By Shireesh Swami / B. Srinivasa Rao
One of the oil public sector units in India decided to convert their existing Motor Spirit (MS) and Kerosene (Kero) storage tanks from existing external floating roof (EFR) to internal floating roof (IFR) in 34 oil depots located across India. A total of 109 tanks storing MS and Kero have been identified and a detailed analysis was carried out for suitability of this roof conversion.
The reasons for this conversion of tanks from EFR to IFR were;
Since the tanks were installed at various timelines and additional roof and roof supporting structure weight was to be imposed, it was decided to ascertain the health and stability of these storage tanks. Teams were identified to carry out the dimensional and design data and the Ultrasonic Testing (UT) of the storage tanks (in service Inspection). Necessary check lists were prepared for collecting required data for carrying out further design and analysis. UT was carried under the supervision of qualified inspectors.
After collecting the required data including data on corrosion rate obtained from UT report, the tank shell design was carried out in accordance with API 653 and 650 to ascertain the stability of the tanks under design loads as well as additional loads due to roof plate and roof supporting structure. The roof supporting structure design was carried out for each of the storage tanks.
The internal floating designs were verified based on Annexure H of API 650 for the additional design loadings and wherever the floating roof design (Buoyancy) was not found within acceptable limits modifications have been suggested.
Analysis & Recommendations
The rate of corrosion per year was computed based on the UT report. Based on the current thickness and latest API 650 code the tank shells were re-designed and verified for the additional design loads. The shell buckling and the local stresses at roof support junctions on shell have been calculated as per the design code. Wherever the existing thicknesses found not acceptable, the modifications needed were communicated.
As a value added solution, it was also suggested to go for Geodesic Aluminium dome roof in order to retain the storage volume of the tanks which with conventional cone roof would have eventually reduced to the extent of depth of roof support structure.
Modifications were suggested for floating roofs where the induced stresses have gone beyond the acceptable limits. A comprehensive report with suggested modifications was prepared and submitted for each of the tanks. The specifications were prepared for carrying out these modifications was issued for implementation.
RLA Studies For Staic Equipment
TCE had adequate skill sets to perform the remaining life assessment (RLA) studies of the installed equipment. The mechanical integrity check of the equipment due to change in design parameters is another service offering. TCE also undertakes in service and out of service inspections as per API-653 and API 510 for storage tanks and pressure vessels respectively. Following are some of the other examples where TCE has carried out RLA studies for the existing tanks in the recent past.
TCE is having in house capability for carrying out performing required inspection and testing to ascertain the rate of corrosion, performing RLA studies and recommending the modifications to be carried out so that the asset can be continued to be in intended service.