Oil and Gas: Technical evolution of ultra-deep water riser integrity monitoring
Roderick Edwards, Vice President of BMT Scientific Marine Services, discusses the need for riser integrity monitoring for offshore platforms, which has accelerated in recent years.
The need for riser integrity monitoring for offshore platforms has accelerated in recent years as operators strive to better understand structural issues and potential risks associated with riser systems in increasing water depths and hostile environments. Planning for monitoring at an early stage of a project is now recognized as essential to significantly reduce risk and cost, while improving the effectiveness of the integrity monitoring system by integrating it into the existing elements of riser systems.
Riser Integrity Monitoring Systems (RIMS) provide operators with data that allows them to assess a riser’s health and indicates critical losses in tension or buoyancy in independently supported riser systems. Furthermore, archived data from RIMS can provide vital support for forensic analysis and for the verification of design procedures. The effectiveness of these monitoring systems continues to improve with technological advances in areas such as sensor reliability, replaceability in subsea service, power consumption and system survival during installation.
The key elements of a riser integrity monitoring system should include:
- Reliable measurement of pipe strain for 20+ years
- Serviceability - evolving technology
- Attachment methods for strain measurement systems
- Intimate connection to pipe surfaces - required for accurate/stable tension measurements
- Attachment to exterior insulation - acceptable for fatigue tracking
- Timely transmission of measurements to the Host Platform
- Reliable measurement of kinematic responses and attitude for 20+ years
- Serviceability is presently well catered for
Over the last decade, BMT has installed eight riser integrity monitoring systems (RIMS) and one TLP Tendon Tension System Retrofit. The systems range from inaccessible/unserviceable strain measurement systems on Hybrid Riser Tower Core Pipe, to Diver serviceable sensors mounted on to the insulation of SCRs, to ROV serviceable systems on Buoyancy Tank Tendons. These systems include over 100 Subsea Strain Sensing Assemblies and have accumulated approximately 2.8 million hours of service. In the next year we will deliver another five RIMS including approximately 120 SSSA’s. This wide assortment of RIMS projects has afforded BMT the opportunity to support developments and evolve the serviceability and reliability of subsea strain measurement systems - some aspects of which are detailed below.
Tools are being developed that permit the reliable, long-term monitoring of the structural response of submerged risers and pipes that is necessary for effective integrity monitoring. These tools are embedded in the earliest installations (2007) that continue to be serviceable for, in excess of six years. This field experience supports a Mean Time Between Failures (MTBF) approaching 900,000 hours for the sensors, meaning that system service lives of 20-25 years seem achievable. In addition, developments in diver and ROV-serviceable sensors also contribute to better system long-term availability. For example, the RIMS for Block 18 within the Greater Plutonio field, offshore Angola, has performed drift free now for six years.
A Measurement Approach
Measurement of riser strain is the most straightforward way to support integrity monitoring. For Free-Standing Risers (FSRs), strain (force) must be measured in an unequivocal load path between the buoyancy can and the riser. On Steel Catenary Risers (SCRs), strain is measured at, or near ‘hot spots’.
Several different methods of strain measurement are available to meet the demands of a deepwater environment such as a system of Linear Variable Differential Transformers (LVDTs). These sensors have been in active use in the industry since the 1930s and have demonstrated long-term stability and reliability. LVDTs have the added advantage of being encapsulated in corrosion-resistant pressure-balanced, oil-filled housing, thereby providing a dual seawater barrier.
With the LVDT system, an accuracy of 0.8% of full scale is achievable if the sensors are calibrated on the pipe or member that will be used in service. An example of calibration of an 18-inch load spool is shown in the table below. However, acceptable accuracy may be achieved with no full scale calibration necessary. An accuracy of 2-3% is achievable with only inexpensive laboratory bench calibrations and no expensive full scale calibrations.
Improvements to Riser Monitoring – Diver Serviceability
BMT’s LVDT-based Strain Sensor Assemblies were proving to provide the accuracy and reliability for long-life. In the sensor configurations designed, four out of six sensors had to fail before the ability to measure bending and tension would be lost. With a respectable MTBF (today, field data supports a MTBF of nearly 900,000 hours), the probability of survival of a single sensor is 76%. A six sensor assembly provides a probability of the survival of the system (3 sensors) of 96.8%. Nevertheless, operators wanted higher reliability and required an ability to replace faulty sensors in the unlikely event of a failure.
BMT first implemented a diver-serviceable Strain Sensor Assembly in 2007 on the P-52 Free Standing Riser Tension System offshore Brazil. The system included sensors equipped with Tronic connectors and large diver-friendly fasteners which are few in numbers and large enough to be operated by a diver with appropriate tools. Other Riser Monitoring System projects including SCRs in West Africa and Brazil and several riser towers have also taken advantage of this design.
With these systems, the sensor assemblies were either permanently installed or used only at depths reachable by divers. However, with the financial investments required and ongoing health and safety issues of saturation diving, as well as the requirements for deeper depths of deployment, it was clear that the development of a Remotely Operated Vehicle (ROV) serviceable strain sensor would prove fruitful.
