Riser stabilisation

Abstract

A method of stabilizing a riser includes applying an oscillating force to the riser, using one or more thrust units on the rise, so as to counteract oscillating movement of the rise, preferably having a first component oscillating at a period between 3 and 20 seconds and a second component oscillating at a period between 1 and 5 minutes, where the thrust units may be positioned at antinodes of the oscillating movement and preferably each comprise three pairs of thrusters respectively configured to provide thrust in three evenly spaced directions.

1 . A method of stabilizing a riser connecting a structure on the seabed to the sea surface, the method comprising:
 applying an oscillating force to the riser, using a thrust unit on the riser, so as to counteract an oscillating movement of the riser.   
     
     
         2 . A method according to  claim 1 , wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes. 
     
     
         3 . A method according to  claim 1 , wherein the oscillating movement of the riser is at a resonant frequency of the riser. 
     
     
         4 . A method according to  claim 1 , further comprising:
 modelling the oscillating movement of the riser;   determining a location for the thrust unit on the riser based on the modelling; and   locating the thrust unit on the riser at the determined location.   
     
     
         5 . A method according to  claim 4 , wherein the determined location is based on a predicted location of an antinode of the modelled oscillating movement. 
     
     
         6 . A method according to  claim 1 , wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions. 
     
     
         7 . A method according to  claim 6 , wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction. 
     
     
         8 . A riser stabilization system for stabilizing a riser connecting a structure on the seabed to the sea surface, comprising:
 at least one sensor for detecting movement of the riser;   a thrust unit for applying a force to the riser; and   a controller configured to, in use, cause the thrust unit to apply an oscillating force to the riser so as to counteract an oscillating movement of the riser detected by the at least one sensor.   
     
     
         9 . A system according to  claim 8 , wherein the oscillating movement of the riser has a period of between 1 second and 30 minutes. 
     
     
         10 . A system according to  claim 8 , wherein the oscillating movement of the riser is at a resonant frequency of the riser. 
     
     
         11 . A system according  claim 8 , wherein the thrust unit comprises at least two pairs of thrusters arranged to provide thrust in non-parallel directions. 
     
     
         12 . A system according to  claim 11 , wherein the thrust unit comprises at least three pairs of thrusters, each being arranged to provide thrust in a non-parallel direction. 
     
     
         13 . A system according to  claim 8 , wherein the sensor is arranged to detect a horizontal velocity of the riser. 
     
     
         14 . A thrust unit for mounting to a riser, the thrust unit comprising:
 a frame configured to be mounted to the riser; and   a plurality of pairs of thrusters, each thruster being mounted to the frame,   wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction.   
     
     
         15 . A thrust unit according to  claim 14 , wherein, for each pair of thrusters, a first thruster of the pair of thrusters is configured so as to be on an opposite side of the riser to a second thruster of the pair of thrusters, when the frame is mounted to the riser. 
     
     
         16 . A thrust unit according to  claim 14 , wherein the plurality of pairs of thrusters comprises three pairs of thrusters. 
     
     
         17 . A thrust unit according to  claim 14 , wherein the pairs of thrusters are mounted to the frame so that, when mounted to the riser, each of the pairs of thrusters are axially offset from the other pairs of thrusters. 
     
     
         18 . A thrust unit according to  claim 14 , wherein a first thruster of each pair of thrusters is mounted in a first thrust plane and a second thruster of each pair of thrusters is mounted in a second thrust plane that is axially offset along the axis of the riser, wherein the thrusters in each of the first and second thrust planes are configured to generate a net thrust in any direction within the thrust plane. 
     
     
         19 . A riser for connecting a structure on the seabed to the sea surface, comprising a system as claimed in  claim 8  or a thrust unit comprising a frame configured to be mounted to the riser; and a plurality of pairs of thrusters, each thruster being mounted to the frame, wherein each pair of thrusters generates a net thrust in a respectively non-parallel direction, wherein the thrust unit is mounted on the riser. 
     
     
         20 . A computer program product comprising computer readable instructions that, when executed, will cause a processor to perform a method comprising:
 receiving indications of a movement of a riser connecting a structure on the seabed to the sea surface from at least one sensor; and   causing a thrust unit to apply an oscillating force to the riser so as to counteract oscillating movement of the riser detected by the at least one sensor.   
     
     
         21 .- 23 . (canceled)