YINGDONG SAND RIDGES IN EAST BEIBU GULF ANDTHEIR IMPACT ON PIPELINE ENGINEERING
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摘要: 北部湾东侧莺东潮流沙脊是整个琼西南沙脊群之一，沙脊从感恩角岸外发育最好，至东方岸外已趋缓和。根据海底特征，由海向岸分为深水平坦段、沙波沙脊段和近岸段共3个区段。沙波沙脊区段主要分布4条沙脊，这些沙脊大都呈NS或NW—SE走向，与主水流平行，呈不甚规则的长条状，长约45 km，宽度较大。脊槽高差约20~30 m，脊顶水深5~8 m，沙脊槽向不对称，向海侧缓，向陆侧陡。成因上属于全新世低海面形成的连滨沙脊，受现代浪流修蚀改造而成。这些沙脊的基础部分基本稳定，只在暴风浪或强潮流情况下表面活动性泥沙做少量迁移。2003年铺设的东方1-1气田输气管线路由经过北部湾莺东沙脊。通过对中国海洋大学东方1-1海底管线多年调查资料分析认为，本区沙波沙脊海底管线被严重掏空，已发现的130余掏空悬跨点平均长达33 m，深度最大可达1 m。管线掏空可归因于底形特征动态变化和管线改变海底水动力环境的综合作用。Abstract: The Yingdong tidal sand ridge, which occurs in the east of Beibu gulf, is a part of the Qiongxinan sand ridge system. It is well developed outside the Ganen Cornor and smmothly extends to Dongfang. The sand ridge area can be divided into three sections, the deep water flat section, the current ridge and sand wave section and the nearshore section, from the offshore to the coast. In the current ridge and sand wave section, there are four sand ridges in NS or NW-SE direction roughly parallel to the main flow. The ridges are about 45km long with a large width. The valleys between ridges are usually 20-30m in depth, while the ridge top is 5-8m below sea level. Sand valleys are asymmetrical, steep on the land side and gentle on the sea side. The sand ridges were formed in the period of low sea level in Holocene and then eroded by modern wave. The foundation of sand ridge is strong and firm. Only in case of storm wave or strong current, can the surface sediment move in a small distance. In 2003, the Dongfang 1-1 seafloor pipeline was constructed, which passed through the Yingdong tidal sand ridge. Through the analysis of the Dongfang 1-1 offshore pipeline survey data acquired by OUC after construction, it is found that there are 130 points where the pipeline are suspended. The suspended part is 33m long on average and the distance between the pipe and the sea bottom is 1m in maximum. The suspension of pipeline is caused by the changes in bedforms and/or water dynamics.
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图 4 (a) 凹槽水流旁蚀沙脊引起管线掏空；(b)沙脊陡坡被底流侵蚀切割造成管线掏空(底形被动掏空)，黑线为海底面，红线和蓝线为管线的上下边缘，黄色部分为掏空坑；(c)KP80.2附近的3D(左)和局部放大后的声呐图(右)
Figure 4.
图 5 (a) 暴风浪时运动沙波尺度增大，凹槽加深引起管线掏空(A.常浪天气；B.暴风浪天气)；(b)KP88.5附近的剖面图，黑线为海底面，红线和蓝线为管线的上下边缘，黄色部分为掏空坑；(c)KP88.5附近的多波束图像，管线通过大小新月形沙波区和丘间冲蚀坑
Figure 5.
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