MXPA97008081A - Method and means to direct a floating structure anclated against the direction of the waves in sea abieve - Google Patents

Abstract

The present invention relates to a method for directing an anchored floating structure, against the direction of waves, wherein the structure, at its forward end, is tied to a buoy or the like, the method is characterized in that it is provided, the floating structure, at its stern end, one or more rudders for the wind, which can be rotated, and which adjust in such a way versus the wind direction, that the floating structure is directed against the direction of the waves, in a way

Description

METHOD AND MEAN TO DIRECT A FLOATING STRUCTURE ANCHORED AGAINST THE DIRECTION OF THE WAVES IN OPEN SEA FIELD OF THE INVENTION The present invention relates to a method for directing a floating structure, against the direction of the waves, wherein the structure is anchored or moored to a buoy at its forward end (in front of the central area of the ship). Here, any type of ship, vessel, boat or floating construction that is designed for use in the open sea.

BACKGROUND OF THE INVENTION Quantities of oil and gas exploited from underground deposits at sea, for example in the North Sea, are commonly transported, currently, to onshore facilities, such as refineries and storage tanks, by pipelines placed on the seabed. In addition, quantities REF: 25975 significant oil and gas are transported by ship, particularly oil and gas produced in small distant fields that have not been put in communication with the existing pipeline system located on the seabed. Using a ship for this type of transport involves the ship being connected or moored to a buoy that is anchored near a platform or a storage facility, underwater, where oil or gas is stored, and oil or gas they are transferred from the storage facility to the ship, through one or more pipes placed through the buoy. Gradually, storage and production vessels have been used for the storage and production of oil and gas in small fields at sea, or in fields where the depths of the sea mean that the use of facilities resting on the seabed is inconvenient or impossible. The boats of this type are anchored by means of a tower that most of the times is placed in the prow of the hull of the ship.

When there is bad weather, with strong winds, the marine currents and the thick seas, which are the forces that act on the boat, buoy and moorings, can be extremely strong. In particular, large forces act on the boats, which are allowed to oscillate freely around a mooring point (buoy, anchor or similar) with large amplitudes from one side to the other. In the open sea (see a subsequent paragraph), the dominant forces acting on a ship that is moored to oscillate freely are normally supported by the forces of the waves, and the greater the amplitude of the oscillation movement becomes, the more influenced the ship by the waves. This is followed by large movements and horizontal forces and also of up and down movements, and undulation, which cause heavy loads to result in wear and damage to the boat and the moorings. It is previously known to direct an anchored ship, against the direction of the waves, by means of lateral propellers placed at the aft end of the ship. However, these facilities are expensive and represent additional costs in relation to maintenance and repair work. In addition, it is a common knowledge, in relation to boats, particularly in relation to small fishing boats in which fishing with ropes and nets, use a stern crab. A stern crab is a sail that is supported by a mast at the stern end of a boat, and serves to hold the boat against the wind direction, and to reduce the wave motion of the boat. When collecting fishing equipment, such as nets and ropes, it is important to keep the boat against the direction of the wind, to prevent the boat from being dragged to the side where the fishing equipment is being collected. Thus, a stern crab is a sail that is placed in a normally parallel direction (except when navigating) with respect to the boat. While a ship is anchored or moored to a buoy or similar, in the open sea, to load or produce oil or gas, the main task is to keep the ship against the direction of the waves, in a stable manner, as mentioned previously, to prevent the ship from starting to oscillate (movement with yaw) with large amplitudes that can cause heavy loads at the moorings. In addition, large amplitudes of the oscillatory movement can be avoided, when the ship is placed with small directional variations.

