EU Joins Forces with TWI to Inspect GRP Pipe Joints

EU-funded researchers developed a high-tech robotic imaging system for
inspecting underground pipes. Commercialisation should facilitate use of
corrosion-resistant materials for transport of hazardous chemicals.

Hazardous fluids such as oil and gas, both of which contain hydrocarbons, are
transported throughout Europe via a network of over 10 million kilometres of
stainless steel piping.

Metal such as stainless steel is subject to corrosion within pipes and as a result
of external factors. Such corrosion is a major cause of catastrophic disasters.

Fibreglass, also known as glass-reinforced plastic (GRP), is a material composed
of a polymer (plastic) together with reinforcing strands of glass. GRP can
provide excellent corrosion resistance to a wide variety of fluids and gases at
ambient or environmental temperatures. In addition, it is generally a much less
expensive material than stainless steel.

Widespread use of GRP in the underground network of European pipes has so
far been limited. Encouraging its application requires the ability to inspect the
integrity of pipe interconnections with confidence.

European researchers initiated the ‘Quality assurance and structural evaluation
of GRP pipes’ (Sure2grip) project to develop non-destructive testing (NDT)
technology for GRP to ensure secure pipe connections. To this end, scientists
developed numerous NDT technologies carried on and controlled by a robotic
scanner.

Thermographic NDT was used to produce a sort of thermal X-ray, a colour-
coded map based on infrared detection capable of detecting non-uniformities.

Phased array ultrasonic technology (PAUT) was also included. It uses a multi-
element array of ultrasound transducers enabling focusing at a variety of angles
and depths over a large area with very little water.

Radiographic systems were also integrated. Low-energy digital radiography
enabled an X-ray using digital sensors rather than film. Dual laser shearography
was developed for the project to visualise two-dimensional strain.

The Sure2grip GRP pipe inspection system thus filled a technology gap until
now prohibiting the widespread use of GRP pipes.

Europe’s underground pipe network carrying hazardous materials could benefit
directly due to the corrosion resistance of GRP. Use of GRP will also likely be
expanded to other applications now due to the excellent NDT technologies
provided by the Sure2grip project consortium.

TWI owns a Laser Optical Engineering SM 1200 Strain Mapper. This is unique in
that it can separately resolve in-plane and out-of-plane strain through the use
of a novel dual laser system. This is especially useful when it is necessary to
differentiate between faults that produce mainly out-of-plane strain, such as
skin to core disbonds, and those that produce mostly in-plane strain, such as
cracks.

TWI (www.twi.co.uk) is using its SM 1200 in two major collaborative projects
funded by the European Commission.

Sure2grip is concerned with the development of an integrated NDT system for
inspecting joints in GFRP pipes. The intention is to be able to detect both initial
and inservice flaws, such as: Build Damage (BD) in the form of weak bonds.
Accidental damage (AD) in service, in particular Barely Visible Impact Damage
(BVID) caused by high velocity impact by solid objects. Environmental
degradation (ED) caused by ultra-violet exposure or salt water osmosis
Renewit aims to develop an integrated NDT system to inspect wind turbine
blades. Shearography will be used to detect impact damage, in particular
defects in the skin-to-foam interface

Laser Shearography uses the coherent, monochromatic properties of laser light
to generate speckle patterns. The component to be inspected is illuminated by
the laser. The surface reflects the light creating a speckle pattern at the viewing
plane, which can be processed to provide information such as the presence of
defects, material degradation or residual stress. The system records the speckle
pattern from an unstressed component surface. The image is recorded using a
video camera, digitised and stored on a computer. The surface is then stressed
and a new speckle pattern generated, recorded and stored.

The computer subtracts the speckle patterns from each other, thus forming an
image made up of series of characteristic black and white fringes, representing
the surface strain in the area of interest. If a defect such as a void or disbond
exists, this will affect the surface strain and the defect can be revealed by the
fringe pattern developed. This can be processed further by the computer to
generate an unwrapped image and a 3D strain map, making the fringe pattern
easier to interpret by the user.