Graphic output from Floatsim traces the movement of one of the two vessels, in this case, over a period of 10 minutes.
Caran Dynamics specializes in advanced design analysis primarily for the marine and automotive. One of its main areas of expertise is the motion and wave load analysis of offshore structures.
Using in house-developed software supplied by, amongst others, the Massachusetts Institute of Technology (MIT), Caran has worked on a variety of notable projects for clients in the North Sea. For example, it helped predict the way the giant 400-meter-high Troll C gravity base structure, complete with topsides, would behave during tow-out and installation in 1998. At the time, Shell reported that this was the largest artificial structure ever moved across the face of the earth. Moving further from home, Caran contributed its know-how to the tow-out into the Atlantic of the Hibernia deck on board two separate barges.
For the past couple of years, the company has spent a good deal of time studying the motions of vessels under dynamic positioning (DP) with the aim of being able to predict how well they will perform under different wave, current and wind conditions.
Less trial and error
Recently, the scope of the work was extended to cast more light on a particularly tricky problem, as Karolina Eriksson, head of the group carrying out the work, explained:
"It is increasingly common to find a vessel using its DP system to maintain station close to a neighboring one, which is permanently moored but obviously still subject to significant movement. The two could well be connected via a gangway. A good example would be a floating accommodation platform operating under DP and linked to a semi-submersible production unit. When you have two vessels moving around, the DP system on the first is obviously not just working to keep the vessel stationary but to ensure it follows perfectly the motions of the second, thus keeping a constant distance between them."
It would clearly be very useful to know, before these operations are attempted, how effectively the DP system will perform under different wind and sea conditions. Operators would like to know under what conditions they are likely to have to disconnect the bridge - combining this information with weather data would allow downtime to be more accurately forecast, which in turn would enable better planning.
Going one stage further, operators might like to know what effect upgrading the DP system might have; for instance, whether it would improve performance to such a degree that it was cost-effective in terms of reduced downtime? Finally, from a safety standpoint, it would be helpful to know what would happen if individual thrusters were to fail.
Caran has developed a program called Floatsim that enables accurate answers to questions like these. Cons-equently, operators are less dependent on previous experience - which is not always available - and do not have to resort to trial and error.
Yungang Liu, PHD, who executed the work, explained that the program takes account of the motion characteristics of both vessels, the performance properties of the first vessel's DP system - how it reacts and how it is controlled - the characteristics of the second vessel's mooring system and the forces generated on both platforms by the waves, currents and the wind.
"We are proud of the program for a number of reasons. It combines a wide range of variables, it makes few assumptions, and it is very accurate. Furthermore, we are confident of being able to modify the program easily if clients come to us with specific requirements," Liu said. Every half second, the program calculates the movement of any given point on either vessel in six degrees of freedom. Graphic output is possible showing the changing envelopes of movement of the two vessels as time goes by. The program can be run on any standard desktop PC.
Eriksson explained that there was still one area where the program could be refined. "The interaction between the individual DP thrusters and also the thrusters and pontoons of the semisubmersible present us with some problems," she said. "We have accurate specifications for the performance of the thrusters, but they are based on their operation when there is nothing surrounding them in the water. In reality, they have a diminished effect as the water moved by the thruster sited on one pontoon impinges on the opposite pontoon. This will have the effect of pushing the vessel in the opposite direction to that desired. We can simulate the effect using computational fluid dynamics but some assumptions inevitably have to be made."
Besides motion analysis, Caran Dynamics has experience of mooring and riser analysis and design. The company also carries out structural analysis of marine components. As part of the larger Caran Group, the range of work that can be undertaken extends to the detailed design of complete marine structures.
For more information contact Karolina Eriksson, WM-data Caran. Tel: +46 31 335 3620, Fax: +46 31 335 9712, E-mail: [email protected]
