Session: 03-04-01 Environmental Effect on Materials Performance

Paper Number: 79458

79458 - Quantification of Pitting and Stress Concentration Factors on Steel Coupons Exposed to Seawater in the North Sea 

Offshore structures are exposed to a harsh environment. Normally they are protected against corrosion by a combination of excess thickness, cathodic protection, paints and / or oxygen depletion. However, the corrosion protection system can also be ineffective for different reasons in the course of the life of the structure. It is therefore possible that unprotected surfaces are exposed to seawater, although it was unintentional at the design of the structure.

Corrosion not only infers a thickness reduction but also a surface degradation by pitting. This surface degradation can potentially induce a decrease of the fatigue resistance of the structure as they are potential sites for crack initiation.

In the present investigation, different samples were exposed inside different monopiles at different heights relative to the lowest average tide level. After exposure, extraction and weighing, the surfaces of the samples were characterized using different experimental means including linear roughness measurement and 3D scans. The surface topology was proven to be very inhomogeneous, leading to the necessity to use information from a large surface. Consequently, 2D roughness measurements and parameters (like Ra) were proven to be not relevant.

As expected, the largest surface degradation was observed for the samples installed at heights corresponding to the tidal zone. Some correlations were observed between the weight loss and 3D roughness parameters. However, such quantification remains very general and does not allow linking the parameters to a degradation of mechanical performance.

Therefore, a processing of the surface profile was applied to decompose it in individual pits, providing information such as depth, length and width. Thanks to the large number of measured pits, a statistical distribution of the pits is possible. It was found that a reasonable agreement was obtained by separating the pits between a population of deep pits and a population of shallow pits, each with a different density of pits per surface area. Both populations were described by an exponential distribution as a function of the pit depth. It was also found that deeper pits tend to be “rounder” compared to shallow pits. Obviously, the samples that were only marginally corroded did not exhibit a population of deep pits.

The individual pit parameters were processed to provide a local stress concentration based on their respective depths and widths. Combining the relation linking the pit geometry to a stress concentration factor and the probability of finding a pit of the given geometry, it is possible to estimate the probability of stress increase as a function of the area of exposed / degraded surface. It is expected that the proposed approach can provide insights relative to the possible shortening of the fatigue life of a structure when corrosion protection is not working properly, only on the basis of measurement of coupons of limited dimensions.

Presenting Author: Philippe THIBAUX OCAS

Authors:

Philippe THIBAUX OCAS
Cedric Vanden Haute Parkwind