the authors have developed a classification system for different types of structural damage such as steel corrosion, protective coatings, structural welds, bolts, steel damage etc. This has been widely used in the industry for many years. The system uses a one to five scale for classifying damage ranging from pristine to unserviceable. The system allows for a consistent rating while being relatively easy to use and adaptable to electronic recording in the field.
Purpose of inspection system
An inspection system for industrial facilities should provide guidelines for setting up and undertaking a regular inspection programme. The purpose of this system is to allow:
• Recording of structural condition in a systematic and repeatable manner
• Provide a better understanding of the risks and criticality of various components
• Help provide a planned approach to remediation and maintenance.
Control against risks
Routine inspections are a control against specific risks.
The main types of structural integrity risks are:
• Corrosion which is mostly related to time and environment
• Fatigue damage which is related to the load and accumulation of load cycles which is closely correlated to throughput rates and cumulative tonnes
• Impact damage, which can occur at any time, and therefore requires a relatively short inspection interval
Risk is often not adequately considered in the inspection process, nor when it comes to prioritising repairs. Failure to consider risk may lead to higher inspection costs and funds not being allocated to the right areas or the wrong repair strategies being adopted. Many systems are based heavily on rating the physical appearance of a defect rather than the effect on which the defect has to the integrity of the structure. This may lead to misdirected expenditure or expenditure at the wrong time in the assets’ life cycle, leading to increased costs.
Condition rating system
In Aspec’s inspection system, structural conditions are recorded in a systematic and repeatable manner using a condition rating system.
The following criteria are used for steel to identify defects and quantify the current condition of the industrial structure components for assessing remaining life. Defects of a condition rating of four or above generally require some form of repairs or ongoing monitoring.
Protective Coating (PC)
Protective coatings are the first line of defence against corrosion which reduces both strength and fatigue resistance. Inspection and repair of paint coating is the main defence against premature failure of this type. Once the paint coating is damaged, corrosion of the underlying steel will occur.
This protective coating assessment is a visual inspection by the structural engineer or inspector and is not intended to replace routine inspection by a protective coating specialist.
Site housekeeping is important to prevent spillage and overloading of structures as well as injuries to site personnel. Areas of poor housekeeping where there is a build-up of material that remains moist for long periods can also cause corrosion. Inspection of housekeeping should include areas such as accessways, conveyor spillage trays and buildings. The state of housekeeping is often an indicator of how seriously the organisation is addressing structural integrity and safety.
The appropriate method of housekeeping assessment is a visual inspection by the structural engineer or inspector.
Steel corrosion (SC)
Corrosion is defined as the deterioration of materials through chemical or electrochemical attack. Industrial processes such as coal wash-plants, fertiliser production, and other industries which produce corrosive materials affect the rate of corrosion. Corrosion can be accelerated in sections which are highly stressed, and the rate of deterioration increases with time.
The appropriate method of steel corrosion assessment is a visual inspection by the structural engineer or inspector with thickness testing as required.
Steel damage (ST)
Steel sections can exhibit excessive deformation if damaged.
Dents, buckles or ‘out-of-straightness’ in beams and columns can reduce the structural strength significantly. Even though the section may still be standing, the risk of collapse (due to high winds, vehicle impacts, spillages) may be unacceptable.
Another form of steel damage is cracking. The main cause of steel section cracking is fatigue or fracture due to issues such as:
• Poor detailing/design
• Excessive loads
• Cyclic loads
• Poor workmanship
• Defective materials
• Changes in the loads
• Temperature effects
Some of the direct consequences of cracking include reduction in strength of the section, redistribution of load to other sections and failure of the section and excessive distortion of other sections.
The appropriate method of steel damage assessment is a visual inspection by the structural engineer or inspector followed by non-destructive testing (NDT) where appropriate.
Steel bolts (SB)
The main cause of damage to bolts is corrosion. This may be caused by temperature variations, wetting and drying of the surface, for example, areas in a splash zone, damp material build-up, that is, air and water near surface and dissimilar metals.
The direct consequences of bolt corrosion include:
• Excessive wear on non-aligned mechanical components
• Reduction in the load carrying capacity of joint and spreading of the load to other parts of the section and other parts of the structure
• Excessive distortion of the section
• Increased vibration
• Failure of the section and excessive distortion or failure of other parts of the structure
The appropriate method of assessment of structural bolts is a visual inspection by the structural engineer or inspector.
Steel welds (SW)
Welding and fabrication defects can greatly affect the performance and longevity of steel structures. Weld defects include geometric imperfections, cracking, inclusions, porosity, incomplete fusion/penetration etc.
The appropriate method of assessment of structural welds is a visual inspection by the structural engineer or inspector and NDT where appropriate.
This one to five scale condition rating system is compatible with the system used in the Infrastructure Report Card published by Engineers Australia in 2010 as shown in Table 1.
Overall level of degradation of assets
Once the inspection has been completed using the above condition rating system, the condition ratings can be aggregated to obtain a sitewide summary of the condition of the structures using the Engineers Australia system. An example is shown below.
Table 1: Condition rating descriptions (Engineers Australia)
Each structure is coloured to indicate its overall condition. Such a summary is beneficial to the owners to plan budget priorities.
A well-developed inspection system is essential to keep a record of the defects and identify where further investigation or repairs are required. Defects of a condition rating of four or above generally require some form of repairs or ongoing monitoring.
Through inspection, risks and criticality of components can be better understood and industrial structures can be efficiently maintained and their useful life extended.