Most of Brazil’s offshore resources are in deep waters so Petrobras has fostered substantial expertise to develop these fields. One area of importance is the design of marine vessels to withstand the extreme waves. While the discovery of 50 billion barrels of oil in recent decades has been a boon to Brazil’s economic outlook, the location of the oil has produced challenging engineering problems. Lying hundreds of kilometers offshore under up to 3,000 meters of seawater, 2,000 meters of rock and 2,000 meters of salt, the oil reserves are some of the most difficult to access on Earth. Engineers are systematically using best design practices and computational fluid dynamics (CFD) to increase the safety of marine structures and vessels used to drill and produce oil from Santos Basin fields.
Besides the task of drilling through these layers to such great depths, the floating production storage and offloading (FPSO) vessels that store oil until it is transferred to a tanker are subject to tremendous forces from waves that can reach up to 12-meters high. The worst case scenario is when unbroken waves crash onto the surface of these vessels, a phenomenon known as “green water.” These waves have enough energy to damage or destroy equipment on deck such as valves, cable trays and fire protection equipment, among other critical structures. Rare wave events could shut down the vessel and cost the company hundreds of thousands of dollars per day.
Full-scale testing is virtually impossible as an enormous ship would need to be built and instrumented at great cost. Traditional scale-model physical experiments to measure the loads from green water on various structures on the deck are time-consuming, costly and limited in the data they provide. To get better data in less time and at lower cost, Daniel Fonseca de Carvalho e Silva, a research engineer with Petrobras, turned to ANSYS computational fluid dynamics software to predict the loads on deck structures due to breaking waves during green water events.
Simulation provided much higher resolution than physical experimentation, enabling the engineer to get a better picture of the forces at each point on the deck. The data obtained by simulation verified the measurements obtained from the experiments, while providing additional information to better understand the physical mechanisms involved in wave interaction with the hull.
CFD simulation of green-water loading throughout hull and deck structures of an FPSO.
The simulations took from 10 to 50 days to complete, compared to roughly three months for the physical experiments. The data obtained helps Petrobras engineers to design the critical on-deck structures to withstand the forces of the green water without costly and unnecessary overdesign of the equipment.