Inspection on the surface level

Materials World magazine
1 Sep 2017

New developments in non-destructive testing technology may deliver a step change in the inspection of externally corroded components and components encapsulated within composite repairs, as Mike Adams and Mike Dixon explain.

Innovation is key in sustaining safe production of marine structures, with new practical solutions challenging the operational mature installations, some of which are approaching their 40-year service life. The hostile environment and remote location of the installations also makes effective fabric maintenance challenging, so it is not unusual for an inspector to encounter steelwork that is encrusted with a thick layer of rust or dotted with rust scabs. 

One of the perennial problems facing an inspection engineer is how to examine a material that has a thick surface coating. Conventional ultrasonic techniques, still the mainstay of inspection, will not typically penetrate these and radiography can introduce ionising radiation to the equation. Modern radiographic equipment seeks to reduce the hazard, but at the cost of a loss of penetration, which generally limits radiography to smaller-bore, thinner-walled pipework. The potential for radiation to interfere with certain instrumentation limits its application further. Alternative techniques have existed for many years, but these have a common limitation – they lack resolution and, while providing reasonable estimates of general wall thickness, can completely miss localised corrosion, such as pitting, that will result in a leak.

Two areas where these limitations create a real problem are components that are covered with a thick encrustation of rust and those that have their service life extended by the application of a composite repair. In both cases, it is necessary to find out what is happening underneath the layer, and it is not possible to remove this barrier while the plant is live.

Internal and external corrosion 

External corrosion is one of the greatest threats to the integrity of pressure systems and structures. The combination of a marine atmosphere and warm process equipment creates an unforgiving environment where coatings break down, resulting in thick blisters of corrosion product. The hostile environment and remote location of the installations makes effective fabric maintenance challenging – even if resources were limitless, the number of personnel who can work on a platform at any one time is strictly limited and weather downtime also has a significant impact upon productivity. It is not unusual to encounter components that have developed a thick encrustation of rust. Usually the actual damage is far less than a casual inspection would indicate, as the rust is many times thicker than the metal that has been lost. However, to be certain, it is necessary to find out what is hidden beneath this encrustation.

Internal corrosion caused by process fluids is also a problem. The conventional approach, at the time when the first-generation installations were built, used unalloyed steel known as carbon steel, with additional wall thickness created as a sacrificial corrosion allowance. This still remains the most cost-effective approach in many cases. In practice, the most common forms of internal corrosion cause localised thinning, so carbon steel pipework can be maintained indefinitely by applying a programme of inspection and repair techniques or replacing individual components as
they wear out.

Engineering composite repairs play a vital role within this strategy. Corrosion rates are not constant but vary with time and location, therefore it is not unusual for inspection to find that a section of pipework has corroded faster than anticipated and requires immediate repair. In many of these situations, it is possible to carry out a temporary repair to extend the life of the pipe by encapsulating the damage in a layer of engineering composite, similar to that used to make aircraft wings. This, if done properly, will restore its mechanical strength, contain any leaks and allow the permanent replacement to be carried out in a planned manner, including a number of repairs into a single planned shutdown. This is safer, more efficient and results in lower production losses than multiple, unplanned shutdowns.

The section of the pipe is covered with a thick layer of composite, the internal corrosion will remain active and new corrosion sites may initiate in places that could compromise the integrity of the repair. This risk can be mitigated by giving the composite repair a short service life, or by inspecting it regularly to sustain confidence in its integrity.

A new pulsed eddy current (PEC) technology is helping to solve this problem, by allowing insulated components to be inspected without removing the insulation. PEC is an electromagnetic technique that uses induction to measure the thickness of an electrically conducting material. A coil in the probe is energised by an alternating current. This creates a magnetic field, which induces an alternating electrical current, known as an eddy current, within the material. When the current to the probe is switched off, the eddy current decays in a manner that is proportional to the volume of material within the probe’s ‘footprint’. As the eddy current also generates a magnetic field, this decay can be sensed and measured by the probe, allowing the signal to be analysed and the thickness calculated. The cycle of energise and decay is repeated continuously, allowing the component to be scanned. 

The benefits of this technique are that close contact with the surface of the component is not required and any electrically insulating material placed between the probe and component is effectively transparent so pulsed eddy current can ‘see’ through insulation, rust and composites.

Technical limitations

Pulsed eddy current is a volumetric effect – it measures the volume of material within the footprint of the probe, so local thinning areas that are smaller than the footprint will not be measured accurately and relatively small areas will not be detected at all. Secondly, as the distance between the probe and the test piece increases as well as the footprint, so the resolution diminishes.

Early PEC equipment had large probes and poor resolution, limiting its use for inspecting pressure systems where localised corrosion is the prevalent mechanism. The latest generation analyses the eddy current decay in a different way, which has enabled the probe and footprint size to be reduced significantly. This has produced a resolution that while not equivalent to ultrasonic examination or radiography, is sufficient to make it a useful inspection tool.

The system offers real-time false colour thickness mapping of the area examined. Data acquisition is rapid, with up to 15 readings per second. The system’s functionality includes both grid-mapping and dynamic scanning modes. It is portable and user-friendly, with cables extending up to 19m, allowing probes to be deployed and manipulated by a rope access technician.

The material thickness was measured using ultrasonic techniques in the areas where the surface condition has not been affected with surface oxidisation. Readings were also taken with the PEC system and a table of readings was created. The results show that where both a UT and PEC reading were taken, the readings have an average variation of 0.4mm, with the highest reading giving a variation of 0.9mm. 

The qualification was conducted in a timely manner from start to completion. It identified the PEC successfully measured thicknesses in the same region where nominal wall thickness was present – no surface scabs present – providing a degree of confidence in the readings taken from the other areas of the sample.  Limitations of the PEC system always need to be considered with the results provided beneath the scabs.

Future work will involve the removal of scabs, allowing for remaining wall thickness to be measured with ultrasonic techniques and feeding this back to what the PEC system measured. Field trials will also be carried out to gain a better understanding of the relationships of surface scab to wall thinning below the scab.

Mike Adams CEng FIMMM is the Lead Materials and Corrosion Engineer at EnQuest, an oil and gas production and development company, investing in maturing and underdeveloped
oil and gas assets. 

Mike Dixon is Contract Manager and Specialist of Non-Destructive Testing at Focal Point Trac International.