New API RP2A Tubular Joint Strength Design Provisions (2005)

Abstract The development of the new API RP2A (22nd Edition) parametric static strength prediction equations for planar circular hollow section (CHS) tubular joints is described. Prediction equations are presented for brace axial, brace in-plane bending, and brace out-of-plane bending loads. The prediction equations are based on screened test databases, augmented and extended by an extensive new series of validated nonlinear finite element (FE) simulations for non-overlapping K joints, Double Tee (DT/X) joints and T joints. The increased reliability (reduced scatter) provided by the new static strength formulation was used to justify a reduction of the load factor of safety to 1.6 from the previous value of 1.7. Introduction API RP2A design guidance for tubular joints, primarily on simple CHS joints, has historically been based on experimental databases. In particular, the most recent previous editions of API RP2A, as well as other offshore codes of practice, have been founded on an experimental database that existed in the early 1980s (1). Many additions to the experimental database have occurred since that time, often because of testing a reference simple joint in the course of examining a complex configuration. In particular, additional experimental information is now available on the effect of additional chord loads on joint capacity, providing the opportunity to incorporate this new information in API RP2A. Unfortunately, the simple joint screened test database does not contain data covering the full range of joint types, joint geometries, and brace and chord loading conditions of interest. For example, except for T joints, test data on brace bending is relatively sparse. Tests with additional (i.e. in addition to equilibrium-induced) chord loads are likewise not sufficient in number and scope to adequately address the effect of chord loads on joint capacity. To provide additional information where needed, and to fill in gaps in the experimental database, numerical finite element (FE) models have been utilized. FE models, properly validated against test results, are now recognized as a reliable, relatively low cost source of static strength data which can be used to supplement and extend the experimental database. Modeling procedures and software are well-established for tubular joints that fail by plastic collapse (2)-(5); however, it must be noted that joint tension failures cannot yet be reliably predicted by numerical methods due to the unavailability of an appropriate and accepted failure criterion for ductile tearing. Therefore, for joint tension capacity, test data must still be exclusively relied upon despite possible shortcomings, e.g. size effect. The new parametric prediction equations more accurately represent the effect of joint geometry, particularly chord diameter-to-thickness ratio, and additional chord loads, as demonstrated by comprehensive comparisons with the screened test database and the validated FE database. Joint Classification Joint classification as K, X or Y is unchanged from the 21st edition, and is used in order to apply the basic capacity equations for simple joints to more geometrically complex joints. API has long recognized that joint classification should be based on axial load pattern as well as joint configuration.