Evaluation of Analysis Methods for Response Prediction of Dent-Damaged Tubular Steel Bracing Members
Authors: Ricles James M. , Bruin William M. 
Published: October 2008

This paper presents the results of a study related to predicting the residual strength of dent-damaged steel tubular bracing members in offshore platforms. Damaged specimens experimentally tested in the laboratory were analyzed to predict their ultimate capacity using different analysis methods. These methods include: beam-column formulations; moment-axial load unity check equations; moment-thrust-curvature methods in conjunction with numerical integration; and nonlinear finite element analysis. The reliability and accuracy of these methods was evaluated by comparing the predicted ultimate capacity of 56 dent-damaged specimens with their measured experimental response. This database of test specimens consisted of both small- and large-scale members with dent-damage ranging in depth from 1% to 30% of their diameter, and diameter-to-thickness ratios (Oft) of 26 to 121. The ability of the analytical methods to accurately predict ultimate capacity was found to depend on the extent of dent-damage. The non-linear finite element method was found to provide the most accurate prediction of member capacity. However, the method of choice for routine analysis is the use of unity check equations that have been calibrated for dent-damaged tubular strength prediction. These equations are easy to use and are shown to provide a reasonable estimate of the capacity of damaged members.

Introduction

At present, there are approximately 3,800 offshore platforms in the U.S. waters, with the majority being in the Gulf of Mexico and the rest scattered along the coast of California and in the Cook Inlet of Alaska1. The average age of these structures is roughly 15 years, with over one-fourth having an age beyond their 20 year designed service life2. As the number of older platforms continues to grow, the oil and gas industry has become increasingly aware of the liability associated with operating an aging offshore infrastructure. Exposure to liability, especially environmental, is becoming an especially large burden for the industry as insurance premiums escalate to balance the risk of platform failure and the subsequent high costs associated with loss of live, environmental devastation, and wasted resources. The financial costs associated with the shut-down, removal, and replacement of these aging platforms with more sophisticated structures are, however, equally prohibitive, and complicated with strict regulatory and environmental requirements. As a result, the oil and gas industry has developed a strong desire to rehabilitate its existing offshore infrastructure, resulting in the need for the development and verification of methods and techniques necessary to assess platform integrity, to strengthen and repair weakened elements, and to extend safe operational life. As part of this maintenance and rehabilitation process, the structural integrity and physical condition of the platform must be thoroughly evaluated before any decisions are made about the safe operation of the structure. This is especially challenging since the assessment of the structural capacity of the platform is complicated by the existence of any deterioration or damage of individual structural elements. Examples of such damage or deterioration typically found in older offshore structures includes global or local corrosion due to environmental exposure, fatigue cracking from continuous wave loading, and dent-damage of structural members due to collision or impact. Despite the ability of the jacket to tolerate a limited number of damaged or missing members, the accumulation of damage reduces the structural integrity of the platform and inherently increases the probability of its failure. Thus, decisions regarding the safety a