Abstract:
A technique of operating a flow line is provided. The flow line includes two or more assembly stations for the assembly of a heavy machine. The technique involves installing a prepared assembly trailer in the flow line before or at a first assembly station. The prepared assembly trailer is connected to at least one flow line tow bar. If the prepared assembly trailer is installed before the first assembly station, then the assembly trailer is moved to the first assembly station. If not, all the flow line assembly trailers are moved to the next assembly station. The steps of moving the flow line assembly trailers are performed timewise stepwise.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/EP2010/061486, filed Aug. 6, 2010 and claims the benefit thereof. The International Application claims the benefits of European application No. 09013689.6 EP filed Oct. 30, 2009. All of the applications are incorporated by reference herein in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a method of operating a flow line and to a flow line of such kind. 
     DESCRIPTION OF THE RELATED ART 
     Production lines for the assembly of e.g. mechanical and/or electrical components are known e.g. from the automotive industry. 
     Here operators or robots, at multiple assembly stations, produce e.g. a car by assembling specific separate parts at each station to the car which is continuously moved slowly on a conveyor belt along the line to the following station. 
     However, for heavy industries producing e.g. large turbines, aeroplanes etc. a similar production line may not be feasible as this would require such a durable construction of the conveyer belt which would make the belt both unsuitable in construction and very expensive. 
     It is an objective of the present invention to provide an advantageous flow line for assembly of heavy machines, preferably a flow line without the above-mentioned disadvantages and most preferred to provide such a flow line technique which is easy to handle and cost effective. 
     SUMMARY OF INVENTION 
     To this end, the invention is realized by a method of operating a flow line comprising two or more assembly stations for the assembly of heavy machines such as wind turbine nacelles, the method comprising the steps of:
         installing a prepared assembly trailer in the flow line before or at a first assembly station,   connecting the said prepared assembly trailer to at least one flow line tow bar,   if the prepared assembly trailer is installed before said first assembly station then: moving the said assembly trailer to said first assembly station,   else: moving all the flow line assembly trailers to the next assembly station, whereby the steps of moving the flow line assembly trailers are performed timewise stepwise.       

     It must be noted in this context that throughout this description and throughout the claims the expression “before”—if not stated in a context of successions of events—is meant to be an indication of place rather than time. It can therefore also be exchanged with the expression “in front of”. 
     Several features of the invention make this process particularly advantageous and easy even though the machines under production are very heavy, i.e. typically above 10 tons: 
     Firstly, instead of a conveyor belt, use is made of transport units, i.e. trailers which can be realized in such way that they also operate under very heavy loads. For example, such trailer can be realized like a railway car, i.e. with metal wheels that are guided along metal rails. Each trailer being a separate unit which is only interconnected with the other trailers at some time during assembly operation, the trailers can also be equipped with some parts that are essential for several steps of the assembly process. For instance, pre-configured cables can be transported on such a trailer all along the assembly so that the staff have the cables at hand at any time during the whole assembly process. 
     Secondly, the transport is not continuous but stepwise at certain pre-defined points in time. This implies a massive reduction of risk of injuries, bearing in mind that the parts assembled in such configuration are very large and also very heavy. A constantly moving conveyor system would therefore pose quite a threat in that sense that some parts might fall down and seriously injure staff or damage objects. During the short time when the flow line is in a state when the trailers are transported, all staff can be ordered outside the sphere of the flow line and also asked to remove any objects such as tools from close proximity to the flow line. 
     Thirdly, use is made of a tow bar, i.e. connection device which is realized in such way that it connects at least two trailers while keeping them at least at a minimum distance, preferably a constant distance. Needless to say, such tow bar can be made up in many different ways, for example as a single piece tow bar in between two trailers or as a multi-part tow bar serially directly connected or indirectly connected via the assembly trailers. In this sense, a coupling as known from railway cars does the trick just as well as two chains which are put together and reinforced by a bar in order to assure that the distance between the trailers remains at least at a certain minimum. 
     One embodiment of a sequence to operate the invented flow line may be:
     Drive with tractor/mover to parking area for empty assembly trailer and connect tractor to assembly trailer   Drive tractor with assembly trailer empty or loaded with set of main cable, centre beams, transport frame and bottom canopy into assembly building   Input of assembly trailer in technical station  1  thereby the trailer have to be aligned in flow direction   Drive out with the tractor   Move (pulling preferred) the assembly trailer to the correct distance to the assembly trailer placed in station  1     Connect the assembly trailer to the tow bar with hydraulic pin from tow bar   Pull the complete flow line (pulled weight 750 t) by 15 meters (50′) in order to move the assembly trailer of station  6  to the technical station  2  with hydraulic cylinder and tow bar that covers stations.   Lift the complete assembly trailer with nacelle by 500 mm   Assembling of the 4 base supports to the centre beams   Lower the assembly trailer in order to get the trailer free and the nacelle standing on base supports   Pull out the empty assembly trailer   Load the next set main cable (OPTION)   Drive the empty assembly trailer out of the building to a parking area.   

