Under the Hood--Extreme: Monitoring Gas Extraction in the Norwegian Sea

The gas field is situated outside the Norwegian west coast, and the gas is transported through pipelines from depths up to 850m to the on-shore production facilities in Aukra. Laying the pipe line, is however one of the most challenging pipe laying tasks due to sea currents and the rough terrain caused by the Storegga rock slide.

Systems Approach

Pipeline vibrations caused by the Gulf Stream, water avalanches and water turbulence due to the uneven sea bed, coupled with the pipeline's internal flow, are a considerable safety risk since they can cause leaks or even pipe breaks. That is why the Norwegian high-tech firm of NAXYS AS, which specialises in underwater instrumentation and condition monitoring, has developed a long-term monitoring system to be installed at the Ormen Lange in 2007.

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The monitoring system offers three basic modes of operation. 1. 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.

2. Event monitoring : A lowest power, intelligent mixed signal circuitry continuously monitors all vibrations and water current levels for limiting values in parallel to data logging. 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.

3. ROV rendezvous : The monitoring system is installed and maintained by remote operated underwater vehicles (ROV). Through acoustic communication with an ROV or a top side modem, all vital parameters can be changed at run time, as well as upload of sampled data or Fourier analysed 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.

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.