The present teachings relate to a gangway, crane or similar suitable for transferring personnel or goods to or from a floating structure such as a sea vessel.

Gangway operations are frequently used for transferring personnel and goods between floating vessels and other fixed or floating structures. In the wind energy business, gangway transfer is one of the most common ways to transfer personnel, equipment or goods between a service vessel and wind turbines for maintenance tasks. It is also a common way of transferring personnel between offshore floatels and other fixed or floating structures.

In offshore gangway operations the gangway is often moved to a certain distance from the connection point before landing. The landing process as such is well known, e.g. from EP3699078, using a contact sensor at the distal end of the gangway, and EP3305650A1 (KM) and US2020/0262521 (KM), using a camera or similar to measure the relative position between the distal end or tip of the gangway and landing point, and where the gangway is stabilized in so called “hover mode” before and possibly after the landing has been made. WO201 5/009163 (ICD) a method for stabilizing the gangway as an aid for the operator controlling the landing process.

US2020/0262521 describes the process of landing where the vessel moves toward the landing site and at a certain stage uses a camera for detecting the position of the landing site relative to the distal end or tip of the gangway. The system uses a Motion Reference Unit (“MRU”) and Global Positioning System (“GPS”) in combination in order to stabilize the distal end of the gangway at desired hover location, typically given by a park data base for the particular area to operate. This will introduce global positioning capabilities controlling the distal end of the gangway. Further, a camera may be used in order determining the relative position and movements between the landing site and the distal end of the gangway. As discussed in US2020/0262521 and EP3305650A1 the movements may be in five degrees of freedom making the adjustments complex.

Use of a local reference system mounted on the distal end of the gangway needs to be included in order to redress the lack of position accuracy in the GPS sensor setup, for example due to inaccurate park data configuration or due to atmospheric disturbances, hence increasing the local positioning capabilities controlling the distal end of the gangway. If not included the position inaccuracy of just using a GPS sensor setup, may lead to the gangway crash into the landing site. However, US2020/0262521 does not address the topic of how to combine these different types of position sensors, in order to end up with position sensor setup having both globally and locally positioning capabilities.

A direct solution of how to combine these globally and locally sensor systems would typically be to use the GPS position measurement while the distal end of the gangway is far away from the landing site, and then switch over to a local position sensor measurement when the distal end of the gangway is close to the landing site. The problem of switching into a local position measurement as described here, is however not a trivial task regarding stability properties, due to the local measurement equipment is located close to the distal end of the gangway. A gangway structure will typically be a flexible arrangement, hence ending up with a non-minimum phase control system. This is caused by physical characteristics (loop dynamics gangway base to tip) of the gangway structure introducing time transients in the camera measurement. For example, given a step change in the gangway base rotation, the tip will not move instantly in the same direction. The rotation of the tip will be a function of time, eventually aligned with the gangway base. Initially the camera will measure a movement in the opposite direction compared to the base movement, due to non-minimum phase effects. As time progresses, the tip will start to move towards the base rotation, eventually aligning with the base. In a feedback control loop, this may end up with a highly oscillating system, and hence be a potential risk of damage for both humans and expensive structures. The classical way to mitigate this problem is to lower the gain in the feedback loop, lowering the ability and performance for the system to compensate for disturbances, however increasing robustness towards unwanted oscillation of the structure. For a gangway structure like this, a decrease in the feedback loop gain may end up with a system that is not able to compensate for the environmental disturbances acting on the vessel, and hence lacking the overall control task of landing the distal end of the gangway on the landing structure.

It is an object of the present invention to provide a solution for accurate positioning maintaining the stability of the structure tip in the last sequence before connecting with the landing site, additionally to apply certain principles to include a camera system without reducing performance. This object is achieved as specified in the accompanying claims.

In other words the invention is related to the question how to use sensors with given physical locations on a gangway, and how to process signals to avoid non-minimum phase problems or instabilities in a gangway control system.

