BACKGROUND 
       [0001]    The present invention relates to an assembly of a first and second elongate tubular element, such as an underwater pile accommodated within a noise mitigation screen, and a centre system suitable for centring the first elongate tubular element and the second elongate tubular element, and wherein the assembly has a common central longitudinal axis along which axis both the first and second tubular element extend. 
         [0002]    The present invention specifically relates to an assembly having a centre system for centring and/or maintaining centred, a tubular element having a tapered section, or sections of different diameter like a telescopic pile, in particular during hammering of the tubular element having a tapered section. 
         [0003]    The invention further relates to a method for centring a first elongate tubular element and a second elongate tubular element, such as an underwater pile accommodated within a noise mitigation screen, at a common central longitudinal axis along which axis both the first and second element extend 
         [0004]    US 2005/0083783 A1 discloses an underwater energy dampening device which device includes a plurality of vertically spaced bubble producing units. With bubbles produced al various depths, the present invention can effectively attenuate sound and other energy from underwater construction projects in high current or deep water areas. 
         [0005]    U.S. Pat. No. 4,102,147 relates to a submersible positioning and guiding apparatus for pile driving. Use of hydraulic cylinders is shown for positioning a pile within a frame. 
         [0006]    However under circumstances the positioning of the pile in the frame is insufficient leaving e.g. parallel and angular misalignment. In addition, control of the hydraulic cylinders is cumbersome. 
       SUMMARY OF THE INVENTION 
       [0007]    The invention aims to provide a assembly wherein at least one of the problems of known centre system is at least partly solved. 
         [0008]    Yet another object of the invention is to provide an alternative centre system. According to a first aspect of the invention this is realized with an assembly of a first and second elongate tubular element, such as an underwater pile accommodated within a noise mitigation screen, and a centre system suitable for centring the first elongate tubular element and the second elongate tubular element, and wherein the assembly has a common central longitudinal axis along which axis both the first and second tubular element extend and wherein the centre system comprises;
       a first coupling means for fixedly coupling the centre system with one of the first and second tubular elements, and   a second coupling means for engaging the other of the first and second tubular element for centring the other tubular element at the common central longitudinal axis, and   a drive system having at least two drive means arranged around the common central longitudinal axis, each drive means having a line of action for applying a centring force at a circumference of one of the first and second tubular elements towards a centred position of said one of the first and second tubular elements at the common central longitudinal axis, wherein the at least two drive means are mutually coupled for evenly driving said one of the first and second tubular elements for maintaining said one of the first and second tubular elements in its centred position,   and wherein the centre system is fixedly coupled with the one of the first and second tubular elements, and engages the other tubular element for centring the other tubular element at the common central longitudinal axis such that an intermediate space between the one of the first and second tubular elements and the other tubular element is optimal with respect to noise dampening.       
 
         [0013]    This provides the possibility of maintaining a pile with a tapered section in its centred position relative to an outer tubular element such that an intermediate space between the pile and the outer tubular element is optimal with respect to noise dampening. The at least two drive means being mutually coupled for evenly driving said one of the first and second tubular elements for maintaining said one of the first and second tubular elements in its centred position ensures that optimal noise dampening is maintained independent of external loads, like flow or swell, to the first and second tubular elements. 
         [0014]    The centre system of the assembly is also useful when using one of the first elongate tubular element and second elongate tubular element as a template for the other. Thus, said one tubular element provides a reference for the other tubular element. 
         [0015]    In this connection a centred position may be interpreted broadly such that it means e.g. that both an underwater pile and a noise mitigation screen are centred at a common central longitudinal axis such that noise transmission from the pile to the surrounding is dampened. In practice a considerable off centred pile or screen still may result in a dampened noise transmission. 
         [0016]    The centre system is capable to accommodate considerable side forces which occur during offshore use. 
         [0017]    In an embodiment of the centre system, the second coupling means couples the centring system and the other of the first and second tubular element ( 2 ) such that longitudinal movement of the other of the first and second tubular element ( 2 ) along the central axis is allowed. 
         [0018]    In an embodiment of the assembly, the at least two drive means are symmetrically and/or evenly distributed around the central axis. This is even more beneficial for centring the first and second tubular element in a controlled manner. 
         [0019]    In an embodiment of the assembly, the at least two drive means are oppositely arranged around the central axis. 
         [0020]    In an embodiment of the assembly, the line of action of each drive means, intersects the central longitudinal axis. This is beneficial for the effectiveness of the assembly. 
         [0021]    In an embodiment of the assembly, the drive system comprises three drive means. 
