Patent Publication Number: US-2023160170-A1

Title: Cage for a monopile of an offshore wind turbine, tower for an offshore wind turbine, offshore wind turbine and method for installing a tower of an offshore wind turbine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application No. 21210049.9, having a filing date of Nov. 23, 2021, the entire contents of which are hereby incorporated by reference. 
     FIELD OF TECHNOLOGY 
     The following relates to a cage for a monopile of an offshore wind turbine, a tower for an offshore wind turbine with such a cage, an offshore wind turbine with such a tower and a method for installing a tower of an offshore wind turbine. 
     BACKGROUND 
     The energy produced by an offshore wind turbine is usually transmitted as electrical energy to another offshore or an onshore installation and finally to an onshore utility grid. For the power transmission of electrical energy, for example high voltage AC cables are used transmitting up to 400 kV. Such electrical cables and their installation are, however, very expensive. Further, offshore wind turbines, in particular of modern wind farms, can be situated quite far from the coast, e.g., with a distance to the coast of 5 km or more. Thus, the energy produced by the wind turbines needs to be transmitted over long distances. 
     An alternative and less expensive way of transporting the energy produced by a wind turbine is converting the electrical energy into a gas, e.g., hydrogen gas, and transporting the gas in pipes to another offshore or onshore installation. The advantage is not only an easier transportation of the produced energy but also that the gas can be directly consumed, for example as fuel in automobiles. For example, a gas producing facility, e.g., an electrolysis equipment generating hydrogen gas from water, is placed at or near the wind turbine to convert the electrical energy into a gas. The produced gas is transported from the gas producing facility at the wind turbine by gas pipes to another offshore or onshore installation and finally to an onshore site. Onshore, the gas can be consumed directly, for example as a fuel in automobiles, or can be converted again into electrical energy in a hydrogen fuel cell to produce again electrical energy for feeding it into a utility grid. 
     SUMMARY 
     An aspect relates to an improved tower for an offshore wind turbine and an improved method for installing a tower of an offshore wind turbine. 
     Accordingly, a cage for a monopile of an offshore wind turbine is provided. The cage is configured to extend, in an installed state, in a height direction of the monopile. The cage comprises one or more supports for holding one or more pipes and/or tubes. The cage further comprises a first inner diameter at a lower portion thereof and a second inner diameter at an upper portion thereof, the second inner diameter being smaller than the first inner diameter. Furthermore, the cage is configured to be arranged on the monopile having a conically shaped outer surface tapered in the height direction such that the upper portion of the cage rests at the conically shaped outer surface of the monopile. 
     By using the cage comprising the one or more supports for holding one or more pipes and/or tubes, pipes and/or tubes can be easier installed and better attached at the monopile. Further, due to the cage being configured to extend, in the installed state, in the height direction of the monopile, the pipes and/or tubes can be guided alongside the monopile, for example from a platform/device of the wind turbine to the seabed. 
     The cage having a smaller inner diameter at an upper portion thereof can be attached to the monopile solely by resting on the conically shaped outer surface of the monopile. In particular, the cage can be lowered from above on the monopile until it gets stuck when the upper portion of the cage with the smaller inner diameter engages the conically shaped outer surface of the monopile. Further, an attachment of the cage to the monopile can be ensured, for example solely, by the weight of the cage. Therefore, attaching the cage to the monopile does not require welding, screwing or another mechanism altering the structure of the monopile. Hence, installing the cage at the erected monopile is easy. Furthermore, a welding process which can weaken the structure of the monopile and, thus, the foundation of the wind turbine is avoided. 
     Further, the proposed cage can be installed on the monopile after piling the monopile into the seabed. Therefore, damaging the cage during the piling of the monopile into the seabed is avoided. 
     The cage is, for example, a rack. The cage is, for example made from a material comprising metal. The cage comprises, for example, several rods. Some or all of the rods may, for example, extend in the height direction of the monopile. The cage comprises, for example, one or more rings (annular elements). The one or more rings may, for example, be connected with each other via the one or more of the rods. The upper portion of the cage comprising the second (smaller) inner diameter may, for example, be and/or include one of the rings. The lower portion of the cage comprising the first (larger) inner diameter may, for example, include another one of the rings. 
