Research on Optical Fiber Sensing Technology Applied to the Stress Monitoring of Deepwater Risers

Author:Xu Jian

Supervisor:yang de xing


Degree Year:2018





In the offshore oil exploration,the drilling platform is one of the most important marine equipment.Marine risers which connect the drilling platform with seabed are affected by various forces from environment and different drilling conditions.In the deep-water drilling riser system,it is important to monitor the maximum stress in the area near the lower marine riser package and blowout preventer(LMRP/BOP)or near the sea level.Otherwise,the overload stress could be damage to risers,persons,equipment and environment.The optical fiber Bragg grating(FBG)has the special wavelength modulation characteristic and advantages of fire proof,explosion proof and electric insulation.Thus,FBG is suitable for the deepwater riser monitoring in the oil and gas environment.In the background of the national supporting to promote the innovation of marine equipment technology,it is of great significance to research the deepwater-riser stress monitoring on the basis of optical fiber sensing technology.In this paper,a full set of optical fiber sensing technology research method including sensing theory and model,sensor design and development,sensor system ocean test was established.Firstly,the theory of the deepwater riser stress monitoring was researched by using FBG sensors.The design and simulation of sensors were put forward.The installation structure was imporved.Secondly,the sensor system engineering prototypes were tested under the actual deep-water conditions after the pool test.Finally,a new-structure sensor was proposed to solve the problems in the marine tests.In the process of developing the sensor,some technical problems were solved.The main contents include the following aspects:(1)The riser maximum stress and bending moment was analyzed in theory when the riser was driven by external force.According to the force model of the riser,a four-quadrant sensor array structure model was applied to monitor the riser stress.The stress monitoring sensor theory was studied and validated by the finite element simulation software.The improved relation formula was put forward.The application method of the FBG sensors was obtained by analyzing the working principle of FBG sensors.Considering the influence of environmental factors on the sensor array,the independent environmental compensation sensor was developed to eliminate the influence of temperature and pressure in the deepwater.The riser sensing formula of maximum stress and bending moment was put forward.This formula is suitable in the conditions while the riser is under the axial force and the bending in any direction.It was theoretically proved that the four-quadrant sensor array can obtain the maximum stress and bending moment of the riser.(2)A variable-cross-section tubular FBG strain sensor that can be used in the deepwater was proposed.The encapsulation technology,performance optimization and stress corrosion resistance of the sensor was studied.The core component of the sensor is a pressure-resistant strain sensitive element with a double glue point encapsulation structure.The axial strain is converted into a wavelength shift of the FBG reflection center spectrum by this structure.Three load(tension,pressure and temperature)tests on the sensitive element show that the wavelength shift has good linear response relationships with three loads.And the sensitive element works well in the water where the pressure is 30 MPa.Under the tension test,the minimum linear correlation coefficient of the sensitive element is 0.99997,the maximum error is 0.35%,and the repeatability error is within 0.55%.All of them satisfy the technical requirement that the error is under 6%.Finally,the development of the sensor array was completed.(3)A method of adjusting the FBG strain sensor sensitivity by changing the diameter ratio of the sensitive element and the prolong element was proposed.The variable cross section,large volume and complicated structure lead to slipping between the load equipment and the sensor.In order to eliminate the slipping influence on the measurement accuracy,a strain sensor sensitivity measurement method based on double optical levers was presented.The experimental results show that the accuracy of the method is 0.91%.The equivalent strain sensitivity measured by the experiment is 0.6817με/pm,and the difference is 1.42% compared with the simulation value of 0.6720με/pm.The results show that this sensitivity measurement method is effective and reliable.(4)The force mode of the sensor array installation structure was analyzed.A metal-tie-type installation structure based on static friction force was developed.Through the optimum design of the belt structure and auxiliary accessories,the friction coefficient and effective contact area between the metal tie and the riser protection layer were improved.The parallel installation between the sensor and the riser axis was ensured.Different installation procedures were designed for various risers.It was proved that the installation method was convenient for assemble and disassemble.There is no welding,no paste and no damage to the riser protection layer.The installation time is within 40 minutes,which effectively improves the working efficiency on the deck.(5)Combined with riser stress monitoring method,the sensor array,the sensor installation structure and the signal processing system,a riser stress monitoring system was built,and its underwater working mode was studied.Two full sets of monitoring systems were tested in the laboratory pool and oceans.Among them,the pool test includes the immersion test and the strength test.The initial value acquisition method and the method of pre-stretching the sensor on the drilling platform were confirmed in these tests.Marine tests include a single-node marine test which has only a deep-water monitoring systems and a two-node marine test which has both deep-water and shallow-water monitoring systems.In two marine tests,the riser stress monitoring systems had worked for 49 days and 19 days respectively according to the drilling log.The test results show that the riser stress monitoring system has the long-term stable-work ability underwater.Among them,the deepwater single-node test completed the drilling task of two wells with depths of 1365 m and 1252 m respectively.And the average maximum stress measured on the monitoring point of the riser is 85 MPa.The two-node test completed a well with a depth of 1432 meters,and the average maximum stress measured on the shallow-water and deep-water monitoring points are 110 MPa and 74 MPa respectively.Compared the test results with the COSL simulation results and working conditions,it shows that the measured average maximum stress in the range of the simulation results.The monitoring system can effectively monitor the riser maximum stress and reflect the influence from different drilling conditions.(6)A simple-wiring cantilever-beam-structure fiber optic sensor structure,which can be used for deep force load monitoring,was proposed.The FBG was sealed inside the cantilever beam element which maintains the characteristics of simple structure and excellent mechanical properties.It worked normally under water pressure of 30 MPa.Experimental results show that the force sensitivity is 2.79pm/N,the temperature sensitivity is 30.93pm/℃,the water pressure sensitivity is 3.79pm/MPa,and the linear correlation coefficients of the various parameters are better than 0.99,which indicates it may effectively compensate for the effects of environmental factors.The influence on sensor error from packaging process and experimental conditions was analyzed by the comparison between the simulation and experimental results.In addition,a metal sealing method was used to study the pressure-resistant encapsulation method for the FBG sensor.This sensor structure simplifies the wiring,decreases the cable winding risk and increases the working stability of the sensor.