WO2015104026A1

WO2015104026A1 - Method to access an offshore construction in rough waters

Info

Publication number: WO2015104026A1
Application number: PCT/DK2015/050003
Authority: WO
Inventors: Jørgen Holm CHRISTENSEN
Original assignee: C-Bed Holding B.V.
Priority date: 2014-01-10
Filing date: 2015-01-09
Publication date: 2015-07-16

Abstract

A method for accessing an offshore construction (1) in rough waters, including at least a first vessel (2) and a second vessel (3), wherein the first vessel (2) is positioned at a distance (5) from the offshore construction (1) and to the side of the offshore construction (1) from where the waves are beating, whereas the second vessel (3) is subsequently positioned such that the second vessel (3) lies in immediate vicinity of or in direct contact with the offshore construction (1) and in lee of the first vessel (2), where it is the object to provide a method for accessing offshore constructions (1), where the method will be capable of operating under difficult weather conditions, thereby expanding the service weather window.

Description

Method to access an offshore construction in rough waters

The present invention concerns a method for accessing an offshore construction in rough waters, including at least a first vessel and a second vessel.

Prior art

It commonly known to service offshore constructions by crews who are transported by means of so-called service vessels that access the constructions from the seaward side.

Such servicing frequently take place by means of small vessels that are capable of picking up/bringing crew/equipment and similar from/to the offshore constructions by the following steps, where the vessel :

1. puts the stem up against the wave direction; and

2. puts the stem against the offshore construction; and

3. maintains the position by means of the propelling machinery of the vessel, and as far as possible in continuous contact with the offshore construction.

A drawback of this method is that by experience waves of a size over about 1.5 m cause problems when accessing the constructions with a service vessel, as the vessel is pitching so much that it is indefensible with regard to safety to let the crew work under such conditions.

By certain wind and wave conditions it is thus possible to service an offshore construction without problems whereas by higher wind speeds and/or wave speeds/wave sizes it is not defensible to proceed by the prior art method. It can be said that there is a service weather window within which work can be performed without any problems under certain circumstances.

Another drawback of this method is that there may be great consequences, technical as well as economical, if the servicing of an offshore construction has to be called off. For example, the case may be an offshore construction that includes a wind turbine with operating problems where the wind turbine can sustain greater damage the more time is passing before the service crew can access the construction, and furthermore the wind turbine will not generate any profit in case of a breakdown or a standstill for other reasons. A third drawback by the prior art is that the current service vessels often have to sail relatively far between the offshore constructions and that the facilities used for crew, workshop and storage, such as a so-called floating hotel, are often provided at a fixed position far from the workplace.

From US 2011/0002739 Al it is known to use a unit which is moored in parallel with a coast with the front against wind direction and wave direction, and where between the unit and the coast it is possible to for a vessel to operate against the wind or with the wind, and to moor/anchor with the bow or the stern towards the wind.

Therefore, by this technique there is not indicated a method for accessing an offshore construction in rough waters, and the technique will therefore not be suited either for this purpose as the unit is to be shipped to the desired position by a vessel, and the unit is to be moored/anchored to the seabed, the unit then being stationary and therefore unable to compensate for changes in wind direction and wave direction. In practice this means that a vessel is to transport the unit by itself, moor/anchor the unit by itself and then bring itself into a position between the unit and an offshore construction where the position is to be parallel with the actual wind direction and wave direction. If the vessel is thus to operate alone, it is to be of a certain size in order to transport and handle the unit, and consequently it will impossible in practice on the whole and in any case neither practical, rapid, flexible, defensible nor in other ways expedient to operate in rough waters between offshore constructions.

It is known from WO 2011/093704 Al to use a mother ship and a number of transport ships in connection with maintenance of an offshore wind turbine farm, where the mother ship is designed as floating harbour with an inner basin, with an opening at the aft end of the ship. When the mother ship is luffing and puts the stem up against the wind, lee is created at the aft end of the ship by which the transport ships, which e.g. are 8-10 times smaller than the mother ship, can safely sail out of the basin or into the basin for mooring.

