Shell E&P Ireland Limited a Shell E&P Ireland limited

Corrib House

52 lower leeson Streel

Dublin 2

Dr. Rlichael Henry Office of hnvironmental Enforce

I , Environmental Protection Agenc I ' John Moore Road ' Castlebar I/

Co. Mayo

0°F CASVEBAR Our Ref: COR-01 -SH-GE-1147

OWIGaiqAk DOChlMENT

2"d April 2009 Gcjcment Logged

Copp to ptiblic file Re: PO738-01 Shell E&P Ireland Ltd ( S E P J ~ L f & ~ ~ 3 $ a ~ ~ $ f # v

Bellanaboy Bridge Gas Terminal

Dear.Dr Henry,

Reference is made to the IPPC lic:ence for Bellanaboy Bridge Gas Terminal, and our meeting on 28th November 2009 in Wexford with Mr Frank Clinton, EPA Licensing Division.

In order to allay persistent local stakeholder concerns, SEPIL is proposing the implementation of a revised arrangement for the discharge of treated produced water arising from the conditioning at the Bellanaboy Bridge Gas Terminal of natural gas from the Comb Gas Field. Please find enclosed a detailed submission, which describes the proposed changes to the discharge regime. It includes details of the proposed new arrangement including the modification to the existing marine outfall, which proposed use is now be limited to the discharge of treated surface water run-off water from the terminal site. The document provides an assessment of the impact of the new lscharge arrangements, including a dispersion modelling report for proposed discharge location for treated produced water, and associated amended discharge monitoring regmes.

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Open Web Doc C ----ic-iy---- - - - r.

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Initials: h a *

Registered in lrelond Number 3 I6588 Registered Ofke: Corrib House, 52 lower leeson Street, Dublin 2, lrelond

Directors: JG Egon. AM Hamilton, TJ Nolon

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SEPIL hereby requests that this proposed modification be considered as an application for a Technical Amendment to the referenced licence. However, SEPIL recognises that this is entirely a matter for the Agency.

If you have any queries or if you require further clarification, please contact either Agnes McLaverty or myself. We will be happy to provide you with any additional information you require or meet with you to discuss the Application at your convenience.

Yours Sincerely,

i Mark Carrigy Terminal Operations Manager

Enclosed: COR-14-SH-0026 Rev 01 Proposal for an Alternative Arrangement for Discharge of Produced Water IPPC Licence PO738-01 (2 Copies)

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. . . .- - .. - .. . . . .-. . . . . . . . _. . . . . .. . . . . -. . . .. .. . . . . . . . . . .. . . . . . . . . . . . .. . . . . . -. . -. -. . . -

i '

Proposal for an Alternative Arrangement for Discharge of Produced Water

Document -No: COR-14-SH-0026 REV 01

Applicant: Shell E& P Ireland Ltd.

IPF'C Licence PO738-01

Shell E&P Ireland Ltd Corrib House

52 Lower Lesson Street Dublin 2

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1. INTRODUCTION 4

2.

4.

5.

1 . I Layout of Document

CURRENT PERMITTED DISCHARGE ARRANGEMENT OVERVIEW

4

5

2.1 Produced Water

2.2

2.3 Outfall Discharges

Surface Water Runoff from Process Areas

3. PROPOSED DISCHARGE ARRANGEMENT

3.1 Infrastructure and Operation

3.2 Umbilical Use

3.3 Produced Water Dtscharge

3.4

3.5

3.6 Biocide Addition

3.7

3.8 Additional Pumps at OTTU

3.9 Disposal Off-Site

EMISSIONS

4.1 Produced Water Discharges

Proposed System lor Treated Water Disposal

Potential for Biological Growth in Umbilical

New Transfer Line to Umbilical

4.2

4.3 Noise Emissions

Surface Water discharges - SWl(b)