     The conically shaped outer surface of the monopile is, in particular, tapered in the height direction from a first outer diameter to a second outer diameter. The first inner diameter of the cage is, in particular, larger than the first outer diameter of the monopile. Further, the second inner diameter of the cage is, in particular, smaller than the first outer diameter of the monopile and larger than the second outer diameter of the monopile. 
     The height direction of the monopile is, in particular a height direction of the monopile and the wind turbine in the erected state. The upper portion of the cage is, in particular, an upper portion with respect to the height direction. Further, the lower portion of the cage is, in particular, a lower portion with respect to the height direction. 
     The one or more pipes include, for example, pipes for transporting energy, e.g., energy produced by the wind turbine. The one or more pipes include, for example, pipes for transporting gas and/or electrical energy. The one or more pipes may also include, for example, pipes for transporting data and/or signals to or from one or more devices of the wind turbine to or from another offshore and/or onshore installation. Further, the one or more pipes may also include, for example, pipes for transporting water, e.g., seawater, to one or more devices of the wind turbine. The water may be used, for example, for cooling and/or electrolysis processes. 
     Pipes for transporting gas are, in particular, gas-tight pipes. The one or more pipes include, for example, pipes made from metal, steel and/or a material comprising metal and/or steel. The material of the pipes may, in particular, be a gas-tight and/or durable material. 
     The wind turbine is an apparatus to convert the wind&#39;s kinetic energy into electrical energy. The wind turbine comprises, for example, a rotor having one or more blades connected each to a hub, a nacelle including a generator, and a tower holding, at its top end, the nacelle. The tower of the wind turbine comprises, in particular, the monopile as a foundation. The monopile is, in particular, driven, e.g., piled or drilled, into a seabed or a ground of a lake or another open water. The tower of the wind turbine comprises, furthermore, a main portion. The main portion is connected at its bottom end to the monopile and is holding at its top end the nacelle. 
     The wind turbine is, for example, an offshore wind turbine. Offshore does not only include marine environments but also lakes and other open waters. 
     According to an embodiment, the cage comprises one or more tubes connected to the one or more supports and extending in the height direction, the one or more tubes being configured for accommodating one or more pipes. 
     The one or more tubes are, in particular, hollow tubes. Each of the one or more tubes has, in particular, a first opening at a bottom end thereof and a second opening at a top end thereof, the first and second openings being openings of an interior channel of a respective tube. 
     The one or more tubes are, for example, configured to accommodate pipes and/or cables and, thereby, guide and protect them. The one or more tubes are, for example, configured to accommodate energy transport pipes/cables, gas pipes, electrical cables, optical cables (e.g., optical fiber cables), data cables, communication cables and/or water pipes. The tubes can be used as guiding means when installing the pipes and/or cables. For example, the pipes and/or cables can be pushed or pulled through the tubes. 
     The guiding function of the tubes is of particular advantage in the case of gas pipes which are made of a stiffer material compared to electrical cables to ensure gas-tightness. Gas-tight pipes comprise, for example, metal and/or steel. Due to the stiffer material, gas pipes are more difficult to bend into the required curvature. Hence, by pushing or pulling the gas pipes through the tubes, an installation of the gas pipes can be easier performed. 
     According to a further embodiment, the one or more tubes are J-shaped tubes, comprise a bell-shaped mouth at a lower end thereof, comprise inside a pre-installed messenger wire and/or comprise a float seal sealing an opening of the respective tube at a lower end thereof. 
     A J-tube comprises, in particular, a first portion extending in the height direction of the monopile, and a second portion continuously connected to the first portion. The second portion is bent (e.g., in a smooth curve) relative to the first portion to form the J-shape. A bending angle of the J-tube may have, for example, a value in the range of 15 to 75 degree, 25 to 65 degree and/or 35 to 55 degree and/or have a value of 45 degree. A bending angle of the J-tube is, in particular, an angle by which the second portion is bent from the height direction. 
     One, some or all of the tubes may have a bell-shaped mouth at a lower end thereof. That means, the first opening of the respective tube may have the bell-shaped mouth. Thus, the first opening is, in particular, larger than a diameter and/or cross-section of the respective tube. Due to the bell-shaped mouth, insertion of a pipe, for example of a relatively stiff pipe, into the tube is simplified. 