By this technique there is not indicated a method for accessing an offshore construction in rough waters either, and the technique will therefore not be ^L ' either for this purpose as it will require cooperation between the mother ship and only a single transport ship, whereby the other transport ships then will be unable to operate and sail out of or into the basin in the mother ship as the sailing in/out will not be in lee.

In practice, this means that the mother ship by itself is to move a transport ship out to the offshore construction, is to bring itself into a position with the stem up against the wind for providing lee at the aft end of the ship, after which the transport ship is to sail out of the basin whereafter the mother ship is to change position in order to provide lee for the transport ship sailing to and operating at the offshore construction, after which the same procedure is repeated in reverse order. This method is not indicated by the prior art where the mother ship may well provide lee for a transport ship, but lee with the purpose of a transport ship accessing mother ship, and where in practice it will largely be impossible and in any case neither practical, rapid, flexible, defensible nor in other ways expedient to operate the said mother ship and transport ship in rough waters between offshore constructions.

Object of the Invention

It is thus the object of the invention to provide a method for accessing an offshore construction of the kind mentioned in the introduction, where the method is able to function under difficult weather conditions, i.e. waves and wind speeds of a certain magnitude, or in other words it is the object of the invention to indicate a method for achieving a larger service weather window.

Moreover, it is an object of the invention to provide conditions that facilitate servicing, ensure a rapid and secure access and transfer of crew and equipment and minimise sailing between the offshore constructions and the necessary facilities for crew, workshop and storage.

It is a further object of the invention to use two individual and dynamic vessels that are capable of adapting their positions according to the existing conditions in connection with accessing an offshore construction. Description of the Invention

According to a first aspect of the invention, the above mentioned object is achieved by a method as indicated in the preamble of claim 1 wherein the first vessel is positioned at a distance from the offshore construction and to the s ^{' '} the offshore construction from where the waves are beating, whereas the second vessel is subsequently positioned such that the second vessel lies in immediate vicinity of or in direct contact with the offshore construction and in lee of the first vessel.

This will enable the first vessel to subdue the waves around the offshore construction and thus subdue the waves in the area in which the second vessel is to access the construction with crew, tools and other equipment, which could be at any position around the offshore construction.

This will therefore enable use of two individual and dynamic vessels that are capable of adapting their positions according to the existing conditions in connection with accessing an offshore construction. The first vessel will then enable the second vessel to operate and thereby access the construction, and notably in such a way that the second vessel can operate all the time irrespective of changes in wind and wave directions as the first vessel will be able to compensate and therefore change its position if so required.

The method will provide a larger service weather window that will enable working estimated 100 more days on an average offshore construction per year.

By an offshore construction is meant wind turbines/marine wind turbines/wind farms/marine wind farms with floating or fixed foundations, devices/ships for laying cables and pipelines as well as tapping points for e.g. oil pipelines, devices/ships performing drainage in the form of e.g. pipe laying/sewerage and deepening or removal of seabed.

By offshore construction is also meant offshore energy constructions such as wave power plants/wave power machines, tidal power stations/tidal power machines, combined offshore energy constructions including e.g. wave power plants and wind turbines. Also, they may be submerged constructions, various submarine plants, submarine installations and pipelines.

The invention can furthermore be applied to a wide range of other constructions located in water, e.g. to constructing and repairing bridges/bridge piers, or to constructing and repairing harbours, jetties and quay constructions. The invention may, for example, reduce the downtime for a turbine located in a wind turbine in a marine wind farm up to 60% by reducing the wave height from e.g. 2.5 m to 1.8 m, additionally adding comfort and minimising the risk of seasickness for the crew.

When constructing and repairing bridges/piers, the second vessel can be replaced by a crew platform/work platform that may be elevated/lowered from the construction itself, and where the first vessel here can function as a lee provider. In a second aspect, the present invention also concerns a method wherein the first vessel is positioned to the side of the offshore construction from where the waves are beating, whereas the second vessel is positioned to the opposite side of the offshore construction. This will enable using the offshore construction as an additional breakwater which is then situated between the first vessel and the second vessel.

