TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractFlow assurance is a key aspect of offshore, particularly deepwater developments. Tremendous progress during the past two decades in the understanding of the issues and the required technology has enabled the developments be designed and operated with low risk of flow assurance problems. However, the problems have not been eliminated due to system component failures or un-designed for operating conditions. Real time monitoring of the production and transportation system can help significantly reduce the occurrence and impact. The present paper discusses the use of non-intrusive fiber optic technology for this purpose.A series of fiber optic sensors for temperature, pressure, heat flux, strain, and acoustic measurements have been developed, which form the basis of the methods proposed in this paper for detecting the formation of hydrate plugs in pipelines, for determining the amount and location of paraffin deposition, for pig detection, and slug detection.

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 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.

Bass Lite deepwater field in the Gulf of Mexico, at water depths of approximately 2,050 m (6,750 feet), commenced operation in February 2008. Natural gas is produced from Bass Lite via a 90-km (56-mile) subsea tieback to the Devils Tower Spar. This project involved several innovations, one of which was the incorporation of a fiber optic sensing system that measures real-time temperature, pressure and strain along the pipeline length. This is a first of its kind innovation that is in actual operation.

Fiber optic sensors have gained increasing use in monitoring offshore structures. The sensors have successfully monitored flowlines, umbilicals, wells, Tension Leg Platform (TLP) tendons, production and drilling risers, and mooring lines. Fiber optic sensors are capable of monitoring strain, temperature, pressure, and vibration. While the success of fiber optic monitoring has been clearly demonstrated, the sensors are now under consideration for automation applications. This paper details the plausibility of using pressure measurements from post-installed fiber Bragg grating (FBG) sensors with Model Predictive Control (MPC) to suppress severe slugging in subsea risers. Prior control schemes demonstrate that slugging is mitigated using a topside choke valve. The most effective methods use a pressure measurement immediately upstream of the touchdown zone of the riser; however, the majority of production risers do not have pressure sensing at that location. With advances in subsea clamp design and bonding it is now possible to install a non-penetrating FBG sensor to monitor pressure near the touchdown zone without shutting down production. Stabilizing the two phase flow both reduces vibration-induced fatigue and has the potential to allow increased throughput with relaxed topside processing constraints. MPC predicts and adjusts for disturbances to avoid pressure and flow instability. The performance of the controller is influenced by sensor location, choke valve response time, and riser geometry. This study demonstrates that severe riser slugging is effectively controlled with MPC and a post-installed, non-penetrating FBG sensor.

This paper addresses the design and development of a fiber-optic monitoring system that can be deployed on existing deep water risers and flow lines; and provides a summary of test article fabrication and the subsequent laboratory testing performed at the National Aeronautics and Space Administration-Johnson Space Center (NASA-JSC). A major challenge of a post-installed instrumentation system is to ensure adequate coupling between the instruments and the riser or flow line of interest. This work investigates the sensor coupling for pipelines that are suspended in a water column (from topside platform to seabed) using a fiber-optic sensor clamp and subsea bonding adhesive. The study involved the design, fabrication, and test of several prototype clamps that contained fiber-optic sensors. A mold was produced by NASA using 3-D printing methods that allowed the casting of polyurethane clamp test articles to accommodate 4-inch and 8-inch diameter pipes. The prototype clamps were installed with a subsea adhesive in a “wet” environment and then tested in the NASA Structures Test Laboratory (STL). The tension, compression, and bending test data showed that the prototype sensor clamps achieved good structural coupling, and could provide high quality strain measurement for active monitoring.