     The messenger wire is, in particular, a pulling rope. The messenger wire includes, for example, a pulling head. The messenger wire can be used to pull an end of a pipe into and through the tube. 
     The float seal of a respective tube is, for example, connected with the messenger wire of this tube. The float seal is used for sealing the opening of the respective tube at the lower end thereof during installation of the tube at the cage and/or during installation of the cage with the tube at the monopile. After inserting a respective pipe through the respective tube, the float seal is floating to the water line and can be easily collected. 
     According to a further embodiment, the one or more supports each include a loop for guiding one or pipes through the respective loop. 
     By providing the cage with one or more supports each having a loop, the pipes can be attached to the monopile by inserting them through the one or more loops. Thus, installation of pipes at the monopile is simplified, in particular when this installation step is carried out partly below the water level (sea level). 
     The loop is, for example, a closed loop. The loop is, for example, integrally formed with the respective support. 
     According to a further embodiment, the cage comprises one or more flexible elements configured for being arranged between the conically shaped outer surface of the monopile and an inner surface of the upper portion of the cage. 
     Thus, a direct contact between the cage and the outer surface of the monopile is avoided. Hence, scratching of the cage at the outer surface of the monopile can be avoided. This is the case even when slight movements of the monopile occur during installation of the main portion of the tower and during operation of the wind turbine under the influence of wind on the rotor or directly on the monopile due to water current and waves. In particular, a direct contact between the cage and the monopile is avoided in the region where the upper portion of the cage is resting on the outer surface of the monopile. 
     For example, the upper portion of the cage comprising the second inner diameter rests via the flexible elements at the conically shaped outer surface of the monopile. 
     The flexible elements are, for example, elastic elements capable to deform under the influence of an external force. 
     According to a further embodiment, the one or more flexible elements comprise rubber, neoprene, nylon, polymer, a fabric, a mat and/or a non-corrosive material. 
     The flexible elements comprising a non-corrosive material helps preventing corrosion of the cage and/or the monopile. 
     According to a further embodiment, the cage comprises in a top portion thereof a landing stage for a vessel. 
     Thus, a landing stage (platform) can be easier installed. 
     The top portion of the cage is a top portion with respect to the height direction of the monopile. The top portion of the cage is, for example, arranged above the upper portion of the cage comprising the second inner diameter. 
     According to a further embodiment, the cage comprises at least two tubes separated from each other by a predetermined angle as seen in the height direction. 
     The predetermined angle is, for example, an angle of 90 degrees or an angle of 180 degrees. However, the angle may also have a different value than 90 or 180 degrees. 
     Having the tubes positioned in a standard angle, the cage can be advantageously positioned such that a route between the tubes of the cage and tubes of a cage of a neighboring wind turbine is short. 
     According to a further embodiment, the cage comprises several rods extending in the height direction and one or more rings connected with each other via the rods, wherein the one or more rings are releasably closed rings. 
     Each of the one or more rings comprises, for example, at least two ring elements (e.g., two half ring elements) releasably connected to each other, e.g., bolted to each other. 
     Thus, the cage can be easily replaced by a new cage. The new cage can be assembled and lowered to lock its position. 
     In embodiments, the one or more rings may also be inextricable closed rings. 
     According to a further aspect, a cage for a transition piece of an offshore wind turbine is provided. The cage is configured to extend, in an installed state, in a height direction of the transition piece. Further, the cage comprises one or more supports for holding one or more pipes and/or tubes, and a first inner diameter at a lower portion thereof and a second inner diameter at an upper portion thereof, the second inner diameter being smaller than the first inner diameter. Furthermore, the cage is configured to be arranged on the transition piece having a conically shaped outer surface tapered in the height direction such that the upper portion of the cage rests at the conically shaped outer surface of the transition piece. 
     The transition piece may be installed on a monopile driven into the seabed or on a floating foundation. 
