Repair Option for API Storage Vessel Case Study

Problem

A contractor had incorrectly cut a door in the bottom course of a 500,000 barrel petroleum storage vessel. The door was too close to the longitudinal welds as specified in API 650 and 653. Code required the vessel be hydro tested, which would result in unscheduled costs and extend the vessel’s downtime.

Solution

As with ASME and ANSI, API codes often allow for properly executed detailed engineering analysis to supersede the letter of the code provided the original intent is observed. KHE developed a Finite Element Analysis (FEA analysis) model of the tank with a sub-model of the area of interest. The tank model incorporated settlement surveys and ultrasonic test data, then was subjected to the required design conditions specified in the Code (hurricane winds with the vessel empty and full.) The model’s response was transferred to the sub-model, which in turn showed the stresses and deflections in the area of interest. The results indicated the improper repair did not jeopardize the vessel’s reliability.

Results

The storage tank was put back into service on schedule. The analysis provided an estimated 25:1 return on investment compared to the direct cost of the hydro test.