Float-over deck (FOD) installation is no longer a novel approach for heavy integrated decks. It is estimated that over 20 FOD installations have been carried out in open (unsheltered) waters over the past 20 years, which have proven the reliability and cost-effectiveness of this method.
FOD installation is an alternative to lift-installing integrated decks using heavy-lift crane vessels. Although the former may not be the most cost-effective solution in areas where such vessels are readily available, it is increasingly becoming an important solution for deck installations in remote parts of the world. This is due to the considerable cost associated with crane mobilisation in such areas. FOD installation also accommodates integrated deck weights in excess of the lift capacity of crane vessels.
This paper provides a database of all documented FOD installations in open waters to date and addresses the technical issues and challenges, with particular emphasis on the complexity, reliability and effectiveness of the technology used. Trends in deck weight and installation sea-states are also illustrated, confirming the growing confidence of the industry towards FOD installation. Indicative cost comparisons with alternative installation methods such as a single heavy lift or a multi-lift modular installation are also presented for some cases. The paper also elaborates on the limitations of installation methods used to date. Finally, a brief introduction is made to several recent innovations, which focus on the improvement of conventional FOD installation systems as well as the development of novel systems suitable for developments in Australia and worldwide.
Introduction
Topsides vary in weight, size and configuration. Small decks have been traditionally installed as one unit using low-capacity crane vessels and jack-ups. Medium to large decks have been either modularised to facilitate installation with small crane vessels, or built as integrated decks and installed either by means of heavy-lift crane vessels (HLCVs) or by floating them over the substructure.
Integrated decks have become popular due to the reduced offshore installation, hook-up and commissioning durations. However, the weight and size of the integrated decks installed to date has been limited by the capacities of the installation methods used. Specifically, although the nominal lifting capacity of the largest twin-crane vessel currently available is 14,000t, large decks often require an extended crane radius which, in combination with dynamic effects and uncertainties in weight and centre of gravity (CG), generally result in a considerable decrease in lifting capacity. As a result, it has been common with large topsides to lift additional smaller modules on to the top of the integrated deck. Limiting environmental conditions and hence workability for the lift have also been defined to ensure successful and safe installation. These conditions are dependent on the characteristics of the three systems involved: the crane vessel, transport barge and deck. With the limited number of HLCVs, deck installations are scheduled based on the availability of such vessels, which may influence the development program (especially in remote parts of the world) due to the long mobilisation time. This, in conjunction with the high cost associated with vessel mobilisation, makes the use of such crane vessels a less attractive option in these regions.
Existing technology allows for float-over deck installations of weight well in excess of that feasible with the largest crane vessels available to date, in both sheltered as well as unsheltered or open waters. However, due to the relatively poor motion characteristics associated with conventional float-over systems in open waters, the limiting environmental conditions are often more severe, yielding smaller allowable sea-states and consequently shorter installation weather windows. Nevertheless, float-over deck installations may still be cost-effective in remote regions.