     According to a further aspect, a tower for an offshore wind turbine is provided. The tower comprises a monopile or a transition piece with a conically shaped outer surface and a cage as described above. Further, the cage is arranged on the monopile or the transition piece such that the upper portion of the cage rests at the conically shaped outer surface of the monopile or the transition piece. 
     According to an embodiment of the further aspect, the conically shaped outer surface is tapered in the height direction from a first outer diameter to a second outer diameter of the monopile or the transition piece. Further, the first inner diameter of the cage is larger than the first outer diameter. Furthermore, the second inner diameter of the cage is smaller than the first outer diameter and larger than the second outer diameter. 
     In particular, the monopile comprises at least one conically shaped section tapered in the height direction from the first outer diameter to the second outer diameter. 
     According to a further embodiment of the further aspect, the tower comprises a gas producing facility and one or more pipes fluidly connected with a gas producing unit of the gas producing facility, wherein the one or more pipes are configured for transporting a gas produced in the gas producing unit to another offshore and/or onshore installation, and the one or more pipes are attached by the supports to the cage or the one or more pipes are guided through one or more tubes of the cage. 
     The one or more pipes are, for example, gas pipes (e.g., gas-tight pipes). 
     The one or more tubes penetrate, for example, a platform of the gas producing facility. 
     The gas producing facility comprises the gas producing unit, the gas producing unit producing gas, for example, by electrolysis. The gas producing unit converts, for example, water (e.g., seawater) into hydrogen gas by using the electrical current produced by the generator of the wind turbine. Thereby, water is separated into hydrogen and oxygen by an electrolysis process in the gas producing unit. 
     According to a further aspect, an offshore wind turbine with an above-described tower is provided. 
     According to a further aspect, a method for installing a tower of an offshore wind turbine is provided. The tower comprises a monopile or transition piece and a cage. The monopile or transition piece has a conically shaped outer surface tapered in a height direction of the monopile or transition piece in the erected state. The cage extends in the height direction and comprises one or more supports for holding one or more pipes and/or tubes. The cage further comprises a first inner diameter at a lower portion thereof and a second inner diameter at an upper portion thereof, the second inner diameter being smaller than the first inner diameter. The method comprises the steps of: 
     lowering the cage onto the erected monopile or transition piece, and 
     arranging the cage on the monopile or transition piece such that the upper portion of the cage rests at the conically shaped outer surface of the monopile or transition piece. 
     The method may further comprise the step of arranging the monopile in a seabed or ground of a lake or another open water such that the monopile is extending in a height direction. 
     The method may further comprise the step of installing one or more pipes at the cage. The one or more pipes are, for example, either directly attached at the cage or inserted into tubes of the cage. 
     The embodiments and features described with reference to the cage of the present invention apply mutatis mutandis to the tower, the offshore wind turbine and the method of the present invention and vice versa. 
     Further possible implementations or alternative solutions of embodiments of the invention also encompass combinations—that are not explicitly mentioned herein—of features described above or below with regard to the embodiments. The person skilled in the art may also add individual or isolated aspects and features to the most basic form of the invention. 
    
    
     
       BRIEF DESCRIPTION 
       Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein: 
         FIG.  1    shows a wind turbine according to an embodiment; 
         FIG.  2    shows a partial view of a monopile of the wind turbine of  FIG.  1   ; 
         FIG.  3    shows a cage of the wind turbine of  FIG.  1   , the cage comprising J-shaped tubes; 
         FIG.  4    shows a ring of the cage of  FIG.  3   ; 
         FIG.  5    shows a cage according to a further embodiment, the cage being arranged on the monopile of the wind turbine of  FIG.  1   ; 
         FIG.  6    shows a partial view of a J-shaped tube of the cage of  FIG.  3   ; 
         FIG.  7    shows a cage of the wind turbine of  FIG.  1    according to a further embodiment, the cage comprising supports including loops for guiding one or more pipes through the loops; and 
         FIG.  8    shows a flowchart illustrating a method for installing a tower of the wind turbine of  FIG.  1   . 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows an offshore wind turbine  1  according to an embodiment. The wind turbine  1  comprises a rotor  2  having one or more blades  3  connected to a hub  4 . The hub  4  is connected to a generator (not shown) arranged inside a nacelle  5 . During operation of the wind turbine  1 , the blades  3  are driven by wind to rotate and the wind&#39;s kinetic energy is converted into electrical energy by the generator in the nacelle  5 . The nacelle  5  is arranged at the upper end of a tower  6  of the wind turbine  1 . The tower  6  comprises a monopile  7  as a foundation, the monopile  7  being driven into the seabed  8 . The tower  6  of the wind turbine  1  further comprises a main portion  9 . The main portion  9  is erected on the monopile  7  and is holding at its top end  10  the nacelle  5 . The wind turbine  1 , its tower  6  and its monopile  7  are extending, in the erected state, in a height direction H. 
