Peter Ho reports on safer and more efficient porosity testing on offshore structures and pipelines.
Ensuring the long-term protection of parts exposed to the extremely harsh conditions found in offshore environments requires intact anti-corrosion protective coatings. Any void, gap or pore in the coatings can considerably shorten the lifetime of protected components. To control the quality of these crucial coatings, porosity testing is mandatory.
All offshore structures - ships, oil rigs, cranes, containers and pipelines (including fittings and valves) are surface coated to shield them from the severe environmental conditions found at sea. As even a tiny pinhole can spoil the protective function, it is necessary to rigorously inspect the coating for integrity. However, even the most careful visual observation still cannot detect all the pores, cracks and thin spots (less than specified thickness) that can form during the coating process. High-voltage porosity testing is the only truly reliable way to inspect the protective coatings on all kinds of offshore structures; the device commonly used for this is often referred to as a 'holiday detector'.
To conduct this important task, Helmut-Fischer has developed a comprehensive range of instruments that meet the safety requirements of ISO 2746, with Fischer Instrumentation (GB) supplying and servicing in the UK. The Poroscope porosity testing range is specifically designed to fulfil the requirements for coating inspection in offshore environments. Features such as the display of the test voltage situated in the test head provide convenience and practicality and three test voltage instruments cover 0.8 - 5kV, 4 - 20kV and 8 - 40kV. The measurement head of the latter allows for the testing of thick coatings.
The test method is based on the fact that all electrically insulating coating materials have a much higher disruptive strength than air does. High voltage is applied using, for example, a brush-like electrode that is moved across the surface of the specimen. In the case of a defect (pore or scratch) a sparkover occurs, which is indicated acoustically and optically by the system.
Portable and practical
The portable HV40 is a sturdy, metal-clad instrument designed for practical application on rugged job sites. The high voltage is generated inside the probe head, improving both operator safety and ease of use. It eliminates the need to drag long, bulky HV insulated cables across the work area - be it a wet ship deck or tank - which also makes the instrument far less sensitive to moisture.
Fig. 3 shows all the types of electrodes - sweeper, flat with rubber tongue, roller and circular ring - that are used for large area enamel, rubber or synthetic coatings, paint coatings and pore testing of foils and exterior pipe walls. Rotating electrodes are for interior pipe wall pore testing up to a diameter of 125mm.
The intactness of corrosion protection on any metal structure in an offshore environment is critical to its performance and longevity and instruments such as the Poroscope HV40 make the job of porosity testing safer and much more efficient.
For more information at www.engineerlive.com/iog
Peter Ho is general manager, Fischer Instrumentation (GB).