LabVIEW 1000 m below the Waves: Synchronized Sampling of Autonomous Units Through Sound

Author(s):
Harald Månum - Bjørge AS
Marco Schmidt - Schmid Engineering AG

Industry:
Water/Wastewater, Oil and Gas/ Refining/ Chemicals

Products:
LabVIEW, Embedded Module for Blackfin Processors

The Challenge:
Norway’s largest industrial project, ever, development of natural gas extraction in Ormen Lange, required a condition monitoring system due to the extreme sub sea conditions.

The Solution:
The combination of National Instrument's “LabVIEW Embedded Module for ADI Blackfin” and the Off-The-Shelf Hardware ZMobile based on Blackfin processors by Analog Devices delivered the stability, versatility, performance and battery life needed to meet both the time-to-market and quality requirements.

Norway’s largest industrial project ever, development of natural gas extraction in Ormen Lange, required a condition monitoring system due to the extreme sub sea conditions. The following solution is an autonomous pipe line free span monitoring system consisting of several synchronized nodes, analyzing mechanical vibrations and water parameters and communicating wireless through accoustic modems . It can operate in harsh conditions for long periods of time, provides high software and hardware reliability, smart error handling and efficient energy management. The combination of National Instrument's “LabVIEW Embedded Module for ADI Blackfin” and the Off-The-Shelf Hardware ZMobile based on Blackfin processors by Analog Devices delivered the stability, versatility, performance and battery life needed to meet both the time-to-market and quality requirements. The technical solution with very accurate synchronized sub sea data acquisition is a worlds first. Also new and unique is the applied graphical programming approach to a large scale application deployed on a low power target.

Gas Extraction at the Ormen Lange in the North Sea

Development of the Ormen Lange gas field is the largest ever industrial project in Norway, budgeted with nearly 66 mrd NOK. The production is started in autumn 2007, it will eventually cover up to 20% of the British gas consumption for the next 40 years (Fig.1).

The field is situated outside the Norwegian west coast, and consists of 24 sub sea wells in 4 seabed templates. The gas is transported through 120km pipelines from depths down to 850m to the on-shore production facilities in Aukra (Fig.2). Driven by pressure from the wells only, natural gas is streamed from underwater camps (Fig.3) to the surface through two parallel 30” pipe lines. Laying of the pipe lines is however one of the most challenging pipe laying tasks ever done, in particular due to the rough terrain and sea currents.

Extreme Conditions Require New Solutions

The pipelines traverse the Storegga rock slide. Measuring over 800 km in length, this is one of the world's longest rock slides on a continental shelf. Rubble has accumulated over thousands of years, creating an extremely rocky sea bed. Due to this rough terrain, several segments of the pipe line are not in contact with the sea bed (Fig.4). The strong sea currents may induce vibrations in these free spans, which may cause leaks or even pipe breaks.
It was thus a direct requirement from the government to have a solution for a vibration monitoring system to identify movements that may result in damage.

This is why Bjørge AS shall install a long-term vibration monitoring system based on NAXYS technology, onto this pipeline when going into production in 2007. The system realized with LabVIEW and deployed on a Blackfin target, must survive rough handling from ship deck cranes and ROV's (Remote Operating Vehicle). Submerged, it faces extreme sub-sea conditions including low temperatures, no remote power, water avalanches and turbulences due to the uneven seabed, and changes in internal pipeline flow.

Pipeline Free Span Monitoring System (PFSMS)

The solution is a sub sea instrumentation network including several autonomous synchronized “Clamp Sensor Packages” (CSP) and a “Master Sensor Package” (MSP) that monitor vibrations in longer pipe line free spans.

Despite the extreme environments, the main challenge of this project was allowing several nodes to start and stop logging with a node-to-node accuracy better than 2.5ms over a distance of at least 100m, as well as simultaneous sampling of 3 analogue channels in parallel to 4 serial IO’s. As all nodes operate wireless using sound for data transfer and signalling, special measures had to be taken in the hard- and software design to obtain the required accuracy and deterministic real time behaviour. A smart energy management scheme ensuring battery power for a minimum of 6 months life time was another vital design factor. And finally, all the electronics had to fit in the geometry of a pressure tight pod, ensuring small size and low weight to allow ROV handling.

The final solution based on Naxys technology consists of 4 systems for:

● Multi node synchronized measurement of vibrations in longer free spans, dynamic and static motion of the pipe line free spans and the measurement of sea-current, -salinity, -pressure and -temperature

● Storage and mathematical analysis of sampled data for up to 6 months

● Wireless node-to-node communication

● In-node error handling and -recovery for fail safe operation

● Acoustic ROV interface for data retrieval and system re-configuration

● At least 6 months service interval

Fig.5 illustrates a typical sea bed installation were one span is monitored. In this case, the NAXYS solution also measures the sideways and longitudinal motion of the 30’’ pipe line with cm accuracy. The CSP's are mounted to the pipeline at regular intervals (Fig.7). Their primary task is recording vibrations in all 3 axis directions. The mechanics, designed for up to 1500m BLS, is based on a clamp that can be attached directly onto the 30 inch pipeline by an ROV.

An electronics pod is then mounted on top, using a locking mechanism that allows for later removal , e.g. for battery replacement. The design allows for easy installation/removal, and yet a stiff connection, allowing for measuring vibrations as small as a few mg, at frequencies less than 0.1Hz. The CSP's are controlled and synchronized by an inertial MSP tower (Fig.6 "Master Sensor Package (MSP)"), installed on the sea bed by a crane and an ROV (Fig.4 and Fig.7). This MSP also records water currents, salinity, temperature and pressure.

