Patent Publication Number: US-2015078836-A1

Title: Method of and system for installing foundation elements in an underwater ground formation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a national stage filing of International patent application Serial No. PCTNL2013/050263, filed Apr. 10, 2013, and published as WO 2013154428 A2 in English. 
    
    
     BACKGROUND 
     The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter. 
     Aspects of the invention relate to a method of installing a foundation element, in particular a (mono)pile, in a hard underwater ground formation by means of a driver, such as an hydraulic driver. The method comprises the steps of placing a foundation element on the underwater ground formation, placing a screen for reducing noise input from the driver into the surrounding water, and driving the foundation element into the ground formation by means of the driver while the screen is positioned about the foundation element. Aspects of the invention further relates to a system for installing foundation elements. 
     SUMMARY 
     This Summary and Abstract herein are provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary and the Abstract are not intended to identify key features or essential features of the claimed subject matter, nor are they intended to be used as an aid in determining the scope of the claimed subject matter. The claimed subject matter is not limited to implementations that solve any or all disadvantages noted in the background. 
     An aspect of the invention to further improve noise mitigation, in particular when driving foundations elements in hard subsea ground formations andor when the noise mitigation screen is made of a material having a high modulus of elasticity, e.g. in excess of 100 GPa. 
     To this end, the method comprises attenuating noise transfer from the (underwater) ground formation to the outer wall of the screen during at least part of the driving, preferably by at least substantially dynamically uncoupling at least the outer wall of the screen from the ground formation. 
     It was found that the medium, typically (sea)water, between the foundation element, e.g. a monopile, and the screen is not the only acoustic transfer path and that noise generated by the pile driver is transmitted from the pile to the outer wall of the screen also via the ground formation the screen is resting on or in. This is particularly true when the ground formation comprises hard materials, such as rock, concrete blocks (scour protection) or compacted sand, andor when the screen is made of a material having a high modulus of elasticity. As noise generated by the pulse-like blows of a pile driver contains a wide spectrum of frequencies, the outer wall of the screen is typically excited at one or more eigenfrequencies, which dampen out relatively slowly. By attenuating noise transfer via the ground formation, such excitation is avoided or at least reduced and overall acoustic radiation is further mitigated. 
     Attenuation is achieved passively e.g. by reducing, relative to prior art methods and systems, the stiffness (modulus) of the interface between the ground formation and the lower end of at least the outer wall of the screen, or actively e.g. by establishing the main eigenfrequencies of the screen and providing means for extinguishing these frequencies in the noise generated by the pile driver. 
     In an embodiment, the lower end of the outer wall of the screen is maintained at a distance from, typically above, the ground formation, e.g. by suspending the outer wall from an inner wall of the screen (if the screen is double walled), from the pile (if the screen is single walled) or from a surface vessel. The distance is preferably in a range from 5 to 200 centimeters, preferably 10 to 100 centimeters, preferably 15 to 70 centimeters. 
     In another embodiment, the screen is at least 50%, preferably at least 70% buoyant, or even has a buoyancy of 100% or more (at 100%, the density of screen equals the density of the (sea)water), in which case the screen floats. In this latter configuration, it is preferred that the screen is held in position by one or more weights on the ground formation, such as concrete blocks or a bubble ring resting on the ground formation, and connectors, such as straps or chains, connecting the screen to the blocks or ring. 
     In yet another embodiment, a noise attenuating material, typically in the shape of a ring having a circumference that corresponds to that of the screen, is being positioned between the lower end of the outer wall of the screen and the ground formation. 
     An aspect of the invention further relates to a system for installing foundation elements, in particular (mono)piles, in a hard underwater ground formation, comprising a driver, a surface vessel, and a screen to be placed about the foundation element to reduce noise input from the driver. The system further comprises means for attenuating noise transfer from the ground formation to the outer wall of the screen during at least part of the driving. 
     In an embodiment, the surface vessel comprises, in addition to a crane for securing the driver, a crane for suspending the screen and maintaining the screen at a distance from the ground formation during at least part of the driving. Note is this respect that known vessels comprise a single crane for handling the pile and screen and for securing the pile driver during operation. 
     In another embodiment, the screen is at least 50%, preferably at least 70% buoyant or even has a buoyancy of 100% or more and to that end e.g. comprises one or more chambers andor is made from a material having a density lower than that of water. 
     In yet another embodiment, the screen is provided with a support, attached to or separate from the screen, of a noise attenuating material, preferably of a material having a modulus that is lower than the modulus of the material of the outer wall of the screen and lower than the modulus of the underwater ground formation. During driving, the support is positioned between the lower rim of the screen and the ground formation. 
     Aspects of the invention will now be explained in more detail with reference to the Figures, which show a preferred embodiment of the present method and system. 
     Within the framework of the present disclosure, the words “hard ground formation” refer to formations, natural or artificial, which, at least at the interface with the screen comprise material having a modulus of elasticity in excess of 10 MPa. Typical examples include, but are not limited to, rock (modulus usually in a range from 10 to 90 GPa), densely packed sand (modulus usually 20 to 150 MPa), and so-called scour protection (modulus usually 10 to 90 GPa), i.e. rocks, concrete blocks or the like dumped at the driving site prior to driving and intended to protect installed foundations element from erosion e.g. by strong currents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-section of a first embodiment, wherein the noise mitigation screen is suspended from a surface vessel. 
         FIGS. 2 and 3  are a perspective view and a cross-section of a second embodiment comprising an inflatable cushion. 
         FIG. 4  is a perspective view of a third embodiment comprising a floating screen. 
     
