YINGDONG SAND RIDGES IN EAST BEIBU GULF ANDTHEIR IMPACT ON PIPELINE ENGINEERING

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.