CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to PCT/SE2018/078846, filed Oct. 22, 2018, entitled “NAVIGATION SYSTEM WITH INDEPENDENT CONTROL OF LATERAL AND LONGITUDINAL THRUST,” which claims priority to Swedish Patent Application No. 1751310-2, filed Oct. 23, 2017, all of which are incorporated by reference in their entirety.

BACKGROUND

1. Technical Field

The present invention relates generally to marine navigation, and more particularly to ship propulsion systems capable of complex maneuvers.

2. Description of Related Art

A ship propulsion system typically has a propeller, screw, or similar vaned apparatus, driven by an engine or motor to generate forward or reverse thrust to propel the ship forward or backward. Steering and navigation require the generation of a lateral thrust, typically at the stern of the ship. A rudder or nozzle may redirect longitudinally flowing water toward a lateral direction, changing the ship's course.

Some propulsion systems have separate components to impart longitudinal and lateral forces. A typical large ship has a fixed-axle propeller connected directly to the engine (for forward/backward thrust) and a rudder aft of the propeller to generate lateral thrust. Some propulsion systems combine lateral with the longitudinal thrust. For example, an azimuthing thruster (e.g., a pod-based propeller) may be rotated toward the port or starboard sides, directing a portion of its thrust laterally. A water jet may include a nozzle that directs a portion of the jet of water laterally. Azimuthing thrusters and water jets provide high maneuverability, but this maneuverability comes at the expense of lower fuel efficiency, especially during straight-ahead steaming (e.g., over the open ocean).

The propulsion system's capabilities are normally specified by the ship's performance requirements, which are typically associated with an expected usage of the ship. Some ships prioritize higher maneuverability; a ferry or cruise ship that operates close to shore might need to navigate narrow fjords or tight harbors. Some ships prioritize higher fuel efficiency; a tanker or container ship that travels from Asia to Europe might need to minimize fuel costs. Reduced pollution is often desired (e.g., from a ferry in a densely populated area). Some ships are required to have higher speed.

The expected type and usage of the ship have historically determined the choice of propulsion system, but the drawbacks of prior propulsion systems have forced compromise. A cruise ship with azimuthing thrusters might navigate tortuous passages well, but consume more fuel than a similarly specified ship having a fixed-axle/rudder propulsion system. Conversely, a tanker having fixed-axle/rudder propulsion might have good fuel economy during open-ocean steaming, but poor maneuverability during berthing. For large ships with fixed-axle propulsion, tugs are typically required for harboring, berthing, and the like, creating additional fuel consumption and emissions resulting from tug usage.

As voyage distance and ship size increase, the propulsion system is typically chosen to maximize fuel efficiency, guiding the artisan toward fixed-axle propeller/rudder systems. When fuel costs trump navigational requirements, the fuel savings of a fixed-axle/rudder propulsion system typically outweigh the maneuverability advantages of alternative propulsion systems having directable thrust (e.g., azimuth, water jet). Thus, many ferries, most cruise ships and typically all large cargo ships (container ships, tankers and the like) use a fixed-axle propeller/rudder system. However, such large ships require tugboat-assisted navigation near shore. Large ships could benefit from the navigational flexibility typically associated with azimuthing or water-jet systems, if not for the expense of increased fuel consumption. By increasing the navigational capabilities of such large ships “close to shore,” large-scale shipping may be made more efficient.

Conversely, ships requiring higher maneuverability (that typically requires pod or jet propulsion) could benefit from better fuel efficiency, provided the maneuverability requirements were met. A ferry or cruise ship that normally requires an azimuthing thruster system would benefit from reduced fuel consumption and lower emissions, provided the propulsion system meets the navigational requirements.

For propulsion systems for “typical” ships designed to transport goods or people between points (e.g., having a bow, stern, self-propelled speed over 10 knots, and the like, e.g., cargo ships, tankers, cruise ships, and ferries) it is challenging to impart an exclusively lateral thrust (e.g., to move sideways without moving forward or backwards). A rudder may redirect water laterally, but the redirected water still has a longitudinal component: the ship turns, but continues to move forward or backward. This limitation may be a problem in certain navigational situations (e.g., a lateral docking maneuver into a tight berth, akin to “parallel parking” in the automotive world). In such situations, tugboats are typically required to provide tight navigation control, turning radii, and lateral movement. Tugboats are costly and emitting; reducing the need for tugboats may significantly improve operational efficiency and reduce pollution.

