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29/09/2008 - MCM Optimise Stripping Gas Consumption in Natural Gas Dehydration

Fast speed of response, practical validation methodology and traceable measurement techniques = BIG savings!

 

In offshore production of Natural Gas, a percentage of the gas stream is blended with dehydrated product in order to achieve the specified gas quality required for network entry into the gas transmission system.

 

In the UK market, this entry specification is normally specified as -26°C Dewpoint at line pressure (nominally standardised at 70 barg). For traceability, this measurement should be taken at atmospheric pressure and translates into a water content of 8 ppm[V].

 

Historically, the offshore industry has performed this analysis by installing aluminium oxide moisture sensors directly at operating pressure. It is well documented that this sensor technology consistently drifts to values dryer than true, even after just a few weeks of use. This is true even in clean gas applications - the effects are therefore only magnified in the presence of contaminating natural gas.

 

When these sensors are installed at line pressure it is extremely difficult to validate their performance. In practice it is only possible to assess if the sensors are "alive" or "dead", a process achieved by exposing them to ambient air and logging a signal response. Such disturbance of the measurement saturates the sensor and renders any data meaningless until the sensor has dried out and re-established equilibrium with the sample gas - with aluminium oxide sensors, this drydown period can often be measuered in hours.

 

Consequently, many of these systems offer little more than a trend for moisture analysis with little quantitative accuracy. As such, because of the tendency to both lose sensitivity and show dryer than true readings, over-stripping is highly likely. One significant consequence is higher than necessary flare emissions, which carries serious environemntal and commercial implications for the shipper.

 

By adopting a more practical validation methodology, and making use of traceable measurement techniques, considerable savings can be made.

 

MCM were involved in recent offshore optimisation trials where consumption of stripping gas was reduced from 1.7 tons / hour to just 1.2 tons / hour. These results were achieved by accurately monitoring the water content of sales gas and running closer to the network entry specifications (about 9ppm[V] of water content).

 

The existing online moisture analysers (aluminium oxide) had significantly degraded in performance, which was resulting in over drying - the inefficiencies had increased the operation's flare emissions, environmental impact and cost of wasted gas. In a time of rising energy costs optimisation of such processes offers a fast payback and optimal profitability for shippers interested in improved productivity and environmental compliance.

 

The key to the success of the trials lay with MCM's unique measurement principle and a carefully-devised operating methodology:

 

  • An Auto-Zero approach was implemented, to offset the effect of gas-borne contamination.
  • Sampling at atmospheric pressure increases traceability and enhances the confidence of measurement data.

 

By choosing to move away from the traditional approach of aluminium oxide technology, the shipper was able to achieve confident results in a much shorter period of time, whilst being able to make crucial improvements on specification and deliver a 40% reduction on the consumption of stripping gas.

 

The savings had more than justified the investment.

 

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