The impact of dropped objects and trawl board on submarine pipelines are simulated by a non-explicit finite element method. The new method works in three mechanics. The impact process is simulated by adjusting the material properties. The damage of the pipeline is solved using Cowper-Symonds equation. Drucker-Prager model is used to analyze the elastic-plastic properties of soil under impact. Then the present work can take into account the interactions among the dropped objects, pipelines and soil. Furthermore, the effects of the weight, shape, impact velocity and seabed flexibility are discussed in detail.
Submarine pipelines are the "lifeline" of offshore oil and gas production system and are used as one of the primary ways to transport oil and gas for offshore development. The risk of pipeline leakage is increasing with the rapid expansion of submarine pipeline networks. Statistically, more than 50 percent of submarine rupture accidents are caused by third-party damage such as ship anchoring and trawl fishing (Famiyesin et al., 2002; Cao et al., 2010; Ivanovic et al., 2011). In order to reduce the damnification to submarine pipelines caused by third-party damage, the pipelines need to be buried into sea floor reasonably. It is necessary to investigate the deformation of the submarine pipelines for designers. DNV-RP-F107 (Det Norske Veritas, 2002) gives an empirical formula for the dent depth of the pipelines impacted by dropped objects (Alexander, 2007). However, this specification does not consider the absorption of the impact energy by seabed and soil covered on the pipelines, resulting in a conservative assessment. Some scholars have explored the response of submarine pipelines to the impact of dropped objects. The interaction between pipe and soil is a complex process which contains complex mechanism and thus evaluating the damage on submarine pipelines caused by dropped objects is quite complicated. Alsos et al. (2012) discussed the importance of impact velocity and mass during impact, and found that global deformations would be triggered, which implied that the dissipated energy going into local denting is reduced to a fractional value. Yu et al.used a three-dimensional numerical method to study pipeline deformations due to transverse impacts of dropped anchors and the dent depth of the pipe was estimated by the local Galerkin discretization method. The results showed good consistency with experiment. Zeinoddini et al. (2013) carried out a parametric study to examine the effect of bed flexibility and the results showed that the flexibility of seabed plays an important role in impact energy dissipation. Ryu et al. (2015) investigated pipe-soil interaction using finite element technology in which the soil was simulated using the Mohr-Coulomb failure criterion. Robert (2017) used a modified Mohr- Coulomb model to simulate the behavior of pipelines in unsaturated soil. The model was developed considering microscopic and macroscopic suction hardening mechanisms and was implemented into a commercial finite program.
