WO2024151169A1

WO2024151169A1 - Method and system for determining position of vessels

Info

Publication number: WO2024151169A1
Application number: PCT/NO2024/050005
Authority: WO
Inventors: Carl Johansson
Original assignee: Kongsberg Maritime As
Priority date: 2023-01-10
Filing date: 2024-01-10
Publication date: 2024-07-18
Also published as: NO20230017A1

Abstract

A module for determining a relative position between an assisting vessel and an assisted vessel connectable to the assisting vessel with a towline The assisting vessel has a winch controlling the towline between the assisting vessel and the assisted vessel. The module estimates the relative position between the assisting vessel and the assisted vessel based on one or more winch parameters. The module may be used in a safety system in towing operations.

Description

METHOD AND SYSTEM FOR DETERMINING POSITION OF VESSELS

INTRODUCTION

The present invention concerns determining relative positions of vessels. In particular, the invention concerns a method, a module and a system for determining a relative position between an assisted vessel and an assisting vessel, e.g. a tugboat. The module and system may e.g. be used as a support system for a captain of the assisting vessel, in a safety system for towing operations or in autonomous or remote control of the vessel.

BACKGROUND

A tug, or tugboat, is a powerful boat or ship that is used for assisting a marine vessel or e.g. a marine floating construction. The tug assists e.g. a marine vessel by towing, pushing, and escorting the marine vessel, i.e. the assisted vessel. By towing, pushing or escorting the assisted vessel, one or more tugs may maneuver the assisted vessel during difficult maneuvering operations, e.g. in a harbor, in a narrow canal or during rescue operations of vessels in distress. During towing operations, the tug maneuvers the assisted vessel by pulling a towline connected between a winch on the tug and the assisted vessel. Tugboats are normally manually captained and the winch is manually controlled by either the captain or one of the crew members. The captain receives, via radio communication, order from the assisted vessel to provide thrust at a certain angle relative to the assisted vessel. The captain maneuvers the tug to that location and then applies the breaks to the winch and then applies thrust. The winch is manually controlled so that the towline is not let in the water or tension is not applied when it is not supposed to. Tugboats may also be connected to the assisted vessel by towline during pushing and escorting operations depending on the circumstances.

For a tug to keep a relative position to an assisted vessel the tug needs to know the position, heading, and speed of the assisted vessel in relation to the tug. Today, this is manually performed. Keeping a safe relative position between the tug and the assisted vessel is important. To keep a safe relative position is also important in situations where the tugboat is an autonomous tug, a remote- controlled tugboat or a tugboat operating in autonomous or semi-autonomous mode. If an error occurs in an autonomously operated tug, or a remote-controlled tugboat loses connection to the remote operation center (ROC), while the tug is connected with a towing line to an assisted vessel, this may lead to potentially dangerous situations.

SUMMARY OF THE INVENTION

The invention provides a module for determining a relative position between an assisting vessel and an assisted vessel connectable to the assisting vessel with a towline, wherein the assisting vessel comprising a winch controlling the towline between the assisting vessel and the assisted vessel, wherein the module is adapted for estimating the relative position between the assisting vessel and the assisted vessel based on one or more winch parameters.

The one or more winch parameters may include at least one of a length of the towline and an angle of the towline with respect to the winch. The one or more winch parameters may further comprise a tension of the towline. The one or more winch parameters may further comprise a speed of the towline when reeling in and reeling out the towline by the winch. The module may be adapted to receive input from at least one sensor for measuring the angle of the towline. The module may be adapted to receive input from a sensor for measuring the length of the towline. The module may be integrated into a dynamic positioning system of the assisting vessel or integrated into a winch control system of the assisting vessel.

The invention also provides a system for determining a relative position between an assisting vessel and an assisted vessel connectable to the assisting vessel with a towline, the assisting vessel comprising a winch and a winch control system adapted to control the towline between the assisting vessel and the assisted vessel, the winch control system being adapted to determining a relative position between the assisting vessel and the assisted vessel based on one or more winch parameters of the winch control system. The one or more winch parameters may comprise at least one of a length of the towline and an angle of the towline with respect to the winch of the assisting vessel. The one or more winch parameters may comprise a tension of the towline. The one or more winch parameters may further comprise a speed of the towline when reeling in and reeling out the towline by the winch. The system may further comprise a first sensor for measuring the angle of the towline. The system may further comprise a second sensor for measuring the length of the towline.

The invention also provides a method for determining a relative position between an assisting vessel and an assisted vessel towed by the assisting vessel, the assisting vessel comprising a winch controlling a towline between the assisting vessel and the assisted vessel, the method comprising determining the relative position between the assisting vessel and the assisted vessel based on one or more winch parameters.

The invention further provides a method for controlling an assisting vessel operating in autonomous, semi-autonomous or remote-controlled mode, wherein the assisting vessel is connected to an assisted vessel by a towline; the method comprising:

- measuring at least one winch parameter;

- inputting the at least one winch parameter to a winch control system of the assisting vessel;

- calculating by the winch control system a corrected assisted vessel position based on the at least one winch parameter; and

- inputting the corrected assisted vessel position to a propulsion control system for the assisting vessel for controlling the position of the assisting vessel relative to the assisted vessel.

