The HOIS joint industry project, managed by ESR Technology, shares NDT good practice guidance documents to address energy industry challenges of corrosion under insulation (CUI) and external corrosion of uninsulated components.


HOIS members have approved open publication of two new key documents: HOIS/OGTC Guidelines for in-situ inspection of corrosion under insulation (CUI) and HOIS/OGTC guidance on the inspection for uninsulated external corrosion scabs These documents, rather than being confidential to HOIS members, can now benefit the wider industry and may be downloaded from:


Corrosion under insulation (CUI) poses a significant operational, safety and economic challenge for the energy industry. The Oil & Gas Technology Centre highlight these headline figures:

  • Corrosion costs the UK economy an incredible £28 billion every year, with that figure rising to £4 trillion globally!
  • Over 20% of the major oil and gas incidents reported within the EU since 1984 are associated with CUI.
  • Many assets in the North Sea are operating beyond their expected design life and 60% of pipe failures are caused by CUI.

The occurrence of CUI is often unpredictable and there is continuing interest in effective inspection/NDT methods to reduce the risk of loss of containment.

The HOIS1 project, supported in this case by the Oil & Gas Technology Centre, highlights the many factors to consider when assessing the applicability of an NDT method for CUI inspection including pipe (or vessel) OD, wall thickness, CUI morphology, the insulation material and thickness as well as the outer cladding (or weather jacket) material and thickness. The guidance document has been developed following an extensive programme of independently assessed blind trials of a range of NDT technologies. It contains a summary of relevant information on CUI within the energy industry. and summarises the main methods currently available (including developmental methods) for NDT. Where available, a summary of performance and reliability information obtained from the HOIS CUI trials is given. Guidance on the application of the methods is given depending on the inspection requirements, including summary tables of methods applicability. Usage of NDT within a strategy for CUI integrity management is also considered and illustrated by case studies, provided by HOIS members, involving successful application of NDT for CUI inspection.

HOIS would like to acknowledge the blind trial participants who invested considerable time, energy and equipment: Acuren, Bilfinger, CAN, Eddyfi, Exxam Systems, GUL, Maxwell NDT/Ether NDE, Oceaneering, Pixel Thermographics, Robinson Research Institute, Russel NDE and Shell and TUV Sonovation. In addition, the Oil &Gas Technology Centre is thanked for their generous co-funding of this work.

External corrosion

External corrosion of uninsulated carbons steel pipework is also a significant problem for energy industry asset owners. As external degradation occurs, corrosion product or scale usually builds up on the affected components to form blisters or scabs. Removal of corrosion product by blasting pressurised live process components has an associated hazard of loss of containment.
In this case, the inspection challenge is to determine the remaining ligament under the corrosion scab. As it is generally not possible to propagate ultrasound through corrosion product; the presence of the scab renders the area of corrosion inaccessible. Hence standard 0° pulse-echo methods are not applicable. Factors that influence the applicability of NDT methods for scab minimum ligament determination include the following:

  • Pipe wall diameter and wall thickness
  • Component geometry
  • Extent of the corroded area in the axial and circumferential directions
  • Accessibility to the corroded area and space around it
  • Surface condition surrounding the corroded area
  • Morphology of the corrosion, especially the presence of highly localised pitting

The HOIS Joint Industry Project started to look at this issue in 2012 and, since then, an extensive series of trials of NDT techniques for inspection of externally corroded un-insulated pipes with the corrosion product in situ has been performed. The most recent trial-based project started in 2017, supported by the Oil & Gas Technology Centre, and was designed to develop and focus the HOIS resources and capabilities for conducting rigorously controlled independent evaluation trials and to compare different inspection methods for these challenging problems. The most recent trials comprised further M-skip scans of additional manufactured components, dual frequency eddy currents, pulsed eddy currents and the GUL QSR1 system.

The trial report is confidential to HOIS members however, the results have been used to develop HOIS/OGTC guidance (HOIS-G-028) on the inspection for uninsulated external corrosion scabs. It summarises the main NDT methods currently available and provides, where available, performance and reliability information obtained from the HOIS scab trials. The final section provides guidance on application for the methods depending on the inspection requirements, including summary tables of method applicability. The guidance is intended for areas of external corrosion that have been located by other means (e.g. visual inspection) for which removal of the corrosion product or scale is hazardous. The component material is limited to carbon steel and the degradation type is limited to loss of wall thickness due to corrosion.


[1] HOIS ( is a well-established joint industry project, managed by ESR Technology, which has been running for more than thirty five years with project partners in the energy sector including asset owner/operators, non-destructive testing service companies, NDT equipment vendors, The Oil & Gas Technology Centre and regulatory authorities such as the HSE. The HOIS vision is to improve the effectiveness of in-service inspection for the energy industry.

Current active HOIS projects include:

  • Determination of remaining thickness under external corrosion scabs: scab characteristics and advanced NDT methods
  • Digitalisation in Oil and Gas
  • Assessment of NDT techniques for carbon and low alloy steel heat exchanger tubing inspection
  • Ultrasonic NDT for NII at elevated temperatures
  • Flange face corrosion recommended practice: updated an extension to include RTJ flanges
  • Optimisation of PEC Technique for CUI – by open trials and modelling
  • Corrosion mapping for small bore pipework
  • Development of a Remote Digital Visual Inspection RP for unmanned pressure vessel IVI (akin to HOIS-RP-103)

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