     One embodiment of a sequence to run the flow line with outside loading of bottom canopy may be:
     Driving of to empty assembly trailer with tractor (both gooseneck and tow bar should be possible) to loading position for set of main cable.   Driving of assembly trailer with tractor to loading position of centre beams   Driving of assembly trailer with tractor to loading position of transport frame   Driving of assembly trailer with tractor to loading position of bottom canopy. Loaded on supports on the assembly trailer   Then go on with the sequence to operate the invented flow line e.g. as described before.   

     A flow line for the assembly of heavy machines such as wind turbine nacelles with two or more assembly stations, comprises according to the invention:
         a number of assembly trailers,   at least one tow bar connecting at least two assembly trailers,   a pulling means, i.e. a moving device, which operates according to the following scheme:       a) if the prepared assembly trailer is installed before said first assembly station then: moving the said assembly trailer to said first assembly station,   b) else: moving all the flow line assembly trailers to the next assembly station, whereby the pulling means is further realized in such way that it moves the flow line assembly trailers timewise stepwise.   

     The invention can in general be described as a flow line for the final assembly of heavy machines such as nacelles of a wind power plant. The flow line comprises equipment which is able to transport the heavy machines through a plurality of work stations. Herefore, for various embodiments of the invention, an assembly trailer will be transported on the floor of the assembly line. During the movement all assembly trailers are moved parallel from one station to the next. 
     Particularly advantageous embodiments and features of the invention are given by the dependent claims, as revealed in the following description. Thereby, features revealed in the context of the method may also be realized in the context of the rotor blade and the other way round. 
     Preferably, the connecting of the prepared assembly trailer is done by activating a hydraulic pin on a tow bar which engages to a hydraulic pin receiving area on the trailer. 
     Furthermore, it is preferred that said prepared assembly trailer is prepared with suitable cables to be installed in said heavy machine. 
     The method preferably further comprises the step of removing one assembly trailer from the flow line, said removed assembly trailer being the trailer moved from the last of the assembly stations. 
     Additionally, according to an advantageous embodiment said flow line tow bar connects one assembly trailer to another trailer so as to form a serial connection between the assembly trailers. 
     Principally, the tow bar may be positioned anywhere in between, underneath or above the trailers. It has proven particularly advantageous if the flow line tow bar is located and operated in a countersunk pit below the assembly trailers. This way it can be made sure that the tow bar does not hinder staff from any operation around the heavy machines during assembly. In addition, this also makes sure that there is no danger of falling over the tow bar during such operations. This embodiment can be further enhanced by at least partially covering said pit, e.g. by means of a cover plate above the pit. 
     The tow bar may be connected to the trailers throughout all their way along the two line. When using one single tow bar for all trailers, which is preferred, particularly because the tow bar can then be positioned in a pit as described before, a connection of the tow bar to the assembly trailer is preferably engaged while all the assembly trailers are moved and disengaged at least temporarily while all the assembly trailers are at a standstill. 
     In such case, a particularly preferred embodiment of the invention can be realized, namely that the tow bar is moved from a resting position to a forward position during the movement of the assembly trailers and backwards to the resting position while the connection is disengaged. 
     The flow line according to the invention is preferably realized such that the pulling means comprises a stationary piston system. 
     The invention further comprises an assembly trailer suitable for performing the method according to the invention as well as a tow bar suitable for performing the method according to the invention and a heavy machine installed on an assembly trailer according to the invention. 
     As for various embodiments of the invention, basic data of flow line may be: 
     For various embodiments of the invention the cycle time will be 2 to 5 hours. 
     For various embodiments of the invention the time for movement i.e. pulling of the complete flow line is maximum. 5 min. preferred 3 min which correspond to an average speed of 5 m per minute. 
     For various embodiments of the invention the technical availability is 98%. 
     For one embodiment of the invention the number of assembly stations is 6. 
     For one embodiment of the invention the number of technical stations is 2 (1 for input of empty trailer, one for output of last trailer). 
     For one embodiment of the invention the pulling weight without assembly trailers is 750 t. 
     For one embodiment of the invention the pulling distance for the complete flow line is the length of one work station with 50′, i.e. 15 meters. 
     For various embodiments the trailers comprise at least one opening to load set of main cable. 
     For various embodiments the trailers comprise front and rear fixtures for centre beams (F/R) of a nacelle support. 
     For various embodiments of the invention, the placement of the buttom canopy on front and rear supports at the assembly trailer. 
     For one embodiment the invention comprises a horizontal placement of the upper edge of the bottom canopy when assembled to the yaw. 
     For one embodiment of the invention, the total weight of product including transport frame, without assembly trailer is max. 110 t. 
     For various embodiments of the invention, requirements to the assembly trailer may be:
         Mechanical-optical indication of the status of connection (e.g. red connected/yellow disconnected   Parking brake/Parking safeguard   Self-Steering of front axle to keep flow direction (passive or active controlled)   Safety bumper at the front   Assembly trailer must be easily turn able either when its handled with a gooseneck or a draw bar turning cycle max. 20 m       

     Other objects and features of the present invention will become apparent from the following detailed descriptions considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for the purposes of illustration and not as a definition of the limits of the invention. 
