The present invention refers to a ship having mainly similar stem and stern and being arranged to be used for some ice-braking activity as well as for stationary drilling activity and the hull of the ship has upper sides inclined with an angle of between 30 and 60 degrees with reference to an imagined horizontal line, up to a level exceeding the deepest active water-line of the ship.

By using the ship for some ice-braking activity as well as for stationary activity at ocean, such as drilling activity, it is important to prevent broken up ice and/or other ice flowing by to be brought in under the ship and make it a mess, but instead compulsory act to move the ice aside in a direction away from the ship.

US-A-5 036 781 defines a ship where free ice pieces (12) are blown away from the ship e.g. by having the propeller flow from the driving propeller (3) of the ship to be diverted by the rudder (6) of the ship, but in first hand air is exhausted such that it is conducted sideways and along the ship (1 ).

Thus inclined rudder propellers (3) are shown therein which can be rotated such that the propeller flow is directed obliquely upwards, out towards ice located in side thereof and the supplied air from a fan (7) creates air bubbles transferred towards the ice to bring it away from the ship.

WO-A2-2010/ 03 170 refers to a craft provided with channels (6) through the hull and in which channels water is driven by propellers (8) to be directed obliquely upwards towards existing ice.

US-A-4 434 741 refers to a drilling platform having an ice breaking activity and the construction and function of which is discussed in the

introduction of the description.

Therefore the main purpose of the present invention is in first hand to solve the problems mentioned above.

This purpose is obtained by a device according to the present invention mainly characterized in that several thrusters are distributed along and carried by the bottom of the ship, essentially tight against the bottom hull of the ship, which bottom hull is angled and has inclined lower bottom sides with an angle of between 10 and 30 degrees with reference to an imagined horizontal line, or in that the thrusters are carried by a cantilever producing a desired flow direction for the thrusters by an angle obliquely up towards the water surface surrounding the ship in water, and in that said angle by which the thrusters produce the flow of water directed towards the water surface is between 10 and 30 degrees with reference to an imagined horizontal line.

A ship being equipped with these inclined bottom sides used for carrying rotatable thrusters make it possible to effectively get away the ice from the ship. The angles a and β mentioned in the claims result in excellent ice discharging qualities shown by practical tests with the ship.

The invention is described below as a preferred embodiment, whereby reference is made to the drawings enclosed, where

Fig. 1 shows a cross-section of an example of a preferred hull design,

Fig. 2 shows schematically a preferred design of the stem of the ship,

Figs. 3 - 4 show a cross-section and a view from above respectively of a craft provided with a moon pool, i.e. an opening with a shaft belonging thereto in the bottom of the ship,

Figs. 5 - 6 show a cross-section and a view from above respectively of a craft provided with wing shaped diverters in front of the thrusters,

Fig. 7 shows a cross-section of a craft having thrusters mounted on an inclined part of the hull,

Fig. 8 shows a view from above of one end of the craft and the thrusters mounted thereto,

Figs. 9 - 10 show a front view and a view from above respectively of the craft with the mounted thrusters rotated with respect to a stop bolt,

Figs. 11 - 12 show views from above of the craft having the thrusters rotated for ice breaking of thin ice at a high speed and for ice breaking of thicker ice at a lower speed respectively, Figs. 13 - 14 show views from above of the craft having the thrusters rotated in different positions,

Fig. 5 shows a front view of a thruster and

Fig. 16 shows a cross-section of a ship having alternative hull shapes and thrusters provided thereto for obtaining of a desired function.

Background

Drilling has been performed by floating units in moving ice at least since the 1970's when Dome Petroleum started extensive exploratory operations in the Canadian Beaufort Ocean. By early 1980's the fleet consisted of four ice strengthened drill ships, two icebreakers capable of breaking about 1.5 m thick ice and four smaller icebreakers capable of breaking about 0.6 m thick ice in the continuous mode of operation. The drill ships had a hull form intended for operation in open water but they were still capable of staying on location until early December when the ice reached a thickness of about 0.6 m when assisted by several icebreakers managing the incoming ice. Due to the small water depth the drill ships were anchored to the ocean floor and thus on occasion presented their maximum length towards the ice movement.

At this time Gulf Canada introduced a competing drilling fleet designed to stay on location until the end of January when the ice reaches a thickness of 1.2 m in the Beaufort Ocean. The drilling unit was a round drill ship with sloping sides intended to break the ice in the downwards direction. The drill ship was assisted by four icebreakers capable of breaking about 1.5 m thick in the continuous mode of operation. The round hull form proved to be unfavourable in open water conditions, increasing down time in a ocean state compared to normal drill ships. The round drill ship was anchored to the ocean floor and drilled successfully in moving ice of about 1 m thickness.

