In many industries, the storage of gases and liquids is an essential part of a company’s business. Storage facilities must withstand many adversities, such as mechanical and metallurgical damage, cracking, external and internal corrosion, and leaking valves and weld quality issues. All of these represent potential threats to an asset if they are not identified and mitigated at the right time. Additionally, storage facilities must adhere to codes and regulations – reason enough to identify rectifications to avoid any potential risks. In this case, we take a closer look at an inspection of a 19-year old storage tank that is part of a process plant in ROSEN’s Asia Pacific region.
Even though storage tanks frequently operate under less extreme conditions than other equipment in process plants, the types of damage mechanisms they can suffer throughout their service lives are manifold. Damage may include fatigue, brittle, chemical attack, erosion, differential settlement and, most importantly, corrosion. In fact, these often work simultaneously, making deterioration of storage tanks a complex and ever-varying issue. That is why tank bottoms often require extra measures for safe and compliant operation.
Since most storage tanks are fabricated using carbon steel, the leading damage mechanism affecting them is corrosion. Carbon steel is prone to reacting with the oxygen in its environment, thereby forming iron oxide – i.e. rust. The parts of a storage tank that are particularly susceptible to corrosion are the bottom and annular plates. In this case, water being the product stored is another factor that facilitates corrosion at the tank base.
CRITICALLY TIGHT SCHEDULE
Recently, a client commissioned ROSEN to inspect a storage tank with a diameter of 50 meters at a facility in Northern Malaysia during a scheduled maintenance shutdown of the entire plant. Since oil and gas terminals change hands often, information about the individual assets in a facility can get lost in such changeovers. In this specific case, the gathering of new and up-to-date data was especially important since the operator was missing the data set from the previous inspection of the tank.
Figure 1 - ROSEN was contracted to inspect a raw water tank with a diameter of 50 meters
The planned maintenance shutdown was scheduled for 168 hours. However, ROSEN was only allocated 24 hours of that timeframe to perform the entire inspection of the tank bottom of that storage tank due to the fact that the raw water tank is critical for the chemical fertilizing process. Additionally, the water tank had to be ready prior to the startup of the terminal, as it also serves as a reserve tank for fire emergencies.
BUMP IN THE ROAD
In order to ensure the integrity of the raw water tank bottom and annular plates, the ROSEN Group utilized their tank bottom inspection tool (TBIT), which uses a combination of magnetic flux leakage (MFL) technology to detect and size corrosion defects, and eddy current (EC) technology to determine the precise location of the defects. In addition, we proposed to perform a weldment vacuum test, as well.
When our experts were about to kick off the TBIT inspection based on the planned schedule, they faced the challenge of other maintenance work – internal blasting – not having been fully completed yet. Although the ROSEN tank bottom inspection tool requires only minimal surface preparation to perform a quality inspection, the client had the tank bottom surface grit blasted for other maintenance activities. As per ROSEN standard practice, the tank needed to be clear from any debris before it could be inspected further. Granite was used during the internal blasting, resulting in a dusty atmosphere. This situation caused a delay of more than twelve hours. Given the critical timeframe, our team was required to wear additional personal protective equipment, consisting of a half-mask respirator and a Tyvek jumpsuit.
The main strategy we used during the inspection was the mobilization of three teams. In order to meet the demands and stay within the given timeframe of 24 hours, ROSEN assigned three separate teams to inspect the bottom of the tank simultaneously, with one TBIT per team. We performed MFL scans of the tank bottom to quantify corrosion levels with minimal surface preparation. The use of eddy current technology allowed for discriminating between top-side and bottom-side features and enabled an inspection of underlying heating coils and pipes, creating a digital map of the tank bottom showing all corrosion features and dimensions, and a repair plan linked to thresholds (provided or calculated) for patch and repair plates. It was crucial to obtain data from the entire tank bottom, including difficult-to-reach areas – i.e. those near lap welds and critical zones for which using the ROSEN TBIT was an advantage – in order to obtain a reliable overview of the overall condition of the tank bottom, including any critical defects.
Figure 2 - Our findings included several severe features, such as seam weld corrosion
The client was provided with inspection findings quickly and now has reliable reference data regarding the storage water tank when planning repairs or inspection and maintenance steps. The report specified some unexpected severe findings the client had not anticipated during the major maintenance. Those findings included product side pittings scattered all over the tank floor at a total of 1,895 locations, which exceeds the tolerances stipulated in the applied inspection code (see Figure 3 for the scanning results of the tank bottom inspection). The high-quality data provided by ROSEN enabled the client to take countermeasures and necessary actions to ensure safe operation before starting up the terminal. Based on the comprehensive tank bottom assessment, ROSEN experts recommended to the client repairs in compliance with API 653.
Figure 3 - Floor map; bottom plate product side features (> 53% metal loss)
THE CUSTOMER BENEFIT
ROSEN was able to deploy sufficient qualified resources with high-quality tools to meet the client’s expectation for a timely delivery and a comprehensive report, bringing the tank back in service and thus avoiding any possible delays, which could have resulted in huge losses if they had prevented the client from meeting the planned schedule.
A HOLISTIC APPROACH
The complex nature of corrosion has compelled our inspection and integrity specialists to continually increase knowledge, build experience and advance technology in automatic corrosion detection and defect quantification. The earlier time-based threats such as corrosion can be identified and repaired, the longer an asset can safely remain in service. This benefits the industry in the areas of safety, environment and cost.
ROSEN launched a new service using high-resolution MFL sensors: TBIT Ultra. The new service provides enhanced accuracy and enables an extension of inspection intervals of storage tanks by finding, automatically quantifying and reporting defects from as small as 10% wall loss (ø 1 – 3 mm), even for pitting corrosion defects. Being able to identify, and if needed repair, defects from 10% wall loss tremendously extends the next inspection date (NID) in many cases, following the common industry standard API 653.