The present invention relates to a method for predicting wave excitation forces that act on an offshore structure in a body of liquid. The present invention further relates to a system for predicting future wave excitation forces that act on an offshore structure in a body of liquid. The present invention further relates to an offshore structure for use in a body of liquid.

Background

Waves play an important role during operations in an offshore environment. It is known that offshore structures move and accelerate in such an environment as a result of waves incident on the structure. Unfortunately, these sea waves behave in an irregular way.

In known prediction systems, a statistical approach is made, such that operations will only be allowed if there is a very low chance of a limiting wave event occurring. As a result, the effective parameters are usually below the critical limits during operation, leaving a huge difference in workability.

Object of the invention

An object of the invention is to provide a more accurate method for predicting wave excitation forces on an offshore structure. A further object of the invention is to provide a method for predicting wave excitation forces on an offshore structure based on a predetermined wave spectrum, consisting of a plurality of waves with different wave heights and wave periods in different directions. A still further object of the invention is to provide a self-improving method for predicting wave excitation forces on an offshore structure. Another object of the invention is to provide a process for the minimization of the predicted wave excitation forces.

Summary of the Invention

This object is achieved by a method for predicting future wave excitation forces, or related physical quantity, that in a body of liquid will be exerted on an offshore structure by surface waves, such as waves on the surface of a sea. The offshore structure may be constructed as a floating structure, such a floating offshore structure is for example known from BE 2014/0224, a solid structure, for example a windmill, or a combination thereof, for example a jackup rig. The method comprises a number of steps using prediction means. In a first step, the method comprises determining a current (directional) wave spectrum of the surface waves for a surface of the body of liquid located around the structure. The weather conditions are also optionally determined. This step of determining a current wave spectrum can include measuring the current wave spectrum using a radar installation or receiving predetermined wave spectrum data using a processing unit.

In a second step, the method comprises predicting a future (directional) wave spectrum of the surface waves for a time horizon, preferably related to the size of the surface area, based on the current wave spectrum.

In a third step, the method comprises determining the wave excitation forces, or a component thereof in one of the six degrees of freedom of the offshore structure, that are exerted on the offshore structure by surface waves at a future time, within the time horizon, based on the future wave spectrum using a response amplitude operator, RAO.

This makes it possible to make a more accurate prediction of the wave excitation forces. Because the wave excitation forces are determined based on the wave spectrum rather than on historical data from measured wave excitation forces, and then statistically approximated, it is possible to more effectively predict the effective wave excitation forces and increase the efficiency and workability of the offshore structure.

In a first aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention which are described herein, the method further comprises using the adjustment means the steps of comparing the predicted wave excitation forces and the effective wave excitation forces at the future time, and deciding whether or not to adjust the RAO based on the comparison. The above steps may be performed through self-learning algorithms or artificial intelligence algorithms, AI, such as machine learning. In an embodiment according to the invention, the method further comprises repeating the preceding steps of comparing, deciding and optionally adjusting the RAO with a predetermined frequency, for example every 5 minutes and/or after the determination or measurement of the effective golf excitation forces.

In a second aspect of the invention, which may occur in combination with the other aspects and embodiments of the invention described herein, the offshore structure is constructed as a floating structure, possibly anchored to the bottom of the body of liquid or to another suitable anchoring point by means of cables, spuds or any other suitable anchoring means. In an embodiment according to the invention, the predicted wave excitation forces are determined based on the future wave spectrum and, by a heading prediction algorithm predicted, heading information of the floating structure. In a further embodiment according to the invention, the method further comprises the steps of comparing the predicted heading information and an effective heading information, deciding on the basis of the comparison whether or not to adjust the heading prediction algorithm, and optionally adjusting the heading prediction algorithm. The prediction model can decide that the wave excitation forces will be lower when the heading of the ship is changed, or another parameter in the control process is changed.

