Experimental And Analytical Studies On The Residual Strength And Repair Of Dent-Damaged Platform Bracing

Abstract

Presently there are over 6000 major offshore platforms in United States waters. A majority of these structures were designed for a 20-year life period, and are still in operation today after 30 and even 40 years of service. This paper discusses an ongoing research study on the evaluation of the residual strength and grout repair of dent-damaged offshore platform steel tubular bracing. This study is highly relevant to the rehabilitation and requalification of offshore platforms, since dent-damage due to dropped objects or vessel collisions is a common occurrence in these structures and shown to cause a reduction in member strength. INTRODUCTION Damage to offshore platforms is a topic of ever increasing concern in the United States, as the age of many of these structures begins to exceed their designed service life and as the stakes associated with platform failure, in terms of lost lives, environmental devastation, and wasted resources continue to climb. Economic realities that make replacement prohibitive have created strong incentives to rehabilitate and extend the service lives of existing platforms. These older offshore platforms that are being requalified for extended service involve technical problems that are unlike those associated with the design of new platforms, A general schematic of an inspection, maintenance, and repair (IMR) procedure is shown in Figure 1. Platforms that are to be requalified for extended service generally must have their current condition assessed and thereby their safety evaluated. First, an inspection must be performed and data collected which describes the current structural state. The data is then used to evaluate the structural integrity, comparing the structural resistance with the demand imposed by the loadings. The results of the evaluation may indicate the need for structural repair or strengthening. Some of the technical problems associated with the assessment of older platforms involve locating and evaluating the effects of defects and member damage on platform safety. Defects could include missing members, misaligned members at connections, improper welding, and other fabrication and construction errors. Damage typically includes dents, corrosion, and fatigue cracked members. This paper focuses on dent-damage of tubular bracing. Dentdamage of tubular offshore platform members often occurs by accidental loading such as workboat collisions, impact of dropped heavy objects, or mishaps during construction and installation. in order to adequately conduct a platform fitness-for-purpose assessment, a reasonable estimate of the residual strength of dented members is needed in order to ascertain whether member repair is necessary. Furthermore, knowledge of the strength of the repair method is required in order to assess the repaired state of the member and platform safety. An ongoing research program concerned with the residual strength and repair of dented steel tubular platform bracing is being conducted at Lehigh University. This program involves both analytical and experimental components, and has the objectives of assessing the residual strength of large scale dented specimens and the use of grout to repair dented members. Discussed herein are results from this research which are related to, and have implications on, platform requalification and reassessment.