Composite Materials Thermographic method devised to test composite joints

Current limitations in use of composites may be lifted upon successful tests

A technique based on thermography which pinpoints defects in composite joints could boost uptake of these advanced materials offshore.

Composites use offshore remains limited, mainly in secondary piping systems. Many leakages have appeared, and in some cases joints have come apart. The problem for inspectors is that these materials are extremely difficult to apply conventional NDT methods to, and ultrasound and X-ray surveys have proven too expensive and unreliable.

At Rogalands Consultants in Norway, senior inspection engineer Kjell Ingebrigtsen had been working on an alternative, more accurate method using thermography. Hearing of this work, Statoil commissioned him in 1993 to perform tests on defective composite pipes. The results were so positive that a patent is now pending on his inspection technique.

Ingebrigtsen has been involved in thermographic inspection work for the past decade. He favors the Agema 470 infrared imaging system, which provides on-the-spot analysis of colour thermal images: "The 470 is relatively small and liquid nitrogen-free, and can be operated in any position. Its low weight allows easy handling, and you can get close to the object - to within 0.5 meters."

Captured thermal images can be transferred to a PC for diagnosis. For the Statoil project Ingebrigtsen used Agema's Irwin condition monitoring program.

Testing was performed at Rogalands' facilities in Sandnes and in Stavanger. Statoil and Conoco provided sample composite joints which varied in diameter from 4-10 inches, with wall thickness from 5-12mm. These test pieces, none of which had actually been in use, had known defects in bonding with either a lack of, or complete absence of, glue in the bond.

The flaws varied in size from 2-40mm wide. In some cases, 70% of the bond was missing. Later in the project, joints with unknown faults were supplied, and the results reported back to the Norwegian manufacturer for verification.

According to Ingebrigsten, the test results clearly verify that infrared thermography is a reliable method for detecting defects in welding/gluing in composite pipe materials, and provides extreme precision in showing up the area at fault.

Basic theory behind the project was that when a testpiece was exposed to deliberate temperature changes, the conductivity of the material would reveal defects in a temperature pattern observable on the thermal image.

The test method devised entails applying induced heat to the inner part of the composite material, with infra-red inspections applied to the outer surface. As heat is conducted through the material, the material structure is `transformed' to the surface, detectable on the outer skin in an isometric or thermal pattern.

Timing of heat conduction is crucial. If heat supplied is too low, the conductivity will allow the temperature to even out through the material, including any of the material's defects. However, if heat conduction time is kept to a minimum, the void space that represents a defect will restrict conduction of heat in this area.

On the thermal image of the test piece, this defect shows on the surface as a cooler area. However, where the material is homogeneous, the temperature will remain almost even. Agema's Irwin program provides a temperature profile over the well jacket, which presents the defect as a plunge in temperature on the graph. This method is particularly helpful when small temperature changes are present during inspections.

For inspection of pipes which are installed and in service, the test method varies depending on the temperature of the liquid carried. Offshore, `hot' applies to oil line, produced water lines and compresses air lines. Here the weld will first have to be cooled down to induce the necessary changes in heat variations.