     In the following description, the invention for heavy machines is described by the example of a wind turbine nacelle as a nacelle is a good example of a machine which is unsuitable to be assembled by production lines of known technique. 
     It must however be emphasized that the invention can be applied for many other heavy machine industries also such as the production of steam generators, aero planes etc. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  a view at a flow line according to an embodiment of the invention both from the top and from the side, 
         FIG. 2  a top view and a side view of the floor of the flow line shown in  FIG. 1 , 
         FIG. 3  a top view of the flow line of the previous figures, with no trailers shown, 
         FIG. 4  a more detailed top view of the flow line of the previous figures, with no trailers shown, 
         FIG. 5  a top view of the second technical station of the flow line according to the previous figures, 
         FIG. 6  a top view of the flow line of the previous figures, with the trailers shown, 
         FIG. 7  a side view of a trailer and a nacelle including an embodiment of a tow bar according to the invention, 
         FIG. 8  a more detailed side view of the tow bar of  FIG. 7 , with three sectional views of details of the tow bar and with a top view with a trailer, 
         FIG. 9  two side views of trailers as used in the context of an embodiment of the invention, 
         FIG. 10  two side views of a trailer as used in the context of an embodiment of the invention, one view with a nacelle attached, 
         FIG. 11  a side view and a top view of an empty trailer as used in the context of an embodiment of the invention, 
         FIG. 12  a side view and a top view of the empty trailer of  FIG. 11  with beams attached, 
         FIG. 13  a side view and a top view of the empty trailer of  FIG. 12  with a main cable attached, 
         FIG. 14  a side view and a top view of the empty trailer of  FIG. 13  loaded with a transport frame, 
         FIG. 15  a side view and a top view of the trailer of  FIG. 14  with a bottom canopy of a nacelle, 
         FIG. 16  a side view and a top view of the trailer of  FIG. 15  in a position to be unloaded, 
         FIG. 17  in two side views the flow line according to an embodiment of the invention with a pulling means, 
         FIG. 18  in two side views the trailer as used in the context of an embodiment of the invention during a lifting process, 
         FIG. 19  two sectional views of a lifting unit of the second technical station in two different positions according to an embodiment of the invention, 
         FIG. 20  in two side views the trailer of  FIG. 18  during a process where legs are attached to the beams, 
         FIG. 21  in two side views the trailer and the nacelle of  FIG. 19  during a process while the trailer is moved away, 
         FIG. 22  in two side views the nacelle of  FIG. 19  during a process of being prepared to be moved away, 
         FIG. 23  a top view of an embodiment of the flow line according to an embodiment of the invention with parts of a security system, 
         FIG. 24  a top view and a detail view of the security system of  FIG. 23 , 
         FIG. 25  a top view of an embodiment of the flow line according to the invention with details of parts of a security system, 
         FIG. 26  a top view of an embodiment of the flow line according to an embodiment of the invention with other details of parts of the security system, 
         FIG. 27  a top view of an embodiment of the flow line according to the invention with more elaborated details of a security system and with three detail views. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
       FIG. 1  shows an embodiment of a flow line  1  according to the invention in a top view and—below—in two side views representing two stages of an embodiment of the method according to the invention. 
     Nacelles  3  are assembled on trailers  5   a ,  5   b ,  5   c , . . .  5   n −1,  5   n  at six different work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6  which are assembled along the flow line  1  in a movement direction p. Before the first work station A 1 , i.e. in an upstream area adjacent to the first work station A 1 , there is a first technical station T 1 , and at the end of the flow line  1 , there is a second technical station T 2 . At the first technical station T 1  a new trailer  5   a  with no nacelle  3  on it yet is supplied and attached to the flow line  1 . The width w 1  of the work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6  is approximately 7,5 meters (25 feet), its length d 1  about 15 meters (50 feet). 
     In order to assemble the nacelles  3 , in the flow line  1 , all trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n  of the complete line have to move by the length d 1  of one work station A 1 , A 2 , A 3 , A 4 , A 5 , A 6  to the next station within 3 minutes (preferred) and maximally  5  minutes. The pulling may be done e.g. by a hydraulic cylinder  4  as a pulling means. After the movement of the complete line, the last assembly trailer  5   n  with the completed nacelle  3  will be disconnected from the flow line  1  at the second technical station T 2 . 
     In this sense,  FIG. 1  shows three stages of the assembly process: In the first stage, represented by the top line shown from above, there is the empty trailer  5   a  at the first technical station T 1  (i.e. upstream the first work station A 1 ) and the last trailer  5   n  at the second technical station T 2  still holds a nacelle  3  which is completely assembled. In the second stage, shown in the second line in a side view, the last trailer  5   n  has been removed from the second technical station T 2 . In the third stage shown in the last line, the flow line  1  has been moved one station further so that every nacelle has changed its work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6 . Now, the first trailer  5   a  has reached a first work station A l  while the second last trailer  5   n −1 has now moved into the position to be taken away from the flow line  1 , i.e. the second technical station T 2 . 