It may be advantageous to impart an exclusively lateral thrust on the ship (e.g., to move the ship to port or starboard without moving forward or backward). Lateral movement without forward movement may enable significantly improved accident-avoidance protocols. By moving sideways without moving forward, a ship might avoid another oncoming vessel. Certain vessels that are designed for dynamically positioned stationary operation use azimuthing propulsion. For example, floating oil platforms may be actively controlled to remain stationary, and so use azimuthing thrusters. However, such vessels have poor fuel economy during steaming (and in many cases, cannot self-propel for significant distance).

Complicated navigational maneuvers typically require an expert captain/skipper, typically located on the ship. Berthing may require that a harbor pilot be helicoptered or ferried out to a vessel so the harbor pilot may navigate the ship into the harbor. Automation of these maneuvers may reduce, or even eliminate, the need for local navigational expertise.

Typically, a significant portion of capital and operating expense is directed toward supporting and ensuring the safety of human passengers. Automation may reduce (or even eliminate) the need for a local (e.g., shipboard) human presence, resulting in reduced cost and increased safety. Cost of implementation is often a barrier. As such, technology that provides a more cost-effective solution to a technical problem may enable the commercialization of technology that might otherwise be economically undesired by the market.

SUMMARY

Various embodiments provide a propulsion system that combines the fuel efficiency of a fixed-axle/rudder propulsion system with the maneuverability of an azimuthing (e.g., pod or water-jet) propulsion system. By increasing both fuel efficiency and maneuverability, ships may be more fuel-efficient, less polluting, less requiring of tug-assisted navigation, and more easily navigated near fjords, harbors, berths, and the like. A ship having at least two (e.g., three) propellers, at least two rudders and/or nozzles, and a particular computing platform as described herein may have the fuel-efficiency advantages of a fixed-axle propulsion system in combination with the navigational capabilities of an azimuthing or jet-based propulsion system. Various aspects may improve performance of ships that steam for some time, then dynamically remain in place, such as FPSO, FSO, FDPSO, and/or FSRU.

A propulsion system configured to independently control longitudinal and lateral thrusts to navigate a ship may comprise first and second thrusters (e.g., fixed-axle propellers), each having an independently controllable thrust, and first and second directors (e.g., rudders and/or nozzles), each configured to independently redirect a portion of the water flowing past a ship toward a lateral direction. Thrusters and directors may be arranged laterally and/or longitudinally with respect to each other. In certain cases, a thruster and director may be integrated, as in a pod-based propeller (in which the propeller spin axis rotates about a steering axis) or a water jet (in which a nozzle directs the flow of water exiting the propeller).

A first thruster is configured to generate a first thrust to propel the ship forward or backward, and a first director is configured to direct flowing water (e.g., the first thrust) over a range of first director angles. A second thruster is configured to generate a second thrust, independent of the first thrust (the same as or different, as desired), to propel the ship forward or backward, and a second director is configured to direct flowing water (e.g., the second thrust) over a range of second director angles, independent of the first director angles (the same or different, as desired).

A platform comprising at least a processor and memory (and typically a communications interface) and in communication with the thrusters and directors is configured to receive a target location to which the ship should be propelled, receive and/or calculate a desired longitudinal heading defining a desired forward/backward displacement of the ship to reach the target location, and receive and/or calculate a desired lateral heading defining a desired lateral displacement of the ship to reach the target location. The headings typically comprise a direction and magnitude (which may be zero). The platform calculates a longitudinal thrust vector expected to yield the desired longitudinal heading when imparted to the ship, and a lateral thrust vector expected to yield the desired lateral heading when imparted to the ship. The platform calculates, identifies, or otherwise selects an operating configuration of the thrusters and directors (e.g., magnitude of each thrust and direction of each director) that meets certain criteria. When propelled pursuant to the selected operating configuration, the ship may then move in the desired longitudinal and lateral headings.

