GB2112326A - Buoyant body - Google Patents

Abstract

An axially symmetrical buoyant body has a submerged bulbous portion 1 e.g. conical, spherical or cylindrical connected to which is an upstanding portion of which one part 4 passes through the water surface 3 below which is a cylindrical portion 2 of reduced cross-sectional area. The shape of the body results in the net vertical force exerted on it by waves being zero at two different wave frequencies. The body may be used on its own as a buoy or as one support leg of a larger floating structure. <IMAGE>

Description

SPECIFICATION Buoyant body The present invention relates to a buoyant body and is primarily concerned with that type of body which can be used on its own as a buoy or which can be used, usually with other such bodies, as a support for a floating structure such as a floating oil rig.

Buoyant structures used in the ocean or other bodies of water are exposed to waves and thus vertical external forces exerted by the waves, referred to herein as the wave force, act on them.

It is desirable that such wave force is kept as small as possible. There are many buoyant bodies which are shaped so that at a certain wave frequency no wave force acts on them.

One such known body on which no wave force is exerted at a certain wave frequency is shown in Figure 1 of the accompanying drawings. This body is axially symmetrical having a lower or submerged bulbous portion connected to which is an upwardly extending portion which passes through the water surface.

The wave force acting on such a body is shown in the graph of Figure 2 (see Journal of the Society of Naval Architects of Japan, Vol. 130, 1971). In this graph dimensionless frequencies of waves are plotted on the x-axis and dimensionless values of the wave force on the y-axis. The values of x and y are expressed by the following relationships: w2 D1 x=, ~ g2

where D1=Diameter of the bulbous portion (see Figure 1) ct)=Wave frequency g=Gravitational acceleration F=Wave force p=Fluid density 7r=Ratio of the circumference of the circle to the diameter CA=Wave amplitude In the graph of Figure 2 the curve indicates the theoretical calculated values whilst the circles indicate experimental values.As may be seen, for a structure having such a conventional shape the wave force becomes zero only at one frequency, and therefore the range of wave frequency within which the wave force is small is itself relatively small. This is a major disadvantage of the conventional shape.

It is thus an object of the present invention to provide a buoyant body on which a small wave force is exerted over a wide range of wave frequencies.

According to the present invention there is provided an axially symmetrical buoyant body having a lower bulbous portion which, in use, is submerged and an upstanding portion which, in use, extends through the water surface, the upstanding portion having a portion of reduced cross-sectional area between the bulbous portion and the point where, in use, it passes through the water surface.

The bulbous portion may be of any appropriate axially symmetrical shape but it will be appreciated that it is only necessary that the body be axially symmetrical over that part of it which is permanently or temporarily under water and that the portion above water may be of any shape or connected to some larger structure such as a floating platform.

Due to the shape of the body there are two wave frequencies at w hich the net wave force on the body is zero and thus a substantial range of frequencies at which this force is substantially zero or at least has a low value.

Further features and details of the present invention will be apparent from the following description of certain specific embodiments which is given by way of example with reference to Figures 3 to 8 of the accompanying drawings, in which: Figure 3 is a partially cutaway side elevation of a first embodiment of the present invention; Figure 4 is a plan view of the structure of Figure 3; Figure 5 is a characteristics curve diagram similar to that of Figure 2 showing the wave force acting on the structure shown in Figures 3 and 4; Figure 6 is a characteristic curve diagram which shows the experimental and calculated values of the wave force acting on an embodying model having the shape shown in Figure 3; Figure 7 is a perspective view showing one application of the structure shown in Figure 3; and Figure 8 is a partially cutaway side elevation showing a second embodiment of the present invention.

The body shown in Figures 3 and 4 is axially symmetrical and comprises a cylindrical portion 4 extending through the water surface 3 below and connected to which is a downwardly convergent frusto-conical portion below which is further cylindrical portion 2 below which in turn is a downwardly divergent frusto-conical portion 1.

The diameter of the cylindrical portion 4 at the water surface 3 is designated D, the maximum diameter of a section of the bulbous or lower frusto-conical portion 1 below the water surface D, and the diameter of a section of the constructed or cylindrical portion 2 D2. These various diameters are in the following relation to one another: D2 < D < The wave force acting on the axially symmetrical body shown in Figure 3 was calculated by the three-dimensional source method (see Journal of the Society of Naval Architects of Japan, Vol. 148, 1980), and the result is shown in Figure 5, by x and y values as defined in connection with Figure 2. As can be seen from Figure 5, for the shape shown in Figure 3 the wave force is zero at two different wave frequencies.The reason for this is that the wave force is to be considered as divided into two components and is thus the sum of the force acting upward and the force acting downward.

