Active Vibration Control of a Monopile Offshore Structure

In the Danish part of the North Sea it has been found that marginal fields can be exploited using monopile offshore platforms which present significant advantages with respect to the costs involved in fabrication and installation and can therefore tip the economic balance favourably. Monopile platforms have been developed for approximately 35 m water depth and to be remotely operated. However, there has recently been a wish to use the monopile concept on 75 m water depth. Using monopiles in such water depths can imply significantly dynamic problems. Therefore, in order to reduce the vibrations, it can be necessary to use an active or a passive vibration control system. However, for a monopile with severe space problems it can be difficult to locate a passive control system such as e.g. a tuned mass damper. Therefore, in order to active control wave introduced vibrations of a monopile structure an active control technique has been proposed in corporation with the consulting company Rambøll, Esbjerg, Denmark. The proposed control technique is based on the relationship between the position of the separation points of the boundary layer flow and the drag term in the wave force on the cylinder. This concept has been experimentally investigated with a test model in stationary flow tests. The idea is to have a large drag coefficient when the cylinder moves opposite of the wave direction implying a relatively large damping excitation. When the structure moves in the wave direction a small drag coefficient should be obtained in order to have a relatively small excitation on the cylinder. The drag coefficient can be controlled if the separation points of the boundary layers can be controlled. It is proposed to control the separation points by blowing compressed air out of the holes in the cylinder. If the natura1 separation points of the boundary layers are rejected by blowing air out of the holes the drag coefficient will increase while it will decrease if it is possible to attach the boundary layer. The results from the experimental test have shown that it is possible to increase the drag coefficient with a factor 1.5-2 by blowing air out of the holes in a cylinder vibrating in a stationary water flow.