The first part of solution includes introduction of second sensor with non-optimal placement from a stability perspective. The resulting secondary measurement is used to increase accuracy of the relative position between the distal end and a known structure. This is a practical approach to increase accuracy based on current state of the art global position sensors.

The second part of solution includes processing of the second measurement signal which separates the non-minimum phase frequency content of the measurement, only using a bias value to correct the global position. The placement of the second sensor is problematic in a flexible structure control system due to non-minimum phase, inherently prone with instability problems but the proposed control system does not use the secondary measurement directly. It uses the filtered value from the secondary measurement to bias the global position measurement of the floating structure, omitting the problem of a non-minimum phase system configuration.

The invention will be discussed below with reference to the accompanying drawings, illustrating the invention by way of examples.

Fig. 1a-c illustrates the prior art

Fig. 2 shows a flow chart according to the invention.

Fig. 3 shows a signal possessing unit according to the invention.

Figure 1a-c illustrates the prior art, i.e. when the distal end 2a of the gangway is located close to the landing site 1 , where the gangway is for illustration purposes shown as a flexible rod 4 with a camera 2 or similar device positioned on or close to the distal end 2a of the gangway. The camera is aimed at a landing point 1 and is configured to detect the position and movements of the landing point 1 relative to the gangway tip 2a, as mentioned in US2020/0262521 and EP3305650A1 taking several degrees of freedom into account, where the measured relative position is corrected using the joints and actuators of the gangway structure. The camera or similar may use a system for aiding the docking of a vessel as discussed in W02020/070114. The landing point may be an automatically detected feature on the installation or may be selected by an operator through user input into the system.

Referring to figure 1c, in order to stabilize the tip 2a position in a desired position relative to the landing point 1 , an actuator base on a vessel 3 is provided being configured to move the gangway 4, here, for simplicity reasons, only shown for one degree of freedom. This is, as illustrated, obtained by providing feedback from the camera 2 or similar detecting the relative position of the landing target 1 from the camera 2, through a controller 5 to the servos 6 in the actuator base 3, which in turn moves the gangway 4 in order to stabilize the relative position between the landing point 1 and the distal end of the gangway 2a. As illustrated in the plot 10 this stabilization process, however, may give rise to an oscillation which may escalate to the degree that landing is impossible or ruin the gangway and/or landing point. The typical solution to solve instability would be to reduce controller gain. This is suitable for a fixed (or a slowly varying) actuator base. This is, however, not the case for a vessel operating in rough sea.

Also as illustrated in fig. 1 a the system prior to the landing process is usually controlled by an inverse kinetic unit 7 steering the gangway to the landing point using the measured global position 8 of the gangway tip 2a as well as the global position reference of the landing point 9 to calculate the global relative position of the gangway tip relative to the landing point. This relative position in global coordinates needs to be mapped into local coordinates 25 before fed into the inverse kinematic unit 7, being the situation when switch 23 is in the illustrated position. Typically, when the vessel has reached the desired position and heading, the stabilization of the gangway is enabled. The actuated gangway then moves the position of the distal end or tip 2a so it ends up hovering at a location around the desired hover point. Thus, when the distal end is close to the landing point the gangway control system switches using switch 23 from using global position sensor to a local position sensor e.g. a camera-based sensor.

As discussed above the present invention is aimed at avoiding the inherent oscillation 10 in the gangway movements, with bandwidth cope with the environmental dynamics. According to the present invention, now referring to figure 2, the relative position error measured by the camera is initially represented in the distal end frame. This error is then mapped into a representation in earth fixed frame. The error measured by the camera (represented in earth fixed frame) is used in order to adjust the desired distal end hover x, y and z point. The gangway system aims to adjust the distal end to the desired hover x, y and z point.

