The present invention relates to a floating marine vessel such as a drillship or other form of ship-shaped vessel for operation in at least partly ice-covered water and, in particular, for stationary operations where the vessel is kept on location e.g. moored to the seafloor by means of a mooring system or by an active thrusters system (Dynamic Positioning system) in areas with drifting sea ice.

Background:

During operation of a marine vessel at a stationary location relative to the seafloor in areas of drifting sea ice it is a challenging task to protect the marine vessel against the drifting sea ice.

Bottom-founded platforms have been successfully developed for shallower water. In deeper water such as in water depth of 75 m or greater, however, bottom-founded platforms become impractical, and floating platforms are frequently employed. A ship-shape vessel is attractive as a floating platform in cold environment and areas with drifting sea ice because it has a large deck area, it has a large under-deck volume, and ice loads on it from drifting sea ice are relatively low when the vessel is aligned with the ice drift direction. Such floating platforms may keep station with the help of a mooring system comprising several mooring lines connected to the vessel, e.g. by means of a turret at the bottom of the hull of the vessel. When the mooring lines are attached to a turret, the vessel may shift heading or rotate in the ice, so as to allow the vessel to be aligned with changing ice drift directions (ice-vane).

Drill ships and other vessels may comprise relative sensitive equipment ex- tending from the bottom of the vessel's hull towards the seafloor, e.g. through an opening defined in the hull. Such an opening is often referred to as a moonpool, e.g. in the context of d llships. For example, during drilling operations a drilling riser may extend from the vessel to the seafloor.

As drifting sea ice impacts the floating vessel, loads are imparted on the hull of the vessel and possibly on the mooring lines. Such loads can become very high when the ice conditions are severe, which may lead to the vessel floating off location or even breakage of the mooring lines and/or damage the hull. The hull shape of conventional icebreakers is typically designed so as to allow the icebreaker to move forward at a speed that exceeds the normal drift speeds of the sea ice. In situations with thick ice the icebreakers may even drive their bows onto the ice and will break it under the weight of the ship. In contrast to this situation, the ice impacting a stationary vessel typically drifts at rather low speed. Additionally, it is generally desirable to prevent ice from submerging under the bottom of the vessel and, in particular, to prevent ice from damaging a riser or other equipment extending from the bottom of the vessel towards the seabed/seafloor, e.g. through a moonpool of the vessel. Generally, hulls for marine vessels for operation in at least partly ice-covered water are typically designed to a set of specified design parameters. These parameters typically include a predetermined design ice thickness (DIT). These design parameters also include an upper ice water line (UIWL) indicative of a maximum draught the vessel is configured to operate at in at least partly ice-covered water. Similarly, these design parameters include a lower ice water line (LIWL) indicative of a minimum draught the vessel is configured to operate at in at least partly ice-covered water.

One example of a hull shape for a marine vessel has been shown in the newsletter "Arctic Passion News 1/201 1 " published by Aker Arctic Technology, Inc of Helsinki, Finland in March 201 1 . The authors of this prior art publi- cation state that the optimal stem solution is a wedge-bow design equipped with an edge and with a vertical water flushing system that moves the ice to the sides. See also EP 2406125. However, it remains desirable to further reduce the risk of ice submerging under the bottom of the vessel and to efficiently prevent ice from damaging equipment extending downwards from the bottom of the hull. It is further desirable to reduce the loads imparted on the hull of the vessel and/or on the mooring system by the drifting sea ice.

Summary:

Disclosed herein are embodiments of a marine vessel for operation in at least partly ice-covered water, the vessel comprising a hull having at least one bow operable to be exposed to the loading effect of ice due to a relative motion between the ice and the vessel; wherein the bow comprises: an upper portion which, during operation of the vessel at operational draughts, is located above the water level; a bottom portion which, during operation at operational draughts, is located below the water level; and an intermediate, ice-breaking portion which, during operation at operational draughts, is located at the wa- ter level.

The operational draughts may be defined for operation in at least partly ice covered water by an upper ice water line (UIWL) and a lower ice water line (LIWL). Usually the UIWL is the same as the maximum draught and the LIWL is the same as the Minimum/Ballast draught. The UIWL and LIWL are normally defined in the classification drawings of a vessel and approved by a classification society. Hence, in some embodiments, the upper portion of the bow is located above the upper ice water line, the bottom portion is located below the lower ice water line, and the intermediate portion is located be- tween the upper and lower ice water lines. The bow comprises two side portions which meet at the foremost end of the bow. The foremost end of the bow is also referred to as the stem; during normal operation, the stem is oriented generally towards the drifting sea ice, such as in a direction within 25° or less relative to the direction of the ice drift, such as within 10° or less, such as within 5° or less. While the invention is described in relation to drifting sea ice, it will be clear to the skilled person that the invention may generally exhibit the same function and advantages in relation to ice originating from land (e.g. ice from ice bergs, growlers or smaller ice chunks) as well as ice that is not drifting but generally exerting a force against the hull and any other ice in the water. It will be appreciated that the side portions may meet at the foremost end so as to abut each other while, in other embodiments, the side portions may meet at the foremost end without directly touching each other but rather being separated by a flat bar or similar structural element which may form the stem or a part of the stem. Hence, the stem may be formed by the touching side portions or by a separate structural element. As was already described for the bow, the stem also comprises an upper portion which, during operation at operational draughts, is located above the water level; a bottom portion which, during operation at operational draughts, is located below the water level; and an intermediate, ice-breaking portion which, during operation at operational draughts, is located at the water level. The intermediate portion may be separated from the upper and/or lower portions by respective transitional portions. For example, an upper transitional portion may separate the upper portion from the intermediate portion. Similarly, a lower transitional portion may separate the in- termed iate portion from the bottom portion.

Embodiments of the intermediate, ice-breaking portion of the stem have an ice-breaking shape and sufficient strength so as to withstand the ice induced force on the hull and in some embodiments sufficient forces so as to break the ice. In some embodiments, the intermediate portion of the stem is raked at an angle equal or smaller than 45° relative to a waterline and wherein the side portions of the bow meet at the intermediate portion of the stem so as to define a V-shape in a waterline. Each side portion may meet the stem at an acute angle (an example of which is illustrated as the angle a in Fig. 7) relative to a longitudinal centerline of the bow, e.g. an angle less than 60°, such as less than 50°, such as less than 40°, such as less than 30°, such as between 25° and 60°, e.g. between 25° and 55°, e.g. between 30° and 50°. In some embodiments the very foremost of the V-shape is ignored in relation to measurement of these angles such as the foremost 4 meters or less, such as the foremost 3 meters or less, such as the foremost 2 meters or less, such as the foremost 1 meter or less, such as the foremost 0.5 meter or less. The foremost part of the V-shaped bow may be defined sufficiently sharp to introduce, during operation, a line of fracture in ice that is not immediately deflected, e.g. large ice floes. At low relative speed between the drifting ice and the vessel, this has turned out to be an efficient way of reducing the ice forc- es. By comparison, conventional icebreakers are often designed with a sloping or rounded stem. It will be appreciated that the terms sharp and round are to be understood in relation to the ability of the stem to introduce a line of fracture in the ice and in relation to the relative length scales, such as typical dimensions of the ice, e.g. as expressed by the design ice thickness. In some embodiments, the foremost part of the V-shaped bow - e.g. a foremost portion of a horizontal cross section of the bow, the portion having a longitudinal extent of no more than the design ice thickness - may define at least one radius of curvature in a horizontal plane of no more than 50% of the design ice thickness, such as no more than 40% of the design ice thickness, such as no more than 30% of the design ice thickness, such as no more than 20% of the design ice thickness, such as no more than 10% of the design ice thickness. In some embodiments, the V-shaped bow may be built with a flat bar at the stem and the flat bar may optionally be sharpened. In some embodiments, the lateral width of a horizontal cross section of the bow at a distance of 100% of the design ice thickness from the stem is less than three times the design ice thickness, such as less than 250% of the design ice thickness, such as between 90% and 200% of the design ice thickness. Similarly, in some embodiments, the lateral width of a horizontal cross section of the bow at a distance of 50% of the design ice thickness from the stem is less than 150% of the design ice thickness, such as less than 125% of the design ice thickness, such as between 45% and 100% of the design ice thickness.

