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Naval engineering

Marine trials

Laboratory expands the study of projects for oil exploration vessels

EDUARDO CESARPhysical tank at Poli-USP: it simulates environmental situations such as waves and windEDUARDO CESAR

Along the Brazilian coast, at the bottom of the ocean, there are large deposits of gas and oil, principally in the Southeast of the country, where everything points to huge reserves in the pre-salt layer. Exploiting this will require floating structures (platforms and ships) and submarine systems with cutting-edge technology, all of which are highly expensive. To find out whether these will run properly and whether the money invested will not go to waste, these pieces of equipment undergo validation testing before they are built and launched. Such tests are conducted in virtual tanks on computers and in physical trial tanks similar to swimming pools. Since December, the two types of testing have been brought together at the University of São Paulo’s Polytechnic School (Poli-USP). Called the Numerical Trials Tank (TPN – Tanque de Provas Numérico), the school’s virtual laboratory has existed since 2002, thanks to a partnering agreement signed with Petrobras in 2006. The TPN has become one of the four nodes of the Thematic Network for Scientific Computing and Visualization known as the Galileu Network. The other three are at the Federal Universities of Rio de Janeiro and of Alagoas and at the Pontifical Catholic University of Rio de Janeiro (PUC/RJ). Another 10 universities and research institutions are part of the Galileu Network.

The expansion and upgrading of the TPN has taken place in response to these new needs, which materialized primarily as a result of the 2007 announcement of the discovery of oil and gas on the seabed, in the layer beneath the salt. The laboratory’s computing system acquired a new cluster of computers, with 1,792 processors running in parallel. To house this new resource, besides the physical tank, new facilities were built, covering a floor area of some 1,600 square meters and with room for more than 80 researchers. All in all, R$9.5 million was invested, R$9 million of which came from Petrobras. The rest was provided by Finep, the Projects and Studies Finance Agency.

The physical tank, called a Hydrodynamic Calibrator (CH-TPN-USP), is 14 meters long, 14 meters wide and 4 meters deep and has generators and wave absorbing devices (flaps). There are 148 of these, distributed along the tank-s entire perimeter. One can therefore create multidirectional, regular or random waves. According to professor Kazuo Nishimoto, from the Department of Naval Engineering and Oceanic Science of Poli, who is the TPN coordinator, thanks to the flaps, which can absorb waves without reflecting them, one can also simulate the conditions of endless sea, as if the tank had no sides and as if the waves could spread without being reflected. The flaps, fans and other systems enable one to represent not only the waves, but also the main environmental conditions that affect ships and marine platforms, such as currents and winds, from a light breeze to hurricanes. Furthermore, one can also reproduce the dynamics of the anchoring lines (the cables fixed to the seabed that keep the platform in place) and of the risers (the rigid, steel or flexible ducts that carry the extracted oil up to the production platform).

In spite of all these advantages, this type of tank also has some inconvenient features. In addition to being costly, it has physical limitations when it comes to simulating what takes place in very deep conditions. Therefore, it cannot reproduce the dynamics of the entire system faithfully. To better represent conditions lying at some 3 three thousand meters or more below the surface of the sea, one would need a tank of such large dimensions that it would be physically unviable. Alternatively, the models of the ships and platforms would have to be so small that this would jeopardize their physical representation and real scale analysis. To overcome such limitations, there are the virtual tanks of numerical simulators, such as the TPN. This is a computer program that can mathematically represent the same conditions of a physical trials tank, with the advantage that there are no size restrictions and that one can get precise results faster. Furthermore, the numerical simulator calculates the dynamics of the floating units, the stresses and tensions in the retaining cables and the risers.

A task handled fast
The cluster of computers can carry out hundreds of simulations of several environmental conditions in a matter of minutes. Nishimoto explains that such a cluster is comprised of a group of dedicated processors that solve a single task in a cooperative and integrated manner. Thus, the calculations are subdivided and distributed across all the processes in the cluster, enabling speedier problem solving. This system was chosen in 2002, when the TPN was created. “We decided that instead of investing in specialized supercomputers, it would be best to develop a cluster with a similar performance but at a far lower cost”, Nishimoto tells us. Today, the TPN’s processing capacity is 55 teraflops, i.e., 55 trillion mathematical operations a second. Just as a measure of comparison, the fastest PCs available cannot reach 0.1 teraflops. Flop stands for floating point operations per second.

The TPN simulations are essential for the success of the exploration of the oil reserves in the pre-salt layer, according to Luiz Levy, the scientific methods manager of Cenpes, the Petrobras R&D Center. “The reserves are far from the coast and very deep, which makes exploring them a challenge”, he says. “With the TPN one can conduct a series of calculations and simulations”. Besides the conventional oil and gas exploration simulations, the Galileu Network can also represent the process of drilling through salt layers. “Drilling these layers is a major challenge, because they deform and can cause the drilling column to collapse”, explains Nishimoto. “For each well one must create a numerical model that simulates the conditions of the sea and the soil and that calculates the dynamics of the vessels involved in the exploration.”

The TPN tests give rise to such a huge volume of data that it is almost impossible to analyze it with conventional methods. To solve this, a visualization computer program, named TPNView, has been created. It is based on real time graphic computing techniques, which provide a precise representation of the environment in the form of virtual reality. It combines visualization along three dimensions (3D) and data analysis tools such as statistics, graphs and diagrams. Despite the TPN’s sophistication, it does not replace the physical trials tank. One complements the other and it is precisely this combined work that makes the USP laboratory a rare one, worldwide. “There are lots of physical tanks in Norway, the Netherlands and Japan, but there is no laboratory that brings together a physical tank with a numerical one based on a cluster as powerful as 55 teraflops. Thus, the TPN is, in essence, unique in global terms.”