         [0022]    In an embodiment of the assembly, the centre system comprises a hydraulic system and wherein the drive system is part of the hydraulic system. 
         [0023]    In an embodiment of the assembly, a first drive means of the at least two drive means comprises a first hydraulic cylinder and a second drive means of the at least two drive means comprises a second hydraulic cylinder, the first and second hydraulic cylinder being mutually volumetrically coupled for evenly driving said one of the first and second tubular elements for maintaining said one of the first and second tubular elements in its centred position. This is beneficial in connection with control of the drive means. Because of the volumetric coupling the first and second cylinder are evenly driven. 
         [0024]    In an embodiment of the assembly, the first and second hydraulic cylinder are mutually hydraulically series coupled for evenly driving said one of the first and second tubular elements for maintaining said one of the first and second tubular elements in its centred position. This is even more beneficial in connection with control of the drive means. Powering one of the first and second hydraulic cylinders, drives the other cylinder as well in an even manner. 
         [0025]    In an embodiment, the assembly comprises a first source of pressurized hydraulic fluid for biasing the said one of the first and second tubular elements towards its centred position. 
         [0026]    In an embodiment of the assembly, the first source of pressurized fluid is directly coupled with only one of the first and second hydraulic cylinder. This is beneficial for ease of operation and construction of the assembly. 
         [0027]    In an embodiment of the assembly, the second coupling means comprises at least one of the following, a levering means, a rolling means, and a sliding means, for engaging the other tubular element for centring the other tubular element at the central axis. In particular the levering means, do even more facilitate the centre system to centre a tubular element having a tapered section. 
         [0028]    In an embodiment, the assembly comprises a plurality of centre systems arranged along the central axis for mutually aligning the first and second elongate tubular element at the central axis. 
         [0029]    The invention further relates to a method for centring a first elongate tubular element and a second elongate tubular element, such as an underwater pile accommodated within a noise mitigation screen, at a common central longitudinal axis along which axis both the first and second element extend, and wherein the method comprises;
       providing a drive system for applying a centring force, having at least two drive means arranged around the central axis,   applying a centring force at a circumference of one of the first and second tubular elements towards a centred position of said one of the first and second tubular elements at the central axis,   evenly driving the at least two drive means for maintaining said one of the first and second tubular elements in its centred position.       
 
         [0033]    The invention further relates to a device comprising one or more of the characterising features described in the description and/or shown in the attached drawings. 
         [0034]    The invention further relates to a method comprising one or more of the characterising features described in the description and/or shown in the attached drawings. 
         [0035]    The various aspects discussed in this patent can be combined in order to provide additional advantages. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0036]    The invention will be further elucidated referring to a preferred embodiment shown in the drawing wherein shown in: 
           [0037]      FIG. 1  in a cross sectional side view an assembly of a centre system and a first and second tubular element; 
           [0038]      FIG. 2  a top view of a centre system according to the invention; 
           [0039]      FIG. 3  a schematic view a hydraulic system of an embodiment of the centre system according to the invention; and 
           [0040]      FIG. 4  a side view of a detail of an embodiment of the centre system according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0041]      FIG. 1  shows in a cross sectional side view an assembly of a centre system  11 , which is schematically depicted, and a first and second tubular element. The  FIG. 1  shows a pile-driving device  1  by means of which a tubular element here a pile element  2  can be driven into the bottom  3  of a body of water  4 . A second tubular element, here an elongate tube  5 , is provided around the pile element  2 . In this case, the elongate tube  5  comprises an outer wall  6  and an inner wall  7 . The pile element  2  and the tube  5  are ideally arranged concentrically with respect to one another along a central axis  13 , with an intermediate space  8  being present between the pile element  2  and the tube  5 . 
         [0042]    In the illustrated embodiment, the pile  2  and tube  5  is of a substantially cylindrical design. In other embodiments (not shown), the tube may of course have another shape, provided that an intermediate space is formed between the outer and inner wall which can reduce the transmission of noise or vibrations to the environment. 
         [0043]    The tube  5  is preferably made from steel walls. The structure of the tube may be self-supporting, which means that no separate supporting structure has to be provided in order to keep the tube in its vertical position. 
         [0044]      FIG. 1  shows that a centre system  11  is provided at the top of the tube  5 . Here, the centre system  11  is schematically depicted. In practice several centre systems  11  may be arranged along the length of the pile  2  for centring and aligning the pile  2  with the tube  5 . This centre system  11  ensures that the source of sound, being the pile  2  while being hammered down by hammering installation  1 , remains centred in the tube  5 , which is an optimal position in view of noise dampening. The diameter of the pile  2  can vary along its length, therefore the centre system is adjustable so that this variation can be allowed and in addition any intended narrowing and/or widening of the pile  2  can be accommodated by the centre system. The pile  2  may for example have a tapered section, or sections of different cross section like a telescopic pile. 
