Thermal-Numerical Simulation of the Gas Offshore Production Undersea Facilities at the Amistad Field
Authors: Gallo Cruz Félix , Sola Andrés Leonardo , Rosero Joshua Andre , Gonzalez Jaime Oswaldo 
Published: October 2019

Abstract
Thermal numerical simulation of two undersea flowlines of natural gas production at Amistad field was developed in order to avoid non-productive time caused by plugging of solid precipitation; due to the operation conditions and recorded problems during gas production in this field, a complete study of thermodynamics, heat losses and multiphase behavior of the production fluid becomes imperative to prevent and control solid formation and flow blockage.
The thermodynamic behavior of gas was analytically determined based on the thermal and transport properties of fluid phases, beginning with the study of the pressure-temperature diagram of the production fluid to define the type of solids that could precipitate within the flow lines (hydrates). The heat transfer analysis was determined applying two methods, one numerical and one analytical, the numerical by the computational fluid dynamics (CFD) method with the support of the ANSYS-CFX software; and the analytical model obtained from the literature that was used to validate it. The hydrodynamic behavior of multiphase flow and pressure losses were determined by the Beggs & Brill (1973) correlation and were contrasted with the open source DWSIM software tool performance. The overall heat transfer coefficient was determined before continuing with the numerical modeling due to its importance and influence in the CFD simulation, which covers three stages: selection of the most optimum pipeline model, mesh refining, and validation of the developed model for a heat transfer phenomenon in two-phase flow. Once the heat loss model was defined by a steady state simulation, a transient simulation was carried out to calculate the gas cooling time in a case of sudden flow shutdown, considering the standard pipe currently used and three polymers of low thermal conductivity as proposed alternate materials instead of carbon-steel or as or as thermal insulation coatings, as appropriate; these are polypropylene, polyurethane and high density polyethylene. Finally, five analytical correlations of hydrate precipitation obtained from the literature and two software precipitation equilibrium curves were selected to define the precipitation scenarios of the system using the simulation results.
According to the P-T diagram, the only solids that could be precipitated in the steel pipelines are methane hydrates due to the composition of the production fluid and the operating conditions of the wells. The thermal gradient of the flowlines is the most relevant result of the numerical heat transfer analysis, this one shows the critical points of the pipes where the fluid reaches its lowest temperature, that is, the temperature of the underwater current. The critical point from the wellhead for both steel lines is 5560 [ft] according to the resulting thermal gradient, although for the second line, it is actually a bit shorter due to its length limit, 5300 [ft]. From the analysis of pressure and temperature conditions at these points, two scenarios of precipitation were established considering all the equilibrium curves. Then, one of the proposed scenarios confirmed the presence of hydrates in these two production lines of the field, providing a clearer realization of the problem that includes the time, distance and critical conditions of pressure and temperature where precipitation occurs. Subsequently, the three analyzes of heat loss of the other proposed materials were compared with the results of the base case to determine the most effective pipeline configuration to avoid the formation of hydrates and it was concluded that precipitation will not occur throughout the entire flowline if any of the polymers is applied as thermal insulator or pipe material, as appropriate.
In addition, the effects of the hydrodynamic phenomenon caused by the multiphase flow were studied, determining a mist flow pattern, where the liquid phase of the fluid is dispersed within the gaseous phase in the form of small water droplets.