With the increase of human activities at sea, it is inevitable that anchors drop into the water due to operating errors, which may lead to failure of pipelines and cause economic damage and environmental pollution. Previous methods of related analysis are mostly based on the DNV-RP-F107 recommended method (hereinafter referred as DNV method). DNV method hardly considers the variation of anchor's size and weight. And it is insensitive to the pipeline geometry and material properties. Based on reliability theory, DNV method is improved to calculate failure probability under the consideration of the above relevant factors. The efficiency of the proposed method is verified by a practical case. Besides, analysis of the influence of various factors on pipeline failure probability is completed in this paper, including anchor weight, size, pipeline geometry and material properties, the distance from the anchor drop point. Meanwhile, considering the variability, the sensitivities of variables to the failure probability are discussed. Study results indicate that the failure probability calculated by DNV method is underestimated in some situations, which can probably cause a loss for pipeline projects. Whereas the proposed method is able to consider much more influences and leads to reasonable results consistent with the actual situation.
Submarine pipeline is seen as the ‘lifeline’ for offshore oil and gas industry. Pipeline safety is one of the most important problems for engineering practice. Recently, anchors dropping into the sea becomes more frequent due to the increasing human activities at sea. The dropped anchors are likely to impact on pipelines and lead to pipeline failures, which can cause economic damage and environmental pollution. In order to reduce the risk and provide safe design, considerable research efforts have been devoted to risk assessment and reliability analysis of pipelines. In general, methods of the relevant research mainly consist of two categories: one is qualitative analysis, which can study the main influence factors on pipeline failures. Among them, fault tree analysis (FTA) is the most popular methodology and has been extensively applied to pipeline failure analysis. (Wang et al., 2007; Dong et al., 2005; Lavasani et al., 2011). The other one is quantitative analysis, which can determine pipeline failure probability and provide reliable reference for safe design. Katteland et al. (1995) developed a model for risk calculation, and applied it to evaluate the risk of all the installations in the North Sea. Det Norske Veritas (2010) proposed a ubiquitously used method for pipeline risk assessment and failure probability calculation (DNV method). Based on statistics of crane accidents, Det Norske Veritas (2013) also gave the falling probability for typical loads and various objects, which provided abundant references for pipeline risk assessment. On the basic of the above research, Liu et al. (2005) proposed a model to calculate the probability of pipeline being impacted under various anchorage conditions. Ding et al. (2010) modified DNV method and made a risk assessment of pipelines due to third-party activities. Yan et al. (2014) proposed a procedure to estimate the pipeline failure probability caused by anchoring activities. Up to now, to the best of the author's knowledge, quantitative analysis methods are mainly based on DNV method. In some situations, this method is hardly to consider the effect of anchor size and weight on pipeline failure probability. What's more, it is insensitive to the effect of pipeline geometry and material properties, which is not consistent with practice and may cause errors. In order to give an insight into those effects, a method based on reliability theory to calculate pipeline failure probability is proposed.
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.