A third aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, provides a method for controlling an offshore structure, comprising a method for predicting wave excitation forces as described above, and performing the step of adjusting forces exerted on the offshore structure in function of the predicted wave excitation force using control means. In an embodiment, the method further comprises adjusting, using the control means, at least one force exerted on the offshore structure in function of the resultant, sum of all engaging forces, exerted at a predetermined critical point of the offshore structure.

A fourth aspect of the invention, which may occur in combination with the other aspects and embodiments of the invention described herein, provides a system for performing a method for predicting wave excitation forces as described above.

In particular, a prediction system for predicting future wave excitation forces, or this quantity related, that will be exerted by surface waves on a floating or fixed offshore structure in a body of liquid. The system includes prediction means adapted to perform the following steps:

determining a current (directional) wave spectrum of the surface waves for a predetermined area, and optionally the weather conditions,

predicting a future (directional) wave spectrum of the surface waves for a time horizon related to the size of the area based on the current wave spectrum, and

determining the wave excitation forces, or a component thereof, in one of the six degrees of freedom of the offshore structure, which are exerted on the offshore structure by surface waves at a future time t in the time horizon based on the future wave spectrum using a response amplitude operator, RAO. In particular, the components of the wave excitation forces related to the six degrees of freedom of the offshore structure can be determined. In an embodiment according to the invention, the prediction means are adapted to perform the following steps of comparing the predicted wave excitation forces and the effective wave excitation forces, deciding on the basis of the comparison whether or not to adjust the RAO, and possibly adjusting it from the RAO. In an embodiment according to the invention, the prediction means are further adapted to repeat the steps of comparing, deciding and possibly adjusting the RAO with a predetermined frequency, for example every 5 minutes or after each determination of the effective wave excitation forces.

In another embodiment according to the invention, the offshore structure is constructed as a floating structure, which is in particular anchored to the bottom of the body of liquid or to any other suitable anchor point, and the prediction means are adapted to determine the future wave excitation forces on the basis of the future wave spectrum and, by a heading prediction algorithm predicted, heading information of the floating structure. In a further embodiment according to the invention, the prediction means are adapted to perform the further steps of comparing the predicted heading information and the effective heading information, deciding on the basis of the comparison whether or not to adjust the heading prediction algorithm, and possibly adjusting the heading forecast algorithm.

A fifth aspect of the invention, which may occur in combination with the other aspects and embodiments of the invention described herein, provides a system for performing a method of controlling an offshore structure as described above.

In particular, a control system for controlling an offshore structure, comprising a prediction system as described above and control means adapted to adjust forces exerted on the offshore structure in function of the predicted wave excitation force. In one embodiment, the control means are further adapted for adjusting at least one force exerted on the offshore structure in function of the resultant, the sum of all forces, exerted on a predetermined critical point of the offshore structure.

A sixed aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, provides an offshore structure, in particular a vessel having an spud pole, comprising a system as described above, namely a prediction system, a control system, or a system for performing a method as described above. In an embodiment according to the invention, the offshore structure is constructed as a floating structure or vessel, which is in particular configured for performing dredging works.

A seventh aspect of the invention, which can occur in combination with the other aspects and embodiments of the invention described herein, provides a method or adjustment system for the adjustment of a response amplitude operator, RAO, for determining the wave excitation forces based on a wave spectrum. The method comprises, using the adjustment means, performing the step of adjusting the RAO on the basis of a comparison between wave excitation forces predicted with the RAO and the effective wave excitation forces. The adjustment system comprises adjustment means for performing the step of adjusting the RAO based on a comparison between wave excitation forces predicted with the RAO and the effective wave excitation forces.

In this description and the appended claims, the term "wave forces" is intended to cover all forces, or components thereof, that are caused by waves, swells, currents, tides in a body of liquid, such as the sea, that result in energy dispersal by cyclical variations in water speed and pressure.

Brief description of the drawings

The invention will be explained in more detail below with reference to an exemplary embodiment shown in the drawing.

