Inspection Of Concrete Platforms: Crack Detection By Current Density Measurements
Authors: Bournat J-P , Stankoff Alain , Auboiroux Marcel 
Published: October 2008

ABSTRACT
Evaluation of concrete wall condition is one of the major inspection tasks that has to be performed on concrete production platforms.
A new survey is proposed allowing crack detection over large concrete areas through a measurement of the D.C. currents due to corrosion or cathodic protection when contact occurs between seawater and the reinforcement bars.
Operations are conducted with a current density sensor and a gyro-doppler navigator installed on a manned or unmanned vehicle. Data is recorded or transmitted to the surface while the vehicle is following a "grid path" that covers the whole surface to be inspected. The data processing result is a chart of the current density measurement as a function of the position. A vertical and longitudinal grid spacing of approximately I meter, along with a sensitivity of the current density measurement of mA/m2, allows detection of deep cracks, approximately 30 m long, over areas larger than 1000 m2.
The system has been evaluated and trials performed on North Sea platforms. The paper includes laboratory tests determining currents to be expected from concrete cracks reaching reinforcement bars. After a description of the equipment and navigation procedures which have been especially developed for this application, the paper is largely devoted to the operating techniques and operational results obtained in the North sea. The ability of the navigation technique allows a complete coverage of the inspected surface, with 20 cm accuracy and cracks producing D.C. currents higher than 4 mA/m2 are clearly identified.

INTRODUCTION
Evaluation of concrete wall conditions is one of the major inspection tasks that has to be performed on concrete production platforms. Until now, only visual inspections were performed by divers or submarines and the detection and evaluation of cracks required a cleaning of the wall prior to the inspection. Moreover, inspection could not be obtained through monotonous sensor measurements due to the poor efficiency of the positionning systems available for operations along concrete walls.
In order to provide the concrete platform operators with a more accurate and objective information, a system has been developped that allows detection and location of corrosion and cathodic protection currents, especially those D.C currents due to the contact between free seawater and the reinforcement bars. These D.C. currents reveal dangerous cracks or abnormal porosity leading to concrete breakup in the long time.
The operations are conducted with a standard current density sensor and a gyro-doppler navigator installed on a manned or unmanned submersible. The data is recorded or transmitted to the surface while the vehicle is following a "grid path" that covers the whole surface to be inspected. Data processing is performed on the mother ship through a computer and produces charts of the current density measurement as a function of the position. Abnormal peaks are interpreted as significant damages and a closer survey of the suspected zones can be decided after this general survey, but only at the specified locations, thus saving time and money.