ABB explain that as offshore shallow-water oil and gas reserves run dry, the industry is turning to deeper waters which sets great technical challenges. It was not until 1934 that the first manned submarine reached a depth of just 914m (0.6 miles). Today, less than one hundred years later, mobile offshore drilling units (MODUs) drill at depths of around 3,000m (1.9 miles), but not without difficulty, as was recently illustrated by the Deepwater Horizon oil spill in the Gulf of Mexico.

To meet these challenges, sophisticated technology has been developed that enables drilling and wellhead maintenance in hostile corrosive environments, at low temperatures and under enormous pressures (pressure at sea level [1 atmosphere] increases by an additional atmosphere of pressure for every 10m [33ft] of descent). A major part of the problem is keeping extraction costs down despite the inaccessibility of deep sea oil and gas wells.

To remain cost effective, every effort is made to minimize the weight, topside, on offshore platforms and floating production storage and offloading (FPSO) vessels. This is achieved by relocating heavy modules to the seabed and conducting many production operations at depth, such as oil and water separation, which facilitates the forced removal of depleting oil and gas from the well, by water re-injection, as it empties. To do this, instruments that measure flow rate for both fiscal and well-head maintenance purposes are required.

Here, gas hydrates are the problem. They form under conditions of high pressure and low temperature, which are exactly the conditions found in many subsea applications. In oil exploration and production, gas hydrates are a serious problem since they block pipelines and subsea transfer lines leading to potential safety issues and losses in revenue through lost production.

The solution is mono ethylene glycol (MEG) - better known as the basis of antifreeze - but how much to add is the question, since whatever is added must be removed and separation is both time consuming and costly. To keep MEG to a minimum, the water to oil ratio is measured constantly so that just the right amount of MEG can be injected to combat wellhead blockage.

The solution sounds easy, but the water to oil ratio changes as the well “matures”, so the amount of MEG added must be constantly readjusted to maintain the optimal mix. And, of course, all of this must be done at depth with minimum human intervention, which means the measurement instruments employed must be sufficiently rugged to withstand the hostile environments of both the deep sea and the oil and gas extraction process.

Ultra-deepwater instrument – The ABB Venturi
The subsea Venturi which can be used to measure process fluids as well as the MEG injection, relies on the quality of the materials used for its construction. This often means using super duplex steel alloys and in some cases, even-more exotic alloys such as Inconel 625 to overlay the base alloy to provide additional corrosion resistance.

Thick walls are also employed, which satisfy the enormous pressure requirements both inside and outside the instrument. Further assurances are provided through stringent material and manufacturing traceability protocols, which include ISO29001 (Oil & Gas approval) and ASME section 9 (welding). This is key to ensure that the instrument is fit for purpose before it is deployed to depths where human intervention is costly and difficult. Typically the instrument will remain maintenance free throughout its life as long as the process stays free of hydrates.

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