Seawater Lift Pumps to submerge or not
Malcolm Walker of AxFlow reviews the competing attributes of semi-submersible vertical line shaft and fully submersible seawater lift pumps.
Due to its proximity it's not surprising that seawater is used widely on offshore platforms as cooling water, for reinjection into the well, and as a firewater supply. The most popular route for this water to get onto the platform is via vertical caissons into which narrow column pumps are inserted. These pumps come in two varieties: semi-submersible vertical line shaft pumps with the motor located above the pump assembly, and fully submersible pumps with the motor positioned below the pump in the sea.
Semi-submersible vertical line shaft pumps are one of the oldest types of pump design. For example, AxFlow's partners Gruppo Aturia have been producing them since the 1890's when they were employed to pull water out of wells for agriculture and drinking supplies. Essentially their designs haven't changed since then, although their applications have widened over the years to include docks, mines and power generation. So, in the 1950s and 60s, when offshore platforms were first constructed they represented the only real choice for the seawater lift pumps.
Their design is simple, with the motor positioned above the pump turning the pump impellers via a vertical rotating shaft that passes down the discharge column into the pump chamber. With a such a well proven design and the correct materials, they can last for decades and it's not unusual for AxFlow to see vertical line shaft pumps in our repair facility that are well over 30 years old. However, in spite of this sterling service over the half a century, the preference when it comes to seawater lift pumps has shifted towards fully submersible pumps.
This move towards submersible pumps has been for one very good reason; submersibles do not contain a rotating column shaft along with all of its associated bearings as the delivery column with a submersible pump is nothing more than that, a pipe with no moving parts. With caisson delivery columns typically in excess of 40 meters, this is an important factor because such shafts not only require significant maintenance, but increase the complexity of the installation process and require guides in the caisson to ensure a perfectly straight alignment. All of this makes the risk, initial cost and maintenance commitment of a vertical line shaft pump much higher than that of a submersible.
The construction of a vertical submersible pump also means that radial flow submersible pumps do not need a separate discharge pipe as is required by a semi-submersible pump. This makes their use in platform caissons feasible and thereby gives them a whole range of low flow high head duties not open to vertical line shaft pumps. For instance, Aturia can go down to 10 m3/hr and achieve differential heads of up to 700 m. Additionally, submersible pumps are quieter, as the motors are in the water and usually have narrower diameters than vertical line shaft pumps, a factor which can be of benefit in some retrofit and decommissioning situations.
However, despite all of the advantages that submersible pumps have over their semi-submersible relatives, vertical line shaft pumps may not be completely dead in the water as there are several situations where they are still the better, if not the only choice. Their motors are accessible and positioned in the rig which means they can be monitored and changed with relative ease. This enables semi-submersible pumps to be driven by a much wider range of motors, diesel engines, hydraulic motors or HV motors. Indeed the motor on a submersible pump can be something of an Achilles heel as it can be damaged by sand if it finds its way inside, and it can over heat if not enough pressure is available to force the water into it.
In general, submersible pump motors are not as efficient as their surface-mounted counterparts because their rotor rotates in fluid, which has more friction than air. Then there are electrical losses down the supply cable and the limitations of the caisson diameter make the subsequently narrow motor diameter windings inefficient. This lower efficiency also makes them more susceptible to problems from overload conditions, low voltage and voltage surges in the power supply. The materials from which they can be manufactured is also limited compared to those of the pump head and can therefore sometimes contravene specific platform standards for wetted materials. Along with the flexibility it gives them when it comes to electrical supplies, this is one of the reasons why Aturia manufacture their own motors. And of course with big flows that require particularly large motors it is just not possible with anything other than a surface mounted motor.
Lastly, there is the thorny issue of up-thrust. Up-thrust is the force exerted by the liquid as it hits the bend at the top of discharge column where it transitions to the horizontal plane and can cause pumps to vibrate excessively to the point where they have been known to rip themselves free of the mounting plate. Up-thrust can become a problem when the pump is able to discharge more seawater than it was originally specified for, and often arises during start-up in the moments before frictional losses have built up to reduce the flow out of the pump. Semi-submersible pumps are better equipped to cope with this up-thrust as the weight of the shaft helps to counter it and the motor bearing at the top of the drive shaft helps to absorb the forces. Submersible pumps have no pump shaft or motor sitting above the discharge column and are usually only held in place by the mountings plate of the discharge column.
In summary, the lower cost of a submersible pump makes it an attractive choice wherever application conditions make it possible. However, it is important to get the duty details correct (in particular the differential head) when drawing up the specification. In particular, applications with high flows should either be approached with great caution, or accommodated by the installation a vertical line shaft pump.