       FIG. 2  shows a top view and a section view of the surface of the floor  7  of the first work station A 1 . 
     The trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n  are moved along two lines  17  over a pit  9  which is covered partially by a plate  15 . Along the floor  7 , pillars  11  are positioned which may be used to fix tools on them and/or to install platforms at a greater height. The width w 2  of the pit  9  is about 500 mm, the shortest distance w 3  from the middle of the pit  9  to the pillars  11  is about 5000 mm. 
     As can be seen in the section view at the right hand side of  FIG. 2 , the pit  9  has a depth d 2  of about 500 mm. The pillars  11  each have a base  13  underneath the floor level which stabilizes them. The pit  9  is the location where a tow bar (not shown) is supplied which serves to interconnect all the trailers  5 , as will be explained with recourse to later figures. 
       FIG. 3  shows a complete flow line  1  without trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n  in a top view. The pit  9  for the tow bar extends right from the beginning of the first technical station T 1  to the end of the second technical station T 2 , i.e. along the complete length d 3  of the flow line  1 , which is about 120 meters (400 feet). 
       FIG. 4  illustrates the same flow line  1  in which the pit  9  is partially covered by plates  15  as was shown in the context of  FIG. 2 . In addition, marked areas  19  are indicated, in which stationary piston systems for pulling all the trailers  5  of the flow line  1  can be installed. In these marked areas  19 , the depth d 2  of the pit  9  may increase to about 800 mm. 
       FIG. 5  shows a top view and a section view of the second technical station T 2 . Apart from the features described in the context of  FIG. 2 , four additional pits or holes  21  for installment of a stationary lifting device are shown. The holes  21  are assembled in the form of an oblong. The lifting device may e.g. comprise hydraulic cylinders which are affixed in the holes  21 . The holes  21  have a length d 4  and width w 4  of 500 mm, i.e. they are sqare-shaped. Their depth h 1  is approximately 1000 mm. 
     Thus, at the technical station T 2  there have to be foreseen lifting units to lift the completely assembled trailer  5   n  in order to assemble the support bases of the support of the readily assembled nacelle  3 . Then the assembly trailer  5   n  has to be lowered to the floor. Thereby the nacelle  3  will be placed on the floor carried on the nacelle support. The assembly trailer  5   n  is now free and has to be pulled out with a tractor. 
       FIG. 6  shows a top view of the complete flow line  1  again which is used to explain a distribution of weights. At the first technical station T 1 , only the trailer  5   a  plus some first equipment parts weighing 13 t is positioned, at the first work station A 1 , there is added that of parts of a nacelle  3 , so that the overall weight is 48 t. At the second work station A 2 , the overall weight is 50 t, at the third work station A 3  it is 110 t, whereas at the fourth to sixth work station A 4 , A 5 , A 6 , it has reached the maximum weight of 115 t. It may be noted that the weight values given exclude the weight of the trailers. 
     If one adds all these weights together, one comes to an cumulative weight of 566 t for a flow line with six work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6 . In accordance, a flow line with only five work stations will have a cumulative weight of about 500 t, one with seven work stations about 650 t and one with eight work or assembly stations about 750 t. This cumulative weight has to be moved during the assembly process. Therefore, it can be clearly understood that a conveyor belt system is useless for this purpose because it cannot resist the forces due to the massive weights. The amount of cumulative weight is also the reason why the flow line is moved stepwise instead of continuously. The pulling speed during such stepwise movement is about 5 m per minute +/−20%. This means that for moving the flow line  1  one work station A 1 , A 2 , A 3 , A 4 , A 5 , A 6  further over the distance d 1 , it takes approximately 3 minutes. This relatively slow movement makes sure that no parts of the nacelles  3  are in danger of falling down from the trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n  and that a pulling system (not shown) can cope with these enormous weights. 
       FIG. 7  shows a completed nacelle  3  assembled on a trailer  5  which is connected to a tow bar  37  so that it can be part of a flow line according to the invention. 
     The trailer  5  has a rigid horizontal structure  42  which rests on the ground via four wheelsets  23 . On the side of the structure  42  facing away from the wheelsets  23  two central beams  40  protrude from the structure  42 . They are orientated in a perpendicular direction to the plane of the figure. The position of the central beams  40  is between two wheelsets  23  in longitudinal direction along the structure  42 . The structure  42  also comprises an opening  44  facing into the same direction as the central beams  40  which houses a main cable  43 , i.e. a set of preconfigured cables for use during the assembly of a nacelle. 
     In addition to the central beams  40 , the trailer  5  also comprises a fixed support  29  at its right hand side longitudinal end and a replaceable support  27  which can be tilted from a vertical orientation into a horizontal orientation on its left hand side longitudinal end. 
     On the central beams  40  there rests a horizontally orientated nacelle transport frame  25  which carries a central part of the nacelle  3 . The fixed support  29  and the replaceable support  27  also serve to carry the nacelle  3  in a region of the so-called bottom canopy  33  of the nacelle  3  and thus to stabilize the connection between the trailer  5  and the assembly. When aligned in the correct, i.e. desired, way, the upper edge  35  of the bottom canopy  33  of the nacelle  3  is horizontal. 