More specifically with reference to figure 2, one difference from the prior art is that the local controller 5 illustrated in figure 1 , directly controlling the gangway 4, is omitted. Instead, the camera 2, being configured to measure the deviation or error in the position of the gangway distal end 2a relative to the target 1 . The data from the camera 2 is transmitted to a hybrid controller unit 20. The hybrid controller unit 20 is connected to the global positioning and attitude measurement system 11 ,12 such as GPS and MRU, preferably through a filter thus receiving the same position and attitude data 8 of the distal end 2a of the gangway 4 as the global position control arrangement utilizing inverse kinetic unit 7. This way steering the gangway relative to global reference and the corrected or biased position of the landing point. The correction in the position of the landing point thus corresponding to the error or deviation measured by the camera.

Although the measurement of the second relative position is here discussed based on a camera 2 or other imaging device the second relative position may be measured using other types of measuring units being capable of detecting the relative position between the gangway tip 2a and the landing point 1 , enabling the system to move the tip into the required position. Also, the information from the local position measuring unit 2 may be filtered so as to remove vibrations, wave movements and movements having higher frequencies so as to stabilize the position data when steering the gangway tip into the predetermined position. These faster variations may be handled by the system at the stage when the gangway tip is actually connecting to the landing point. The filter may be preset or may be dynamically adjusted according to the measured movements of the gangway tip.

The hybrid controller unit 20 thus combine the global position of the base 3 with the local relative position detected by the camera 2, adding a correction 17 to the control signal from the operator 18 and/or predetermined landing position 19. This earth fixed corrected reference signal 9’ is then combined with the estimated global position 8 of the gangway tip 2a, calculating 21 the difference between the global estimated tip value 8 and earth fixed corrected reference value 9’, based on the data from the camera and the data base stored location of the target. Estimated tip value 8 is calculated by the forward kinematic base to tip positioning unit 22. The control signal 21 is then used to calculate the joint positions and velocities by first using the global to local coordinate mapping unit 25, followed by using the inverse kinetic unit 7, which is used as references to control the position of the distal end of the gangway 4. Once the system is stabilized in the desired position relative to the landing point the landing operation maybe performed automatically or manually using the local servos 6 controlling the gangway 4 and the camera 2.

The hybrid controller unit 20 may according to the invention include a forward kinetic base to camera unit 13 feeding the received global position and attitude to a mapping unit 14. The mapping unit 14 also receive the local position data from the camera 2 and provide a combined data set representing the position error measured by the camera in global coordinates. The output from 14 is combined with the desired hover distance 18, e.g. prestored or chosen by an operator, corresponding to the desired distance of the gangway tip 2a relative to the landing point and a measured error between the desired and actual position. The resulting error is used for building a correction or bias data signal. The correction is calculated using a signal possessing unit 16, e.g. a filter, but is not limited to being a filter.

As illustrated in figure 3. an example of a signal possessing unit can be to multiply the error signal xe,ye and ze, derived from 14 and 18 with constants Kx,Ky and Kz respectively, integrating the resulting signal to get the bias xb, yb and zb. The data corruption signal 15 detecting if the data from the secondary position measurement is corrupt, is fed into the signal possessing control unit 24. The signal possessing control unit 24 can be used to reset the signal possessing unit, to turn the signal possessing unit on and off, etc. The functionality of the signal possessing control unit 24 is not limited to mentioned control functions. The signal possessing control unit 24 can control each signal or state individually or all simultaneously. In short the system and method according to the invention controls the gangway based on the measured global position and attitude 8 of the gangway tip and a corrected reference 9’ adjusted for the relative position of the landing point from the distal end of the gangway.

