Technical Field

[0001] The present invention relates to an offshore transformer assembly, such as used in association with offshore wind parks or electrification of offshore hydrocarbon production facilities. Other applications may for instance include gas to wire solutions, where an offshore electric power generation facility running on gas is used to produce electricity that is guide to land.

Background Art

[0002] For offshore power distribution systems, such as used in association with offshore wind parks, it is known to arrange transformer substations on an offshore structure, such as a jacket resting on the seabed. Such offshore structures need to be large and robust, as transformers are large and heavy equipment.

[0003] Subsea transformers are presently available up to the 20 MVA range with 18/30(36) kV wet-mate connectors and/or 76/132(145) kV dry-mate connectors for the corresponding subsea ac-power cable interfaces. The typical power limitations and cost-levels associated with subsea transformers call for more cost-effective options for a highly competitive energy sector gradually expanding offshore.

[0004] Moreover, due to the size and weight of transformers, heavy lifting and transport equipment is needed when installing or replacing such offshore

transformers.

[0005] Offshore wind parks can typically have transformers of 100, 200 or even 300 MVA.

[0006] Publication EP2553174 addresses this problem with a solution where the transformer can be transported to a position below a jack-up platform with a barge, so that strand jacks on the platform can lift the transformer up to the platform.

[0007] Publication CN204663059U discloses another marine transformer substation, where a transformer is arranged on top of an offshore jacket structure.

[0008] Publication WO2001046583 discloses an offshore wind power installation with a tower and a rotor head. Connected to the tower body, some distance below the rotor head, there are arranged subsystems such as a switchgear and//or a transformer.

[0009] Publication US2008164966 discusses cooling of an offshore transformer. A transformer is installed with its transformer portion below the sea surface and with its electrical connections arranged above sea surface.

[0010] Solutions of the prior art require heavy lifting equipment to hoist the

transformers into their operation positions.

[0011] Furthermore, as illustrated with the publications cited above, large vessels are typically required for transporting the transformers to and from the operation site. [0012] An object of the present invention may be to provide an offshore transformer substation assembly that alleviates one or more challenges associated with the prior art solutions.

[0013] Moreover, it may be an object of the present invention to provide a method for transport and installation of the transformer to the operation site, which does not require a transport vessel as large as is necessary with the known methods.

Summary of invention

[0014] According to the present invention, there is provided a method of installing an offshore transformer assembly with a power range above 20 MVA and/or with a mass of more than 50 metric tons onto an offshore main structure which extends vertically from a distance below the sea surface to a distance above the sea surface, and which comprises a transformer landing section at a distance below the sea surface. According to the invention, the method comprises the following steps:

a) transporting the offshore transformer assembly with a floating transport means, such as a vessel or a barge, to the location of the offshore main structure at sea; b) after step a), landing the offshore transformer assembly on the transformer landing section;

c) after step a), connecting dry-mate connectors of the offshore transformer assembly with dry-mate connectors of the offshore main structure at a position above the sea surface;

According to the invention, the offshore transformer assembly comprises a

transformer inside an encapsulation and a conduit that extends vertically upwards from the encapsulation. Furthermore, according to the invention, step a) comprises transporting the offshore transformer assembly with the transformer in a position below the sea surface and with the upper portion of the conduit in a position above the sea surface.

[0015] With the present invention, a solution is provided where the heavy part of a transformer assembly, namely the transformer itself, comprising inter alia electrical windings and a core, is arranged at a low vertical elevation. For the main structure that shall carry the weight of the transformer, this means that its upper part needs to be less robust. Moreover, the main structure will need less space on the upper portion, since it needs not receive the transformer there.

[0016] Furthermore, by arranging the lower portion of the transformer assembly, namely the transformer itself and the surrounding encapsulation, into the sea and below the sea surface, water will be displaced and thus reduce the effective weight of the transformer assembly. This also results in a reduced structural demand for the main structure that carries the transformer. This also reduces the size requirements for the floating transport means (e.g. a vessel, ship or barge).

[0017] By transporting the offshore transformer assembly in such a partially submerged state, the center of gravity of the transported mass is relatively low. This also reduces the size requirement of the floating transport means, which is used for transport of the offshore transformer assembly.

[0018] For transformer embodiments typically with a power range above 50 MVA and/or with a mass of more than 50 metric tons, there are only a few proven topside transformers that can be replaced with the permanent crane(s) of the offshore installation / offshore platform.

[0019] For larger transformers, typically with a power range of above 100 MVA and/or with a mass of more than 100 metric tons, there are few or perhaps no proven topside alternatives, where one can replace the transformer with the permanent crane(s) of the offshore installation / offshore platform.