DESCRIPTION OF THE INVENTION The present invention provides a method and a device that provide a solution to this matter. According to the present invention, the method is characterized in that the floating structure is provided with a rudder, at its stern end, which is adjusted versus the wind direction, in such a way that the floating structure is directed against the direction of the waves, as defined in the independent claim, appended, 1. Furthermore, according to the invention, the device is characterized by the arrangement of a rudder driven by the wind, which can be rotated, preferably positively driven , which is adapted to fit in any desired angular position, according to the ship's length axis, as defined in appended claim 2. The dependent claims, 3 and 4, describe advantageous features of the invention. In the following, the invention is described in detail, with reference to the drawings, which illustrate modalities thereof, and in which: Figure 1 shows, in a side view and from above, a boat provided with a rudder for the wind, according to the invention, Figure 2 shows a modality, of a rudder for the wind, included in the invention, Figure 3 illustrates a theoretical situation for a ship moored by a tower, as shown in Figure 1, where the wind and waves approach the ship with different directions, The Figure shows, based on experiments performed on models, a graphic presentation of: a) the yaw movement, of a scale boat, when the wind direction versus the direction of the ocean current and waves, is 20 degrees, and where the scale boat is not provided with a wind rudder , Y b) the yaw movement, from the same boat to scale, as above, when the wind direction versus the direction of the sea current and waves is 20 degrees, and where the boat is provided with a rudder for the wind , at an angle of 30 degrees to the length of the boat.

As mentioned above, Figure 1 shows a ship 1, in a side view and from above. At its bow end the ship is provided with a tower 4 which is placed on the hull, for the turning movement and which is moored to the seabed by the anchoring lines 3 (not shown). Thus, the ship is positioned in such a way that it turns or oscillates freely around the tower. An essential feature in accordance with the invention is that a wind rudder, which can rotate, is located at the aft end of the ship, where the rudder extends above the deck or possible facilities that are located. located on the deck. The rudder 5 for the wind is preferably driven by means of an electric or hydraulic motor, and is adapted to rotate to any desirable position (angle), relative to the axis of length or longitudinal axis of the ship. The cross section of the rudder should conveniently have the shape of an alar profile or a drop, as shown in the drawing, to achieve increased "lift" and reduced air resistance. On the other hand, other shapes may be employed, such as a planar or approximately planar shape. Figure 2 shows the cross section of a rudder with an alternative shape, which has a shape such that an approximate bearing surface effect is achieved, for the wind directions coming from both sides of the ship. In this figure the following symbols are used: aR = Steering of the rudder in relation to the ship ß = Wind direction in relation to the ship t = Wind direction in relation to the rudder C - Direction of the bow fin, in relation to the direction of the rudder d = Direction of the aft fin, in relation to the direction of the rudder FR = Held rudder force DR = rudder drag force In Figure 2a the rudder has a shape to maintain a "lift" on the port side (PS in the figure) when the wind arrives from the port side of the boat. Figure 2b shows, in an inverted situation, the shape of the rudder profile, as the wind arrives from the starboard side of the ship, and when a "lift" is desired on the starboard side. That profile maintains a great "lift" even when the angle of attack is 0 degrees, and represents a maximum force in its transverse direction at approximately 8-15 degrees, depending on the shape of the profile. The rudder is divided into three articulated sections that can rotate, one with respect to the others, in a way that allows the center line of the profile to form a curve that characterizes the shape of a wing. It has a main section 10 which is allowed to rotate around the mast 11 supported by ship 1. A section 8, closest to the bow, of the profile, the "leading edge", is allowed to rotate about the axis 6 Both axes 6 and 9 are fixed to the main section 10. Waves in the open sea are generated mainly by the wind, and generally under strong stormy conditions (storm and stronger situations) the direction of the waves will be similar to the direction of the wind, within a band of angles of 15 to 20 degrees, on both sides. This angle can be made greater under windy, weak conditions, due to the so-called "old sea". The marine currents are also generated mainly by the wind. This current generated by the wind, will advance, as a result of the rotation of the earth, towards a direction of up to 20 degrees with respect to the direction of the wind. However, there may be contributions to this current, caused by marine, global (Gulf Stream) and local currents. In those cases, the angle between the current and the waves can reach up to 40-60 degrees, even under strong wind conditions. Since wind and currents generally act at an angle that differs from the direction of the wave, a boat that is not provided with a rudder for the wind will be oriented in an averaged direction, which differs from the direction of the wave. The wave forces will then be significant since the waves, as mentioned above, will cause heavy loads in the transverse direction of the ship. In addition, the waves vary considerably over time, and in this way the ship will perform large yaw movements that can cause strong dynamic loads at the moorings. Figure 3 illustrates a theoretical situation where a ship is moored by a tower, as shown in Figure 1, and where the wind and waves come to the ship from different directions, as indicated by the arrows. The symbols in this Figure are as follows: F3 = Transverse component of the force of the wind on the ship Fc = Transverse component of the loads exerted by the currents, on the ship Fw = Transverse component of the force of the waves, on the ship D3 = Longitudinal component of the force of the wind , on the ship Dc = Longitudinal component of the loads exerted by the currents, on the ship Dw = Longitudinal component of the force of the waves, on the ship Ft = Mooring force to the tower? = Direction of the ship in relation to the orientation or course of the wave M3 = Moment of turn with yaw, of the force of the wind, on the ship Mc = Moment of turn with yaw, of the loads exerted by the currents, on the ship Mw = Moment of turn with yaw, of the forces of the waves, on the ship FR = Component transverse to the ship, of the force of the wind, on the rudder for the wind DR = Longitudinal component of the force of the wind on the rudder the wind CDG = Center of gravity of the ship.

