Page 179 - Marine Structural Design

Part I1

Ultimate Strength

Chapter 9 Buckling and Local Buckling of lhbular Members

9.1 Introduction
9.1.1 General
Equations for buckling strength of tubular members may be found from several books (such as
Chen and Han, 1985) and offshore design codes (such as AISC (1978) and MI RP 2A). This
chapter will address the interaction between beam-column buckling mode and local (shell)
buckling mode based on Yao et aZ(l986,1988).
In the past 40 years, many kinds of offshore structures have been built and are in service for
drilling and production in the oil and gas industry. Semi-submersible drilling units are one of
the most commonly used offshore structures owing to their high operation rate and good
performance in rough sea. However, this type of offshore structure has no self-navigating
systems, and cannot escape from storms and rough sea conditions. Therefore, the structure
must have enough strength to withstand extreme sea conditions (100 years storm).
Consequently, no buckling and/or plastic collapse may take place under ordinary, rough sea
conditions if the structural members are free of damages.
On the other hand, the bracing members of drilling units are ofien subjected to accidental
loads such as minor supply boat collisions and dropped objects from decks. Furthermore, a
fatigue crack may occur after a service period. Such a damage will not only cause a decrease
in the load carrying capacity of the damaged member, but will also change the internal forces
in undamaged members. Consequently, under rough sea conditions, buckling and/or plastic
collapse may take place in the undamaged members as well as in the damaged members. This
can cause a loss of integrity of the structure system. From this point of view, the ultimate
strength limits and the load carrying capacity of tubular bracing members in serni-submersible
drilling units should be assessed carefully.
Many studies have been performed during the last decade regarding the ultimate strength of
tubular members. For example, Chen and Han (1985) investigated the influence of initial
imperfections such as distortions and welding residual stresses on the ultimate strength of
tubular members, and proposed a practical formula to evaluate the ultimate strength. Rashed
(1980) and Ueda et al (1984) developed the Idealized Structural Unit (element) for a tubular
member, which accurately simulates its actual behavior including overall buckling and
plastification phenomena. They showed that accurate results are obtained within very short
computation time when applying this model.
However, these results can only be applied to tubular members with small diameter to
thickness ratios, e.g. D/t less than 30-50, which are typical bracing members in jackets and
jack-ups. Local shell buckling need not be considered in these members. On the other hand,