Key points
The exploration and production of oil and gas takes place in many of the world’s most inaccessible and hostile environments. In today’s turbulent market, operators are deploying increasing numbers of Floating Production Storage & Offloading (FPSO) vessels which enables them to extract what resources remain in remote and deep-water areas more cost-effectively than using traditional and expensive offshore rigs.
Amarinth is at the forefront of the design and manufacture of centrifugal pumps and has established itself as the industry’s “go to” expert for pumps that can deliver the duties demanded by FPSO operators and which will operate reliably in what is the harshest of any process industry environment.
Floating Production Storage & Offloading vessels
Oil has been produced from offshore locations since the 1950s. Originally, oil platforms sat on the seabed, but as exploration has been forced to move into deeper waters and more distant locations a more cost effective means of extracting the oil was needed.
The first example of Floating Production Storage & Offloading vessel (better known by their acronym of FPSO) was simply a converted tanker tethered over a well site.
Later versions were then purpose built with sophisticated “turret” systems to allow the vessel to turn into the prevailing weather without tangling the umbilical hoses connecting the subsea well heads with the vessel. Initially this production technique was limited to oil but now there is a new generation designed for LNG.
With advances in subsea production techniques, deep water fields can now be developed economically using FPSO vessels which can deal with multiple subsea well heads without the cost previously associated with large fixed or jack-up platforms. Once the field has been exhausted then the vessel can be towed to a new location or back to dry dock for maintenance or re-fitting.
The construction of an FPSO vessel breaks down into two main sections – the hull and the topsides. The hull is usually based on a standard tanker design and is for storing the produced oil or gas until it can be offloaded onto a tanker or, less frequently, transported through a pipeline.
Many of the FPSO hulls are produced in shipyards based in south-east Asia. The topsides is where all of the hydrocarbon (oil or gas) processing modules are located which separate fluids into crude oil, natural gas, water, and impurities and clean up the oil before it is exported.
Both the hull and the topsides modules require pumping solutions that involve, for example, pumping sea water ballast in the hull, separating and compressing the gas before stabilising the crude oil, dehydration of seawater and produced water, and removing sand, solids and acid gas, ready for re-injection of water back into the well or for environmentally safe disposal.
Significant challenges on short lead times
Specialist module manufacturers have developed their businesses to service the FPSO market, such as:
- DPS – gas sweetening and filtration
- Merpro – sand wash and filtration
- Cyclotech – filtration and gas sweetening
- Petreco – gas compression and amine packages
These, along other organisations, require reliable, robust, bespoke pumping solutions to maximise the efficiency of their equipment and processes. The pumps abroad a FPSO vessel must withstand an extremely harsh environment, from the acidic composition of the oil and gas, corrosive sea water, highly abrasive sand, and all aboard a vessel that is constantly in motion from wind and waves, and they must operate reliably 24 hours a day requiring minimal downtime for scheduled maintenance.
Space is at a premium on FPSO vessels, particular headroom, which means that pumps must be designed to operate in a low Net Positive Suction Head (NPSH) available environment otherwise they will be prone to cavitation, seriously shortening their working life.
For other duties, pumps must possess low-shear capability to prevent the emulsification of oil in the produced water prior to processing, and in some cases must operate reliably with sea water washing over them.
Furthermore, despite many of these considerable design challenges, the target time to fit out a vessel is often less than 52 weeks from start to finish, and out of that the time given over to the procurement of equipment can often be as little as 20 weeks.
This places immense pressure on the pump manufacturer to align the design and delivery of the pumps to meet the critical build schedule of the FPSO as any delay would prove extremely costly.
Design for low NPSH
Selecting a pump is a balance of many factors, including the volumes and properties of the fluid to be pumped, total static lift, pipe size, pipe losses, the efficiency of the pumps and how frequently the pump will be run. Within the process industry where space is at a premium, then engineers may also have to deal with an additional and difficult factor, a lack of suction head.
On FPSO vessels, the lack of headroom between decks means that a low suction head is always a significant design consideration. Not taking the low suction head into account can cause catastrophic cavitation to occur in the pump.
Cavitation occurs when the pump cannot get enough fluid flow into the impeller and the resulting reduction in pressure causes the liquid to vaporise and form bubbles. These bubbles can grow dramatically and choke an inlet, further reducing the flow of liquid and the performance of the pump.
In addition, the bubbles can implode with tremendous force, literally tearing away at the metal surface. The resulting increase in stresses, vibration and noise can lead to downtime and premature component replacement and in some cases complete pump failure.
To avoid this catastrophic situation, the pump designer needs to ensure that the Net Positive Suction Head available at the pump, NPSH(A), exceeds that required by the pump to operate without cavitation occurring, NPSH(R).
