Stability Analysis of Orthogonally Stiffened TLP Columns
Authors: Wang Y.Y. , Luyties W.H. , Balint S.W. 
Published: October 2008

After completion of fabrication and prior to installation of the Auger tension leg platform (TLP), the designers became concerned that the hull's ring stiffening might not be adequately braced against lateral-torsional buckling under hydrostatic pressure. Such a condition for stiffened shells was not clearly covered by existing design codes. The primary code under which Auger columns were designed, API Bulletin 2U, provided guidelines for compactness, but nothing on the allowable unsupported length of the compression flange. Lt, Furthermore, it was unclear if the notion of Lt, was applicable to a circular member. Insufficient capacity would mean retrofitting more than 2500 tripping brackets and delaying installation.

A finite element analysis procedure was developed at EWI to evaluate the capacity of the hull to resist buckling in the as built arrangement. The modeled hull structure was stiffened by either circumferential rings and stringers or circumferential rings only. The analysis incorporated fabrication tolerances for out-of-roundness, axial offset, and web tilt of the circumferential stiffening rings.

Models with different combinations of fabrication tolerances were loaded by hydrostatic pressure, axial load, or both. The maximum sustainable pressures were computed and compared to the design pressure. The analysis results indicate that the cylinders with only the circumferential stiffeners (such as the Auger inner shell design) can sustain hydrostatic pressures more than twice the design pressure. Cylinders with circurnferential and longitudinal stiffening (such as the Auger outer shell design) can carry even more. Therefore, tripping brackets are not needed to prevent lateral torsional bucking of the circumferential rings. The fabrication tolerances used in the analysis are compared with the requirements in API 2U. which is among the most comprehensive code available in the design of stiffened cylinders. The degree of conservatism embodied in API 2U is examined based on the comparison.

Introduction

The cylindrical hull columns of a TLP are subjected to external hydrostatic pressure when submerged in seawater. The inner shell is stiffened by circumferential rings, while the external shell is stiffened by rings and longitudinal stiffeners. Between fabrication and installation of the Auger TLP, the designers became concerned that the ring stiffening might not be adequately braced against lateral-torsional buckling to resist hydrostatic pressure. The dimensions met the AISC definition of compactness, but not the guidelines given in API 2U Section 7 for web height to thickness. In addition, the compression flange of the ring tee was not supported between bulkheads. The basis for the original design of the rings was that tripping could not physically occur with compact section rings due to the geometric constraints of stiffened cylinders. Tripping of rings would involve circumferential shortening and lengthening of opposite edges of the flange which would provide a restraining force. This assumption was validated qualitatively with Clarence Miller of CB&I, who was the principal author of API 2U. However, no conclusive data was available. Insufficient capacity would mean retrofitting more than 2500 tripping brackets and delaying installation.

Finite element models were constructed to compute the maximum sustainable external pressure under a variety of fabrication conditions.