Author: Stephen Auld – Global Business Manager – Subsea Asset Monitoring
For deep water oil and gas field developments, complex and highly engineered riser systems are required to make sure that hydrocarbons are transported from the seabed to the surface safely and efficiently.
The deeper the water, the more complex they become. A floating production, storage and offloading (FPSO) vessel will have risers connected to multiple riser bases on the seabed through one of a variety of mechanisms, e.g. steel lazy wave, buoyancy support, steel catenary or hybrid risers, which take their weight in the water column. In depths reaching 3000 metres, there will be miles of risers, all needing support to make sure no undue stress is put on the risers, the FPSO’s top riser assembly or the riser bases on the seafloor.
Monitoring the integrity of these systems and the fatigue being experienced by the risers they’re supporting is a critical task that can extend riser life, save money and prevent downtime. It can also give operators the critical information they need in the event of a hurricane, which may require them to unlatch the FPSO from the riser system.
The monitoring technology required can also be complex and costly. Each riser system will need multiple sensors, as well as positioning systems, supported by seabed transponder arrays, and, potentially, through-water communications technology. The sensors will collect depth, temperature, inclination, load and attitude and sound velocity data, which will all need transferring to the FPSO’s control room. In cases where these sensors are not hardwired, one or more transceivers will be deployed to relay this data to the surface so that a full picture of the subsea architecture status can be built.
However, there’s a smarter way to monitor subsea assets, using fewer seabed transponders and sensors that are more intelligent, because they can process data at its source, all while using less power, which means batteries need changing less often.
As an example, working with engineering consultancy BMT, we performed a study on a deep water FPSO hybrid riser monitoring system in the US Gulf of Mexico. The incumbent system, to monitor five flexible risers, has two omnidirectional transponders and a modem at each of the buoyancy systems supporting each individual riser. Positioning is supported using a Long BaseLine (LBL) array, via three sets of four seabed transponder arrays, from which the riser buoyancy transponders get their ranges. Each array is supported by a separate transceiver mounted on the FPSO’s turret, but, importantly, there’s no interaction between each of the arrays. This system also has external strain sensors, an internal heading and motion sensor, plus separate modems.
However, BMT and ourselves, using Sonardyne equipment, were able to design a simpler arrangement with fewer transponders and more intelligent and longer battery life sensors. Our alternative scheme would reduce set-up and life-of-field costs as well as reducing the number of maintenance trips needed.
Analysing areas for potential savings concluded that the number of LBL transponders on the seabed could be reduced from 12 to six, while maintaining line of sight to each riser, by sharing the LBL array instead of having three completely separate arrays.
Additionally, by retrofitting our low-power SMART (Subsea Monitoring, Analysis and Reporting Technology) to measure strain and motion (strain using external strain gauges and motion using an internally housed high-performance inertial measurement unit), and communicating all from one unit would enable the previously installed large battery pack’s life to be extended well beyond five years, compared with just two years, in the current incumbent system.
Furthermore, by fitting an additional passive USBL array, SMART can also offer high accuracy heading, to ~0.1°, calculated from the bearing to the seabed array using the navigation pings already being transmitted, so no change or additional software is needed. Configuring SMART in this manner eliminates the need to use the existing, larger and more power hungry inertial measurement unit, therefore further extending battery life and reducing maintenance visits.
Significantly, all of this would cost one third of the incumbent systems’ through-life maintenance charge.
SMART’s advanced data acquisition and processing system, which is built around a highly capable micro-processor with the latest peripheral electronics, enables data processing subsea – what some call ‘edge analytics’ – with low-power electronics for long-duration logging and telemetry in a single unit.
What’s more, the LBL transponders, could be installed prior to asset installation, to aid subsea infrastructure positioning operations, and then be used again during life-of-field operations, from inspection, maintenance and repair and brownfield operations, to decommissioning, as they have multi-user functionality.
What all of this means is that operators can see the status of their subsea assets throughout the life of the field and have access to data to perform fatigue analysis, so they can intervene before they have a problem, at lower installed and operational costs than existing systems.
This is just one of the ways we and BMT are working together to bring further savings to the subsea asset monitoring environment through smarter, more efficient monitoring solutions. By working collaboratively on integrated solutions, we can offer greater project and product efficiency and performance.