Copyright 2002, Offshore Technology Conference This paper was prepared for presentation at the 2002 Offshore Technology Conference held in Houston, Texas U.S.A., 6–9 May 2002. This paper was selected for presentation by the OTC Program Committee following review of information contained in an abstract submitted by the author(s). Contents of the paper, as presented, have not been reviewed by the Offshore Technology Conference and are subject to correction by the author(s). The material, as presented, does not necessarily reflect any position of the Offshore Technology Conference or its officers. Electronic reproduction, distribution, or storage of any part of this paper for commercial purposes without the written consent of the Offshore Technology Conference is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of where and by whom the paper was presented.

Abstract The injection of oil contaminated seawater into a sensitive injection formation requires careful selection of process equipment. The treatment of backwash effluents from such plant pose many problems. This paper looks at how one such project deals with the problems using non-standard oilfield equipment to arrive at a robust solution. Introduction A gravity base platform allows storage of stabilised crude oil within the platform subsea tanks. Periodically when the subsea storage tanks are full a tanker is moored alongside and evacuates the crude oil. To maintain platform stability the discharged oil phase is replaced with raw seawater via a subsea valve. When crude oil from the separation train is sent to the subsea storage tanks the ballast seawater is displaced. This displaced seawater is re-injected as part of the water injection system.

The displaced seawater contains some oil originating from the subsea ballast tanks and it is the treatment and disposal of this ballast stream that presents the problem. Due to legislation in force the design basis specified use of the displaced water phase to provide water injection. Displaced water, or any of its consistent components, is not allowed to be returned into the sea even after treatment. The injection reservoir formation is relatively ’tight’ and requires that injection water is treated to a high specification.

This creates three problems for the design of the water injection system:

• highly filtered seawater is required for injection into a

tight reservoir

• the injection process creates backwash effluent which is oil contaminated

• the volume of effluent water must be minimised

because of the necessity for onshore disposal as hazardous waste.

Description of Water Injection System Pressure maintenance is required to maintain the hydrocarbon reservoir pressure at the necessary levels to fully exploit the field development. This is achieved by seawater injection. The reservoirs are relatively tight and require a very high quality of water to prevent permeability decline in the injectors. The source of water for the water injection system is primarily the displaced water from the platform subsea tanks, with sea water supplementing the displaced water as necessary to fulfil the total water injection requirements. Process Systems There are 2 principal unit operations associated with a standard water injection system;

• media filters to achieve the necessary filtration requirement

• vacuum deaeration to reduce the oxygen

concentration to levels required to minimise corrosion

The primary complicating factor was the handling of the displaced water stream and it’s integration into the overall scheme. The treatment of this water for high quality water injection is difficult and presents particular challenges offshore. Figure 1 shows a process schematic for the water injection system. The displaced and sea water streams are filtered separately and combined upstream of the vacuum deaeration unit. The water injection system delivers water at a rate sufficient both to maintain injection pressure and to dispose of any displaced water. The displaced water flowrate depends on the platform subsea storage tank operating mode and on the oil production rate,

OTC 14287

Treating Oil-Contaminated Seawater and Minimizing Backwash Effluent Discharge Kevin Lynch, Axsia Serck Baker; Wayne Evans, Axsia Serck Baker

