Method for installing an off-shore structure

Abstract

An off-shore structure is installed in a body of water. The body of water extends above a floor, such as a seabed, and has a water surface. A buoyant pontoon and a top structure are provided separately from each other. The top structure has a deck, a plurality of legs that are connected to the deck, and a jacking system for displacing the legs relative to the deck. The pontoon is submerged below the water surface. The submerged pontoon is connected to the floor with tendons. The top structure is aligned above the submerged pontoon connected to the floor. Then, the legs are jacked-down relative to the deck until the legs abut against the submerged pontoon connected to the floor. Subsequently, the jacked-down legs are attached to the submerged pontoon and the deck is jacked-up relative to the pontoon until the deck is standing above the water surface. A number of risers are installed for providing a fluid communication between the floor and the deck after jacking up the deck above the water surface.

1 . Method for installing an off-shore structure ( 1 ) in a body of water ( 3 ) extending above a floor ( 5 ), such as a seabed, the body of water ( 3 ) having a water surface ( 4 ), the method comprising:
 providing a pontoon ( 7 ) having buoyancy and a top structure ( 9 ) having a deck ( 18 ), a plurality of legs ( 20 ) that are connected to the deck ( 18 ), and a jacking system for displacing the legs ( 20 ) relative to the deck ( 18 ),   submerging the pontoon ( 7 ) below the water surface ( 4 ),   connecting the submerged pontoon ( 7 ) to the floor ( 5 ) with tendons ( 32 ),   aligning the top structure ( 9 ) above the submerged pontoon ( 7 ) connected to the floor ( 5 ),   jacking down the legs ( 20 ) relative to the deck ( 18 ) until the legs ( 20 ) abut against the submerged pontoon ( 7 ) connected to the floor ( 5 ),   attaching the jacked-down legs ( 20 ) to the submerged pontoon ( 7 ) connected to the floor ( 5 ),   jacking up the deck ( 18 ) relative to the submerged pontoon ( 7 ) connected to the floor ( 5 ) until the deck ( 18 ) is standing above the water surface ( 4 ),   characterized in that a number of risers ( 37 ) are installed for providing a fluid communication between the floor ( 5 ) and at least the pontoon ( 7 ) after jacking up the deck ( 18 ) above the water surface ( 4 ).   
   
   
       2 . Method according to  claim 1 , wherein the method comprises drilling a first group of well heads in the floor ( 5 ) after jacking up the deck ( 18 ) above the water surface ( 4 ), and wherein subsequently risers ( 37 ) are installed for providing a fluid communication between said first group of well heads and the deck ( 18 ). 
   
   
       3 . Method according to  claim 2 , wherein the method comprises drilling a second group of well heads in the floor ( 5 ) after drilling the first group of well heads, wherein subsequently risers ( 37 ) are installed for providing a fluid communication between said second group of well heads and the deck ( 18 ). 
   
   
       4 . Method according to  claim 1 , wherein the pontoon ( 7 ) is submerged by decreasing its buoyancy, and after connecting the submerged pontoon ( 7 ) to the floor ( 5 ) with tendons ( 32 ) the buoyancy of the pontoon ( 7 ) is increased so as to tension the tendons ( 32 ) before the legs ( 20 ) of the top structure ( 9 ) are jacked-down. 
   
   
       5 . Method according to  claim 4 , wherein the buoyancy of the pontoon ( 7 ) is adjusted after the legs ( 20 ) of the top structure ( 9 ) have been jacked-down and abut against the pontoon ( 7 ). 
   
   
       6 . Method according to  claim 1 , wherein the buoyancy of the pontoon ( 7 ) is adjusted after risers ( 37 ) are installed. 
   
   
       7 . Method according to  claim 1 , wherein the pontoon ( 7 ) is submerged below the wave active zone ( 6 ). 
   
   
       8 . Method according to  claim 1 , wherein the pontoon ( 7 ) comprises a hollow column ( 28 ) extending substantially vertically along a side of the pontoon ( 7 ), the hollow column ( 28 ) having an upper end ( 29 ) and a lower end ( 30 ), the upper end ( 29 ) extending above the water surface ( 4 ) and the lower end ( 30 ) being submerged. 
   
   
       9 . Method according to  claim 8 , wherein drilling is carried out through the hollow column ( 28 ) after the pontoon ( 7 ) is submerged and connected to the floor ( 5 ) and before the top structure ( 9 ) is aligned above the pontoon ( 7 ). 
   
   
       10 . Method according to  claim 9 , wherein the risers ( 37 ) are guided through the hollow column ( 28 ). 
   
   
       11 . Method according to  claim 8 , wherein the lower end ( 30 ) of the column ( 28 ) is closed. 
   
   
       12 . Method according to  claim 8 , wherein the pontoon ( 7 ) is submerged to a depth such that the lower end ( 30 ) of the column ( 28 ) is located below the wave active zone ( 6 ). 
   
   
       13 . Method according to  claim 1 , wherein the pontoon ( 7 ) comprises a number of openings ( 16 ) that extend through the pontoon ( 7 ), wherein the risers ( 37 ) are guided through the openings ( 16 ) of the pontoon ( 7 ). 
   
   
       14 . Method according to  claim 1 , wherein the tendons ( 32 ) have lower ends ( 34 ) and upper ends ( 33 ), the upper ends ( 33 ) being provided with buoyancy members ( 35 ) that can be removed, and wherein the lower ends ( 34 ) are connected to the floor ( 5 ) before submerging the pontoon ( 7 ), and wherein the buoyancy members ( 35 ) are removed from the upper ends ( 33 ) after submerging the pontoon ( 7 ) and connecting the upper ends ( 33 ) to the submerged pontoon ( 7 ). 
   
   
       15 . Off-shore structure ( 1 ) installed by a method according to  claim 1 . 
   
   
       16 . Method for operating an off-shore structure ( 1 ) in a body of water ( 3 ) extending above a floor ( 5 ), such as a seabed, the body of water ( 3 ) having a water surface ( 4 ), the off-shore structure ( 1 ) comprising:
 a submerged pontoon ( 7 ) connected to the floor ( 5 ) with tendons ( 32 ) that are under tension due to the buoyancy of the pontoon ( 7 ),   a top structure ( 9 ) having a deck ( 18 ) that is standing above the water surface ( 4 ), a plurality of legs ( 20 ) that are connected between the deck ( 18 ) and the submerged pontoon ( 7 ), and   
     which method comprises:
 drilling a first group of well heads, and 
 installing a first number of risers ( 37 ) for providing a fluid communication between the first group of drilled well heads in the floor ( 5 ) and at least the pontoon ( 7 ), 
 operating the first number of risers ( 37 ), 
 
     characterized in that after the first number of risers ( 37 ) are operating a second group of well heads is drilled and a second number of risers ( 37 ) is installed for providing a fluid communication between the second group of drilled well heads in the floor ( 5 ) and at least the pontoon ( 7 ).