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Simulation of Cable Dynamics For Moored Ocean Platforms
Modeling Aids Design of Large, Underwater Power Cable

AUTHORS:


Joana Prat

Marisa Zaragozá

Joaquín del Río Fernández



OBSEA’s buoy at the Vilanova coast.
Observation platforms at sea need power cables for energy. Such platforms and cables are exposed constantly to the dynamic behavior of sea waves, wind and current.

It is useful to carry out numerical simulations of how a large underwater power cable connecting an ocean surface platform to a seafloor technology would behave with regard to marine conditions prior to design and installation at sea. This would help a manufacturer identify critical parameters that could affect the cable in the field, for instance, in projects that would require mooring wind turbines in the ocean.

We conducted a simulation experiment using a small, existing platform, OBSEA, located 4 kilometers offshore the Vilanova i la Geltrú coast in a fishing protected area of the Catalan coast of Spain. OBSEA is a cabled seafloor observatory connected to a station on the coast by a power-and-communication cable. The station located onshore provides the power supply and a fiber-optic communication link, while carrying out alarm management tasks and storing data in real time. The marine observatory is located at 20 meters depth and gathers data on waves, current and pressure, among other environmental factors. A buoy to gather meteorological data is moored to the seafloor observatory by three chains of 30 meters length each.


Methodology and Simulations
OBSEA’s buoy is currently moored to the seafloor observatory by an umbilical. Instead of using an umbilical in our simulation, we modeled the buoy to simulate a large power cable connecting the buoy to a technology on the seafloor. The purpose of this simulation was to predict the behavior of such a cable and a moored platform at sea using real-world conditions.

The numerical simulations were done with the help of OrcaFlex software, a marine dynamics program developed by Orcina (Ulverston, England) for static and dynamic analysis of a wide range of offshore systems. OrcaFlex provides fast and accurate analysis of umbilical cables under wave and current loads and externally imposed motions. It is a 3D nonlinear time domain finite element program capable of dealing with arbitrarily large deflections from the initial configuration.

Simulations of OBSEA’s buoy show the dynamics of the buoy, power cable and chains under real-world conditions, i.e., the orbit of the buoy and cable and chain tension as a function of time or variations of curvature.

Real data of meteorological conditions on December 16, 2011 were used for the OrcaFlex model, with some restrictions. A periodic sea wave, a constant wind and a time-constant profile of current were used. The day was chosen to represent typical wind conditions on the Vilanova coast instead of averaged values between time periods to offer more realistic ocean conditions for modeling. We fixed the real-world conditions at the time of 9 p.m. UTC, characterized by a maximum wind speed of 11.27 meters per second and 117° direction of advance. The significant wave height was 3.05 meters, with a period of 7.52 seconds and 17° direction of advance. The current intensity profile was divided into three layers: on the top, a linear boundary layer of 3 meters, with a maximum of 0.92 meters per second and 87° direction of advance; a middle layer of 10 meters depth, where current intensity was constant at about 0.55 meters per second; and a third layer of 7 meters above the seabed, with intensity decreasing linearly to zero. The current direction depended on depth.

The OBSEA buoy simulation was moored with three chains on the seabed in a circle of 20 meters radius. The chains were equally spaced at 120°. The buoy consists of one cylinder 4 meters long and 0.8 meters in diameter, and another small cylinder on the bottom that is 0.9 meters long and 0.05 meters in diameter. It weighs 650 kilograms in air. At the bottom is a free link to three chain branches 0.65 meters long, 0.03 meters in diameter and 130° declination, equally distributed. To continue this article please click here.



Joana Prat has been an associate professor of the Departament de Matemātica Aplicada IV at Universtitat Politécnica de Catalunya (UPC), Spain, since 2001. She is a member of the research group SARTI. Her research focuses on applied methods to marine problems: nonlinear fluid dynamics, modeling of submarine moored cables and modeling of bottom trawl gear.

Marisa Zaragozá has been an associate professor of the Departament de Matemática Aplicada IV at UPC since 2001. She is a member of the research group COMGRAF, which is interested in the theoretical problems arising from the design and analysis of interconnection networks.

Joaquín del Río Fernández received his B.S. and M.S. degrees in telecommunication engineering and electronic engineering in 1999 and 2002, respectively, from UPC. Since 2001, he has been an assistant professor at UPC and a member of the research group SARTI. His research focuses on electronic instrumentation, interoperability in marine sensor networks, wireless sensor networks and the OBSEA cabled observatory.







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