In a third aspect, the present invention also concerns a method wherein the longitudinal axis of the first vessel and the longitudinal axis of the second vessel are parallel, however preferably such that the longitudinal axis of the second vessel intersects the longitudinal axis of the first vessel at an angle between 1° and 90°, and preferably at an angle between 45° and 90°.

This enables positioning of the second vessel in any thinkable position in relation to the first vessel, depending on the physical conditions around the offshore construction. They could be additional constructions on/below/above the water surface or the draught of the vessel in relation to the water depth as well as the size of the first vessel in relation to the wave frequency. In a fourth aspect, the present invention also concerns a method wherein the first vessel can use a Dynamic Positioning System.

This will enable keeping at least the first vessel at its desired position as such a computer-controlled system can automatically maintain the position and direction of the vessel by means of the vessel's own main and auxiliary machines for driving propellers and thrusters, for example. By a desired position is meant a position lying within a range of plus/minus 0.5 m of the determined position, where the determined position of the first vessel is located at a distance fn offshore construction in the interval 1-100 m, preferably in the interval 25-75 m and further preferred in the interval 35-65 m.

In a fifth aspect, the present invention also concerns a method wherein the first vessel can have a number of lee providers/floating barriers/breakwaters in the form of one or more heavy members that may pivot about points/axes at one or both sides of the first vessel, e.g. the bow and/or quarter of the first vessel.

This will enable further subduing of the waves, also providing possible screening from the wind where the waves may come from one direction and the wind from another direction.

In a sixth aspect, the present invention also concerns a method wherein the draught of the lee providers/floating barriers/breakwaters can be adjusted.

This will enable lowering these lee providers/floating barriers/breakwaters to a draught longitudinally of, transversely of, or in many other constellations relative to the first vessel such that the waves that e.g. goes under the first vessel that usually does not have any appreciable possibility of adjusting the draught can be further subdued by the lee providers/floating barriers/breakwaters.

In a seventh aspect, the present invention also concerns a method wherein the first vessel is a floating hotel and/or a storage barge and/or a workshop ship and/or a crane barge and/or a ferry. By ferry is here meant a vessel/type of ship that previously had the function as transport means between two ferry ports but where the vessel/type of ship her in addition to a transport function also has at least one other function, namely as lee provider/floating barrier/breakwater.

This will enable having all necessary measures in the form of food and accommodation, storage and workshop facilities within a very short distance from the offshore constructions, and such that the second vessel does not have to sail very far for crew and such.

In an eighth aspect, the present invention also concerns a method wherein the first vessel has a length of 25-200 metres and width of 5-30 metres.

The first vessel has a length in the interval 25-200 m, preferably in the interval 75-150 m, and further preferred in the interval 80-125 m, and a width interval 5-30 m, preferably in the interval 10-25 m and further preferred in the interval 15-20 m.

This will enable operation as a floating hotel and/or storage barge and/or a workshop barge and/or a crane barge and/or a ferry, and with such a size that the vessel is not just floating upon the wave crests but actually will be able to subdue them.

In a ninth aspect, the present invention also concerns a method wherein the first vessel is positioned along the wave crests.

This will enable subduing of the waves as the first vessel rolls longitudinally of the wave crests, thereby creating smooth waters for the second vessel. Besides, the first vessel provides more lee for the wind that if the first vessel was positioned such that the latter broke the waves transversely.

In a tenth aspect, the present invention also concerns a method wherein the first vessel is positioned transversely of the wave crests and possibly with a number of lee providers/floating barriers/breakwaters at a position along the wave crests.

This will also enable subduing the waves, but if the vessel is longer than it is wide the subduing will, however, be less, unless the vessel has a number of lee providers/floating barriers/breakwaters which in applied condition have a size that is at least equal to the length of the vessel.

The Drawing

The invention will now be explained more closely in the following by description of non-limiting embodiments with reference to the drawing, where: Fig. 1 shows the last part of the method including two vessels.

Fig. 2 shows the last part of the method including two vessels and a number of lee providers/floating barriers/breakwaters. The following reference numbers are used on the drawing for the reference numbers used in the detailed part of the description:

1 offshore construction 2 first vessel

3 second vessel

4 wave crest

5 distance

6 lee providers/floating barriers/breakwaters

7 longitudinal axis of first vessel

8 longitudinal axis of second vessel

Detailed Description of Embodiments of the Invention

An example of the last part of a method for accessing an offshore construction in rough waters according to the invention is shown on Figs. 1 and 2.

