Span Monitoring And Evaluation Of Distributed Submarine Pipelines Based On Active-thermometry

Submarine pipelines systems are the lifeline of subsea oil and gas transmission.However,buried subsea pipelines are inevitable to the problems of scour and free-spanning subjected to complicated submarine environments like current,storm.Failure of pipelines can potentially cost millions of dollars and serious environmental damage.It is quite difficult to utilize current monitoring or inspection techniques to monitor the subsea pipeline suspension in real time for long term,because of their shortages,such as difficult installation,high cost,local monitoring,etc.Aiming at this problem,advantages of fiber optical sensing technique,such as high durability,distributing monitoring ability,are considered and utilized in this paper.And then,one novel kind of scour monitoring method base on active thermometry and fiber optical sensing technique is proposed from the new point of view.Suspension scour length is monitored by means of heat conduction difference detection between water and sand sediment exposure of subsea pipeline.To verify the feasibility of this theory,our team has done a lot of research previously.Researchers designed a series of experimental devices based on FBG optical fiber sensor,DS18B20 digital temperature sensor and distributed Brillouin fiber optical sensor,and scour monitoring tests for pipeline were conducted.Results show the good applicability and feasibility of the Active thermometry on scour monitoring for subsea pipelines.Although the preliminary feasibility studies had good progress and provided a new idea for overcoming the scour monitoring problem,but there still remains some problems need to be solved and optimization.On one hand,the structure of the sensors designed previously is complex and the encapsulation is simple,it is difficult to deal with the turbulent subsea environment,which means that cannot be applied to engineering;on the other hand,the price of BOTDA is expensive,so the economic effect is poor.Given this,this paper presents a low cost distributed monitoring method based on RAMAN sensing technology,and first uses armored cable.Taking the metal line inside the cable as the heating belt and the optical fiber as temperature sensing unit,which improve the overall performance of the sensor greatly.In addition,the armored cable provides great convenience for the layout of the sensor because of its strong packing and convenient installation.This paper carried out a series of research work based on above introduction.Firstly,heat transfer models of different media(water and sediment)and temperature measuring principle using RAMAN were analyzed;then,establishing the finite element models of line heat source and coaxial cable to simulate the temperature change in the process of heating and cooling.Theoretical analysis and numerical simulation proved the feasibility of this active thermometry in scour monitoring for subsea pipeline.Finally,experiment based on RAMAN and armored cable was conducted in laboratory,and the data was processed using “Three-index”.Results showed that,the temperature measuring technology based on RAMAN and armored cable can recognize the water and sand accurately,thus realized the scour monitoring and the calculation of the span length.The new method proposed in this paper can solve the remaining issues in prior research significantly.