Comparative Evaluation of Fracture Mechanics Methodologies as Applied to Offshore Structural Design and Integrity Analyses

Abstract

ABSTRACT A comparative study was performed to evaluate the three most widely used elastic-plastic fracture mechanics methodologies. The three methods are the crack opening displacement (COD) design curve, which is widely used in the offshore industry, the British Central Electricity Generating Board (CEGB) R6 method, and the J-resistance curve method. Analyses of various flawed geometries based on these three methods were performed using consistent material fracture properties. The results of both the COD and R6 methods were compared with those of the J-resistance curve, which is the most rigorous of the there be methods. The results of the COD concept, following BSI PD 6493 procedures, appear to be conservative at applied stress levels below 60 percent of the material yield strength and unconservative above this value. However, the results of the R6 method appear to be consistently conservative. The behavior of COD design curve solutions can be improved when the structural geometry and the actual stress-strain relationship are reflected in the analyses. INTRODUCTION The predicted gradual depletion of fossil fuels is confronting the oil industry with the challenges of drilling and producing oil and gas wells from offshore reservoirs in deep water and adverse environmental conditions. Figure 1 shows the historical and projected depths for exploration and production activities. In order to optimize structural configuration and reduce weight, advanced analysis methods rather than simple methods with high safety factors are currently being utilized. Fracture mechanics is one of these advanced methods which has recently become an important tool for design, material selection, and establishing fabrication limitations for offshore structures. Fracture mechanics methodologies are also used for the in-service integrity assessment of offshore structures. In general, failure of structures results from the subcritica1 growth of a pre-existing defect (e.g., welding defect) under cyclic environmental and operational loads to such a size that crack instability occurs. Fracture mechanics concepts are used to establish conditions of such crack instability. For offshore structures, it is necessary to utilize elastic-plastic fracture mechanics methods because offshore structural steel is selected such that its brittle-ductile transition temperature is below the minimum operating temperature of the structure. At present, the most common approach for fracture mechanics analysis in the offshore industry is based on the COD design curve method as described in the British Standard Institute document PD6493. There are two other elastic-plastic fracture mechanics methods that have been recently developed, mainly in the power generation industry. These are the R6 method, which was developed in the British Central Electricity Generating Board, and the J-integral crack instability method, which was developed in the U.S. using Rice's path independent line integral. Based on Dugdale strip yield model the COD and R6 methods were 1nitia1ly intended to develop simple failure criteria applicable to cases with large scale yielding without rigorous recognition of ductile tearing.