     The monopile  7  comprises, for example, several lengthwise sections  11 ,  12 ,  13  continuously connected with each other.  FIG.  2    shows an enlarged partial view of the monopile  7  and its lengthwise sections  11 ,  12 ,  13 . At least one of the several lengthwise sections  11 ,  12 ,  13  (section  12  in  FIG.  1   ) has a conical shape  14  with a conically shaped outer surface  15 . In the example of  FIG.  2   , the other sections  11 ,  13  are cylindrically shaped. However, in other examples, one, some or all of the other sections  11 ,  13  may also be conically shaped such that they are tapered in the height direction H. 
     The reference sign  16  in  FIG.  1    denotes a water line (i.e., sea level). The monopile  7  is partly arranged above the sea level  16 . 
     The wind turbine  1  further comprises a gas producing facility  17 . In the shown example of  FIG.  1   , the gas producing facility  17  comprises several units  18 ,  19  arranged on one or more gas producing platforms  20  of the wind turbine  1 . The electrical energy generated by the generator in the nacelle  5  of the wind turbine  1  is converted into a gas, e.g., a hydrogen gas, by the gas producing facility  17 . The gas producing facility  17  comprises a gas producing unit  19  converting, for example, water (e.g., sea water) by electrolysis into hydrogen gas. The produced (hydrogen) gas is transported by one or more pipes  21  from the wind turbine  1  to another offshore or onshore installation (not shown). The one or more pipes  21  are fluidly connected with the gas producing unit  19 . The other offshore or onshore installation (not shown) is, for example, another offshore wind turbine and/or an offshore or onshore collector station. 
     The wind turbine  1  further comprises a cage  22  arranged on the monopile  7 . The cage  22  is configured for easily attaching the one or more gas transporting pipes  21  to the monopile  7  and guiding the pipes  21  from the gas producing unit  19  to or close to the seabed  8 . In  FIG.  1   , the shown pipe  21  is laid on the seabed  8 . In other examples, the pipe  21  may also be buried in the seabed  8  and only emerge again at another offshore installation or close to the shoreline. 
     As shown in  FIG.  2   , the first lengthwise section  11  of the monopile  7  has a cylindrical shape with a first outer diameter D 1 . The second lengthwise section  12  is conically-shaped (conical shape  14 ) and is tapered in the height direction H from the first outer diameter D 1  to a second outer diameter D 2  to form the conically shaped outer surface  15 . That means, the second outer diameter D 2  is smaller than the first outer diameter D 1 . Further, the third lengthwise section  13  of the monopile  7  has a cylindrical shape with the second outer diameter D 1 . 
     The cage  22  being arranged on the monopile  7  ( FIG.  1   ) is in particular arranged around the conically shaped section  12  of the monopile  7 . Further, the cage  22  is resting on the conically shaped section  12 . 
     Furthermore, the cage  22  in the example of  FIG.  1    is arranged partly above the sea level  16 . 
     As best visible in  FIG.  3   , the cage  22  comprises, for example, several rods  23  extending, in the installed state of the cage  22  on the monopile  7 , parallel to the height direction H. The cage  22  shown in  FIG.  3    comprises six rods  23  extending parallel to the height direction H. However, in other examples, the cage  22  may also comprise another number of rods  23 , another arrangement of the rods  23  and/or another shape of the rods  23 . 
     Furthermore, the cage  22  shown in  FIG.  3    comprises several rings (annular elements)  24 ,  25 ,  26 ,  27 . Each ring  24 ,  25 ,  26 ,  27  is connected and fixed to each of the six rods  23 . In particular, the rods  23  are fixed to an outer surface of each ring  24 ,  25 ,  26 ,  27  (e.g., the outer surface  28  of the ring  26 ,  FIG.  4   ). 
