Evaluating scale effects in the modeling of buried offshore pipes in Chabahar carbonate sand using centrifuge testing

Abstract

A major instability mode of buried offshore pipes, used to transport hydrocarbon materials, extracted from reservoirs, discovered at the seabed, is "upheaval buckling". Although research on factors that affect the buckling of buried pipes is extensive, there are still uncertainties about the effects of the soil particle size in centrifuge models. This research has used 3 varying-diameter pipes and 3 types of carbonate soils with different grain sizes, to study such effects in 4 axes utilizing a centrifuge device. According to the results, in the "modeling of models" tests with similar soils, scale effect is ignorable if the ratio of pipe diameter to the average soil-grain size ($D/d_{50}$) is greater than 40, but if different grain-size sands are used, the scale effect is clearly evident in the results assuming that the pipe geometry is kept constant at model and prototype scales. It seems that the common criterion used by past researches for $D/d_{50}$ has not been enough alone, and minimizing the scale effect needs adding another criterion such as$L/d_{max}$, which is the ratio of the shear band length to the maximum soil-particle size. Results have also shown that increasing the pipe's burial depth will make the particle-size effect more obvious, but increasing the soil's relative density will make the mentioned effect less evident on the peak dimensionless breakout factor ($N_{u.max}$).