Buoyancy unit with controlled heave

Buoyancy unit comprising a body (14) in which there are formed a buoyancy chamber (20) and a chamber (22) which is intended to become filled with water. According to the invention, the wall of the chamber (22) has at least two openings (24, 28), at least one opening of which comprises a nozzle (24, 28) allowing the passage of water between the chamber (22) and the outside, the nozzle being dimensioned in such a way as to appreciably slow the flow of water. The unit may form a buoyancy column intended to hold the deck of a floating rig up out of the water.

BACKGROUND OF THE INVENTION 
The present invention relates to a buoyancy unit with controlled heave, and 
more particularly to a unit of this kind intended to hold the deck of a 
floating oil rig up out of the water. 
Floating oil rigs are subjected to wave motion which, during the passage of 
a wave, alters the draught of the rig. This variation causes the rig to 
move along the vertical axis, as the rig attempts to maintain constant 
draught. 
Each sea has its own geographical conditions which give the wave motion 
associated characteristics: mean height of the waves and frequency of the 
wave motion. The shape and dimensions of a rig give a specific response 
time during the passage of a wave. In order to avoid problems of resonance 
it is necessary for the natural periods of the wave motion and of the rig 
to be sufficiently different. 
DESCRIPTION OF RELATED ART 
Document FR-A-2,681,831 describes a floating oil rig with controllable 
heave comprising a deck and a buoyancy unit. In order to control the 
response of the rig to the movement of the sea in which it is installed, 
the buoyancy unit comprises a tidal chamber open to the sea and connected 
to a gas tank by a circulation conduit fitted with a restriction. 
In spite of its advantages, the buoyancy unit described in document 
FR-A-2,681,831 has the drawback of having a complicated construction, 
which increases the cost of manufacturing it. 
SUMMARY OF THE INVENTION 
The subject of the present invention is therefore a buoyancy unit with 
controlled heave which is of a simple and reliable construction and makes 
it possible to limit the vertical movements of a floating rig. 
In order to achieve this objective, the present invention proposes a 
buoyancy unit comprising a body in which there are formed a buoyancy 
chamber and a chamber which is intended to become filled with water, 
characterized in that the wall of the chamber has at least two openings, 
at least one opening of which comprises a nozzle allowing the passage of 
water between the chamber and the outside, the nozzle being calibrated in 
such a way as to appreciably slow the flow of water. 
This type of rig is particularly suitable for seas where the wave motion 
has a long period, for example the Gulf of Guinea. 
A rig fitted with the buoyancy unit according to the invention has 
advantages of manufacturing cost and enables the rig to be assembled 
directly on site, the buoyancy columns being manufactured in the region of 
installation and only the deck being constructed in a remote shipyard. In 
addition, such a rig can be installed without having to use heavy 
equipment. The deck is an independent barge which arrives on site fully 
equipped. 
The advantages, as well as the operation of the present invention will 
emerge more clearly on reading the following description given in 
non-limiting manner with reference to the attached drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 1, an oil rig with controlled heave is depicted overall as 10. The 
rig 10 comprises a floating deck 12 which is mounted on the upper ends of 
buoyancy columns 14. The deck 10 comprises at least three columns 14, and 
six in the example illustrated. The six buoyancy columns are more or less 
identical and will be described in greater detail hereinafter. The columns 
may have a rectangular cross-section, or preferably a circular 
cross-section. A drilling mast, depicted diagrammatically in 16, is 
mounted on the deck 12. 
The deck 12 is fitted with six bearing cages 18 arranged around its 
periphery, each one intended to receive the upper end of an associated 
buoyancy column 14. The upper end of each column 14 preferably bears on 
the lower surface of the associated cage 18 via a set of blocks made of 
elastomer and arranged in a circle on the lower surface, which makes it 
possible to spread the load on the end of the column 14 and form a joint. 
Other types of joint, for example a spherical bearing surface, can also be 
used. 
Each column 14, having the form of a tube which defines a body with closed 
ends, is preferably made of steel, concrete, or possibly fibreglass. Each 
column is arranged vertically in the water, its upper end having an air 
draught "h" relative to the water surface 15. A buoyancy chamber 20 
defined at the upper end of the column 14 (looking at the drawing) is 
dimensioned so that its centre of buoyancy is above the centre of mass of 
the column 14, in order to give it greater stability. Each column is 
preferably weighed down at its lower end. 
Defined in the lower part of the column 14 is a chamber 22 that forms a 
water trap. This chamber opens to the outside of the column via at least 
two openings, an upper opening 24 arranged adjacent to a partition 26 
delimiting the bottom of the buoyancy chamber 20 and via a lower opening 
28 arranged towards the lower end 29 of the column 14. At its lower end, 
the column has a chamber 30 intended to contain ballast. 
At least one of the orifices 24 and 28 is dimensioned to form a nozzle, 
intended to slow the flow of water between the chamber 22 and the outside, 
as will be described in greater detail hereinafter. 
