Use of a Punching Technique with Small Samples to Investigate Temperature Dependent Shale Strength

Abstract

A flexible pipe consists of several layers of steel and fluid barriers, where the fluid barriers keep the inside (steel) of the flexible pipe dry. Unfortunately, the most frequently damaged component of the flexible pipe is the external sheath, which is the outer fluid barrier. Water in the annulus is an indication of leakage, and replacement is then needed.

To protect against or avoid severe damage of the flexible pipe, several inspection methods to detect damage of different parts of the flexible pipe are available. To detect the presence of air or water inside a gap between two steel armour plates in this work, an inspection method was necessary.

The performed inspection was normal incident pulse-echo ultrasonic inspection. Both unfocused (plane) and focused transducers were used to scan a simplified flexible pipe model. The model consisted of a high density polyethylene (HDPE) external sheath and steel armour plates beneath it. The gaps between two steel plates beneath the HDPE-sheath was filled with either air or water and inspected.

A realistic gap size between steel plates was found in theory to be in the range from 1 to 2 mm if the gaps were evenly distributed over the circumference of the flexible pipe. To detect whether air or water was present, only the focused transducer fitted this purpose. The focused transducer detected difference between air and water for a 2mm gap between steel plates when the transducer gradually inspected from steel over the gap and to steel again. From circle segment area consideration, it should be possible in theory to see a 180° phase-shift for 1.2mm gap between steel plates to differ air and water.

Another way to differ air and water was to detect the plexiglass-container layer beneath both the HDPE-plate and the water layer. If air was present the layer could not be detected, but if water was present a 0.5mm gap between the steel plates could be identified by amplitude consideration. In the lab-test this layer was a plexiglass-container, but for a real flexible pipe this layer is an antiwear layer or another steel layer beneath the first steel layer.