     As shown in  FIG.  3   , the cage  22  comprises a first inner diameter E 1  at a lower portion  29  (e.g., at the rings  24  and  25 ) of the cage  22 . Further, the cage  22  comprises a second inner diameter E 2  at an upper portion  30  (e.g., the ring  26 , see also  FIG.  4   ) of the cage  22 . The second inner diameter E 2  is smaller than the first inner diameter E 1  of the cage  22 . 
     Furthermore, the first inner diameter E 1  of the cage  22  is larger than the first outer diameter D 1  of the monopile  7 . In addition, the second inner diameter E 2  of the cage  22  is smaller than the first outer diameter D 1  of the monopile  7  and larger than the second outer diameter D 2  of the monopile. 
     Therefore, when the cage  22  is arranged on the monopile  7  by lowering it from above onto the monopile  7 , the first section  11  of the monopile  7  is inserted into the lower portion  29  of the cage  22 . In other words, the lower portion  29  of the cage  22  is arranged on the first section  11  of the monopile  7  such that it surrounds the first section  11 . Furthermore, the cage  22  is further lowered until the upper portion  30  (e.g., the ring  26 ) of the cage  22  having the smaller second inner diameter E 2  rests at the conically shaped outer surface  15  of the second section  12  of the monopile  7 . In particular, an inner surface  31  ( FIG.  4   ) of the upper portion  30  (e.g., the ring  26 ) is in contact and rests at the outer surface  15  of the conically shaped section  12  of the monopile  7 . 
     In the example of  FIGS.  1  and  3   , the rods  23  of the cage  22  are arranged parallel to the height direction H. Further, the smaller second inner diameter E 2  of the cage  22  is realized by configuring the ring  26  in the upper portion  30  of the cage  22  with a smaller diameter E 2  than the rings  24 ,  25  in the lower portion  29  of the cage  22 . 
     In other examples, as illustrated in  FIG.  5   , rods  23 ′ of a cage  22 ′ may also have a configuration such that they are inclined with respect to each other in an upper portion  30 ′. In particular, the cage  22 ′ shown in  FIG.  5    has a section  32  tapered in the height direction H. 
     The cage  22  is configured for simplifying attachment of the one or more gas transporting pipes  21  to the monopile  7  and guiding the pipes  21  from the gas producing unit  19  to or close to the seabed  8 . 
     The cage  22 ,  22 ′ ( FIGS.  3 ,  5   ) may comprise one or more flexible elements  33 , as shown exemplarily in  FIG.  5   . The one or more flexible elements  33  are arranged between the conically shaped outer surface  15  ( FIG.  2   ) of the monopile  7  and the inner surface  31 ′ of the upper portion  30 ′ of the cage  22 ′. Due to the flexible elements  33 , scratching of the cage  22 ,  22 ′ (i.e., its inner surface  31 ,  31 ′) at the monopile  7  (i.e., its outer surface  15 ) is avoided. The one or more flexible elements  33  comprise, for example, one or more mats. Furthermore, a material of the flexible elements  33  may comprise, for example, rubber, neoprene and/or another flexible and non-corrosive material. By using a non-corrosive material for the flexible elements  33 , corrosion of the cage  22 ,  22 ′ and/or monopile  7  is avoided. 
     The purpose of the cage  22 ,  22 ′ is to provide an improved attachment means for pipes of the wind turbine  1  such as the pipe  21  shown in  FIG.  1   . In the embodiment shown in  FIGS.  1  and  3   , the cage  22  comprises one or more tubes  34  connected and fixed to supports  35  of the cage  22 . The tubes  34  may be pre-installed at the cage  22  such that during installation, the cage  22  with the pre-installed tubes  34  is lowered onto the monopile  7 . The tubes  34  shown in  FIGS.  1  and  3    are J-shaped tubes having a first portion  36  extending in the height direction H and a second portion  37  being bent with respect to the first portion  36  to form the J-shape. The tubes  34  are configured for accommodating the pipes  21 , as illustrated in  FIG.  1   . Having the tubes  34  allows to install the pipes  21  by pushing or pulling the pipes  21  through the tubes  34 . 