[0020] Notably, in offshore wind farm facilities, permanent topside cranes are a maintenance burden, which one seeks to avoid.

[0021] In an embodiment of the invention, step a) can comprise transporting the offshore transformer assembly at a first vertical level and that the transformer landing section is at a second level, which is lower than the first level.

[0022] The floating transport means may comprise lifting beams at an aft portion / the stern. During step a), the offshore transformer assembly may depend from the lifting beams on winch wires.

[0023] Step b) may comprise engaging the offshore transformer assembly with landing guiding means that are part of the offshore main structure.

[0024] In some embodiments, the offshore main structure can be the tower of an offshore windmill.

[0025] The offshore transformer assembly, which is installed with the method according to the invention, may comprise an encapsulation and a transformer inside the encapsulation. Primary cables can be connected to a primary side of the transformer and secondary cables can be connected to a secondary side of the transformer. The primary and secondary cables extend out of the encapsulation through an encapsulation aperture. The offshore transformer assembly can further comprise a conduit which is arranged with a sealed connection to the encapsulation aperture. The primary and secondary cables extend from the transformer and upwards through the conduit, and at least to an exit port at a vertical distance above the transformer.

[0026] In some embodiments of the present invention, the offshore transformer assembly may have a power range in the range above 5 MVA, 10 MVA, or alternatively in the range above 20 MVA.

[0027] In some embodiments, the primary and/or secondary cable can terminate in a primary and/or secondary dry-mate connector, which is arranged in the exit port. In other embodiments, the cable itself may exit the conduit through the exit port. In such embodiments, the cable may terminate in a dry-mate connector that is arranged some distance from the exit port. The dry-mate connectors can for instance be of a type as typically used on topsides offshore or in coastal regions onshore. Alternatively, subsea connector types could be used for enhanced reliability for particularly exposed embodiments.

[0028] The term sealed connection means that a water tight connection exists between the conduit and the aperture. That is, the conduit and the enclosure together form a water barrier protecting the inside of the conduit and the enclosure from water intrusion through the connection interface between the conduit and the enclosure.

The sealed connection can typically be in the form of a welded connection, where the conduit is welded onto the enclosure. In other embodiments, the sealed connection may be provided with a combination of a bolted flange and a gasket.

[0029] In some embodiments, the exit port can be arranged more than 5 meters, more than 10 meters, or even more than 20 meters vertically above the transformer.

[0030] Advantageously, the exit port can be located at a level above the design wave-height applicable for the installation in question. However, it could be

acceptable to locate it at or slightly above the reach of a 10-year wave-height since some occasional white-water splashing technically could be acceptable. The mechanical design should however advantageously comply with a 100-year wave- height or similar criteria. In embodiments comprising an oil conservator tank / oil expansion tank and“breather openings”, these should advantageously also be designed at a vertical level to handle a 100-year wave-height.

[0031] The encapsulation and the conduit can in some embodiments be filled with oil. Such an oil will contribute in cooling the transformer in the encapsulation and to electrically insulate. The oil can advantageously be made to circulate within the encapsulation and the conduit. Such oil circulation could be natural or forced, and the transformer oil circulation loop may in some embodiments include a permanent or temporary oil filtration and/or dehydration unit. For some embodiments, the oil circulation loop could also include a heat exchanger arranged topside or subsea.

[0032] In an embodiment of the present invention, the offshore main structure can comprise a conduit guiding means that is arranged above the sea surface, which is configured to attach to an upper portion of the conduit.

[0033] A particular advantage of the present invention may emerge when using the method for installing transformers with offshore windmills at very shallow waters. A problem for installing transformers in such shallow waters may be that a larger ship will have excessive draught, so that it cannot be used. With the method according to the invention, however, one can use a significantly smaller floating transport means, which will have less draught.

Brief description of drawings

[0034] While the present invention has been described in general terms above, a more detailed and non-limiting example of embodiment will be given in the following with reference to the appended drawings, in which Fig. 1 is a cross section side view of an offshore transformer assembly;

Fig. 2 is a perspective view of an offshore transformer installation, where two

transformer assemblies are installed on a main structure at sea;

Fig. 3 is a cross section side view of an embodiment substantially corresponding to the embodiment shown in Fig. 2;

Fig. 4 is a perspective view of a ship that is installing a transformer assembly;

Fig. 5 is an enlarged perspective view of a portion of the main structure;

Fig. 6 is a perspective view of a transformer assembly and the main structure in a situation where the transformer assembly is being landed;

Fig. 7 is a perspective view corresponding to Fig. 6, where the transformer

assembly is nearly landed;

Fig. 8 is a side view of an offshore transformer installation;

Fig. 9 is a principle cross section side view through a main structure in the form of a spar platform, wherein a transformer assembly is landed on the main structure;

Fig. 10 is a principle top view of a part of a semi-submersible platform equipped with a transformer assembly; and

Fig. 11 is a principle top view of a part of a semi-submersible platform with a

transformer assembly.