At least some embodiments of a vessel having a V-shaped stem that is raked at an angle of 45° or less allow the drifting sea ice to be efficiently divided or even split by the V-shaped stem and the ice is driven downwards and side- ways, thus reducing the load on the stem while effectively guiding the ice along the hull of the ship towards the aft. The raked stem and its adjacent bow region thus provide an effective ice-breaking bow.

In some embodiments, the intermediate portion of the stem is raked at an angle of 45° or less relative to the waterline to form, together with its adjacent lateral bow regions, an ice-breaking bow; and the bottom portion of the stem comprises a forwardly protruding ledge portion, in the following also referred to as forward ledge portion; wherein the intermediate portion is operable to divide the ice and to push the ice downward and sideways and wherein the forward ledge portion is configured to prevent downward moving ice from moving below the bottom of the hull. Hence, the forward ledge portion is operable as an ice protecting barrier. In some embodiments the intermediate portion of the stem is V-shaped as described above. In some embodiments, an upper surface of the forward ledge portion is sloped in an upward and forward direction whereby a more efficient barrier in view of ice transport aftwards is provided. In alternative embodiments an upper surface of the forward ledge portion is sloped in a downward and forward direction.

In some embodiments the upper surface of the forward ledge portion is gen- erally flat so as to efficiently stop or prevent at downward movement pass or around the forward ledge portion of ice that submerges along the stem. The flat top surface may be delimited towards the front and/or the sides by an edge formed between the flat top surface and upright side walls of the forward ledge portion. Generally, the size of the forward ledge portion may be chosen according to the design ice thickness, e.g. large enough to prevent ice floes from sliding below the hull and small enough so as to avoid build-up of ice above the ledge portion. In some embodiments, the forward ledge portion extends outwardly forward by between 25% and 150% of the design ice thickness, e.g. between 50% and 100% of the design ice thickness, or between 50% and 150%, or between 75% and 150%. In some embodiment the forward ledge portion extends outwardly forward 250% of the design ice thickness or less, such 200% or less, such 150% or less, such 100% or less, such 50% or less. In some embodiment the forward ledge portion extends outwardly forward between 0.4 m and 4 m, such as between 0.5 m and 3 m, such as between 0.5 m and 2 m, such as between 1 m and 1 .5 m.

In some embodiments, the forward ledge portion is positioned, shaped and sized such that it does not significantly interfere with the bow wave generated at operational speeds at e.g. 5 knots or higher through water. In some embodiments, the forward ledge portion is positioned, shaped and sized such that it does not significantly alter the water resistance of the hull at operational speeds at e.g. 5 knots or higher, while providing an effective barrier for submerging ice at low speeds, in particular at typical drift speeds of sea ice, e.g. at speeds below 5 knots, such as below 3 knots, such as below 1 knot through water.

It is typically desirable to provide a large ice-breaking portion of the stem. Accordingly, in some embodiments, the forward ledge portion forms the bottommost part of the stem or is at least positioned close to the bottommost part of the stem, e.g. less than 4m, such as less than 3 m, such as less than 2 m such as less than 1 m from the bottommost part of the stem where the distance is measured between the top surface of the forward ledge portion and the bottom of the hull, such as a flat bottom of the hull.

Accordingly, in some embodiments, the upper surface of the forward ledge is located well below the lower ice water line, e.g. at least 30% of the design ice thickness, such as at least 60% if the design ice thickness, such as at least 100% of the design ice thickness, such as at least 150% of the design ice thickness below the lower ice water line, or more than 150%. Hence, the distance between the lower ice water line and the upper surface of the forward ledge portion may be at least 1 m, such as at least 2 m, such as at least 3 m. Consequently, sufficient space is provided around the water line for the ice breaking portion of the stem and bow to operate while the forward ledge portion provides a lower barrier preventing ice from submerging under the bottom of the hull. The location of the forward ledge portion well below the lower ice water line further reduces the effect of the forward ledge portion on the bow wave of the vessel at higher operational speeds.

In some embodiments, the stem comprises a transitional portion between the intermediate, raked portion and the bottom portion of the stem. In some em- bodiments, the transitional portion of the stem curves downward between the ice-breaking, raked part of the intermediate portion of the stem and the for- wardly protruding ledge portion. Alternatively, the raked portion may extend downwards all the way to the forwardly protruding ledge portion with only a small or even no transitional portion. In any event, the angle between the raked portion or the downward transitional portion and an upper surface of the forward ledge portion may be an acute angle. The transitional portion may, during operation, be located below the water. In particular, the transitional portion may be located below the lower ice water line. A plumb transitional region between the ice-breaking, raked intermediate portion of the stem and the forward ledge portion provides space for a transversal propulsion device such as a bow thruster to be position close to the bow. Hence, in some embodiments, the hull comprises one or more lateral openings adjacent the transitional region for accommodating one or more transversal propulsion devices. The stem angle (an example of which is referred to as the angle φ in Fig. 3) defined between the intermediate portion of the stem and the waterline may be smaller than 35°, such as smaller than 30°. In some embodiments, the angle is larger than 10°, such as larger than 15°, such as larger than 20°. For example, the stem angle may be between 10° and 45°, such as between 15° and 30°, for example 20°, 25°, 30°, or 35°. In some embodiments the intermediate portion forms an angle with a waterline of between 10° degrees and 45°, such as between 15° and 30°, e.g. between 20° and 35°. It will be appreciated that the intermediate portion of the stem may be substantially straight, i.e. the angle between the stem and a waterline may be substantially constant between the upper and lower water lines. Alternatively, the angle may vary along the intermediate portion while remaining between 10° degrees and 45°, such as between 15° and 30°, e.g. between 20° and 35°, relative to a waterline along a major portion of the region, or even the entire region, between the upper and lower ice water lines.

Generally, the raked intermediate portion of the stem, which may be V- shaped so as to provide a sharp ice-dividing edge, constitutes the ice breaking portion of the bow. It is configured to push ice floes downwards and sideways/ backwards and to reduce pressure on the bow and to reduce station keeping forces.

The intermediate, ice-breaking portion of the stem may be long enough to encompass the operational draughts, optionally with the addition of at least 1 .5 times the design ice thickness. Hence, in some embodiments, the inter- mediate portion extends at least all the way from the lower ice water line to the upper ice water line. In some embodiments the intermediate portion may extend upwards beyond the upper ice water line and/or downwards below the lower ice water line. For example, the intermediate, ice-breaking portion of the stem may extend up to twice the design ice thickness above the upper ice water line, such as between 10% and 50% of the design ice thickness. Similarly, the intermediate, ice-breaking portion of the stem may extend up to the design ice thickness below the lower ice water line, such as between 30% and 60% of the design ice thickness. Consequently, during operation of the vessel at normal operational draughts the operational waterline is located between the upper and lower ice water lines and the intermediate portion extends both above and below the operational waterline. Hence, the raked, ice-breaking portion is configured to interact with the ice-water interface during operation at normal operational draughts, optionally including some or all of the part of the drifting ice which extends out from the water and/or the submerged part of the drifting ice.