         [0045]    The dimensions of the centre system vary, depending on the dimensions of the sound source, being the pile  2 . The pile may have a characteristic diameter of 4-6 m or more, the diameter of the tube  5  will in practice be 7 m or more, so that the distance between the sound source and the inner side of the tube is sufficiently large to prevent contact noise (that is to say transmission of noise by direct contact between the sound source and the tube). 
         [0046]      FIG. 2  shows a top view of a centre system  11  according to the invention. Here, the centre system  11  is fixedly coupled with tube  5 , and engages the pile  2  for centring the pile  2  at the central axis  13 . The first coupling means  30   a ,  30   b ,  30   c  fixedly couple the centre system  11  with the tube  5 . The second coupling means  31   a ,  31   b ,  31   c  engage the pile  2  for centring the pile at the central axis  13 . The pile  2  is able to slide with respect to the second coupling means  31   a ,  31   b ,  31   c . The drive system has three drive means  28   a ,  28   b ,  28   c  evenly arranged around the central axis  13 . Here, each drive means  28   a ,  28   b ,  28   c  has a line of action  32   a ,  32   b ,  32   c  for applying a centring force at a circumference of the pile  2  towards a centred position of the pile at the central axis  13 . 
         [0047]      FIG. 3  shows a schematic view a hydraulic system  14  of an embodiment of the centre system  11  according to the invention. The hydraulic system  14  is divided in number of hydraulic sections  18 ,  19 ,  20  and  21 , separated by respective cylinder pistons  22 ,  23 ,  24 . The hydraulic sections, though separated with respect to hydraulic fluid itself, are series coupled with respect to displaced volume of hydraulic fluid. When during operation, the piston  22  of the first hydraulic cylinder is forced to move over a certain stroke, e.g. because of a tapered section of a pile  2 , a certain volume of hydraulic fluid is displaced in the second hydraulic section  19 . The piston diameter  23  of a subsequent hydraulic cylinder  26  is chosen such that the displaced volume of the second hydraulic section  19  forces the piston  23  of the second hydraulic cylinder to move over a stroke that is equal to that certain stroke of the piston  22  of the first hydraulic cylinder  25 . This way, the pile  5  is maintained in its centred position at the central axis  13 . 
         [0048]    It is conceivable that the hydraulic system  14  comprises two accumulators as a source of pressurized fluid for driving the hydraulic cylinders. Each end of the hydraulic system is coupled with one of the two accumulators. One accumulator  37  is shown, the other accumulator (not shown) is coupled with hydraulic section  21  which section in  FIG. 3  is coupled with valve unit  38 . Providing two accumulators is beneficial since in that case no supply conduits to the centre system are required during driving of the pile while still providing a controlled actuation of the hydraulic cylinders of the centre system  1 . 
         [0049]      FIG. 4  shows a detail of an embodiment of the centre system according to the invention. 
         [0050]    A lever arrangement  34  is applied as a first coupling means to fixedly couple the centre system  11  with the tube  5 . A contact wheel  35  is applied as a second coupling means to engage the pile  2  for centring the pile at the central axis  13 . The pile  2  is able to slide with respect to the wheel  35 . The running surface  32  of the wheel  35  engages the outer circumference  33  of the pile  2 . Here, a drive means  28   a  is coupled with the tube  5  and the lever arrangement  34  for applying a centring force at a circumference of the pile  2  towards a centred position of the pile at the central axis  13 . The drive means  28   a  applies the centring force by means of the lever arrangement  34  and the wheel  35 . The drive means  28   a  is hingeably coupled with the pile  2 . The drive means  28   a  is couples with the lever arrangement  34  is a sliding manner. 
         [0051]    It will also be obvious after the above description and drawings are included to illustrate some embodiments of the invention, and not to limit the scope of protection. Starting from this disclosure, many more embodiments will be evident to a skilled person which are within the scope of protection and the essence of this invention and which are obvious combinations of prior art techniques and the disclosure of this patent.
The invention relates to an assembly for centring a first elongate tubular element and a second elongate tubular element, such as an underwater pile accommodated within a noise mitigation screen, at a common central longitudinal axis along which axis both the first and second element extend, and wherein the centre system is provided with first coupling element for fixedly coupling the centre system with one of the first and second tubular elements, and second coupling element for engaging the other tubular element for centring the other tubular element at the central axis.