- keeping the towline at a constant tension;

- measuring a change of position of the assisted vessel with respect to the assisting vessel based on at least one winch parameter; and

- inputting the measured change of position to a propulsion control system of the assisting vessel for controlling the position of the assisting vessel relative to the assisted vessel. The one or more winch parameters may comprise at least one of a length of the towline and an angle of the towline with respect to the winch of the assisting vessel. The one or more winch parameters may comprise a tension of the towline. The one or more winch parameters may comprise a speed of the towline when reeling in and reeling out the towline by the winch. The method may further comprise controlling a position of the assisting vessel relative to the assisted vessel by reeling in or feeding out the towline while keeping the tension of the towline. The method may further comprise measuring a rate of change of the position of the assisted vessel with respect to the assisting vessel. The method may further comprise inputting the measured rate of change of position of the assisted vessel to the propulsion control system of the assisting vessel, and either maintaining the relative position of the assisting vessel and the assisted vessel, or moving the assisting vessel to a new relative position with respect to the assisted vessel. The method may further comprise keeping the tension of the towline at a minimum constant tension.

The invention also provides use of the system or module above in a safety procedure during loss of connectivity between a remotely controlled assisting vessel and a remote control facility.

The invention also provides use of the system or module above as a safety support system for a captain of the assisting vessel for providing information about the assisting vessel relative to the assisted vessel or for assisting in maintaining the relative position between the assisting vessel or the assisted vessel.

The invention further provides use of the system or module above as support for a captain of the assisting vessel in an autonomous follow ship mode.

The invention further provides use of the system or module above for autonomous control of the assisting vessel during towing operations of an assisted vessel.

The invention further provides use of the system or module above for towing of a wind turbine e.g. a floating wind turbine. The invention may provide a safety solution for keeping a safe relative position between an assisted vessel and an assisting vessel, e.g. a tugboat. If a situation occurs during the tow and the safety system kicks in, the DP system may be allowed to take control of the tugboat and move the tugboat away from the assisted vessel or keeping a safe relative position between the tugboat and the assisted vessel. The invention enables keeping control of the tugboat in all situations, avoiding and preventing potentially dangerous situations. An example is in situations with loss of connectivity between the assisting vessel and a remote operation center (ROC). If loss of contact with the ROC, the assisting vessel may be moved away from the assisted vessel. Further examples are as a safety system preventing the assisting vessel for coming too close to the assisted vessel, or as a support system when the assisting vessel is in autonomous mode. The security system may function like an anchor for the assisting vessel. Knowing the position of the assisted vessel relative to the assisting vessel also provides a further safety system in autonomous operations. This enables maintaining control of the assisting vessel even if an error should occur in the autonomous control system. The assisting vessel may be manned or unmanned. The security system also provides a safety system for a manned assisting vessel, as the security system may take control of the assisting vessel or may assist the captain if a dangerous situation occurs or assist to prevent a dangerous situation. The relative position may be calculated based on onboard winch parameters for the assisting vessel and no interaction with the assisted vessel or remote facilities or other remote entities are required, which provides a true onboard safety system The security system may kick in automatically and requires no manual input. The invention may also be used in a safety system for towing in narrow passages.

BRIEF DESCRIPTION OF DRAWINGS

Example embodiments are described with reference to the following drawings, where:

Figure 1a is a schematic illustration showing three tugboats assisting an assisted vessel;

Figure 1 b is a schematic illustration illustrating a tugboat with a winch assisting an assisted vessel, where the tugboat is illustrated in different assisting positions; Figure 1c is a schematic illustration illustrating the different positions a tugboat may have in relation to the assisted vessel;

Figure 2a, 2d is a schematic illustration illustrating a tugboat with a winch assisting an assisted vessel, where the assisted vessel has a shifted position and is in a relative position (X, Y) with respect to the tugboat;

Figure 2b, 2e is a schematic illustration illustrating a situation where the tugboat from Fig.2a and the assisted vessel have slowed down and the relative position between the assisted vessel and the tug has changed; and

Figure 2c, 2f is a schematic illustration illustrating a tugboat with a winch towing an assisted vessel and where the assisted vessel has further turned with respect to the tugboat.

Figure 3 is a schematic diagram of a propulsion control system, winch control system, winch and sensors and operational parameters of the assisting vessel; Figure 4a is a schematic illustration illustrating an assisting vessel with a winch assisting a vessel, and where the assisting vessel is connected to the assisted vessel with a towline of length L. The towline has a fixed baseline C, a variable height H of the bow of the assisted vessel above the baseline of the towline, where the fixed baseline C and the variable height H provides the legs of a right triangle, and the length L of the towline the hypotenuse in the right triangle. The towing line length L is variable and is measured. The height difference between the assisting vessel and assisted vessel provides a vertical angle of the towline with respect to the horizontal plane;

Figure 4b illustrates an assisting vessel with a winch assisting a container vessel, and where the assisting vessel is connected to the assisted vessel with a towline of length L. The height difference between the assisting vessel and assisted vessel provides the vertical angle of the towline as explained for Figure 4A.