     At the right hand side of the trailer  5 , there is a hook  31  attached by which the trailer  5  can be connected to a tractor. Instead of a hook  31 , a gooseneck can also be used. 
     Looking at the underside of the trailer  5 , there is shown a tow bar  37  which rests in the pit  9  which was shown in  FIG. 2 . A hydraulic pin  39  protrudes from the longitudinal extension of the tow bar  37  in the direction of the trailer  5 , i.e. out of the ground. The hydraulic pin  39  connects the tow bar  37  to the trailer  5  once it is inserted into a pin receiving area  41  of the trailer  5 . 
       FIG. 8  shows the tow bar  37  in a more detailed side view and in a top view with trailers standing above it. The tow bar  37  has a length d 5  of approximately 120 meters, i.e. a length essentially equal to that of the flow line itself. This one single tow bar  37  thus connects all trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n  of the flow line  1 . Apart from the hydraulic pin  39  the tow bar  37  further comprises floor rolls  45  and side guiding rolls  47  which make possible a smooth gliding along the longitudinal extension of the tow bar  37 . From the top view at the bottom of  FIG. 8  it can be understood that with the tow bar  37  being in the pit  9 , workers can walk all around the nacelle  3  without being hindered by the tow bar  37 . For that purpose, the upper edge of the tow bar  37  must be essentially even with the ground or a covering plate (not shown) must cover the pit as shown in  FIG. 2 . 
       FIG. 9  shows the preferred and maximum dimensions involved in an arrangement of a trailer  5  and a nacelle  3  as depicted in  FIG. 7 . The preferred length d 6  of the bottom canopy  33  is 8980 mm, while its maximum length d 7  is 9350 mm. Its preferred maximum width w 6  is 2500 mm, while its biggest possible maximum width w 5  is 2800 mm. The height h 2  of the trailer  5  up to the upper end of the rigid structure  42  is 800 mm. 
       FIG. 10  shows in a side view both the combination of a trailer  5  with a nacelle  3  and a trailer  5  alone. From the bottom drawing, it can be seen that both the fixed support  29  and the replaceable support  27  can be adjusted in height. 
       FIG. 11  shows the trailer  5  in a side view and a top view. Apart from the features already discussed in the context of previous figures, there are shown two adapters  49 ,  51  for positioning the central beams  40  which are not drawn in this figure for reasons of clarity. The first adapter  49  comprises four pin-like railings protruding from the plain of the plain of the rigid structure  42 . They are aligned in such way that a beam  40  can be fixed on them in a rectangular way with respect to the longitudinal extension of the trailer  5 . Accordingly the second adapter  51  is aligned, however it comprises a base plate of a height of 108 mm from which its pins protrude. This way it can be made sure that the bottom canopy  33  is orientated such that its upper edge is horizontal (cf.  FIG. 7 ). 
       FIG. 12  shows the trailer  5 , again in a side view and a top view, with the beams  40  attached to the adapters  49 ,  51 . 
       FIG. 13  shows the trailer  5 , again in a side view and a top view, furthermore equipped with a main cable  43  comprising—as can be seen in the top view—a set of single cables that are all assembled on a wheel drum as a de-winder so that they can be readily rolled off for installation purposes during the assembly of the nacelle. This way some part of the assembly parts, i.e. the cables are readily at hand any time during the assembly. 
       FIG. 14  shows the same trailer in the same views with the nacelle transport frame  25  attached. The transport frame is carried by the central beams  40 , and, as can be seen in the top view, it has an opening  53  through which cables from the main cable  43  can be led. 
     This effect becomes clearer when looking at  FIG. 15 . which shows in a side view on top the trailer  5  in the same state as in  FIG. 14 , however with the fixed support  29  and the replaceable support  27  being adjusted to the necessary height, the replaceable support  27  being in an upright position. 
     In the bottom drawing, the bottom canopy  33  of a nacelle  3  is shown as it is fixed to the transport frame  25 . The loaction of the opening  53  of the transport frame  25  corresponds to that of a canopy opening  55  of the bottom canopy so that cables from the main cable  43  can be introduced into the nacelle  3  through these openings  53 ,  55 . 
       FIG. 16  shows the trailer  5  in two side views with the fixed support  29  and the replaceable support  27  being lowered, the replaceable support  27  being in a horizontal orientation. As can be seen in the bottom drawing, the nacelle  3 , of which only the bottom canopy is shown, can be lifted off the trailer  5  as it is only fixed to it via the transport frame  25  resting on the beams  40 . 
       FIG. 17  shows the process of moving the flow line  1  one station further. To this end, a pulling means  4  comprising a stationary piston system is attached to the tow bar  37 . As the tow bar  37  connects all trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −2,  5   n −1,  5   n , by pulling it using the stationary piston system  4 , all the trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −2,  5   n −1,  5   n  will be pulled at the same time along a distance of 15 meters. After the moving process, the tow bar  37  is in a forward position and the connections of the tow bar  37  to the trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −2,  5   n −1,  5   n  can be disengaged. When all the trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −2,  5   n −1,  5   n  are at a standstill again, the stationary piston system  4  can move the tow bar  37  back into its previous position, i.e. its resting position. 