The frequency at which the wave force becomes zero is the frequency at which the two force components are equal and opposite. The force acting downwards is mainly caused by the pressure of the waves acting on the upper surface of the bulbous portion, that is to say the inclined surface of the frusto-conical portion 1 in Figure 3, and the constricted or cylindrical portion 2 plays little part over the entire range of frequencies. On the other hand, the force acting upwards is mainly caused by the wave pressure acting on the lower surface of the bulbous portion 1 and the wave pressure acting on the lower surface of the portion near the free water surface, that is to say the inclined surface of the frusto-conical portion 4 shown in Figure 3.At a low wave frequency (of long wavelength), the influence of the constricted portion 2 on the wave force is small, but at high frequencies (of short wavelength), the force acting on the lower side of the portion 4 becomes relatively large with the result that the constricted portion 2 exerts an increased upwardly acting force. The reason is that when the wavelength is short, the wave pressure is large near the free water surface and decreases abruptly with increasing water deDth.

Having regard to the above, one can now consider the curves of the upward and downward acting forces at different wave frequencies.

Firstly, it is clear that due to the presence of the bulbous portion 1, irrespective of the presence of the constricted portion 2, the two curves intersect once at a certain frequency, and that as a result of the presence of the constricted portion 2 the curves intersect again at a higher frequency due to the above mentioned effect of the constricted portion 2. In case of the conventional wave-free shape, a large wave pressure near the free water surface does not exert a vertical force, because the conventional shape has vertical sides near the free water surface so that the effect mentioned above cannot occur.

With regard to the wave force characteristics of the shape shown in Figure 3, experimental results are shown below.

The dimensions of the model used for the experiment are shown in the following table:

D | D D1 | D2 | T 0.2 0.4 1 0.08 1 0.6 In the above table, D, D1,and D2 indicate the diameters of the respective portions of the model and T indicates the draft in metres respectively.

The experiment was performed in a seakeeping tank measuring 70 m long, 30 m wide and 3 m deep, and two incident wave heights of 2go=0.04 m and 0.08 m were adopted.

The experimental results and the results of the calculation made for a model having the above dimensions by the three-dimensional source method are shown in Figure 6. In this figure, X' and Y' are expressed by the following: a; D X'=, ~ g2

In Figure 6, the calculated values are indicated by the continuous curve, the experimental results at a wave height 2go=0.04 m by hollow circles and the experimental results at 2to=0.08 m by solid circles.

The calculated values include two wave-free points, that is to say two frequencies at which the net wave force is zero, as mentioned above, and the experimental values were almost zero at these wave-free points derived from the calculation, in both cases of 2go=0.04 m and 0.08 m.

The axially symmetrical body shown in Figure 3 can be used by itself as a buoy or as an immersed support leg for a floating structure such as a petroleum rig, a floating crane or the like. Figure 7 illustrates the latter application in which a floating platform is supported by four such buoyant bodies.

In the construction of Figure 3 the major components of the body are conical, but alternatively they can be spherical or cylindrical as shown in Figure 8. From the practical standpoint, a body having cylindrical components is to be preferred. This corresponds to a shape in which an annular ring is fitted near the free water surface to a conventional wave-free body having a cylinder as its lower bulbous portion. Thus the concept of the present invention can be used, to modify and improve conventional structures.

As mentioned above, since the floating body of the present invention is an axially symmetrical body with a constricted portion provided between the portion at the water free surface and the bulbous portion positioned below the free water surface, the wave force becomes zero at two different wave frequencies and is thus relatively small over a wide range of wave frequencies.

Furthermore, within the range of small wave force, if an additional component having a viscous damping effect, for example, a bilge keel as is fitted to ships or the like, is fitted to the body, the heaving motion of the body as a result of the waves will be small also.

Claims (8)

Claims

1. An axially symmetrical buoyant body having a lower bulbous portion which, in use, is submerged and an upstanding portion which, in use, extends through the water surface, the upstanding portion having a portion of reduced cross-sectional area between the bulbous portion and the point where, in use, it passes through the water surface.

2. A body as claimed in Claim 1 in which the portion of reduced cross-sectional area is of cylindrical shape.

3. A body as claimed in Claim 1 or Claim 2 in which the upstanding portion is of cylindrical shape at that point where, in use, it passes through the water surface.

4. A body as claimed in any one of the preceding claims in which the bulbous portion is of cylindrical shape.

5. A body as claimed in any one of Claims 1 to 3 in which the upstanding portion is ofdownwardly convergent frusto-conical shape immediately above the portion of reduced crosssectional area and the bulbous portion is of downwardly divergent frusto-conical shape immediately below the portion of reduced crosssectional area.

6. A body as claimed in any one of the preceding claims in which the maximum diameter of the bulbous portion is greater than the diameter of the upstanding portion at that point where, in use, it passes through the water surface which in turn is greater than the diameter of the portion of reduced cross-sectional area.

7. An axially symmetrical buoyant body substantially as specifically herein described with reference to Figures 3 and 4 or Figure 5 of the accompanying drawings.

8. A buoyant structure supported by one or more legs each constituted by a body as claimed in any one of the preceding claims.