One of the Dome drill ships was fitted with a powerful air bubbling system along the sides of the ship which created a strong surface current directed away from the parallel mid body and thus made in easier for the broken ice to get around the unit. Since the 1990's a more efficient method to reach the same result in the form of rotatable thrusters creating a strong surface current has been tested successfully on several icebreakers intended for operation in first year ice conditions.

When drilling a 400 m core close to the North Pole in about one thousand meters water depth it was proven that with sufficient icebreaking support it is possible to stay on location even in moving multi-year ice of considerable thickness.

Several dynamically positioned drill ships are known for operation in deep water, each fitted with six rotatable thrusters mounted below the bottom of the ship in order to optimize the station keeping capability of these units. One of these drill ships is provided with a hull and propulsion ice strengthening which allows it to operate virtually in any ice conditions when assisted by icebreakers.

If an assisting icebreaker is able to provide an ice free environment around this icebreaker they will be able to operate in very severe ice conditions when occasionally assisted by additional icebreakers.

The Invention

A novel hull form in combination with thrusters below the bottom of the ship that direct their propeller stream towards the ocean surface in order to keep ice from close contact with the sides of the ship establish the principal feature of this invention.

The body plan of the hull is shown in Fig. 1 which manifests that the bow and the stern are essentially identical in order to operate in opposite directions while drilling in the dynamic positioning mode. This is important when the ice movement discontinues for some period of time and, as frequently is the case, starts again in the opposite direction.

Fig. 1 displays an inclined bottom portion over the entire length of this section under which rotatable thrusters may be mounted in such a manner that their propeller stream encounters the ocean surface around the entire ship. Between the sloped bottom and some distance above the deepest operating water line the side of the ship along its entire length is sloped outwards at a large angle, in Fig. 1 shown as 45 degrees, in order to facilitate turning of the ship in heavy ice conditions.

Fig. 2 indicates a fairly conventional wedge shaped and pointed bow and stern portion. This hull form is not optimal for the breaking of ice but due to the efficiency of the bottom mounted thrusters in transporting broken ice along the hull the performance of the unit in heavy ice will nevertheless be exceptional. The wedge shaped stem and stern portion is essential when staying on location as it causes the broken ice to move sideways and not downwards and under the bottom of the ship.

As the thrusters protrude below the bottom of the main hull it is possible to introduce a central wing shaped ice diverter close to a amidships portion in order to deflect broken ice away from the moon pool area, as shown in Figs. 3 and 4, without increasing the minimum water depth in which the unit is able to operate. This also makes it possible to introduce two additional wing shaped diverters at both ends of the ship in order to provide protection for the thrusters by stopping progress before riding too high up on thick multi-year ice as shown in Figs. 5 and 6. By protruding well below the bottom of the hull the three ice diverters also function as very efficient roll motion reducers in a ocean state. If there is a docking facility available that has the capability to accommodate the draft including the total vertical extent of the ice diverters then these may be connected permanently to the bottom of the main hull, otherwise the ice diverters have to be connected to the hull in the floating mode in the same manner as the thrusters.

In order for the thrusters to flush the hull and surroundings equally on both sides as well as at both ends of the ship it becomes obvious that they must be located in pairs both transversely and longitudinally from which it follows that the number of thrusters must be divisible by four or in other words that their number must follow the series 4, 8, 12 and so on. In the examples used in this presentation the number of thrusters is kept constant at eight.

The body plan with the thrusters in the icebreaking mode is shown in Fig.

7 and the lines plan with the thrusters in the icebreaking mode is shown in Fig. 8. The thrusters closest to the bow are directed in or close to the longitudinal direction in order not to interfere with the thrusters closer to amidships. The central ice diverter will direct the propeller stream towards the ship's side, as indicated in Fig. 8, and thus this propeller stream will also augment the sideways transport of broken ice.

The body plan with the thrusters in the station keeping mode is shown in Fig. 9 and the lines plan with the thrusters in the station keeping mode is shown in Fig. 10. As the thrusters are attached to the inclined bottom portion of the hull the propeller stream will meet the ocean surface some distance from the hull and thus the outward surface current so created will transport broken ice away from the hull which creates a lower concentration of broken ice at this location, reducing the ice load on the unit and at the same time make it easy to turn the ship into the direction of the incoming ice.