A criterion may include an operating condition of the propulsion system and ship for which a summation of (a first long.vector comprising a net longitudinal thrust generated by the first thruster and first director) plus (a second long.vector comprising a net longitudinal thrust generated by the second thruster and second director) results in the calculated longitudinal thrust vector that is expected to yield the desired longitudinal heading. A criterion may include an operating condition for which a summation of (a first lat.vector comprising a net lateral thrust generated by the first thruster and first director) plus (a second lat.vector comprising a net lateral thrust generated by the second thruster and second director) results in the calculated lateral thrust vector. By simultaneously fulfilling these criteria, the selected operating condition yields an arrangement of thrusters and directors in a way that imparts the desired longitudinal and lateral thrusts to the ship, yielding the desired longitudinal and lateral headings.

The selected operating condition may include information selected from a database (e.g., via a lookup table). The database is typically vessel-specific, and typically contains, for a wide range of environmental conditions, thrusts, and director angles, a concomitantly wide range of lat.vectors and long.vectors. The selected operating condition may be calculated dynamically (e.g., iteratively as needed). Various aspects comprise optimization subroutines (e.g., numerical minimization between a difference between a desired heading and actual heading). A “closed loop” algorithm that iteratively reduces a deviation between desired and actual headings (e.g., PID, proportional/integral/derivative and/or an algorithm comprising a numerical method for minimization (Newton's, Least-Squares, Steepest-Descent, Monte-Carlo, and the like) may be used. A benchmark operating condition (e.g., for no wind, no waves, full load, temperature, and the like) may be combined with various sensors (e.g., wind, load, power) to calculate an operating condition. Selecting an operating condition may comprise extracting information from a lookup table and/or calculating information (e.g., using sensor data). Pursuant to a desired net thrust (e.g., in combination with environmental effects such as load, wind and current), the platform selects an operating condition that yields the desired thrust vectors. This operating condition is typically used to navigate the ship, and the resulting motion (e.g., the deviation between actual and desired lateral and longitudinal headings) is used to recalculate/reselect an updated operating condition. Closed-loop control of the propulsion system (desired vs. actual effect) may be used to iteratively update the present operating condition in order to achieve a desired heading. Typically, the lookup table is updated as the effect of different operating conditions on ship headings is manifest. By using canonical “reduced” vectors as proxies for the complex flow fields typically associated with CFC models, iterative updating of tables may reduce, or even eliminate, the need for repeatedly recalculating complicated CFD models.

Independent control may include configuring the first thruster to generate a forward thrust while the second thruster generates a reverse thrust. Independent control may include configuring the first director to “point” to starboard, while the second director “points” to port. A configuration may result in a substantially zero longitudinal thrust yet a nonzero lateral thrust to port or starboard. Thrust from the first thruster may be different than thrust from the second thruster. The director angle of the first director may be different than that of the second director.

A ship (e.g., with a bow, a stern, an appreciable self-propelled maximum speed (e.g., over 5 knots, including over 10 knots), and a draft greater than 3, or even above 5 meters), may comprise a propulsion system having thrusters configured to generate thrust and directors configured to direct the thrust in lateral directions. A ship need not have a draft greater than 3 meters, and need not have a self-propelled maximum speed that exceeds 10 knots. A computing platform in communication with the thrusters and directors may coordinate the thrusters and directors to achieve a desired combination of longitudinal and lateral thrust that yields a desired longitudinal heading and a desired lateral heading independent of the desired longitudinal heading.

The ship may have a starboard thruster comprising a fixed-axle propeller configured to generate a first thrust to propel the ship forward or backward and a starboard director comprising a rudder and/or nozzle and configured to direct the first thrust over a range of starboard director angles. The ship may have a port thruster comprising a fixed-axle propeller configured to generate a second thrust, independent of the first thrust, to propel the ship forward or backward, and a port director comprising a rudder and/or nozzle and configured to direct the second thrust over a range of port director angles, independent of the starboard director angles. The ship may have a center thruster comprising a propeller (e.g., a fixed-axle propeller) and configured to generate a third thrust to propel the ship forward or backward, independent of the first and second thrusts.

The ship includes a platform comprising a processor, memory, and communications interface (e.g., to communicate with a command console that issues navigation instruction data) configured to receive a target location toward which the ship should be propelled. Using the target location data, the platform calculates a desired longitudinal heading defining a desired forward/backward displacement of the ship to reach the target location and a desired lateral heading defining a desired lateral displacement of the ship to reach the target location. Using these headings, the platform calculates a longitudinal thrust vector expected to yield the desired longitudinal heading when imparted to the ship and a lateral thrust vector expected to yield the desired lateral heading when imparted to the ship.

According to the desired thrust vectors, the platform selects (e.g., calculates, extracts information from a lookup table and/or other database) an operating configuration of the thrusters and directors (e.g., magnitude of thrust from each propeller and director angle of each director) that meets certain criteria. The thrusters and directors are typically operated independently. The starboard director angle may be the same or different than the port director angle, and the magnitude of the thrust generated by the starboard thruster may be the same or different than the magnitude of the thrust generated by the port thruster.

The selected criteria may include a summation of (a first long.vector comprising a net longitudinal thrust generated by the starboard thruster and starboard director) and (a second long.vector comprising a net longitudinal thrust generated by the port thruster and port director) and (a third long.vector comprising a longitudinal thrust generated by the center thruster) results in the calculated longitudinal thrust vector (e.g., within a thrust tolerance). The criteria may also include a summation of (a first lat.vector comprising a net lateral thrust generated by the starboard thruster and starboard director), and (a second lat.vector comprising a net lateral thrust generated by the port thruster and port director) results in the calculated lateral thrust vector (e.g., within a thrust tolerance). The platform may then send the selected operating configuration to the command console and/or directly to the propulsion system to propel the ship in the desired longitudinal and lateral headings.

Maneuverability may be enhanced by imparting relatively large lateral thrusts as compared to longitudinal thrusts. A lateral thrust vector may have a magnitude that is larger than the longitudinal thrust vector, particularly at least twice as large, particularly at least ten times as large. The longitudinal thrust vector may correspond to substantially zero forward/backward thrust, yet the lateral thrust vector corresponds to a nonzero lateral thrust. Such a combination of vectors may be used to move the ship sideways without moving forward or backward. In some cases, a ship may be rotated around its prow to change direction within a very tight radius. In some cases (e.g., with a bow, swingdown, and/or tunnel thruster) a ship may be propelled to rotate around its midpoint (e.g., “spin”).

A magnitude of the longitudinal component of the thrust generated by the center thruster may be within 20%, particularly within 10%, particularly within 5%, of the magnitude of the sum of the first long.vector (650) and second long.vector comprising the net longitudinal thrusts generated by the starboard and port thrusters. The magnitude of the longitudinal component of the thrust generated by the center thruster may be substantially equal to the magnitude of the sum of the first and second long.vectors, particularly wherein the thrust of the center thruster is in the opposite direction to the thrusts of the starboard and port thrusters. “Substantially equal to” typically allows for a small deviation from a desired value that does not affect performance. In certain navigation operations, the selected operating configuration is such that the starboard and port thrusters are configured to thrust in the opposite direction of the center thruster. For example, the starboard and port thrusters may thrust forward while the center thruster thrusts backward, or vice versa. The thrusts may have a magnitude such that that their longitudinal and/or lateral components cancel each other.

The platform may be configured to iteratively (substantially continuously) receive location information, update the headings, update the desired thrust vectors, and correspondingly update the operating configurations. The platform may receive an updated position of the ship from at least one ship sensor (e.g., a GPS sensor), calculate an updated longitudinal heading defining the desired forward/backward displacement of the ship, calculate an updated lateral heading defining the desired lateral displacement of the ship.

Using these updated headings, the platform then calculates updated longitudinal and lateral thrust vectors expected to yield the updated longitudinal and lateral headings and selects an updated operating configuration such that a summation of the first long.vector, and second long.vector and optionally third long.vector results in the updated longitudinal thrust vector, and a summation of the first lat.vector and the second lat.vector results in the updated lateral thrust vector. The platform may then send the updated operating configuration to the thrusters and directors (e.g., via a command console) to propel the ship in the desired updated headings. A “closed loop” iteration, in which ship position data are used to dynamically update headings and subsequent thrust vectors, may allow the ship to “home in” on a desired location. By combining substantially autonomous navigation with high fuel efficiency, environmental impact may be reduced while safety is increased. Without the need for a human crew (e.g., requiring air, rescue facilities, and emergency aid), a ship may safely navigate challenging environments.

