GB2038261A - Stabilising system for a semi- submersible crane vessel - Google Patents

Abstract

In a stabilising system on a semi- submersible heavy lift crane vessel water ballast tanks are arranged above and below the waterline (A1, B1 and A2, B2) and the flow of water ballast is selectively controlled by valves 16, 17, 21, 22 in response to the unbalancing forces during the handling of loads. The regulating valves 21, 22 of the underwater ballast tanks A2, B2 are water inlet valves which are operable to open and close these tanks to the sea. <IMAGE>

Description

SPECIFICATION Stabilising system for a semi-submersible crane vessel The invention relates to a semi-submersible crane vessel, having a stabilising system comprising water ballast reservoirs above and below the surrounding water level and in which the discharge of water from the upper water reservoir on the one hand and the input of water into the lower water ballast reservoir on the other hand is selectively controlled by regulator valves in response to the momently generated unbalancing forces brought about when the cranes are handling loads.

A similar system is known from German Offenlegungsschrift P 2802249.3.

The invention provides a semi-submersible vessel with at least one heavy lift crane; the vessel having a stabilising system comprising water ballast reservoirs arranged in use to be above and below the surrounding sea level in which the discharge of water from the upper water reservoir on the one hand, and the input of water into the lower water ballast reservoir on the other hand, is selectively controlled by regulator valves in response to the instantaneously generated unbalancing forces brought about when the cranes are handling loads, characterised in that the regulating valves of the underwater ballast reservoirs are water inlet valves for said reservoirs, which valves are operable to open and close the said underwater reservoir to the sea.

An advantage of the invention is that it can lead to a further simplification of the device for stabilising a vessel during load handling by one or more cranes mounted on the vessel, whereby the deviations from pre-defined attitudes of the vessel are kept within small limits (preferably 1"); the load movements being permitted nonetheless to be carried out with the utmost speed which the particular crane(s) will allow without exceeding the stabilisation capacity of the device.

Preferably pumping means is provided by which the water can be pumped out of the lower water ballast reservoir into the water ballast reservoir above the sea level.

A specific example of a semi-submersible crane vessel according to the invention is now described with the aid of a few calculations and schematical drawings in which: Figure 1 is a perspective view of a floating body on which acts a vertical load; Figure 2 is the same body in horizontal position with indication for the required compensating forces for this position; Figure 3 is a cross-section through the vessel along lines Ill-Ill of Figure 4, which shows two cranes in front elevation; Figure 4 is a plan view of Figure 3 on a smaller scale, showing Figure 3 turned through a 90" angle about a vertical axis of the drawing; Figure 5 is a simplified diagram of one crane with indications for making the calculations;; Figure 6 is a plan view of the work platform of the vessel as depicted by Figures 4 and 5 turned through 90 about a vertical axis; and Figure 7 is a view showing the filling of the water ballast chambers at various stages of the load manoeuvre as described in the examples.

Compensating principle ofa floating body If on a floating body, as shown in Figure 1 by broken lines, a vertical load G is applied, the following changes occur in the position of the body measured in accordance with fixed axis system X - Y - vertical displacement (Z) - angular displacement 5 x with respecttothexaxis (roll axis) - angular displacement 4 y with respect to they axis (pitch axis) In order to counteract all these changes, at least three compensating forces have to be used, which act at three different points.

If the compensation of two of the degrees of freedom is sufficient, two compensating forces only are necessary.

With one compensating force, only one degree of freedom can be affected.

When the compensating forces only act in one direction, still other possiblities exist.

It is thus possible, for example, by means of three or more downwardly acting compensating forces to compensate the two angular displacements.

Compensating system with two compensating forces For application of a stabilising system, as aforementioned, for so-called semi-submersible floating cranes two compensating forces will be sufficient, whereby the two angular displacements are held at zero.

The vertical displacement is then not compensated. The position of the compensating forces A and B is determined by construction when this, as described hereafter, uses the water ballast chambers which are situated in pairs above each other and whereby the vertical axis of the chambers runs through points 2 and 3 (coordinates a, b for point 3 and a, - b for point 2).

