Design of offshore tubular members against excessive local indentation under lateral impacts

Abstract

Tubular members are widely used in offshore jacket and jack-up platforms. Such structures are often exposed to the risk of lateral impacts from service vessels and dropped objects. The impact responses can generally be classified into three stages: local denting, beam bending and membrane stretching. To maximize energy absorption capacity of braces/legs, local indentation should be limited to a small value because of significant degradation effect of local indentation on the bending capacity of tubes. This paper reviews the existing analytical denting models and the design requirements for tubes to resist excessive local indentation. Extensive numerical simulations are carried out using LS-DYNA for two ship sterns crushing braces/legs with varied tube diameters, lengths, thicknesses, and contact lengths. Based on numerical results, the existing denting models and compactness criteria are discussed. A concept of transition indentation ratio of tubes (wtran/D) from local denting to global bending is proposed. The existing compactness criteria are discussed based on the concept and numerical simulation results. Finally, a compactness criterion is recommended.