Joachim Haberl and Gregor Mori, CD (Christian Doppler) Laboratory of Localized Corrosion, Montanuniversität, Leoben, Austria;

and Astrid Krenn, Alicona, Grambach/Graz, Austria

Offshore facilities surrounded by rough water environment are subject to rough conditions. Facilities such as piping face corrosion and wear attacks that can cause severe damage. By understanding corrosive attack and its distribution, damage can be reduced or even obviated.

With Focus-Variation, a new technology for optical surface measurements, meaningful corrosive studies can be carried out. The numerical verification of corroded surfaces in combination with 3D visualization in true color leads to new measures in the development of material with enhanced corrosion resistance and to the increase of production rates.

A profound percentage of failures in oil and gas production are caused by erosion corrosion. Production rates are often limited by crucial fluid velocities, depending on critical erosion corrosion rates. The degradation of material surface due to corrosive and mechanical attack is mainly due to impinging liquid and solid particles on components such as gravel pack screens, nozzles, valves etc. The investigation of this corrosive phenomenon is a very complex and difficult task, and therefore hardly carried out.

However, the only way of mitigating or even avoiding erosion corrosion is the numerical verification of the corroded surface, since this knowledge is crucial to increasing the lifetime and productivity of wells by applying higher resistant alloys.

Area and profile analysis

The first step was to screen different materials with different mechanical properties, with respect to their corrosion resistance. Following this, several evaluation methods of specimens, exposed to a high velocity multiphase flow, were performed in a pilot plant. Among other measurement tools, the degradation and corrosive attack of stainless steel was measured with the optical measurement device “InfiniteFocus.” The system is based on the technique of Focus-Variation and used for advanced surface measurement and characterization in all fields of material science, corrosion and tribology.

Corroded surfaces showing typical features such as steep flanks and high reflection surface properties are measured with a vertical resolution of up to 10 nm. The high resolution measurement tool from Alicona simultaneously captures the entire surface topographic information in combination with its true color information. Both the topographic and color information are registered to the 3D data file.

The depth measurement of the corrosive attack was mainly carried out with the profile and area analysis. The Sv parameter, calculated with the area analysis, shows the deepest void of the corroded samples. Combined with the profile analysis to measure its real depth, this is the most distinctive indicator when evaluating erosion corrosion. These two parameters are regarded as the most appropriate characteristics for the evaluation of degradation. Conventional computations such as the calculation of the average depth are by far not as meaningful as the numerical verification of a voids’ depth. Values as average data are less representative since they do not comprise any information about the deepest degradation depth.

Depth measurement and corroded surfaces

The operating principle of Focus-Variation, developed by Alicona, combines the small depth of field of an optical system with vertical scanning to provide topographical and color information from the variation of focus. Depending on the topography of a surface, the information from the variation of focus is analyzed in relation to the distance to the optics.

Using conventional optical measurement techniques, a high vertical resolution can only be reached with a small vertical scanning range; whereas the use of InfiniteFocus yields a high vertical resolution over the entire scanning range, allowing a dynamic of 1:430000. Unlike other techniques, Focus-Variation simultaneously captures the entire surface topographic information in combination with its true color information. Both the topographic and color information are perfectly registered to each other. Additionally, a quality measure is determined for each measurement point.

The advanced technology is the core of the optical 3D measurement device InfiniteFocus. The main component of this optical metrology instrument is a precision optic consisting of various lens systems. It can be equipped with different objectives allowing measurement with different resolutions. With a beam splitting mirror, light emerging from a white light source is inserted into the optical path of the system and focused onto the specimen via the objective. Depending on the topography of the specimen, the light is reflected into several directions as soon as it hits the specimen. If the topography includes diffuse reflective properties, the light is reflected equally strong into each direction.

In case of specular reflections, the light is reflected mainly into one direction. All rays emerging from the specimen and hitting the objective are bundled in the optics and gathered by a light sensitive sensor behind the beam splitting mirror. Due to the small depth of field of the optics, only small regions of the object are sharply imaged.

To allow a complete detection of the surface with full depth of field, the precision optic is moved vertically along the optical axis. This means that each region of the object is sharply focused. A sensor captures a series of 2D datasets during this scanning process. Thereby, all sensor parameters are optimized at each vertical position according to the reflective properties of the surface.

After the scanning process, the 2D datasets are evaluated to generate 3D information as well as an image with full depth of field. This is achieved by analyzing the variation of focus along the vertical axis. Due to the large amount of data, mechanical restrictions can be eliminated allowing measurement results with a high resolution. Once all height measurements are determined, an image with full depth of field is computed. The technique of Focus-Variation has been accepted in the draft of a new ISO standard 25178, which is a recently developed standard for the classification of topographical measurement techniques.

Corrosion becomes measurable

Focus-Variation is used for the analysis of different forms of corrosion, corrosive data and clarification of individual corrosive mechanics. The knowledge about circumstances causing erosive and corrosive attack of material surfaces and protective layers of components leads to the establishment of new guidelines to develop more resistant material.

In particular, profile and area measurements are used for further mathematical computations of degradation rates. Both measurement results as well as the resulting 3D visualization in true color are exported and used for further documentation. Also, measurement data and 3D visualizations of fractured surfaces are used for further research and development activities.

The technology is a well established method to characterize surfaces with complex geometries. Typical surface features coming along with corrosive specimen are steep flanks and highly reflective properties. The vertical resolution up to 10 nm can also be achieved at very inhomogeneous and rough geometries.