4.4 Air Emissions

4.5

4.6 Control and Monitoring 4.7

4.8 Treated Water Sampling

4.9 Marine Environmental Monitoring

4.1 0 Proposed Toxicity Monitoring ,

EXISTING ENVIRONMENT & IMPACT OF THE ACTIVITY

5.1 Metocean Conditions

5.2 Stratification and Water Quality

5.3 Seabed Sediments Physico-chemical Characteristics

5.4 Biological Characteristics

5.5

Resource Use and Energy Efficiency

Onsite Treated Water Sampling and Monitoring

Impact on Marine \Vater Quality

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/ > i F

6. SECTIONS IN THE LICENCE REQUIRING AMENDMENT 20

6.1 IPPC Licence Ct$ncIition I .2 20

6.2 IPPC Licence Condition 5.4 20

6.3 IPPC Licence Condition 6.13 21

6.4 IPPC Licence Condition 6.14

6.5

6.6

IPPC Licence Schedule C.2.2 Monitoring of Emission to Water

IPPC Licence Schedule C.6- Receiving Marine Water Monitoring

APPENDIX A Map Showing Location of Discharged Water

APPENDIX B Details of Discharges to Surface Waters

APPENDIX C Dispersion Modelling Report

Abbreviations:

SEPIL Shell E&P Ireland Ltd

EIS Environmental Impact Statement

OTTU Onshore Terminal Termination Unit

PW Produced Water

sw Surface Water

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I

I 2nd February 2009 Shell E&P Ireland Ltd Ji_ T’ I

I 1. INTRODUCTION 1 1

Shell E&P Ireland Limited (SEPIL) wish to seek a technical amendment to the IPPC licence issued in respect of the Bellanaboy Bridge Gas Terminal (PO738-01). The amendment is being sought in relation to the method of discharge of produced water from the Bellanaboy Bridge Gas Terminal. I

The proposed new arrangement involves the discharge of most of the treated produced water through spare cores in the main control umbilical to the central manifold located at the Corrib gas field some 65km offshore and in 350m depth of water. A small amount of produced water will need to be removed offsite by a licensed waste contractor, but this may not be required in later years when produced water volumes decline. Discharged produced water will remain treated to the Emission Limit Values conditioned under the IPPC licence. Surface water run off from process areas on site will be treated and discharged as permitted under the IPP(’ I ’ icence.

‘ I

1 .I Layout of Document

Section 2 gives a brief outline of the current licensed scheme. Section 3 provides information on the proposed new arrangement for the discharge of treated produced water. Section 4 presents an overview of likely changes in emissions. Section 5 reviews the existing marine environrnent and the impact of the proposed changes. Finally Section 6 summarises the changes which are being sought to the current IPPC licence.

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2. CURRENT PERMITTED DISCHARGE ARRANGEMENT OVERVIEW

2.1

The current permitted scheme involves the combined discharge of treated surface water from process areas on site and treated produced water via outfall pipe to the diffuser located approximately 12.7 km from the landfall location in 68.5m water depth.

Produced Water

Produced water is covered in detail in the Section 10.4.4 of the Terminal EIS and Section F1.2.1 of the IPPC application.

Water is a by-product of natural gas production. The water originates from two sources: water which always exists as water vapour within the gas (water of condensation) and water which exists in the rock formation and which may be produced as the reservoir pressure declines (formation water). Water that exists as a vapour in the gas will condense out through cooling and expansion as the gas is produced. Reservoir simulations carried out for the Corrib Field predict that formation water will be produced in very low quantities.

The produced water tre,atment system treats the water to the limits set out in the IPPC licence, which are equivalent to concentrations of Irish Environmental Quality Standards or EQS. The treated produced water is then combined with the treated surface water runoff from process areas on site and discharged at the permitted ouffall location.

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2.2 Surface Water Runoff from Process Areas

2.3

Water that falls on to paved process areas onsite is directed to the open drain system. The open drain system collects surface water run off and directs it to the open drain sump. The water is then fed into the surface water treatment plant where it is treated to the Emission Limit Values set out in the IPPC licence which are equivalent to EQS concentrations and combined with the treated produced water for discharge. If water from the surface water treatment system is out-of-spec the combined volume in the discharge sumps is recycled through the produced water treatment system via the raw methanol storage tanks. This is covered in more detail in Section 10 of the Terminal EIS and section F1.2.1 of the IPPC Licence application and item ‘Fh’ of a request for further information to the Agency in April 2006.

