CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a United States national phase filing under 35 U.S.C. §371 of International Application No. PCT/EP2006/010614, filed Nov. 6, 2006. The complete disclosure of the above-identified application is hereby fully incorporated herein by reference. 
     TECHNICAL FIELD 
     The invention relates to a subsea installation with a Variable Speed Drive (VSD). Furthermore, it relates to subsea applications such as subsea vessels and subsea vehicles with such subsea installation. 
     BACKGROUND 
     Adjustable speed ac induction motor drive is known from U.S. Pat. No. 4,137,489 and makes use of a common voltage converter for applying a voltage of variable magnitude to a dc link and a polyphase current source inverter having a variable frequency output with the dc link current magnitude. 
     As far as the invention is concerned, subsea installations, vessels and vehicles are usually used in exploring and exploiting gas and oil fields at wellheads located at the seabed. Variable Speed Drives are used in this context for applications requiring translatory or rotational movements by electric motors. 
     Prior art Variable Speed Drives for such subsea installations, vessels and vehicles make use of common voltage-controlled alternating current/alternating current (AC/AC) converters based on insulated gate bipolar transistors (IGBT), the converters comprising a rectifier, a direct current (DC) link buffered by a capacitor bank, and an inverter. 
     Disadvantageously, capacitors are sensitive to environment pressure. Prior art subsea applications therefore have to provide pressure compensation with an internal air pressure equal to the atmospheric standard pressure of approximately 1013 hPa (1 atm). Typically, the alternating current/alternating current converter and its control electronics are placed in a special housing to keep the pressure-compensated volume small. Only the interior of the housing has to be kept at the atmospheric standard pressure then. As this housing has to be pressure-resistant against the overpressure of the subsea environment or of a surrounding main vessel, several constructional problems occur. Securely withstanding the pressure requires thick walls (up to more than 100 mm depending on pressure and dimensions of the housing) and/or support rods, which takes much space, and poses high prerequisites regarding the sealing of the housing, in particular regarding penetrators for electrical connections into and out of the housing. Additionally, the converter circuits need a complex cooling system to dissipate waste heat from within their housing. 
     SUMMARY 
     According to various embodiments, a subsea installation can be specified with a variable speed drive of the type initially mentioned, having lower constructional requirements. According to further embodiments a corresponding subsea vessel and subsea vehicle can be specified. 
     According to an embodiment, a subsea installation may comprise a Variable Speed Drive with an alternating current/alternating current converter having a current-controlled capacitor-less direct current link, wherein at least the converter is arranged within a liquid-tight housing. 
     According to a further embodiment, the direct current link may comprise a smoothing inductor. According to a further embodiment, the direct current link may consist of the inductor only. According to a further embodiment, the housing can be filled with an electrically isolating liquid. According to a further embodiment, a wall of the housing may have a thickness of maximally 10 mm. According to a further embodiment, a volume compensator can be located on the housing. According to a further embodiment, the housing may be arranged within a liquid-tight main vessel. According to a further embodiment, the main vessel may be at least partially filled with an electrically isolating liquid. According to a further embodiment, the converter may comprise a thyristor-controlled rectifier and a thyristor-controlled line-commutated inverter. 
     According to another embodiment, a subsea vessel may comprise such a subsea installation with a Variable Speed Drive as described above. 
     According to yet another embodiment, a subsea vehicle may comprise such a subsea installation with a Variable Speed Drive as described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, exemplary embodiments of the invention are explained in further detail with drawings. 
       In the drawings, 
         FIG. 1  shows a circuit diagram of a variable speed drive; 
         FIG. 2  shows a circuit diagram of another variable speed drive; and 
         FIG. 3  shows a subsea installation. 
     
    
    
     Like parts are denoted by like reference signs in all figures. 
     DETAILED DESCRIPTION 
     According to various embodiments, a subsea installation with a variable speed drive comprises an alternating current/alternating current converter having a current-controlled capacitor-less direct current link. This not only reduces the number of electronic components in the converter, hence reducing the probability of failure. Moreover, complex measures for pressure compensation can be completely omitted or can at least be limited to the control electronics of the converter, as according to various embodiments no pressure-sensitive capacitors are used. The converter can be exposed to overpressure therefore, in particular to subsea environmental pressure. The converter can be equipped with any kind of power switches, such as thyristors, metal oxide semiconductor field effect transistors (MOSFET) and insulated gate bipolar transistors. 
     Preferably, said direct current link comprises a smoothing inductor. A smoothing inductor is typically used for high-power current-controlled direct current links. The direct current link may comprise other pressure-proof components. 
     Particularly, said direct current link may consist of said inductor only. This way, the variable speed drive has a simple design and a low failure probability and takes only little space. 