Fig. 1 1 shows the underwater hull while breaking thinner ice at high speed. Providing extra room for broken ice at the stern is not necessary at high speed operation and thus the stern propellers are turned to maximize forward thrust without their propeller streams influencing the operation of other propellers.

When operating at slow speed in heavier ice it becomes critical to provide extra room for broken ice at the stern by directing the propeller streams of the propellers close to the stern away from the ship, as shown in Fig. 12. As long as the thrusters create an unobstructed path for the broken ice to move behind the ship the forward progress will continue and may be augmented by also using the stern thrusters to create an oscillating sideways movement of the stern in order to further loosen the grip of the ice. The ice clearing action provided by the thrusters makes it impossible for the ship to become stuck in ice even if the ice is under heavy pressure.

Fig. 13 shows the ship while reducing the amount of ice in an already broken channel in order to make it possible for a wider or otherwise less capable ship to navigate in heavy ice. The same procedure may be used when assisting another drill ship to stay on location in heavy ice. This capability may prove to be essential in the unlikely event that a relief well has to be drilled in heavy ice conditions.

The principal operation of the thrusters while working in dynamic positioning mode is shown in Fig. 14. Having eight thrusters makes it possible to create the necessary transverse and longitudinal forces without having to direct the propeller streams towards the centre of the ship, thus making it possible to always keep a surface current directed away from the unit. If mooring winches provided with underwater fairleads are added to the unit it will be capable of staying on location also in shallow water. As the thrusters are capable of keeping the unit on location even if one of the mooring wires should fail it will be possible to operate with six mooring systems. The mooring pattern with mooring lines at 90 degree angles against each other will create a system that only allows minor excursions from the desirable drilling locations.

The thrusters located at the forward end of the unit when advancing in heavy ice will come in contact with substantial pieces of ice and will thus have to be capable of dealing with considerably larger forces than those experienced in open water operation. The nozzle makes it impossible for large pieces of ice to contact the propeller blades and thus the propeller and the gear driving the propeller have to be strengthened only in order to deal with the largest piece of ice that is able to gain entrance into the nozzle. In order to reduce the size of design ice piece and at the same time make it more difficult for a large piece of ice to block the flow of water into the nozzle a support structure in front of the nozzle has been developed. Four wing shaped structures placed at 90 degree angles in front of the nozzle reduce the size of ice piece that is able to flow into the nozzle and at the same time add considerable strength to the nozzle. The almost vertical support structure is the widest, it provides passage for the driving shaft and any pipes and cables needed at the propeller hub, and is provided with a virtually vertical leading edge in order to turn sideways large pieces of ice. The three remaining support structures are provided with a backward leaning leading edge in order to push large ice pieces away from the centre of the propeller. The outermost part of these three structures protrudes well in front of the nozzle in order to reduce the tendency for the ice piece to remain in front of the nozzle and thus to impede the free flow of water into the propeller.

The loads on the main thrusters bearing and the turning gear caused by the nozzle coming into contact with large pieces of ice will be considerably higher than those caused by operating in water without ice being present. In order to unload the turning gear the top of the nozzle is provided with a wing shaped structure extending a considerable distance behind the nozzle. The trailing edge of the wing shaped structure will come into contact with a stopper when the nozzle is turned in the optimum direction for ice breaking operations. The turning gear presses the wing shaped structure against the stopper with the force the turning gear is designed for and the stopper is placed in a location that causes the load on the stopper to increase when large pieces of ice come in contact with the thrusters' configuration. The stopper resists the entire addition to the turning moment caused by contact with ice and thus the turning gear is not loaded beyond its design moment. The stopper is also capable of carrying the additional longitudinal load caused by ice forces and thus the main bearing resisting longitudinal forces is not overloaded by ice contacting the thrusters configuration. The distance between stopper and main bearing cannot be manufactured with as small a tolerance as the main bearing and thus it is important that contact between stopper and wing shaped structure depends on the rotation of the thrusters unit, which will correct any difference in tolerances.

More in detail the present invention includes a ship 2 having essentially identically shaped stem 23 and stern 24 and being arranged to be used for certain ice breaking actions and also for a stationary drilling action as well. The hull 11 of the ship 2 has sides 10 inclined with an angle a of between 30 and 60 degrees referred to an imagined horizontal line 1 up to a level 100 exceeding the deepest active water-line 15 of the ship. According to the invention there are a number of thrusters 3 distributed along and carried by the hull 11 on the bottom 102 of the ship 2, such that active water flow 12 of the thrusters is directed by an angle β obliquely up towards the water surface 15 surrounding the ship 2 in the water 103.

