Mechanism and model tests on protection of submarine pipelines using rock armor berms
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摘要: 近海海底管线会在港口锚地处因船舶走锚而受到损害,碎石结构能够防护管线免受走锚的危害,因此研究碎石结构的防护机理对保护管线有重要意义。采用极限平衡方法分析了走锚状态下土楔对形状复杂的商用霍尔锚的作用力,提出了锚冠和锚爪的受力计算模式,采用3种不同几何相似比的模型锚在模型槽中进行了拖锚试验。模型试验的结果与计算结果相互吻合。锚体整体受力分析的结果表明,霍尔锚在从砂土进入碎石保护层后因力矩不平衡引起锚体翻转,使锚爪尖位置上移而远离埋设的管线,从而使管线避免走锚的伤害。该机理也通过模型试验得到验证。此外,不同试验锚重的归一化结果还说明,霍尔锚锚爪在碎石中的稳定入土深度约为爪长的0.45,拖曳力为2.5倍锚重,可为碎石结构设计参考。Abstract: Submarine pipelines are likely to be exposed to the risk of damage from drag anchors of ships that are out of control during severe storms. Rock armor berms are commonly adopted to protect the pipelines by deflecting drag anchors. To understand the protection mechanism, both theoretical studies and model tests are carried out. A limit equilibrium method is put forward to deal with the interaction between the drag Hall anchor and the developed soil wedges. The polygons of forces on the soil wedges induced by both the anchor crown and the anchor fluke are established respectively. The total drag force on the anchor is the sum of the drag force on the anchor crown and that on the anchor fluke. The minimum total drag force on the anchor can be solved by using the hill climbing algorithm. Model tests are carried out using three model anchors with different linear scales. The drag forces measured in model tests agree well with those calculated by the theoretical approach. With the obtained forces, the moment on the anchor can be calculated. It is found that when the anchor is dragged into the rock armor berm from the foundation soil, unbalanced moment on the anchor will take place,which will cause the drag anchor to rotate and the fluke tip to rise up. The deflecting of the anchor trajectory can make the fluke tip keep a safe distance from the buried pipeline, which explains the protection mechanism. Using the proposed approach, the position of the fluke tip and the drag force on the anchor under equilibrium conditions can be predicted, which may be helpful to the berm designers.
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Keywords:
- drag anchor /
- rock armor berm /
- Hall anchor /
- submarine pipeline /
- limit equilibrium method /
- model test
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