Offshore structure installation

Abstract

Improved embodiments, apparatus and method relating to the manufacture and installation of offshore steel support structures, known as jackets, for Wind Turbine Generators, including transition pieces thereof.

1 . A pre-installation subframe comprising pile guides through which piles may be driven into a sea bed and installation points for cooperative engagement with a structure, characterized in that the pre-installation subframe is a lightweight frame structure constituting the lower section of a jacket for supporting a wind turbine, the upper section of the jacket constituting the structure. 
     
     
         2 - 6 . (canceled) 
     
     
         7 . A jacket for supporting a wind turbine, characterized in that the jacket comprises an upper section and a lower section and the lower section constitutes a pre-installation subframe which may be piled to a sea bed separately from the upper section and the preinstallation subframe is a lightweight frame structure comprising ‘n’ hollow tubular components for acting as installation points, ‘n’ crossbraces connecting the installation points, ‘n’ further hollow tubular components for acting as pile guides, each of which is laterally displaced relative to an installation point, and ‘n’ mudmats, each connecting one pile guide to one installation point, wherein ‘n’ is an integer of three or more. 
     
     
         8 - 9 . (canceled) 
     
     
         10 . A method of modular manufacture of a plurality of offshore jacket structures as claimed in  claim 1  wherein the base diameter of the frusto-conical body of the transition piece of each jacket is a common size and each jacket is manufactured to one of two common cross-brace geometries such that a range of overall jacket heights may be produced by making the minimum of alterations, those alterations being main leg length, preinstallation subframe crossbrace length, the height of a cylindrical element of a transition piece, and the lowermost crossbraces of the upper jacket section, which may be either removed or have their lengths and brace angles altered. 
     
     
         11 - 12 . (canceled) 
     
     
         13 . A method of installing a jacket for a wind turbine tower comprising the steps of;
 a. Providing a pre-installation subframe;   b. Providing level sensors fitted to two of the cross-braces of the pre-installation subframe;   c. Providing a levelling system beneath each mudmat of the pre-installation subframe;   d. Providing a sealing system within each pile guide of the pre-installation subframe, towards the upper end of each pile guide;   e. Placing the pre-installation subframe upon the sea bed;   f. Installing piles into the sea bed through the pile guides of the pre-installation subframe;   g. Determining the highest lying mudmat by means of readings from the level sensors;   h. Activating the levelling system beneath the highest lying mudmat so as to raise the mudmat and create a seal between the mudmat and the sea bed;   i. Determining a second mudmat at the far end of a pre-installation subframe crossbrace fitted with a level sensor and also connected to the highest lying mudmat, and inflating the levelling bag beneath said second mudmat to raise this second mudmat until the said level sensor indicates that the attached pre-installation subframe cross-brace is horizontal;   j. Activating the levelling system beneath the remaining mudmat until the other level sensor indicates that a second pre-installation subframe cross-brace is also horizontal;   k. Activating the sealing system within each pile guide; and   l. Pumping grout into the gap between each pile guide and its associated installed pile.   
     
     
         14 - 17 . (canceled)