Outfall Discharges

Treated water from the produced water and surface water treatment systems is fed into the combined treated water discharge sumps. From here the water is then discharged via the marine outfall pipe to the outfall location.

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3.

3.1

3.2

PROPOSED DISCHARGE ARRANGEMENT

Infrastructure and Operation

The proposed new arrangement utilises spare cores in the umbilical to discharge the majority of produced water at the manifold module located at the Corrib gas field some 65km offshore and in 350m water depth. The remainder will be removed offsite by road tanker by a licensed waste contractor. Appendix A presents a map showing the location of the proposed discharge points at sea.

To facilitate this alternative discharge method for treated produced water, some minor rationalisations of planned terminal infrastructure are required, plus the addition of two new discharge pumps and transfers lines. The following sections outline these changes.

Umbilical Use

The main umbilical is used to control the subsea wells by providing hydraulic and electrical power as well as the injection of production chemicals into the pipeline to protect against corrosion and the limitation of gas hydrates. Two spare cores in the main umbilical have been identified for produced water disposal, one chemical spare and one hydraulic spare. Hydraulic and metallurgy studies undertaken by SEPIL demonstrate that the cores are suitable for discharges of produced water. The modifications are entirely reversible should the cores be required for chemical or hydraulic duty. A cross section of the main umbilical showing the cores identified for the disposal of produced water is shown on the following page. The two lines will terminate under the manifold protection cover where the water will be discharged.

Figure 1: Dispiosal of Produced Water via Umbilical Spare Cores

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3.3 Produced Water Discharge

Shell E&P Ireland Ltd I

The produced water flow rate will vary with gas production, pipeline slugging and possible formation water breakthrough. The maximum annual average expected rate is expected to be ca. 80 m3/day, although peak production could reach 144 m3/day. Expected produced water flow rates are covered in Section 10.4.8 of the Terminal EIS and Section F Table 7 of the IPPC licence application. Figure 2 below presents the expected produced water volumes.

Hydraulic studies carried out have shown that ca. 40 - 43 m3/d of water can be discharged via Core 10 (spare methanol core). A further ca. 20+ m3/d of water can be discharged via Core 6 (spare 19mm hydraulic core). Thus a total of 60-65 m3/d can be discharged through the umbilical cores. This leaves a surplus of ca. 15-20 m3/d which will need to be removed offsite by truck.

Flow assurance studies, and reservoir modelling have shown that produced water production is expected to decrease significantly after the first few years of operation. After year 4, produced water is expected to decrease by approximately 20% which may eliminate the need to dispose of surplus water off-site. After year 6 produced water is projected to decrease by ca. 50%, at which point only Core 6 (25.4mm core) should only be required to dispose of the produced water.

Studies have shown that formation water may occur later in field life, if this occurs it is expected to be from only one well. If formation water does occur however, production from this well can be shut in if the volumes cannot be disposed by via the umbilical. Predicted produced water volumes are shown in the graph below based on data from the Terminal EIS.

Figure 2 - Projected Produced Water Volumes

3.5

3

2.5

L . 2 f E 1.5

1

0.5

0

m

I - I Off site disposal not I I

1 2 3 4 !5 6 7 8 9 10 ll 12 13 14 15 16 17 18 19 20 21

Year

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Surface water SurfaceWater treatment system -> Discharge Sump

(T-83026) 1 05m3

3.4 Proposed System for Treated Water Disposal

In the current licensed scheme, water leaving the produced water treatment system enters the treated produced water sump (144m3 volume) and is then fed to the outfall discharge sumps which incorporate 2 no. 105m3 interconnected sumps. The discharge sumps also receive treated surface from the surface water treatment system.

> Outfall

In the proposed new arrangement, in order to ensure segregation of the treated produced water and treated surface water; the connection between the two outfall sumps will be closed and isolated. This will provide a dedicated sump for discharges of treated surface water via the sea outfall, ;and discharges of treated produced water via the umbilical cores. This arrangement is shown in Figure 3 below.

Drobuced water

Biocide addition ...........

