Residual Strength of Damaged Offshore Steel Tubular Bracing
Authors: Ricles J.M. , Lamport W.B. , Gillum T.E. 
Published: October 2008

Six steel tubular braces with dent damage and of various diameter to thickness (D/t) ratio were tested to examine their residual strength under direct axial loading, and combined axial and flexural loading. The specimens were divided into three series, each containing two specimens of the same Dlt ratio with similar dent depths. One specimen of each series was tested under direct axial loading while the other specimen was tested under combined loading. The axial test results show a pronounced degradation in residual strength, indicating a susceptibility of prototype members to failure under design loads. The degradation in specimen strength was found to be further reduced under combined loading. Existing damage strength equations were modified and found to reasonably assess the residual strength of the dented braces subjected to combined flexural and axial load.

INTRODUCTION

A large number of existing offshore steel platforms are known to have suffered some form of damage [1,2]. One common form of damage is denting, caused by impact from dropped objects or vessel collisions. The need to maintain the platform's reliability and safety makes the existence of a damaged member a great concern. To make optimal design decisions regarding safety, as well as economy, it is important to have knowledge about the ultimate and post ultimate strength behavior of dented structural members and the effect of such damage on the platform's load carrying capacity.

Numerous analytical and experimental studies have been conducted on dented tubular steel members. Those dealing with residual strength and published in the literature include the work of Smith et al [3,4], Padula and Ostapenko [5], and Taby and Moan [6]. Various analytical studies have also been conducted, and include the work of Macintyre [7] involving nonlinear finite element analysis of dented members, in addition to Duan et al [8,9] who developed a moment-curvature approach to predict nonlinear behavior. Based on experimental data, there have been computer programs and design strength equations developed to predict the residual strength of dented members. One is that of Taby [10], who developed a semi-empirical method for predicting the behavior of dented members using a stress resultant approach applied to a beam element. Ellinas [11] proposed a simple design oriented format to predict the ultimate strength of a dented member subjected to pure axial load. Based on the equilibrium of a dented brace under pure axial load and considering secondary effects, he proposed that the ultimate stress ?ud corresponding to residual strength of a dented member be evaluated by solving the following quadratic equation:(available in full paper)

where ?e is the undamaged Euler buckling stress of the member, Ad and Sd, respectively, are the reduced cross sectional area and elastic section modulus of the member in the dented zone, ed is the eccentricity that exists in the dented zone between the centroid of the dented and non-dented cross sections, a. accounts for geometric out-of-straightness effects