The use of floor scanning techniques for the examination of storage tanks

Version No:

1

OG Status:

Fully Open

Author Section:

HID CI1G, HID CI5B

Issue Date:

09 June 2009

Review Date:

09 June 2012

Target audience:

HID Discipline Specialists (Mechanical Engineering), HID Regulatory Specialists, FOD Specialist Group (Mechanical Engineering) 

Purpose

This SPC draws inspectors’ attention to a contract research report (RR481¹) on the use of floor scanning for the examination of atmospheric storage tank floors. It also uses the results of consultation with industry on this subject to give HSE’s opinion on good practice.

Background

Floor scanning techniques have been around for a number of years. A number of operators routinely use them, but they have not been universally adopted. Consequently, while floor scanning techniques are referred to in a number of guidance documents ^{2, 3, 4, 5,} there is no accepted norm for when they should be used.

Many inspection regimes for tanks bases rely only on internal visual inspection, and/or spot ultrasonic thickness measurements. In such situations, the internal visual inspection will not detect deterioration of the underside of the tank base, and spot ultrasonic thickness measurements will not provide a comprehensive picture of the tank base condition.  

A number of failures of tank bases have occurred unexpectedly. HSE and industry believe that some of these could have been prevented by the use of more modern inspection techniques for examining the tank floors. An example is given in Safety alert: rupture of an (atmospheric) crude oil storage tank [PDF 290KB]

Application of floor scanning techniques

In this circular, the term floor scanning is used to refer to those techniques which will give an indication of floor condition across a large proportion of the storage tank floor. It therefore includes magnetic flux leakage (MFL) and saturation low frequency eddy current (SLOFEC) techniques. It may also be applicable to other techniques developed in the future. References to floor scanning include not only the use of the screening technique itself, but also any follow up work that may be required on suspect areas, e.g. ultrasonic thickness measurements.

The contract research report RR481¹ has shown there is at least one floor scanning technique (MFL) which is viable. It goes on to set practical guidelines for its application.

It is expected that floor scanning will form part of the inspection regime for all tanks within the following scope;

• Atmospheric, or low pressure, tanks for the storage of liquids, with flat bottoms (including cone up or cone down designs), where the base is in contact with the ground, and which are constructed of ferromagnetic materials; and
• Where, in the case of tanks of diameter greater than 10m, it is used for the storage of dangerous substances (as defined by the CHiP regulations6), or, in the case of any size tank (provided the application of floor scanning is possible), where the site is subject to the COMAH regulations and a loss of containment from that tank has the potential to cause a major accident; but
• Excluding tanks on refrigerated storage duties.

To allow the above expectation to be adhered to, operators should seek to achieve the following;

• Where obstructions are present across the floor, e.g heating coils, they should be removed to allow the floor scanning to take place.
• Where the tank has a floating roof, arrangements should be made for it to be landed with sufficient clearance for the floor scanning process to take place.
• Competent floor scanning operators should be used.

Where cathodic protection (CP) is used, there is still a need to ensure it is effective and make an assessment of the condition of the tank floor. Therefore floor scanning will still be appropriate. However, the protection offered by the CP, may be a factor considered when determining intervals of inspection.

It should also be remembered that floor scanning techniques are only part of a comprehensive inspection package for an atmospheric storage tank. It is important to have a good understanding of the degradation mechanisms to which each specific tank is susceptible to, and provide a regime consisting of appropriate inspection techniques. Further guidance on these wider issues is given in the referenced documents ^{2, 3, 4 & 5}. 

HSE expects duty holders to adopt relevant good practice as a minimum. The above measures are considered as representing good practice in this area. Therefore these measures, or others that have been demonstrated to be at least as effective, would normally be considered enforceable as part of the package of measures that reduce risks ALARP.

Action for inspectors

Mechanical Engineering Discipline Specialists should use the above information when inspecting atmospheric storage tanks at COMAH sites and other hazardous installations.

Regulatory Specialists are not expected to attempt a detailed technical inspection using this guidance. It may be useful, however, in supporting interventions associated with the HID Chemicals Industry Strategy particularly Key Goal 1 ‘Prevention of Major Accidents’

Further information

Please contact HID CI1F, VPN 519 6286, for further information.

References

1. Recommended practice for magnetic flux leakage inspection of atmospheric storage tank floors RR481. [PDF 1.84MB]
2. EEMUA publication number 159:2003 - Users’ guide to the inspection, maintenance and repair of above ground vertical cylindrical steel storage tanks.
3. EEMUA publication number 183:1999 – Guide for the prevention of bottom leakage from vertical, cylindrical, steel storage tanks.
4. API standard 653 – Tank inspection, repair, alteration and reconstruction.
5. API recommended practice 575 – Guidelines and methods for inspection of existing atmospheric and low pressure storage tanks.
6. Chemicals (Hazard Information and Packaging for Supply) Regulations 2002 SI2002/1689. ISBN 0 11 042419 0.
7. A guide to the Control of Major Accident Hazard Regulations 1999 (as amended) L111. ISBN 0 7176 6175 X. HSE Books PO Box 1999, Sudbury Suffolk CO10 2WA.