Method for use in offshore load transfer and floater and hydraulic device for the same

Abstract

A method for offshore load transfer and a floater for use in such load transfer are described. A hydraulic device for use in a horizontal positioning system for keeping the floater in position is also described. During the mounting and dismantling of large structures offshore, the floater will be exposed to wave and wind forces. By means of hydraulic jacks the floater is kept in a target position, within a horizontal frame. The jacks act as a positioning system and may also act as a sea fastening or sea securing system for the load when it is carried by the floater.

1 . A method for use in offshore load transfer, wherein a floater is taken to a transfer site at sea and loads are transferred between the transfer site and the floater or vice versa, which floater is positioned horizontally relative to a marine structure (or load) at the transfer site before and during the load transport operation, wherein the floater is kept in a target position, within a horizontal frame, relative to the marine structure by means of a horizontal positioning system, with the additional function of reducing low frequency motions by small forces and allowing wave frequency motions through limited forces, thereby protecting marine structures from large bending moments and high shear forces, which system comprises a plurality of hydraulic jacks mounted on the floater by means of cardanic suspension or similar devices, thereby allowing the jacks to tilt in the vertical and horizontal plane, each such hydraulic jack including a cylinder and a piston which is movable longitudinally in the cylinder and has a piston rod projecting from the cylinder, which piston rod is connected to the marine structure (or the load), the cylinder on each side of the said piston being hydraulically parallel connected to a plurality of hydro-pneumatic accumulators each one having a different gas pressure, the number and type of accumulators being related to the required number of different spring characteristics to be obtained in the hydraulic jacks, wherein the spring characteristics provided by a combination of said accumulators have a high spring stiffness from neutral position up to a certain deflection value (related to both sides of the neutral position), and a lower spring stiffness in other parts of the deflection range.  
   
   
       2 . A method according to  claim 1 , wherein after the floater has lifted the load from the marine structure, the horizontal motion of the load relative to the floater is dampened by the spring characteristics of the hydraulic jacks.  
   
   
       3 . A method according to  claim 1 , wherein the load is secured to the floater by closing stop valves to the hydraulic cylinder.  
   
   
       4 . A floater for offshore installation and removal of structural elements on/from a marine structure, comprising a horizontal positioning system for keeping the floater in a target position, within a horizontal frame, relative to the marine structure, which horizontal positioning system has the additional function of reducing low frequency motions by small forces and allowing wave frequency motions through limited forces, thereby protecting marine structures from large bending moments and high shear forces, which system comprises a plurality of hydraulic jacks mounted on the floater by means of cardanic suspension or similar devices which allow the jacks to tilt in the vertical horizontal plane, each hydraulic jack including a cylinder and a piston with a piston rod intended for connection to the marine structure, which piston is movable longitudinally in the cylinder, the cylinder on each side of the said piston being hydraulically parallel connected to a plurality of hydro-pneumatic accumulators which have different gas pressure, the number and type of accumulators being related to the required number of different spring characteristics to be obtained in the hydraulic jacks , wherein the spring characteristics are obtained by a combination of accumulators having a high spring stiffness from neutral position up to a certain deflection value (related to both sides of the neutral position), and a lower spring stiffness in other parts of the deflection range.  
   
   
       5 . A hydraulic device for use in a horizontal positioning system for keeping a floater in a target position, within a horizontal frame, relative to a marine structure, comprising: 
 a cylinder having two opposite ends;    a piston movable longitudinally in the cylinder between said two ends;    a piston rod connected to the piston and projecting from the cylinder at both ends thereof;    which piston defines two opposite chambers in the cylinder, each of these chambers being hydraulically parallel connected to a first hydro-pneumatic accumulator which has a first, lower gas pressure;    a second hydro-pneumatic accumulator which has a second, medium gas pressure; and    a third hydro-pneumatic accumulator which has a third, higher gas pressure, the spring characteristic provided by the combination of the accumulators having a higher spring stiffness from a neutral position up to a certain deflection value (related to both sides of the neutral position), and a lower spring stiffness in other parts of the deflection range.    
   
   
       6 . A hydraulic device according to  claim 5 , by comprising a hydraulic pump with variable displacement and hydraulically connected to each of said chambers in the cylinder.  
   
   
       7 . A hydraulic device according to  claim 6 , wherein: 
 said hydraulic pump is reversible, with two opposite outlets/inlets,    the pump is hydraulically connected to a respective one of said chambers in the cylinder via a line from a respective outlet (inlet), and    two adjustable safety relief valves are connected between these lines.    
   
   
       8 . A hydraulic device according to  claim 7 , wherein a flow restrictor is provided in each of said lines.  
   
   
       9 . A hydraulic device for use in a horizontal positioning system for keeping a floater in a target position, within a horizontal frame, relative to a marine structure, comprising: 
 a cylinder having two opposite ends;    a transverse wall in the cylinder, between said ends, whereby two chambers are formed in the cylinder;    a respective piston movable longitudinally in a respective chamber;    a piston rod connected to the two pistons and projecting from the cylinder at both ends thereof;    each piston defining two respective sub-chambers in the respective cylinder chambers, between the transverse wall and the respective piston and between the respective piston and a respective one of said cylinder ends, each of said sub-chambers adjacent to the transverse wall being hydraulically parallel connected to a first hydro-pneumatic accumulator which has a first, lower gas pressure;    a second hydro-pneumatic accumulator which has a second, medium gas pressure; and    a third hydro-pneumatic accumulator which has a third, higher gas pressure, wherein the spring characteristic provided by the combination of the accumulators has a higher spring stiffness from a neutral position up to a certain deflection value (related to both sides of the neutral position), and a lower spring stiffness in other parts of the deflection range, the two other sub-chambers being connected to each other via a fixed or adjustable flow restrictor.    
   
   
       10 . A hydraulic device according to  claim 9 , wherein each of said cylinder ends forms a piston in a respective external chamber.  
   
   
       11 . A hydraulic device according to  claim 10 , wherein said external chambers are flow-connected to each other.  
   
   
       12 . A hydraulic device according to  claim 5 , wherein: 
 the first hydro-pneumatic accumulator comprises a first accumulator cylinder having two opposite first accumulator cylinder ends, a first accumulator piston movable longitudinally in the first accumulator cylinder and defining a first hydraulic chamber and a first gas chamber in the first accumulator cylinder, and an end stop in the first accumulator cylinder for the first accumulator cylinder piston between the said first accumulator ends;    the second accumulator comprises a second accumulator cylinder and a second accumulator piston movable longitudinally in the second accumulator cylinder and defining a second hydraulic chamber and a second gas chamber in the second accumulator cylinder; and    the third accumulator comprises a third accumulator cylinder and a third accumulator cylinder piston movable longitudinally in the third accumulator cylinder and defining a third hydraulic chamber and a third gas chamber in the third accumulator cylinder,    the dimensions and fill pressures being chosen so that the spring characteristics provided by a combination of said accumulators have a high spring stiffness from neutral position up to a certain deflection value (related to both sides of the neutral position), and a lower spring stiffness in other parts of the deflection range.    
   
   
       13 . The use of a hydraulic device according to  claim 5  for dampening the horizontal motion of a load relative to a floater and securing the load (sea securing) relative to the floater.