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which varies throughout the platform life. Displaced water rates are typically highest at the start of platform life corresponding to high initial oil production rates due to high initial reservoir pressure. However the displaced water rate can vary for a variety of reasons and the water injection system had to be designed to respond to provide the necessary injection rate regardless of what fluctuations occurred in the displaced water rate. The design of the subsea tank minimised the risk of severe oil contamination of the displaced water but concentrations of up to 300 mg/l were expected. The risk of severe oil carryover into the water injection stream was assessed and, although low, such an event would severely foul the filter media and reduce the effectiveness of the deaeration tower. An Induced Gas Flotation vessel (IGF) was therefore installed as the first process stage. This ensured that any significant oil carryover could be removed and recovered. As the displaced water arrived at the platform surface at just over atmospheric pressure it was necessary to pump this into the water injection system. The IGF vessel was hence designed with the necessary residence time to ensure adequate pump suction volume for control purposes. The Induced Gas Flotation vessel consists of a two cell mechanical flotation unit with a quiescent gas disengagement zone to allow any entrained gas to escape and to allow a reasonable working volume for the downstream filter feed pumps. Flow into the vessel is not controlled and is entirely dependent on the displaced water rate (i.e. crude oil rate). A reverse demulsifier chemical can be added upstream of the IGF unit to increase the oil removal efficiency if required. The oil concentration at the outlet of the IGF is specified to be typically less than 20mg/l, sufficient to avoid excessive fouling of the downstream dual media filters. Filtration Process The filtration specification was set at 98% removal of 5um and above, with a maximum filtrate oil-in-water specification of 5 mg/l or less. The design of seawater filters is itself a complex subject due to the plankton and algal nature of the solids and the filtration mechanisms occurring within a dual media filter. This is a subject which has been well studied by Axsia Serck Baker over many years. The displaced water was known to contain small amounts of oil and possibly other chemicals associated with production. The surfactant properties of these contaminants can have a negative effect on the efficiency of a dual media depth filter. In addition any water recycled via the backwash treatment system is recycled upstream of the Induced Gas Flotation vessel and this stream will contain a wide variety of other chemicals.

A dual media filter system was chosen for the filtration process. The dual media consists of different grades of anthracite and garnet allowing effective depth filtration. This type of filter has a long history of treating water to very high filtration specifications, is easy to operate and to backwash, and has a high solids holding capacity. Two choices were possible for treatment of the waters namely, 1) treatment of a common combined displaced and sea water stream or 2) separate treatment of each water stream. The former approach seemed the most straightforward. However, it is normal practice offshore that seawater filter backwash effluent is dumped directly overboard. This would not be possible where there is oil present. By treating the two waters as a combined stream, it would become necessary to store and treat all of the backwash effluent produced. Considering that the displaced water stream volume falls significantly after approximately 3 years of operation it was decided to maintain separate filtration systems for both water sources. The design of the separate filtration systems also took advantage of the changing profiles to allow re-allocation of the filter vessels from displaced water to sea water service as the displaced water rate falls. The filter design had to take account of the differing chemistry of the sea and displaced waters. The displaced water contains not only oil but several other chemicals added in the production train and in the water injection system. This required considerable attention to detail in designing the filters for interchangeable duty The design flux of the displaced water filters is approximately half the flux of the seawater filters. The design flux is a relatively simple concept referring to the velocity of the water stream as it passes down through the vessel. In general terms the lower the flux the better the performance. The lower flux of the displaced water filters allows improved solids retention to counteract the surface effects of the oil and chemicals. The second major difference concerns the backwash sequence. An intermittent detergent dosing step is included in the washing sequence of the displaced water filters to assist with the removal of oil from the media and internals. This backwash effluent is discharged to a holding tank rather than to the sea, as is the case on seawater duty

Filter Operation & Control Logic The displaced and sea water sources differ in that the injection water demand must always at least equal the displaced water rate if this source of water is to be disposed of downhole. The sea water flow is varied in order to balance the injection demand, i.e. if injection demand is greater than the displaced water demand then sea water will make-up the balance. If the injection water rate is increased above the displaced water rate, the level in the vacuum deaeration tower will fall and the sea water flowrate through the sea water filter vessel(s) will be increased to make-up the demand and vice-versa.

