Background to New Edition of API Bulletin 2U- Stability Design of Cylindrical Shells
Authors: Balint Stephen W. , Capanoglu Cuneyt , Kamal Rajiv 
Published: October 2008

Abstract
API Bulletin 2U has recently been revised. New formulations for local buckling of ring-stiffened and ring-and stringerstiffened cylindrical shells, when subjected to both longitudinal and circumferential hoop stresses, have been developed. The new formulations are in line with classical buckling equations and are compatible with the test data. The definition of plasticity reduction factor has been revised to make it more consistent throughout the document. It has been found that the presence of vertical stringer, especially when spaced closely, play an important role in modifying the distribution of stresses in rings and shells due to applied external pressure and axial loading. New formulations have been developed to determine stresses in the shell at rings and midway between ring spacing for ring stiffened and ring and stringer stiffened shells when subjected to external pressure and axial load. Sample computations have been added to the bulletin to help and illustrate to the user the sensitivity of various parameters affecting design decisions.

Introduction
In 1987, the oil industry in the United States was pushing off the continental shelf into deep water. Until this time, offshore production sites could be supported by bottom-supported fixed or compliant structures, typically steel jackets with small tubular members. Guidance for the design of small diameter tubulars up to a D/t ratio of 300, typically governed by local and column buckling equations and the interaction relationships, are found in the recommended practice provided in API RP 2A. However, the deepwater locations called for another type of structure - floating compliant units. These structures require large volumes of entrapped space to provide the buoyancy necessary to support the production risers, drilling and production equipment, mooring, and export lines. Large diameter, orthotropically stiffened cylindrical shells effectively provide the desired buoyancy. Unlike small diameter tubulars, large diameter, orthotropically stiffened cylinders have three failure modes. These modes, from least critical to most critical (i.e., hierarchical order) are: local buckling (i.e., shell plate only), bay instability (i.e., shell plate with stringer) and general instability (i.e., shell plate, stringers and ring).
Prior to the existence of API Bulletin 2U, orthotropically stiffened large diameter cylinders for semisubmersible-based Mobile Offshore Drilling Units (MODUs) were designed and constructed based on either classical buckling equations or DnV's panel buckling equations. These equations were assumed to be conservative but their level of conservatism were uncertain due to the limited amount of test data.
In preparation for the design of Jolliet Tension Leg Wellhead Platform (TLWP) and other floating compliant structures, Conoco gained support of other oil companies and the American Bureau of Shipping (ABS) to form a joint industry project (JIP) to test a series of large diameter orthotropically stiffened cylindrical shells in the United States and the United Kingdom in 1983. This effort was intended to generate the test data that would support the development of guidelines needed to assist designers and regulators.