NL1006223C2 - Method for measuring the tensile force exerted on a dredging arm thereof during operation of a dredging vessel. - Google Patents

Description

Method for measuring the tensile force exerted on a dredging arm thereof during operation of a dredging vessel.

The invention relates to a method for measuring the tensile force exerted on a dredging arm thereof during operation of a dredger vessel.

Such dredging vessels are nowadays increasingly equipped with a "Dynamic Positioning System", or in short the DP system, with the. aim to steer the ship very precisely along a desired path. This is particularly desirable when dredging a trench into which an oil or gas line or a cable is to be laid. The more accurate the trench can be dredged, the faster work is completed. Of course, it makes no sense to dredge more soil than is necessary, so not next to the trench and no deeper than necessary to avoid the formation of undesired pits.

When using the DT system, the loads exerted on the ship are measured as much as possible, such as the influence of the wind and the forces caused by the dredging arms. If the wind or the dredging forces change, this can be reacted to immediately by applying any stern thrust screws with the available "actuators" such as propellers, rudders and bows. This prevents the ship from deviating too much from the desired runway. It is important that responding to disturbances is even before a deviation in the ship's path can be measured.

A disturbance can also occur in the flow of water past and below the ship. This will in particular be the case at the dredging site, as variations in the bottom depth will also allow the flow velocity and direction under the keel of the ship to vary. The flow velocity cannot generally be measured, since the instruments used in normal ships do not function properly due to the disturbances caused by the dredging process.

The flow rate must therefore be estimated as is generally done in the 1006223 2 with a so-called "Kalman filter". This filter contains a mathematical model of the ship to be steered and all forces working on the ship are entered into the filter. These forces are derived from the wind, from the force of the propellers, from the bow and stern thrusters, from the rudder positions and from the forces exerted by the dredging arms.

Using these known forces, the displacement and rotation of the ship is predicted and compared with the actual change measured with positioning equipment such as, for example, DGPS and the compass. Since the equations of motion only lack the influence of the current, the difference between the actual displacement and twist and the predicted displacement and rotation can be used to calculate the current. Technically this is called "estimating". This estimated flow can then be used in the control algorithms to ensure that the vessel continues to accurately follow the desired trajectory.

When used on dredging vessels, for example with hopper dredgers the forces exerted on the ship by the dredging arm or the dredging arms, which may be in the form of suction tubes. These forces can be a factor of 10 greater than the force to travel. Thus, approximately 90% of the propulsion power may be required to compensate for the forces in the dredging arms and 10% to propel the ship itself. In order to arrive at a good prediction of the movements of the ship, it is therefore necessary to measure the forces occurring in the dredging arms as accurately as possible. For this purpose, use is made of sensors which are generally incorporated in the hinge points of the suction tubes. However, these disappear when dredging underwater and are hoisted on deck while sailing. The sensors are therefore exposed to large environmental changes and experience shows that they last on average only for a short time. The sensors are expensive, so regularly replacing them is expensive. A defective DT system cannot be obtained with a defective sensor due to the relatively very large influence of the suction tube.

1006223 3

Furthermore, the measurements are carried out far from the suction head, which in fact should compensate for disturbing influences of, for example, the weight of the parts of the suction tube, of the forces exerted on the suction tube 5 by the lifting wires, the pressure differences by the suction process and of such. In addition, the weight of the suction tube can vary due to variation in the solids concentration of the dredged mixture.

The object of the invention is now to eliminate these drawbacks and to that end provides that the tensile force exerted on a dredging arm is derived from an algorithm using process data which are measured during the dredging process.

Such process data are, for example: a) the concentration of the dredged mixture in the suction tube; b) the speed of the dredged mixture in the suction tube; c) the pressure for the dredge pump; and d) the pressure difference across the suction head.

In particular, the pressure difference measured over the suction head appears during the dredging, seen in time, to have the same dynamic course as the tensile force that is directly measured. It would therefore be obvious to use this pressure over the suction head as an alternative to the direct measurement of the tensile force. However, the problem arises here that the pressure difference across the suction head is not only proportional to the pulling force but also depends on the type of soil. Namely, the pulling force is proportional to the force with which the suction head is pulled over the ground, since the suction head is pressed onto the ground as a result of the pressure difference occurring over the suction head. The frictional force between the suction head and the ground will therefore have to be overcome with the movement of the suction head.

