ARRANGEMENT FOR CONTROLLING WAVE FORCES ON BODIES SUBMERGED IN A FLUID

Abstract: Device for affecting the wave forces acting on a body completely or partially submerged in sea. Utilized in a wave power plant two bodies (28, 29) are connected with two hydraulic piston pumps (32), the bodies being provided to have circumferential currents in different directions. The bodies are suspended in buoys (35).

Description: Arrangement for controlling wave forces on bodies submerged in a fluid. The background of the invention. This invention relates a method for controlling wave forces on bodies submerged in a fluid and means for carrying out this method. The invention can for instance be applied for reducing heave motions of semi-submersible offshore plattforms or to increase the energy-absorption of wave energy converters. It is a well known fact that submerged bodies in the ocean experience wave forces from surface waves. These wave forces are fully determined by the wave paramters and the geometry and position of the submerged body, until now it has not been known any means to increase or decrease the wave forces in a given sea state on a given submerged body, in a controlled manner. The object of the invention The object of the present invention is to establish a method and means by which it becomes possible to change the wave forces in a controllable manner. It is a particular object of this invention to be able to change the wave forces on semi-submersible plattforms in such a way that the heave motions of the plattforms are reduced. It is another object of this invention to increase the wave forces on the wave interacting members of a wave energy conversion device, in order to increase the energy absorption of the device. It is a particular object to obtain a wave energy converter without the need of an external, fixed reference point, at the bottom of the sea. Principle of the invention: The present invention is based on the unexpected discovery that the wave forces on a submerged cylindrical body is substantially changed if the cylinder is being rotated around the axis of the cylinder. Depending on the direction of propagation of the incident wave relative to the axis of the cylinder and the rotational direction and the rotational velocity the cylinder, the forces on the body can be made substantially smaller or substantially larger than the wave forces without rotation. In special cases the wave forces can be practically eliminated. The method in accordance with the invention is specified in claim 1. Other characteristics of the invention, including those arrangements which are necessary to implement the invention, are specified in the other claims. The submerged body is preferably rotationally symmetric, as for instance a cylinder which rotates around its axis. Experimentally it has been found that by constantly rotating the cylinder the wave forces could be increased by a factor of as much as 6 above that of a non-rotating cylinder. Moreover, the wave forces could also be reduced by reversing the rotational direction of the cylinder. The explanation of the phenomena is given below. It is well known that rotation of a cylinder in a stationary flow gives rise to lifting forces (Magnus effect). However, a surface wave on a fluid does not establish a stationary, uniform flow in the fluid, and therefore has not created the additional forces which are obtained by the method in accordance with the invention. Example: Below the invention is explained in more detail by figures where: Fig. 1-3, shows schematically cross-sections of three different bodies with arrangements in accordance with the invention. Fig. 4 shows schematically a cross-section of a wave energy converter, anchored to the sea floor. Fig. 5 shows schematically a cross-section of a floating wave energy converter, and Fig. 6 shows schematically a cross-section of a semi-submersible offshore platform. In Fig. 1 is shown a circular, massive cylinder 11, being suspended or floating in water, with horizontal axis 12. The cylinder 11 is rotated around its axis 12 by a motor (not shown i Fig. 1) as shown by arrows 13. The cylinder 11 is simultanouesly exerted by waves with incidence as shown by 14. Depending on the direction of incidence of the waves relative to the axis of rotation of the cylinder, the direction of the rotation and the rotational velocity, the wave force on the body 11 can be made considerably larger or considerably smaller than the wave force without rotation. This can be explained as follows: The rotation of the body 11 sets up a circulation of water around the circumference of the body. The combination of the circulating water and the oscillating water in the waves, gives rise to the rotationally induced wave forces. When the rotational period is equal to the wave period, which is a moderate rotational speed, a doubling of the total wave force has been measured. With larger rotational speed the measured force on a cylindrical body has been measured to be as high as 6. In fig. 2 is shown an alternative way to establish fluid circulation around a rotating body. Here, the rotating body is shaped like a cylindrical tube 15. In the wall 16 of the cylinder, narrow slots or vents 17 are made, through which water is pumped , as shown by the arrows 18. in this way a circulation is set up around the cylinder 15. By controlling the velocity of the water pumped through the vents 17, the wave forces on the cylinder can be controlled. Fig. 3 shows a third method to obtain circulation around a cylindrical body 19. Here, propellers 20 and 21, driven by motors, are placed on opposite sides of the body. The propellers 20 and 21 are driven in such a way that circulation of water is efficiently established around the body. The rotational axis or the axis of circulation may have different orientations relative to the direction of incidence of the wave or relative to the vertical direction. It is not important for the utilization of the invention that the body is completely submerged. By , rotating the body is it possible to achieve fluid current also for partially submerged bodies. A completely submerged body will however have certain advantages because the wave forces will increase approximately linearly to the wave amplitude. The examples in Figs. 1 to 3 show the principle of the invention, only. This principle may for in stance be used to convert wave energy into useful form. In Fig. 4 is shown an example of a wave energy converter. A cylinder 22 which can be rotated around an axis, is connected by struts 23 and 24 which extend inclined downward from the cylinder to each side and is connected to a pump elements 25 and 26 respectively. The hydraulic pumps 25 and 26 are conviently connected to the sea bottom. This device corresponds to the so-called Bristol cylinder. hen the cylinder is set in motion by the waves, fluid will be pumped thorugh a hydraulic motor or turbine (not shown) connected to an electric generator (not shown) . When, according to the invention, the cylinder 22 is given a proper rotation around its axis, the energy absorption will increase by a factor of at least 2. In the example it is at 27 a pipe for pumping fluid from the pumps 25 and 26 to the shore. Fig. 5 shows an alternative form of a floating wave energy converter. Two parallel cylinders 28 and 29 are forced to rotate in opposite directions. Hydraulic pumps 32 are connected between the axes 30 and 31 of the cylinders. Hydraulic medium can be transferred to the shore through a pipe line 33 for conversion in known manner. The cylinders are suspended in the example in wires 34 connected to pontoons or buoys 35. Thus the device will be independant of the connection to the sea bottom. When the distance between the cylinders 28 and 29 changes, the pumps 32 will pump fluid through the pipe 33. The fluid can also be led to a turbine arranged in one of the buoys 35. It is practical to arrange several such sets of cylinders 28 and 29 in a longitudial or transverse • arrangement. In the example in Fig. 6 is shown' how the invention can be used in connection with floating platforms. The problem with such platforms is that wave forces induces unwanted platform motions. By controlling the wave forces on the pontoons of the platform, the total wave force on the platform can be reduced. in Fig. 6 is shown a platform 36 which is schematically illustrated by a deck 37 and vertical shafts 38 and 39 which are connected to cylindrical pontoons 40 and 41. The pontoons 40,41 are in principle made like the body 15 in Fig. 2 with vents in the cylindrical tube through which water is pumped. The ejected water from the vents in turn establishes a circulation of water around the pontoons, as illustrated by the arrows 42. Hence, according to what has been written above, the wave forces on the pontoons 40,41 and consequently on the platform as a whole, can be controlled. In practice it is probably necessary to have several vent systems with automatic control (not shown) due to the constantly changing sea. The present invention can also be used in connection with other offshore constructions where it is desireable to influence the wave forces on the construction. It is an advantage if the diameter of the submerged body or the bodies, particularly in wave energy converters, is of roughly the same size as the average wave height.

