Life-Cycle Design Of Semisubmersible Platforms
Authors: Van Roosmalen C.I. , Reijmers J.J. , Salza Paolo , Wittenberg Lutz 
Published: October 2008

This paper describes a method and prototype design system developed to include life-cycle cost considerations in the preliminary design of semi submersibles (semTs) as a basis for floating production. Life-cycle costs addressed include initial as well as in service costs. The latter are based on the estimated probability of repair.

Introduction, floaters for oil and gas production

In the early days of offshore oil and gas production the interest was focused on locating and developing large fields which were easily accessible and exploitable. Today, however, new fields are often small and may be located in more remote or deeper waters. Development of new fields today requires advanced technology to overcome the difficulties of working in deep water and flexible production facilities which can be deployed elsewhere when fields are depleted. Furthermore, cost effective means of exploitation are needed for operations to be profitable at all. This requirement leads to a demand for low initial cost and high reliability to limit in-service cost. A further requirement is flexibility to accommodate different types of production equipment and ease of conversion.

Apart from specific technical requirements, today's situation calls for a life-cycle cost effective solution. Semi-submersible based floating production units could be such a solution as they can be moved with relative ease and costs do not rise extremely with increased water depth. Floater based production units enable motion sensitive processes to be continued in all kinds of weather.

In contrast to floater based drilling rigs which can be drydocked regularly, production floaters must remain on station for 15 to 25 years. Removal for preventive maintenance or repair is out of the question for a production floater. Inspection, maintenance and repair (IRM) must be minimized and execution at sea should be possible. This requires a robust structure and the hi$h cost of offshore maintenance activities should be taken into consideration in the design process.

To address IRM aspects early in the design process a method and prototype design system, the Heavy Floater Expert System (HFES), was developed. The objective of the HFES is to be able to optimize the life-cycle cost of a floater from initial design to scrapping.

In the development of the HFES the design stages were identified where choices are made which have an impact on life-cycle cost. The system allows evaluation of design choices which affect life-cycle cost, based on structural reliability, in a design stage where these choices are still open.

The HFES allows a fairly detailed design definition, including preliminary scantlings, early in the design process. Subsequent reliability analysis includes determination of: internal forces and moments at different wave frequencies; the influence matrix to convert forces and moments to stresses; long-term stress range distribution based on relevant sea states; crack growth calculation based on Paris' equation; reliability, taking into consideration inspection, repair and maintenance strategies.