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Author:
Allie Yuxin Lin
Marketing Writer
In 2010, an explosion occurred on the Macondo Prospect off the Gulf of Mexico, taking the lives of 11 men and releasing 5 million barrels of oil into the water. The “worst oil spill in US history” led to thousands of lost habitats and created a graveyard of coral reefs stretching a mile deep beneath the blowout site.
After this devastating event, four major oil and gas companies joined forces to create a nonprofit devoted to providing containment technology, such as containment domes, funneling caps, and capping stacks. A subsea capping stack is not in the water during drilling; rather, it is the centerpiece of a containment system kept at a nearby onshore location. Since it is only deployed after the subsea blowout preventer has failed, it serves as the second line of defense in preventing oil spills. A capping stack’s primary purpose is to stop or redirect the flow of hydrocarbons, buying time for engineers to permanently seal the wellhead.
This giant piece of equipment can weigh up to 100 tons, which makes maneuvering the device to seal the small opening of the blowout preventer quite difficult. Computational fluid dynamics (CFD) can model capping stacks to inform well control decisions and response operations, prevent incidents, and minimize risk.
In order to effectively model a capping stack deployment, we need a transient CFD simulation to capture the dynamic interactions between the jet of hydrocarbons released from the well and the capping stack. Transient simulations most accurately mimic the reality of freely-flowing gasses and liquids. Capturing the multi-phase physics of this problem can be accomplished by CONVERGE’s volume of fluid (VOF) model. Additionally, an accurate simulation requires modeling the combined dynamics of the rigid capping stack and the flexible cable attached to a crane which is used to maneuver the capping stack into position. CONVERGE’s autonomous meshing technology, including Adaptive Mesh Refinement (AMR), makes the software well suited to capture the complex geometry of the capping stack, the associated flow features, and the fluid’s interaction with the stack. In these types of simulations, your targets are constantly changing due to both the transient evolution of flow features as well as the motion of the geometries. AMR allows you to adapt to the changes in the flow by refining the mesh automatically throughout the simulation. In a system with both fluid and solid components, the fluid exerts forces on the solid, which are distributed around the structure. CONVERGE’s fluid-structure interaction (FSI) modeling calculates these fluid forces, predicts how the structure will react, and moves the solid accordingly.
For our case study, we used CONVERGE to simulate capping stack placement onto a blowout preventer. In this simulation, we employed FSI modeling and AMR based on void fraction. A void fraction is a mathematical representation of the gaseous fraction of the volume of a single cell in a generated mesh. Using void fraction to predict which regions need finer mesh allowed us to capture the important physics. What’s more, this method helps maintain a sharp interface between the liquid and the gas, avoiding excessive numerical diffusion, which is caused by the discretization of the continuous fluid transport equations. Although numerical diffusion is generally unavoidable in CFD codes, using AMR based on void fraction reduces the amount that is introduced into the system. In addition to modeling the capping stack, we also used CONVERGE’s mooring cable model to capture the interaction between the cable and the surrounding water. CONVERGE’s suite of advanced models and features allows us to efficiently model these problems while maintaining a high degree of accuracy.
The power of CFD stems from its ability to predict if something will happen before it happens. The power of CONVERGE is that it does this in a streamlined, effective, and accurate way. If you want to learn more about this case, or how CONVERGE can solve other oil and gas industry problems, take a look at this webinar or contact us today!
[1] United States Government Accountability Office, “Oil and Gas: Interior Has Strengthened Its Oversight of Subsea Well Containment, but Should Improve Its Documentation,” GAO-12-244, Feb 29, 2012.