Claims: Claims: 1. Method for controlling wave forces on a body totally or partly submerged in a fluid, c h a r a c t e r i z e d in that a circulation of fluid, is efficiently established around the body. 2. Method according to claim 1, c h a r a c ¬ t e r i z e d in that the body is rotated around an axis. 3. Method according to claim l, c h a r a c t e r i z e d in that each body is equipped with slots or vents placed around the circumference of the body and through which fluid can be pumped with a velocity which has a significant component in a direction tangential to the body surface. 4. Method according to claim 1, c a r a c t e r i z e d in that the body is equipped with two or more propellers placed around the circumference of the body and which propellers establish a circulation of fluid in the same direction around the body. 5. Device for carrying out the method according to claim 1, with a body which is fully or partially submerged in waves, c h a r a c t e r i z e d inn that the body is provided with means (17-20,21) to create a surcomferential fluid current in one direction. 6. Device according to claim 5, c h a r a c t e r i z e d in that two bodies (28,29) are joined with a couple of hydraulic piston pumps (32) or similar mechanisms for energy conversion, said bodies being provided to have surcomferential fluid currents in opposite directions. 7. Device according to claim 6, c h a r a c t e r i z e d in that it is suspended in pontoons (35). 8. Device aaccording to claim 5, adapted to be used on a floating arrangement, particularly a platform with submerged pontoons, c h a r a c t e r i z e d in that the submerged pontoons (40,41) are provided with means to 8 create a surcomferential fluid current arround each body, the effect of these means being controllable to reduce the movement of the pontoons. 9. Device according to one of the claims 5-8, c h a r a c t e r i z e d in that the period of the surcomferential current is longer than the wave period. 10. Device according to one of the claims 5-9, c h a r a c t e r i z e d in that the body or bodies have a diameter which is approximately equal to the wave height. •^s- * !;

Inventors: BUDAL KJELL, BUDAL, KJELL
Applicants: BUDAL KJELL, BUDAL, KJELL
Application year: 1986
Main classification: F03B13
IPC codes: F03B13