CFD techniques improve stability of platforms and mooring systems

Graphical output of CFD analysis carried out by Caran Dynamics on a floating production unit in order to determine current loads on the hull. [14,469 bytes]

With more than 60 vessels - including FPSOs and barges - in its vessel database, marine engineering specialist Caran Dynamics believes it can now support rig designers and owners in a more efficient way by comparing new designs with old concepts. All data is systematically stored and can be used to perform a variety of studies on vessels such as motion, stationkeeping, risers, stability, structural analysis, environmental forces, and ventilation.

In the area of environmental and ventilation forces, Caran has introduced computational fluid dynamics (CFD) to its range of services which it uses to calculate the forces subjected on platforms by wind and currents. CFD studies have replaced the previously standard wind tunnel tests and Caran believes it has the leading edge on this new technology.

Using sophisticated software packages such as FLUENT/UNS, FIDAP and RAMPANT, Caran performs studies such as evaluating the drag resistance of semisubmersible platform hulls and the air intake of exhaust from gas turbines into accommodation modules relative to their position on the platform.

Fluid and structural dynamics manager, Mickey Johansson, explains that by using a new mesh technique, a computer model can be created very rapidly to calculate wave loads on structures. "Motion analysis models can be translated easily in order to calculate environmental loads using our historical database," says Johansson. "We believe we have the most comprehensive database of this type available to the offshore market and therefore have the ability to perform these calculations very efficiently."

Umoe spar

As an example of work performed using CFD analysis, Caran has conducted current and wave analysis on the spar platform concept developed by Umoe in order to determine the optimum position for the strakes which are attached to the uppermost sections of the spar. The strakes produce friction which dampens the vertical motion of the spar caused by the movement of the sea.

Caran has also used CFD analysis to calculate the loads on the cruise version of the recently launched Mermaid advanced azimuthing pod propulsion system designed and built by Kamewa and Cegelec as well as conducting analysis on a variety of water jets.

Caran continues to work closely with GVA Consultants performing both riser, stationkeeping, and motion analysis on a number of its semisubmersible designs including, most recently, the Troll C and Visund platforms. For the Visund platform, the analysis of the original design resulted in a recommendation that the original 12-point mooring system be increased to a 16-point mooring system which actually ended up being more cost effective. "In addition, we looked at the fatigue life of the mooring chains, the offset positions of the rig so that the risers don't get strained, and the optimization of wire-to-chain ratio," says Johansson.

On Troll C, Caran performed a stationkeeping analysis looking at the interface between the risers and the mooring system. Recommendations to the project following this analysis resulted in substantial cost savings. Analysis was also carried out on the stresses subjected to the Ramnas chain links caused by being run out over the fairlead.

Slamming forces

"Every FPSO project is different because of the different weather and environmental forces to which the structure is subjected," continues Johansson. "On the Esso Jotun project, for example, we also looked at the effect of green water coming on deck if the bow went under the water line and also the effect of slamming forces caused by the bow rising and falling in heavy weather."

For all 3D structures it has worked on, Caran uses the multi-body radiation-diffraction panel program WAMIT developed by the Massachusetts Institute of Technology to calculate wave induced loads and motions. In order to provide fast and accurate end-user results, however, Caran has developed its own POSTWAM package which calculates additional quantities of interest, short-term responses in irregular long-crested and short-crested seas, as well as providing graphics output facilities.

In addition, POSTWAM is used as an interface program for efficient and reliable transfer of motion data to mooring and riser programs such as MIMOSA, MODEX, ARIANE-3Dynamics and FLEXCOM-3D.

Impact analysis

Caran has also extended its experience of impact analysis developed for the automotive industry for use in the offshore market. The company has applied this knowledge when it looks at the stress distribution and risk of structural collapse following incidents such as a vessel crashing into a semisubmersible or a crane collapsing and hitting part of a pontoon. Caran has carried out such an analysis on the Troll C platform and expects this type of work to become more common in the future.

For further information contact Mickey Johansson, Caran Dynamics: telephone +46 31 335 6017, fax +46 31 335 9712, email mickey.johansson@caran.se or http://www.caran.se.

Copyright 1998 Oil & Gas Journal. All Rights Reserved.

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