New transfer line for injection of P W into Umbilical Cores

Flexible hose for surplus PW to tanker '

Figure 3: Proposed New Arrangement for Treated Produced and Surface Water

Treated produced water from the water treatment plant first enters the Treated Produced Water Sump (T-6005). If the quality of the water complies with the IPPC license requirements, the water will be pumped to the Produced Water Discharge Sump. From here it will be transferred 'to the Umbilical Cores for final discharge. Any surplus water will be disposed of off-site. If treated produced water in (T-6005) fails to meet IPPC licence requirements, it will be re-circulated as before to aqueous Methanol tanks. The Treated Produced Water Sump (T-6005) acts as a preliminary buffer tank for checking the quality of produced water.

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Treated surface water will enter the Surface Water Discharge Sump (T-8302A) and if it complies with the IPPC licence requirements, will be discharged to the sea via the current outfall. If the water fails iio meet specifications, it will be re-circulated through the surface water treatment system again. '

The following measures will be incorporated into the new arrangements to ensure satisfactory operation:

Biocide Addition: It will be necessary to incorporate Biocide dosing into the treated produced water discharge to prevent fouling in the Umbilical cores. An additional flow proportional sampler will be installed at the Treated Produced Water Discharge Sump (T-8302B)

. A new transfer will be installed to transfer the treated produced water from the Treated Produced Water Discharge Sump (T-8302B) to the Onshore Terminal Termination Unit (OTTU) on the Terminal site. An additional pump will be installed to pump treated produced water to the OTTU.

. A 75mm flexible hose will be employed to transfer surplus treated produced water to the road tanker utilizing the current treated Produced Water Discharge Pump (P- 8302B)

. Two additional pumps will be required at the OTTU to pump treated produced water through the Umbilical Cores for discharge at the manifold.

3.5 Potential for Biological Growth in Umbilical

The treated produced water from the produced water treatment process will have a significant potential for biological growth, which is likely to be bacteria as opposed to fungal or algal growth. The biological growth, in the form of biofilm, has the potential to block or restrict the umbilical cores. Blockage or restriction is most likely to occur at system restrictions, such as umbilical connectors.

In addition, biological growth could induce corrosion (microbially induced corrosion - MIC), if biofilm becomes established. MIC normally takes the form of pitting corrosion that can rapidly penetrate pipe walls.

The treated produced water will be an ideal medium for biological growth due to the presence of readily metabolised nutrients such as methanol, sulphate, nitrogen, acetate, and other trace elements necessary for biological growth. The water treatment process will have removed natural biocides and biostatics such as heavy metals, BTEX and other hydrocarbons, and the IOW salinity will also favour biological growth. The biological growth potential of the treated produced water will far exceed that of potable water or seawater.

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It is not feasible to exclude bacterial and fungal spores from the produced water treatment system, therefore some form of biological control will be necessary.

3.6 Biocide Addition

An important requirement of the proposed discharge system for treated produced water is the need to control biological growth, core restriction and corrosion within the system. Single point sterilisation technology, such as UV water treatment, will not prevent biological growth in the umbilical. There would be a significant potential for biological growth to become established downstream of the single point location and to proliferate unchecked. As a consequence, biocide addition will be necessary to provide biological control.

Biocides can be applied on a continuous or an intermittentlbatch basis. The following points are made in relation to continuous versus intermittentlbatch addition:

Continuous:

= . . Bacteria can become'tolerant to the applied biocide

Low concentration is required relative to intermittent application (e.g. 5-60 ppm)

Total application of biocide and discharge of biocide will be greater than intermittent application (e.g. 3 m3 per annum).

Intermittentlbatch:

Frequency can range from daily (e.g. 4 to '8 hours per day) through to weekly, or less frequent. . . . Bacteria are less likely to become tolerant to the applied biocide

High concentration is required relative to continuous application (e.g. 100-200 ppm)

Total application of biocide and discharge of biocide will be less than intermittent application (e.g. I m3 per annum).

SEPIL believe that intermittent/batch addition of biocide is likely to be more effective for biological control. An upper limit of 200 ppm of as supplied product is anticipated to be the upper concentration limit. Intermittentlbatch addition will also result in lower total discharge of biocide (approx 1 m3 per annum) to the marine environment.