The arrows of forces, indicated by Fw, Fc, and Fs, represent the transversal components of the forces originated by the waves, by the currents and by the wind, respectively, that act on the ship. FR and DR represent the transversal and longitudinal components of the wind forces that act on the rudder for the wind. The longitudinal components of the wind forces, waves and currents, acting on the ship, are indicated, similarly, by the force arrow indicated as D3 + Dw + Dc. The wind, waves and current will also cause a yaw force, for now (around the vertical axis of the ship), as represented in the Figure by an arrow marked with Ms + M "+ Mc that acts around the center of gravity (COG) of the ship. The magnitude of the forces and the strength of the moments acting on the ship depend on the shape of the ship, both below and above sea level, and the relative direction between the ship and the wind, the waves and the current, respectively. The mooring force, indicated by FR, acts through the center of the tower. The forces of moment, which act in relation to the mooring systems to the tower, are generally of such a small magnitude that they can be neglected. It can be defined that a ship is moored in a directionally stable manner, if it is altered from an initial position to another position significantly different, from an initial position, by the influence of a minimum transverse force (disturbance). This unique feature of an unstable static situation. An unstable dynamic situation is characterized because the ship will start rotating (with yaw) with an increasing amplitude, if the ship is given a small transverse disturbance (influenced by a force in a limited period of time). The forces that can generate unstable behavior of the ship can be caused by wind, waves, current or other type of influence, acting on the ship. A moored boat is stable or unstable, with respect to its direction, depending on the coefficients of forces and transverse torques that are caused by wind, waves and currents, along with the location of the tower and its mooring forces. The criterion of dynamic directional stability is further determined by the moment of inertia of the ship, with respect to the yaw motions, and transverse movements of the ship. The magnitude of the forces originated by the waves, wind and currents that act on the ship, depend on the geometry of the ship and its direction averaged with respect to the direction of the waves, wind and currents. In a given situation, if the ship is directionally unstable, large movements with yaw should be anticipated, as mentioned above. If the case is that the ship is directionally stable, the feedback force (wind, currents and waves) will generally be small compared to the ship's inertial forces. Thus, the response period for the yaw movement will become 100 seconds and longer, depending on the forces of the wind, currents and waves. This implies, in addition, that if a component of the force (for example, the force of the wave) alters its magnitude or direction, the direction of the ship can be significantly altered. In particular, the yaw movement will be influenced by (varying slightly) by the forces of the waves. As the wind often acts in a direction that differs with respect to the direction of the waves, and also represents the most dominant force influencing the direction of the ship, the average direction of a ship not provided with a rudder for the wind will be determined mainly by the direction of the wind. Thus, the direction of the ship will be deflected somewhat with respect to the direction of the waves. This is an unfavorable situation since the waves that come against the bow of a boat, with a skewed direction, causes large dynamic forces that generate yaw motions, which results in very large dynamic loads, in the mooring lines of the anchored ship. The waves that arrive against the ship, with an oblique angle, can also cause the ship, great turning movements. The use of one or more rudders for the wind, will provide, in accordance with the invention, a force acting in a direction that is inverse to the sum of the forces FW, FC and FS, and which contributes to the following: to improve the directional stability of the ship, since the rudder acts to increase the "elastic rigidity in the yaw angle" of the ship, an increase in the forces that will return the ship to an average direction, after a turn that moves it from its direction , Y - to alter the average direction of the ship, in such a way that the direction of the waves versus the bow, will be straight, whereby the dynamic forces that influence both the yaw angles of the ship, and the average load of the wave, will decrease. The rudder for the wind can be adjusted and controlled in alternative ways, for example by: - a periodic adjustment of the rudder, according to the changes in the average direction of the boat, versus the wind and the waves, or - the continuous adjustment of the rudder for the wind, which also takes into account the yaw movements of the boat, for maximum use of the capacity of the rudder for the wind. In addition, the wind rudder should be dimensioned to support a transverse force that is large enough to keep the bow of the boat, against the waves, under the most likely combinations of wind, waves and current, for both loaded and with water ballast In addition, adjustment and control of the rudder can be performed manually or automatically, in a manner similar to that of a lateral propeller, on a dynamically placed ship, which will indicate through a data control, based on continuous records, for example from the boat direction, wind, current and waves. Experiments were performed with a scale boat, moored to a tower, where the boat was provided with a fixed rudder for the wind, in accordance with the invention. The experiments were conducted in a scale tank where the waves, propagated to a direction of 20 ° versus the direction of the wind, and where the direction of the current was similar to that of the waves. The rudder for the wind was fixed in a position that formed an angle of 30 ° with the axis of length, of the boat to scale, and had an area that was approximately 20% of the cross-sectional area, of the surface water, of the boat . In the course of the experiments, the boat was placed at an average angle of 3.3 ° versus the direction of the waves, such that the angle of attack of the wind versus the rudder for the wind was 30-20 + 3.3 = 13.3 °. Under these conditions, the maximum yaw angle of the boat was 11.43 °, while the minimum yaw angle was -4.1 °. In the last mentioned case, the angle of attack of the wind versus the rudder for the wind was 30-20-4.1 = 5.9 °, and in the first mentioned case the similar angle was 30-20 + 11.4 = 21.4 °. Experiments were also carried out with a scale boat that was not provided with a rudder for the wind. In these experiments the directions for the wind and for the waves were the same as before. In this situation the boat had an average angle of 13 ° versus the direction of the waves. In addition, the maximum yaw angle was 28 ° and the minimum yaw angle was 0.4 °. Figure 4 a) and b) shows a graphical representation of the yaw movements of the boat, respectively, without and with a rudder for the wind, as recorded during a period of time, under the experiments.