DETAILED DESCRIPTION

Example embodiments are described with reference to the drawings. The disclosed embodiments are non-limiting and other embodiments may also be envisaged. Elements from different embodiments may also be combined.

Example embodiments are described exemplifying a tugboat as an assisting vessel. Other marine vessels than tugboats as e.g. marine naval ships, fishing vessels, supply ships, and anchor handling vessels may also be used as an assisting vessel. The assisted vessel may be all kinds of marine vessels that need assistance by towing. The marine vessel may e.g. be a tank ship, a cargo vessel, a container vessel, an oil/gas tanker, or any other frighting ship, an offshore vessel, fishing vessel, passenger vessel, a mobile offshore unit, a wind turbine to be towed to a offshore wind installation site etc..

Figure 1a illustrates an example of a towing situation with three tugboats 2 assisting an assisted vessel 1 . The assisted vessel 1 typically has its own propulsion control system and thrusters 4, 5 and steering systems 3. The assisted vessel may also not contribute to the propulsion. The assisted vessel may also be without its own propulsion system. The assisted vessel may have no propulsion control system, thrusters and steering systems. The tugboats are illustrated in different positions for assisting the assisted vessel. A tugboat 2 is connected to the aft of the assisted vessel 1 by a towline. A tugboat 2 is connected to the fore end of the assisted vessel by a towline. These two tugboats in Figure 1a are each provided with a winch on the fore/bow of the tugboat and are each connected to the assisted vessel by a towline 13. The third tug in Figure 1a is in close contact with the assisted vessel pushing and/or guiding the assisted vessel. Other positions for the tugboats are also possible. There may also be only one tugboat and the three positions illustrating three possible assisting positions for the tugboats. The tugboats may be remotely controlled by a central control facility 40. The central control facility may control each tugboat by a remote human captain or by automatic autonomous control or a combination of automatic and remote human captain or a combination of autonomous control and a captain onboard the tugboat(s). The tugboats may be remote controlled from another vessel or from a land based remote operation center (ROC) facility 40 as explained below. The tugboats may be unmanned or manned. The tugboats include redundant propulsion 8, 9 and steering systems. The assisted vessel 1 may be provided with an onboard control module 20 communicating with the central control facility 40. The onboard control module may transmit parameters related to the assisted vessel to the central control facility 40. Parameters related to the assisted vessel may e.g. be parameters related to thrusters, rudder, and propellers and/or position and heading and speed of the assisted vessel. The onboard control module may include transceivers, antennas and a control module for communicating with the onboard propulsion control system, e.g. a DP system, and onboard control systems for maneuvering the assisted vessel.

The tugs illustrated in Figure 1a may be provided with a remote control system 50 for communicating with the remote operation control facility 40 on land or on another vessel. The remote control system 50 may include transceivers, antennas and a control module for communicating with the onboard propulsion control system, e.g. a DP system, and onboard control systems for maneuvering the tug. The remote control system 50 may also include remote winch and machinery control. The onboard control module may transmit a various number of parameters to the central control facility concerning the tugboats; e.g. thruster parameters, steering parameters, winch parameters (e.g. tension, length, speed), length of the towline, tension of the towline, the towline angle, position of the tugboat etc.

The tugboat may be manned or unmanned. The unmanned or manned tugboat may be operated in autonomous or semi-autonomous mode. The assisted vessel may be manned or unmanned and may be operated in autonomous or semi- autonomous mode or fully manually by the human captain.

The assisted vessel 1 may be controlled by the three tugboats performing control by the towline 13 both by controlling the aft of the assisted vessel, by controlling the fore of the assisted vessel and by pushing/escorting the assisted vessel. The three tugboats may work together to control the assisted vessel to fulfill their mission with the assisted vessel. Escorting and pushing can still have a towing line connected even though the towing line is not used to transfer force at that point.

Fig. 1b illustrates a tugboat 100 assisting an assisted vessel 104. The tugboat 100 is illustrated in three different positions for assisting the vessel. The tugboat in Fig.1b is connected to the assisted vessel by the towline 103. The illustrated first position 105 of the tugboat is straight behind the assisted vessel. In the illustrated second position 106 the tugboat is further behind and at an angle in view of the travelling direction of the assisted vessel. In the illustrated third position 107 the tugboat is closer to the assisted vessel, and the length of the towline is shorter, but the tugboat is further offset from the travelling direction of the assisted vessel. The tugboat may e.g. assist the assisted vessel in turning, maintaining the speed, heading/positions, or reducing speed. The assisted vessel illustrated in Fig.1b, as an example only, is a tank ship. The assisted vessel has its own propulsion control system and thrusters and steering systems. The tugboats further include redundant propulsion 9 and steering systems 8. The tugboat may also be provided with a DP system, and an onboard control module as described above for the tugs illustrated in Figure 1a.

The tugboat 100 in Fig.1b is connected to the assisted vessel 104 by a tow equipment 102 including a towline 103. The towline 103 may on the assisted vessel end be connected to the assisting vessel 104 by a towline connector 110, such as a bollard. The tow equipment 102 may comprise a spooling device, such as a winch, to spool in and spool out the towline 103. An assisted vessel may be towed, but the assisted vessel may also be pushed or escorted. In an escorting or pushing operation, the tugboat may push or apply forces directly to the assisted vessel. In escorting and pushing operations, a towline may still be connected between the assisted vessel and the tugboat even though the tow line is not used to transfer forces between the tugboat and the assisted vessel.

