Statistical Corrosion Assessment

Benefits

Atteris employed an in-house statistical assessment of a limited amount of available corrosion inspection data to infer the corrosion risk in a subsea pipeline. The resulting low risk profile of the pipeline allowed Client to defer the requirement to undertake a subsea-subsea in line inspection by several years. This reduced the inspection cost by an estimated $15 million (AUD).

Background

A subsea pipeline in Northern Australia had been wet parked for a number of years since its installation and pre-commissioning operations.

The wet-parked duration was significantly longer than the original design intent and as a consequence the Client elected to inspect the line prior to start-up and operation. Inline inspection (ILI) pigging was uneconomic so Client’s strategy was to perform a number of spot-inspections.

The Client engaged Atteris to provide engineering support to the offshore inspection team during the inspection period, so that Client could be confident that a sufficient number of inspections were done to ensure a high confidence in the pipeline’s condition.

Work Performed by Atteris

Atteris initially identified all possible corrosion mechanisms and corrosion zones (girth weld, seam weld, top of line, bottom of line, etc.) within the pipeline. Client’s inspection team then inspected a number of girth weld locations at a number of points along the pipeline to obtain a number of datasets for each corrosion zone classification. For each zone classification a statistical assessment was performed. To improve confidence in the cumulative distribution functions (CDFs) a statistical method called “Bootstrapping” was performed for each zone’s CDF. The lower bound confidence interval was then used to determine the likelihood of an unacceptable corrosion feature.

Through this advanced inspection and assessment methodology Atteris were able to give Client a high level of confidence in the corrosion status of the pipeline based on a modest number of spot inspections performed by a vessel of opportunity. This deferred more expensive topsides to subsea in line inspection operation which reduced inspection costs by approximately $15 million (AUD).