In some embodiments the upper portion of the stem is blunt and and/or plumb and the stem may curve backwards between the upper portion and the ice-breaking intermediate portion. In some embodiments the upper portion may be raked or have another suitable shape.

Generally, the vessel may be oblong having two ends - a bow and a stern. In some embodiments, the vessel comprises a midship portion between the ends, also referred to herein as mid-body. The mid-body of the hull may comprise at least 10%, such as at least 20%, such as at least 40% of the length of the hull, e.g. at least 50% such as at least 60%. Nevertheless, it will be appreciated that, in some embodiments the bow and stem may be combined without an intermediate mid-body. The length of the hull may be at least 3 times its width, e.g. at least 4 times its width, e.g. at least 5 times its width, e.g. at least 6 times its width, e.g. at least 7 times its width. The hull sides of the mid-body may be parallel, inclined, stepped or have a different shape. In some embodiments, the hull sides of the mid-body of the hull may be upright, substantially parallel. The stern may have any suitable shape and the vessel may comprise any suitable propulsion system e.g. including propellers, thrusters or both. In some embodiments, the vessel may comprise two bow-like ends adapted to face the direction of the ice drift and comprising an icebreaking portion as described herein. In particular, the stern of the vessel may have a bow-like shape and be adapted to function as a bow. For the purpose of the present description, the term bow thus refers to any bow-like end of a vessel adapted to face the direction of the ice drift and comprising an icebreaking portion as described herein.

In some embodiments the hull comprises a mid-body where the hull sides are substantially upright, e.g. they may form an angle with a horizontal plane of more than 75°, such as more than 80°, such as more than 85°. The plane hull sides of the mid-body may extend below the upper ice water line or even below the lower ice water line, such as all the way to the bilge keel.

In some embodiments, the sides of the bow may have substantially plane upward-outwardly sloped portions which meet at the stem to form the ice- breaking intermediate portion of the stem. Accordingly the frames positioned proximal to the stem may comprise substantially straight intermediate sections at the level of the intermediate portion of the stem. The sloping angle and/or length of the straight sections relative to a horizontal plane may gradually change from the stem towards the aft, e.g. to a substantially upright, flat hull side at the mid-body of the hull, so as to form an ice-guiding face configured to guide or plough ice floes upwardly and sideways and/or to cause a bottom edge of partly submerged ice floes to tilt laterally outwards away from the hull. The frames of the bow region of the hull, proximal to the stem, may comprise bottom portion, below the lower ice water line, an upper portion, above the upper ice water line, and an intermediate portion, between the lower and upper ice water lines. At least a part of the intermediate portion of the frames may be substantially straight and slope upwardly outwards. In the area proximal to the stem, the sloping angle relative to the horizontal may e.g. be be- tween 15° and 45°, such as between 20° and 40°. In some embodiments this sloping angle may be any of the angles or intervals cited for the intermediate section of the stem. The sloping angle may increase from the stem towards the mid-body of the hull. The upper portion of the frames of the bow region may have a substantially straight portion which slopes upwardly outward at a larger angle, relative to a horizontal plane, than the intermediate portion of the corresponding same frame. The sloping angle relative to the horizontal may e.g. be larger than 50°, such as larger than 60°, e.g. larger than 70°. The transition between the intermediate, sloped portion and the upper, sloped portion may be formed as a curved transitional portion of the frame. The curved transitional portion may be located approximately at the upper ice water line. In some embodiments the location of the curved, transitional portion may vary between frames typically from a position at or above the upper ice water line, for a first frame, to a position at or below the upper ice water line (or even at or below the lower ice water line) for a second frame positioned relatively closer to the stern (typically proximal to the mid-body) than the first frame. For example, the frames most proximal to the stem may have a transitional, curved portion located above or at the upper ice water line, while the frames in the region proximal to the mid-body may have a transitional, curved portion located at or below the upper ice water line. Moreover, the curved transitional region of the frames proximal to the stem may be closer to a longitudinal centre plane of the hull than the corresponding curved, transitional region of the frames proximal to the mid-body of the hull. In some embodiments, the hull comprises a lateral ledge portion extending from the forward ledge portion along the sides of the hull towards the mid- body part of the hull, e.g. so as to form a ledge comprising the forward ledge portion and the lateral ledge portion. The lateral ledge portion may extend along a smaller or larger portion of the sides of the bow or even further so as to guide ice floes that impact the forward or lateral ledge portion towards the side and aft. For example, the lateral ledge portion may extend along at least 10% of the length of the hull, such as at least 20%, such as at least 25%. For example, the mid-body of the hull may comprise generally flat upward side walls and the lateral ledge portion may continue all the way to a region with flat upward side walls of the hull, e.g. to a region where generally flat, upright hull sides extend from a bottom, such as a flat bottom or slightly inclined bottom, of the hull upwards. The angle of the upper surface of the lateral ledge portion relative to the waterline may gradually change from the stem towards the aft. For example, while the upper surface of the forward ledge portion may slope forwardly upwards at the stem, the upper surface of the lateral ledge portion extending along the sides of the hull may slope laterally outward and slope upward to a lesser degree or even downward. The lateral ledge portion may be shaped and sized so as to guide ice whose downward movement has been stopped by the forward ledge portion aftwards while preventing the ice from submerging under the bottom of the hull. To this end, the dimensions of the lateral ledge portion may be chosen according to the design ice thickness so as provide an effective barrier. For example, the lateral width of the lateral ledge portion may e.g. be between 25% and 100% of the design ice thickness, such as between 30% and 75%, such as between 30% and 40% or between 40% and 50%, or between 50% and 80%. For example, the lateral width of the lateral ledge portion may e.g. be between 0.4 m and 4 m, such as between 0.5 m and 3 m, such as between 0.5 m and 2 m, such as between 0.5 m and 1 .5 m, such as between 1 m and 1 .5 m. Simi- larly, the lateral ledge portion may be positioned relative to the forward ledge portion so as to guide ice whose downward movement has been stopped by the forward ledge portion aftwards while preventing the ice from submerging under the bottom of the hull. In particular, the lateral ledge portion may be positioned sufficiently close to, or even extend from the forward ledge portion, so as to guide ice whose downward movement has been stopped by the forward ledge portion aftwards while preventing the ice from submerging under the bottom of the hull.

In some embodiments, the lateral ledge portion forms the bottommost part of the stem or is at least positioned close to the bottommost part of the bow, e.g. less than 4m, such as less than 3 m, such as less than 2 m such as less than 1 m from the bottommost part of the bow where the distance is measured between the top surface of the lateral ledge portion and the bottom of the hull, e.g. a flat bottom of the hull. The top surface of the lateral ledge portion may be generally flat, so as to efficiently stop or prevent at downward movement of ice pass or around the lateral ledge portion.

