Impacts from anchoring and trawling gears are great threats to the safety of subsea pipelines. Although recommendations have been provided to access the damage risk from external hazards, there has been a lack of methods to calculate the impact frequencies. In this paper, based on AIS data in part of the South China Sea near Hainan island, anchoring points and trawling trajectories were first identified and studied with cluster analysis using DBSCAN algorithm, and then the impact frequencies were calculated. The methods proposed in this paper could provide assistance in improving the risk analysis of subsea pipelines.
Subsea pipelines play a key role during the exploitation of offshore hydrocarbon resources, providing means of continuous transportation of gas and oil. The safety of subsea pipelines is of great importance as their damage may lead to the potential human safety problem, environmental pollution and economic losses. In recent years, more subsea pipelines have been installed, and meanwhile, the fishing activity and marine vessels are also increasing. The number of damage accidents on subsea pipelines caused by impact from ship anchors and trawling gears has been increasing.
The impact frequency is an important criterion for design of subsea pipelines, as shown in Table 1 (DNVGL, 2018). The DNVGL-RP-F107 (2017) provides a basis for risk assessment of subsea pipelines by external hazards including dropped objects, anchoring and bottom trawling. The impact frequency of trawling gear and anchoring on subsea pipelines are mainly dependent on the vessel density and routines of activities. Based on AIS data, Mustafina (2015) studied the ship traffic density and the frequency of ships crossing the pipelines, however, no further investigation was performed to study the impact frequencies according to the AIS data. Fuad (2020) studied the risk of bottom trawling impact on subsea pipelines using the DNVGL algorithm, in which the frequency of a fish trawler crossing over the pipelines was calculated based on interviews on operation and site survey. Wu (2015) developed a simplified approach to estimate the probability of otter board hooking at pipelines based on the fishing activities. In addition, the response of the pipeline to different types of impact loading needs to be considered during design. Al-Warthan (1993) studied the area of DEM analysis of pipeline responses to the impact loading types.
