Eddy current detection of fatigue cracks in drill pipes

Master Thesis

2012

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http://hdl.handle.net/11427/6084
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thesis_sci_2012_molteno_m.pdf (9.78 MB)
Authors
Molteno, Matthew Robert
Supervisors
Tait, Robert B
Wilkinson, Andrew John
Becker, TH
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University of Cape Town

Department
Department of Mechanical Engineering
Faculty
Faculty of Engineering and the Built Environment
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Abstract
The Vertical Marine Mining technique is a lucrative method used by De Beers to recover bottom lying diamonds from the seabed, mainly off the coast of Namibia. The method employs a 200m length pipe-drilling structure rotated from the surface and acting as a conduit through which the disturbed sediment is transported to the surface for processing. However, cyclic operating stresses combined with corrosion have tended to cause circumferential fatigue cracks to develop on the outside surface of the drill pipe and propagate inwards. As no early warning system exists, these cracks propagate undetected until failure occurs. High operational costs and losses associated with down time, provide a strong motivation for a system to detect fatigue cracking prior to failure, especially given that fatigue crack growth rate and fatigue lifetimes can be estimated using Fracture Mechanics principles. Therefore what is required is an early warning of fatigue crack initiation with non-destructive testing (NOT). The eddy current method is an ideal NDT technique as it does not require contact with the test surface and is highly sensitive to fatigue cracks .. However, this method is generally only sensitive to surface and near surface defects. This proves to be a major limitation - the external drill pipe surface is obstructed by flanges and fittings such that continuous inspection of the external surface would be impossible. Therefore two specialised eddy current methods to allow the detection of external fatigue cracks from inside the drill pipe were reviewed, namely: Pulsed Eddy Current (PEC) and Remote Field Eddy Current (RFEC). It was found that unlike PEC, RFEC is highly sensitive to external circumferential planar defects in ferrous pipes. This, above all, highlighted the suitability of RFEC for drill pipe inspection.
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Includes bibliographical references.

Keywords
Mechanical Engineering

Reference:

Molteno, M. 2012. Eddy current detection of fatigue cracks in drill pipes. University of Cape Town.

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Masters