On Fig. 1 is shown a first vessel 2 positioned at a distance from the offshore construction 1 and to the side of the offshore construction 1 from where the waves are beating, whereas a second vessel 3 is positioned such that it lies in immediate vicinity of or in direct contact with the offshore construction 1 and in lee of the first vessel 2.

The distance of the first vessel 2 from the offshore construction can be from 1 to 100 m, depending on whether the second vessel 3 is to be able to access the construction 1 from any position around the offshore construction 1, and also depending on the size of the second vessel 3. If the second vessel 3 is then to be capable of accessing the construction 1 in such a way that the second vessel 3 will lie between the construction 1 and the first vessel 2, and if the second vessel 3 e.g. 20 m long, the distance can e.g. be 30 m. If the second vessel 3 is to access the construction from the opposite side of the first vessel 2, the distance can be from 1 m and up, but where a distance of half the length of the first vessel between the first vessel 2 and the offshore construction 1 by experience is, however, to be preferred, where the first vessel has a length of 25-200 m.

If the second vessel 3 is not to access the construction 1 from an arbitrary position around the offshore construction 1, the first vessel 2 can, however, also lie in immediate vicinity of or in direct contact with the offshore construction, be moored to the offshore construction or to one or more buoys in the vicinity.

On Figs. 1 and 2 appears that the first vessel 2 is positioned to the side of the offshore construction 1 from where the waves are beating, whereas the second vessel 3 is positioned to the opposite side of the offshore construction 1, which in most cases will be optimal, but in no way limiting to the invention.

On Figs. 1 and 2 also appears that the longitudinal axis 7 of the first vessel 2 and the longitudinal axis 8 of the second vessel 3 are at an angle of 90°. However, there are many other possibilities, from the longitudinal axis 7 of the first vessel 2 and the longitudinal axis 8 of the second vessel 3 being parallel or even coinciding, to the longitudinal axis 8 of the second vessel 3 intersecting the longitudinal axis 7 of the first vessel 2 at an angle from 1 to 90°. This means that the first vessel 2 and the second vessel 3 can have positions between parallel and/or coinciding longitudinal axes 7,8 and mutually perpendicular longitudinal axes 7,8.

This also means that the second vessel 3 can be positioned all the way around the offshore construction 1 if the distance 5 between the first vessel 2 and the offshore construction 1 is large enough and obviously if the conditions allow for it.

At least the first vessel 2 can use a Dynamic Positioning System (not shown on the Figures) which will be an obvious choice for this method. Other systems or no systems are also options, of course, but in case of no other systems being used the method is hampered to a high degree.

It will obviously be an advantage, though not a requirement, if the second vessel 3 had such a system as well, though a manual steering and control of the vessel possibly will be preferred by direct contact between offshore construction 1 and second vessel 3.

It appears on Fig. 2 that the first vessel 2 can have a number of lee providers/floating barriers/breakwaters 6 in the form of one or more heavy members that may pivot about points/axes at one or both sides of the first vessel 2, e.g. the bow and/or quarter of the first vessel 2.

These lee providers/floating barriers/breakwaters 5 can be mounted on the side of the ship, preferably above the water surface for subsequently being moved into the desired position. Whether there are one or more lee providers/floating barriers/breakwaters 5 on a ship side or whether there are one or more lee providers/floating barriers/breakwaters 5 on both ship sides must depend on wish and need at the given conditions. The lee providers/floating barriers/breakwaters 5 can alternatively be on-board on the first vessel 2 for subsequently being lowered into the water and moved into position when/if the need arises.

The draught of the lee providers/floating barriers/breakwaters 5 can be adjusted (not appearing on the Figures) and may advantageously extend to a given depth and to a given height from the water surface whereby the desired subduing action is achieved. This can be done by e.g. pumping ballast in the form of e.g. sea water into tanks on-board the lee providers/floating barriers/breakwaters 5.