       FIG. 18  illustrates according to one embodiment of the invention, the take-out of the trailer  5  in a side view. 
     In the initial situation, shown in the top drawing, the_assembly trailer  5  has been pulled 15 m to the technical station T 2 . The tow bar  37  has been disconnected. The 
     fixed support  29  and the replaceable support  27  are lowered, the replaceable support  27  being in a horizontal orientation. The trailer  5  is positioned such that the position of its wheelsets  23  correspond with those of hydraulic lifting units  59  which are inserted into the ground in the holes  21  which have been mentioned in the context of  FIG. 5 . This position is defined by markings on the ground (not shown). The correct positioning can be checked by a sensor (also not shown). The hydraulic lifting units  59  have “elefant feet” at their top facing in the direction of the trailer  5 . 
     In a lifting situation, these hydraulic lifting units  59  have been lifted so that the trailer  5  including the center beams  40 , the transport frame  25  and the nacelle  3  is in a lifting position. The hydraulic lifting units  59  thus carry a weight of 110 t plus the weight of the trailer  5 . The stroke of the hydraulic lifting units  59  is approximately 400 mm and at a maximum 500 mm. 
       FIG. 19  illustrates for various embodiments of the invention, the lifting of the assembly trailer  5  at the second technical station T 2 . 
     Beforehand, it can be stated that the designs of the assembly trailer  5  and that of the lifting units  59  have to be concerted. The lifting units  59  are installed in the holes  21  in the ground of the flow line  1  at the technical station T 2 . They comprise each a hydraulic cylinder  61  in which a piston  65  can move upwards and downwards. The piston  65  has an end directed away from the cylinder  61  which comprises a so-called elephant foot  63 . In a resting position as shown in the left hand side drawing, the elephant foot  63  is in one plane with a lifting plate  64  which closes the lifting unit  59  in the upward direction. The design of the lifting units  59  should enable their installation in one step, the access to the lifting units  59  can be allowed through the lifting plate  64 . The operation of four lifting units  59  at the same time enable a save and synchronized lifting process of the assembly trailer  5 . Each lifting unit  59  can carry a weight of approximately 600 kN, because they must be designed to carry the complete pulling force of a pulling tractor (in case the tractor is running over the lifting unit during pulling). 
     The upper and lower end position of the lifting units in operation are scanned by position switches (not shown). 
     As mentioned, the stroke of the lifting units is 500 mm, assuming that the assembly trailer  5  is 800 mm in height, and accordingly more if the height of the assembly trailer  5  is smaller. 
     In order to secure the lifted assembly trailer  5 , mechanical fall arresters (not shown) are connected to the lifting units  59 . 
       FIG. 20  illustrates for various embodiments of the invention, the assembly of nacelle supports  67 . 
     As shown in the top drawing, once the trailer  5  with the nacelle  3  has been lifted by the lifting units  59 , four legs  67  are attached as nacelle supports to the two central beams  40 , one at either end of each beam  40 . 
     The bottom drawing shows that the lifting units  59  are then lowered into their resting position, while the transport frame  25  now rests via the beams  40  and the legs  67  on the ground. This lowering of the assembly trailers  5  is reached by synchronous lowering of the four hydraulic lifting units  59 . All four legs  67  of the nacelle support must be placed to floor at the same time. Therefore the height difference of approx. 110 mm between the front and the rear of the nacelle  3  has to be taken in consideration. 
     When the legs  67  of the nacelle support are seated on the floor, the assembly trailer  5  releases from the central beams  40  and the nacelle support carries the transport frame  25  with the nacelle on top. 
       FIG. 21  illustrates for various embodiments of the invention, the further process after the trailer  5  and the nacelle  3  have been separated. 
     The assembly trailer  5  is driven away by a tractor or a fork lift (not shown). The assembly trailer  5  and all superstructure on it have to stay within a maximum height of 1000 mm, because the clearance under the central beams  40  is 1100 mm. 
     The nacelle  3  is now parked on the legs  67  of the nacelle support and ready for take over by a transport trailer. 
       FIG. 22  illustrates for various embodiments of the invention, the driving under, lifting and driving out of the nacelle from the second technical station T 2 . 
     The top drawing shows the nacelle  3  on the support with a transport trailer  69  being driven underneath it, in between the four legs. 
     The bottom drawing shows the transport trailer  69  in a lifted position which means that the nacelle  3  including the legs  67  has been lifted of the ground by the transport trailer  69 . Now, the transport trailer  69  can drive away or be driven away by a tractor and carry the nacelle to a destination of delivery. 
       FIG. 23  illustrates for various embodiments of the invention, a safety system definition and operating areas. 
     The system indicates that the flow line  1  is in condition for moving and all safety requirements are fulfilled and the safety system is in an ok-status. 
