As Philip Woodcock explains, non-destructive testing (NDT) is an increasingly commonly used technique in the maintenance of offshore windfarms but do we really know what it is?
Non-destructive testing is the process of inspecting, testing or evaluating materials, components or assemblies for discontinuities or differences in characteristics without destroying the serviceability of the part or system. In other words, when the inspection or test is completed, the part can still be used.
According to the British Institute of Non-Destructive Testing (BINDT), it is “the branch of engineering concerned with all methods of detecting and evaluating flaws in materials”.
NDT is used throughout the lifecycle of an offshore windfarm, but this article concentrates on the techniques used during operations to monitor the condition of blades, towers, transition pieces, foundations and secondary steel but does not look inside the nacelle or cables.
The defining feature of NDT is that the process does not cause detrimental effects on the material or structure under test. However, some techniques may require cleaning of marine growth or damage to coating protection systems that will result in additional maintenance to restore protection.
Visual inspection is the original NDT technique and one that is undertaken at all times consciously and unconsciously by professionals. It can be divided into general visual inspection (GVI) and close visual inspection (CVI).
Visual inspections are undertaken on the topside and subsea elements of a wind turbine or other structures such as offshore substations and can be remotely or physically performed. Here, one is looking for surface breaking defects.
Remotely operated vehicles (ROVs) routinely undertake subsea GVI and, with tooling to clean away marine growth, can also conduct CVI. The use of drones is becoming more and more common for GVI of blades and towers, but at least for the time being, drones are payload limited. However, their ability to detect defects is advancing rapidly. Whichever technique is used, a key element of visual inspection is recording and reporting on results, either through still or video photography, both of which are dependent on suitable light conditions.
The acoustic method works by exciting a vibration in a material by local impact and then measuring the response of the vibrations. In its simplest form, it is like a railway wheel tapper of old, where one hears or measures a sound when coming across an occlusion.
Electromagnetic effects resulting from the interaction of electricity and magnetism form the basis of a number of NDT techniques, including eddy current testing, magnetic particle inspection (MPI), magnetic flux leakage testing and alternating current field measurement (ACFM). Electromagnetic induction tests are applied to all stages of metal and alloy fabrication and processing.
MPI is used for the detection of surface and near-surface flaws in ferromagnetic materials and is primarily used for crack detection. The component being tested is magnetised, and if it is sound, the magnetic flux is predominantly inside the material. If, however, there is a surface-breaking flaw, the magnetic field is distorted, causing local magnetic flux leakage around the flaw.
Eddy current testing uses a coil carrying an AC current placed close to the specimen surface or around the specimen. The current in the coil generates circulating eddy currents in the specimen close to the surface, and these in turn affect the current in the coil by mutual induction. Flaws and material variations in the specimen affect the strength of the eddy currents.
ACFM is a non-contact electromagnetic technique capable of both detecting and sizing (length and depth) defects in metals. The basis of the technique is that an alternating current flows in a thin skin near the surface of any conductor. By introducing a uniform current into an area of the component under test, when there are no defects present, the electrical current will be undisturbed. If a crack is present, the current flows around the ends and down the faces of the crack. As the technique requires no electrical contact with the surface, it can be used to inspect through paint and coatings. The technique is widely used for weld and thread inspection and for subsea inspection of offshore platforms. It can also be used on both magnetic and non-magnetic components.
Liquid penetrant testing, also known as dye penetrant or ‘die-pen’ testing is a simple low-cost method of detecting surface-breaking flaws such as cracks and porosity. It requires a clean surface free of grease and cannot be used under water.
Radiography uses X-rays or gamma-rays to produce an image of an object on film. This is a well established technique, which gives a permanent record and is widely used to detect internal flaws in metals, coatings and composite materials such as blades. However, radiography is potentially dangerous and must be performed either inside a protective enclosure or with appropriate barriers and warning signals to ensure that there is no radiation hazard to personnel, thus increasing cost and reducing workability.
Ultrasonic methods such as ultrasonic thickness measurement (UTM) of NDT use beams of mechanical waves (vibrations) of short wavelength and high frequency transmitted from a small probe and detected by the same or other probes. The technique detects internal, hidden discontinuities that may be deep below the surface.
Thermography is a technique of obtaining an image of the heat distribution over the surface of an object. This technique is commonly used for checking the adhesion of joints in blades. Due to the weight of equipment, this cannot be performed by drones at this time.
In the offshore wind industry, access to the work site provides many more challenges than one would find onshore in a workshop environment. For work above water, NDT technicians at least need training in offshore safety and working at heights as prescribed by the Global Wind Organization.
However, much of topside NDT cannot be reached from ladders or platforms and thus rope access techniques as governed by IRATA are needed. This limits the number of available technicians and also the working time due to weather limitations.
Many see drones performing GVI as the future due to the reduced risk of harm to technicians as well as reduced cost and time needed to perform an inspection. However, as highlighted above, drones are limited in the payload that they can carry to perform tasks other than GVI, the time that they can spend aloft and the spatial accuracy for recording locations of defects.
These, however, will be overcome as technology is evolving rapidly. MME Group’s senior marketing communications officer Michaël Roerade confirmed this. “A limited part of visual inspections will be carried out with autonomous drones,” he explained. “However, the nature of NDT and especially interpretation and assessment of the results is something that – for the foreseeable future – will remain the realm of human operators.”
ROVs can be used for GVI and, if they have tooling to clean away marine growth and paint, CVI. ACFM can be performed by ROVs for surface crack detection, lack of fusion and strain measurement. However, according to Rana Diving’s project manager Francesco Morini, “for most subsea structures, especially the bracings of jackets, a diver takes less time and delivers a better job than trying to get a ROV to hold an accurate enough position to deliver creditable results”.
NDT destructive testing provides a key element in the lifecycle management of offshore windfarms. Although many of the techniques used offshore are similar to those used in onshore wind, the means of access, training and certification of the technicians makes it more challenging and expensive to complete.
The use of remote access techniques such as drones and ROVs will increase from general visual inspections to more in-depth techniques as technology improves and gets smaller. Until then, there will be a need for access by divers and rope access technicians who can not only inspect but interpret findings based on experience.