The biocide selected for the Corrib application will have a relatively low environmental impact when discharged to the marine environment. SEPIL will endeavour to use a biocide that degrades easily, based on OSPAR guidance on chemicals released to the marine environment. The selecteld biocide will exhibit at least 20% biodegradation after 28 days.

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It is important to emphasise however that biocide addition can only be optimised when the terminal is operational and treated produced water samples are available. Optimisation will establish the optimum applied concentration and dosing frequency and help establish minimal total discharge to the environment. It will also help select a biocide product with a minimal environmental impact.

The primary methods of verifying biological control will be the monitoring of pump discharge pressure versus discharge rate and laboratory culturing of treated produced water grab samples to verify the efficacy of the biocide addition.

3.7 New Transfer Line tot Umbilical

An additional 4-inch lined carbon steel line and pump will transfer treated produced water from the Treated Produced Water Discharge Sump via the existing pipe rack to the Onshore Terminal Termination Unit (OTTU). The OTTU is the termination module for the umbilical on the terminal site.

3.8 Additional Pumps at OTTU

Two new injection pumps rated at 20kW and 35kW each, will pump the treated produced water from the OTTU module through the umbilical lines to the manifold. One pump will be rated at 25 m3/day, max, at 61 0 bar discharge and the second pump will be rated at 45 m3/day at 345 bar discharge. The pumps will be variable speed pumps and will run continuously.

3.9 Dis posa I Off -S i te

Treated produced water which cannot be discharged through the umbilical cores, will be removed off-site by a licensed waste contractor. There are a number of licensed waste contractors that could be considered such as Shannon Environmental Services, Enva, Veolia. Greenstar and other such contractors.

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4. EMISSIONS

This section describes the principal emissions and discusses any changes that are likely to be of relevance.

4.1 Produced Water Discharges

The concentration of the constituents in the final treated effluent will be as outlined in the IPPC licence and which are equivalent to Irish Environmental Quality Standards for the marine environment. These treatment targets are unprecedented for discharges such as this. Notwithstanding this, the water will also discharge in 350m water depth and will have virtually an infinite number of dilutions available. Therefore concentrations of produced water will not be discernable from the natural background within a relatively short distance from the manifold. No additional environmental impact is anticipated as discharges are already below levels designed for the protection of human health, animal species and ecosystems even without further dilution.

Biocide will be dosed into the treated produced water before final discharge. Typical biocides for this application will rapidly degrade due to hydrolysis. Consequently, the toxicity of the water will reduce whilst treating the produced water discharge system i.e. sumps, discharge pumps and umbilical cores. The toxicity-to-bacteria half-life is expected to be in the order of several hours. By the time the produced water is discharged at the subsea location at the Corrib field, the concentration of the biocide will have reduced, with the degradation of the biocide to carbon dioxide, bromide and ammonia. On discharge, the positively buoyant water will rise within the water column and rapidly be diluted and dispersed, such that the concentration of biocide within the water will have reduced to levels that will present little potential for impact to the marine environment. Toxicity testing of the final discharged water will be carried out in accordance with requirements of the IPPC Licence.

Appendix B presents details of the projected composition of the treated produced water and treated surface water, as discharged. These data are in line with the original IPPC licence application and with the current IPPC licence discharge limits.

4.2 Surface Water Discharges - SW1 (b)

Treated surface water will comprise treated rainfall runoff from process areas on site. This will be treated to the ELL% set out in the licence and discharge via the marine outfall pipe as permitted under the IPPC licence. The treated surface water will comprise of 30m3/hr maximum discharge rate. The design flow rate in the outfall will have been reduced by approximately 17% as a result of the treated produced water being discharged separately via the umbilical cores. No produced water will be discharged via the outfall thus there will be a reduced environmental loading at the outfall location and little potential for environmental impact.

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4.3 Noise Emissions 1

Two additional pumps for pumping treated produced water from the OTTU module at the terminal to the manifold, will give rise to some noise. The specification of these pumps will be such that they will cornply with the noise emission limits which the EPA have specified in the IPPC license.