Detailed description of the invention

[0035] Fig. 1 shows an electric offshore transformer assembly 10. The assembly has a transformer 11 arranged inside a watertight encapsulation 13. A conduit 15 extends upwards from the encapsulation 13. The inside of the conduit 15 is in fluid communication with the inside of the encapsulation 13, so that it constitutes a prolongation of the encapsulated chamber inside the encapsulation 13.

[0036] The conduit 15 may advantageously be a rigid pipe. It may however in some embodiments be in the form of a flexible tube.

[0037] The encapsulation 13 has an aperture 17, to which the lower and open end of the conduit 15 is connected, thereby connecting the inside of the conduit 15 with the inside of the encapsulation 13. While the aperture 17 in the shown embodiment constitutes a wide-open opening between the interior of the conduit 15 and the inside of the encapsulation 13, in other embodiments there may be arranged for instance a baffle in the opening.

[0038] In the shown embodiment, the aperture 17, to which the conduit 15 connects, is arranged in an upper face or roof of the encapsulation 13. In other embodiments however, the aperture 17 may be arranged elsewhere on the encapsulation 13, such as in a side wall.

[0039] To the transformer 11 , which in the shown embodiment is an electric three phase transformer, has a primary side and a secondary side. To the primary side there are connected three primary electric cables 19a. To the secondary side there are connected three secondary electric cables 19b.

[0040] As shown in Fig. 1 , the primary and secondary electric cables 19a, 19b extend upwards through the aperture 17 and through the conduit 15. In embodiments where the aperture 17 is not wide-open, such as with a baffle, there will still be one or more openings for guiding the primary and secondary electric cables through the aperture.

[0041 ] At the top of the conduit 15 there is a conduit head 21. At the conduit head 21 , the primary electric cables 19a are terminated in a set of primary connectors 23a. Correspondingly, the secondary electric cables 19b are terminated in a set of secondary connectors 23b. The primary and secondary connectors 23a, 23b can advantageously be dry-mate connectors, configured to be connected with connector counterparts (not shown) in a dry environment (i.e. not in a body of water).

[0042] In the shown embodiment, the primary and secondary connectors 23a, 23b are arranged at a primary exit port 27a and a secondary exit port 27b, respectively. In alternative embodiments, the primary and secondary electric cables 19a, 19b may exit the conduit 15 at the primary and secondary exit ports 27a, 27b, respectively, and extend a further distance until their terminations at the primary and secondary connectors 23a, 23b. In such embodiments the primary and secondary connectors 23a, 23b are more flexible, as they are connected to the rest of the assembly 10 via the primary and secondary electric cables 19a, 19b. In order to prevent oil from exiting the conduit, a (not shown) sealing means must be arranged at the primary and secondary exit ports 27a, 27b. Such a sealing means will seal against the primary and secondary connectors 23a, 23b and/or against the primary and secondary cables 19a, 19b.

[0043] While in the shown embodiment, the primary and secondary exit ports 27a, 27b are arranged at different positions on the conduit 15, they may in other embodiments be arranged at the same location.

[0044] The vertical distance between the position of the interface between the transformer 11 and the primary and secondary cables 19a, 19b, and the primary and/or secondary exit ports 27a, 27b, may be more than 5 meters, and

advantageously more than 10 meters.

[0045] The volume inside the encapsulation 13 and the conduit 15 is filled with oil to cool the transformer 11 and to electrically insulate. On top of the conduit head 21 there is arranged an oil conservator tank 25. The upper surface level 27 of the oil is arranged inside the oil conservator tank 25. Inside the oil conservator tank 25, there may be arranged a compressible gas bladder (not shown) to allow for some variation of oil volume.

[0046] In some embodiments, in addition to oil, other insulation means, such as paper, may be arranged in between the primary and secondary electric cables 19a, 19b inside the conduit 15. Moreover, as will be appreciated by the skilled person, the electric cables themselves are provided with insulating jackets / layers.

[0047] Fig. 2 shows an offshore installation 100 standing on a seabed (not shown). The offshore installation 100 comprises a main structure 101. In the shown

embodiment, the main structure 101 is in form of a jacket. In other embodiments, the main structure 101 can be a floating structure, such as a spar platform or a semi- submersible. Thus, although the following example embodiment is given with a jacket 101 standing on the seabed, the discussed features may also be present with any other type of main structure 101.