Reliable Low Power Battery Operated System with Acoustic Communication and Built-in Logging

The links between CSP and MSP units are wireless through acoustic modems. The synchronization scheme is very robust and ensures sampling synchronization even for large and unpredictable delays in the MSP synchronizing signal, e.g. due to different sound propagation delays. The whole monitoring system offers three basic modes of operation.

● Long Term Data logging: The MSP wakes up at a configurable time interval, typically every 3 hours. At first, it measures the distance to each CSP for compensation purpose. Following, distributed analogue data recording at 10-20 Hz for 10-30 minutes is initiated by sending a group call to all CSP nodes. The MSP then starts reading water current, salinity temperature and pressure through serial interfaces. When logging has finished, the data is processed and stored to removable memory. After programming the next wakeup, (RTC) both the MSP and CSP's go to sleep and the whole process is repeated.

● Event monitoring: A lowest power, intelligent mixed signal circuitry continuously monitors all vibrations and water current levels for limiting values. If any CSP detects a high water current or acceleration while asleep, it wakes up and sends a signal to the MSP to initiate the logging scheme.

● ROV rendezvous: The monitoring system is installed and maintained by remote operated underwater vehicles (ROV, Fig.4 and Fig.7). Through acoustic communication with a ROV or a top side modem, all vital parameters can be changed at run time, as well as upload of sampled data or Fourier analyzed data for a requested time period. ROV's are able to request data from either a CSP or MSP at any time and in parallel to its current mode of operation. This reliable communication interface is a key feature of the embedded hard- and software.

Built-in Redundancy

Redundancy was a big challenge in this system. Every action is monitored for errors to occur. In an error case, a node performs a self correction and informs its caller about the situation. All nodes communicate to decide if the error is within the node itself or any other nodes. If the real MSP fails, any CSP can become the new MSP to sustain the operation. The pipeline monitoring system has a lifetime of several years, and will be submerged for at least six months at a time, thus the highest demands are placed on hard- and software reliability, in-program error handling and efficient energy management.

Graphical Design of the Embedded System with LabVIEW and Prototyping with Off-The-Shelf Hardware

Full support for the new graphical embedded system design approach, using LabVIEW Embedded was a key requirement for Bjørge. It helped the system engineers to manage the high complexity of the application and to meet the challenging project's schedule. Under the hood, Schmid Engineering's “ZMobile BSP for LabVIEW” with its 9 palettes providing 150 re-usable VI's delivered the necessary real-time features and thread-safe IO libraries.

A synchronized, autonomous system with this accuracy has never been made before and called for a series of engineering challenges. LabVIEW Embedded, the VDK realtime kernel and the ADI Blackfin processors resources have been pushed to their limits as the large scale program demanded for multiple parallel loops (Fig. 8), multiple high accuracy interrupts and determinism of the program execution. The challenges were met and the system may produce an accuracy of simultaneousness better than the original requirement.

The final application equals around 50'000 lines of hand written “C” source code, generates a DXE executable of 10 Megabytes in size resulting in an Intel HEX loaderfile of 5 MB which is burned into a 8MB boot flash memory. The application involves typical features of a “cutting edge” embedded system:

1. Main application tasks implemented by 10 independent Timed Loops / VDK-Threads, including 4 interrupt callback threads and 2 parallel timed loops

2. Software modularity with more than 100 re-usable sub-modules/sub-VI's

3. Inter-task data communication and synchronization by 5 real time FIFO's, 110 global variables and 110 VDK semaphores

4. Application logic based on a classic state machine with 16 program states

5. An embedded file handler for configuration files and mass data storage on a removable 4GB medium (CF-Card)

6. Signal conditioning, max/min and FFT analysis

7. Impulse triggered DAQ synchronization technique with µs accuracy and full compensation of software latencies and modem propagation delays.

8. Smart power and battery management scheme

9. Programmable Shutdown, Wakeup and Watchdog logic

10. 3x simultaneous analog channels to sample a tri-axis accelerometer sensor

11. 4x asynchronous Serial IO for measuring water current, salinity, pressure and temperature

Deploying LabVIEW Application Code to the Low Power Mixed Signal Platform ZMobile

Each CSP and MSP holds a pod with all the electronics, batteries, sensors and acoustic modem antennas (Fig.9). The pod is water tight and designed for sea depths down to 1500 meters (150 bar). All internals are mechanically decoupled from the vibrations of the pipeline in order to reduce stress on the components. The embedded system HW relies on the compact, low-power target ZMobile (Fig.10), enhancing a Blackfin BF533 processor with versatile mixed signal circuits. Most of the functionality was already provided by the Off-The-Shelf platform. A custom specific add-on board completed all missing circuits, connectors and interfaces for Bjørge (Fig.11).

This two board approach and combination of standard and customer solution saved precious engineering time and lowered project risks.

With a project the size of the Ormen Lange and involving tight timelines and rugged conditions, innovative approaches were required to solve the engineering challenges. Through NAXYS technology, Bjørge AS was able to solve these challenges by re-using Off-The-Shelf hardware based on a Blackfin Processor by Analog Devices and applying graphical programming to generate the code required to deploy to this low-power embedded target. The LabVIEW Embedded Module helped to shorten development time and ensured parallel operation and complex interrupt handling demanded for this application.

The oil and gas market now gains from a solution that allows synchronized measuring between 'n' nodes over distances far longer than 100m with high accuracy. The wireless acoustic communication, in-node data analysis and storage along with a smart power scheme allows lowest power consumption and ensures continuous detection and handling of critical static and dynamic motions. Thanks to acoustic links, critical data can be sent directly to the engineers desk. This provides new possibilities within all kinds of synchronized vibration monitoring.

Author Information:
For more information on this Case Study, contact:
Marco Schmidt
Schmid Engineering AG
Mezikonerstr. 9
Münchwilen 9542
Switzerland
marco@schmid-engineering.ch

Browse All Case Studies »