Influence of Framing Configuration on the Robustness of Offshore Structures
Authors: Gebara Joseph , Westlake Hugh , DeFranco Sam , O&apos;Connor Pat 
Published: October 2008

Design of steel offshore structures has traditionally been based on elastic analysis to determine the distribution of forces through the structure, for an envelope of design cases. Checks are then performed on a component basis to ensure that no element of the structure fails to meet the governing criteria. Uncertainties in estimating the design loads and component strengths are allowed for by the application of safety factors. The analyses undertaken are linear and take no account of the reserve strength, and the robustness of the system. This paper discusses how the use of X-bracing leads to more robust designs that are: lighter, less expensive to fabricate and easierto install. In this paper we will use the results of previous studies, [1,2] and a number of pushover analysis on two, third generation (1990's), central North Sea platforms, to show that:The .....use of X-bracing..." [3] increases structural robustness in the jacket, without sacrificing cost, or constructability.Omitting the horizontal members on the vertical framing will not compromise the robustness of an X-braced jacket, subjected to hydrodynamic loading, if adequately designed X-braced horizontals are used.

Introduction

The main aim of structural design and fitness for purpose assessment is to ensure that a structure adequately fulfills its requirements with respect to serviceability and safety. Most offshore jacket structures possess an inherent reserve strength that is greater than the strength of the critical components. This is derived from a variety of sources, the principal one being the nonlinear structural interaction between components through plastic deformation and load redistribution, which in redundant systems allows mobilization of alternative load paths. Some structures display considerable levels of reserve strength whereas others suffer from a sudden drop in capacity as soon as one critical member fails. The reduction in ultimate capacity of a jacket structure due to damage to individual members can be measured by the so-called Damage Strength Ratio (DSR). This is effectively identical to the Reserve Strength Ratio (RSR), but is calculated with specific member(s) removed. DSR analyses have been used to prioritize inspection and also to check the strength or reliability implications of damage events. As the industry moves towards system based design it is important during the design cycle that the reliability of different framing configurations be assessed. In this paper we examine the results of several pushover analyses performed for two platform designs which were selected to have similar water depth, deck payload, hydrodynamic loading and vertical framing but different horizontal framing patterns. The discussion will focus on the advantages of considering system behavior, and robustness at the conceptual design level without compromising steel weight, fabrication and inspection costs. Consideration will also be given to the effects of structural robustness on inspection/repair philosophy, and changes in operational requirements during the design life.

Definitions

Redundancy

Fixed offshore structures are typically statically indeterminate, When the number of unknown forces and/or moments in a structure exceeds that which can be determined by statics, the structure is said to be indeterminate. The forces and/or moments that cannot be determined using statics are said to be superfluous or redundant.