Now if the magnitude G and the acting point (x; y) of the vertical load force is given, the largest of the two compensating forces A and B essential for keeping the vessel in a horizontal position can be calculated.

For that purpose, the resultant moment Mx with respect to the x axis must be zero, and My with respect to they axis must also be zero.

For that reason: (A-B)b=Gy (A + B) a = Gx therefore: ab a b The variation of draught can be calculated from the total load variation and the total area of the cross-section of columns (Aw)

From this it can be seen that for x = a the draught remains unchanged. Before a crane of the vessel is put into operation, the size and position and associated values of the compensating forces are calculated; they are indicated as zero values Ao and B,. These are stored in a computer memory.

Now when the magnitude and/or position of the load G is changed, the values of the compensating forces to be calculated also change.

These changes AA = A - A,, respectively AB = B - B,, are fed into the control system which adjusts the compensating forces.

Construction of the compensating system (Figure 3) As mentioned at the beginning, the invention is applied to a vessel (see Figures 3-5) having two underwater hulls 4, 5, upon which are placed hollow columns 6-11 which above the water level 12 carry a platform 13 for supporting at least one heavy crane 14, 15 (for example a 2000 ton and a 3000 ton crane).

Below each crane the stabilising system is provided with two water ballast reservoirs positioned above each other, whereby the pair arranged in column G beneath crane 14 is indicated by A and wherein A1 denotes the upper and A2 denotes the lower reservoir.

The pair of reservoirs in column 7 is similarly indicated by B1, B2. Compensating forces A, B (Figure 2) are applied in a known manner by allowing water to flow freely in and out, respectively in the reservoirs A1, A2 and B1, B2. An upwardly directed force is applied by allowing water to flow out from a reservoir A1 or B1 which lies above sea level.

A downwardly directed force is generated by allowing water to flow into a reservoir A2 or B2 which lies below the sea level.

Because according to the invention use is made of variations in level during the crane load manoeuvres very large quantities of water are displaced in a short time, which is essential when it is required to keep the vessel in a horizontal position at all times during the lifting, turning and moving of loads by the cranes.

It can thus be seen that the water ballast reservoirs A1, B1 are situated above the surrounding sea level 12 and the outflow is regulated by selectively operating control valves 16 and 17, which will be explained later.

The discharge is effected by vertical discharge pipes 18, 19 (only one is shown for each reservoir A1, B1) which extend through the lower reservoirs A2, B2 and through the bottom of the hulls 4, 5. Similar water inlet control valves 21, 22 are arranged in the bottom of the reservoir A2, B2 for the inlet pipes 23, 24 of which likewise only one is shown for each reservoir. Although water could be discharged from these chambers A2, B2 by introducing compressed air when the valves 21, 22 are open, according to the invention the ballasting operations can be carried out without the necessity of using compressed air, such as hereafter explained as an example of a specific manoeuvring cycle.The reservoirs A2, B2 can be emptied after such a cycle by pumping the water from a lower chamber to an upper chamber through the pump lines 25, 26 (indicated by the broken lines); in practice these will be situated inside the columns 6,7. The upper level of the reservoir A2, B2 can be in open contact with the atmosphere, as is indicated by the pipe sockets 27, 28. Naturally, extra pipe line(s) (not shown) are connected to the upper reservoirs, thus making it possible, to fill these with water from the surrounding sea.

The valves and associated pipes naturally have to be sufficiently large and numerous in order to realize the required flow volume in a completely unobstructed outlet from A1, B1 and inlet to A2, B2.

In an efficient embodiment a displacement of about 4000 m3 in 40 seconds was effected.

The system must be "charged" before use by pumping out the lower reservoirs A2, B2 and pumping full the upper reservoirs A1, B1.

It is therefore advantageous to place the reservoirs in pairs one above the other.

By pumping water from the lower to the upper reservoir, the equilibrium is not disturbed.

The pumping can thus take place already during the compensating procedure when the lower reservoir is not completely empty any more and the upper reservoir is not completely full.

