External Pressure Tests of Ring-Stiffened Fabricated Steel Cylinders

Abstract

Summary This paper summarizes the results of 20 hydrostatic external pressure tests on ring-stiffened fabricated steel cylinders. The objectives of these tests were to develop a comprehensive data base for fabricated cylinders and to determine the adequacy of the current API design rules for fixed offshore platforms. platforms. The test specimen parameters were selected to represent (1) typical offshore platform member sizes (D/t of 32 to 128), (2) commonly used platform steels (A36 and A572), and (3) routine platform fabrication procedures. The specimens were designed to fail at different stress levels. Fourteen specimens were designed to fail by local buckling of the shell between rings and six were designed to fail by general instability. Out-of-roundness measurements were taken at the middle of each bay and adjacent to each stiffening ring. The out-of-roundness values exceeded the API tolerance limit of 1% for many of the test specimens. The test results are compared with theoretical methods used to predict the buckling pressure based on measured out-of-roundness values. pressure based on measured out-of-roundness values. In general, poor correlation is found. On the basis of the test results, empirical reduction formulas to account for out-of-roundness effects are proposed. Introduction The buckling criteria for cylinders under external pressure vary widely with different specifications. pressure vary widely with different specifications. The ASME Boiler and Pressure Vessel Code rules are based on an allowable pressure equal to one-third of the theoretical buckling pressure. For inelastic buckling, the tangent modulus is used in place of Young's modulus. The initial geometry of the cylinder must meet two requirements. Overall out-of-roundness is limited by D max - D min less than or equal to 0.01D nom where D max, D min, and D nom are the maximum, minimum, and nominal diameters, respectively. The maximum deviation from a true circular arc over a distance corresponding to one-half of the theoretical buckle wave length is limited to a value between 0.3 t and 0.1 t, where t is shell thickness. The specified out-of-roundness criterion is intended to limit the reduction in buckling stress to 80% of theoretical. The resulting factor of safety is 3 x 0.8 = 2.4.The British Standard Inst. (BSI) BS5500 rules for unfired pressure vessels are based on an allowable pressure equal to one-third of the theoretical pressure equal to one-third of the theoretical buckling pressure for elastic buckling. For inelastic buckling, a lower bound curve on test data was determined and a factor of safety of about 1.5 was used to determine the allowable pressure. Cylinders must be circular to within 0.5% on radius measured from the true center.The Det norske Veritas rules for offshore structures are based on an elastic buckling stress equal to the theoretical buckling stress reduced by a factor that varies with the shell geometry. This factor varies from a maximum of 1.0 to a minimum of 0.6. An empirical reduction factor is applied for inelastic buckling. The out-of-roundness requirements are the same as the ASME code for maximum deviation from a true circle measured over one-half of a wave length and the same as the BSI rules for overall out-of-roundness.The hydrostatic collapse criterion given in the tenth edition (and several earlier editions) of API RP 2A was based on the theoretical equation developed by Timoshenko to account for out-of-roundness. API Spec. 2B rules limit the out-of-roundness to D max - D min less than or equal to 0.01 D nom [or 0.25 in. (6.35 mm) for cylinders up to 48 in. (1.22 m) in diameter or 0.5 in. (12.7 mm) for cylinders greater than 48 in. (1.22 m) in diameter, whichever is less]. JPT P. 2528