MOLDI™: A Fluid Permeation Model to Calculate the Annulus Composition in Flexible Pipes — Validation With Medium Scale Tests, Full Scale Tests and Field Cases

Abstract

In flexible pipes, the steel wires in the annular space can sometimes be in contact with a corrosive environment containing condensed water or seawater and acid gases (CO2 and H2S), coming from the bore by permeation through internal polymer sheaths. In order to choose the most suitable steel wires to avoid hydrogen embrittlement and to evaluate their resistance to corrosion and corrosion fatigue, it is necessary to know the annulus composition. A finite element model named MOLDI™ has been developed in order to precisely calculate this environment. This model is composed of two major modules: The first one describes the time dependent permeation of fluids through the pipe layers. Concentration and pressure versus time are calculated assuming Fick and Henry’s laws. The shielding effect of steel wires on the plastic sheaths and the temperature gradient through the structure are taken into account. The second module uses a thermodynamic flash algorithm to calculate the phase equilibrium in the annulus, including water condensation. In order to validate this model, a permeation testing program has been initiated in 1997. It is based on medium and full scale permeation tests and field cases. For each test, the pressure build-up and the composition inside the annulus are measured and compared with the ones calculated with MOLDI™. Medium scale prototypes consist in a polymer tube surrounded by a steel part simulating the annulus geometry of a flexible pipe (shielding, free volume). One of the prototypes has been specially designed in order to be able to maintain a chosen temperature gradient between the bore and the external side. The full scale permeation tests are conducted on a piece of manufactured flexible pipe. At the beginning of the test, the bore is filled up with a pure gas or with a fluid mixture (gas–water–crude oil) under pressure and temperature. The permeation rate from the bore to the annulus is determined by recording the pressure build-up in the annulus versus time. The gas composition in the annulus is measured using a Gaseous Chromatography device. Field cases have also been used to validate MOLDI™. To do that, the annulus of flexible risers has been monitored on site and the measurements compared with MOLDI™ simulations. The validations conducted at the time being, using different polymer sheath materials and fluid mixtures, have shown that our permeation model predictions are in good agreement with experimental results. Consequently, MOLDI™ is considered as validated and is now used to calculate the annulus composition from the field conditions.