Tow equipment 102 may be arranged in a fixed position on the tugboat 100. The fixed tow equipment 102 may also comprise a towing point device 111 that can guide the towline 103 from the spooling device to the towed vessel 104. The towing point device 111 may also be a rotating element or may be a fairlead as illustrated in Fig. 1 b. The fairlead 111 in the bow of the tugboat 100 in Fig.1 b allows the transverse force of the towline 103 to be transmitted to the hull of the assisted vessel 104. In Fig. 1 b, the tugboat 103 is illustrated in different positions 105, 106, 107 connected to the aft of the assisted vessel 104 by a towline. The winch may be arranged at the stem or the bow of the tugboat or in other positions on the tugboat suitable for towing or escorting as illustrated in Fig. 1a and Fig.1c. As can be seen from Fig. 1c, the tugboat may be arranged alongside the assisted vessel at the aft, mid, or fore positions. The tugboat may be connected to the assisted vessel by a towline. The tugboat may be arranged with the bow or the stem facing the side of the assisted vessel either in distance from or in close contact with the hull of the assisted vessel. The tugboat may be arranged in different positions connected to the aft of the assisted vessel by a towline e.g. pulling the aft of the assisted vessel, pulling the aft to starboard or port side, or assisting the assisted vessel in maintaining the speed, heading and/or position. The tugboat may be arranged in different positions connected to the fore of the assisted vessel by a towline e.g. pulling the fore of the assisted vessel forward or to starboard or port side, or assisting the assisted vessel in maintaining the speed, heading and/or position. Other positions for the tugboat in relation to the assisted vessel may also be possible. The tugboat may be connected to the assisted vessel by a towline. The connection point for the towline is marked with a dot in the drawings. The assisted vessel may also not contribute to the propulsion or lack own propulsions means. In that case the towing or moving of the assisted vessel is only dependent on the pushing and/or towing force provided by the assisting vessel(s). This is for example the case when towing an offshore wind turbine to a final installation site.

The tugboat 100 may have a propulsion control system 101. The propulsion control system 101 may be a dynamic positioning (DP) system. The propulsion control system, which may be a dynamic positioning system, controls the position, speed, and heading of the tugboat by using the tugboats own propellers and/or thrusters with respect to one or more position references. The tow equipment 102 may be connected to the propulsion control system 101 . The tow equipment 102 may be manually controlled, automatically controlled, or semi-automatically controlled by the winch control system based on a plurality of input parameters. The plurality of input parameters may include at least one of a length L of the towline 103, a tension T of the towline 103, a speed of the towline and at least one angle A, B of the towline 103. The angle of the towline may be an angle A in the horizontal direction (Fig. 2a-f). In addition to the angle A in the horizontal direction also an unput parameter may be provided for an angle B in the vertical direction (Fig 4a and b). Speed of the towline may be calculated based on the length of the towline and time for spooling in/out the towline to the desired position. Speed of the towline may also be used to calculate the speed of the assisted vessel 104. The input parameters for the towline may be input into the winch control system 113 (Fig.3). The winch control system may output an assisted vessel position (AVP) to the propulsion control system 101 of the tugboat. The assisted vessel position may be calculated by the winch control system based on one or more winch parameters. The one or more winch parameters may include at least one of a length L of the towline 103, a tension T of the towline 103, a speed S of the towline and at least one angle A, B of the towline 103. The propulsion control system controls the propulsion systems 115 of the tugboat and moves the tugboat in reaction to the output assisted vessel position. The propulsion control system

101 may based on the output assisted vessel position calculate a waypoint to which the tugboat moves. The winch control system 113 may also control the winch 112. Winch parameters as e.g. winch tension T, the towline length L and winch speed S may be controlled based on a number of input parameters.

The input parameters may e.g. be provided by sensors, systems or processing devices. Input parameters may also be provided in the form of a manual input from e.g. a human or robot e.g. by a manual lever input, manual input into a system or processing device provided by e.g. a mouse, keyboard, touch screen, switch etc.

The angle A of the towline in the horizontal plane may be measured with respect to the longitudinal direction of the tug. The longitudinal direction of the tug may be e.g. with respect to the winch or towing point device 111 depending on the position on the winch and tow point device on the tug. Automatic control of the tow equipment based on such input parameters may allow for continuous adjustment of the tow equipment 102 and thereby the winch and towline. The tow equipment may also be adjusted intermittently, at specific times, or as required based on the input parameters. The tow equipment may also be adjusted semi-automatically or manually based on the input parameters. The adjustment of the tow equipment

102 continues while the tug 100 tows the assisted vessel 104. The propulsion control system (e.g. a dynamic positioning system (DP)) 101 may be adapted to receive information of the length L of the towline, the tension T of the towline and/or the angle A of the towline from the tow equipment 102, e.g. by wired or wireless communication. A winch of the tow equipment 102 may, for example, keep record of the length L of spooled out towline 103. A sensor for measuring the length L of the towline may be included in the winch 112 or winch control system

