A Unified Design Procedure for Tubular Members

Abstract

ABSTRACT A new unified procedure for designing tubular members under combined axial load (compression or tension), bending, and hydrostatic pressure on offshore structures, fixed or compliant, is presented. The equations are applicable to members at all water depths, but are limited to members with outside diameter to wall thickness ratios less than 80. In addition to a pure hydrostatic check against hoop collapse, the procedure recommends only one check each for the axial compression and tension cases. The new design equations have been calibrated to test data from several recently completed test programs, and are thus more reliable than currently available technology. The new equations are also simpler to use. The procedure can replace several subsections in the 18th edition of API RP 2A-ASD and the draft RP 2A-LRFD (yellow copy). The recommended procedure provides unambiguous guidance for determining the member axial, bending, and hoop stresses from a frame analysis in order to check the design adequacy or determine the unity check (member utilization) of the member. The recommended procedure includes a reduced safety factor for hoop compression, but other safety factors in RP 2A-ASD are unchanged. Further examination of the reliability question may be warranted, but it is not envisioned that such an evaluation would alter the proposed procedure so as to become more conservative. A comparative design study using the new equations on a 2500-foot compliant pile tower resulted in about 5% tower steel-weight saving (2300 tons) compared to the approach proposed by Chen, Bubonic, and Sohal. Even larger weight savings can be expected for TLP tendons although a comparative design example is not yet available. The new equations are currently being reviewed as part of an ongoing API PRAC 89-56 study on analysis of the cylindrical shell database and validation of design formulations. INTRODUCTION Both API RP 2A-ASD1 and RP 2A-LRFD2 provide incomplete guidance for the design of tubular members under combined axial compression, bending, and external hydrostatic pressure loads, as shown in Fig. 1. The guidance in both documents is limited to checking local cross-sectional buckling. (Shear forces are not shown in Fig. 1. These forces have negligible impact on the strength of beam-columns.) In addition, there is an inconsistency in both documents relating to the allowable bending capacity term. However, this inconsistency generally results in a conservative design. In addition to hoop stress, external hydrostatic pressure will impose a capped-end axial compression force in a member if the ends are capped, as shown in Fig. 2. On the other hand, external radial pressure only imposes hoop compression in a tubular member and no capped-end compression. Since the hoop and capped-end axial stresses from external pressure are always in compression, they are assumed to have a positive sign.