Unbonded flexible pipelines used for offshore fields developments usually rely upon a stainless steel carcass as innermost layer for collapse resistance and a polymeric pressure sheath for internal leakproofness. For some dynamic flexible risers, this pressure sheath is a multi-layer construction made of 2 or 3 layers. In this multi-layer configuration, during operation, the fluid transported in the bore of the pipe can penetrate the annular space between the pressure sheath layers by means of flow paths through the end-fitting. Then, when the pipe bore is depressurized, the pressure of the fluid trapped between the sacrificial sheath and the pressure sheath does not decrease as fast as in the bore. This is due to the small annular flow path between the sheaths. Depressurization of the pipe bore should thus be performed at a limited and controlled rate to avoid an excessive differential pressure between the pipe bore and the annulus that could potentially cause the collapse of the carcass. The allowable depressurization rate is a key parameter the field operators need to know to avoid such an issue.
A model was developed and implemented in a numerical software to calculate the evolution of the differential pressure during the depressurization. It is based on a one dimension gas/liquid transient transport model. More specifically, it is composed of the mass and momentum conservation laws. The thermodynamics properties of the fluid are computed with a simple model. In addition, the dynamic behavior of the annulus is coupled with the conservation equations and the pressure of both the bore and the external environment. A Finite Volume Method is chosen to both discretize and keep the conservative properties of the model on a discrete level.
In addition, a large test campaign was carried out on a full scale pipe to validate the model. A test consists in a pressure increase followed by a depressurization. The pressures in the bore and in the annulus were permanently recorded using a specific device which enables to know in real time the differential pressure between the pipe bore and the annulus. Various conditions were tested by varying the fluid viscosity, the initial bore pressure and the depressurization rate.
After explaining the physics of the depressurization for this specific flexible pipe construction, this paper will present the numerical model developed to calculate the maximum differential pressure and the test campaign performed to validate this model.