The Study On Anchoring Damage To Subsea Pipelines

The Bohai Sea is the main offshore oil producing area of our country, with the ex-ploitation of oil in Bohai gulf, the application of subsea oil pipelines are widespread. Recent years, the damage of vessel anchoring to subsea pipelines is more and more fre-quent. Once the pipelines have a leakage accident, they will destroy the marine envi-ronment and cause a great economic loss. Thus, the research on protecting the subsea pipelines is extremely urgent.Firstly, according to the risk assessment of pipeline protection of Det Norske Veri-tas (DNV-RP-F107), this thesis investigates on two forms of subsea pipeline dam-age,they are impact damage and dragging damage.By taking the subsea pipelines in Bohai Sea as the research object, the Bohai Sea’s submarine geology,atmosphere and hydrology, which affect the damage level of pipelines, are also detailed introduced. The typical vessel is selected based on the port condition around Bohai Sea, then its equip-ment number is calculated according to China Classification Society standards, thus the anchor weight is determined.Secondly, this thesis studies the impact damage of anchoring operation to the sub-sea pipelines. The research mainly includes the impact probability, impact energy and energy absorbing ability of pipeline coating. In order to determine the impact probabili-ty, the mathematical model of pipeline impact is set up based on the regulation, then the impact probabilities are calculated by utilizing the basic principle of probability theory. As to the impact energy, the velocity and kinetic energy of anchor are calculated ac-cording to the kinetic energy theorem, when they are hitting the seabed. Regarding the energy absorbing ability of pipeline coating, by using the soil energy absorption formula in the recommend practice, the embedded depth of anchoring is calculated.Thirdly, this thesis studies the dragging damage of anchoring operation to the sub-sea pipelines. The dragging forces, which consist of anchor holding force and anchor chain holding force, contribute the dragging damage to pipelines. The factors that in-fluence the magnitude of dragging forces are the coefficient of holding power, the anc-hor weight, the unit weight of anchor chain and the length of anchor chain. The former three items can be obtained by consulting the regulations, and the length of anchor chain is determined by the vessel tonnage and forces exerted on vessels, in which the forces mainly consist of wind pressure and hydraulic pressure. The magnitude of dragging forces can be calculated by above parameters, and the security checks of pipelines are made according to the calculating result.Finally, based on the above research, this thesis offer two calculation methods for design burial depth of new subsea pipelines and the distance of anchorage prohibited area to the existing subsea pipelines, and the result datas in the Bohai Sea are suggested. Some other protective measures are also proposed for subsea pipelines and cables.