The additional pump to transfer treated produced water from the treated produced water discharge sump to the Onshore Terminal Termination Unit (OTTU) will be small and not give rise to additional noise emissions.

4.4 Air Emissions

Due to the addition of the two water injection pumps there will be a slight increase in power requirements at the Terminal. This additional load is expected to increase emissions of carbon dioxide from the power generators by approximately 225-250 tonnes per year. Increases of oxides of nitrogen and carbon monoxide will be negligible and will not result in any significant environmental impact.

4.5 Resource Use and Energy Efficiency

As mentioned previously there are two additional electric pumps required to discharge produced water through the cores. The pumps rated at 35kW and 20kW each. The additional power required is anticipated to result in the increase usage of fuel gas of 85 tonnes per annum. The additional power requirement does not change the mode of operation or rating of the /power generators permitted under the IPPC licence.

4.6

Additional biocide will be required to prevent fouling within the cores and this will result in increased use of chemicals. Biocide will be intermittentlbatch mixed in the treated water discharge sump prior to discharging. The total usage of biocide is not likely to exceed 1 m3 per annum for intermittent dosing.

Control and Monitoring

The original design for the control and handling of produced water and surface water is

described in Section 2.5.8 and 2.5.11 of the EIS and RFI to the Agency on 4'h of April 2006, item Fh pg 55. The segregation of the treated produced water and treated surface water and the separate discharge of each, is the principal change. In addition, off specification treated surface water and off specification treated produced water will now be re-circulated separately b,ack to their respective treatment systems.

Monitoring will generally be carried out as described in the original IPPC licence application and as required by the current IPPC Licence with the exception of ambient marine monitoring which will be discussed later in this section. It is also proposed to change the frequency of rnonitoring of Nitrogen in the produced water.

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Surface Water

(T-8302A) 1 05m3 -) Discharge Sump

Surface water treatment system

4.7 Onsite Treated Water Sampling and Monitoring

Treated water from the water treatment plants will be continuously monitored using a combination of online measurements such as flow, pH, conductivity, suspended solids and periodic lab analysis. Conductivity, pH and suspended solids have been selected as the most reliable indicator of treatment performance. Water exceeding defined threshold indicating the treatment targets are not being met will be recycled through the surface water treatment system or the produced water treatment system.

Outfall

4.8 Treated Water Samplling

As well as online measurements and periodic grab sampling, 24hour composite sampling of the discharge will take place as required by the licence. The samples will be analysed as required under Schedule C2.2 Monitoring of Emissions to Water.

Flow proportional On-line pH, conductivity, On-line pH, conductivity, composite sampling suspended solids, TOC

Biocide addition suspended solids, TOC

...........

t Off spec water will be re- circulated to methanol , tanks and further processing through WV treatment system

Figure 4: Proposed Monitoring and Control of Treated Produced and Surface Water

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4.9 Marine Environmental Monitoring

Shell E&P Ireland Ltd

Schedule C.6 'Ambient Monitoring - Receiving Marine Water Monitoring' of the IPPC Licence set outs a requirement for monitoring water chemistry, sediment and biota. Given the proposed new arrangements for discharging treated produced water at the Corrib manifold 65km offshore i3nd in 350m of water, and treated rainfall runoff water only being discharged at the outfall location it is proposed to remove this requirement from the licence.

The existing environment and impact of the activity in the vicinity of the manifold is discussed further in Section 5.

,

The proposed amended monitoring programme will focus on the quality and toxicity of treated produced water leaving the terminal rather than attempting to sample local environmental conditions. This is considered a more pragmatic approach given the very high level of treatment of produced water, the high dilution and dispersion afforded to the produced water discharge and the impracticalities of monitoring waters in such a deep and distant location.

It is proposed that the monitoring of treated produced water i.e. SWl(a), as set out in Schedule B.2 and C2.2 in the IPPC licence, is broadly retained with the exception of nitrogen sampling, which is discussed further in Section 6.5 of this document.