OTC 14287 TREATING OIL-CONTAMINATED SEAWATER AND MINIMIZING BACKWASH EFFLUENT DISCHARGE 3

The filter system consists of 4 vertical, downflow, filter vessels. The vessels are manifolded in such a way that three vessels are initially dedicated to displaced water service and one to sea water service. Each filter has a dedicated flow control valve and flowmeter on the inlet line. During normal operation the water level in the downstream vacuum deaerator sump varies in response to changes in injection water demand. The level controller modifies the flow control set point on the seawater filters so that, irrespective of source, the filter package passes sufficient clean water forward to meet the injection demand rate. The displaced water filters are similarly equipped with dedicated flow control valves which respond to level changes in the upstream IGF vessel. An increase in the displaced water rate causes a rise in the IGF level and a level controller modifies the set point of the displaced water filter flow control valves to allow more flow through. Process control is undertaken directly by the platform DCS. The only operator intervention is to allocate the correct number of filters to each service, this being done infrequently according to the predicted crude oil flowrate. Filter Backwashing During operation, the filter media becomes fouled with particulate matter and needs to be cleaned to maintain efficiency and prevent particulate breakthrough to the filtrate. Backwash frequency in seawater applications is difficult to predict accurately because of differing source water and platform operating conditions. Backwashing can be initiated either by operating until the filter differential pressure has reached a preset maximum or by backwashing at pre-set intervals. Long experience has indicated that regular backwashing of filter media before it has reached a high differential pressure is by far the best approach as this results in prolonged media life, easier and more efficient backwashing and reduced risk of particulate breakthrough. This is the case for filters in both duties, with a differential pressure override initiating backwash in the event of unusually high loading. Backwash Procedure The dual media filters are backwashed according to the following generic sequence:

• drain down of water above the bed • air scour

• upflow backwash

Depending on the source water, foulant type and mode of operation a second cycle is not unusual and, if necessary, a pre-service flush step to condition the bed prior to return to

normal service. The backwash water source for this application is seawater taken downstream of the platform 100um coarse strainer system. The main disadvantage of using unfiltered seawater for backwashing is that a pre-service flush is usually required to prevent the carryover into the filtrate of high particulate counts before the media bed is fully conditioned. Although seawater is used to backwash both the displaced and sea water service filters it was still necessary to segregate the displaced water filter draindown and backwash effluent water for further treatment. The control logic ensures that only water from the on-line seawater filters is allowed to return overboard. Any water from the displaced water filters is automatically routed to the displaced water backwash effluent treatment system. Backwash Volumes A realistic estimate of backwash water volumes was necessary to size the backwash effluent treatment system. This calculation for any system needs an estimate of backwash frequency and backwash volumes. The volume of backwash water can be estimated from the backwash sequence and generally comprises at least 3 components:

i) drain down water from above the bed ii) backwash water used to wash the bed iii) pre-service flush volume to condition the

bed prior to return to service

The following are conservative estimates per backwash cycle

Draindown 3.0 m3 Upflow backwash 20.0 m3 Pre-service flush 7.0 m3 Total 30.0 m3

The frequency of backwash is more difficult to judge but a conservative estimate of up to 3 times per day for each displaced water filter formed the design basis of the backwash treatment system.

This equates to a volume of less than 6% of the total plant daily cumulative throughput. It should be stressed that this is very much a design figure for this particular application which will be optimised during plant commissioning. Backwash Effluent Treatment System The purpose of the backwash effluent treatment system is to:

• collect backwash effluent water from the displaced water filters

• separate the water, oil and solid phases

• minimise the volume of solids to be disposed

of onshore.

4 LYNCH, K., EVANS, W. OTC 14287

A process schematic of the treatment system is shown in figure 2. Backwash Treatment Design Challenges The design challenges associated with backwash treatment are many including, ♦ handling short duration, high instantaneous flowrates ♦ separation of different phases given complex

chemical pre-treatment ♦ presence of oil ♦ maintaining a cost effective, workable solution High Flowrates Figure 3 shows the flow profile for the backwash treatment plant based upon the maximum backwash frequency for the displaced water filters (9 vessel backwashes per day total). Over 90m3 of water is received by the surge tank over an 8 hours period. Given the sudden flow variations the first requirement was to smooth out the flows to give stable flowrates to the treatment equipment. This is achieved by using a balancing or surge tank of sufficient capacity to allow a smaller but continuous flow output. The surge tank also provides a buffer volume to cope with upstream or downstream problems without causing a complete plant shutdown.

