A new concept in the design and installation of fixed offshore platforms has been developed. This paper details the development of the procedure and chronicles the design, evaluation, fabrication and installation of the first application. Also the installation histories of this structure and an identical, traditional structure are compared.
Reflecting upon traditional fixed offshore platform design history, appreciating existing technologies, more efficient structural configurations were envisioned. Traditional design and installation procedures basically developed more than twenty years ago. 'exhibit certain intrinsic constraints. The traditional installation sequence requires jackets to temporarily support themselves on the seafloor, vulnerable to environmental conditions, while foundation pilings are assembled and installed through the jacket legs. Jacket and foundation designs are geometrically coupled, as jacket legs must accept the passage of pilings. Optimal designs often conflict as it is advantageous to minimize jacket leg diameters to reduce wave forces but necessary to increase piling diameters to enhance lateral capacities, reduce penetration, and improve driving characteristics. A design spiral may often result from the interdependence of jacket leg diameters, piling diameters and the wave forces affecting each. Traditional installation procedures create difficult salvage operations as flotation hydrodynamics of jacket structures are altered during installation. Buoyant chambers are destroyed and/or the weights of jackets are increased by grouting pilings within the jacket legs. Consequently, upon removal from the seafloor, jackets, not easily refloated, are difficult to salvage in a reverse installation sequence.
The challenge was to use the offshore industry's extensive design experience and existing technologies in the development of more efficient structures utilizing, with certain refinements, proven design, fabrication and installation procedures and existing offshore fabrication equipment. Ideally, the resulting structures would be nearly identical in form and function to their traditional predecessors. The remarkably simple and straightforward solution is the pre-installation of foundation pilings. The resulting, more classic construction sequence, as compared to "foundation first" land based construction, would permit a number of advantages. The design of jackets and foundations would be, essentially, geometrically uncoupled and piling designs would become more selected, parametric optimizations. Jacket legs would only be the diameter necessary for load1ng and framing considerations while wave forces would generally be reduced. Skirt pilings, added to limit the diameter of main pilings, could, at times, be eliminated. The new installation sequence could permit the optimization of offshore equipment utilization, in certain applications, by the use of smaller, less expensive equipment to pre-install foundations and the use of heavy-lift vessels to install jackets and decks. Installation procedures would not significantly alter jacket flotation hydrodynamics, permitting simpler, reverse sequence salvages.
The Pre-Installed Foundation concept (PFC) Jacket has been developed as an alternative to traditionally designed and installed fixed platforms. The procedure, utilizing only existing technology, optimizes structural configurations and the offshore installation sequence resulting in more efficient designs providing advantages in fabrication, installation, and removal.
