MOLDITM: A fluid permeation model to calculate the annulus composition in flexible pipes

Abstract

In the offshore petroleum industry, flexible pipes, used as risers or flowlines, are made with internal and external polymer sheaths that ensure internal fluid and outer seawater integrity. These polymers exhibit permeability towards some gases which can create potential damage mechanisms (CO2 corrosion and hydrogen attack) and reduce the lifetime of the steel layers located between the internal and the external plastic sheath. These damage mechanisms are associated with water condensation, which must therefore be predicted. The following paper describes a finite element model named MOLDITM used to predict diffusion of gases through layers of flexible pipe versus time. The temperature gradient in the pipe is taken into account, as permeation rates depend on temperature. It is coupled with a thermodynamic flash calculation that gives vapor and liquid phases, compositions and volumes in the annulus. Associated mathematical models and resolution methods are examined, as well as the coupling of diffusion and phase prediction. MOLDITM was programmed under Matlab, with a friendly graphical user interface that helps input and output of data and results. This paper illustrates the possibilities of this software.