As part of an overall program to improve the reliability of platforms, a literature survey was made to determine the effect of various material properties on the corrosion fatigue performance of carbon steel in seawater. When this survey disclosed that little improvement could be obtained without stopping the corrosion, a more thorough effort was directed towards the effect of cathodic protection. Pertinent data available in the literature was either performed in non scaling solutions (e.g., NaCl or KCl) or did not include sufficient current density, potential and velocity data to allow us to reach any conclusions about the effectiveness of cathodic protection in reducing corrosion fatigue of offshore platforms. While high current densities (e.g.,300-l000ma/sq ft) were shown to result in air fatigue performance, we hoped that significant improvement could be obtained with less current under conditions more closely approximating the platform environment.
A test system was devised for testing AISI 1020 specimens in synthetic seawater under tension-tension load at constant potential while recording the current. A circulating system was used to insure air saturated water, and excess solid calcium carbonate was added to prevent depletion of this critical component. The potential was varied from the corrosion potential (-.4) to -1.15 V versus Ag/AgCl and the results compared to the air fatigue performance over a range of applied loads. Improved performance was obtained with cathodic protection, even at low current levels although some investigators had shown a deleterious effort.
Metallurgical study of structural failures in four offshore platforms has shown that the primary mode of failure was brittle fracture and that failure occurred almost exclusively near weld areas (Ref. 1). Although some laminations in the steel and some weld failures were found, it has been reported that the primary cause of failure was the extremely high notch brittleness of the steel used in these structures. In order to prevent recurrence of this type of failure, changes in steel chemistry and microstructure have been recommended that greatly improve the mechanical and metallurgical properties of the steel. It has been further recommended that a notch toughness specification be included in the overall steel specifications. The recommended changes should significantly reduce the occurrence of failures of this type in future platforms.
Once brittle failures are eliminated or, reduced to an acceptable minimum, it is necessary to consider the problem of fatigue, failures. The combination of a corrosive medium (aerated seawater) and cyclic loading due to the wind and wave action makes this type of failure a very real possibility. The critical area will be the welded tubular joint(s).
In order to evaluate the possible contribution of this failure mode, it appeared necessary to obtain answers to several questions:
What effect do the various compositional, microstructural, and mechanical properties of the steel have on its corrosion fatigue behavior in seawater?
What effect does cathodic protection have on the corrosion fatigue behavior?
Is there a combination of properties, cathodic protection, and coatings for optimum fatigue performance?
