TANK SETTLEMENT AND ROUNDNESS SURVEYS  (API 653 B)
 
 

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Petrospection is accredited by the National Association of Testing Authorities, Australia (NATA) as a metrology laboratory to perform vertical tank settlement surveys and assessments to API 653, Appendix B and similar specifications and to perform roundness surveys to in-house procedures, ISO 7507 and similar specifications

Petrospection have developed the capability to perform settlement and roundness surveys by expansion of our tank calibration services and we are the only company in Australia providing the service with NATA endorsement. 

Assessment and monitoring of tank settlement has gradually gained greater importance in tank maintenance programmes and is now a routine component of most ten yearly inspections, due to its inclusion in the API Standard 653 assessment requirements. Terminal operators are realising that the stress analysis associated with a settlement survey is as important information as that provided by an NDT survey in determining a tank's suitability for service. Decisions about expensive tank repairs, particularly floor replacement and foundation repairs, may hinge on the measurements taken during a settlement survey.  

It is likely that there are tank operators in Australia with tanks in service that fail to meet the API653B standard, due mainly to the lack of providers of a comprehensive survey and processing service such as offered by Petrospection.

Petrospection have developed software in consultation with industry to produce a report containing the graphical representations and calculations required for convenient assessment of tank shell, floor edge and localised floor subsidence against API 653 Appendix B criteria.

The API 653B Standard considers tank bottom settlement to be the result of either one or a combination of the following three settlement components:

1. Uniform settlement

2. Rigid body tilting of a tank (planar tilt)

3. Out-of-plane settlement (differential settlement)

In the case of uniform settlement, there is normally little to concern the tank operator. As mentioned in the Standard, even where individual tanks settle differentially, the only problems are usually associated with pipeline alignment or drainage.

The API 653 Appendix B Standard provides recommendations on allowable limits for the following forms of differential settlement: -

Due to either planar or out of plane tilt, there may be associated problems of over filling; jamming of floating roofs because of ovality of the tank shell (identified by a tank roundness survey); or rupture of the shell or bottom plates from excessive stress in the tank shell and/or floor (identified by the tank settlement survey).

The causes of any of these manifestations of settlement have to do with compression or movement of the tank foundation or supporting ground, and may involve such factors as non homogenous fill, changes in ground water level and others.

The Petrospection report contains a tank shell roundness and verticality section, which is particularly useful for assessing floating roof/blanket seal clearances and for associating floor subsidence with affects to the shell.

All measurement data is provided along with the graphical representations and a schematic drawing for orientation of the measurement points.

A typical report will contain the following pages and summary reports: -

Additional pages containing detailed analysis will be included where necessary.

Other guidelines and standards are usually referred to for comparison in circumstances where our analysis identifies non compliance with the API 653B standard.

Petrospection can also provide a Tank Floor Subsidence Report, which utilises the data from two tank settlement surveys to produce a report of the subsidence or elevation of the floor in the intervening period.