NPSH(A) is in principle a straightforward calculation, taking into account the suction static head, pipe friction losses, atmospheric pressure and the vapour pressure of the liquid.
The first step in designing a system with low NPSH is to review the physical location of the pumps and tanks. Ensuring the pump is as close to the tank as possible will minimise pipe losses.
Similarly, reducing the number of bends, valves and filters in the pipework between the tank and the pump suction will result in greater NPSH(A). The designer must also ensure that the pipework is of the right size for the required flow rates and that the minimum level of liquid is sufficiently above the suction tank outlet, which reduces vortices and potential air entrapment at low levels.
When designing for extreme challenges such as FPSO vessels, every calculation must be completed fully and accurately. Simply introducing “safety margins” without due thought to the process would result in a complex engineering solution with large costs when a simpler solution may be available.
For example, API 610 recommends that that the pump manufacturer gives serious consideration to the difference between NPSH(A) and NPSH(R) when calculated using the vapour pressure of the lightest fraction in the oil being pumped.
In many process operations aboard FPSO vessels, the headroom is simply not available to engineer a solution if the calculations are only based on the lightest fraction. In such cases, the designer must understand the application in more depth and what is behind the accepted practices if a cost-effective solution is to be found.
In the case of pumping a combination of crude oil or a mix of oil and water, the designer could investigate the lightest bulk vapour pressure of the fluids that the process will encounter rather than just considering the lightest fraction of oil (as specified in API 610), which in many cases may be only a very small percentage of the mix.
By halving the vapour pressure used in the NPSH calculation, which would be quite feasible with mixed fluids in the oil industry, it could reduce the static head required by 10m which is enough to deliver a much more cost effective solution for the operator.
In addition to designing the pump for low NPSH, innovative packaging of the pump system can also help with fitting pumps in the tight spaces available between decks.
When the Triton FPSO vessel required a sophisticated filtration system to meet OSPAR regulations, which targets operators with reducing the amount of oil they released into open water in the North Sea, ten high specification produced water pumps required a small enough footprint so that they could squeeze within the tight space constraints of the vessel.
The lack of decks restricted the headroom available and so Amarinth designed the pumps to operate with a low NPSH. To achieve the dimensional space constraints, the final design was a hybrid based on a proven heavy-duty chemical pump incorporating up-rated API 610 bearing brackets.
The pumps were fitted with sophisticated ‘upstream pumping’ (USP) mechanical seals that utilise seawater to provide a supply of cool, clean buffer fluid to the seal faces to ensure reliable operation with the abrasive media.
In another example of innovate packaging, the Quad 204 FPSO vessel called for five API 610 OH2 A-Series super duplex pumps with Plan 53B buffer fluid seal support systems.
To fit within the space constraints Amarinth designed an integrated pump and seal support system package on a bespoke baseplate that effectively wrapped the seal support system through 90 degrees around the pumps whilst still maintaining its interoperability and achieving the footprint limitations set by operator.
Low shear capabilities
To clean up produced water that results from the oil and gas extraction from subsea wells, any oil present in the water must be separated and collected before the water can be disposed of.
This is achieved in a variety of ways, but for these to work efficiently, the mixing of the oil and water must be minimised to prevent restructuring and emulsification during the transfer of the fluid through the separation systems.
Where space is available, low speed progressive cavity (PC) pumps have traditionally been used which minimise any shearing of the pumped liquid.
However, PC pumps increase in length and size as flow rates increase, and there is a limit to their capacity. On a FPSO vessel, large fluid volumes need to be pumped and so PC pumps would be too large to fit in the available space.
Centrifugal pumps can pump higher flow rates than PC pumps with the additional benefit of requiring a smaller footprint. Furthermore, centrifugal pumps achieve this with a smooth continuous delivery and low pulsation effect, and no pressure relief valves are required to prevent system over pressurisation.
However, centrifugal pumps tend to shear liquids and the higher the speed of the pump the more shear results. Shearing occurs along a boundary layer when the velocity of the fluid is changed creating a velocity gradient across the fluid.
This causes shear stress between the slower and faster moving flows in the liquid. In a centrifugal pump the shearing effect is highest at the impeller and the resulting turbulence in a fluid of oil suspended in water causes the oil and water to emulsify and when this exceeds a certain level the separators are not able to extract the oil droplets from the water.
For low shear applications, such as required on FPSO vessels, Amarinth approached the use of centrifugal pumps afresh. Using the latest in Computational Fluid Dynamic (CFD) modelling, hydraulic design engineers investigated how to reduce the shearing action of impellers in rotodynamic pumps at high flow rates and determined that the key ratios to reduce the shearing action on the fluid are the inlet area, outlet area and the proportions of the impeller hub.