Figure 1 is a schematic side view of an offshore structure in a body of liquid according to an embodiment of the present invention;

Figure 2 illustrates the operation of the prediction means of the offshore structure in accordance with a first embodiment of the invention;

Figure 3 shows a perspective view of a simplified representation of the floating vessel during the in Figure 1 shown operation;

Figure 4 shows in top view the floating vessel shown in Figure 3;

Figure 5 shows predicted spectrum data according to a second step of the operation according to the first embodiment;

Figure 6 shows predicted wave excitation forces according to a third step of the operation according to the first embodiment;

Figure 7 illustrates the operation according to a second embodiment of the invention; Figure 8 illustrates the operation according to a third embodiment of the invention;

Figure 9 illustrates the operation according to a fourth embodiment of the invention; and

Figure 10 illustrates the operation in accordance with a fifth embodiment of the invention.

Detailed Description of Embodiments

The present invention will be described below with reference to specific embodiments and with reference to certain drawings, but the invention is not limited thereto and is only defined by the claims. The drawings shown here are only schematic representations and are not limitative. In the drawings, the size of certain elements may be exaggerated and not drawn to scale for illustrative purposes. The dimensions and the relative dimensions do not necessarily correspond to actual practical embodiments of the invention.

In addition, the terms first, second, third and the like in the description and in the claims are used to distinguish between similar elements and not necessarily to describe a sequential or chronological order. The terms are interchangeable under appropriate conditions and the embodiments of the invention may be used in sequences other than described or illustrated herein.

In addition, the terms top, bottom, over, under and the like in the description and the claims are used for illustrative purposes and not necessarily to describe relative positions. The terms thus used are interchangeable under suitable conditions and the embodiments of the invention described herein may be used in orientations other than those described or illustrated herein.

Furthermore, the various embodiments, although referred to as "preferred forms," are to be understood rather as an example of how the invention may be practiced than as a limitation on the scope of the invention.

The term "comprising", used in the claims, is not to be interpreted as being limited to the means or steps mentioned thereafter; the term does not exclude other elements or steps. The term is to be interpreted as specifying the presence of the aforementioned features, elements, steps or components referred to, but does not exclude the presence or addition of one or more other features, elements, steps or components, or groups thereof. The scope of the expression "a device comprising means A and B" should therefore not be limited to devices that consist solely of components A and B. The meaning is that with regard to the present invention only components A and B of the device are listed, and the claim should further be interpreted as including equivalents of these components.

The offshore structure shown in Figure 1 is a floating structure on a body of liquid 1, such as a sea or a river, the structure being provided such that the surface waves 2 exert wave excitation forces on the structure.

The floating structure is a cutter suction dredger 101 which is provided with an spud pole near its front side and a cutting arm with a cutter head. In the operating state of Figure 1, in which the cutting arm is lowered and the cutting head is used for loosening and removal of soil material from a hard bottom 3 of the body of liquid 1, there should be a counterforce present for the cutting head, therefore, before the cutting head is switched on, the spud pole is fixed in the bottom of the body of liquid.

The cutter suction dredger 101 is provided with prediction means 201 for predicting wave excitation forces which will be exerted at different locations of cutting head suction dredger 101 by the surface waves 2. The prediction means 201 comprise one or more processing units for processing information related to a current wave spectrum. The prediction means 201 may further include radar, light detection, and distance systems (LIDAR), buoys, and/or satellite imaging systems for measuring and/or determining the current wave spectrum. The operation of this is further illustrated in Figures 2-5.

The cutter suction dredger 101 may further comprise adjustment means 301 for adjusting a response amplitude operator, RAO, which determines the wave excitation forces on the basis of a wave spectrum, by means of self-learning algorithms.

The cutter suction dredger 101 may be further provided with control means 401 for the control of the cutter suction dredger 101. The adjustment means 301 and/or control means 401 may comprise one or a more processing units for processing information.

Figures 2- 6 illustrate the operation 1000 of the in figure 1 shown prediction means 201 according to a first of embodiment of the invention.

