Enhancements of TOPCAT: 3-Dimensional Loadings, Reliability, and Deck Structure Capacities
Authors: Jin Zhaohui , Bea Robert 
Published: October 2008

This paper summarizes the formulations, development and applications of recently developed analysis modules for TOPCAT (Template Offshore Platform Capacity Analysis Tools). These modules have significantly increased the scope of analyses that can be performed using TOPCAT. The modules include those for evaluations of the loadings and platform capacities associated with spatial 3-dimensional wave loadings, evaluations of the long-term reliabilities and reliability sensitivity factors of the primary components and elements that comprise template-type platforms, and evaluations of the vertical loading capacities of truss and box girder deck structure systems.

Introduction

The newly developed TOPCAT modules have been applied to determine the 3-dimensional loadings developed on template type platforms whose geometries are large relative to those of the waves (spatial loading effects) and for conditions in which the waves approach the platforms diagonally relative to the principal axes of the platform. The new loading - capacity modules include a shallow-water wave kinematics and force analysis that utilizes Cnoidal wave theory to determine the wave kinematics. The wave force module has been based on the API RP2A 20th Edition guidelines and verified with results from wave force measurements performed in 10 m of water in the Gulf of Mexico.1,2 The platform element (legs, joints, braces, piles) capacities are evaluated for the 3-dimensional loading effects.

The reliability analysis module has been applied to determine the long-term (exposure period) probability of failure of the primary components that comprise template-type platforms (decks, jackets, pile foundations). The formulation has been based on a 'fragility' approach in which the probabilities of failure of the platform components are determined conditional on a user specified range of wave heights, associated wave periods, currents, and wind speeds. These probabilities of failure are then integrated to determine the long-term reliabilities based on user specified probabilities of occurrence of the specified wave heights. This formulation allows the evaluation of the effects of truncations in force characteristics (e.g. due to shallow water wave breaking or deformable sea floor effects) and the effects of changes in the pattern of wave forces developed on the structure as the wave heights increase (e.g. wave crests in lower decks). The reliability analysis module also allows the evaluation of reliability sensitivity factors to indicate the relative importance of the parameters that determine the reliabilities of the primary components that comprise template-type platforms.

The deck element module allows the evaluation of the capacities of truss type decks whose elements are wideflanged shapes and tubular members. The deck element module also allows the evaluation of the capacities of boxgirder type decks whose elements are plate girders and box girders. This module has proven to be important in allowing rapid evaluations of the ultimate limit state capacities of deck sections and thereby evaluate their factors of safety or expected performance for increased deck loadings.

The new TOPCAT modules have been verified with results from experimental programs and analytical studies.