The Behavior of Concrete Over Thin Film Epoxy Coatings on Offshore Pipelines
Authors: Archer G.L. , Adams A.J. 
Published: October 2008

Problems have been experienced on lay-barges with the slippage of concrete weight coating when applied over TFE coatings. Tests were carried out on 42 inch concrete coated pipe samples, incorporating three different slip prevention methods, to determine the shear strength between the concrete and the pipe. Analyses were carried out to determine the resulting increase in the flexural stiffness of the pipe string developed by the concrete due to the varying shear strengths produced. It was found that the use of a rough band, consisting of flint chippings embedded in a layer of liquid epoxy on top of the TFE, was the optimum method. The advantages of this technique are that it is simple, cheap and only need be applied to a small area of the pipe so that the increase in stiffness can be controlled. The rough-band technique was used recently on an offshore pipeline project and found to be convenient and fully effective.

INTRODUCTION

When laying offshore pipelines from laybarges of current designs, it is necessary to maintain a substantial axial tension on the line to prevent unacceptably high bending stresses and buckling in the over and sag bends. Most lines require a concrete coat to provide both stability and protection from damage on the sea bed. The axial tension is applied through tensioners on the barge, usually two or three in number, which grip the outer surface of the concrete by means of caterpillar type tracks. Thus the tension is applied through the concrete and anti-corrosion coating into the pipe and there needs to be a reliable 'bond' or shear strength between these materials. In recent years, there have been two developments which have made the shear strength level more critical. Firstly, the trend to larger pipes and deeper waters which require higher laying tensions. Secondly, the use of thin film epoxy (TFE)* anti-corrosion coatings which are harder and smoother and present a different type of surface to the concrete than more traditional coatings such as coal tar. Some years ago British Gas made a decision to change from coal tar to TFE coatings because of the superior anti-corrosion properties and resistance to damage during handling. These developments have led to problems with concrete slippage on some recent projects in the UK, and hence the need for research towards finding a solution. This paper describes some experimental work carried out by the Engineering Research Station of the British Gas Corporation to find a cheap and reliable method of preventing slippage for use on future offshore projects. The programme consisted of push-off tests, bend tests and simulated laying tests on a barge on full size specimens. The resulting influence on pipe stiffness and laying stresses is also discussed and evaluated where possible. The paper concludes with a description of the application of some of the findings to a recent offshore pipeline project.

DESCRIPTION OF COATED PIPES

All the pipes involved in the test programme were submerged arc, longitudinally welded of API 5LX 60 steel with diameter of 42 in and wall thickness in the range 0.56 to 0.75 in.