The Subsea Bedforms Evolution Characteristic On Continental Shelf And The Effect To The Seafloor Pipeline Condition Off Dongfang, Hainan Island

Since the Holocene Epoch the continental shelf has experienced the large-scale ingression, meanwhile the exposed continental shelf has gradually submerged by the sea water, and the tidal current, which leading to the formation of the seabed landform and the deposition, becomes the most important and the most active sea power essential factor to the epicontinental sea. It has molded the different scale longitudinal and transverse direction bedforms, especially scouring deep grooves may be the largest hypsography; it has also driven long sand drift and formed a large-scale modern tidal current accumulation -- tidal current sand ridge and tidal current sand mat. Research indicates that the past century global sea level has risen 10 to 20 cm, and in the future it must accelerate to rise. The sea level rise will exacerbate storm surges strength, and Hainan Island is just the one of most serious regions affected by the storm in our country.Hainan Dongfang 1-1 natural gas fields located at the southeast of Beibu Gulf, outside the bank of Dongfang city, Hainan Province, where the water depth approximately 70m. The gas pipeline from gas fields to Hainan Island is about 109 km, routing the deep water smooth area, the sand wave and sand ridge growth area and the shallow water eroding area. The exceptionally complex seabed bottom shape is a good place to study the continental shelf bottom shape change. From 2003 to 2007 Ocean University of China has repeatedly carried out many investigations on the pipeline security, and obtained a wealth of multi-beam, shallow sonar and post-mortem data.These investigations provide detailed survey data for the research of the seabed bedforms change characteristic and the influence to the pipeline condition of this region.The study area is divided into Shallow water area, Sand wave and Sand ridge area and the Deepwater area according to the different seabed terrains. (1) Shallow water area: the influence of waves and tidal current is comparatively obvious. The river silt supply is small as a result of the Hainan Island vegetation cover. The hydrodynamic force erosion plays the main role in this region, at the same time the movement of surface sediment is comparatively active. The sediment transporting by hydrodynamic force may temporarily bury the erosion pit or ditch, consequently causes the false appearance of the safety of the pipeline. (2) Sand wave and Sand ridge area: the displacement rate of sand ridge is small and the sand wave migration rate is 0 ~ 50 m/a. Under the normal state, sand wave distribution, form and migration direction are controlled by local tidal current. According to different tidal current function, sand wave area can be divided into East, Middle and West regions. East region: Water depth 20~40 m, VEbb>VFlood, and the ebb current direction is south while the flood tide current direction is north. The primarily migration direction of the Sand wave is SW, at the same time sand wave which at the west of the sand ridge receives the sand ridge terrain influence and shows the migration trend to the NE; Middle region: Water depth 28~37m, the sand wave assumes the symmetrical figure and relative stabilization; West region: Water depth 36~52 m, VFlood>VEbb, the flood current is the superior potential flow, and the sand wave migratory direction should be NE. In the conditions of strong hydrodynamic force caused by typhoon, Sand wave and Sand ridge area seabed bottom shape will have the large changes. The seabed terrain, sand wave migration and the growth of regional erosion ditch create subsea pipeline suspended span. Although the big section of the suspended span discovered by preliminary investigation concentrates in this region, but the overall state of the pipeline develops to a good direction according to the form of the suspended span and the route area geology evolution. (3) Deepwater area: the mainly component of seafloor sediment is clay and silty clay, and it belongs to a relatively stable environment. However, the deposit coarsening and the both sides water depth enlarging around the pipeline, and hundreds of erosion pit seriously threat the pipeline safety. This is mainly because the pipeline laying causes the change of the hydrodynamic force conditions.This article has analyzed the seabed bottom shape change and pipeline response from the Ideal Scour and the Pore Effect, and elaborated the influence of pipeline laying to the seabed bottom shape change. Because the pipeline has not completely entered the mud after laying, the bulge subsea pipeline changed the surrounding hydrodynamic force condition and formed flow field which was propitious to scour.The pipeline beam flux function created the rapid expansion of erosion pit, which seriously influence the pipeline security. This phenomenon is especially obvious in the Deepwater area.Analysis indicates that the study area seabed bottom shape is mainly controlled by the tidal current and the strong bottom current which caused by the storm, and the pipeline laying creates a big influence to the surrounding bottom shape. The South China Sea is a resource-rich sea area. It is known as"the second Persian Gulf", it has been given the future hope of the sources in the thought of energy strategy changes from the land to the sea and listed as one of the top-ten country oil gas strategy electoral district. The seabed pipeline is a important component of subsea oil and gas field , it is also the the primary means of transmission. Therefore the research of continental shelf bottom shape change not only has the very vital scientific significance, but also has the widespread production application value.