4.10 Proposed Toxicity Mlonitoring

Toxicity will be carried out on the treated produced water as required by the Licence. Treated produced water will be sampled with a composite sampler prior to being pumped through the umbilical cores. To conduct toxicity testing, these samples will be taken to an approved laboratory facility where they will be used in standard ecotoxicology tests. Such tests will involve a number of serial dilutions of the treated produced water, with a number of marine species exposed to each concentration. The species may include a marine phytoplanktonic alga, a marine zooplanktonic species, a sediment re-worker (such as Corophium) and a flatfish larvae, however the testing regime and species shall be determined in conjunction with the EPA.

The first of these exposure experiments will be carried out as soon as practical after start- up. Other test dates/events will be agreed with the EPA depending upon the operational programme for the Terminal and results from the first exposure work. Results will be presented to the EPA irnmediately following the end of the exposure experiment, with interpretations and conclusions to follow within an agreed period following the test.

As required by Condition 6.12 of the IPPC licence, further details will be submitted and agreed with the Agency in advance of start up.

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I

~ -

i

2nd February 2009

5.

Shell E&P Ireland Ltd

EXISTING ENVIRONMIENT & IMPACT OF THE ACTIVITY

5.1 Metocean Conditions

j The environment offshore at the manifold and throughout the area of the Corrib Field is characterised by conditions reflecting its location at the continental margin. The field receives the full fetch of the North Atlantic, which is responsible for the wave regime at the field (predicted 50 year significant wave height of 22m).

Currents in the area are influenced by the predominant ocean circulation, in particular peripheral currents formed by the North Atlantic Current, which results in both deep water and surface water currents off lrelands western margin that move from the south and south west. Currents tend to weakly reflect the directional dominance of the ocean currents, despite also being influenced by the effects of storm surges, and tidal circulation. As such there is a residual current moving in a north-easterly direction.

Water depth at the point of discharge is approximately 350m below chart datum (CD), with a mean neap and spring tidal range of 1.4m and 3.lm respectively, representing less than 1% change in total water depth throughout the tidal cycle. Bathymetry in the region is irregular and characterised by a number of ridges and depressions thought to be the result of iceberg scouring.

5.2 Stratification and Water Quality

Temperature and salinity stratification occurs in the area of the Corrib Field during the summer months. A se,asonal thermocline develops due to the temperature difference between surface and deeper water layers, resulting in restricted mixing between the two layers, this thermocline usually occurs at a depth of around 80-1OOm.

. 5.3

The temperature range for surface waters ranges between around 9°C and 15"C, while at depths greater than 100m there is expected to be little variation from around 10°C. Seawater surface salinities are consistently around 35.1 ppt throughout the year, increasing to 35.4ppt below 1 OOm.

While water quality measurements have not been recorded in the Corrib Field, samples have been taken from the surface and bottom water to the east of the Field, at approximately the 12 nautical mile Irish territorial limit, in water depths of approximately 125m below CD. Results from the 12 nautical mile area, for both trace metals and hydrocarbons reflect the pristine nature of the environment, and show very little difference from typical background levels for oceanic seawater. It is predicted that water quality in the area of the Corrib Field itself is very similar to that recorded to the east, and if anything will contain lower concentrations of contaminants due to its location further offshore, and further from any anthropogenic inputs.

Seabed Sediments F'hysico-chemical Characteristics

A number of seabed surveys have been undertaken in the Corrib Field, showing that the seabed sediments are predominantly sands (fine or very fine sand), overlying sandy clay.

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2nd February 2009 Shell E&P Ireland Ltd

Levels of metals in the seabed sediments at the Corrib Field are generally low, reflecting the overall pristine nature of the receiving environment. This is further confirmed by the similarity of the results obtained to OSPAR published background concentrations. One exception to this was the elevated levels of barium observed at a number of sample locations, which can be attributed to well drilling activities. Due to its low solubility,

1

elevated barium concentrations are rarely of toxicological concern. Levels of Polycyclic Aromatic Hydrocarbons (PAHs) in sediments were below OSPAR published background concentrations, again confirming the overall pristine nature of the environment.

