To minimize capital costs, operating companies are choosing to produce oil and gas through longer and longer subsea tie-backs. Transportation of multiphase gas, oil and aqueous mixture over long pipelines under high pressure and low subsea temperatures present some unique challenges. One of the most important challenges in flow assurance for such more common long pipeline designs is the possibility of blockage due to hydrate formation and/or wax deposition.
This paper discusses results using the newly developed wax deposition and hydrate transport models for a pre-FEED study. The models demonstrated are integrated into a generic and well proven multiphase dynamic simulator framework designed to be used for pre-FEED and FEED studies for a green oil field as well as to trouble shoot production challenges and simulate production of an already producing field.
LedaFlow™ is the multiphase, transient simulator that was used during the study. It contains a framework which easily allowed implementation of the new modules for wax and hydrate.
Wax deposition and hydrate transport are two important challenges for pipeline design and operations with waxy oil fluids that are prone to hydrate formation. An accurate model for quantifying the wax deposition and hydrate transport processes is essential to assure secure and safe transportation of these hydrocarbons. In addition, providing accurate predictions of not only the pressure drop along the pipeline but also of the flow behavior during the transport will help to optimize the development schemes, avoiding over-design. There are several options for modeling both processes, however, for the evaluation reported here not only the process itself is important, but to model dynamic operational scenarios is also important. For this study a waxy and hydrate forming stream was selected. This stream comes from a single stage separator in an offshore facility.
The new wax model includes some key features, such as the non-Newtonian nature of the waxy oil, the growth of the wax layer and the effect on heat transfer throughout the pipeline. It is based on a known model from the literature, however, it brings a fundamental new framework for simulating the effect of wax on multiphase flow as it benefits from all the functionalities of the multiphase flow simulator like transient operations, accurate thermal modeling including use of heat tracing. All of this has been considered in our study of normal operations. In the hydrate transport model, mass transfer during hydrate formation, the hydrate former/water depletion in the pipeline and the slurry viscosity with hydrate particles have all been considered.