Efficient Development of Innovative Offshore Structures
Authors: Birk Lothar , Clauss Günther  F. 
Published: October 2008

Hydrodynamic shape optimization is one of the key tools for the development of new system concepts in offshore engineering. This paper presents a fully automated numerical procedure for optimum adjustment of shapes to enviromnental conditions. Nonliear programming algorithms vary the design parameters to find a minimum of the objective function within a few iterations. The paper presents the optimization of a mono-tower, a buoy, a semisubmersible and a TLP. The resulting hull shapes are characterized by minimum wave loads and motions.

Introduction

To ensure the future supply of hydrocarbons offshore technnology stretches its frontier into deep water regions and areas subject to harsh environmental conditions. A large variety of different system concepts have to be developed for specific offshore operations at these frontiers. For reducing economic risks offshore operators improve cash flow by achieving an early start of production. In Norway, for example, the lead time to bring fields on strmun has been cut down by 60–70% to a period of 15–20 months within a few years1. To support this process the development of system concepts and design have to be performed in decreasing space of time. It should be emphasized that these preliminary design decisions govern major parts of later expenses2.

The design of complex systems like offshore structures is an iterative process involving different fields of engineering. The main ohjective in the design of a new structure is the development of a solution, which serves best the operational requirements and meets all economic and engineering constraints. Primary factors influencing the design of offshore structures are environmental conditions and their impact on system behaviour. Wave induced motions and loads influence operability as well as structural dimensioning and the associated construction cost. The quality of the final design is based on the knowledge and the experience of the design team. Expensive and time consuming trial and error parameter studies are necessary to achieve a good design. Although more systematic approaches, like Nonlinear Programming (NLP), are not new in designing offshore structures they are rarely used.

Chou3 presented a remarkable paper proposing an analytical procedure for the optimum design of offshore structures. The minimization problem is solved by means of calculus of variations. An optimtun displacement distribution along the vertical axis is developed. Due to the analytical formula used for the evaluation of forces and motions the geortmtry is restricted to hydrodynamically transparent bodies of revolution with vertical axes4. Chow derived optimal shapes for a buoy and a platform supported by four columns applying ISSC spectra.

The same restriction with respect to geometry applies to the work of Saitos5. The heave exciting force of a buoy is minimized for selected wave frequencies. A semisubmersible is optimized with respect to heave motion in a design sea state applying the objective function proposed by Chou. Forces are calculated on the basis of a slender body assumption. Viscous effects are neglected.

Kagemoto6 optimized the arrangement of vertical floating cylinders in waves using the penalty function method SUMT (Sequential Unconstrained Minimization Technique) of Fiacco and McCormick7. Alternativeiy three objective functions were investigateed: the total horizontal force, the total vertical force and the mean drift force for a specific wave length. Force evaluation is based on the hydrodynamic interaction theory of Kagemoto and Yue8 which restricts the procedure to vertical cylinders. Arrangmnents of va