The Christmas tree is a set of valves, remotely operated and installed in the oil well head, that control the well's production in an intelligent and optimized way, without the need for maintenance. Another function of the Christmas tree is to measure the pressure and temperature of the well and its currents. Chains can be either production extraction or injection chains. In Brazil, wet Christmas trees are the most common due to greater off-shore production. The name Christmas tree comes from the dry type in cold regions of the USA, which after the incidence of snow looked like a pine tree.
In the pre-salt, they operate at great depth, below 2000 m, so they suffer high external pressure from the ocean and the average temperature is 4 ˚C. They can pump 5,000 to 15,000 barrels of oil a day. A 5,000 psi tree weighs 86 tons and is approximately 7 meters. Following installation from the well, the Christmas tree has the following parts: BAP – Production Adapter Base, Lower Orientation Hopper, High Pressure Housing, Flow Line Mandrel (MLF), MCV Nozzles, SC – Column hanger, Main forged body, Outer jacket with castle, Sub extender, ANM – Wet Christmas Tree. A typical configuration of wet Christmas tree valves is: production master valve, a production side valve, a production and ring interlock valve, a master ring valve, a side valve.
If the extraction of the oil well is interrupted, the temperatures of the lines and of the Christmas tree itself drop due to ocean temperatures (cool down), 4 ˚C, since there is no longer the high temperature flow flowing through the pipeline. Oil flow must be carried out above 40 ˚C due to the presence of paraffins, which solidify at lower temperatures and impede flow. Therefore, an important study is to determine the temperature cooling curve at various points on the Christmas tree from the shutdown of hot runoff. To delay cooling and save time, the Christmas tree has thermal insulation. The material, thickness, shape and installation locations of the insulation need to be studied so that the design can be optimized.
Thermal analysis of Christmas trees is done with CFD software (CFD++) or with advanced thermal modules from FEA (Adina Thermal), which make it possible to calculate not only the transient in solids but also in liquids. iChrome Nexus software is used to maximize time and design thermal insulation coupled with FEA or CFD.
ATS has already done designs and simulation involving steady-state and transient thermal analysis for subsea systems for Halliburton and for Oceaneering, which was on a research and development project for Shell Houston.
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