As follows, from the values of the previous digits and from Figure 4 a) and b), the yaw motions (the oscillation movement, from side to side) are substantially smaller for the boat provided with a rudder for wind. In this way, the differences between the greater yaw amplitudes are more than 30%. This reduction in the amplitude of the yaw movements also resulted in a reduction of the mooring loads which upon measuring were found to be approximately 25% for the boat provided with a rudder for the wind. However, as regards the rudder for the wind, which was applied in the experiments, it should be mentioned that this rudder was not optimized regarding size or shape. Meanwhile, the results of the experiments illustrate the positive influence of the movements and forces that will be obtained exclusively, applying a rudder to the wind, in accordance with the present invention. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention. Having described the invention as above, property is claimed as contained in the following:

Claims (4)

1. A method for directing an anchored floating structure, against the direction of the waves, wherein the structure, at its forward end, is tied to a buoy or the like, the method is characterized in that it is provided, to the floating structure, in its aft end, one or more rudders for the wind, which can be turned, and which adjust in such a way versus the wind direction, that the floating structure is directed against the direction of the waves, in a way stable.

2. A means for directing an anchored floating structure, against the direction of the current and / or waves, wherein the structure, at its forward end, is tied by a buoy or the like, the means being characterized in that one or more rudder (is ), which can (and) rotate and which is (are) positively driven, is (are) placed in relation to the aft end of the floating structure and is also adapted (s) to adjust at any desired angle versus the length axis of the floating structure.

3. A means in accordance with claim 2, characterized in that the rudder or rudders for the wind have a wing or wing profile, or drop-like sections.

4. The means according to claims 2 and 3, characterizes or because the rudder (s) is (are) divided into three articulated sections that can be rotated with respect to one another, in a manner that allows the center line of the sections form a camera.