Case Study: Mechanical Integrity and Engineering Review of a Partially Submerged FRP Vessel

January 28, 2021

A major Canadian manufacturer of steel products asked UTComp to investigate the condition of an FRP tank used to store spent hydrochloric acid (HCl). The client needed to know whether the straight-sided cylinder made of a vinyl ester / isophthalic polyester composite could be relocated and repurposed at the same facility. In October 2020, the UTComp team inspected the tank using UltraAnalytix® to report on its mechanical integrity and followed up with a detailed engineering review to assess the tank’s suitability for future uses intended by the client.


Fiber-reinforced polymers (FRP) have been used for decades to make storage vessels and process tanks for a variety of industries around the world. Lightweight, strong, corrosion-resistant and less expensive to buy and maintain than metal alloy tanks, FRP tanks are manufactured in a wide assortment of shapes, sizes and storage capacities. Their properties make FRP tanks and vessels a preferred choice for the storage, transport and treatment of acids, alkalis, salts, oils and solvents.

Steel making involves a number of chemical and physical processes designed to remove impurities from iron and add substances such as manganese, nickel, tungsten, and chromium to produce the alloys desired for specific uses. Hydrochloric acid (HCl) is commonly used in pickling, a step in the steel finishing process that uses an acid solution to clean rust and scale from the surface of strip steel, wire and other forms of steel. Pickling weakens the acid over time; once the HCl solution is no longer effective, the spent acid is drained or pumped to a storage tank, where it is held for regeneration or treatment as hazardous waste.

Because of their high strength-to-weight ratios, durability and corrosion-resistance, FRP tanks have been used successfully for many years to safely store HCl. Composites made with vinyl ester and isophthalic polyester resins provide the chemical resistance, strength and flexibility required to safely store corrosive agents like HCl and withstand the environmental loads and variable pressures that can result from filling and emptying the tank.

In order to comply with environmental regulations, tanks used to store HCl and other hazardous chemicals often require secondary containment systems that collect spills or leaks and prevent harmful substances from entering the environment. The HCl tank evaluated in this case study was located outdoors, with the lower portion of the tank below grade in a concrete-walled containment pit. The client was considering repurposing the tank and raising it to ground level, where it would be exposed to additional stresses from wind, rain and snow as well as new seismic loads. However, the tank had been partially submerged in approximately 1.5 metres of water from surface runoff. This raised additional questions about whether the tank had been damaged by the water and could withstand being lifted to a new concrete pad at ground level.

UTComp was  asked to perform a non-destructive analysis of the tank using UltraAnalytix® to assess its condition and determine its suitability for the change in service.

Project Summary

The UTComp engineering team inspected a vinyl ester/isophthalic straight-side tank used to store spent hydrochloric acid (HCl).

In October 2020, the UTComp engineering team inspected the 18-year-old vinyl ester / isophthalic straight-side tank, located at a steel manufacturing facility in Hamilton, Ontario, Canada. The tank measured approximately 8.9 metres tall by 4.1 metres in diameter, with its lower third portion housed below grade in a concrete containment pit. Because the tank had been partially submerged for an unknown period of time, it was essential to determine whether the water affected the strength and condition of the FRP laminate. In particular, the inspection focused on the FRP band wrapped around the base of the tank and over 12 steel C-shaped hold-down lugs, the corrosion barrier and the condition and bonding of reinforcement pads (repads) at key locations.

Mechanical Integrity

The team conducted internal and external visual inspections of the empty tank to identify surface defects and damage, paying particular attention to the bottom 5 feet (1.5 metres) of the tank that had been underwater.

  • External: looking for any flaws such as cracks and blisters that penetrate into the structural layers of the FRP
  • Internal: examining the lower sections of the shell wall, tank floor and penetrating nozzles for cracks, scratches and abrasions.

The inspection included the FRP band wrapped around the base of the tank and over the hold-down anchor lugs.

The interior inspection found damage to the corrosion barrier, including a 3-inch crack in the centre of the floor.

UltraAnalytix is a scientifically proven, non-destructive method for measuring the thickness of FRP and calculating the loss of strength and stiffness over time as a percentage of its design stiffness (PDS).

UltraAnalytix readings were taken above and below the water line on the shell, the tank floor, repads, and the FRP overwind at the base. Additional measurements of the hold-down lugs and lifting lugs were also taken.

Data from the inspection was used to inform the detailed engineering review and stress analysis.

Engineering review

A UTComp Engineering Review is conducted as part of a detailed Suitability for Service analysis for in-service assets. Because changes in service often occur during a storage vessel’s lifetime that may damage or weaken FRP, UTComp’s standard practice is to recommend an Engineering Review at 75% of service life. The review includes a detailed assessment of the equipment and may also be recommended where design standards of the vessel are unknown or deficiencies in the reinforcing pads are found.

In addition to UltraAnalytix inspection results, a UTComp engineering review includes detailed information such as specifications and drawings provided by the client about the vessel including:

  • Size of the vessel
  • Required operating pressures and temperatures
  • The fluids or substances to be contained by the vessel
  • Codes and standards that must be met by operating equipment
  • Environmental loads for the installation – wind, rain, snow, earthquake, explosion, etc.
  • Owner-specified limits on safety factors or allowable stresses.