In a first step 1001 of the operation 1000, the wave movements in a range of a few a thousand meters around the offshore structure 102 are measured by means of a wave radar system 202, as shown in Figures 3 and 4. In particular, the wave spectrum 1002 of the surface waves 2 around the offshore structure 102 for a predetermined measurement period, for example half an hour, and the effective individual waves, namely the direction, the wave height and the wave period, are measured. In a second step 1003, the operation includes predicting the wave spectrum 1004 of the surface waves 2 at a future time. In particular, the wave spectrum 1004 of the surface waves 2 around the offshore structure 102 for a predetermined prediction period (also called time horizon), e.g. two minutes or related to the size of the measured liquid surface, is predicted based on the measured wave spectrum 1002. Figure 5 shows the wave height of such a wave spectrum 1004 at a certain point of the offshore structure in function of time for a predetermined prediction period.

In a third step 1005, the operation comprises determining wave excitation forces 1006, or a component thereof, in one of the six degrees of freedom of the offshore structure, which are exerted by surface waves at one or more points of the offshore structure at a future time, located in the prediction period, based on the wave spectrum 1004 of the surface waves 2 at the future time, or based on the wave spectrum of the surface waves 2 during the prediction period, using a response amplitude operator, RAO, 1005. Figure 6 shows such a wave excitation force 1006 at a certain point of the offshore structure in function of time for a predetermined prediction period, as well as the effective wave excitation force 1008.

Figures 7 - 10 illustrate the operation of the shown in figure 1 prediction means 201 and/or adjustment means 301 according to further embodiments of the invention.

Figure 7 shows the operation 1010 for further adjusting the self-learning or replacement of the RAO. In a first step 1011 of the adjustment operation 1010 the effective wave excitation forces 1012 are determined by means of force measurements 1011 at various locations on the offshore structure 102. In a second step 1013, the adjustment operation 1010 comprises comparing the predicted wave excitation forces 1006 and the effective wave excitation forces 1012. In a third step 1014, the adjustment operation 1010 comprises deciding on the basis of the comparison whether or not to adjust the RAO, and, in the case of a positive decision, adjusting the RAO on the basis of the comparison 1013 between the predicted wave excitation forces 1006 and the effective wave excitation forces 1012 at the future time.

Figures 8 and 9 show the operation 1010 for predicting the wave excitation forces, while the offshore structure is constructed as a floating structure, and the third step 1005 comprises determining the wave excitation forces 1006 on the basis of the future wave spectrum 1004 and, by a heading prediction algorithm predicted, future heading information 1021 of the floating structure. Figure 9 further shows the operation 1020 for self-learning adjustment of the heading prediction algorithm 1022 based on the effective heading information 1023. The operation 1020 includes the steps of comparing the predicted heading information and the effective heading information at the future time, based on the comparison, decide whether or not to adjust the price forecast algorithm, and, in the case of a positive decision, adjust the price forecast algorithm based on the comparison between the predicted price information and the effective price information.

In addition to the prediction means 201 and/or adjustment means 301 shown in Figure 1, control means 401 for controlling an offshore structure may also be present. The operation 1030 (shown in Figure 10) of the in Figure 1 shown control means 401 according to any one of the embodiments of the invention, comprises in a first step, the measurement and prediction of other forces, i.e. non- wave excitation forces, on the offshore structure using force measurements performed at different locations of the structure.

In a second step, the operation of the control means 401 comprises determining the force equilibrium at a predetermined critical point on the basis of the force measurements and, by means of the prediction means 201 predicted, wave excitation forces.

In a third step, the operation of the control means 401 comprises manual or automatic state adjustment of the offshore structure in function of a comparison between the equilibrium of forces at the critical point and predetermined threshold values in order to avoid exceeding a further critical threshold. This state adjustment may for example change the heading of the structure in time, or change the other forces that engage (such as ground forces, anchor forces, etc.).

This makes it possible to predict the critical force on the spud pole of the floating and/or anchored cutter piston 101 in an advantageous manner, and to be able to keep the critical force low in bad weather conditions. For example, by temporarily changing the heading to or reducing the anchoring forces when a huge wave approaches.