Experimental Study On Bionic Grass Protection Measures For Submarine Pipelines

Submarine pipeline is an important channel for transportation of marine oil and gas that connecting offshore oil and gas fields and land.Since the environment that the pipeline located is affected by various natural and human factors,the stable flow field around the pipeline is easily destroyed,causing erosion of the seabed,causing the pipeline exposed or even suspended.The suspended pipeline will deform under the action of gravity,and generate vortex-induced vibration under the action of current,which will cause fatigue damage of the pipeline structure and bring huge safety hazards.The bionic grass protection technology is a submarine anti-scour technology developed based on the principle of ocean bionics.Practice has shown that bionic grass can effectively reduce the current velocity,protect the seabed from erosion and promote sedimentation in the grass.Taking Zhoushan Cezi Island-Zhenhai submarine pipeline as an example,this paper proposes a new type of pinnate leaf bionic grass with better flow resistance capacity and lower cost.Through the flow experiment in water flume,the flow resistance capacity of three different types of bionic grasses were observed and compared,and the corresponding relationship between height and density and the flow resistance capacity of pinnate leaf bionic grass was explored.The results showed that the pinnate leaf bionic grass had the best flow resistance capacity.Under the approximate conditions,the flow resistance capacity of pinnate leaf bionic grass(the reflux rate was about 90%)was better than the strip-shaped bionic grass(the flow inhibition rate was about 78.7%).Relative grass height(bionic grass height/water depth)is positively correlated with the flow resistance capacity.Under certain condition,the height and density of bionic grass have the optimal value.exceeding the optimal value,the grass height and density continue to increase,and the growth efficiency is limited.The growth of flow resistance capacity is limited and the benefits are small if the grass height and density continue to increase while exceeding the optimal value.After laying the bionic grass,the internal flow velocity of the bionic grass was greatly reduced,and a velocity increase zone of 10 cm height appeared at the upper edge of the grass.The maximum increase was about 25%,sectional flow velocity exhibits an inverse S-shaped distribution.According to the measured hydrological data in Zhoushan area,the pipeline sediment scour experiment was designed to verify the protective effect of the pinnate leaf bionic grass on the pipeline.After sediment particle analysis and sediment start-up test analysis,the sediment in Caofeidian area was selected as the scour experiment prototype sand.Using the series scale model test method,the scouring results of the pipeline before and after the bionic grass laying were observed under the three scales of 15:1,20:1 and 25:1,and the fitting results under the double logarithmic coordinates system were extrapolated into the prototype.The results show that in the hydrological environment of Zhoushan area,the pinnate leaf bionic grass can effectively protect the pipeline: under the unidirectional constant flow,when the maximum daily tidal current velocity and the extreme maximum flow velocity are simulated,the sediment under the bionic grass will be scoured at different extents,and the pipeline will not be exposed;when simulating the tidal reciprocating flow,the siltation and erosion of sediment in the bionic grass is basic balanced,and the effection of pipeline protection is well.The pinnate leaf bionic grass has good effectiveness of reducing flow velocity and promoting deposition,and has a high value of application.In this paper,the protective effectiveness and its influencing factors of pinnate leaf bionic grass are analyzed,which provides a reference for its future promotion and application.