The pressure difference across the suction head is furthermore influenced by the excavation process, the mixing speed and the concentration of the dredged mixture. Furthermore, the pulling force is influenced by the possible installation of teeth and water jets at the bottom of the suction head, which together have to loosen the soil. The entire dredging process around the 1 006223 4 suction head is therefore a very complex event that cannot be represented on board by simple relationships, partly because the data regarding the soil condition, and therefore the composition of the dredged mixture, is not continuously available. stand.

According to a further elaboration of the invention, using a Kalman filter, the ratio between the actual pulling force exerted on the dredging arm and the measured pressure difference over the head is now continuously estimated. 10 The fact that the filter can estimate both the just mentioned value and the previously mentioned value of the flow rate is because the pulling force on the dredging arm changes continuously and rapidly due to the specific dredging properties - whereby changes from 0 to 100% can be made in a few seconds. while changes in the flow occur only in the time order from minutes to hours. This difference in dynamics allows the Kalman filter to use rapid variations in measured values to adjust the estimate of the ratio of the traction force on the dredge arm to the pressure differential across the suction head and to allocate slow deviations to changes in water flow.

The invention is further elucidated with reference to the drawing, in which:

Fig. 1 shows a schematic diagram of the operation of the Kalman filter; and

Fig. 2 schematically shows a dredging vessel in which the different loads acting on the ship and the components used for steering the ship are indicated.

In the diagram of Fig. 1: X = X coordinate of the ship in longitudinal direction; Y = Y coordinate of the ship in transverse direction; PSI = course of the ship with index K: the X, Y, PSI 35 with index m estimated by the Kalman filter: the X, Y measured for example by a DGPS and PSI measured with a compass; rudder = rudder angles; thruster = strength of bow (and stern) propellers; props = power of propellers; 1006223 5

Fps = force in port suction tube;

Fsb = force in starboard suction tube;

Pps = pressure over port suction head;

Psb = pressure over the starboard suction head; 5 Kps = estimated ratio between Pps and Fps, so that estimated fps = Kps * Pps;

Ksb = estimated ratio between Psb and Fsb, so that estimated fsb = Ksb * Psb.

The difference vector e * = (Xm-XK, Ym-YK, PSIm-PSIK) between 10 estimated and measured values is used to estimate the current and ratios Kps and Ksb:

Set Ex = Xm-XK longitudinal position error

Ey = Ym-YK transverse position error

Epsi = PSIm - PSIK course error 15 then e * = (Ex, Ey, Epsi)

The estimated current vector v * = (Vx, Vy) is obtained by integrating Ex, Ey, for example in the most basic form: v = ƒ (Ae) .dt 20 with eg A = au 0 0 0 a22 0_ with Vx = longitudinal and transverse flow.

The ratios Kps are obtained by integrating the product of E and differential pressure Pps over the suction head, for example in the most basic form:

Kps = ƒ (Pps *. (B. e)). dt with eg B = bu 0 b13 0 bj2 b23 and Pps * = (Pvx, Pvy) Suction power (in the direction of the suction pipe) decomposed in the longitudinal direction (= Pv *) and in the transverse direction (= Pvy).

Same algorithm for the starboard suction tube:

Ksb = ƒ (Psb *. (B.e)) .dt

In principle, if there is no dredging, the dredging force is zero. Due to the flow rate of the water to the dredge pump, there may still be a small pressure difference across the suction head, which could lead to incorrect corrections. This can be prevented by disabling the adaptation algorithm when the suction head is not on the ground.

1 006223 6

This can be deduced from various dredging signals, for example from the force in the hoisting wires or signals derived from them, the position of the heave compensator or the measured concentration.

5 claims 1006223

Claims (3)

A method for measuring the tensile force exerted on a dredging arm thereof during operation of a dredger vessel, characterized in that the tensile force is derived from an algorithm using process data measured during the dredging process. 2. Process according to claim 1, characterized in that as process data, either individually or in combination, can be used: a) the concentration of the dredged mixture in the suction tube; b) the speed of the dredged mixture in the suction tube; c) the pressure for the dredge pump; and d) the pressure difference across the suction head. Method according to claim 1 or 2, characterized in that, using a Kalman filter, the ratio between the actual pulling force exerted on the dredging arm and the measured pressure difference across the suction head is continuously estimated. 20 ------------ 1 006223