Research On Technologies Of Detection And Treatment For Submarine Free Span Pipeline

As a kind of marine oil and gas transportation mode, with its characteristics of high efficiency and low cost, Submarine pipeline has been widely used in today’s ocean engineering, and is an important part of marine oil and gas development. Under the complex undersea environment which accompanies long-term erosion of current and geological disasters, submarine pipeline is easy to hang over the seabed because of the washing away of sediment around. The vortex formed by current passing through free span will speed up the suspended sediment erosion around pipeline, and increase the length of free span. Periodic vortex release will lead to periodic vibration of free span which reduces the service life of the pipeline vibration, seriously will lead to fatigue damage. If the free span could not be found and managed in time, its length will increase under the current erosion ceaselessly. Then the fatigue fracture of pipeline caused by vibration may lead to serious marine pollution and great economic losses. In order to prolong the life of submarine pipeline, ensure the safety of oil and gas transmission, and avoid accidents caused by fatigue fracture, it is important to research the dynamic characteristics of free span under action of current, detection and manage method of submarine free span pipeline.Dynamic performance of free span, detection and manage methods of free span are studied in this paper.(1)Dynamic characteristics of free span under the effect of internal and external fluids are studied based on theory combining with calculation example. Vibration differential equation of free span with effect of flow inside and outside pipeline is established according to the Hamilton theorem. Based on the vibration differential equation, computational formula for allowable scouring length of free span is derived considering effect of internal flow. According to the results of calculation example, allowable scouring length and dynamic response under vertical vibration and cisoid vibration are compared to research the change law of dynamic response with internal and external fluids.(2)The principle and working mode of side-scan sonar are studied. A certain type of side-scan sonar is selected and detection test of submarine pipeline is made. According to the results of detection, whether side-scan sonar can be used or not in detection of submarine free span pipeline is estimated.(3)Sonar image is low resolution and full of noise because of complicated marine environment. In order to judge the status of pipeline accurately, the sonar image must be processed first. The image denoising test with several kinds of wavelet image denoising methods which are most widely used and several methods aiming at sonar image denoising is done, and suitable method for pipeline sonar image denoising is determined, according to the result.(4) In order to analyze the pipeline sonar figure better, methods for image segmentation and feature extraction are studied using in pipeline sonar image to extract the accurate information of pipe.(5) Base on the research of submarine plough trench cutter, a method for free span pipeline covering using submarine plough is presented. The preliminary design of submarine free span pipeline covering equipment for the method is finished. The design includes whole structure of the equipment and all parts implement different functions.