Seawater Lift Pump for Sand Filter Flushing

We were recently approached by a company who had a very particular requirement. They wanted a fully submersible seawater lift pump able to handle some of the toughest conditions on the planet in the North Sea.

The pump we provided would be helping to flush sand filters onboard their offshore oil rig, and would play a big part in keeping production rolling, and the platform clean and safe.

So what is a sand filter and how does it work?

Sand filters are one of the oldest and most common ways to treat water and remove suspended particles. Low cost, highly efficient, durable and reliable, sand filters have helped make water treatment both easy and possible in a wide range of domestic and industrial applications.

As its name suggests, a sand filter or sand water filter, simply uses sand to filter and purify water.

The unfiltered water is pumped into the top of the sand filter and slowly sinks through the filter bed of sand, gravel or other filter medium. The small pores ensure that dirt particles larger than 100 microns can’t pass through, leaving them behind in the fine pores of the sand.

This simple yet incredibly effective filter technology is especially suitable for recirculation systems, irrigation systems, pre-filtration of disinfection equipment, rinse water processes, basin water filtration and ground water cleaning.

Moreover, this nature-inspired method is much cheaper than using other water treatment processes such as ultrafiltration and nanofiltration to remove particles suspended in water.

In addition, sand filters are easy to install, maintain and repair, offer high water purification power and a good range of sand filter capacity and water passage rate options.

What are they used for?

Sand filters are used in a broad spectrum of applications in industrial, domestic and leisure environments, thanks to their simplicity, reliability and relatively low cost:

• Wastewater treatment

One of the most important uses of sand filters is for wastewater treatment after consumption. The treated water can be used again to reduce the cost of watering green spaces and lower the water consumption of industrial plants

• Cooling tower sand filter

Sand filters are also used in power station cooling towers.  The sand filter feeds the cooling water to the media bed. Here, the particles get trapped and the treated water then goes to the bottom of the tank to be discharged through the outlet

• Agricultural water treatment

Mud, leaves, twigs and random pieces of dirt can soon block sprinkler irrigation systems and cause them to stop working – potentially damaging the crops they’re watering. Sand filters are an easy, low cost solution to the problem

• Drinking water purifier

Excessive salt in any drinking water, caused by sediment in a plumbing system can damage your kidneys over time. This can be solved by using a water purifier with a built-in sand filter

• Pool water purifier

Both private and public swimming pools obviously need clean, purified and pollution-free water. Pool sand filters can achieve this cost-effectively

Why are sand filters important on an offshore oil rig?

Sand filter systems are absolutely vital to the smooth running of any offshore oil or gas platform. Sand is produced by the drilling process, and this can quickly work its way into various systems, disrupting production, damaging equipment and surface facilities, and severely impacting profitability.

Sand filters help to ensure that the sand by-product is kept out of critical systems.

What is seawater lifting and what’s it used for?

Offshore platforms are surrounded by trillions and trillions of gallons of water which can be used to flush out the sand filters on the platform.

In addition, the water can be used for a range of vital functions including cooling diesel generators and equipment, for reinjection into wells, helicopter pad cleaning, bird dispersal and as a firewater supply.

But there’s a problem.

The typical atmospheric pressure at sea level is 1013.25 millibars or 14.7 pounds per square inch. No pump on the planet has the power to overcome this and draw the seawater up onto the platform from the sea below - it’s against the laws of physics. 

This means that the water must be pumped up from much lower down by a seawater lift pump when the heigh of prime is above 9.8M.

Positioning the pumps

The most popular way to get seawater onto the platform is by using vertical caissons into which narrow column pumps are inserted.

Three types of pumps are used to pump seawater up to what is known as the topsides of the platform:

• Semi-submersible vertical line shaft pumps or which have the motor located above the pump assembly. These can be installed in the platform legs and use a motor located outside the fluid and pump column immersed in the fluid to extract the seawater.

• Self Priming Pumps providing the vertical suction is less than 8M.

• Fully submersible pumps with the motor positioned below the pump in the sea in which there are 2 variants. Borehole pumps which are a type of multistage submersible pump designed to deliver high flows at high pressures or submersible pumps which contain a single impeller designed to deliver high flows are relatively low pressures.