     As shown in  FIG.  6   , the tubes  34 ′ may have a bell-shaped mouth  38  at a lower end  39  thereof. The bell-shaped mouth  38  allows to easily insert a respective pipe  21  into the tube  34 ′. 
     As shown in  FIG.  3   , each tube  34  may further have a pre-installed messenger wire  40  guided through the respective tube  34  before installation of the cage  22  with the tubes  34  at the monopile  7 . The messenger wire  40  is used for pulling the pipe  21  through the tube  34 . Furthermore, each tube  34  may have a float seal  42  ( FIG.  3   ) sealing an opening  43  (e.g., the bell-shaped mouth  38 ,  FIG.  6   ) of the respective tube  34  at a lower end  39  thereof. The float seal  42  is configured for preventing water entering the tube  34  before the pipe  21  is inserted into the tube  34 . The float seal  42  may be connected to the messenger wire  40 , as shown in  FIG.  3   . 
     In a further embodiment of the cage  22 ″, the cage  22 ″ comprises instead of tubes (such as the tubes  34 ) supports  44  for attaching one or more pipes  21  directly at the cage  22 ″, as shown in  FIG.  7   . For example, each of the one or more supports  44  includes a (e.g., closed) loop  45  for guiding the one or pipes  21  through the respective loop  45 . 
     In embodiments, the cage  22  ( FIG.  1   ) may further comprise in a top portion  46  thereof a landing stage  47  for a vessel. 
     Using the cage  22 ,  22 ′,  22 ″ arranged on the monopile  7  of the wind turbine  1 , pipes  21  such as energy transportation pipes  21  can be easier installed at the wind turbine  1 . 
     In the example of  FIG.  1   , the cage  22  is arranged on the monopile  7 . In other examples, the cage  22 ,  22 ′,  22 ″ may also be arranged on a transition piece  48  ( FIG.  2   ) extending in a height direction H′. The transition piece  48  comprises a conically shaped section  49  tapered in the height direction H′, as indicated in  FIG.  2   . Further, the conically shaped section  49  has a conically shaped outer surface  50 . The transition piece  48  may be installed on a foundation driven into the seabed such as a monopile or a floating foundation (not shown). 
     In the following, a method for installing a tower  6  of an offshore wind turbine  1  is described with respect to  FIG.  8   . 
     In a first step S 1  of the method, a monopile  7  is erected at a seabed  8  such that the monopile  7  is extending in a height direction H. The monopile  7  has a conically shaped outer surface  15  tapered in the height direction H. 
     In a second step S 2  of the method, a cage  22 ,  22 ′,  22 ″ is lowered onto the erected monopile  7 . The cage  22 ,  22 ′,  22 ″ extends in the height direction H and comprises one or more supports  35 ,  44  for holding one or more pipes  21  and/or tubes  34 . Furthermore, the cage  22 ,  22 ′,  22 ″ comprises a first inner diameter E 1  at a lower portion  29  thereof and a second inner diameter E 2  at an upper portion  30 ,  30 ′ thereof. The second inner diameter E 2  is smaller than the first inner diameter E 1 . 
     In a third step S 3  of the method, the cage  22 ,  22 ′,  22 ″ is arranged at the monopile  7  such that the upper portion  30 ,  30 ′ of the cage  22 ,  22 ′,  22 ″ rests at the conically shaped outer surface  15  of the monopile  7 . 
     In a fourth step S 4  of the method, pipes  21  are installed at the cage  22 ,  22 ′,  22 ″. The pipes  21  are directly attached at the cage  22 ″, e.g., by inserting a respective pipe  21  through loops  45  of supports  44  of the cage  22  ( FIG.  7   ). Alternatively, the cage  22  comprises tubes  34  ( FIG.  3   ), e.g., J-shaped tubes  34 , and the pipes  21  are inserted into the tubes  34  ( FIG.  1   ). The pipes  21  are, for example, pulled or pushed through the tubes  34 . 
     Although the present invention has been disclosed in the form of embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention. 
     For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements.