Mechanical Separation with Chemical Separation to Treat Waste Water Offshore

Offshore drilling rigs operate in very sensitive environments surrounded by water. Waves can wash seawater onto the rig. Monsoons can completely cover the deck with a thick layer of water within a couple of minutes. Oil spillages can end up on the drill floor because of a mistake or a mechanical failure.

There are four major drains on a rig:

  • bilge water: this water come from leakages or from washing the floor in the engine room. It is mainly clear water containing only a bit of fuel or lube oil and a very small concentration of solids. There shouldn’t be any emulsion in this stream.
  • open deck drain: this refers to all the water that ends up on the main deck like melted snow, rain, seawater and water washing the deck. This is water with a very low contamination of oil and solids; nevertheless, the volume of water can be significant.
  • tank wash: on each rig, there is an important number of tanks collecting all sorts of fluids, such as the different types of drilling mud. This drain contains some water with a high content of oily solids and some emulsion.
  • drilling floor drain: this water is usually heavily contaminated with a lot of drilling mud, cuttings and oil. This stream can be very erosive and quite difficult to clean.

there are various ways to handle contaminated water. Some operators try to use settling tanks and chemicals, together with filters. The oil company BP accumulated a total of 2,800m³ of water containing only 2% of solids and 2% of oil on a drilling rig in UK sector.

The geographical situation is an important parameter that affects the amount of water on the rig. Rain seasons, snowy winters, dry seasons bring huge quantities of water or nothing at all. Sometimes this low contaminated water can be pumpdirectly overboard if the policy of the drilling contractor decides to do this; even if all water gathered on th ted to ensure that this drain complies with the legislation as specified in many parts of the world.

The quality of the piping and the number of oil spills on the drill floor are other factors that influence the amount of water to be treated. Each drilling rig is unique and the process solution treating the waste wter must be designed especially for it.

Traditional water treatment process line on a drilling rig
Fig1: Traditional water treatment process line on a drilling rig

The Separation Challenge

Emulsification. In the context of oily water treatment, emulsification normally refers to oil and solids contained in the water and strongly bound in the form of a stable emulsion. According to the general definition, an emulsion consists of two immiscible liquid phases, one being dispersed in the other, continuous phase. For instance, emulsions can be of the water-inoil or oil-in-water type.

Activities associated with drilling, transportation, and the cleaning of the different parts of the rig can vigorously moil and water phases together. This can result in the creation of stable emulsions, which are highly undesirable from a treatment quality perspective.

One type of emulsification can start in the drilling mud, where oil and water are mixed with various strong chemicals. Then, this physical phenomenon could be increased due to high temperature and high pressure. When the holes are drilled, there is usually a substantial pressure reduction with a pressure gradient over chokes and valves where the mixing of oil and water can be intense. The use of detergents to clean the deck or the tanks can further contribute to the emulsification of the oil in water.

The complexity of emulsions comes mostly from the composition of the oils; in particular, from the surface-active molecules contained in the oils. These substances occur naturally and contribute to emulsion stabilization by creating a barrier at the oil/water interface that prevents water droplet coalescence.

To make these systems even more complex, emulsions in oil-in-water may also contain solids. Therefore, it is necessary to have considerable knowledge of emulsion chemistry and the processes involved in emulsion formation and breakdown to control and improve the processes at each stage.

Factors Influencing Separation Efficiency. Many separation techniques are based on insolubility and a density difference between the components to be separated; Stokes’ Law, ν=d²(Ρs-Ρl)/18μ G. Describes the separation velocity of a solid particle or a liquid droplet in a continuous liquid medium under the influence of gravity.

Stokes’ Law notes that separation efficiency is a function, not only of the density difference, but also of the droplet or particle size and the viscosity of the continuous phase. The droplet/particle size in particular is crucial to separation, because the Stokes’ velocity increases exponentially with the diameter. Maintaining a high temperature, and therefore lowering the viscosity, is also important for the efficient removal of oil and solids from the water.

Traditional Process To Treat The Drain

The Conventional Approach. In previous decades, there has always been the problem of contaminated water on offshore sites. Although the actual water treatment may vary from location to location, one traditional way of dealing with oil-in-water has been to use large settling tanks, usually several placed sequentially in a train. During the first stage, the heavy solids separate in the sand traps. Following this, the rest of solids are captured in the first part of the tank. The mixing of chemicals is meant to break the emulsion. Thanks to the filter, however, the free oil is trapped in the last part of the process line. If the quality of the water is not satisfactory, it would be sent ashore for further treatment, so the oil concentration complies with legislation. When stable emulsions are encountered, long retention times and large quantities of heat and chemicals are generally necessary to achieve the set requirements.

Overlook decanter centrifuge
Decanter centrifuge manufactured by aipu solids control china Pic 1.

Because conventional static systems rely on gravity to exploit the density difference between water, emulsion, oils and solids, very large separation systems are needed as the density of each phase can be close to the others. In this context, oil-in-water treatment offshore poses further demands on the design of the rigs, because of limited space and payload on platforms.
Problems with emulsions, the concentration of solids, and small density difference further complicate the separation task in gravity-based equipment. A consequence of this is huge, heavy, and expensive process line that still has problems reaching the limits fixed by the legislation. To add further to the difficulties, the rigs need to rely on the supply vessels to transport the huge amount of water ashore.

Nowadays, legislation requires that the operators pay considerable attention to their oil-in-water treatment. It is very complicated and costly to use these traditional systems in order to keep within the limits of oil concentration levels in the water rejected overboard. Regularly blocking the filters, these systems require a lot of maintenance.

Over the years, these basic separation systems have proved how inefficient they are as a means of treating the different drains. The presence of small particles and strong emulsions is not easily separated by a 1-G separation line. The important quantities of oil-in-water stored create unnecessary additional weight on the drilling rigs. As a result, the dependence on the supply vessels, in general, and the transportation of the dirty water are key problems for the rigs.

The Alternative Approach: Going from Gravity to Centrifugal Force. Because of the very small size of the dispersed matter in heavy crude oil, high separation efficiency is needed for successful treatment. The basic principle of a centrifuge is to induce a high centrifugal force by rotation of the liquid, therefore creating acceleration by rotation. According to Stokes’ Law, the acceleration caused by gravity is then replaced by the centrifugal force: ν=d²(Ρs-Ρl)/18μ rω². This can increase the separation velocity several thousand-fold. As a result, the centrifuge is able to separate out even the smallest, micrometer-sized particles and water droplets:

treat mud procedures offshore rig
Fig2: The principle of the new process solution that has been developed

A cost-efficient process solution to treat the oil-in-water drains

This process solution has been developed so it can treat any type of deck drains as described in this paper. The goal was also to build a cost-effective system which could perform anytime to meet the requirements as specified by legislation.

The Decanting Stage

The purpose of the first step in the process is to remove solids from the water and dry it. The decanter centrifuge features a slender cylindrical/conical bowl with a relatively large length to diameter ratio. A central characteristic of the decanter centrifuge is the screw conveyor fitted inside the bowl to facilitate the continuous removal of separated solids. Typical bowl speeds are in the range of 1000-5000 rpm where the G-force is from 300 to more than 3000 G. This makes it possible to separate particles down to a 20 micrometer diameter.

decanter centrifuge bowl
Fig3: A decanter centrifuge

Description. The product is fed through the feed pipe (1), where the velocity has only one horizontal component. The feed inlet (2) is designed so the product gets a smooth acceleration and will avoid any drops from spilling. Simultaneously, the solids are not destroyed and can keep the particles as big as possible to make the separation easier. It then forms a layer around the wall of the rotating bowl. The clear liquid phase runs through a series of discharge weirs (3) at the end of the large cylindrical section. As they are heavier, the solids are collected at the bowl wall and are continuously removed by the screw conveyor and pushed on the beach (4) to squeeze the cake and remove the maximum of humidity. When it is dry, it is discharged through the ports (5) at the narrow end.

To carry this out, the decanter centrifuge should be built for heavy-duty jobs so it can handle large amounts of feed solids and can cope with abrasive and coarse particles. Feed solids up to 30% volume by volume and particle sizes up to 5 mm diameter are easily processed. The liquid coming out of the decanter should contain only traces of free solid particles.

The screening part

The density difference between the water, oil, and emulsion can be very small so the centrifugal separation is a difficult task. So the need to mix small volumes of chemicals is inevitable in order to make the emulsion easier to remove. Fine solid particles and encapsulated water droplets pass through the initial decanter as part of the solids removal process.

Dynamic mixing in the dosing system ensures a homogenous chemical distribution and limits the extent to which solids can settle in the small mixing tanks. The emulsion can then be continuously and easily removed by a simple separation device from the oil and water phases.

The disc-stack centrifuge

In a disc-stack centrifuge, water with very small oil droplets can be treated efficiently to meet the requirements from the legislation without excessive chemical use. The disc-stack centrifuge offers high separation efficiency in a compact size that is also suitable for offshore use where available space and payload are at a minimum.

At this stage of the process, the oil-in-water contains a very small amount of solids. An intermittent discharging centrifuge is the right type of equipment to handle the low concentration of solids.

Description. In the disc-stack centrifuge, the oil-in-water is continuously treated by inducing a high centrifugal force in a rotating bowl. The internal components consist of a stack of closely packed conical discs that provide the necessary separation area.

Although the residence time in a centrifuge is in the region of just a few seconds, the degree of separation is high thanks to the centrifugal force. The separation velocity in a centrifugal separator can be up to three times greater than that under the influence of gravity

Fig. 4 shows the internal components of the centrifuge bowl. The flow of liquid in the bowl is briefly described in the following summary:

The liquid to be treated is fed through the stationary inlet pipe (1), through the inlet device (2) and the bushings (3) into the rotating separator bowl, where it enters the disc stack (4). In the stack, heavy particles and water separate outward along the discs and towards the periphery of the bowl.

Bringing the centrifuge feed to full rotational speed in a fraction of a second, without shearing, droplet splitting, and foaming is a challenge, because that would enhance emulsion formation by producing even smaller droplets. For this reason, the design of the inlet device is crucial. By equipping the inlet zone (2) with a so-called disc inlet, consisting of a stack of closely packed circular discs, the liquid entering the separator can be gently accelerated, with minimum splashing and foaming. Non-rotating liquid flows upwards into the space between the discs and the inlet pipe and then moves outwards to the periphery of the discs as it is being set in rotation. The number of discs required to set the liquid in rotation depends on the flow through the separators–a small number of discs for low flow rates, a greater number for high flow rates. This increases separation efficiency because droplets and particulate contaminants are not broken up. In addition, less gas becomes trapped in the liquid.

shows the internal components of the centrifuge bowl
Figure 4 shows the internal components of the centrifuge bowl.

The performances of the process line. On the Maersk Drilling rig, all the drains are mixed and collected in one tank. To remove very heavy solids (density above 3), the drains are processed through sand traps and then pumped into a buffer tank of 30m3. The mixture is pumped from this reservoir and feeds this new process line.

The system was installed in the hull of the rig, right under the main deck and beside the hatch. The sludge created by the system can be easily collected in 200 liter drums and then shipped ashore. The separated product can also be re-injected in the wells.

This improved system was installed 15 months ago and is still being used. It was the only way to treat the drains on the rig and proved to be successful. For example: during 6 hours of treatment, the system produced 400 liters of sludge and pumped the water overboard (12 m3) with a constant concentration of oil under the maximum limit of 15 ppm. The consumption of chemicals was limited to 2 liters per m3 of drains treated.

The rig engineer has reported that the system required minimum maintenance. To save power consumption and use of chemicals, some stages could be bypassed depending on the type of drains.

Conclusion

With continually expanding markets, more offshore drilling rigs being built, and high treatment costs of the deck drains remaining a major issue, the industry has come to rely even more on separation equipment to cope with the more complex mixture of oils and water. To be more independent and increase cost efficiencies, contractors are looking into how the water can be treated in an economic way onboard the rigs. Cleaning the water offshore presents a unique set of constraints because of the limited available space and the need for a very high level of reliability.

The treatment of deck drains is associated with a number of challenges. The different streams of waste water have a tendency to contain solids in a wide range of concentrations and different types of emulsions, because of the presence of detergents and emulsifiers. Stable emulsions contain droplets and particles so small that separation occurs only after a very long time.
This complex mixture of drains requires high temperatures and chemicals to initiate the separation. Even after all this up-front investment, the quality of the water coming out of the system cannot be guaranteed to meet legislation specifications.

The process solution developed and tested in cooperation with Maersk Drilling is a combination of 3 separation stages to remove the different contaminations. Each stage is complementary and helps the next stage to perform at its best.

Separation in a centrifuge is highly efficient. The use of disc-stack centrifuges for removing the free oil from the water is widespread and well proven as a reliable technology. For instance, there are tens of thousands of disc-stack centrifuges installed in the marine sector for treatment of intermediate and heavy fuel oils through removal of water and dispersed fines. In addition, the use of the decanter centtrifuge at the first stage ensures that the bulk of solids are removed and dried. Then the emulsions are gently removed in a screening step so the disc stack is not blocked in the third part of the process.

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