     The first five work stations A 1 , A 2 , A 3 , A 4 , A 5  constitute working areas for assembly and logistic operators. During the pulling of the flow line this area is locked by a barrier tape. The sixth work station A 6  constitutes a working area for cleaning and painting. An operator working in the area of this work station A 6  should be specially trained and equipped, e.g. with a breathing protector. 
     At both technical stations T 1  and T 2  only right trained operators should be admitted to work. 
     For the purpose of control, two operator panels  71  are positioned between the technical stations T 1 , T 2  and their neighbouring work stations A 1 , A 6 . The trained operators working in the technical stations T 1 , T 2  and in the sixth work station A 6  can use these operator panels  71  in order to make sure than in preparation of, during and in the aftermath of the pulling process of the flow line  1  no personell is present in close proximity of the flow line  1 . Additionally, clearance buttons  73  between all the work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6  are installed so that staff can signal they have cleared the areas of the work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6 . 
       FIG. 24  illustrates a safety system according to one embodiment of the invention. 
     The safety system comprises a barrier tape  79  which can be stretched in between posts  75  which are equipped with roller shutters  77  on top in which the barrier tape  79  can be fixed. As depicted at the right hand side drawing, such barrier tape  79  can be spread between the first technical station T 1  and the first work station A 1  during assembly operation time in order to prevent walking in of operators into the first technical station T 1  during the supply of a next assembly trailer. 
     When the barrier tape  79  is taken out of the bracket of the roller shutter  77  an optical and an acoustical alarm signal is activated automatically (security query). There is also an emergency button at each post  75 . Optionally a radio controlled dead-man&#39;s control can activate a security alarm. 
       FIG. 25  illustrates a safety system according to various embodiments of the invention in the region of the second technical station T 2 . 
     The outlet gate of the paint booth at the sixth work station A 6  is normally closed and has a security trigger with a connection to the security system. 
     During the operation of the tractor, i.e. while a completed nacelle  3  is pulled away from the second technical station T 2 , and in preparation of this (lifting of the assembly trailer  5   n , assembly of the nacelle support legs  67 , take-out of the assembly trailer  5   n ) the security trigger at the roll shutter is activated. 
     The rest of the second technical station T 2  including the complete driving area of the tractor is locked by barrier tape  79 . When the barrier tape  79  is taken out of the bracket of any of the roller shutters  77  an optical and acoustic alarm signal is activated automatically and the engine of the tractor is powered off directly. There is also an emergency button at the posts  75 . When the tractor is pulling out the empty assembly trailer  5  of the second technical station T 2 , or driving in the transport trailer  69  or driving out the nacelle  3  with the transport trailer  69  the barrier tape  79  at the end of the flow line  1  is open and the security trigger at the brackets is deactivated. Again, a radio controlled dead-man&#39;s control can be used to activate a security alarm by a foreman or team leader. 
       FIG. 26  illustrates the same safety system as in  FIG. 25  while the assembly trailer  5  is lifted together with the nacelle  3  (cf.  FIGS. 18 and 20 ). 
     The station is locked with barrier tape  79  as described before and the safety system is working. The outlet gate of the paint booth at the sixth work station A 6  is closed and has a security trigger with is in connection to the security system. During operation of the lifting units  59  the security trigger at the roller shutter  77  is activated. The upper and lower end position of the lifting units  59  is scanned by position switches (not shown). 
     The assembly of the legs  67  to the center beams  40  is only allowed when the end position of the lifting units  59  is achieved and a mechanical fall arrester is in position. The tractor is only free to pull out the empty assembly trailer  5  when the lifting units  59  are in home position. This must be verified by the safety system. 
       FIG. 27  illustrates a safety system for a complete flow line  1  pulling according to various embodiments of the invention. 
     The complete flow line  1  area is locked by a barrier tape  79  during pulling of the flow line  1 . At all the work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6  and at the technical stations T 1 , T 2  nobody, with exception of a tractor driver inside the tractor cabin is allowed be inside the locked area. The inlet and outlet roller shutter of the paint booth at the sixth work station A 6  are open, as checked by a safety switch. 
     The tow bar  37  is connected to all the assembly trailers  5   a ,  5   b ,  5   c  . . . ,  5   n −1,  5   n , which is shown with a mechanical-optical indication. When the barrier tape  79  is taken out of the bracket of a roller shutter  77  an optical and acoustical alarm signal is activated automatically due to a security query and the engine of the tractor is powered off directly. The same happens if an emergency button is pressed. Such emergency stop button can be realized on middle posts  83  which also house tape retractors. 
     In addition, there is a radio controlled dead-man&#39;s control in operation to activate a security alarm by foreman or team leader. 
     Before the pulling takes place, the end of the operation time (e.g. 5 h) of a cycle is indicated in all stations by a count-down time display. 
     Before the end of the operation time the technical and work stations T 1 , T 2 , A 1 , A 2 , A 3 , A 4 , A 5 , A 6  are prepared for pulling of the flow line  1 : 
     Swiveling all pivot cranes and manipulators in base position 
     Placement of all tools in tool carriers, nacelles  3  are made clear with no cables (power, compressed air, vacuum cleaner, etc.) lying inside the nacelles  3 . 