Figure 3 Effect of Effluent Discharge on Surge Tank Volume The sizing and design of the surge tank takes account of a large number of considerations such as:

• sufficient time to accommodate one full package backwash

• promotion of phase separation

• reduction of treatment capacity of downstream equipment

In general the first consideration is the most important and does have a significant effect on platform layout. A single package backwash (3 filters) typically accounts for nearly 90 m3 of liquids. This had considerable space and weight implications but afforded the maximum process security for this particular platform. The surge tank allows solids to fall to the bottom take-off point while any gross oil can be skimmed off manually by means of a floating skimmer. Gravity Thickening Tank The variable inflow into the surge tank is not conducive to effective settling characteristics so a second, smaller, purpose-designed gravity thickener, commonly found in the municipal waste water industry, was used to concentrate the settled bottom phase from the surge tank. The tank is designed on a rise rate of less than 1.5m/hr and receives flow from the upstream surge tank by gravity feed. The inlet flow is controlled to minimise flow variations in the thickener and to prevent disturbance of the settling particulates. Water enters near the top (A) through a central diffuser box designed to distribute the fluid radially over the vessel cross sectional area (figure 4). Clear liquid (supernatant phase) flows over a fixed weir into an internal launder from where (D) it is periodically pumped back to upstream of the IGF vessel.

Figure 4 Cross section of gravity thickener

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OTC 14287 TREATING OIL-CONTAMINATED SEAWATER AND MINIMIZING BACKWASH EFFLUENT DISCHARGE 5

The settled solids falls to the base of the tank (C ) where a slow speed scraper pushes the sludge into a central hopper. To aid separation of the particulate matter a dedicated polyelectrolyte can be added upstream of the tank to improve separation efficiency. The thickener is designed to produce a consistent slurry flow of between 0.4 – 1.0 wt% solids. The degree of thickening is effectively controlled by the discharge rate which is set be adjusting the downstream centrifuge feed pump. The flow into the thickener is set in accordance with the volume of backwash fluid being received. A slow discharge rate will mean relatively thick slurry feed to the centrifuges while a faster rate means more dilute slurry is achieved. The inlet flowrate is independent of the outlet flowrate meaning that the excess water flows over the top weir and into the internal launder. Decanter Centrifuge The gravity thickener process step can clarify the majority of the water phase producing a continuous volume of wet slurry. However seawater solids, particularly algal or plankton based, are difficult to separate and dewater as their bulk density is often close to water and a certain amount of water is locked within cell boundaries.

A decanter centrifuge was chosen as the final process step to primarily dewater the slurry generated in the upstream gravity thickener. However in the design of the system, given the known difficult separation characteristics of seawater based solids, the centrifuge may have to perform two different tasks:

• clarification and treat the full water backwash effluent volume (11 m3/hr at 0.03wt% )

• thickening and treat a much smaller volume (0.5

m3/hr at 0.7wt%) Although the two choices represent the same solids loading for the centrifuge there is a practical limit to how thick a sludge can be produced with a very dilute feed or how clear a centrate can be produced with a more concentrated feed. The centrifuge was sized for the maximum liquid hydraulic capacity. Other factors have to be borne in mind such as the seasonal variation of seawater solids, the unknown and difficult chemistry and the presence of oil as well as the assumption that the gravity thickener will work efficiently under all conditions. In any event the conservatively sized centrifuge is far easier to turndown than to turn-up an undersized unit. The centrifuge system consists of a duty/standby system comprising feed pump and centrifuge. The feed pumps supply the centrifuges at the necessary volume flowrate (0.5 to 11.0 m3/hr) and pressure (1.0 barg).