113. The length L of the towline may also e.g. be measured by a winch control system of the tow equipment controlling the reeling in and reeling out of towline. The angle A of the towline in the horizontal direction may be detected by e.g. use of one or more cameras, one or more laser ultrasound short range radars, lidar or an array of laser distance sensors. The one or more of these sensors may be arranged on or near the winch 112 or in another appropriate position on the tugboat 100. The angle A may also be measured by use of a friction less mechanical sensor (e.g. a mechanical arm) attached to the tugboat. The mechanical sensor may be attached on the deck of the tugboat near the winch. The tow equipment 102 may further comprise a tension sensor 108 to measure the tension T of the towline 103.

The propulsion control system 101 e.g. in form of a dynamic positioning system, may also receive information about the relative height AH (Fig.4a, 4b) and/or the vertical angle B of the towline connector 110 on the towed vessel 104 with respect to the tow equipment 102 on the tugboat. The relative height AH is a parameter that relates to the displacement of both the tug 100 and the towed vessel 104, the position of the tow equipment 102 on the tug 100 and the position of the towline connector 110 on the towed vessel 104. The relative height AH may be variable as the tugboat 100 and the assisted vessel 104 move due to e.g. waves and heave motions. Knowing the relative height AH and/or the vertical angle B may also increase the precision of the output from the winch control system 113 of the tugboat 100 to the propulsion control system 101 of the tugboat 100. The winch control system 113 and/or the winch 112 may adjust for the vertical motions between the assisting vessel 100 and the assisted vessel 104 due to waves and heave based on the relative height AH, by paying out or paying in the towline 113. The length L of the towline 103 is measured as explained above. The length L of the towline may be input to the winch or winch control system 113. Figure 4a and 4b illustrates tugboat 100 with a winch 102 assisting an assisted vessel 104, and where the tugboat 100 is connected to the assisted vessel 104 with a towline 103 of length L. The relative height AH between the tow equipment 102 of the tugboat 100 and the towline connector 110 on the assisted vessel 104 provides a vertical angle B of the towline 103 with respect to the horizontal plane. The winch control system or the winch may calculate the relative height AH and the vertical angle B based on the measured length L of the towline and the distance between the tugboat and the assisted vessel. The distance C between the tugboat 100 and the assisted vessel 104 may be kept fixed by use of positioning data from the tugboat and the assisted vessel or by measuring the distance between the tugboat and the assisted vessel and controlling the tugboat 100 to maintain this distance, or a combination of positioning data from the tugboat or the assisted vessel and measuring a distance between the tugboat and the assisted vessel. The vertical angle B of the towline may also be measured by e.g. use of one or more cameras, one or more laser ultrasound short range radars, lidar or an array of laser distance sensors. The one or more of these sensors may be arranged on or near the winch

112 or in another appropriate position on the tugboat 100. The vertical angle B may also be measured by use of a friction less mechanical sensor (e.g. a mechanical arm) attached to the tugboat. The mechanical sensor may be attached on the deck of the tugboat near the winch.

Figure 3 is a schematic diagram of an example embodiment showing relationships between a propulsion control system 101 , propulsion 115, winch control system 113, winch 112 and sensors 114, and operational parameters of the assisting vessel. Mission control may be implemented in an onboard control system on the assisting vessel. A tow controller and vessel systems for navigation and manoeuvring, sensor systems and different operational parameters may be provided on the assisting vessel. A module may convert winch parameters to a relative position between the tugboat and an assisted vessel. The module may be implemented in the winch control system. The winch control system 113 may provide assisted vessel position (AVP) information to the propulsion control system 101. The module may alternatively be integrated in the propulsion control system 101. Winch parameters related to the tension, length, speed and angle of the towline may be provided to the propulsion control system. Figure 3 illustrates relationships between the propulsion control system 101 of a tugboat, the propulsion system 115 of the tugboat, the winch control system 113 of the tugboat, the winch 112 on the tugboat and towing line angle sensors 114 of the tugboat. Sensor(s) as described above for measuring at least one angle A, B provides towing line angle inputs to the winch control system 113. The winch control system

113 may also control the winch 112. The winch control system 113 may provide control parameters for operation of the winch to the winch. The control parameters for controlling the winch may e.g. be winch tension which relates to the tension T of the towline, winch length which relates to the length L of towline 103 and winch speed which relates to the speed S of the towline when reeled out or reeled in. The winch control system 113 may also receive parameters for winch tension T and towing line length L and winch speed S from the winch. The parameters for winch tension, towing line length and winch speed may also be measured by sensors near or attached to the winch and input into the winch control system from these sensors. Based on at least one of the parameters for winch tension, winch length, winch speed and towing line angle(s), the winch control system may calculate a corrected assisted vessel position. The corrected assisted vessel position may be an input to the propulsion control system of the tugboat. Alternatively, as explained above, the corrected assisted vessel position may be calculated by the propulsion control system based on at least one of the parameters for winch tension, winch length, winch speed and towing line angle(s) provided by the winch control system. The propulsion control system of the tugboat may operate the propulsion system for manoeuvring the tugboat to the corrected assisted vessel position correcting the relative position between the tugboat and the assisted vessel. The propulsion control system may be part of a mission control system for the tugboat.

