CFD Thermal Analysis of Subsea Equipment and Experimental Validation

Abstract

Abstract This paper presents a Computational Fluid Dynamics (CFD) approach for characterizing thermal performance of subsea equipment. With the CFD approach proposed, local convective coefficients between the subsea equipment and surrounding seawater are numerically determined in a coupled way; both well stream and various trapped fluids are modeled as real fluids with conduction and convection contributions; and temperature-dependent thermal properties of associated materials are also implemented. The CFD thermal approach has been validated against experimental data conducted at Southwest Research Institute with very good agreements, followed by successful applications in deepwater subsea tree, manifold, and flowline connector projects. Introduction As the subsea industry goes into deepwater, it is now very common for a subsea equipment to be installed in a water depth from 3,000ft (915m) to 10,000ft (3000m). The corresponding operating pressure ranges from 10,000psi (69MPa) to 30,000psi (207MPa), and the operating temperature from 300°F (149°C) to 350°F (177°C). With the High Pressure and High Temperature (HPHT) environments where the subsea equipment is exposed to a low temperature and high pressure environment, flow assurance becomes critical for subsea production. This is becasue the cold surroundings favor large heat loss, while the high pressure condition is prone to hydrate formation. It is, therefore, a common practice to apply thermal insulation to reduce heat loss during production and prevent the onset of hydrate formation during unplanned shutdown and start-up events. However, complexities of the subsea equipment/system may prevent an equipment from being fully covered by insulation. As a consequence, cold spots may be created that will lead to formation of hydrate, and other flow blockages. The deep water operation also presents challenges for design of subsea equipment/system, thus makes the existing technologies currently used in subsea industry may not be applicable under HPHT conditions. The most obvious example is the insulation materials since the conventional insulations have a maximum operating temperature around 250 °F (121 °C). Problems may also arise from the non-metallic seal technology as the existing elastomeric seals are very sensitive to high temperatures. If the local temperature near the seals is over a certain critical value, the sealing performance will be reduced significantly and lead to severe consequences. Moreover, the subsea equipment often incorporates electronic sensors, which have specific qualification temperatures. It is, therefore, essential to ensure that the temperature around the sensors does not exceed the temperature limits to prevent damage due to overheating. Therefore, both operators and vendors are jointly faced with the challenges of developing environmentally and economically viable solutions for deepwater and HPHT applications, which deliver thermal performance, structural reliability, and production assurance. This implies that the requirements for thermal insulation and accurate prediction of temperature evolution during production and cool-down operation are increasing, and a reliable and accurate thermal analysis is essential to ensure that the designated thermal performance can be achieved Nowadays thermal analysis for subsea equipment has been largely conducted using Finite Element Analysis (FEA) approach [1-5]. With the conventional FEA approach, the well stream inside the production bore is simplified as solid body; the heat convection between the subsea equipment and the surrounding seawater is approximated by singular constant or empirical correlation; and convection contribution of trapped fluids are either ignored or empirically estimated. In addition, thermal properties of materials involved are frequently treated as temperature-independent constants. All these simplifications introduce uncertainty to the results of thermal analysis.