Grout Repair Of Dent-Damaged Steel Tubular Bracing

ABSTRACT A test program consisting of 13 steel tubular brace specimens with dent damage and various diameter-to-thickness (D/t) ratio was conducted to assess the residual strength and repair of damaged tubular braces in offshore platforms. Specimens were inflicted with a dent having a depth of 0.1 OD, with internal full grouting and grouted steel clamps, respectively, used to repair specimens. A baseline of unrepaired, damaged specimens subjected to concentric axial loading showed a significant reduction in residual strength due to the dent. A further reduction was found in specimens subjected to combined loading. The use of grout to repair specimens was found to inhibit a growth in dent depth under axial loading and restore the capacity of the dented member to a level greater than the undamaged strength. A comparison with existing and modified analytical methods for strength prediction of unrepaired and grout repaired braces shows these methods to provide reasonable results. INTRODUCTION At present time there are more than 6,000 offshore platforms worldwide. Most of them were designed for a 20 year life but are still in operation after 30 to 40 years after installation. Many platforms have suffered some form of damage. Damage records indicate that the majority of accidents result from either ship collisions or impact with debris, including dropped objects. These accidents account for 177. and 12?40, respectively, of all recorded accidents [1]. Under impact, the memberâ??s circular cross section is susceptible to localized denting and ovalization. Previous investigations on dented members have shown that even minor damage can cause severe degradation in a memberâ??s strength, which in turn affects platform integrity. Economic necessity and the need to ensure structural integrity and safety creates strong incentives to develop techniques to rehabilitate damaged offshore structures. Previous studies on the residual strength include both experimental and analytical research. The experimental studies reported in the literature include the work of Smith et al. [2,3], Taby [4], as well as Landet and Lotsberg [5]. Analytical work on dented members includes the nonlinear finite element studies conducted by Maclntyre and Birkemoe [6], the elastic-plastic beam-column analysis model of Smith [2,3], as well as a moment-curvature approach to predict nonlinear behavior by Duan et al. [7] and Padula and Ostapenko [8]. Based on experimental data, there have been computer programs and closed form solutions developed to predict the residual strength of dented members. One is that of Taby [9], who developed a semi-empirical method for predicting dented member behavior using a stress resultant approach applied to a beam element. The method was implemented into a computer program called DENTA. Ellinas [1 O] proposed a quadratic equation for ultimate strength prediction based on equilibrium of concentrically axially loaded dented members. In addition to studies of dented members, some investigations have been conducted on grout repaired members having dent damage. Boswell et al. [11] has conducted a series of mostly small-scale tests on complete grout filled dented braces as part of a United Kingdom Department of Energy project, as well as tests by Parsanejad [12].