     In a preferred embodiment, at least said converter is arranged within a liquid-tight housing. This housing is preferably filled with a liquid. As liquids are nearly incompressible the liquid serves for supporting the housing on all sides against any external pressure. As a consequence, the housing itself is nearly incompressible even at a high external pressure such as in the deep sea, and the converter components are indirectly exposed to the external pressure. For this reasons, the construction complexity of the housing can be significantly reduced. The seals and fittings, in particular of penetrators, can be simpler and are considerably more reliable due to low differential pressure between inside and outside of the housing. Additionally, the liquid can serve as a coolant for dissipating waste heat from the converter circuits to the outside of the housing. The complexity of a cooling system is therefore reduced, too. Because the constructional requirements are lower than in prior art, compact and lightweight Variable Speed Drives are possible by this embodiment. 
     Advantageously, said housing is filled with an electrically isolating liquid. In this case, no precautions have to be taken against short circuits between bare electrical contacts of the converter. For example, the liquid can be oil. 
     Preferably, a wall of said housing has a thickness of maximally 10 mm, in particular of maximally 5 mm. Therefore, the housing has a low heat capacity, but a high heat conductance for efficiently dissipating waste heat. Besides, it needs less material, and less space in the subsea application. Generally, the walls of said housing only need to have a thickness designed to withstand normal handling and operation conditions. 
     Malfunction due to a leakage can be retarded by a volume compensator that is located inside of said housing. The volume compensator prevents external liquids from intruding into the converter housing for a certain period of time by providing a slight overpressure inside the housing. 
     In one possible embodiment, said housing is arranged within a liquid-tight main vessel. Other parts of the subsea application can be arranged in the main vessel. In this embodiment, the converter is contained in the housing independently from such parts and from the internal pressure of the main vessel. In particular, the main vessel can be filled with a liquid, hence applying the pressure-resistant principle of the converter housing to the main vessel, too. The main vessel can thus have a reduced construction complexity as the nearly incompressible liquid serves for supporting the main vessel on all sides. It is possible to locate the control electronics of the converter in a small pressure-compensated containment inside or outside of the main vessel. In particular, this pressure-compensated containment can be arranged inside the converter housing. 
     Preferably, said main vessel is at least partially filled with an electrically isolating liquid, for example, oil. The main vessel may comprise bare electrical contacts in its interior as described above for the housing of the converter, so short circuits are avoided this way. Besides, in case of a leakage of the housing, the liquid of the main vessel may enter the housing without electrically influencing the operation of the converter. 
     In a special embodiment, said converter comprises a thyristor-controlled rectifier and a thyristor-controlled line-commutated inverter. Instead of thyristors, it may comprise arbitrary other semiconductor valves. This allows for operating a synchronous machine using the Variable Speed Drive. 
     In addition, other embodiments comprise a subsea installation, a subsea vessel and a subsea vehicle comprising a Variable Speed Drive as described above. 
       FIG. 1  shows a sample circuit diagram of a variable speed drive  1  for a subsea installation (not shown in this figure) according to various embodiments. It comprises a three-phase alternating current/alternating current converter  2 , control electronics  3  and an asynchronous induction motor  4 . The input of the converter  2  is connected to a three-phase voltage source  5 . The converter  2  supplies the motor  4  with a pulsed three-phase alternating current. 
     The converter  2  comprises a rectifier  6 , a direct current link  7  and an inverter  8 . The exemplary rectifier  6  is a full wave three-phase bridge rectifier comprising six thyristors  9 . The exemplary direct current link  7  is current-controlled and comprises only a smoothing inductor  10 , but no capacitors. The exemplary inverter  8  comprises six thyristors  9 , serving for creating pulse-width modulated output waveforms that cause a sinusoidal current in the motor  4 . The pulse-width modulation is performed by the control electronics  3 . Any algorithm for generating the output waveforms can be used by the control electronics  3 , for example, space vector modulation. 
     The converter  2  can be exposed directly or indirectly to a pressure significantly higher or lower than 1013 hPa without influencing its function, because no pressure-sensitive capacitors are used. 
     In another embodiment (not shown), the converter  2  may be contained in a liquid-tight housing that is filled with an electrically isolating liquid. It supports the housing on all sides against any external pressure. Hence, the differential pressure between the interior of the housing and the exterior is negligible. As a consequence, the complete design of the converter  2  is insensitive to leakages, because the probability of an external medium intruding into it is strongly limited. Hence, the converter  2  has a high reliability at low production costs. 
       FIG. 2  shows a sample circuit diagram of another Variable Speed Drive  1  according to various embodiments. It is built similar to the one of  FIG. 1 . However, the inverter  8  comprises six insulated gate bipolar transistors  11  instead of thyristors, and the motor  4  is a synchronous machine. Using insulated gate bipolar transistors  11  allows for applying other modulation patterns for the inverter  8 . The ability to switch off the current at desired times enables the converter  2  to drive other kinds of motors and loads than synchronous machines, too. 
