Study On The Risk Assessment Technology Of Submarine Pipeline Anchor Failure

With the vigorous development of offshore oil and gas,the total length of offshore oil and gas pipelines is increasing.Due to the economic development,the fisheries around the offshore pipelines are busy and there are many waterways in China.The distribution of submarine pipelines and waterways is interlaced and complex,and the risk of pipelines being impacted by ship anchor is greatly increased.However,the existing risk assessment methods for falling objects impacting on pipelines do not take into account the complex marine environment around pipelines,and the probability of falling objects is different from that of ship anchoring,and the degree of damage to pipelines is also different.Therefore,it is necessary to study the risk assessment technology of submarine pipeline anchorage failure,to provide technical support for the scientific management of sea routes and submarine pipelines,and to ensure the safe operation of submarine pipelines.The main contents and conclusions of this paper are as follows:(1)Firstly,the marine environment is analyzed to understand the hydrological conditions of submarine pipelines and surrounding sea areas,the distribution characteristics of sea routes and vessels,and to provide technical support for the establishment of graph theory model;secondly,the accident database of submarine pipelines is analyzed,the single-layer pipeline structure is determined,and the pipeline section is the middle section of pipeline connecting the offshore platform;secondly,the anchor strike of submarine pipelines is determined according to DNV standard of Norwegian Classification Society.Failure risk assessment method and process.(2)Establish the calculation model of failure probability of submarine pipeline anchorage.Firstly,the calculation model of anchoring impact probability of submarine pipeline is established:the graph theory model is used to describe the position relationship of coast,sea route and submarine pipeline,and the probability calculation model of intersection probability of route and submarine pipeline is established based on probability theory;the probability of anchoring of ship is calculated by Bayesian network model,the geometric relationship between route and pipeline is analyzed,and the flow rate of route is considered to establish the model.The calculation model of anchoring probability of ships at the intersection of routes and submarine pipelines;the method of calculating the probability of falling objects impacting on pipelines based on DNV standard of Norwegian Classification Society.According to the characteristics of anchor structure,the drift offset angle of anchor is modified,and the probability calculation model of anchoring impacting pipelines after entering water is established.Combining three calculation models,the calculation model of impact probability of submarine pipeline anchoring is formed.Then,the probability calculation model of anchorage failure conditions for submarine pipelines is established.The ANSYS LS-DYNA finite element software is used to establish the stress analysis model of ship anchor impacting on submarine pipeline.The stress-strain distribution and time-history change of pipeline after anchoring are analyzed by the model.According to the recommended method,the concave diameter ratio at the collision center is used to measure the degree of pipeline damage.Considering the influence of anchor quality,wall thickness,depth of embedment,thickness of concrete layer and other factors on the damage degree of pipeline,the relationship curves between each factor and concave diameter ratio are obtained by simulation.Statistical analysis and simulation results show that the damage degree of pipeline(concave diameter ratio)is divided into five grades,corresponding to five pipeline deformation forms.According to the reliability theory,the relationship curve between influencing factors and concave diameter ratio is fitted,and the critical values of influencing factors corresponding to concave diameter ratio at all levels are obtained by fitting function,which is used as the basic variable of reliability analysis to calculate the failure condition probability of pipeline anchorage.Combining the impact probability and failure condition probability of submarine pipeline anchoring,the calculation model of failure probability of submarine pipeline anchoring is established.(3)Forming a method for evaluating the failure consequences of submarine pipelines.The possible consequences of submarine pipeline leakage are analyzed by event tree,and the estimation methods of liquid and gas leakage are given.The failure consequences of anchoring impact on submarine pipeline are evaluated from three aspects of safety,environment and economic loss,and the failure consequences are classified.(4)Based on the research results of failure probability and consequence of submarine pipeline anchorage,the risk assessment method of submarine pipeline anchorage failure is formed.According to the relevant standards,the failure probability and consequences of submarine pipeline anchorage are classified and the risk assessment matrix is established.The risk mitigation measures are proposed by comparing the acceptable risk standards.