In some embodiments, the vessel may comprise laterally protruding ice- protection barriers along the sides of the mid-body of the hull so as to prevent ice from submerging under the bottom of the hull, also at a mid-body portion of the hull. In some embodiments this barrier is arranged to prevent ice impacting the side of the ship (e.g. during a turn in ice) to go under the hull. These barriers may be formed as bilge keel portions of the hull or as structures attached to the hull forming a laterally extending ledge. In some embodiments, the laterally protruding ice-protection barriers may be formed as a continuation of the lateral ledge portion along the sides of the hull. For example, the forward ledge portion, the lateral ledge portion and a laterally protruding ice-protection barrier of the mid-body of the hull may together form an ice- protection barrier extending from the stem and along at least 30% of the length of the hull, such as at least 50%, such as at least 70%. For example, the laterally-ice protection barrier may extend aftwards to or even beyond the position of the moonpool or a similar position under the hull that is to be pro- tected from submerging ice, e.g. at least 1 m beyond said position, such as at least 2 m, such as at least 4 m. In alternative embodiments, the laterally-ice protection barrier may not extend all the way to the position of the moonpool or a similar position under the hull that is to be protected from submerging ice. For example, the laterally-ice protection barrier may end at least 1 m forward of said position, such as at least 2 m, such as at least 4 m.

The height at which the laterally protruding ice-protection barrier is located relative to the base of the hull may be constant or it may vary, e.g. increase, towards the aft of the vessel .

The dimensions of the laterally protruding ice-protection barrier may be chosen according to the design ice thickness so as provide an effective barrier. For example, the lateral width of the laterally protruding ice-protection barrier may e.g. be 25% and 100% of the design ice thickness, such as between 30% and 75%, such as between 30% and 40% or between 40% and 50%, or between 50% and 80%. The position of the laterally protruding ice-protection barrier on the hull may be at least the design ice thickness below the lower ice water line. In some embodiments two or more laterally protruding ice- protection barriers may be employed. In some embodiments, the laterally protruding ice-protection barrier forms the bottommost part of the mid-ship section of the sidewalls of the hull, or it may at least be positioned close to the bottommost part of the sidewall, e.g. less than 4m, such as less than 3 m, such as less than 2 m such as less than 1 m from the bottommost part of the side wall where the distance is measured between the top surface of the laterally protruding ice-protection barrier and the bottom, e.g. a flat bottom, of the hull. The top surface of the laterally protruding ice-protection barrier may be generally flat, so as to efficiently stop at downward movement of ice submerging along the hull. In one embodiment, the laterally protruding ice- protection barrier has a bottom surface substantially flush with or having another smooth transition to the bottom, e.g. a flat bottom, of the hull. In some embodiments, the upper surface of the laterally protruding ice- protection barriers may be substantially horizontal or slope laterally outwards and downwards, e.g. at an angle (an example of which is illustrated as angle σ in Fig. 4) between 0° and -35°, such as between -15° and -25° relative to the waterline where negative angles represent an outwardly downward slope. This angle may in principle be less than 25°, such as less than 15°, such as less than 10°, such as less than 5°, such as less than 0°, such as less than - 5°, such as less than -15°, such as less than -20°, such as less than -25°, such as less than -30°, such as less than -35°.

In some embodiments and in some conditions ice flow may extend past the upperside of the lateral ledge. In particular the ice floe may lean against a side of the hull at a point above the lateral ledge and on the outward side of the ledge. While the ledge may still provide some guidance there is a risk that further submersion of the floe (e.g. due to push from other ice floes) may cause the leaning ice floe to submerge under the bottom of the ship where it's buoyance will cause it to press upwards against the bottom. Such ice floes may end up in the moon pool and/or impact equipment extending from the moonpool towards the seabed. As further measure to prevent this the vessel may be equipped with a flushing system which can provide a force directed outwards and/our downwards from the outwards side of the ledge. In this context a flushing system is taken to mean a system to provide one (but typically more than one) stream of water (or other liquid) to impose a relative- ly located force on the ice floe. The water stream is typically provided by pumping water from a one location on the hull (preferably in a location that is ice free and where a potential flow cannot create an undesirable flow of ice) out through a pipe ending or another type of nozzle. In this way a force away from the vessel may be applied to a portion of the ice floe below the upper surface of the ledge. In this way, the chance of an ice floe (particularly one that is leaning against the hull and extending beyond the lateral so that the guiding by this is ledge is reduced) rotating away from the hull can be increased. Ice floes that are leaning against the ledge and hull (in some cases even above the waterline) will often have a tendency to rotate anyway due to their buoyancy but inertia and/or other ice floes pressing against them may prevent this rotation. This is particularly a problem if the ice floe is otherwise at risk of being blocked by mooring lines in which case it may stop and act as a barrier for further ice causing a build up. Accordingly, in some embodiments the invention relates to a marine vessel for operation in at least partly ice-covered water, the vessel comprising a hull having at least one bow op- erable to be exposed to the loading effect of sea ice due to a relative motion between the ice and the vessel; where the bow defines a stem at which respective side portions of the hull meet, the bow having an upper portion which, during operation of the vessel at operational draughts, is located above the water level; a bottom portion which, during operation at operational draughts, is located below the water level; and an intermediate portion which, during operation at operational draughts, is located at the water level and where the hull comprises a lateral ledge portion extending from a position on the bow along the sides of the bow towards a mid-body portion of the hull so as to guide ice floes that impact the ledge portion towards the side and aft, wherein the vessel further comprises a flushing system having nozzles along at least part of said lateral ledge portion to provide a stream of water said stream arranged to provide a force directed outward and/or downward from an outward side of the lateral ledge portion. The force may be arranged to increase the change that the lower end of the ice flow will rotate away from the hull and that the ice submerge under the bottom of the hull. The lateral ledge portion typically extends from some where between the stem and the moonpool to a position alongside or aft of the moonpool.

This flushing concept may also be suitable for vessels without a ledge in which case the force is preferably provided (and typically the nozzles provid- ed) low on the hull side. Accordingly, the nozzles are placed low on the hull side, such as lower than 50% of the hull side, such as lower than 60% of the hull side, such as lower than 80% of the hull side, such as lower than 80% of the hull side, such as lower than 90% of the hull side, such as lower than 95% of the hull side.

In some embodiments the flushing system may be arranged to provide water using one or more pumps rated at 1 .5 to 5 Mwatts, such as 2-4 Mwatts. In some embodiments the flushing system is arranged to vary where the pressure is directed such as in a cycling patern e.g. cycling front to aft.

As explained above it is often advantageous to mitigate the rotation of the ice floe before it is at risk of engaging the mooring lines. Also, in some embodiments ice may build up in the transition from the bow to the parallel mid section of the ship increasing the risk that a flow may be pushing down beside the ledge. Accordingly, in some embodiments, the flushing system comprises one or more nozzles located in the aftmost 50% of the lateral ledge portion, such as in the aftmost 40%, such as in the aftmost 30%, such as in the aftmost 20%, such as in the aftmost 10%. It may further be an advantage to include nozzles in the laterally protruding ice-protection barriers along the sides of at least a part of the mid-body, such as in at least the forward most 10% or more, such as in at least the forward most 20% or more, such as in at least the forward most 30% or more, such as in at least the forward most 40% or more, such as in at least the forward most 50% or more. In some embodiments the flushing system is arranged so that it may provide a substantially homogenous force distribution along the sections specified in the previous sentences. In some embodiments the flushing system is arranged as an active system that monitors the ice floes under water and/or the pressure exerted on the hull to determine where the application of a pushing force is beneficial to reduce the risk of ice going under the bottom of the hull.

