NO831103L - COMPENSATION OF HOUSE CANCER - Google Patents

Description

HIRE

This note outlines how floating constructions

can be designed so that they get little heave movement in waves.

The background for the idea that underlies this work,

is wave energy research. A main problem here is namely to

provide a fixed reference on which the conversion mechanism can work,

against. Without such a reference, the power plant becomes very inefficient.

Within wave energy research, three methods have been established to obtain a reference point. The most obvious is to use the seabed. Examples of this are fluctuating liquid columns that stand on the bottom. This is often an expensive solution because the construction is very large. This solution is therefore limited to relatively shallow water where the wave energy is usually somewhat reduced due to bottom friction. Another method is to use an elongated, ship-like structure that extends over at least one average wavelength. The forces along the structure then have varying signs and will therefore partially compensate each other. The result is that the hull has small heaving movements and thus represents the necessary fixed reference. The bending moments on such a construction will be large. This solution is therefore also expensive. The third method is to use a floating construction with a gyroscope on board. The hull can carry out rolling, dampen movements in relation to the gyroscope axis and thus produce energy. This can be a technically complicated and expensive solution. We note that a significant part of the cost of converting wave energy into useful energy lies in obtaining the necessary fixed reference.

In the following, we will deal with the idea of a fourth method to obtain a reference. The proposal is to design a passively floating structure in a special way so that he gets small vertical movements in the waves.' Models of the construction have been built and measurements of these are presented. The measurement results show that it is possible to make the floating structure heave significantly less than the wave fluctuation over a large frequency interval.

The idea presented in this note can be used in wave power plants. It is also conceivable that he could have an area of application within offshore petroleum technology,

METHOD OF OPERATION

Figure 1 shows the wave force on a buoy (body no. 1) and

a submerged body (body no. 2), respectively, as a function of the wave frequency. It should be noted that the relative dimensions of the bodies and the immersion of body no. 2 can be chosen so that the forces on the two bodies are approximately equal over a certain frequency interval. Furthermore: if the bodies are small in relation to the wavelength, the forces on the two bodies will be 180° out of phase. The idea is then this: If we connect the two bodies with struts (fig. 2), so that they form one body, the net wave force on this composite body will be close to zero. The forces on the floating body are compensated by the forces on the submerged body. A body that is not exposed to wave forces is also not seen in motion. One should therefore expect that the response to such a construction will be small in a certain frequency interval. What frequency interval does this happen?

.a dimensional ionization question.

Force compensation can also be established on structures that are not small in relation to the wavelength. Let's imagine a long, ship-like, floating body. This body is connected by a long dive. body, slide as illustrated in figure 3. The submerged body is designed as far as possible, so that the wave force acting between two vertical cut planes on the body is compensated by the wave force acting on the foot body between the same two cut planes (see figure 3). In this way, local force compensation can be obtained along the entire elongated construction. One then has to wait for the composite body to be trusted. heaving movement and slight stomping movement. Note also that since the force combination is local, the bending moment on the composite body will be smaller than on the individual bodies without braces between them.

In addition to the submerged body, which e.g. can be a plate, compensates for wave forces, he will also affect the dynamic conditions by increasing the swing moss of the construction. A plate will also provide well-bred cushioning and thus increase the effective damping of the vertical movements of the structure. This applies separately" to large waves.

MODELS

We have built two models. One is rotationally symmetrical (buoy) and the other elongated (ship). As submerged bodies, flat plates are used in the models. The scales are intended to be 1:100 and 1:125, respectively. Figures 4 and 5 show sketches of the models. When dimensioning the models, as a first approximation, it has been assumed that the wave force on the composite structure is equal to the sum of the forces on each of the isolated bodies. Note that in order to obtain good force compensation, the area of the submerged plate must be significantly larger than the water surface area of the floating body. Furthermore, the immersion of the plate must be of the order of magnitude A/2ir, where X is an average wavelength.

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MEASUREMENT RESULT

Figure 6 shows the heave flow for the buoy as a function of the wave period in model scale. If the plate is removed, the light buoy will have approximately the same fluctuation as the wave, i.e. s/n = 1. You can then see from the figure that for periods T < 1.4 s the composite construction will have a significantly smaller heave motion than the isolated buoy. Figures 7 and 8 show the heave stroke and pitch angle, respectively, for the ship. The measurement result for the ship without plate is also shown.

As you can see, the effect of the submerged part of the construction is great.

FURTHER MEASUREMENTS

Previously, measurements have only been made with the inside in all directions

to the waves in the longitudinal direction of the ship. Although the ship can be oriented so that the bow points towards the waves, it will be of interest to measure the movement of the ship also for other wave incidence directions. Furthermore, the behavior of the construction in extreme waves must be measured.

ALTERNATIVE CONSTRUCTIONS

It should be noted that the force compensation method can also be used on floating platform structures. An example is shown in figure 9. The platform is raised above the wave level by means of columns attached to buoyancy bodies with a relatively large water plane area. The wave forces are compensated using a submerged plate under each of the floating bodies. Because the buoyancy bodies have a large flat planar area1, the stability of the platform will be good.

Claims (1)

Design of floating construction for reduction of vertical movements of the construction in waves, where the construction consists of a floating part and a fully submerged part and where the rotating parts are firmly connected to each other by means of struts, trusses etc., characterized by the design of the floating and the submerged part and the vertical distance between the two parts are chosen so that the vertical wave forces acting on the structure are locally compensated, i.e. that the vertical wave forces acting on a local area of the floating part, where the area in horizontal extent is small in relative to a wavelength, the vertical wave forces acting simultaneously on a corresponding directly underlying area of the submerged part are approximately equal in magnitude and oppositely directed, so that the sum of the wave forces on each pair of areas is zero or close to zero over a substantial and characteristic frequency interval for the waves.