A Unified Structural Design Method For Floating Offshore Structures
Authors: Hughes O.F. , McNatt T.R. 
Published: October 2008

ABSTRACT
The paper presents a unified, rationally based structural design methodology for the practical design of floating marine structures such as mobile offshore drilling units (MODUs), tension leg platforms (TLPs), ships and advanced marine vehicles. The methodology is totally computer-based and consists of three major capabilities: design-oriented, full structure finite element analysis; comprehensive evaluation of structural limits including failure modes; and integrated design optimization, to achieve least weight, least cost and/or lowest vertical center of gravity.

INTRODUCTION
The unified structural design method presented provides the capability to incorporate computer-based, global structural design into the design process, including preliminary design. The method is tailored specifically to thin-walled, stiffened plate structures such as those used in MODUs and TLPs. As shown in Figure 1, the method combines finite element analysis with comprehensive evaluation of structural adequacy and multi-objective design optimization. The calculation of failure modes (buckling, yield, plastic hinge, etc.) is conducted for all principal structural members and supports design analysis or optimization using the modem Load and Resistance Factor Design format. The method has been fully implemented within a single, unified computer program which operates on all major classes of engineering computers (workstations, PCs, etc.), and has been applied and validated, over the past eight years, for a variety of large, complex floating structures such as SWATHs, catamarans, commercial and combatant ships, and submarines.
The unified structural design method can be applied to all classes of MODUs, TLPS, ships and other floating production support facilities. The structural design of these types of platforms can be enhanced by the application of this method, which computes not only deflections and stresses, but also evaluates all of the structure's local and global failure modes as well. The method's optimization capability applies a user defined objective of least weight, least cost and least vertical center of gravity (in any desired combination) to develop a safe, cost effective, and structurally optimal design.
The results of numerous structural designs and validations of the method against other analysis tools and test data will be discussed. These designs and validations have been conducted by major technical organizations worldwide. The unified structural design method is implemented in the computer program MAESTRO, and is an important advance in the design of large, complex floating structures. While many programs offer powerful stress analysis, none offer complete failure evaluation of all principal members and multi-objective, design optimization of the entire structure.
Technology and economics drive the designs of major capital equipment, such as floating facilities, toward higher levels of sophistication and refinement. But these higher levels can only be achieved by utilizing the improved design methods and tools that have been developed and proven. The unified structural design method represents a clear step forward in giving offshore structural design teams a practical tool for developing safe, cost effective and optimal structures.