A gouge, Figure 1, can be created when a pipeline is impacted. The impact causes:

  • plastic flow;
  • metal transfer; and, even,
  • re-melting due to the heat of friction.
Figure 1. Gouge in Pipeline.

(Photograph courtesy and copyright of Penspen Ltd.)

Impacts on pipelines will generate heat, as the impactor drags along the surface. This heat can change the microstructure of the line pipe material. This then creates a ‘hard layer’ (low ductility and low toughness) of material around the gouge. Typically, the hard layer has a maximum depth of ~ 0.5 mm.

The material around the gouge is called ‘martensite’ – this is a very hard, brittle form of steel caused by the rapid heating and cooling of the line pipe steel during and after impact. Martensite cracks easily. The cracks can be in any orientation.

The cracks may not be visible to the naked eye, and can sometimes be hidden by folded metal, Figure 2.

Figure 2. Cracks and Crack-like Features in a Gouge.
These folds are ‘crack-like’ and are sometimes described as ‘spalling’ or ‘pancaking’. Spalling and pancaking are caused by severe abrasion which smears the surface of the pipe, producing shallow surface laps, Figure 3.

A ‘magnetic particle inspection’ could reveal numerous, small circumferentially- orientated indications, at the base of a gouge, which would be typical of spalling. Dressing (grinding or filing) the base of the gouge removes the deformed layer, and makes it easier to detect more severe, longitudinally-orientated cracking.

Figure 3. Spalling in a Gouge.

(Photograph courtesy and copyright of Penspen Ltd.)

Bibliography:

  1. M Zarea et al, ‘Full Scale Experimental Database of Dent and Gouge Defects to Improve Burst and Fatigue Strength Models of Pipelines’, Proceedings of the 2012 9th International Pipeline Conference. IPC2012. September 24- 28, 2012, Calgary, Alberta, Canada. IPC2012-90620.
  2. P Hopkins et al, ‘Recent Studies of Significance of Mechanical Damage in Pipelines’, AGA and EPRG Research Seminar, San Francisco, USA, September 1983.