[0048] The main structure 101 shown in Fig. 2 extends from the seabed and vertically upwards beyond the sea surface 103, which is indicated in the drawing.

[0049] In this embodiment, two electric offshore transformer assemblies 10, such as the assembly shown in Fig. 1 , are installed on the main structure 101. The main structure 101 comprises a transformer landing section 105, onto which the two transformer assemblies 10 are landed.

[0050] On the transformer landing section 105, there is provided a landing guiding means 107, which facilitates the installation of the transformer assembly 10 in the correct position. In the embodiment shown in Fig. 2, the landing guiding means 107 comprises upwardly facing guide funnels that are configured to receive downwardly extending transformer assembly feet 29 (Fig. 5) arranged on the lower side of the transformer assembly 10.

[0051] In the embodiment shown in Fig. 2, the main structure 101 has a balcony-like transformer landing section 105, which extends some distance laterally out from the main portion of the main structure 101. Notably, the transformer landing section 105 and the main structure 101 is designed so that a transformer assembly 10 may land by being lowered in a vertical direction towards the transformer landing section 105.

[0052] On an upper deck 108 of the main structure 101 , there is arranged a control arrangement 109. The control arrangement 109 may comprise various components such as switchgears, gauges, and control means.

[0053] Electrical input cables and output cables 111 enters the control arrangement 109. In an embodiment where the offshore installation 100 is used in association with an offshore wind park, the input cables 111 may typically be cables that conduct electric current from a plurality of wind power towers (not shown) and into the control arrangement 109. Moreover, the output cables 111 may typically be conducting stepped-up electric power for export to a grid or other consumer. Thus, the imported power may be stepped up in the transformers 11 in the transformer assemblies 10. [0054] Fig. 3 shows a cross section side view of an embodiment that substantially corresponds with the embodiment shown in Fig. 2.

[0055] In some embodiments, the primary and/or secondary electric cables 19a, 19b that extend upwards inside the conduit 15 may be supported within the conduit. In other embodiments, they may be loosely arranged within the conduit 15.

[0056] In Fig. 3 there is indicated dry-mate connectors 123a, 123b that are part of the offshore installation 100. The dry-mate connectors 123a, 123b are configured to be connected to the primary and secondary connectors 23a, 23b of the transformer assembly 10. The dry-mate connectors 123a, 123b are connected to the control arrangement 109 with electric cables.

[0057] The transformers may in some embodiments have a mass of more than 100 metric tons. Flence, the center of gravity of the offshore installation 100, with one or more transformers 13 installed on it, will be substantially affected by the weight of the transformer. By positioning the one or more transformers 13 at a vertical distance below the sea surface 103 and thus below the upper deck of the main structure 101 , the center of gravity is significantly lower than with prior art solutions, where the transformers are arranged at a higher level.

[0058] In the perspective view of Fig. 2, a conduit guiding means 110 is shown, located substantially at the vertical elevation of the upper deck 108. The conduit guiding means 110 will be connected to the conduit 15 after having installed the transformer assembly 10 into position, resting on the transformer landing section 105. The conduit guiding means 110 retains the conduit 15 in its correct position.

Moreover, in embodiments where the conduit 15 is flexible, the upper portion of the conduit 15 may be suspended in the conduit guiding means 110.

[0059] When transporting a transformer assembly 10 at sea, for installation, it can be transported in different manners. Notably, common for the different ways of transporting the transformer assembly, is that one does not need to hoist it a substantial vertical distance upwards, since it shall be installed with the transformer 11 itself below the sea surface.

[0060] According to the invention, the transformer assembly 10 will be transported in a position where it is partially immersed into the sea. Such a situation is shown in Fig. 4. Flere, a ship 200, which in the shown embodiment is a modified anchor handling vessel, is used for transport and installation of the offshore transformer assembly 10. In the shown embodiment, the transformer assembly 10 is hung off at the stern of the ship 200, with the transformer 11 below the sea surface 103 and with the upper part of the conduit 15 above the sea surface.

[0061] When lowering the transformer assembly 10 down towards the installed position, it can be lowered with a winch (not shown) on the ship 200.

[0062] In the embodiment shown in Fig. 4, a pair of winch wires 201 extend from two (not shown) winches on the ship, over two lifting beams 203, and down to the transformer assembly 10. As the skilled person will appreciate, the operator does not need to hoist the transformer assembly out of the sea at all, when at the installation site. This feature reduces the necessary rating of the lifting beams 203, the winch and the winch wires 201.

