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  1. ROSEN Group
22 October 2008

Inspection for Shallow Internal Corrosion

Offshore pipelines are different from their onshore equivalent. This is not only with regards to their design and operation but also with regards to the deterioration process of these structures. While onshore pipelines are threatened by third party damage and the external corrosion process, the main threat for offshore pipelines, besides occasional anchor damages, is internal corrosion.

Naturally, the accessibility of offshore pipelines is more restricted than their onshore counterparts. These circumstances are already taken into account during the design and construction of the offshore pipeline. Heavy wall pipe, a wall thickness of 30mm is not uncommon, is used to withstand a high pressure regime, but also to allow for controlled reduction of the wall thickness during the lifespan of the pipeline.

The developments in design, construction and maintenance of this particular sub-group of offshore pipelines triggered the deployment of a new in-line inspection technology by ROSEN. An eddy current-based sensor technology was developed, allowing a highly resolved and accurate image of the internal pipeline surface.

Additionally, the system provides very detailed information about the internal diameter and the internal shape, such as ovality and dents of the pipeline. The results from the in-line inspection is used to determine the corrosion status of the pipeline, but primarily to determine the corrosion growth rate of a corrosion attack in the initiation phase. Corrosion of any size with a diameter of at least 10mm and a minimum depth of 1.0mm is detected. Internal shallow defects with a maximum depth of 10mm are sized with a high accuracy of +/- 0.5mm.

The compact design of this inspection tool is advantageous with regards to the passage. The robust but flexible sensor suspension is supportive even for challenging bore or multi-diameter pipelines. The absence of a magnetizer unit, as known from magnetic flux leakage inspection, is enabling a low friction application. This can be a low flow or low pressure autonomous pigging application, as well as a tethered tool operation.

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