A petroleum refinery, located along the Gulf Coast, has a massive 50 year-old conventionally reinforced concrete structure that supports a critical crude unit vacuum tower. Supporting well over 600,000 lbs of process equipment weight while in service, the embedded metal in the eight column support structure began corroding which caused extensive deterioration of the support columns and beams. The distress was significant enough to cause a reduction in operation serviceability.
Recognizing the need to address the conditions of this critical support structure, STRUCTURAL TECHNOLOGIES’ forensic investigation team was contracted to perform investigation services that included a comprehensive condition assessment and structural analysis. Through evaluation, a root-cause analysis report was compiled to identify the source of deterioration and provide solution recommendations that would restore the support structure to as-designed serviceability.
Especially significant in 1960s construction in the US was the practice of including calcium chloride ad mixtures into the concrete to accelerate the rate of strength gain. Thought to lower the cost of the product, this practice was well-intentioned but ultimately introduced aggressive electrolyte into the concrete that exacerbated corrosion of the reinforcing steel. This combined with the close proximity to the coastline, resulted in heavy misting and fog affecting the integrity of the structure.
Through conducting a series of non-destructive and semi-destructive tests, existing concrete materials were determined to be carbonated (pH <9.8), delaminated, and contaminated with levels of chlorides 2 to 5 times the threshold value established by the American Concrete Institute. This finite element analysis lead to the development of an innovative hybrid repair program that would both lower the repair construction costs, as well as decrease project duration. The program included active shoring and support of existing process equipment, as well LifeJacket® galvanic cathodic protection systems for the column and beam repairs.
STRUCTURAL, also a Structural Group company, was then brought on to ensure constructability and execute the repairs.
Discovering Additional Challenges during Construction
During the repair of the columns and beams, STRUCTURAL discovered corrosion-under-insulation involving the vessel skirt base ring and anchor bolts. A strategic repair approach was developed to address the required repairs without impacting the schedule. Repairs were made to the vessel skirt that included strengthening of the vessel skirt ring base, base ring stiffeners, anchor bolts and anchor bolt chairs. Due to the severity of the corrosion, a more substantial shoring support structure was designed and installed. This enhanced support structure contacted additional process equipment members while the vacuum tower was in operation.
Delivering a Solution
The successful investigate-design-build services provided by STRUCTURAL TECHNOLOGIES and STRUCTURAL lead to an innovative and proven repair concept integrated from other industries that was less expensive and allowed for a shorter project timeline. Initially designed to be performed during a short duration outage, the repair design was flexible enough to be undertaken on-line as ultimately required by the owner.