[0063] Fig. 5 to Fig. 7 show the process of landing a transformer assembly 10 onto the transformer landing section 105 of the main structure 101 in greater detail. The transformer assembly 10 can be hoisted by means of the ship 200 as shown in Fig. 4, or by any other suitable means, such as a ship with a crane (not shown).

[0064] Fig. 5 depicts a portion of the main structure 101 , such as a jacket resting on the sea bed. The transformer landing section 105 extends laterally, like a balcony, out from the main body of the main structure 101. In this shown embodiment, the main structure 101 is configured to receive two transformer assemblies 10. The transformer landing section 105 is provided with the landing guiding means 107, as were discussed above. The landing guiding means 107 are configured to receive transformer assembly feet 29, which are indicated in Fig. 6.

[0065] The main structure 101 further comprises landing alignment means 113, which in the shown embodiment are in the form of substantially vertically extending rails. The landing alignment means 113 of the main structure 101 is configured to engage with a transformer assembly alignment means 31 of the transformer assembly 10. In the embodiment shown in Fig. 6 and Fig. 7, the transformer assembly alignment means 31 are formed as two bars that extend laterally out from the transformer assembly 10, to form an angle that constitutes a corner. When lowering the transformer assembly 10 down towards the transformer landing section 105, the landing alignment means 113 of the main structure 101 is received in the corner, and thus facilitating correct alignment of the transformer assembly 10 with respect to the main structure 101. In the embodiment shown in Fig. 4, a somewhat different solution is disclosed.

[0066] In the situation shown in Fig. 7, the transformer assembly 10 is still being lowered down towards the transformer landing section 105, and the transformer assembly feet 29 have barely entered into the receiving landing guiding means 107. The transformer assembly alignment means 31 are still in sliding engagement with the landing alignment means 113 of the main structure 101.

[0067] During all the situations shown in Fig. 5 to Fig. 7, the transformer 11 of the transformer assembly 10 may remain below the sea surface (which is not shown in these figures). The primary and secondary exit ports 27a, 27b (Fig. 1 ) may however always remain above the sea surface.

[0068] Fig. 8 depicts an advantageous embodiment of the present invention.

According to this solution, two electric offshore transformer assemblies 10 are arranged on the main structure 101. Contrary to the embodiment shown in Fig. 2, however, one transformer assembly 10 is arranged on respective sides of the main structure 101. In this manner, the weight of the two assemblies is distributed more evenly on the main structure 101. In similar embodiment, one may of course arrange more than two transformer assemblies 10 to the main structure. For instance two on one side and one on the opposite side, or even four or more transformer assemblies 10.

[0069] According to an aspect of the present invention, there is also provided an offshore transformer structure 101 that has at least two transformer landing sections 105, and wherein at least two of the transformer landing sections are arranged at opposite sides of the offshore transformer structure.

[0070] According to an alternative embodiment of the present invention, the main structure 101 , namely the offshore transformer structure, is in form of a floating spar platform 201. Such an embodiment is depicted in Fig. 9, which shows a principle cross section side view through a cylindrical main body part of such a spar platform 201. In this embodiment, the transformer landing section 205 is arranged in a niche in the main structure 201. In this way, the transformer assembly 10 will be protected from supply ships, inter alia, which approaches the spar platform.

[0071] The sea surface 103 is indicated in Fig. 9, at a vertical position between the encapsulation 13 and the conduit head 21 of the transformer assembly 10.

[0072] As can be appreciated from the embodiment depicted in Fig. 9, the entire transformer assembly 10 is arranged within the outer perimeter of the cylindrical shape of the main structure 201 of the spar platform. It will be clear to the skilled person, however, that in other embodiments, the transformer landing section 205 may extend out from the cylindrical shape of the main structure 201.

[0073] The skilled person will appreciate that also such a spar platform may comprise more than one transformer landing section 205, which in some

embodiments may be arranged on opposite sides of the main structure 201.

[0074] Fig. 10 and Fig. 11 depict top views of a corner section of a floating semi- submersible platform (a“semi”). The semi has a plurality of columns 301 , extending vertically between a pontoon and an upper deck section (not shown).

[0075] In the embodiment according to Fig. 10, a transformer landing section 305 is arranged such that the transformer assembly 10, when landed, extends beyond the perimeter of the column 301. In the embodiment according to Fig. 11 , however, the transformer landing section 305 is designed in such way that the entire transformer assembly 10 can be arranged within the perimeter of the main shape of the column 301. This is similar to the depicted embodiment in Fig. 9, which shows the spar platform 201.