As mentioned above, the upper surface of the forward ledge portion of the stem may slope forwardly upwards or downwards. For example, the angle may be between the sloping angle of the laterally protruding ice-protection barriers and 15°, e.g. between -35° and 15° (where negative angles represent a forwardly downward slope). In some embodiments the upper surface of the forward ledge portion of the stem slopes forwardly upwards, e.g. at an angle between 0° and 15°, such as between 0° and 10°, such as between 5° and 15°, e.g. between 5° and 10°. In some embodiments, the forward sloping angle of the upper surface of the forward ledge may be negative, e.g. equal or larger than the laterally outwardly sloping angle of the upper surface of the laterally protruding ice-protection barriers, e.g. between 0° and -35°, such as between -15° and -25° relative to the waterline where negative angles represent an outwardly downward slope. The forward sloping angle of the upper surface of the forward ledge may in principle be less than 25°, such as less than 15°, such as less than 10°, such as less than 5°, such as less than 0°, such as less than -5°, such as less than -15°, such as less than -20°, such as less than -25°, such as less than -30°, such as less than -35°.

Generally, in some embodiments, the forward ledge portion, the lateral ledge portion and/or the laterally protruding ice-protection barriers may be attached to, be part of the bulk structure, or be part of the frames or be integrated into or attached to the hull in another suitable form. In some embodiments, the forward ledge portion and/or the lateral ledge portion and/or the laterally protruding ice-protection barriers are passive elements that are fixedly installed. In particular, they may be static elements that are non-movable relative to the hull and/or they may comprise no moving parts. Generally, the forward ledge portion and/or the lateral ledge portion and/or the laterally protruding ice- protection barriers may be shaped and sized so as to prevent ice from sliding downwards along the hull pass the forward ledge portion and/or the lateral ledge portion and/or the laterally protruding ice-protection barriers, respectively. In particular, their respective upper surfaces may be shaped and sized so as to effectively engage ice sliding downward along the hull and to prevent the ice from sliding further downward underneath the bottom of the hull. In some embodiments, the forward ledge portion of the stem, the lateral ledge portion and the laterally protruding ice-protection barrier together form a substantially uninterrupted ice-protection barrier extending from the stem aftwards along at least a part of the midship section of the hull, e.g. to or even beyond the position of the moon pool so as to prevent ice from submerging under the bottom of the hull and from reaching the moonpool. The ice-protection barrier may extend along a substantial portion of the length of the hull such as more than 50%, such as more than 60%, such as more than 70% of the hull. While the ice-protection barrier may extend along the entire length of the hull, in some embodiments it will only extend along a portion of the hull such as less than 90%, such as less than 80%, such as less than 70%, such as less than 60% of the length of the hull. It will be appreciated that, in alternative embodiments, gaps or other discontinuities in the lateral ledge portion and/or the ice protection barriers may be provided, such as gaps small enough so as not to substantially diminish the barrier and guiding function of the lateral ledge portion or the ice protection barrier, respectively. In some embodiments, any such gaps may have a longitudinal extent smaller than 200% of the design ice thickness, such as less than 100% of the design ice thickness, such as less than 50% of the design ice thickness. For exam- pie, any such gaps may have a longitudinal extent of less than 5 m, such as less than 2 m such as less than 1 m. In some embodiments the lateral ledge portion and/or the laterally protruding ice-protection barrier may be formed as multiple barrier sections separated by gaps and/or positioned at respective heights, e.g. to form a structure resembling a stair-case.

The marine vessel may be a drillship or other form of floating ship-shaped vessel which may have a moonpool through which equipment is lowered towards the sea floor, e.g. a drilling riser extending from the vessel to the sea- floor. The marine vessel may be held stationary by any suitable mechanism such as a DP-system or a mooring system comprising mooring lines. The latter may be anchored at the seafloor and connected to the drillship, e.g. at a turret such that the vessel may rotate around a vertical axis which may be aligned with and preferably coaxial with the hoisting axis along which equipment is lowered to the seafloor. The moonpool and/or the turret may be located at different positions along the length of the hull, such as substantially midship or further towards the bow or the stern. In some embodiments, the turret is positioned between 25% and 40%, such as between 30% and 35%, of the overall length of the vessel away from the stern which has been found to reduce heave motions. In alternative embodiments, the turret is positioned between 25% and 40%, such as between 30% and 35% of the overall length of the vessel away from the stem which has been found to facilitate rotation of the moored vessel relative to the drifting ice. To further facilitate rotation of the moored vessel relative to the drifting ice the turret may be positioned within 40% of the ship's lengths towards the bow or the stem, such as with 30%, such as with 25%, such as within 20%, such as within 15%, such as within 10%, such as within 5%.

In some embodiments, the hull comprises a generally flat bottom such as at least at its mid-body portion. In some embodiments the flat bottom extends at least 10%, such as at least 20%, such as at least 30% of the length of the beam of the vessel. In some embodiments, the bottom may have a slight in- clination, e.g. of 1 ° or less, while in other embodiments at least a part of the bottom may have an even larger inclination, such as 5% inclination or less, such as 10% or less, such as 15% or less, such as 20% or less, such as 30% or less or even larger. In some embodiments the inclination is over more than 10% of the length, such as over more than 20%, such as over more than 30% such as over more than 40%, such as over more than 50%, such as over more than 60%, such as over more than 70%, such as over more than 80%, such as over more than 90%. In some embodiments, the hull comprises a downwardly protruding barrier portion protruding downwards from a flat bottom or an inclined bottom of the hull. The barrier portion may completely or partly surround the moonpool or another position at the bottom of the hull from which equipment extends towards the seafloor. The height of the barrier portion relative to a flat portion of the bottom of the hull may depend on the thickness of the ice floes. For example the barrier may be between 5% and 100% of the design ice thickness or even more. In some embodiments, the barrier is at least 0.4 m high, such as at least 0.5 m, such as at least 0.8 m, such as at least 1 m, such as at least 1 .2 m, such as at least 1 .5 m, such as at least 2 m, e.g. between 0.5 m and 1 .5 m high, but it can be even higher. The barrier may comprise a main portion and a forward portion. The main portion defines a periphery at least partly surrounding the position under the hull that is to be protected. The forward portion may have a shape that nar- rows towards the bow end, e.g. so as to form a tip or plough-shape. Generally, the barrier may be configured to prevent ice floes that have submerged under the bottom of the hull from reaching the moonpool and, instead, to be guided laterally away from the moonpool. In some embodiments, the hull comprises multiple downwardly protruding barrier portions, e.g. for protecting multiple locations under the hull.

In some embodiments the vessel comprises an air bubbling system with holes in the downwardly protruding barrier and/or the bottom of the hull arranged to discharge compressed air (or another gas e.g nitrogen). Typically the air flow will seek to the surface but may in some cases flow along the bottom with the current on both cases creating a current on the bottom. The water flow (and the water flow derived from it) will drive ice floes or rubble under the bottom of the hull outwards and away from the centreline thus reducing the risk of this ice entering the moonpool. In some embodiments the holes are arranged to provide such air flow along the bottom in front and/or next to the moonpool, so that holes are placed in the respective areas. In one embodiments the holes placed in the side of the downwardly protruding barrier. The general use of bubbling is discribed in US Patent 3,580,204 which provides air volumes measures and air volume specifications and examples that may also be applied so that in one embodiment air is blown through said openings at a volumetric rate sufficient to cause an upwardly directed flow of water along the side of said ship to produce a substantial ridge of water at the water surface along the side of said hull and thus generate a water flow which moves laterally away from the side of said hull. In one embodiment this ridge may rise height of from 25 to 50 cm. above the water level. In some embodiment the air flow is arranged to reduce the friction between ice and hull with more than 5%, such as more than 10%, such as more than 15%, such as more than 30%, such as more than 50%.

The bubbling system preferably creates the flow in front of the moonpool to increase the chance that ice floes do not enter the moonpool, so that bubbles are created more than 30% of the distance between the forward moonpool edge and the stem forward of the moonpool, such as more than 40%, such as more than 50%, such as more than 60%, such as more than 70%, such as more than 80%, such as more than 90% of this distance. In some instances more than 30% of this distance has air outlets such as at least 50%, such as at least 70%, such as at least 90%.

In some embodiment the bottom of the vessel is slanted upwards at least part of the way from the centreline to the side of the flatbottom hull. In some embodiments the hull is slanted at least part of the way from the centreline to the side of the flat bottom hull to further enhance the airflows rise to the sur- face. The slant would typically be small such as 10 degree or less, such as 7 degrees or less, such as 5 degrees or less, such as 4 degrees of less, such as 3 degrees or less, such as 2 degrees or less, such as 1 degrees or less; however at the same time the slant is often more than 0.5 degrees, such as more than 1 degree, such as more than 2 degrees such as more than 3 de- grees. In some embodiments the slant is from the centreline but typically begins somewhere from 70% to 20 % from the centreline end extends to the side of the flatbottom hull. Since the aim in many embodiments is to avoid ice in the moonpool slanted section is not present aft of the moonpool, such as aft of the center of the moonpool. Generally, in some embodiments, the bow is a passive structure but may comprise one or more active devices such as moveable ledges, a flushing system in the bow and/or the like. Generally, the drifting sea ice may have a variety of forms such as ice floes of varying sizes, it may be level ice, comprise ridges etc. The drifting ice may be managed or unmanaged ice, i.e. the drifting ice may or may not have already managed by e.g. one or more vessels with icebreaking capacities before reaching the vessel. It will be appreciated that embodiments of a vessel described herein may be designed for a variety of different ice thicknesses. For example, the design ice thickness may be between 0.5 m and 5 m, such as between 1 m and 4 m. In some embodiments the design ice thickness is 1 m or more, such as 2 m or more, such as 3 m or more, such as 4 m or more; however, at the same time the design ice thickness may be less than 6 m, such as less than 5 m, such as less than 4 m.

The present disclosure relates to different aspects including the marine vessel described above and in the following, further aspects of a marine vessel and to corresponding methods and/or products. Each aspect may yield one or more of the benefits and advantages described in connection with one or more the other aspects, and each aspect may have one or more embodiments with all or just some of the features corresponding to the embodiments described in connection with one or more the other aspects and/or disclosed in the appended claims.

Brief description of the drawings:

In the following, one or more embodiments of the invention will be described in more detail and with reference to the drawing, where: Fig. 1 schematically shows an example of a cross section of an marine vessel.

Fig. 2 shows an example of a hull for a marine vessel.

Fig. 3 shows longitudinal cross sections of the bow of the hull of fig. 2.

Fig. 4 shows portions of lateral cross sections of the bow of the hull of fig. 2. Figs. 5 and 6 illustrate a function of the ice protection barrier of the hull of Fig. 2.

Fig. 7 shows a plurality of waterlines of the hull of fig. 2 at different heights above the base.

Figs. 8 A-D show horizontal cross sections of examples of the intermediate, V-shaped foremost portion of the bow.

Description of embodiments:

Fig. 1 schematically shows an example of a marine vessel floating in a body of water 105 with drifting sea ice 127. In particular, fig. 1 shows a cross section of a drillship, generally designated 100, that is anchored to the seafloor 1 1 1 by mooring lines 108. The drillship comprises a hull 101 which may be substantially oblong or ship-shaped, e.g. as described in connection with figs. 2-7 below. The drillship further comprises a drill floor 102 formed on top of a platform supported by legs 130 or another form of substructure. The platform defines the drill floor from which drilling operations are conducted and spans across a moonpool 107 formed in the hull of the drillship so as to allow equipment to be lowered towards the seafloor. One or more holes in the drill floor, each typically in the form of a rotary table, define one or more well cen- tres through which drilling operations can be performed. The well centre(s) may be located next to or generally under a drilling support structure 104. In the example of fig. 1 , the drilling support structure is a mast positioned adjacent to the well centre, but other forms of drilling support structures, such as a derrick structure, are possible as well. The drilling support structure is sup- ported by the legs 130 or a similar substructure and it extends upwardly relative to the drill floor 102. The hoisting system comprises a hook or similar device from which a string of tubulars 109 may be suspended and lowered and raised through the well centre and the moon pool 107. To this end, the hoisting system may comprise a topdrive 103 to which an upper end of the drill string may be connected and which may impart torque on the drill string. The hoisting system may be a draw-works system where the hoisting line is fed over stationary sheaves carried by the drilling support structure or another suitable type of hoisting system such as a hydraulic hoisting system comprising cylinders that extend upwardly from the drill floor and support the load to be lowered or hoisted.

In some embodiments the drillship is configured to perform drilling operations through a marine riser string 1 10 extending from the drillship to a blow-out- preventer (BOP) 1 12 that is placed on the seafloor. Hence, the drillship is connected to a subsea well via the marine riser string 1 10. The drillship is moored via a turret 106 and a plurality of mooring lines 108. The turret allows the drill ship to align its longitudinal axis with any given ice drift direction. To this end, the ship may rotate around the vertical axis defined by the marine riser. It will be appreciated that other types of subsea operations may involve other devices or systems for keeping the vessel in position, for connecting the vessel to the seafloor or otherwise extending from the hull towards the seafloor where the vessel is maintained stationary floating above a location on the seafloor. In some embodiments the drillship is configured to perform drilling operations through a surface BOP and a pressurized riser. In some embodiments the drillship is configured to perform riserless drilling operations into the seafloor.

When the drillship floats in drifting sea ice 127 the drillship is typically oriented such that the ice approaches the bow of the hull 101 . To this end, embodiments of the hull may have an ice-breaking shape and sufficient strength so as to break the ice. In any event, the drifting sea ice imparts loads on the hull and/or the mooring lines 108 or other station keeping systems; it is desirable to maintain these loads on acceptable level. Moreover, it is desirable to keep ice from submerging under the vessel and from reaching the moonpool area 107 so as to avoid damaging the riser 1 10, the turret 106 and/or the mooring lines 108.

Such vessels are frequently used for drilling operations for exploration of hydrocarbon reservoirs, such as oil or natural gas reservoirs, in subsea formations, for establishing bored wells into such reservoirs and/or for subsequent production of hydrocarbons. It will be appreciated that the size and shape of the vessel, its equipment, and/or the type of equipment extending downwards from the vessel may vary according to the specific application.

Fig. 2 shows an example of a hull for a marine vessel, e.g. for the marine vessel 100 of fig. 1 . The hull is generally oblong and ship-shaped defining a longitudinal axis and having a bow 200 and a stern (not shown). The hull is symmetric with respect to a vertical center plane extending longitudinally between the bow and the stern, though non-symmetric hulls may be used as well. The side portions 201 of the bow meet at the foremost end of the bow 200 in the center plane so as to form the stem 210 of the hull.

The hull defines an upper ice waterline (UIWL) 202 and a lower ice waterline (LIWL) 203. A waterline generally divides the hull, for a given draught, in a lower portion that is below the water level and an upper portion that is above the water level when the vessel is operated at said draught. The upper and lower ice waterlines thus represent a range of operational draughts. Accordingly, the stem has an upper portion 21 1 which, during normal operation, is located above the water level, a bottom portion 215 which, during normal operation, is located below the water level, and an intermediate portion 212 between the upper and lower portions and which, during normal operation, is located at around the water level, e.g. partly extending below and partly above the water level. The hull has a generally flat bottom 253 in which a moonpool 251 or other opening, recess or connection area is defined, where subsea equipment may be connected or from which equipment may extend downward. It will be appreciated that, in alternative embodiments, the bottom of the hull may be shaped other than flat.

In the following, with continued reference to fig. 2 and with further reference to figs. 3-7 various features of the hull will be described in more detail.

In particular, fig. 3 shows longitudinal cross sections of the bow at different distances from the longitudinal center plane. Fig. 4 shows portions of lateral cross sections of the bow of the hull at different distances from the bow. Fig. 5 shows longitudinal cross sections of the bow and of the mid-body section at different distances from the longitudinal center plane. Fig. 6 shows portions of lateral cross sections of the mid-body of the hull at different distances from the bow. Finally, Fig. 7 shows horizontal sections of the bow and mid-body of the hull at different heights above the base of the hull.

In the present example, the upper portion 21 1 of the stem is generally plumb and blunt and, in particular, more upright than the intermediate portion and more blunt than the intermediate portion. However, it will be appreciated that other shapes of the upper portion may be used, as the upper portion does not normally interact with the ice. The stem then curves backwards to form a raked intermediate portion 212. As can most easily be seen in Fig. 3, the intermediate portion 212 is substantially straight and forms an angle φ with the waterline 304 of between 10° and 45°, such as between 15° and 20° or between 20° and 30°, e.g. about 15°, about 20°, about 25°, about 30°, about 35°.

As can most easily be seen in Fig. 7, which shows a plurality of waterlines of the hull of fig. 2 at different heights above the base, the intermediate portion 212 of the stem is V-shaped in a horizontal cross section so as to form a sharp, ice-dividing edge. For example the sides of the hull may meet at the intermediate portion 214 of the stem such that the their horizontal cross sections form an acute angle with the longitudinal center line of the corresponding cross section, as indicated by angle a in Fig. 7. The angle a may be 70° or less, such as 60° or less, such as 50° or less, such as 40° or less, e.g. about 55°, about 51 °, about 50°, about 49°, about 48°, about 47°, about 46° or less. For example, the angle a may be between 25° and 60°, such as between 25° and 55°. Alternatively or additionally, an ice-splitting member may be attached to the intermediate portion of the stem or the intermediate por- tion of the stem may comprise or be formed by such an ice-splitting member. The straight intermediate portion of the stem may extend from around the water level to one or several meters below the water level, e.g. by the design ice thickness or even twice the design ice thickness or more. The intermediate portion of the stem and the laterally adjacent sidewall forming the bow provide an efficient ice-breaking bow.

In particular, it has generally turned out that the raked intermediate portion of the stem with an ice-dividing edge efficiently divides or even splits ice at low relative speeds between the ice and the hull, e.g. at speeds less than 2 knots, such as less than 1 .5 knots, e.g. less than 1 knot, i.e. in particular when the vessel is held at station in drifting sea ice. The sharp edge splits larger floes of ice and the raked intermediate stem portion and raked bow pushes the incoming ice downward. The floes are then guided sideways along the sides 205 of the intermediate portion of the bow. To this end the sides 205 of the ice-breaking intermediate portion of the bow are substantially flat as defined by substantially straight sections 406 of the frames at the bow, as is most easily seen in Fig. 4 which illustrates how ice floes 354 are guided sideways away from the stem. The flat-sectioned frames and hull sides thus plough ice to the sides. As the angle of the side walls relative to the waterline gradually increases, ice floes are also guided from a horizontal to a sloped or even vertical orientation which further facilitates their upward movement and prevents them from submerging under the bottom of the hull. This is schematically illustrated by ice floes 354 in Fig. 4. The upper portion 482 of the hull side may be more upright than the intermediate portion and separated from the intermediate portion by a curved transitional portion 483.

Generally, as was already described above with reference to the corresponding curved transitional portion of the frames that separates the upper portion and the intermediate portion of the frames, the flat portion of the sides of the bow may be shaped so that a transitional portion separating the flat portion from an upper, more upright portion moves downwards and aft, and the angle of the flat portion relative to a horizontal plane increases when moving aft. This has the effect of guiding ice floes down and aft by pushing them down in the foremost part of the straight portion and gradually raising them towards vertical, such as to vertical.

The stem comprises a transitional portion 213 connecting the bottom part of the intermediate portion 212 of the stem with the bottom portion 215. The transitional portion 213 curves downward to form a substantially downwardly directed transitional region connecting the raked portion with the bottom por- tion 215. This creates space for one or more thrusters 252 or other lateral propulsion devices.

In general, the bottom portion 215 of the stem may be formed as a forwardly protruding ledge 214 which efficiently functions as an ice-protecting barrier preventing ice floes that are pushed downwards by the ice-breaking bow including intermediate portion 212 of the stem to submerge under the hull, as is illustrated by ice floes 354 in fig. 3. The submerging ice floes 354 hit the forward ledge portion and are guided sideways rather than further downward. To this end the hull comprises a lateral ledge portion which continues from the forward ledge portion along both sides of the bow from the stem in the aft direction, e.g. all the way to the mid-body where the hull sides are flat and upright. The lateral ledge portion 214 is located below the water level, preferably low on the hull so as to allow the icebreaking bow to work efficiently before the ice is diverted by the forward and lateral ledge portions. In some embodiments, the forward and/or lateral ledge portion is more than the design ice thickness below the water line, such as at least twice the design ice thickness, such as at least 4m below the water level, such as at least 6m such as at least 5m, such as at least 6m, such as at least 8m. The upper surface 316 of the forward ledge portion is sloped in a forward-upward direction, as illustrated by angle γ in Fig. 3, so as to facilitate submerging ice floes to be effec- tively prevented from sliding below the forward ledge portion. This is schematically illustrated by ice floes 354 in fig. 3. For example, the angle γ relative to the horizontal may be between 0° and 15°, such as between 2° and 15°. The forward ledge portion may extend at least 25% of the design ice thickness away in the forward direction from the transitional region 215, e.g. be- tween 50% and 100%, such as between 1 m and 4m, such as between 1 m and 2m. Alternatively or additionally to the forward ledge portion and lateral ledge portion, fins or similar structures may be employed. In some embodiments, the forward ledge portion may comprise a pump or other device for flushing/jetting a fluid, such as water or air, upwards against the ice ap- proaching the stem so as to facilitate the breaking up and/or diversion of the ice.

In the present embodiment, the hull comprises a lateral ledge portion that extends as an ice protection rail- or bulge-like barrier 421 from the forward ledge portion along the sides of the bow. In some embodiments, the lateral ledge portion only extends a small distance along the sides of the bow, e.g. less than 20% of the length of the beam of the vessel, such as less than 10% of the length of the beam. In the example of figs. 2-7, however, the hull further comprises a bilge keel protrusion 220 which protrudes laterally away from the hull and extends along a substantial portion of the mid-body of the hull. This prevents ice from submerging under the bottom of the hull also at mid-ship direction as illustrated by ice floes 354 in fig. 6. In the example of figs. 2-7, the forward ledge portion 214 of the stem, the lateral ledge portion 421 and the laterally protruding bilge keel 220 together form an uninterrupted ice-protection barrier extending from the stem aftwards to or even beyond the position of the moon pool so as to prevent ice from submerging under the bottom of the hull and reach the moonpool. It will generally be appreciated that embodiments of an ice-protection barrier described herein may also be provided on other examples of hulls. For example, in general a hull may comprise a forward ledge portion at its stem, a lateral ledge portion at the sides of the bow and a laterally protruding ice-protection barrier at the mid- body of the hull - e.g. in the form of a bilge keel portion 220 - that together form an ice-protection barrier extending from the stem aftwards along the hull beyond a position where the bottom of the hull is to be protected against submerged ice. The ice-protection barrier may extend along a substantial portion of the length of the hull such as more than 50%, such as more than 60%, such as more than 70% of the hull. It will be appreciated that, in alternative embodiments, discontinuities in the ice protection barrier may be provided. Generally, the forward and lateral ledge portions are shaped so as to prevent ice coming down the bow from submerging below to the bottom of the hull. In some embodiments, the lateral ledge portion extends along the entire bow where the hull sides have a sloped intermediate portion pushing the ice downwards and sideways.

The bilge keel is located on both sides of the mid-body portion of the hull; in some embodiments the bilge keel does not extend all the way to the stem. The bilge keel may be shaped so as to prevent ice that moves down the bow and/or flat side from going ahead and in the ice drift reversal. In some embodiments, e.g. as in the present example, the upper surface 316 of the forward ledge portion of the stem slopes forwardly-upward, while the upper surface 423 of the lateral ledge portion 421 at the bow and/or of the bilge keel 220 is outwardly inclined by a smaller angle than the forward incli- nation of the surface 316 of the forward ledge portion 214. The inclination may even be such that the upper surface 423 of the lateral ledge portion 421 at the bow and/or of the bilge keel 220 is inclined outwardly downward, as can be seen in Fig 4 and 5. In this way, ice floes being pushed downwards by the ice-breaking portion of the stem and bow are guided along the flat sides 205 of the bow sideways and backwards while being prevented from submerging under the bottom of the hull. Alternatively, the sloping angle may be outwardly downwards along the entire length of the barrier. In Fig. 4 the outwardly downward slope is illustrated by angle δ. The outer surface 424 of the lateral ledge portion 421 at the bow and/or of the bilge keel 220 may comprise nozzles from a flushing system as described above.

As can most easily be seen in Fig. 7, the hull comprises a generally flat bottom 253 including an ice-protecting downwardly protruding barrier 230 that surrounds the moonpool 251 and protects the moon pool from ice which, de- spite the ledge portion 214 of the stem and ice protection barriers 421 and 220, may submerge under the bottom of the hull and slide along the bottom towards the moon pool 251 . The barrier 230 defines an edge extending downward from the flat bottom of the hull at which ice gets caught and guided sideways and towards the aft at a safe distance around the moon pool as illustrated by dots in Fig. 7. The barrier 230 may have various sizes and shapes, some of them extending entirely around the moon pool, some only extending partly around the moonpool, e.g. only around a forward portion of the moonpool. The barrier shown in Figs 2 and 7 defines a generally drop shaped plateau having a plough-shaped tip portion 231 forward of the moonpool from which its edges extend sideways and backward towards both sides of the hull. In this example, the aft direction from the moon pool the barrier plateau again increasing narrows to form another plough pointing in the aft direction; however other shapes may be used. This may protect the moon pool in case of ice drift from the aft or in case of maneuvering of the vessel. The height of the barrier 230 relative to the bottom 253 may depend on the design ice thickness of the ice floes; for example, the height may be between 50 cm and 1 .5 m. Generally, it will be appreciated that an ice- protecting, downwardly protruding barrier as described herein may also be provided with other forms of hulls, e.g. hulls having a different form of bow, and/or with hulls with or without one or more ledges and/or lateral ice- protection barriers as described herein. It will be appreciated that a downwardly protruding barrier may also be employed in combination with a hull having a bottom that is shaped other than flat, e.g. a bottom having one or more inclined portions. The arrows 701 point to examples of locations for nozzles of the flushing system discussed above.

Figs. 8 A-D show horizontal cross sections of examples of an intermediate, V-shaped foremost portion of the bow. FIG. 8A shows an example where the plates forming the hull sides abut and are welded together at the stem 810 forming sharp ice-dividing edge, while FIG. 8B shows an example where the plates forming the hull sides 805 are separated by a flatbar 880 which forms the stem and provides two ice-dividing edges. The example of FIG. 8C is similar to the one of FIG. 8A, but where the foremost portion of the sides 805 curve inwards so as to form a small radius of curvature while still providing a stem that is sharp enough to facilitate introduction of a line of fracture in the ice. The example of FIG. 8D is similar to the one of FIG. 8C, but where the stem is further provided with an ice-breaking structure 881 . Hence, in all the examples of FIG. 8A-D, the sides of the hull meet at the foremost part of the bow so as to define the stem and to form a V-shaped bow. The bow is V- shaped at the scale of the design ice thickness, i.e. sharp enough so as to facilitate creating a line of fracture in the ice. In particular, at a predetermined distance L - e.g. at 50% of the design ice thickness or at 100% of the design ice water thickness - from the stem, the width W of the bow is less than 2.5-L, such as less than 2-L, such as less than 1 .5-L. In the example of Figs 8C and D, the radius of curvature R is 50% of the design ice thickness or less, such as no more than 30% of the design ice thickness. In the example of FIG. 8D, the radius of curvature of the inwardly curved sides may be larger since the ice-breaking structure 881 provides one or more sharp edges.

In summary, the examples described in connection with figs. 2-8 are exam- pies of a hull of a marine vessel having the following general ice-protection features for reducing ice under the hull and protecting the moonpool area of the hull against ice:

Ice-breaking bow: shaped with small wedge in stem to split ice floes and comprising flat sectioned frames to plough ice to the side. The bow splits the ice and/or moves the ice to the side of the hull and thus reduces ice transport towards the flat bottom.

Ice protection ledge: A forward ledge portion stops ice coming down towards the flat bottom along the stem and a lateral ledge portion redirects the ice movement away from the flat bottom.

Ice protection barrier: The ice protection barrier starting as lateral ledge portion in the bow area and continuing into a laterally protruding bilge keel of the mid-body of the hull stops ice coming down towards the flat bottom both ahead and during a turn, and redirects the ice movement away from the flat bottom. The ice protection barrier thus prevents ice from going into the bottom, also in mid-ship direction. Ice plough around the moonpool: The ice plough around the

moon pool/turret prevents ice from hitting the turret from all directions. This may thus be regarded as a final barrier which deflects ice that has come below the flat bottom and that prevents the ice from entering the moonpool. In general, other embodiments of a hull of a marine vessel may include one or more of the above features while lacking one or more other features, thus providing varying protection against going below the hull and/or ice reaching the moonpool.

For the purpose of the present disclosure, the terms "preventing" ice from submerging under the bottom or from reaching the moonpool etc. are intended to refer to measures that substantially reduce the risk of ice submerging under the bottom and/or reaching the moonpool etc., as it will be appreciated that embodiments of the hull described herein, while preventing most of the ice from reaching the moon pool, may not necessarily guarantee that all ice floes are prevented from reaching the bottom. It will further be appreciated that the various ice-protecting features described herein may cooperate with one another to further reduce the risk of ice reaching the moon pool.

Although some embodiments have been described and shown in detail, the invention is not restricted to them, but may also be embodied in other ways within the scope of the subject matter defined in the following claims. In par- ticular, it is to be understood that other embodiments may be utilized and structural and functional modifications may be made without departing from the scope of the present invention.

The mere fact that certain measures are recited in mutually different depend- ent claims or described in different embodiments does not indicate that a combination of these measures cannot be used to advantage.

It should be emphasized that the term "comprises/comprising" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.