Using 3D CAD software, engineers were able to outline new impeller designs which were then imported into vane creation software which controls the shape of the vane and the geometry of the water passage through the impeller.
Once engineers had tuned the impeller and vane design, this was exported directly back into the 3D CAD system where Finite Element Analysis (FEA) was carried out on the impeller and vanes to ensure they were capable of handling the dynamic loads and mechanical stresses.
Using Computational Fluid Dynamics (CFD), the engineers could then visualise the fluid flow through the pump volute and impeller and predict the point at which shearing would occur and then design the impeller to reduce the shearing effect.
This low shear centrifugal pump design was used by Amarinth to deliver a solution for an Induced Gas Flotation (IGF) system aboard the Espirito Santo FPSO vessel. The IGF system cleans up produced water to meet strict environmental controls before returning it back to the sea.
The pumps had to be low shear to minimise any emulsification of the oil and water before it entered the filtration units but if progressive cavity (PC) pumps were used these would be some 4m in length which would not fit within the very restricted space available on the process module.
The solution was to use proven API 610 OH2 horizontal pumps but with the necessary modifications to ensure low shear operation. In addition to the low shear properties of the pumps, the limited space between decks further complicated the design as the pumps had to handle a low NPSH(A) and avoid any cavitation, which if it occurred could lead to premature or catastrophic failure of the pumps.
Finally, to withstand the harsh offshore conditions and the corrosive effects of the produced water the pumps were manufactured using duplex material with API 682 III Edition double seals and Plan 53A seal support systems.
Protection against water ingress
One of the more unique aspects of designing pumps for FPSO vessels is that for some applications, such as Mono-Ethylene Glycol (MEG) Heating Circulation or Condenser Circulation, the pumps need to be positioned on the deck with waves washing over them and therefore require a high level of water ingress protection.
This was the case on the FPSO vessel Pioneiro de Libra which was destined for the ultra-deepwater Libra oil prospect field, located in the Santos Basin, about 230km off the coast of Rio de Janeiro, Brazil.
The vessel would be subject to extreme sea states and so a drain pump in the wash-down sump in the bow of the vessel was liable to submersion during heavy seas.
Amarinth designed a vertical pump that could withstand the demanding conditions by modifying its proven vertical sump pump to be fully IP66 rated, which is defined as “dust tight and protected against heavy seas or powerful jets of water”.
The IP66 rating covered the pump, motor and transmitters and even the control panel which was also open to the elements. Level transmitters were installed to automatically start the pump when the sump has filled, pumping the water to the cleaning package before its return to the sea, and to switch off the pump once the sump has been emptied.
Vessel stability
FPSO vessels are subject to large fluctuations in loads as they process and then store the oil and gas in their hulls until ready to offload and then start again with an empty hull.
To maintain sea worthiness and stability in what can often be heavy seas, FPSO vessels have self-contained ballast systems which require pumps that can move large volumes of sea water into the ballast tanks but which are compact enough to fit within just 1m to 4m of headroom.
For the FPSO currently being built to service the Tortue field development project located off the coast of Mauritania and Senegal, which is thought to contain a potential 50 trillion cubic feet of natural gas, Amarinth was asked to provide compact vertical inline pumps for the filling and emptying of the ballast tanks in the hull to maintain the equilibrium of the vessel as the amount of processed liquified natural gas in its internal hull tanks changed.
Due to the unique needs and constraints of FPSO vessels, end users will create specifications for some of the pumps that are a combination of their own specifications used across multiple projects (such as BP GIS, Shell DEP, NORSOK) along with selected aspects of the API standard appropriate to the project in hand.
In the case of the ballast pumps, which are located in the hull of the vessel, the unique project specification laid down by the end user required Amarinth to design bespoke vertical inline pumps within tight deadlines to meet such a set of exacting requirements.
The pumps were supplied with complex pipework designed to fit within the particularly tight space constraints of the vessel and also Variable Speed Drives enabling them to efficiently move the right amount of sea water at any time to compensate for both slow and rapid changes of volumes in the process tanks.
Pumping solutions for the process industry as a whole
Designing pumps for use aboard FPSO vessels is a huge challenge for engineers with each vessel throwing up its own unique demands. In delivering solutions to the oil and gas industry for these extreme environments however, pump manufacturers such as Amarinth are able to leverage the expertise, skills and technology acquired to design and manufacture innovative and more robust, reliable and cost-effective pumps for the benefit of the process industry as a whole.
