Abstract

With the advent of a new generation of increased capacity semi-submersible crane vessels (SSCVS) in the 1980s (the DB102 and the M7000), new concepts in twin-crane lift installed jackets and integrated decks began to emerge. During the late 1980s projects such as Veslefrikk, Gyda and Kittiwake took advantage of the increased lift capacity available. Whilst each of these projects broke lift records, the SSCV lift capacities were not fully utilised. However, in the early 1990s lifting at the limit has really come of age.

This paper critically examines the close correlation between installation vessels and the trend towards larger and heavier lifts, especially for integrated decks. The limitations of the SSCVs are reviewed, and the basic methods of "tailoring" jacket and deck design to maximise the SSCV lift capacities are identified. These methods are then reviewed and questions raised for the future.

Introduction

When the 1993 offshore work season draws to a conclusion, an extraordinary number of heavy integrated decks will have been installed and brought on-stream. It is now time for the industry to take stock and evaluate whether these heavyweight structures have been as cost effective as their owners had originally expected.

Similarly, but perhaps economically more justifiable, a large number of heavy lifted jackets (5000 Tonnes plus) will have been installed, providing clear cost savings when compared to barge launched structures.

DEVELOPMENT OF INTEGRATED DECKS

The concept of an integrated topsides facility is perhaps as old as the offshore industry itself. As the North Sea province matured, and exploration moved further north, production facilities grew in complexity and size. They could not be confined to single liftable structures, and hence the principle of modularisation was born.

In the early 1970s, the weights of such modules were limited by the lifting capacity of the second generation derrick barges, such as Thor (2000 short ton capacity), and the consequential costs of hook-ups were unavoidable. The advent of the Balder and Hermod (third generation heavy lift vessels), in the late 1970s, was a quantum leap in offshore lifting technology, and designers immediately availed themselves of the increased lifting capacity such semi-submersible crane vessels (SSCVs) afforded them, and module weights increased dramatically (see Figure 1).

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