     All support structures respectively all kid sets are moved away from respectively out of the nacelles  3 . 
     All platforms are moved out of the flow areas. 
     After this all barrier tapes  79  are closed. 
     At the end of the operation time in all technical or work stations T 1 , T 2 , A 1 , A 2 , A 3 , A 4 , A 5 , A 6  a foreman or team leader gives a signal that the station is fully prepared for pulling of the flow line  1  and that nobody is inside the locked area (also not inside the nacelles  3 ). 
     In the following example, it is assumed that not a pulling means  4  as shown in  FIG. 17  is used for pulling the complete flow line  1 , but a tractor. The safety system has signal lights to indicate: 
     GREEN During assembly operation 
     GREEN Flashing Last 5 minutes of assembly operation, all operators has to leave the marked area and have to close all barrier tapes  79   
     YELLOW Work stations A 1 , A 2 , A 3 , A 4 , A 5 , A 6  are prepared for pulling, which means that all operators are out of the stations and that the barrier tapes  79  are closed (brackets scanned by a security query) and the foreman or team leader pressed a button to give clearance for puling. Safety is ok and all safety requirements are fulfilled. 
     RED The tractor starts engine and can start pulling of the flow line  1 . 
     RED off Red is off when the engine of the tractor is off 
     GREEN Green is shown when the pulling is completed and the engine of the tractor is off 
     As indicated just before, to be operated together with the invention may, for one embodiment of the invention, a terminal tractor be used. The said terminal tractor may comprise:
     a coupling to an assembly trailer  5 ,   a gooseneck,   an after-treatment of exhaust gases (Diesel or LPG)   connectors for radio controlled safety circuit to stop engine   a hydraulic unit with quick-connectors to assembly trailers  5 ,  5   a ,  5   b , . . . ,  5   n −2,  5   n −1,  5   n , if required.   

     For various embodiments of the invention, the assembly trailers  5 ,  5   a ,  5   b ,  5   n −2,  5   n −1,  5   n  may e.g. be assembly trailers for loads of 110 t comprising:
     coupling to a tow bar at both front and end,   self steering device to secure straight pulling, if required,   couplings for pull rods of tractor,   gooseneck connectors at both front and end,   four jacking points for lifting the complete assembly trailer,   height adjustable and/or swivelable supports for the bottom canopy  33  of a nacelle  3 ,   loading adapters  49 ,  51  for center beams  40 ,   openings  44  for a main cable  43 ,   mechanical-optical indications of the status of connection (e.g. red connected/yellow disconnected)   

     Also for one embodiment of the invention, in order to list a complete flow line system, the system may comprise:
     one stationary piston system  4  to move the complete flow line  1  by e.g. 15 meters,   including a hydraulic and a control unit,   one tow bar  37 , crossable with wheel load of e.g. 10 t,   two guidings for front and rear connections to the tow bar  37  to feed in an empty assembly trailer  5   a  in the first technical station T 1  at the start of the flow line  1 , one pusher (for tractor or easy mover) or winch to connect an empty assembly trailer  5   a  to the tow bar  1  at the first technical station T 1 ,   four moveable lifting units  59  to lift and lower the loaded assembly trailer  5  (such as each 600 kN, 500 mm stroke),   one moveable hydraulic and control unit including connection to a safety system for moveable lifting units,   one safety system, including   a safety system (PLC or PC) in cabinet,   Software for safety system,   operator panels,   clearance buttons (such as two for each technical or work station left and right hand),   multiple emergency buttons,   multiple tape barriers (posts, tape retractor and brackets) left and right side of the work stations, all brackets with security query,   a tape barrier (posts, tape retractor and brackets) between the first technical station T 1  and the first work station A 1 , all brackets with security query,   a tape barrier (posts, tape retractor and brackets) around the second technical station T 2  including complete working area of tractor during pulling, all brackets with security query,   safety switches to record the position of inlet and outlet roller shutters and all other doors of the paint booth,   death-man&#39;s controls to stop the engine of the tractor during the pulling phase,   a radio-controlled safety circuit to stop the engine of tractor,   a safety circuit to stop the pulling of the stationary pulling system  4 , if installed, count-down time displays to show remaining assembly operation time, for each station, signals lights (green, yellow, red) to indicate operation status of the flow line  1 .   

     Optionally the system may also comprise:
     a moveable hydraulic and control unit including a connection to a safety system to connected to lifting units of the assembly trailers  5 ,  5   a ,  5   b , . . . ,  5   n −2,  5   n −1,  5   n,      lifting units  59 , installed in holes  21  to lift and lower the loaded assembly trailer  5 ,  5   a ,  5   b , . . . ,  5   n −2,  5   n −1,  5   n  (such as each 50 t, 500 mm stroke)   a hydraulic and control unit including a connection to a safety system for stationary lifting units  59 .   

     Although the present invention has been disclosed in the form of preferred 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. In particular, this applies to the shape and kind of tow bar, but also to the pulling means used to pull the whole flow line as well as the last trailer in the line and/or the completely assembled nacelles. 
     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.