    
    
     It is noted that the Figures are schematic in nature and that details, which are not necessary for understanding the present invention, may have been omitted. 
     DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS 
       FIG. 1  shows an embodiment of a system  1  for installing a monopile  2  in an underwater ground formation  3 , e.g. a seabed. In this example, the monopile  2  has a circular cross-section and a diameter of five meters and is intended to serve, after installation, as the foundation of a wind turbine. 
     The system  1  comprises an hydraulic driver  4  (depicted in  FIG. 2 ), e.g. an IHC Hydrohammer S-1800, connected to a power pack on board of a surface vessel, such as a ship  5  or jack-up barge, a driver screen  6  for securely mounting the driver on the monopile and an anvil (hidden from view by the driver screen) for transmitting impact energy from the driver  4  to the monopole  2 . 
     The system further comprises a noise mitigation screen  7 , made of e.g. steel, to be placed about the foundation element to reduce noise input from the driver  4  into the surrounding water. In this example, the screen  7  comprises an inner wall  8  and an outer wall  9 , i.e. is double walled, has a circular cross-section and an inner diameter of six meters. The double wall provides one or more chambers  10  for air or a porous material and renders the screen  7  50% buoyant. 
     The upper rim of the screen  7  is provided with a detachable extender, which is used to adjust the effective length of screen  7  to the depth of the water at the location where the foundation element is to be installed. In general, it is preferred that, once in place, the sound-insulating screen  7  extends to above the water level. 
     The ship  5  comprises a first crane  11  to lift and manipulate the monopile  2  and the screen  7  and a second crane (not shown) to secure the hydraulic driver  4  during driving. 
     Installation of a monopile is carried out for instance as follows. The cables of the crane are attached to the upper end of a monopile stored on the deck of the ship and the monopile is lifted overboard, manipulated to an upright position, lowered onto the seabed and, if required by the circumstances, allowed to penetrate the scour protection and possibly the seabed under its own weight. At this stage, the monopile  2  is driven, e.g. by means of a vibratory device, into the seabed to a depth of some meters to further stabilize the monopole  2 . 
     The driver  4  is positioned on top of the monopile  2  and the screen  7  is lifted over the monopile  2  and the driver  4 . Alternatively, the screen  7  is placed and the driver  4  is subsequently placed inside the screen  7  and on top of the pile  2 . In this position, the pile  2  is driven to the required depth. Finally, the driver  4  is removed, the screen  7  lifted over the pile  2  and placed back on deck or into the sea, and installation is completed. 
     A distance in a range from 20 to 100 centimeters is maintained between the lower rim of the screen  7  and the seabed, thus dynamically uncoupling the two. As a result, substantially no noise or vibrations are transmitted to the screen  7  via the seabed. Although noise leaks through the annular opening resulting from the distance, the energy of this noise is significantly less that that of the noise generated by excitation of the outer wall of the screen  7  by the seabed, i.e. overall acoustic radiation is further mitigated. 
     In the embodiment shown in  FIGS. 2 and 3 , the screen  7  rests on an annular inflated cushion  12 , which has a modulus of elasticity that is at least one order of magnitude smaller than the modulus of the seabed and the modulus of the screen  7 . Thus, the cushion effectively attenuates at least the higher frequencies, i.e. serves as a low pass filter. 
       FIG. 4  shows a system comprising a screen  7  having a buoyancy in excess of 100%, e.g. achieved by spacing the inner and outer walls of screen  7  farther apart, and a weight  13  for holding the floating screen  7  upright and spaced from the seabed over a distance in a range from 20 to 100 centimeters. In this example, the weight  13  is provided by a ring for generating a bubble screen inside the double walled screen  7 , i.e. the outer diameter of the ring  13  is smaller than the inner diameter of the buoyant screen  7 . The screen  7  is attached to the ring  13  by means of straps  14 . 
     The invention is not restricted to the embodiment described above and can be varied in numerous ways within the scope of the claims. For instance, the outer wall of the screen can be suspended from the inner wall of the screen (resting on the ground formation), e.g. by interconnections between the inner and outer walls. It is preferred that such interconnections are made of or provided with a dampening material.