Integrated, Float-over Deck Design Considerations
Authors: Hartell William David , Beattie S. Michael 
Published: October 2008

Offshore platforms with integrated, lift installed decks have become increasingly common design concepts in the last ten years based on the availability of super heavy lift crane vessels in certain parts of the world. High installation costs for these lifting vessels have led operators to investigate alternate installation methods. The integrated, float-over ('IFO') deck concept has been utilized several times in the last I 7 years. Recent successful design and installation experiences with integrated, float-over decks in the Far East have encouraged operators and designers to consider this concept more frequently for applications in other areas. The purpose of this paper is to describe some of these recent experiences and how the experience might be applied for similar offshore developments in the future. Design and installation considerations are reviewed and recommendations made for future applications of the concept.

The concept consists of a steel topsides deck structure which is conventionally loaded out onto a floating transportation barge. After transportation to the offshore location, the barge is maneuvered ("docked") inside the legs of a platform substructure and installed ("mated") by lowering down the transportation vessel. Depending on the weight of the integrated deck and the directionality and magnitude of prevailing environmental conditions, a passive, partially active, or fully active mating system is utilized for the load transfer operation.

The separate benefits of the integrated deck concept and the float-over deck concept make this an attractive combined concept worthy of application for different sizes and weights of topsides and varied offshore locations, The benefits for design, fabrication, installation, hookup/commissioning, operations, and schedule are summarized for the integrated, float-over deck concept compared with modular, lift installed decks. The paper discusses the development of various details for particular applications. The development of a platform structural configuration based on the weight of the topsides, transportation vessel selection, and required size of vessel opening in the structure are discussed. Installation equipment including vessel systems (tendering, mooring, and rapid ballasting) and load transfer systems (structure tendering, alignment, guides, and passive/active mating/lowering details) are reviewed. Installation (naval architecture/structural) and operational (structural) analyses and design are described.

Introduction

In 1995, two large IFO decks were successfully installed on top of steel substructure jackets for the M Fields in the South China Sea for Sarawak Shell. Another IFO deck, designed to sit on top of a concrete gravity-based substructure, is currently being fabricated and is scheduled for installation in late 1996 on the North West Shelf of Australia for Ampolex, Some aspects of the designs for these three IFO decks are reviewed in this paper along with a general discussion of lFO deck design considerations based on previous industry projects. Safely docking an IFO deck and transport vessel inside an opening in a platform substructure and mating the deck and substructure requires detailed planning and engineering.