The regulating system The change in compensating forces calculated for a specific change in the load is converted into a desired water level in each reservoir or into a desired volume of water to be displaced (setpoint). During the regulating of the water level, this water is continuously measured. When the measured value deviates from the desired, nominal value, a valve is opened.

Each reservoir may be provided with a plurality of valves of mutually different capacity. Type and number of activated valves is then defined as a function of the difference between the value, necessary for the vessel being on an even keel and the measured value of the water level.

A second possibility exists in the volume control. In this case the position of the valves is measured. By using the associated valve characteristics which, as a constant factor, are fed into the computer, the volume flow is integrated to the time until the desired volume has been reached.

The momently varying values of the water level required (or volume) for maintaining the horizontal position are calculated directly from the indications of the various sensors on the cranes (angle of swing, maximum angle, load).

By means of various known factors (such as jib length, distance between swivel point of crane and swivel point of jib, position of centre of gravity of the crane housing and counter-weight, etc.) it is possible to calculate the required changes in the compensating forces.

In these calculations the influence of various load forces can be taken into account.

With reference to the measuring values given in Figures 3 and 5, the undermentioned formulae, defined for calculating both the compensating forces AK and BK, when two cranes 14 and 15 are operating, take into account the following: - weight of the load in the main hosting block GA, GB - nett weight of the cranes, distributed in the weight of the crane housing GH - crane top GT and counterweight Ge In the formulae the letter A or B is added in the indexes to indicate the crane to which the given information is related, i.e. A for crane 14 and B for crane 15 in Figure 3.

See also the directions in Figure 4 for the distances measured to axis X and Y.

Naturally the formulae can be elaborated to include compensation of the loads in auxiliary hoisting blocks GA 1,2, 3; GB 1,2 and for cases where more than two cranes are in operation.

Examples of calculations for determining compensating forces: c d c' d' AK=GA (a+b)- 1 GB(a+b) + 1/2 (GA+GTA) (PA+1A COS#A) + GHA RHA+ GCARCA.

- (G3+GTB) (PB+1B COS(PB) + GHBRHB+GCBRCB

c d c' d' BK = - GA (a+b) + GB (a + b) -(GA+GTA) (PA+1A COSS9A) + GHA RHA+GCARCA

+ (GB+GTB) (PB+1BCOS#B) + GHBRHB+GCBRCB.

A special method is used for placing loads on decks and picking them up again.

In the case where the crane hook is situated above its own deck are, during changes in the load G, the values A0 and B0 adapted.

Further Figure 3 shows schematically the processing of data into the computer, i.e. in box I (sensor input values), box 11 (comparison with calculated fixed programme values) and box Ill (converted into commands for the valves, the latter being indicated by VAl,2 and V8 1,2).

Example of the compensating procedure Figure 6 shows schematically that crane 14 (only the jib is indicated), is arranged at one corner of the work deck structure above column 6.

For the sake of simplicity, only one crane is described, and the nett weight of this crane is not being compensated for (see Figures 5 and 6).

This permits simple formulae to be used in calculating the compensating forces which, were described before under the heading "Compensating system with two compensating forces".

For calculating the position of the load we use the slewing angle and the radius of the crane and assume that the centre of the crane coincides with the operating line of the compensating force A1 i.e., with the axis of column 6 coincident with the axis of crane 14.

The following formulae then apply: x = a - R cOS*A Y=b+R sin#A R R A = G (2 - c COS#A + b sin#A) c b B=1/2G(---RCOS6A Rsin6A) We may assume as a numerical example with a=43m b=34m Volume of all reservoirs : 4000 m3 Crane capacity: G = 2700 tons when R = 32 m.

Figure 1 a shows the filling state of the reservoirs Aa,2 and B1,2 at the starting point I of Figure 6, wherein the projection of the crane jib extends outboard parallel to the Y axis. The reservoirs A1 and B1 are completely filled and the reservoirs A2 and B2 are empty.

Now assume that a load of 2700 tonne is picked up from a barge alongside the vessel. From the formulae it is clearthatthe desired compensating forces areA: 3970,6tonne and B: - 1270,6tonne. The vertical position of the vessel remains unchanged because during pick up of the load the valve A lets out 3970,6 tonne of water, while valve B2 lets in 1270,6 tonne of water. The filling state of the reservoirs is then as shown in Figure 7b.