BMT developed an ROV-serviceable Subsea Strain Sensor Assembly (SSSA) mainly as a response to client need. It is true to say that using an ROV is a less expensive, safer and more available way to replace sensors. In addition, clients rely less on MTBF and redundancy to achieve full design life as sensors may be replaced much more easily. In order to evolve from the proven diver-serviceable design, it was necessary to change how the sensor attaches to the pipe and in turn, make it ROV-friendly. Both ends of the sensor were modified, as were the attachment features that are pre-installed on the pipe. This allows for a single, socket-head screw connection to the pipe on each end of the sensor which could be easily reached by an ROV.
This design allows sensors to be serviced individually, without affecting or interrupting data from the others. This is significant for two reasons. Firstly, it allows the monitoring system as a whole to remain operational, continuing to collect and record data despite a sensor failure. Secondly, it allows for the preservation of the absolute (as opposed to dynamic) bending or tension measurement when a sensor is removed and replaced. Sensors installed manually prior to the riser's installation can be "zeroed" when the riser is on the back of a vessel, in an unloaded condition.
To qualify the ROV serviceable sensors, full-scale bending and tension tests were conducted in a laboratory in Berkeley, California, the results of which were noteworthy. In tension, the uncertainty of the sensor’s measurement was less than ± 2.5 με, 0.5 % of the full-scale strain. In bending, the uncertainty of the sensor’s measurement was less than ± 1.5 με, or 0.4 % of the full-scale strain.
Additional testing in Saipem’s Houston ROV Tank was also conducted to help prove that the removal of a failed sensor and the replacement of a new sensor was an easy process. Again, the tests were successful. The ROV was able to remove and reinstall a sensor in less than 1 hour and 40 minutes. The first ROV serviceable SSSA was installed on a Riser Monitoring System in a project offshore Angola one year ago and the system is presently functional. Two additional Riser Monitoring projects which will implement this technology are currently in progress.
It is worth noting that the approach used to attach a subsea strain sensor to a pipe very much depends on the function of the system. The most demanding application for the strain sensors is to monitor the tension in the tether that connects a buoyancy tank to a FSR or the tether on a TLP. Accuracy in the estimation of the tensile strain of less than 1% is generally demanded. Creep or drift is unacceptable since it could contribute to a misunderstanding of the uplift force and finally - cross talk due to bending strains is also unacceptable. For such an application, the most successful approach is placing displacement sensors between forged flanges that are part of the tether. This approach was first used on the tethers of the Jolliet TLP and produced a very stable and accurate assessment of tendon tension from 1988 until recently when hurricane-induced cable and connector failures required replacement of the cables and sensors.
Both the Greater Plutonio Block 18 and P-52 Riser Towers employed forged flanges as foundations for the strain measurement system. These systems have been providing excellent, stable, subsea tension measurements for five and six years respectively. However, the flanged foundations are very intrusive, expensive to produce and create fatigue issues.
For monitoring systems which have been implemented post 2007, sensor assemblies that mate to a less obtrusive bossing have been provided. This is in effect, a discrete flange. The bossing can be forged or welded and can be carefully designed to minimize stress concentration factors in fatigue sensitive structures. The same bosses support both diver and ROV serviceable foundation fixtures.
In some cases, particularly SCRs, any kind of forged or welded feature is anathema to the riser designer and a clamp seems to be the only acceptable way for attaching strain sensors to a pipe. We have experimented with various clamped on attachment schemes and found that both tensile and bending strain can be measured very accurately with a clamped system. In full scale calibrations, the clamps have been shown to perform well on pipes with Fusion Bonded Epoxy (FBE) anti-corrosion coating. This is welcome news since removing and restoring anti-corrosion coatings on submerged pipes is very concerning. When looking to implement a clamped system care must be taken, particularly for pipes that will endure variations in internal pressure and temperature. Finally, while clamping to the FBE does provide adequate intimacy with the pipe steel for good measurements - for insulated pipes, the insulation removal and restoration of thermal protection is a major problem. This has led to the development of a solution to address tension and bending measurements on insulated pipes.
A patented solution
To avoid the pitfalls of clamped systems, BMT initiated an internal research and development program to evaluate ways in which to connect sensors to the exterior of insulated pipes. By performing extensive full scale tension, bending and internal pressure testing over a five year period we were able to demonstrate that our plastic welding system of attaching strain sensor foundations to insulation layers as thick as 100 mm 5 LPP and as thin as 3-5 mm 3 LPP results in a very accurate estimation of not only static and dynamic bending moment, but also the accurate estimation of the tensile strain in the underling pipe.
Over the past eight years, tools have been developed and evolving technology permits the reliable, long term monitoring of the structural response of submerged risers that is necessary for effective Integrity Monitoring. Not only have the earliest installations continued functional for in excess of six years, but the failure rate experienced to date is very low (3 failures in 2,647,334) for an MTBF of 882,448, which is a good indication that a system life of 20+ years is achievable.
Furthermore, attachment schemes have been developed that suit the full range of requirements that are encountered in SCRs, FSHRs and TLP Tendons, and diver and ROV serviceability of the sensors permits improved system long term availability. Deepwater, hostile environment Riser Integrity Monitoring Systems are most certainly past the R&D and “science experiment” phase and have proven their worth for long term service.
This article was first featured in Oilfield Technology. Visit their website to read similar articles.