The propulsion control system 101 of the tugboat may include a dynamic positioning system. External environmental forces such as wind, current and waves, may also be taken into account by the dynamic positioning system of each tugboat. Further control parameters for the dynamic positioning system may also include at least one of wind direction and wind speed, current direction and speed, and/or wave height direction and speed.

A feature of the winch 112 during towing of an assisted vessel is maintaining a constant tension of the towline 103 in order to safely control the assisted vessel. Constant tension of the towline may be provided by the winch control system based on a number of input parameters as explained above. For a tug to keep a relative position to an assisted vessel (e.g. a container ship) the tug should know the position, heading and speed of the assisted vessel in relation to the tug. Keeping the relative position may be especially important in a situation where an unmanned, remote controlled tug loses connection to the remote operation center (ROC) while the tug is connected with a towline to the assisted vessel. Loss of connectivity may occur between the remote operation center (ROC) for the tug and the onboard control system of the tug for a number of reasons. This may be due to the tug entering into a signal shadow area due to terrain formations, other nearby vessels or constructions, or due to waves, wind and rain. Loss of connectivity may also occur due to e.g. equipment failure. When loss of connectivity occurs, the remote operation center loses control of the tug, which may potentially lead to dangerous situations. Loss of connectivity may last for a few seconds and up to permanent loss of connectivity (may then be equipment failure). The position of the assisted vessel may be difficult to get from the onboard sensor suite for remote controlled ships. Remotely uncontrolled, the tug may change its position and speed with respect to the assisted vessel connected to the tug via the towline which may e.g. result in collisions between the tug and the assisted vessel. Safety procedures may therefore be implemented in the onboard control systems of the tugs, if such loss of connectivity should occur. In particular, the tug should know the relative position between the tug and the assisted vessel. The onboard control systems and propulsion control system of the tug are fully functional and may safely control the tug if the onboard control system is provided with information of the relative position between the tug and the assisted vessel. During the safety procedure the tug may follow the movements of the assisted vessel until the assisted vessel comes to a stop.

If connectivity is lost, a first safety step may be that the onboard control system of the tug reduces the tension of the towline 103 to a minimum constant tension and maintains that minimum constant tension. This minimum constant tension is a positive value and is roughly the same as the built in or expected friction of the towline in the system. If it was not set to a positive value, the friction would make the winch feed out the full towing line all the time. The onboard control system of the tug may control the winch directly or through the winch control system. The winch keeps the tension constant through feeding out or reeling in the towline 103. As explained above, the tension of the towline may be measured with a tension sensor 108. Through the use of one or more winch parameters such as e.g. tension, towline length, at least one towline angle A, B with respect to the winch or towing point device 110, or towline speed, the change of position of the assisted vessel with respect to the tug may be determined as earlier explained. The change of position of the assisted vessel with respect to the tug may be determined by the winch control system or by the propulsion control system. If the tension is maintained constant, a longer towline between the tug and the assisted vessel means that the distance between the assisted vessel and the tug has increased. Knowledge of the towline angle A with respect to the longitudinal direction of the tug determines if the increased distance is due to the assisted vessel has turned or slowed down. The longitudinal direction of the tug may in some embodiments be represented by the winch or towing point device 110. Further, the rate of change of the towline length and the towline angle (horizontal angle A and/or vertical angle B) can give additional information about the movement of the assisted vessel. The onboard control system of the tug may use the information about the rate of change of the towline or towline angle(s) to move the tug until the towline length and towline angle(s) return back to the previous relative position. The onboard control system of the tug may also use the information about the rate of change of the towline or towline angle (s) to adjust the relative position between the assisted vessel and the tug enabling the tug to move to a new relative position to e.g. move out of the way of the assisted vessel. The tug may be able to move to a new relative position until the assisted vessel comes to a stop.

The safety system may be implemented in the form of a position module which converts the winch parameters to a relative position between the tug and the assisted vessel. The position module may be integrated in the propulsion control system (e.g. a DP system) of the tug. The position module may be integrated in the winch control system of the tug. The estimated relative position between the tug and the assisted vessel may be output from the position module of the winch control system to the propulsion control system. Other implementations of the safety system e.g. in the onboard control system of the tug may also be envisaged.

The safety system determines a relative position between a tugboat (assisting vessel) and an assisted vessel connectable to the tugboat with a towline. The tugboat has a winch and a winch control system adapted to control the towline between the assisting vessel and the assisted vessel. The winch control system may be adapted for determining a relative position between the assisting vessel and the assisted vessel based on one or more winch parameters of the winch control system.

The system or module may be used as a safety support system for a captain of the assisting vessel for providing information about the assisting vessel relative to the assisted vessel or for assisting in maintaining the relative position between the assisting vessel or the assisted vessel. The system or module may be used as support for a captain of the assisting vessel in an autonomous follow ship mode. The system may also be used for autonomous control of the assisting vessel during towing operations of an assisted vessel.

