On the design of slip-on buckle arrestors for offshore pipelines
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Offshore pipelines are susceptible to the damage that leads to local collapse. If the ambient pressure is sufficiently high, local collapse can initiate a buckle that propagates at high velocity catastrophically destroying the pipeline. Buckle arrestors are circumferential local stiffeners that are placed periodically along the length of the pipeline. When properly designed, they arrest an incoming buckle thus limiting the damage to the structure to the distance between two adjacent arrestors. Slip-on type buckle arrestors are tight-fitting rings placed over the pipe. They are relatively easy to install and do not require welding. As a result they have been widely used in shallow waters. It has been known that such devices often cannot reach higher levels of arresting efficiency. The somewhat deficient performance is due to the fact that a buckle can penetrate such devices via a folded-up U-mode at pressures that are lower than the collapse pressure of the intact pipe. Because of this they have not seen extensive use in deeper waters. The aim of this study is to quantify the limits in arresting performance of slip-on buckle arrestors in order to enable expanded use in pipelines installed in moderately deep and deep waters. The performance of slip-on buckle arrestors is studied through a combination of experiments and analysis. The study concentrates on pipes with lower D/t values (18-35) suitable for moderately deep and deep waters. The arresting efficiency is studied parametrically through experiments and full scale numerical simulations. The results are used to generate an empirical design formula for the efficiency as a function of the pipe and arrestor geometric and mechanical properties. The performance of slip-on arrestors is shown to be bounded by the socalled the confined propagation pressure. That is the lowest pressure that U-mode pipe collapse propagates inside a rigid circular cavity. Therefore, a quantitative study of this critical pressure is undertaken using experiments and numerical simulations. A new expression relating this critical pressure to the material and geometric parameters of the liner pipe is developed. This in turn is used to develop quantitative limits for the efficiency of slip-on buckle arrestors.