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Physical Model Test Report of 10” Subsea Oil Pipeline in the Pinghu Oil and Gas Field

Filed under: 研究成果 — cjk3d @ 09:08

1 Introduction
The Pinghu Oil and Gas Field Project is a national important development project and one of ten capital construction projects in Shanghai. The Pinghu Field is situated in the East China Sea, 400 kilometers ESE of Shanghai. The development of the Pinghu Field involves the construction of two sub sea pipelines. The first pipe, a 14 Gas pipeline, is 366.8 kilometers long between the Pinghu Field and the shore at Nanhuizhui , Shanghai, China. The second pipe is a 10 Oil pipeline, 302.8 kilometers long, running between the Pinghu Field and Daishan. The project was completed in Dec. 1998. Gas has been sent to Pudong, Shanghai through the Gas pipeline since April 1999. The 10 Oil pipeline was completed in Oct. 1998. There were several different activities includsive of trench excavation, post lowering, backfilling, and remedial dumping during installation of the 10 Oil pipeline for offshore section. After post lowering, backfilling was performed from KP0.156 to KP2.69, and excavation was reburied naturally by bed silt outside KP2.69. In the section between KP2.1~KP2.6 where the seabed is complicated and extrudes out of the surrounding area and, due to the current effects, the berm dumped was being scoured on both sides, and in some area the pipeline was exposed or even free of span. In order to protect the pipeline, additional dumping operations were performed on the top of the existing berm. On 15th Oct. 2000 and 14th Nov. 2001, the pipeline fractured continuously at KP2.314 and KP2.615 due to strong current effects. The accident made the oil production pause and led to large economic loss.
In present, the topography near the old 10 pipeline route not only is very complicated, but also has influenced the stability of the nearby seabed. In this case it is difficult to analyze the change of flow field and to forecast the further change of seabed only based on a few hydrologic survey data.
Nanjing Hydraulic Research Institute was entrusted by China National Offshore Oil Corporation (CNOOC) to perform a further research on the changeable tendency of seabed by running physical model tests and to provide technical assistance for repairing the oil pipeline.
The physical model test includes two parts: the first part is fixed bed model test in order to analyze the features of flow field and the scouring reason of the seabed nearby the pipeline; the second part is to conduct a movable bed model test in order to forecast the tendency, the scour range and depth of seabed near the new oil pipeline route, and to provide engineering measures for the safety of the new oil pipeline.


8 Conclusions
1.The stability of Houshayang Bay and its surrounding areas is mainly controlled by offshore tide, wave action and sand movement. Because the tidal current in this sea area is not affected by runoff, all kinds of tidal disparities change slowly. As the current is an intermittent dynamic force, even if it goes with a strong wave process, the amplitude of scour and deposition is usually lower than 0.5m, and the sea bed will no soon be stable. Therefore, the sea bed was relatively stable before 1997.
2.Because of the gravel backfilling scheme designed to protect the pipeline, with no timely construction supervision monitors, the sea bed beside the 10 pipeline sufferde intense scour. The ununiform scour along the pipeline leads to the erosion of sea bed under the pipeline. Finally the pipeline is fractured because of the suspension. Results of the movable bed experiments show that the scour trough beside the pipeline has already been in a relativly stable state except the -20m scour pit near KP2.7 which will develop a little.
3.The sea bed inside the -5m depth contour of Houshayang Bay is stabledue due to the protection by. The sea bed outside the -15m depth contour is also relatively stable. After the remedial construction of 10 pipeline in January 2001, local washout happened to the north of the old pipeline from KP1.8 to KP2.2. The -10m and -15m depth contour extends northward and the maximum scour depth exceeds 5m. Meanwhile the scour at KP1.7 of the new pipeline is about 3m. Data of flow measurements in the fixed bed model shows that in this region the bottom velocity of flood tide is very strong and greater than that of ebb tide. This may be an important reason for the recent scour. The seabed between -5m and -15m depth contours is still being adjusted for the strong tidal current. Generally speaking, the scour amplitude will not be too great and little deposition will occur in the local region.
4.Natural backfilling speed of the trench inside KP1.4 will be very slow under the tidal action. Based on the experiences of other projects, similar shallow dredging will quickly lead to refilling under the action of wave. It takes approximately two months for the parallel section from KP1.4 to KP3.1 to be naturally refilled. Refilling rate of the vertical section of KP3.1 may be relatively slow and a period of about six months is needed.
5.The new pipeline lies in the parallel section 875m north of the 10D pipeline. This region is relatively stable, and has hardly any change during recent years. The -10m scour pit north of KP1.9 developed a little. The -15m depth contour goes on evolving southward and connects with the -15m line north of the 10D pipeline. The virtual scouring depth is not large because the depth in this region is already lower than -14m.
6. Based on the forecasting by the movable bed experiment, the maximum scouring or deposition depth will not exceed 3m. If 3m-depth dredging is designed for the new pipeline, the oil pipeline will be safe without any protective measures.

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