Here is a short history of deploying flowmeters, of various technologies, subsea
The first custom design involved the LM pelton wheel. A compact product was required for installation on a submarine. The pelton wheel is a radial turbine meter type suitable for low viscosity liquids and, in this incarnation, with a stainless steel body rated to 20 bar internal pressure and capable of 0.1 to 28 l/min.
Challenger Oceanic, 1992
Litre Meter contributed a plastic bodied LM pelton wheel. The internal and external pressures were identical so the plastic housing was quite strong enough with simple hose connections. The electronics, however, was inside a submersible pod where the pressure was 1 bar. To withstand the pressure differential (300bar, 3000m) the sensor wall, thin for good signal acceptance, was constructed in a titanium alloy, previously purposed for the nose cone of a Harrier Jump Jet. The meter was installed on Alvin and used as part of a sampling system in subsea fumaroles, in places such as the Marianas Trench. Particles were collected and the volume of water measurement helped determine the percentage of solids.
North Sea, 1993:
For a simple solution to the flotation and righting of an offshore vessel two VFF 4 flowmeters were designed with subsea totaliser displays. The VFF4 was the original VFF flowmeter size capable of measurement, at that time, of 4 l/minute as they were fitted with carbon graphite rotors. A simple reed switch sensor was used. With a battery powered LCD display mounted behind a clear acrylic window the diver were able to assess the amount of hydraulic fluid being used and thereby the angle of the platform as it slowly rotated from it’s horizontal transport position. The challenge was to design a compact subsea display against a tight cost restraint.
Heron and Egret, 1996:
As Litre Meter extended it’s capability with increased design freedom internal pressures increased to 690bar. The application required a very low flow of chemical to be measured and the smallest meter at the time, the MF15, was selected, again with a carbon graphite rotor. The next problem was designing for 300 bar external pressure and provide a loop powered 4-20mA output with a customer specified Tronic connector. Two sensors were utilised with a well designed electronic board that provided redundancy by continuous monitoring each sensor. Typically, scale inhibitor was measured at 0.5 to 2.5 litres per hour. A stainless steel housing was constructed incorporating the connector and fully O-ring sealed. The housing was tested by Sira at 300bar.
Subsea visual rate display, 2005 onwards
By upgrading an existing turbine design Litre Meter engineers were able to add a submersible battery powered display for use by an ROV mounted camera. Starting with a standard ‘topsides’ meter a subsea collar was welded on. Then a stainless steel canister was constructed with a heavy duty acrylic window to withstand the pressure at 800m external depth. A large height LCD rate and total instrument was incorporated. As the turbine used a magnetic sensor requiring no power, a simple battery powered solution was provided to the client. Flow rates were typically 5 to 90 l/min of water but the design has been incorporated in to different size turbines since.
Galley Tieback, 2006:
Hydraulics measurement was needed, in a very specific envelope. The fluid was Castrol Brayco running at a wide ranging 1 to 50 litres per hour. Internal pressure was up to 414bar and the external pressure was 20bar. A 4-20mA loop powered electronics was integrated within a cylindrical housing and utilised a Tronic connector.
When hydraulic fluid is measured valve position can be interpreted. When subsea this is considerably more difficult as the measurement has to be compact and resist high pressures inside and out. With an innovative manifold base little of the VFF MF30 meter remained. The sensor was standard but all other aspects of the design were optimised to use least space, maintain a pressure rating in and out and incorporate a custom electronics interface. Forward and return lines were monitored with pressures at 760bar and 345bar respectively. External pressures could be up to 300bar. Glass to metal seals for the sensor outputs were utilised together with the redundant sensor design used before.
Subsea valve, 2006 onwards:
The VFF meter in it’s various sizes were becoming more and more popular for chemical injection particularly at high pressures and low flows. The MF15 was redesigned as the LF15. This enabled a higher accuracy, lower flow ability and crucially a smaller chamber diameter. This, in turn, allowed a more compact meter design at the same pressure or, usefully, a higher pressure rating for a specific cap diameter. The design brief was simple: Could we design a 1,035 bar meter for 90 litres per hour and 345 bar external rating in the same envelope as a can of Guinness? – we’re still working on this! What we did end up with is a fairly compact device (100mm diameter and 125mm long) where we supply just the chamber and cap and the customer integrates these parts into a subsea module. The unique chamber design allows pressure independent measurement whether at 1 bar or 1,000 bar.
With an expected lifetime of 30 years all efforts were made to increase the ability of the rotor and chamber. Extensive testing on various bearings and coatings refined the design, retaining it’s simplicity but greatly improving resistance to high flow shocks and significantly improving low flow performance. A full 2.5 years continuous running at twice the previous maximum flow demonstrated it’s durability. Careful calibration showed that low flow measurement at the start and the end of the 30 months were identical thereby proving that 90 litres per hour was the new maximum with that design and coating.
Hundreds are in service subsea around the world. The meter is used to provide feedback on the flow rate of chemical. The pulse output (with a redundant sensor) requires no power and the meter has excellent particle size acceptance.
North Sea, 2012
The VFF4, when specified with a PVD coated Nitronic rotor and chamber, can measure up to 8 litres per minute. With a pressure rating of 300 bar (external and internal) a compact 3 wire 4-20mA meter was designed with 1/2″NPT connections. Hydraulics footprinting was then possible subsea. A Seacon Micro Wet-Con 4 pin connector was utilised with dual reed switches for redundancy. Field not known.
North Sea, 2013
Apart from the Pelton Wheel, turbine and VFF rotary piston flowmeter designs there are other meter types that an provide a significant design benefit to subsea modification. The ZHM gear meter offers a wide range and high pulse output rate. A SubConn subsea connector was incorporated and a high pressure 690bar design modified for 0.3 to 6 litres per minute. The application was again chemical injection and 409 bar was the external rating.
North Sea, 2015 onwards
When a clean sheet of paper is used then a streamlined solution can result. The subsea turbine flow meter developed for hydraulics measurement from 3 to 33 l/min (HM009) is typical of this approach. The turbine blades are standard whilst the sensor and Canbus electronics board are encapsulated by welding into the integral stainless steel housing. The customer specified subsea connector provides easy integration electronically. Internally, the meter can withstand 1,035 bar (tested to 1550 bar, 22,500 psi) and externally the specification allows for 300 bar.