The crane now moves from position I to position II of Figure 6, and from the formulae it is possible to calculate that the desired compensating forces in position II are: A = 3704,7 tonne and B = 1004,7 tonne.

During this manoeuvre the valve A2 lets in 265,9 tonne and valve B1 lets out 2275,3 tonne of water. The resulting reservoir contents are shown in Figure 7c. Trim and keel angles remain equal to zero whereas the draught of the vessel has been decreased by the amount Z = (2275.3 - 265.9)/Aw After depositing the load from position II onto a platform outside the vessel, the compensating forces A and B, in the starting position, have again become equal to zero; the value they had at the beginning of this crane operation.

Thus during this final stage valve A2 has been letting in 3704.7 tonne of water and valve B2 has been letting in 1004.7 tonne of water. The final filling state of the reservoirs is shown in Figure 7d.

Trim and keel angles are again equal to zero, whereas the draught of the vessel has during the setting of the load been increased by the amount Z = (3704.7 + 1004.7 - 2700)/Aw This increase is of benefit to this mode of crane operation.

Claims (9)

1. A semi-submersible vessel with at least one heavy lift crane; the vessel having a stabilising system comprising water ballast reservoirs arranged in use to be above and below the surrounding sea level in which the discharge of water from the upper water reservoir on the one hand, and the input of water into the lower water ballast reservoir on the other hand, is selectively controlled by regulator valves in response to the instantaneously generated unbalancing forces brought about when the cranes are handling loads, characterised in that the regulating valves of the underwater ballast reservoirs are water inlet valves for said reservoirs, which valves are operable to open and close the said underwater reservoir to the sea.

2. A vessel according to claim 1, characterised in that a pumping means is provided by which water is pumped out of the lower water ballast reservoir and into the water ballast reservoir above sea level.

3. A vessel according to claim 1 or claim 2, characterised in that at one end of the work deck structure is mounted on each underwater hull a column provided with a ballast reservoir above and one below the sea level; a crane being mounted above each of the said columns.

4. A vessel according to claim 3, characterised in that the centre of each crane is substantially aligned with the general axis of the upper and lower water reservoirs arranged in the columns beneath the crane.

5. A vessel according to claim 3 or claim 4, characterised in that at manoeuvring of an operative crane during the lifting of the outboard load by a crane arm extending in the transverse direction of the vessel a ballast chamber above the water level is emptied in response to the increasing load moment and then a part of the lower ballast chamber at the opposite column is filled by opening of the water inlet valve, whereafter during the swivelling of the crane through 90 to a second outboard position, stabilisation is achieved by expelling water through an outlet situated in the last mentioned ballast chamber and an additional control of water inlet for the lower ballast chamber underneath the operative crane, while during the depositing of the load, after a completed swivel movement, stabilisation is achieved by the water inlet in the lower ballast chamber beneath the operative crane and water inlet in the lower ballast chamber of the opposite column.

6. A vessel according to one of the aformentioned claims, characterised in that the compensating forces to be obtained by the water outlet from the upper water ballast reservoirs and water inlet in the lower ballast reservoir are continuously calculated during a crane manoeuvre as a function of the water level or volume in the water ballast reservoir required for the given crane arm position and load, and the particular valves are activated as a function of the difference between the desired and the measured values of the water level or water volume in the particular water ballast reservoir.

7. A vessel according to claim 6, characterised in that a volume regulation is used in which the momently positions of the valves are measured and the valve flow characteristics are introduced and the volume flow is time integrated until the desired volume, required for the compensation of forces is obtained for each water ballast reservoir.

8. A vessel according to claim 6 or claim 7, characterised in that each water ballast reservoir has several valves of different capacity and the choice of the number of valves per water ballast reservoir to be activated are computed with sufficient frequency as a function of the difference between the desired value and the measured value of the water level, or water volume, during the crane manoeuvre.

9. A semi-submersible crane vessel substantially as herein described with reference to and as shown in Figures 3 to 7 of the accompanying drawings.