Using CFD to Investigate Safety Issues with Cryogenic Restarts
Authors: Davidsson Niklas , Friedemann John D. , Gharaibah Emad 
Published: October 2011

The low temperatures resulting from Joule-Thomson cooling during start-up of pressurized gas filled wells after blow-downs presents significant material and safety challenges. The equipment downstream of the choke will be subject to cryogenic temperatures, with several reliability issues arising as a consequence:Freezing of fluids including inhibitors,Embrittle of materials andIce formation on the external surfaces and enclosed galleries.

Piping temperatures and hydrate issues have traditionally been estimated using PVT (pressure, volume and temperature) models and 1-D pipeline simulators, but the icing issue as well as the need to investigate the temperatures of seals or other temperature sensitive equipment requires a 3-D modeling approach.

This paper describes a 3-D numerical approach for estimating the temperature in the surrounding equipment during well start-up. This allows for capture of all of the relevant physical aspects for the production fluid (super-sonic flow, pressure drop and Joule-Thomson cooling) in combination with resolving the geometrically complex surrounding equipment, and the heat flow through these. The benefits of the approach are demonstrating the critical pressure drops for external ice formation and mitigating the need for exotic materials.

Different options for determining the temperature fields in the subsea equipment are investigated. The first approach involves resolving the full physics through a compressible CFD simulation, including the thermodynamic behavior of the production gas. The full internal details of the choke cage and trim may however not be fully known at the time of the study so a two-fluid simulation approach using two incompressible gases, with different temperatures, is developed and tested. Here, the downstream choke fluid temperature is set based on the traditional PVT calculation approach. Mass flow rates and other boundary conditions are identical to the compressible simulation, which is used to benchmark the quality of the two-fluid approach.

The results of the compressible CFD simulation show that the model successfully captures the relevant physics. The Joule-Thomson effect gives a temperature of −77°C at the simulation outlet with even colder temperatures are observed in the expansion regions downstream of the choke restrictions. This temperature change corresponds well the value of −80°C derived from literature data. The model also shows that ice will form at the majority of the external surfaces during the start-up scenario defined for this study. The two-fluid approach gives a decent match of the temperature field to the field observed in the compressible simulation.

Because of fluid acceleration, the local temperatures inside and shortly downstream of the choke the expanding gas are below those from the traditional first-law PVT approach. As this effect is omitted in the two-fluid simulation, solving the full compressible momentum and energy equations is the preferred and most correct approach for studying Joule-Thomson cooling of subsea equipment.