The system or module may also be used in a safety system for towing in narrow passages. Monitoring of relative distances between the assisted vessel and the assisting vessel is important to maintain safety margins internal in the tow and against other obstacles. The system or module provides information about the relative position between the assisting vessel and the assisted vessel. The system or module may also provide information about the rate of change of the relative position between the assisting vessel and the assisted vessel. The safety system may take actions accordingly to ensure the safety margins are maintained.

Fig. 2a-2f illustrate an example of a towing situation where the safety system has been implemented and safety steps are executed. Figure 2a, 2d illustrates a tug 100 with a winch 102 towing an assisted vessel 104, where the tugboat has lost connection with the remote operation center and the assisted vessel has shifted position and is in a relative position (Xi, Yi) with respect to the tug 100. When the connection is lost, the first safety step is executed. The initiation of the safety step may be provided by the onboard control system of the tug. The towline 103 is then kept at a minimum constant tension T by the winch control system. The minimum constant tension has been described above. The winch control system may go to the minimum constant tension a bit slowly so that the propulsion control system has time to adjust to the new load dynamics. The length of the towline is Li and the angle in the longitudinal direction between the towline and the longitudinal direction of the tug represented by a winch tow point in Fig.2a, is Ai. The length of the towline, the angle of the towline and the tension of the towline may be measured as described above. As is illustrated in Fig. 2a-2f, the towline angle Ai-s and the towline length L1-3 form a right-angled triangle where the coordinate X_m and the coordinate Ym may be calculated based on:

Xm — Lm SinAm

Ym⁼ Lm COSAm

Fig. 2b, 2e illustrates a development of the situation started in Fig. 2a, 2d where the tugboat had lost connection with the remote operation center. In Fig. 2b, 2e the assisted vessel has slowed down and the towline angle and the relative position between the tug and the assisted vessel has changed. The relative position between the assisted vessel and the tug has changed to a relative position (X2, Y2). The tension of the towline 103 is held constant at the minimum tension T by the winch control system 102 by feeding out the towline. The towline length has increased by +10 in Fig.2b,2e and the towline length is L2=LI+10. The angle between the towline 103 and the longitudinal direction of the tug represented by the winch tow point has been reduced by -10° and the new towline angle is A2= A-i-10. The length of the towline, the angle of the towline and the tension of the towline may be measured as described above. Towline length and towline angle are parameters input to the winch control system. The towline length and towline angle may be winch parameters as these are measured by sensors related to the winch. By knowing these parameters, the position module may calculate the relative position (X2, Y2) between the tug and the assisted vessel. Based on the knowledge of the relative position between the tug and the assisted vessel, the onboard control system of the tug may react and move the tug to maintain the relative position between the tug and the assisted vessel also during loss of connectivity, situations with lack of exact position information or total loss of position information from either tug and/or assisted vessel. Alternatively, the onboard control system may also move the tug to a new relative position to e.g. move the tug out of the way of the assisted vessel.

Fig. 2c, 2f illustrate a further development of the situation in Fig. 2b, 2e where the tugboat still has no connection with the remote operation center and the assisted vessel has further turned with respect to the longitudinal direction of the tug. The towline tension is still kept constant at a minimum tension T by the winch control system by reeling in the towline by 5 meters to a line length of l_3= Li+5.The towline angle A has increased by 20° to an angle of As= Ai+20. The length of the towline, the angle of the towline and the tension of the towline may be measured as described above. By calculating the relative position (X3;Ys) of the assisted vessel based on the towline length and the towline angle, it is possible to calculate the rate of change of how much the assisted vessel has turned and how fast the assisted vessel has turned. The rate of change of how much the assisted vessel has turned and how fast the assisted vessel has turned is as illustrated in Fig.2b,2c,2e,2f in relation to a longitudinal direction (dotted line) of the tugboat. The rate of change may also be output from the module for determining the relative position directly or via the winch control system to the onboard control system.

In addition to the horizontal angle A as described above for the situations in Fig.2a-2c, 2e-2f, the vertical angle B may also be measured in the example situations in Fig.2a-2c, 2e-2f, for increased precision in the calculation of the relative position and the rate of change of position of the assisted vessel.

Based on the knowledge of the relative position between the tug and the assisted vessel and optionally the rate of change of position, the onboard control system of the tug may react and move the tug to maintain the relative position between the tug and the assisted vessel also during loss of connectivity. Alternatively, the onboard control system may also move the tug to a new relative position to e.g. move the tug out of the way of the assisted vessel.

The tug may also be provided with a dynamic positioning system. The calculated relative position between the tug and the assisted vessel and optionally the rate of change of the relative position, may be input into the dynamic positioning system of the tug. This information may be used by the dynamic positioning system of the tug to move and position the tug in relation to the assisted vessel in a safe manner. When the connection between the tug and the remote operation center is resumed, the remote operation center again resumes control of the tug. The tow parameters and the relative positions between the tug and the assisted vessel during the loss of connectivity may be transmitted from the tug to the remote operation center as part of the operation log. The dynamic positioning system may also be configured to take control of the tug and move and position the tug in relation to the assisted vessel in a safe manner based on the calculated relative position also for manned tugs. In this way, the system may function as a security system and assistant for the captain also on a manned tug.