Figure 5 shows a picture of the decanter centrifuge which consists of 3 main components: the bowl, the scroll or screw conveyor and the drive motor. There are 3 main connections – the feed, the centrate and the solids discharge. The drive motor rotates the bowl at high speeds to increase the ’g’ force acting on the particulate matter. The centrifuge is fed with slurry through a hollow shaft inside the screw conveyor (scroll). The feed pipe sprays the slurry into the bowl section to ensure good inlet distribution. The rotating bowl causes a centrifugal force which results in a phase separation with the heavy, solid phase going to the bowl wall and the lighter, water phase accumulating in a concentric annulus known as the ’pool’. The depth of this annular pool is set by an adjustable weir plate located at the centrate end of the machine. This weir plate allows the clear water phase to spill-over into a collection launder. The deeper the pool the longer the retention time so the better the centrate quality at the expense of a wetter sludge discharge. Similarly the shallower the pool the dryer the sludge discharge at the expense of poorer centrate quality. The scroll turns at a differential speed to the main bowl and the helix pushes the settled solids towards the conical end of the machine for disposal into a small solids launder and disposal chute. The chute discharges onto a conveyor belt which transports the solids to a disposal skip. Characterising a decanter centrifuge for seawater solids service is somewhat difficult due to the difficult nature of the solids. However consideration was given to: ♦ Bowl diameter, length and shape: Increasing the bowl

diameter increases the solids handling capacity and will also dictate a lower bowl speed than a smaller diameter bowl. This may be important when trying to stay within the limitations of the bowl material of construction. Increasing the cylindrical length of the bowl will generally improve the centrate quality (i.e. longer separation time). The shape of the bowl also has some importance as sudden steep changes in bowl angle may result in erosion at the point of change. Most bowls have a cylindrical section and a conical section. The conical section is designed partly to cause a reduction in the g force and allow the solids to discharge from the bowl in a controlled manner. For this application a dual angle 305mm diameter bowl was chosen, the first section at 4o and the second shorter section at 8o. The cylindrical section is 550mm with the conical section length about 660mm. The conical end of the bowl also acts as a ’solids beach’ with the water level being determined by the pool depth.

6 LYNCH, K., EVANS, W. OTC 14287

The bowl is fitted with metal ribs to reduce abrasive wear. In this way the ribs can be examined periodically to see if erosion is occurring.

♦ Bowl Speed: The faster the bowl then the better the separation and the dryer the sludge, in theory, but it is useful to have the facility to vary the bowl speed for the application. A working range of 1500 to 3000 g appears appropriate given the manufacturer’s experience on separating thin (i.e. low solid content) organic wastes.

♦ Scroll drive mechanism, shape and speed: For this

application a hydraulically powered scroll was chosen. This offered independence from the main bowl drive allowing the scroll to be operated without the bowl in operation. This was seen as an important feature as it is not desirable to allow solids to settle in a static bowl. Start-up of an unbalanced bowl is very difficult so the independent scroll allowed greater chance of clearing the bowl before a start-up or after a shutdown. Scroll differential speed (typically 0 – 20 rpm) is also an important variable when setting up the machine. The slower the differential, the drier the sludge as there is a longer drying time on the beach, but at the expense of centrate quality. The shape of the scroll is also important and particularly the pitch and angle of the screw. The scroll was supplied with a narrow pitch which is more efficient than a wide pitch with difficult to move solids.

Centrifuge Optimisation There are a large number of variables available to get the best from the machine. However it is useful to define what purpose the centrifuge best fulfils, i.e.

