Method and apparatus for the offshore installation of multi-ton prefabricated deck packages on partially submerged offshore jacket foundations

A method for the offshore installation of multi-ton prefabricated deck package on a jacket foundation, comprising the steps of:

• a. transporting the prefabricated deck package to the site of the jacket base on a first transport barge;
• b. attaching a triangular truss-like lifting assembly to the deck package at multiple positions on the deck package that are at least on generally opposite sides of the deck package, and at upper and lower positions on the deck package respectively and at vertices of the triangular truss-like lifting assembly, the triangular truss-like lifting assembly including at least three chords, including a horizontal chord normally in tension during the lifting process which has a variable length and a diagonally extending chord normally in compression during the lifting process;
• c. structurally supporting the triangular truss-like lifting assembly with one or more lift barges;
• d. removing the transport barge as a support for the deck package so that the deck is supported by the triangular truss-like lifting assembly and lift barges, with the deck package froming a load carrying portion of the triangular truss-like lifting assembly;
• e. alinging the deck package with the jacket so that the deck package can be placed on the jacket;
• and
• f. lowering the deck package upon the jacket by changing the length of the horizontal chord of each triangular truss-like lifting assembly.