Now, as the rig 10 moves vertically, each column 14 is displaced more or 
less vertically, in the direction of the arrow 30. This vertical movement 
tends to displace the mass of water held in the chamber 22 towards the top 
or towards the bottom of the column, in the opposite direction to the 
movement of the column. This vertical movement actually creates a pressure 
gradient in the water inside the chamber 22. This pressure gradient 
consists of a slight depression at one end of the chamber 22 and a slight 
overpressure at the opposite end, compared to the pressure of the water 
surrounding the column. 
The difference between the pressures at the ends of the chamber 22 and the 
pressure of the water outside causes the water in the chamber to 
accelerate and this tends to drive it out of the chamber at one of its 
ends to be replaced with water entering the chamber from the opposite end. 
The friction caused in the nozzle 24, 28 as the water entering or leaving 
the chamber 22 passes, converts some of its kinetic energy into heat. This 
conversion reduces the heave energy of the column/water unit, the result 
of this being an attenuation of its vertical movement. The heat generated 
by the friction of the water through the nozzles is dissipated into the 
sea water. 
The restriction of the nozzle 24, 28 may be variable, for example 
controlled from the deck 12 of the rig 10. Alternatively, the nozzle 
control may be connected to a measurement unit fitted with accelerometers, 
which is intended to measure the rate of vertical displacement of each 
column 14 in order to optimize the attenuation of the vertical movement, 
and which controls the nozzles of each column separately so that the 
columns are displaced as one, thus making the deck stable. 
As the lower part of the column is open to the water, this makes it 
possible for its structure to be lighter. Only the upper part that forms 
the chamber 20, which is intended to support the weight of the deck needs 
to have reinforced construction. 
The openings 24, 28 may each comprise a calibrated nozzle so as to slow 
still further the flow of the water. Alternatively, one of the openings 28 
may be formed by the open bottom 29 of the column, which enables the 
construction of the column to be simplified. This type of construction 
requires the column 14 to be longer. 
To supplement the foregoing description, a numerical example is given 
hereinafter, without implied limitation. 
The dimensions of the column 14 are marked on FIG. 2. The mass of the 
column, empty, is M.sub.2 and the chamber 22 contains a mass M.sub.1 of 
water. The column 14 experiences a regular wave motion, which tends to 
displace the column under the effect of the pressure at C and tends to 
displace the mass of water M.sub.1 under the effect of the pressures at A 
and B. The x-axes of the column and the speeds V.sub.1 and V.sub.2 are 
taken as being positive upwards. 
The following assumptions are made: 
x=1.182 m: the column has risen by more than one meter from its equilibrium 
position. 
V.sub.1 =+0.101 m/s 
V.sub.2 =-0.126 m/s. 
For simplicity, it will be assumed that the cross-sectional area of the 
column is 1 m.sup.2. 
Water flow rate: 0.101-(-0.126)=0.227 tonne/second. 
The cross-sectional area of the orifices is 
##EQU1## 
15.3 is an estimated value which was chosen bearing in mind the rates of 
displacement of the column and of the water, the rate of displacement of 
the water being of the order of half the rate of displacement of the 
column. 
This value was estimated in order to optimize damping. The speed at which 
water is ejected from the nozzle 24 is 0.227 m/s.times.15.3-3.47 m/s. 
______________________________________ 
The external pressures are: 
P.sub.A = 10.38 kPa 
P.sub.B = -4.36 kPa 
P.sub.C = -3.85 kPa 
Inside the column, there are: 
P.sub.A' = -4.35 kPa 
P.sub.B' = -10.39 kPa 
______________________________________ 
Now it is known tha .DELTA.p across an orifice is 
##EQU2## 
It is convenient to write down the pressure forces acting on the column 
(which is 1 m.sup.2 in cross-section) in two different ways: 
______________________________________ 
Pressure 
A -4.43 kN Wave forces: 
-10.38 kN at A 
at: B +10.39 kN +0.52 kN at B & C 
C -3.85 kN Damping +12.06 kN at A & C) 
Upthrust -11.59 kN Upthrust -11.59 kN 
Resultant -9.40 kN -21.45 kN 
on the 
column 
______________________________________ 
It can thus be seen that the damping forces are far from insignificant in 
terms of amplitude, because the damping device reduces the external force 
from -21.45 kN in situations when the orifices 24, 28 are closed and 
therefore when the rates of displacement of the column and of the water 
V.sub.1, V.sub.2, are equal to -9.40 kN. 
Thus the buoyancy unit according to the invention allows the heave of the 
rig to be reduced considerably. 
The energy dissipated per second is, on average, 0.92 kW. The system tends 
towards steady state and the dissipated power becomes 1.11 kW. In this 
example, if upthrust is neglected, the work done by the forces generated 
by the wave swell pressures is 1.24 kW at the moment in question (-9.87 
kN.times.0.126 m/s). 
The excitation period chosen is T=18 s. The maximum pressures for a wave 
swell 7 m crest-to-trough is 390=11.2 kPa, 3160=4.7 kPa, 3170=4.15 kPa. 
The buoyancy unit according to the invention may form a subassembly 
intended to be attached to a floating structure in order to dampen the 
overall heave. Furthermore, the buoyancy unit may be arranged other than 
vertically. For example, the unit may be installed, generally 
horizontally, under the hull of a ship in order to dampen the roll, yaw or 
pitching. Alternatively, the unit may be arranged inside a ship or inside 
a floating structure with the openings open to the water.