♦ maximise sludge dryness or

♦ maximise centrate quality In practise there is a balance between the two and, as stated above, maximising one objective can be detrimental to the other. For this particular application the main objective was to produce the lowest volume of sludge thereby minimise handling, shipment and onshore disposal costs. The unknown factor is how dry a sludge can be produced without significantly affecting centrate quality. While the sludge volume is critical to reducing operating costs, the fact that the centrate is recycled to the water injection system means that control of centrate quality is important in keeping injection water quality at the required levels.

Centrifuge Installation The centrifuge must run at atmospheric pressure and requires an atmospheric discharge for both centrate and solid discharges. As the centrate is pumped back into the system this had serious layout issues as the centrifuge had to be positioned at an elevated position on the platform. The solids discharge has the same implication although for this application a conveyor belt was installed under the discharge chute with minimal height requirements. Stage Volume Reduction Without the backwash effluent treatment plant the platform would have to dispose of a maximum of 270m3 per day from the displaced water filter plant. This is obviously a large volume of water, particularly so when all displaced water waste must be shipped and disposed of onshore. The backwash effluent treatment plant consists of a two-stage reduction in volume. The first stage is the gravity thickener which is designed to reduce the volume by over 90% to between 10 and 20 m3/d of sludge (0.4 – 0.8 wt%). This is still a considerable volume to ship onshore. The 2nd stage is the centrifuge which is designed to reduce the disposal volume to a conservative target of 0.5 m3/day (15wt% approx.) and better if achievable. Achieving this level of volume reduction does come at a price in terms of additional equipment cost, additional weight and utility requirements and operating cost. As an example the dry weight of the main water injection process equipment packages (excluding injection pumps and drivers) was about 97 te dry and 170 te operating. The backwash treatment plant process equipment system was about 25te dry and 195te operational (80% of this being the surge tank volume). Backwash Treatment System Control The system was chosen for a number of reasons to be manually controlled as it is envisaged that once set up the system will run continuously. The operator first needs to consider the backwash frequency of the displaced water filters and manually sets the set-point for the thickener feed flowrate. The feed flowrate to the centrifuge is next set and the centrifuge optimised to meet the objectives of minimizing sludge volume without significantly affecting centrate quality. Hence as long as the displaced water flowrate is relatively consistent the operator can be assured that the backwash system will work satisfactorily. Based on the number of backwashes per day Axsia provided the set-up criteria for equipment flowrates. Chemical Regime Apart from the usual water injection system chemicals an additional polyelectrolyte is included for optimization of the thickener and centrifuge. Although satisfactory performance may be achieved without the use of an additional

OTC 14287 TREATING OIL-CONTAMINATED SEAWATER AND MINIMIZING BACKWASH EFFLUENT DISCHARGE 7

polyelectrolyte, chemical optimisation should always be considered. Unfortunately it is unlikely that the polyelectrolyte or coagulant used for the filter plant will be effective for the thickener or centrifuge and hence the need to allow for separate make-up and dosing equipment. This is especially through of the centrifuge where, due to the high shear forces, most seawater filter polyelectrolytes will have little effect. Conclusions The treatment of oily seawater streams is a difficult issue both in terms of selecting filtration equipment for high injection specification and disposal and treatment of oily backwash effluent. High quality water is achievable using dual media depth filters as long as the issues which make treatment difficult are addressed and resolved. Treatment of oily backwash effluent streams is not entirely predictable. The design of the centrifuge should consider that gravity separation may not be very efficient and that the full water flowrate may have to be treated by the centrifuge. Recycled water from the backwash plant must be of sufficient quality so as not to prove detrimental to injection water quality. Gravity thickening and centrifuges can provide high reductions in backwash effluent volumes. Backwash treatment systems should be considered early on in the project due to the space and weight issues encountered with such plant.

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Figure 1 Process schematic of Water Injection System

Figure 2 Process schematic of Backwash Effluent Treatment System

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OTC 14287 TREATING OIL-CONTAMINATED SEAWATER AND MINIMIZING BACKWASH EFFLUENT DISCHARGE 9

Figure 5