The angle β with which the thrusters 3 produce the water flow in the direction 12 towards the water surface 15 between 10 and 30 degrees with reference to an imagined horizontal line 104, or more preferred between 15 and 20 degrees, with which angle β also said lower bottom sides 14 can be inclined.

As mentioned above the present invention refers to a ship 2 being arranged to be used by drilling work at ocean and having at least some ice breaking capacity. Said ship 2 is equipped with a number of rotatable thrusters 3. Said thrusters 3 are on one hand arranged to divert free ice 4 in the dynamic positioning of the ship 2 and on the other hand when used as ice breaker in desired directions 5, 6 to divert loose broken ice 7 in a direction away from the ship 2. Said thrusters 3 are carried rotatable and bear to a stopping 8.

Said thrusters 3 are carried directly or indirectly on the hull 1 1 and with such direction that they act obliquely up 12 and out from the length centre line 13 of the hull. Either the thrusters 3 are carried on a inclined bottom hull part 14, having an extension mainly along the whole length of the ship, and are inclined with an angle β of between 10 and 30 and most preferred between 15 and 20 degrees with respect to the water line 15 of the ship, or the thrusters are carried of a cantilever 105 being arranged in such a way that it generates the desired flow direction 12 towards the water surface 15 for the thrusters 3.

Each one of said thrusters 3 has an end part 16 projecting mainly in a horizontal direction and in the end position of the thrusters belonging thereto it is arranged to bear to said stopping 8. The bear part 7 of said end part 16 and the stopping 8 have congruent shapes, preferably a straight inclined surface 18, 19.

The arrangement and distribution of thrusters 3 at the ship 2 has been chosen in such a way that the result will be an equal drive and an equal action thereof. Therefore, there preferably are an even number of thrusters 3 distributed at the bottom hull 1 1 , in view from amidships across the ship 2, along an imaginary transverse line 20, preferably in the series 4,6,8,10,12 and so on or in the series 4, 8, 12, ... and so on. Furthermore the thrusters 3 suitably are distributed in pairs lengthwise 5, 6 as well as sideways 27 under a bottom part 1 of the ship 2.

At the embodiment shown there are an equal number of thrusters 3 carried at each side A, B about a vertically downwards directed 21 protruding, centred underlying longitudinal thickening 22, similar to a keel, on the hull sides 1 1 of the ship, the thrusters 3 being rotatable around an axis, preferably 360 degrees.

To make ice breaking possible with the ship 2 in both travel directions 5, 6 thereof, the ship 2 has a pointed wedge shaped stem 23 and stern 24.

A number of said bodies 22, 25 are located at and arranged to extend along the centre 13 of the ship. Of these bodies, having a pointed shape at stem and stern, one body, the centre body 22, has mainly vertical side surfaces.

The main thing is that the propeller flow 12 from the thrusters 3 is directed towards the water surface 15 with an angle β of between 10 and 30 degrees and that the ship sides above that have an angle of between 30 and 60 degrees to make it possible to rotate the ship at the operation with dynamical positioning and to minimize the force of the ice when drilling at an anchored position.

All the three bodies 22, 25 are located along the centre 13 of the ship and common for them is that they in open water, i.e. in waves, drastically reduce the rolling of the ship, which is very important when drilling. The centre body 22 acts as an ice deflector as it pushes the broken ice in side length travelling forward as well as astern, and thereby the risk of having ice in contact with an existing drilling tube, connecting a derrick with the equipment at the bottom of the ocean, is drastically minimized. The bodies 25 at stem and stern act as stops to prevent that the ship at approach ice breaking rides too high up on multi-year ice, which could result in damages to the thrusters 3.

The nature and function of the present invention ought to have been understood by that described above and by help of that shown by the drawings. By thrusters is in first hand to be understood a motor driven propeller which with or without an angle gear is connected to the ship 2 and its hull 1 1. Also other drive force generators than thrusters 3 shown and described above can be used, such as for instance water jets etc. The thrusters 3 are rotatably carried about a respective axis 26 extending transversely to the inclined bottom part 14 of the hull 1. Preferably the thrusters 3 are rotatable about 360 degrees before a stop position is obtained between the stopping 8 and its bear part 17 and the bear part of the end part 16 of the respective thrusters 3.

Of course the invention is not restricted to the embodiments described above and shown on the drawings enclosed. Modifications are possible, in particular when it comes to the nature of the different parts or by making use of equivalent technique without departing from the scope of the invention such as it is defined by the claims.