What is a caisson and which type of pump is best?

In geotechnical engineering, a caisson is a narrow structure that houses large pumps to draw seawater onto the platform for water filtering, cooling, production, fire services, cleaning and so on.

In this context, the name caisson comes from the French word caisson meaning box and the Italian cassone meaning large chest. It can also be a watertight retaining structure used to work on the construction of a concrete dam or the foundations of a bridge pier, or for ship repair.

Is a semi-submersible pump the best option?

Semi-submersible vertical line shaft pumps are one of the oldest types of pump design, dating back to the 1890s. Indeed, when offshore platforms were first built In the 1950s and 60s, they were the only real choice for seawater lift pumps.

They have a very simple design. The motor is above the seawater lifting pump and turns the pump impellers via a vertical rotating shaft which goes down the discharge column into the pump chamber.

In addition, their motors are easily accessible on the rig itself, which means they can be monitored and changed without difficulty. This also means that semi-submersible pumps can be powered by a broader range of motors including hydraulic and high voltage, as well as diesel engines.

With such a straightforward design, they can work for decades and some are still operating perfectly well after 30 years in service. Nevertheless, when it comes to seawater lift pumps, fully submersible pumps have definitely become more popular in recent years. Here’s why.

The disadvantages of semi-submersible seawater lifting pumps

Semi-submersible seawater lifting pumps have caisson delivery columns which usually stretch over 40 metres. These shafts not only need major maintenance, they are difficult to install and need guides in the caisson to ensure everything lines up perfectly.

All of this makes these pumps much more costly to run.

The initial installation cost, ongoing maintenance requirements and future dismantling of this type of design is much higher than that of a fully submersible pump.

Fully submersible pumps for seawater lifting

For many applications, a fully submersible pump is the perfect choice. They have lower running costs and the pump is always ready and primed.

They don’t have a rotating column shaft with all of the associated couplings and bearings which need regular servicing – the delivery column is just a pipe with no moving parts.

What’s more, fully submersible pumps are quieter. This is because the motors are in the water and usually have smaller diameters than vertical line shaft pumps. This can help with decommissioning and any retrofitting.

The disadvantages of fully submersible pumps for seawater lifting

However, fully submersible pump motors are not as efficient as surface-mounted semi-submersibles as the rotor rotates in fluid, which has more friction than air. As they’re fully submersible, the materials from which they can be made are more limited than those for semi-submersibles.

The motor and pump are constantly exposed to the elements usually in rough conditions. In addition, the motor on a fully submersible pump can be a problem as it can easily be damaged by sand if this gets inside. It can also overheat if not enough pressure is available to force the water into it. Occasionally, the pump will require an external cooling sleeve.

What was our specification for this demanding application?

At North Ridge Pumps, we can specify a wide range of pumps for a variety of industrial applications.

Moreover, we’re not limited to just one pump technology. Our unrivalled expertise enables us to offer a tailor-made solution, not something off the shelf. So we always choose the most cost-effective pump for any specific client, project and application.

For seawater lifting applications like this, the pump specification usually requires materials able to deal with prolonged contact with seawater such as nickel aluminium bronze, if high pump pressures are also required.

Alternatively, if seawater temperatures are particularly warm, duplex stainless steel will be chosen.

For this client, who required a seawater lift pump able to withstand rough conditions on a North Sea oil rig, we designed and developed an 8” fully submersible borehole pump made from duplex stainless steel. With 45m of cable, the pump will deliver 100m³h at 100m head.

What is duplex stainless steel?

Duplex stainless steels have a duplex micro-structure containing both austenitic and ferritic grains which give them a blend of attractive properties. They are stronger than 316 grade stainless steel, have better fatigue properties, high resistance to chloride stress corrosion cracking, and higher corrosion resistance in certain other media.

What’s more, these materials are highly heat resistant, withstanding temperatures over 300°C and cannot be hardened by heat. This is why duplex stainless steel pumps are often used for pumps within the offshore, chemical and petrochemical sectors.

If you have any application needing the transfer of fluids, solids or viscous liquids, and aren’t sure which pump to specify, speak to North Ridge Pumps to see how we can help.

For more information on Seawater Lift Pump for Sand Filter Flushing talk to North Ridge Pumps Ltd