Having described preferred embodiments of the invention it will be apparent to those skilled in the art that other embodiments incorporating the concepts may be used. These and other examples of the invention illustrated above are intended by way of example only and the actual scope of the invention is to be determined from the following claims.

Claims

1 . A module for determining a relative position between an assisting vessel and an assisted vessel connectable to the assisting vessel with a towline, the assisting vessel comprising a winch controlling the towline between the assisting vessel and the assisted vessel, wherein the module is adapted for estimating the relative position between the assisting vessel and the assisted vessel based on one or more winch parameters.

2. Module according to claim 1 , wherein the one or more winch parameters include at least one of a length of the towline and an angle of the towline with respect to the winch.

3. Module according to claim 1 or 2, wherein the one or more winch parameters further comprising a tension of the towline.

4. Module according to one of claims 1 -3, wherein the one or more winch parameters further comprising a speed of the towline when reeling in and reeling out the towline by the winch.

5. Module according to one of claims 1 -4, wherein the module is adapted to receive input from at least one sensor for measuring the angle of the towline.

6. Module according to one of claims 1 -5, wherein the module is adapted to receive input from a sensor for measuring the length of the towline.

7. Module according to one of claims 1 -6, wherein the module is integrated into a dynamic positioning system of the assisting vessel or integrated into a winch control system of the assisting vessel.

8. System for determining a relative position between an assisting vessel and an assisted vessel connectable to the assisting vessel with a towline, the assisting vessel comprising a winch and a winch control system adapted to control the towline between the assisting vessel and the assisted vessel, the winch control system being adapted to determining a relative position between the assisting vessel and the assisted vessel based on one or more winch parameters of the winch control system.

9. System according to claim 8, wherein the one or more winch parameters comprising at least one of a length of the towline and an angle of the towline with respect to the winch of the assisting vessel.

10. System according to claim 8 or claim 9, wherein the one or more winch parameters comprising a tension of the towline.

11 . System according to one of claims 8-10, wherein the one or more winch parameters further comprising a speed of the towline when reeling in and reeling out the towline by the winch.

12. System according to one of claims 8-11 , further comprising a first sensor for measuring the angle of the towline.

13. System according to one of claims 8-12, further comprising a second sensor for measuring the length of the towline.

14. A method for determining a relative position between an assisting vessel and an assisted vessel towed by the assisting vessel, the assisting vessel comprising a winch controlling a towline between the assisting vessel and the assisted vessel, the method comprising determining the relative position between the assisting vessel and the assisted vessel based on one or more winch parameters.

15. A method for controlling an assisting vessel operating in autonomous, semi- autonomous or remote controlled mode, wherein the assisting vessel is connected to an assisted vessel by a towline; the method comprising:

- measuring at least one winch parameter;

- inputting the at least one winch parameter to a winch control system of the assisting vessel; - calculating by the winch control system a corrected assisted vessel position based on the at least one winch parameter; and

16. Method for controlling an assisting vessel operating in autonomous, semi- autonomous or remote controlled mode, wherein the assisting vessel is connected to an assisted vessel by a towline; the method comprising:

- keeping the towline at a constant tension;

- inputting the measured change of position to a propulsion control system of the assisting vessel for controlling the position of the assisting vessel relative to the assisted vessel.

17. Method according to claim 14, 15 or 16, wherein the one or more winch parameters comprising at least one of a length of the towline and an angle of the towline with respect to the winch of the assisting vessel.

18. Method according to one of claims 14-17, wherein the one or more winch parameters comprising a tension of the towline.

19. Method according to one of claims 14-18, wherein the one or more winch parameters comprising a speed of the towline when reeling in and reeling out the towline by the winch.

20. Method according to one of claims 14-19, further comprising controlling a position of the assisting vessel relative to the assisted vessel by reeling in or feeding out the towline while keeping the tension of the towline.

21 . Method according to one of claims 14-20, further comprising measuring a rate of change of the position of the assisted vessel with respect to the assisting vessel.

22. Method according to one of claims 14-21 , further comprising inputting the measured rate of change of position of the assisted vessel to the propulsion control system of the assisting vessel, and either maintaining the relative position of the assisting vessel and the assisted vessel, or moving the assisting vessel to a new relative position with respect to the assisted vessel.

23. Method according to one of claims 14-22, the method further comprising keeping the tension of the towline at a minimum constant tension.

24. Use of the system or module according to one of claims 1-13 in a safety procedure during loss of connectivity between a remotely controlled assisting vessel and a remote control facility.

25. Use of the system or module according to one of claims 1 -13 as a safety support system for a captain of the assisting vessel for providing information about the assisting vessel relative to the assisted vessel or for assisting in maintaining the relative position between the assisting vessel or the assisted vessel.

26. Use of the system or module according to one of claims 1 -13 as support for a captain of the assisting vessel in an autonomous follow ship mode.

27. Use of the system or module according to one of claims 1 -13 for autonomous control of the assisting vessel during towing operations of an assisted vessel.

27. Use of the system or module according to one of claims 1 -13 for towing of a wind turbine, e.g. a floating wind turbine.