Instead of using the camera measurement directly in the gangway servo control loops, as is the case in the prior art, the camera measurement or local measuring device 2 is thereby used to determine the position of the gangway base 3 necessary to obtain the desired hovering distance or position. The distal end 2a is then positioned using forward kinematics from the base 3. The camera is in this way not directly used in the servo control loops, eliminating the problem of oscillation due to the fact that measurement is located at the end of a flexible structure (non-minimum phase system). The camera’s primary function is to eliminate the position bias typically present in a gangway control system purely using measurements from a GPS unit and MRU unit, compared to a data base with stored locations. The camera measurement may be fed into a filter/observer to determine the bias present in the global position measurement. This global bias is forwarded to the gangway control system to determine actual gangway base location relative to the landing point. The new earth fixed desired hover x, y and z point used by the gangway system, now incorporates the filtered/observed bias in order to fulfill the goal of hovering at a desired distance away from platform. From this point, typically the next step is to engage a control approach that fulfills the connect procedure placing the gangway in connection with the target point.

To summarize the present invention relates to a control system for controlling a gangway structure on a floating structure to a desired position relative to a fixed or moving landing point 1 as well as a method for controlling the movements of the end point 2a of the gangway. The desired position will usually be at a distance from the landing point 1 but may in some cases be at the landing point. It should be noted that although the word gangway has been used throughout the specification the solution according to the invention also applies to corresponding systems such as crane or similar suitable for transferring personnel or goods to or from a floating structure such as a sea vessel.

The system includes a global position measuring system 11 , such as GPS, measuring the position of the floating structure 3 and the position of the landing point 1 , and thus, based on these measurements, calculate a first relative position between the floating structure and the landing point based on their global positions. The control unit system according to the invention is configured to move the gangway relative to the landing point. The system also includes a local position measurement unit 2 positioned at, close to or at least in relation to the distal end 2a of the gangway 4 and is configured to measure a second relative position between the landing point 1 and the gangway distal end 2a.

The local position measurement unit, constituted by a camera or similar system, e.g. as described in W02020/070114, is connected to the control system unit so as to communicate the second relative position to the control system unit. The control system being configured to calculate in a control unit 20 an adjustment or bias signal 17 and to adjust the first relative position based on the second relative position. Thus results in a corrected reference signal 9’. The control system is configured to, based on the corrected reference signal, adjust the position of the gangway so the gangway distal end 2a is moved into the desired position relative to the landing point 1 .

The corrected reference signal is derived by a combination of the measured first and second relative position, the local relative position between the camera 2 and target point 1 and the global relative position between the global position of the landing point and the position of the gangway end, so that the combination of the first and second measurement may provide a closed loop system.

The bandwidth of the communicated second relative position measurement may be filtered by a predetermined filter, e.g. depending based on a selected bandwidth so as to remove the effect of at least some of the movements of the gangway and/or gangway tip. The filter may be predetermined or adjusted in real time based on measured movements by suitable sensors on the gangway.

The second relative position is added to the reference position of the landing point, thus biasing or correcting the reference position, or the measured global first reference position of the floating structure vessel, thus biasing or correcting the global position data of the floating structure.

The second relative position may preferably be measured using an imaging device, such as a camera, configured to recognize a feature on the landing point but other principles may be contemplated such as laser scanning, beacons, etc, being available to the person skilled in the art. The feature may be selected by an operator or automatically recognized by the system.

The method for controlling a gangway on a floating structure is based on the utilization of a global positioning system measuring the global position of the floating structure, where the gangway with a distal end configured to be positioned in a predetermined position relative to a landing point and the landing point having a known global reference position. The method further includes the steps of: providing a first relative position between the floating structure position and the landing point position, e.g. defined by the difference between the global position of the structure and the landing point, e.g. measured using a global positioning system, measuring a second relative position between the landing point and the predetermined position of the distal end of the gangway relative to the landing point, e.g. using a camera or other available system measuring the relative position between two objects, preferably in three dimensions, calculating a corrected or biased first relative position based on the second relative position, e.g. by adjusting the measured global position data of the landing point or floating structure, and controlling and moving the distal end of the gangway on the floating structure into the predetermined position based on the biased first relative position, preferably using the floating structure propulsion system and/or gangway control actuators.