Key findings, discussion & analysis

During the exterior visual inspection, the team observed some rusting and chipped paint on the hold-down anchors. Plus, some wrinkling and surface water damage was evident on the lower section of the outer shell that had been submerged. Some unused level gauge support brackets attached to the side of the tank were loose to the touch and in need of minor repairs.

The interior inspection found damage to the corrosion barrier including a three-inch crack, approximately the width of a knife blade, in the centre of the floor that must be repaired.

This tank is vented at the top, but does not have an overflow. If the liquid level in the tank exceeds the tangent line (the point where the curved head meets the straight wall of the cylinder), the hydrostatic pressure will act vertically upwards on the dome, causing an upward force at the tank bottom and on the hold-down lugs. This raised questions about the risk of tank failure at the bottom. Therefore the inspector recommended that active liquid level controls be implemented or verified to ensure that the liquid level does not exceed the upper head tangent line.

Vessel thickness and PDS

UltraAnalytix measurements were taken from two sections of the vessel and three reinforcement pads. Table 1 below shows the results obtained from UltraAnalytix readings of the shell and floor of the tank.

Table 1

Reinforcement pad condition and bonding

Repads are used to repair or reinforce vessels by increasing FRP thickness and strength in locations that must support additional stresses or loads, such as the the manway/hatch area or around penetrating nozzles. Repads less than 6 mm thick are considered inadequate and frequently identified as the root cause of failures.

The quality of the bond between repads and the FRP vessel is also critical, and is calculated as the Percentage of Theoretical Bonding. It has been estimated that secondary bonds, such as those that attach repads to a tank, may have as little as 50% of their theoretical sheer strength. Therefore, a reinforcement is considered bonded when UltraAnalytix data show a percentage of theoretical bonding greater than 50%. But if readings show two or more consecutive points or more than 25% of the points are not bonded (i.e. below the 50% threshold), immediate repairs would be recommended.

UltraAnalytix data collected from repads at two nozzle locations and the manway showed all were at least 6 mm thick and none were below the 50% bonding threshold.

However, UltraAnalytix results for the 12 anchor lugs indicated poor bonding between the FRP overwind and five of the lugs. But even though they are not bonded, the hold-down lugs are designed with a steel lip at the top that will prevent them from pulling out in the event of an uplift.

In the figure below, the red line illustrates the various data points above and below the bonding threshold of 50%. Any points below the threshold are considered unbonded.

Figure 1: Repad bonding of 12 anchor lugs

The table below summarizes results for the 12 anchors.

Table 2: Repad bonding of 12 anchor lugs

Engineering review

This project required a detailed engineering review to assess the current condition of the HCl storage tank and determine whether it is suitable to be moved from its below-grade location and use to a ground-level concrete pad and new service. The review involved using UltraAnalytix data and the client’s original drawings and specifications for the equipment to calculate the current factor of safety and percentage of design stiffness (PDS). The anchoring brackets were analyzed on the basis of strain at the FRP band overlapping the C-shaped lugs.

The data were analyzed to determine if the tank could withstand the dead load, live load, snow/rain and wind loads, and seismic loads according to the 2015 National Building Code of Canada (NBCC). The engineering review also determined the future intended Operating Strain and Maximum Case Strain under the building code.

Techniques for lifting, moving, and anchoring tank were outside the scope of this engineering review.

Key findings

The information below summarizes the UTComp system data used in the calculations:

Table 3: FRP thickness and PDS

  • Critical PDS: 40%
  • Current Minimum PDS: 65%
  • Current Tank Knuckle PDS: 67%

Vessel Operating Pressure Limits: atmospheric

  • Internal pressure: 0 kPa
  • External pressure: 0 kPa

Location and Environmental Loads: Hamilton, Ontario, NBCC 2015

The following values were calculated:

  • Vessel Operating Safety Factor at Full Capacity: 8.238
  • Current Operating Strain: for expected operating conditions*
  • Current Maximum Strain: for environmental load with load factors per NBCC 2015*

Table 4: Environmental loads

Trilam results:

Trilam software for laminate analysis was used to calculate theoretical as-new material parameters for the vessel, assuming wind angle of 15 degrees from horizontal.

Table 5: Theoretical as-new modulus values

Conclusions and recommendations

After analyzing results of the inspection, UltraAnalytix data and design information provided by the client, the UTComp team determined the HCl tank to be suitable for service. Despite concerns about possible water damage, the inspection detected no defects or damage to the tank’s exterior or support structures or pipe and nozzle connections.

However, some unused level gauge support brackets on the tank’s exterior had loose or damaged connections, and damage to the interior floor was detected.

Because the tank had been designated for a change in service, UTComp did not estimate its remaining service life. Instead, data collected during this evaluation will act as the new baseline for the asset — the new 100 per cent. UTComp recommends inspection of FRP assets every three years; the team will predict the tank’s remaining service life following the next inspection.


UltraAnalytix evaluation is recommended after all significant process excursions or environmental events. UTComp recommends that a competent engineer be engaged for recommended engineering activities including replacement, review, design and inspection of repairs.