Study On Health Monitoring System And Assessment Of Submarine Pipeline

Submarine pipeline system is a main pattern in storage and transportation of offshore oil and gas. It plays an important role in the offshore oil and gas exploration and development, more and more attention is pain on the safety of submarine pipeline. But there is no real-time monitoring system available for submarine pipeline nowadays. To improve its safety, Zhejiang University and CNOOC applied for the project of distributed optical fiber sensing technology for long-distance submarine pipeline, which sponsored by National Hi-Tech Research and Development Program of China (863 Program). This technology develops a health monitoring system for submarine pipeline based on Distributed Optical Fiber Sensor (DOFS). In this paper, there include the related researches carried out: experiment study on sensing optical fiber selection for DOFS, laying method of sensing optical fiber on real pipeline construction, 3-level health monitoring algorithm and Health Monitoring System (HMS) of real pipeline and so on. Besides, the study of residual corrosion reliability on existed pipeline is also made. The major contents are summarized as follows:(1) Based on the safety assessment of BZ28-1 submarine pipeline for re-use after 16-year partially using and idling, a new assessment method for existed pipeline is put forward, which is composed of 'partial inspection + pressure test'. With the partial inspection data, the probability model of residual wall thickness in space and time domain is established. The relationship between failure pressure, maximum operation pressure and minimum residual wall thickness is calculated, and it is used as the govern law for pressure test. The assessment results of BZ28-1 submarine pipeline are certificated by American Bureau of Shipping (ABS) and China Classification Service (CCS).(2) For the blank of special sensing optical fiber, a new optical fiber structure based on Enhanced Performance Fiber Unit (EPFU) is put forward as the special optical fiber for DOFS. Experiments on 5 kinds of optical fiber available are carried out, all the optical fibers sensing characteristics, including sensing precision, mechanic characteristic and so on, are summarized for further application reference.(3) Special study on optical fiber laying method is conducted for real submarine pipeline construction. The influence on flowing speed, micro pipe diameter, sensing optical fiber diameter and compounding specification is summarized. The calibration experiment on optical fiber after flowing and grouting filling is carried out,and the results prove the method is reliable. Then the detail optical fiber laying design of real pipeline construction is made, which is conducted on QK17-2 submarine pipeline, and the practical construction indicates the method with 'flowing and grouting filling' is successful.(4) The Time Series Method (TSM) is applied in on-line health monitoring system as an abnormity detector to extract the abnormality signal from mass strain signals with high computation speed. The simulated experiment of free-spanning vibration is conducted to check the algorithm. The results prove the algorithm is effective by using fixed 3-order AR model and precise by using moving best AR model under FPE role.(5) Based on the mechanic model of free-spanning pipeline, and the strain model is introduced into the solving of vibration formula, which presents the theory bases of Empirical Mode Decomposition (EMD) and Random Decrement Technology (RDT). The program based on EMD and RDT to extract the natural frequency of submarine pipeline from random motivation is developed in MATLAB environment. The simulated free-spanning vibration signals and pipe model experiment signals are used to verify the algorithm, and the results show it is effective and precise. With free-spanning vibration mechanic model, the free-spanning length is gained.(6) With the multi-axial fatigue life model and Miner linear fatigue cumulated rule, the strain signals from DOFS along the pipeline are transformed into the fatigue life. And the mono-axial fatigue rain-flow counting method is modified to count the multi-axial fatigue life. The monitoring results can be used as the life quantization of pipeline fatigue life.This research work are sponsored by National Hi-Tech Research and Development Program of China (863) (Grant No.2001AA602022-l, 2004AA602210-1) and Research Center of CNOOC.