Collapse capacity for a pipeline with thick coating

Abstract

When a pipeline is installed at deep waters the primary design load is the external pressure, hence making the collapse pressure the major design parameter. Pipeline collapse formulas are functions of elastic and plastic collaspe pressure acompanied the initial ovality and yield stress. The most common formulas for collapse are the Timoshenko, Shell and Haagsma equation where the latter is the least conservative and used in the offshore standard DNV-OS-F101. In any pipeline system onshore or offshore there is a need of pipeline protection. The purpose of coating is to isolate the pipeline steel from the seawater, the soil and to introduce a high resistance path between anodic and cathodic areas. Such coatings can be ranging from a few up to around hundred millimeter, thus the cros section of the coated pipe can be many times initial cros section of the bare steel pipe. Existing design formulas for collapse are neglecting the effect of pipeline coating and a study is proposed to investigate if todays practice of neglecting the coating is ac- ceptable. In this project ABAQUS finite element software has been used to examine a coated pipe section with various thicknesses, elasticity and ovality and compared the collapse limit to the values of a uncoated pipe. Based on the assumption of elasticity and material model of coating the results has shown to increase the collapse capacity when the coating thickness and elasticity increases. The values has shown a increase in range of 5 to 25 %. The ovality of the coating however has shown to not effect the pipe collapse capacity in any significant way. Also based on a proposed simple analytical collapse model of serial resistance where the collapse capacity of the steel pipe is added to the collapse capacity of the coated pipe without the steel, the model has shown a good agreement with ABAQUS collapse data and thus is recommended for use in collapse calculation of coated pipelines. For future work a study is proposed to investigate the possibilities of steel pipe wall thickness reduction when adding thick coating to a line pipe