However, it will also be possible to use a first vessel 2 in which extra ballast can be received, e.g. sea water, and thereby increase the draught. Adjusting the lee providers/floating barriers/breakwaters 5 can also take place in e.g. hydraulic or other suitable ways.

The lee providers/floating barriers/breakwaters 5 can thus be adjusted depending on whether it is waves or wind which are the most problematic, and it may e.g. be so that the lee providers/floating barriers/breakwaters 5 at one side of the first vessel 2 were far down in relation to the water surface for the sake of the waves, whereas the lee providers/floating barriers/breakwaters 5 on the other side of the first vessel 2 were not so far down in relation to the water surface relatively and for the sake of the wind. The first vessel 2 can be a floating hotel and/or a storage barge and/or a workshop ship and/or a crane barge and/or a ferry (not appearing on the Figures), but any other vessel with or without these facilities that can maintain a desired position will be a possibility as well. The first vessel has e.g. a length of 25-200 metres and width of 5-30 metres which is regarded as necessary in order to function optimally as breakwater, but larger or smaller vessels may of course also be used, though expectedly with less success. On Figs. 1 and 2 the first vessel 2 is positioned along the wave crests 4 at a distance from the offshore construction 1, where the distance 5, as mentioned before, can be greater or lesser depending on desires/needs and conditions at the site. Since the first vessel 2 has the task of subduing the waves as much as possible, it is therefore positioned along the wave crests 4, but can of course be positioned in any thinkable way as an alternative to the optimal position. This means that the first vessel 2 can also lie transversely of the wave crests 4 if it is the most optimal position under the given circumstances.

By a position of the first vessel 2 transversely of the wave crests 4, a number of lee providers/floating barriers/breakwaters 6 can possibly be positioned along the wave crests 4.

This may particularly be an advantage in case that a Dynamic Positioning System is not used on-board the first vessel 2 which therefore more easily will maintain its position transversely of the wave crests 4 than along the wave crests 4.

Claims

1. A method for accessing an offshore construction (1) in rough waters, including at least a first vessel (2) and a second vessel (3), characterised in that the first vessel (2) is positioned at a distance (5) from the offshore construction (1) and to the side of the offshore construction (1) from where the waves are beating, whereas the second vessel (3) is subsequently positioned such that the second vessel (3) lies in immediate vicinity of or in direct contact with the offshore construction (1) and in lee of the first vessel (2).

2. Method according to claim 1, characterised in that the first vessel (2) is positioned to the side of the offshore construction (1) from where the waves are beating, whereas the second vessel (3) is positioned at the opposite side of the offshore construction (1).

3. Method according to any of claims 1 to 2, characterised in that the longitudinal axis (7) of the first vessel (2) and the longitudinal axis (8) of the second vessel (3) are parallel, however preferably such that the longitudinal axis (8) of the second vessel (3) intersects the longitudinal axis (7) of the first vessel (2) at an angle between 1° and 90°, and preferably at an angle between 45° and 90⁰.

4. Method according to any of claims 1 to 3, characterised in that at least the first vessel (2) can use a Dynamic Positioning System.

5. Method according to any of claims 1 to 4, characterised in that the first vessel (2) can have a number of lee providers/floating barriers/breakwaters (6) in the form of one or more heavy members that may pivot about points/axes at one or both sides of the first vessel (2), e.g. the bow and/or quarter of the first vessel (2).

6. Method according to any of claims 1 to 5, characterised in that the draught of the lee providers/floating barriers/breakwaters (6) can be adjusted.

7. Method according to any of claims 1 to 6, characterised in that the first vessel (2) is a floating hotel and/or a storage barge and/or a workshop ship and/or a crane barge and/or a ferry.

8. Method according to any of claims 1 to 7, characterised in that the first vessel has a length of 25-200 metres and width of 5-30 metres.

9. Method according to any of claims 1 to 8, characterised in that the first vessel (2) is positioned along the wave crests (4).

10. Method according to any of claims 1 to 8, characterised in that the first vessel (2) is positioned transversely of the wave crests (4) and possibly with a number of lee providers/floating barriers/breakwaters (6) at a position along the wave crests (4).