Detection And Safety Evaluation For A Central Landing Pipeline In Chengdao Oilfield

Chengdao Oilfield is located in the northern sea area of modern Yellow RiverDelta, which is a delta lobe abandoned in1976. This area suffers serious seafloorscour and instability geological hazards. Over the past twenty years, ChengdaoOilfield has grown into a large-scale and independently developed oilfield built onshallow sea of our own country. The annual output of crude oil is more than threemillion tons. So far three central platforms and over104satellite platforms have beenestablished. Besides, Chengdao Oilfield is densely covered by submarine cables andpipelines including160sections of oil pipe which stretches280km in total length.Due to a longer time in construction and operation, production safety issues areincreasing highlighted in projects. Therefore, it is extremely urgent to carry outregular detections on submarine oil lines. Based on the overall research in Chengdaooilfield area, this study focuses on experimental research on the pipeline which startsfrom CB30A wells group to the landing point of HaiWulian. With the length of7km,the pipeline is one of the main landing pipelines to transportcrude oil from oilfield tomainland Because built across the underwater fore-slope of Yellow River delta, thissubmarine pipeline suffers a serious problem of free spanning.This work summarizes the related technologies and methods both at home andabroad by optimizing and integrating external pipe inspection by acoustic method,internal pipe inspection by Magnetic Flux Leakage Testing, indoor experiment etc. tostudy the spatial modes of occurrence of submarine pipeline, erosion around pipeline,geometric deformation of pipeline and the corrosion level of pipeline wall. Besidesthat, this research combines historical data and engineering geological investigationand numerical simulation to investigate the ocean hydrodynamics environment andseabed erosion-deposition situation of Chengdao Oilfield area, to reveal the adversegeological factors which are affecting the safety of submarine pipeline, such as seabederosion and liquefaction at the routing area. Furthermore, the free spanning genesis is discussed and the safety of free spanning pipeline is evaluated, based on thesubmarine pipeline detections and numerical simulation. Finally, specific treatmentmeasures are proposed for safety control of submarine pipelines in different positions.Results are shown as followings:(1) Given the particular marine sedimentary environment of the Yellow RiverDelta, the study integrates advanced technology nationally and internationally toachieve a precise detection in the modes of occurrence and corrosion which coversinternal and external submarine pipeline. Therefore, a submarine pipeline safetytesting system is established and the system emphasizes multi-beam, side scan sonar,high-resolution sub-bottom profiler and magnetic flux leakage correction inspection.A positive outcome has been obtained in the preliminary production test.(2) The study accurately identifies the characteristics of submarine pipelinerouting landform and the pipeline occurrence modes from CB30A to HaiWulianlanding point. The pioneering inspection focuses on the internal part of submarinelanding pipeline at Chengdao oilfield by Magnetic Flux Leakage Testing, and get thecorrosion level.15free spanning pipeline sections are detected with totally980.9meters long, which accounts for about13.68%of the total length. And seabed unevenscour is the main reason for pipeline spanning.11corrosion points were found, and itis the first time to find that long free spanning probably will accelerate the pipelinecorrosion. According to the average corrosion rate, the time left from maximumcorrosion point to minimum allowable wall thickness is only2years. Thus, a timelymaintenance and pipeline change is needed.(3) The numerical model of hydrodynamics around the Chengdao area isestablished to compute the wave, current and water level condition with differentrecurrence interval. Combined with engineering geological investigation andhistorical data, seabed erosion-deposition situation of Chengdao area was analyzed.The adverse geological factors which are affecting the safety of submarine pipelineare revealed. The causes of seafloor scour and soil instability in the pipeline routingarea also explored.(4) The stability of spanning pipelines was evaluated through the limitations regarding maximum allowable stress under static or quasi-static loads and the onset ofVortex Induced Vibrations (VIV) under different hydrodynamic actions. Staticanalysis results show that four sections of the CB30A-Haiwulian submarine pipelineexceed the criterion regarding maximum allowable stress. VIV analysis results showthat14sections of the investigated pipeline are unsafe in the5-year return periodwave conditions.(5) The study carries out a contrastive analysis on various treatments byfocusing on the spanning features of CB30A-Haiwulian submarine pipeline andyears-long investigation findings on domestic and overseas oilfields to summarizeappropriate solution to different types of pipelines at Yellow River Estuary and makespecific proposals on the maintenance and landing of pipeline spanning as well as fora long-term safety control in oilfield production.Research innovations:(1) A combined method of acoustic scan and magnetic leakage is applied tosubmarine pipeline detection for the first time at a domestic level to precisely verifyoccurrence states and erosion of submarine pipelines.(2) Combine submarine inspection, numerical hydrodynamics environment,engineering geological investigation and free spanning pipeline stability analysis toreveal the mechanism of submarine pipeline free span and to evaluate its safety, whichprovides basis for pipeline management and further maintenance.(3) Considering local geological conditions and safety concerns on submarinepipelines, a proposal is raised to deal with the pipelines of different spanning types.