     In  FIG. 3 , an example of a subsea installation  20  is depicted. It comprises a liquid-tight main vessel  21  surrounded by seawater  22  and located at the seabed  23 . The subsea installation  20  is part of a wellhead for drilling (not shown) into the seabed  23 . A liquid-tight housing  24  for a converter  2  of a variable speed drive  1  is arranged inside the main vessel  21 . The converter  2  is located inside the housing  24 . The control electronics  3  of the converter  2  are located inside a separate air-filled and pressure-compensated containment  25  that is located, for example, inside the converter housing  24 . The housing  24  is filled with an electrically isolating first liquid  26 . The main vessel  21  is filled with an electrically isolating second liquid  27 . Both liquids  26 ,  27  are oils. 
     The subsea installation  20  is connected to a subsea power line  28  providing it with electrical energy in the form of a 10 kV three-phase alternating current. The power line voltage is transformed to a 240 V three-phase installation voltage to which the input of the converter  2  of the variable speed drive  1  is connected, wherein the transformer can be seen as a voltage source  5 . The converter  2  is designed as the one described in  FIG. 1 . However, in other embodiments (not shown) it may be any kind of alternating current/alternating current converter comprising a current-controlled capacitor-less direct current link. The converter  2  outputs a 3 kV (root mean square value) three-phase pulse-width modulated alternating current to a drilling motor  4 . 
     The voltages given above just describe an exemplary subsea installation  20 . Other voltages may apply depending on the length of the power line  28 , the distance from the voltage source  5  to the converter  2 , and the required voltage and power of the motor  4 , respectively. If appropriate, the transformer can be omitted. 
     Both liquids  26 ,  27  serve as coolants for the respective electronic components. The waste heat rising at the converter  2  inside the housing  24  is dissipated via the first liquid  26  to and through the housing  24 . From there, it is dissipated via the second liquid  27  to and through the main vessel  21  and, finally, to the seawater  22 . For this purpose, a circle flow of the first liquid  26  can be created inside the housing  24 , either by the force of gravity or by at least one dedicated pump (not shown). In the same way, a circle flow of the second liquid  27  can be created inside the main vessel  21 . Heat exchanging devices may be arranged in the first liquid  26  and/or in the second liquid  27  to increase efficiency, wherein one liquid  26 ,  27  is extensively conducted through the other  27 ,  26  without merging them. 
     The liquids  26 ,  27  support the housing  24  and the main vessel  21 , respectively, on all sides against the external subsea overpressure. Due to this, the differential pressures between the interior of the subsea installation  20 , i.e. inside the housing  24  and inside the main vessel  21 , and the surrounding seawater  22  is negligible. The seals and fittings (not shown), in particular of penetrators, of the housing  24  and of the main vessel  21  can therefore be designed for a low differential pressure. In the subsea installation  20  the intrusion of seawater into the converter  2  even is nearly impossible, because the housing  24  is located within the main vessel  21 . In case of a leakage of the main vessel  21  no seawater will enter the housing  24 . In case of a leakage of the housing  24  only, the second liquid  27  of the main vessel  21  can enter the housing  24 . However, it will not lead to malfunction of the converter  2 , as might be the case for seawater  22  directly intruding into the converter  2 , because the second liquid  27  is electrically isolating. Thus, the subsea installation  20  will maintain operation. Due to these reasons, the Variable Speed Drive  1  is applicable to a wide range of power and voltage. 
     In other embodiments (not shown), the variable speed drive  1  can be used for driving a pump motor  4  or any other kind of motor  4 . 
       FIG. 4  shows a typical application of a Variable Speed Drive  1  in a subsea installation (not shown). The motor  4  and driven equipment are located apart from the converter  2  that is arranged in a single liquid-tight housing  24  filled with a first liquid  26 . The control electronics  3 , if needed, are preferably located in a pressure-compensated containment  25  inside of the housing  24 . The housing  24  is directly exposed to seawater  22 . It can be equipped with a volume compensator  29  giving the first liquid  26  a slight overpressure with respect to the surrounding seawater  22 . In case of any leakage the differential pressure will prevent the seawater from entering the housing. The volume compensator  29  can be monitored as necessary. 
     A volume compensator  29  can also be used in embodiments that comprise a main vessel  21 , such as the one described in  FIG. 3 . The volume compensator  29  is arranged normally on and alternatively inside the housing  24  then as described above. Additionally or alternatively, a volume compensator can be arranged inside the main vessel  21 , outside of the housing  24 .
A Variable Speed Drive ( 1) for subsea installations ( 20), subsea vessels or subsea vehicles, as well as to a corresponding subsea installation ( 20), subsea vessel or subsea vehicle, has an alternating current/alternating current converter ( 2) having a current-controlled capacitor-less direct current link ( 7).