This paper presents a comparison between field data from a West Africa deep offshore field operated by TOTAL and software available for design engineering. Multiphase flow simulations have been performed for several production loops to evaluate the software ability to reproduce steady state operations (slugging, pressure and temperature profiles, etc.) and the full preservation sequence including shutdown and fluid displacement by dead oil circulation.
Results show that software predictions are generally in good agreement with field observation especially with regards to pressure variation and timing. However the structure of the interface (i.e. mixing zone) between the cold dead oil and the live oil is not well reproduced for the loop operated in hybrid mode (i.e. only one branch in production, the other one being already preserved with dead oil).
In oil dominated systems, one of the most common means of preservation against hydrate formation in deep water subsea production networks is to displace the live oil by circulation of stabilized dead oil. In a conventional design, the dead oil is pumped from the cargo tanks and the production lines and risers are looped to allow circulation from both ends.
However ensuring the displacement of a multiphase production fluid sitting in deep water risers and flowlines involve complex transient phenomena such as thermal effect (cooldown and warm-up, cold spots effects as manifolds, subsea connectors), occurrence of important pressure variations (depressurization, packing, unbalanced liquid head in risers) and multiphase flow effects (phase segregation, fluid displacement, liquid slugs).
In design phases, it is required to model in detail the full preservation sequences to optimize the thermal specifications of the equipment and demonstrate the capabilities of the subsea system to stay free from hydrate formation during the preservation planning considered and to design relevant circulation pumps, etc. Flow Assurance engineers then use commercial multiphase flow software for the design of field development facilities.
