Advanced Deepwater Monitoring System

This study investigates new methods to improve deepwater monitoring and addresses installation of advanced sensors on ”already deployed” risers, flowlines, trees, and other deepwater devices. A major shortcoming of post installed monitoring systems in subsea is poor coupling between the sensor and structure. This study provided methods to overcome this problem. Both field testing in subsea environments and laboratory testing were performed. Test articles included actual flowline pipe and steel catenary risers up to twenty-four inches in diameter. A monitoring device resulting from this study can be installed in-situ on underwater structures and could enhance productivity and improve safety of offshore operations. This paper details the test results to determine coupling methods for attaching fiber optic sensor systems to deepwater structures that have already been deployed. Subsea attachment methods were evaluated in a forty foot deep pool by divers. Afterword, structural testing Address all correspondence to this author. was conducted on the systems at the NASA Johnson Space Center. Additionally a 7,000 foot deep sensor station was attached to a flowline with the aid of a remote operated vehicle. Various sensor to pipe coupling methods were tested to measure tensile load, shear strength and coupling capability. Several adhesive bonding methods in a subsea environment were investigated and subsea testing yielded exceptionally good results. Tensile and shear properties of subsea application were approximately 80 percent of those values obtained in dry conditions. Additionally, a carbide alloy coating was found to increase the shear strength of metal to metal clamping interface by up to 46 percent. This study provides valuable data for assessing the feasibility of developing the next generation fiber optic sensor system that could be retrofitted onto existing subsea pipeline structures. NOMENCLATURE A Cross-Sectional Area of Tendon ( m2 ) CTES Coefficient of Thermal Expansion for Pipe 1 Copyright c © 2013 by ASME (μm/m−C) CT ET Coefficient of Thermal Expansion for Sensor (μm/m−C) E Young’s Modulus (GPa) FEA Finite Element Analysis FG Gauge Factor ST Temperature Sensitivity T LP Tension Leg Platform ε Strain ratio of deformation per original length ( m m ) λb Bragg wavelength (nm) λ0 Baseline wavelength (nm) ∆λS Wavelength shift from strain gauge (nm) ∆λT Wavelength shift from temperature compensation gauge (nm) με micro-Strain ( 10−6 m m ) μεT micro-Strain due to thermal expansion ( 10−6 m m ) FIBER OPTIC LOAD SENSORS This project details a new innovation to replace Tension Leg Platform (TLP) load monitoring systems that use load cells, with fiber optic load advanced sensors that are installed on the tendons. This project involved several innovations including postinstalled subsea load sensing with fiber optics, laboratory testing of the adhesive and sensor strength in sub-sea conditions, and verification of sensor adhesion through bend, tension, and compression cycling up to 70% of ASTM A36 steel yield strength. The newly installed load sensors are a first of its kind innovation that is in actual operation. Much progress has been made in the last decade to monitor subsea operations. Critical parameters such as stress, strain, temperature, pressure, vibration, leaks, and flow assurance issues are of great interest. The first subsea fiber optic sensing system was installed as part of the Troika project in the Gulf of Mexico in 1997. Troika used Fiber Bragg Grating (FBG) sensors with a very early and elementary signal conditioning unit. This early project was tasked to monitor the pressure without placing any penetrations into the 14 mile pipe-in-pipe subsea tieback. In addition to pressure, the temperature and strain were monitored in real time. Since then other deployments utilizing fiber optic sensors were deployed on deepwater drilling risers and later on deepwater steel cantenary risers [1–5]. An important aspect is that the sensors do not require penetrations into the flow stream, pressure vessel, or pipe wall. The sensors use light to interrogate the fiber optic measurement devices so no electric current is required. The sensors can be installed many miles away with little attenuation of the signal strength. The fiber optic strands themselves are vulnerable to damage so they are ruggedized with multiple layers of protective sheathing for installation and long-term service. Monitoring of subsea pipelines and structures provides needed information for managing offshore oil and gas operations and helps prevent environmental damage and catastrophic failure. Another aspect of this project is in the analysis, display, and dissemination of results for early fault detection. When retrofitted measurement devices are installed on existing subsea equipment, fatigue life must take into account prior information that is not typically available. This study employs methods for extrapolating fatigue life to estimate the prior unknown operating data. Vibration analysis and strain are used to predict time to failure. Temperature, pressure, strain, and other non-penetrating sensors are used to predict flow assurance. Dynamic data reconciliation is used to align the observed measurements with detailed process mathematical models of the system. The software complements the measurement devices to synthesize and deliver real-time process information to operators, engineers, and management. Web-based streaming of the results from the production facility allows world-wide access to current conditions and summary reports. TESTING OF LOAD SENSORS Three fully integrated test articles were used for the laboratory tests and consisted of three 24” diameter test pipes with two nominally 36” long segments and one nominally 14’ long segment. The two 36” test articles consisted of steel (ASTM A232 steel for the ends and ASTM A36 steel for the mid-section). The manufactured fiber optic sensor stations were integrated with the pipes for each of the above test articles (see Figure 1), with a combination of friction and adhesive attachments. All of the fiber optic sensors were integrated with the software monitoring system to collect data at two samples per second for each of the sensors in raw format and calculated results for strain and temperature. FIGURE 1. CLAMP AND FIBER OPTIC SENSOR STATIONS. 2 Copyright c © 2013 by ASME Each of the test articles were constructed to emulate the configuration of the TLP clamp system. Preparation of surface bonding included cleaning, grinding, and bonding. Bonding was performed in a simulated subsea environment with the aid of a small water pool. Resistive strain gauges were bonded directly to surface of the steel to verify the fiber optic strain sensor readings. After the attached sensors were installed, a repeat stress analysis was conducted with a two mil thick coating of polyurethane between the sensor and the bare steel pipe. This testing was to ensure that time and temperature shift factors were included to determine stress relaxation modulus effect of the polymer layer. A 70% of yield stress was achieved and stress relaxation modulus occurred from zero time to one month of constant stress loading. This analysis compared the elastic region of the carbon steel to the viscoelastic properties of the polyurethane coating. FIGURE 2. INSTALLED CLAMP AND FIBER OPTIC SENSOR