Corrosion Fatigue of Heavy-Duty Coated and Cathodically Protected Steel for TLP Tendons
Authors: Kobayashi J. , Takeshi Y. , Yoshida K. 
Published: October 2008

Fatigue properties of 80 ksi yield strength level steel in seawater treated with the combined protection system, which consists of a heavy-duty double-layer organic coating and catholic protection, were investigated using the specimens with the system. Corrosion fatigue tests were carried out in such extraordinary cases as partial breaking of the organic coating and disbanding as well as under normal conditions.

The experimental results indicate that the fatigue properties of tendons with the combined corrosion protection system under the normal catholic protection in seawater are almost the same as those in air, even if there exists partial breaking of the organic coating and/or disbanding. These results represent that when the combined corrosion protection system is used in seawater, it allows the utmost advantage to be obtained from the merit of high strength steel, and gives the steel the same fatigue strength as in air.

1. INTRODUCTION

The tension leg platform (TLP) is one of the most promising means of deep-sea oil production. In July of 1984, with the world's first successful installation in the Hutton Field in the North Sea [1], TLP systems became practical.

As the TLP is designed to be moored to the foundation templates on the sea bed by vertical tension legs which are called tendons or tethers, these tendons are the most important elements for the safety of the system. With respect to this important tendon element, one of the authors already reported on the suitable structure, fabrication, steel and welding materials for the cost-saving and high reliable tendons at the 85' and 86' OTC [2]-[3].

Since the tendons are generally subjected to high stresses and pulsating stresses due to wave motion in the seawater environment, the highest care against failures brought by corrosion fatigue must be taken. In general, the fatigue strengths of carbon and low alloy steels in seawater are almost independent of the tensile strength, while those in air are approximately proportional to the tensile strength. In addition, the steels do not seem to have an endurance limit in seawater. The most effective means for improving fatigue properties in seawater would be reducing or excluding the corrosion. Therefore, one of the authors already presented that the combined corrosion protection system which consists of a heavy-duty double-layer organic coating and catholic protection is the most recommendable and reliable one [2].

Such combined corrosion protection systems are popular in pipelines, but they seem to be uncommon in offshore structures. And also, there seems to be less research on the effectiveness of the ombined corrosion protection systems, especially on heavy-duty coating against corrosion fatigue. Therefore, the authors carried out the corrosion fatigue tests in synthetic seawater using specimens with the combined corrosion protection system in order to evaluate their effectiveness. As it is important to ensure the effectiveness of the catholic protections in such cases where the steel surface is exposed directly to seawater because of partial breaking of the organic coating or disbanding, the experiments for such extraordinary cases were also performed.