I 5.4 Biological Characteristics

The survey of the Corrib Field for the 2001 offshore EIS recorded 261 different benthic taxa in the general vicinity of the manifold location. These sites showed the greatest similarity to the reference sites located outside the Field, with some stations from other well sites in the Field containing some species indicative of organic contamination. In a second survey in 2008 the majority of sites in the Field showed greater similarity to each other, indicating that the sediment at previously impacted (from drilling operations) sites had recovered such that the benthic communites present were similar to those found throughout the area.

Benthic communities in the Field are of moderate to high diversity, with moderate dominance and evenness. A review of the results shows that the community composition is fairly constant throughout the survey area, with annelids making up the highest percentage of animals at all sites.

5.5 Impact on Marine Water Quality

Shell E&P Ireland Ltd have carried out dispersion modelling studies of the produced water discharging at the Corrib manifold. Refer to dispersion modelling report in Appendix C. The results of the mixing zone modelling indicate that, for the range of conhitions assessed, predicted concentrations of each of the discharge constituents will be reduced to within 10% of background levels up to 300m downstream, to within 5% of background levels up to 500m downstream and to within 1% of background levels up to 1.6km downstream of the discharge location. For many of the discharge constituents, significantly smaller mixing zones are required to reduce concentrations to background levels.

There are some difficulties in modelling a discharge such as this, as discussed previously the water will discharge under the manifold cover which has 7 large openings. In effect the manifold will act somewhat as a large diffuser with water being released through these openings. Modelling of the resulting release would be extremely difficult to carry out, therefore the model was constructed using a point source discharge releasing vertically into the water column. Although this not an actual true representation of the discharge it nonetheless is a very good representation of the dispersion and dilution afforded to the discharge once released.

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2nd February 2009 Shell E&P Ireland Ltd

I . I - - Contaminant concentrations in the produced water discharge will be within the limits imposed in the current IPPC Licence, which are equivalent to Environmental Quality Standards for the listed parameters. Many of the components in the discharge will be at or below their respective environmental quality standards prior to discharge. In addition to this, the modelling confirms that the impact of produced water at the Corrib manifold will be very low indeed and not discernible. Any changes to the concentrations of metals and organic material both in water and sediments around the outfall location will also be extremely difficult to detect. Therefore there will be no significant environmental impact arising from the discharge of treated produced water at the Corrib manifold.

I I ' I

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2nd February 2009 Shell E&P freland Ltd

6. I

6.1

SECTIONS IN THE LICENCE REQUIRING AMENDMENT

This section outlines conditions in the licence that will be required to be amended to take into account the revised discharge method for produced water. The agency may consider it appropriate to amend or add additional conditions. Where practical, potential wording for consideration by the EPA has been identified.

,

IPPC Licence Condiition I .2

For the purposes of this licence, the installation authorised by this licence, is the area of land outlined in bold on Drawing No. 01 01 59-22-DR-0001 of the application and the outfall pipeline as shown in Drawing No. 188 Rev.01. Any reference in this licence to “installation” shall mean the area thus outlined in bold and the “outfall pipeline. The licensed activities shall be carried on only within the installation.

Proposed amendment to Condition:

For the purposes of this licence, the installation authorised by this licence, is the area of land outlined in bold on Drawing No. 010159-22-~R-0001 of the application and the surface water outfall pipeline and umbilical discharge line as shown in Drawing ‘Corrib Bellanaboy Bridge Gas Terminal Treated Water Discharge Locations’ (Doc MXD:EP20080322400004, Rev. B). Any reference in this licence to “installation” shall mean the area thus outlined in bold and the outfall pipe and umbilical discharge lines. The licensed activities shall be carried on only within the installation.

6.2 IPPC Licence Condition 5.4

Emissions from emission point references SWl(a) and SWl(b) shall discharge through a diffuser at a location not less than 500 meters outside the boundary of the Broadhaven Bay cSAC and at least 12.7 km offshore from the landfall location and shall be located within 100 metres of the discharge location as shown on Drawing No. 188 Rev.01.

Proposed amendment to Text

Emissions from emission point references S W1 (a) shall discharge via the umbilical cores to a location under the central manifold cover at the Corrib gas field. Emissions for SW1 (a) that cannot be discharged through the umbilical due to hydraulic capacity limitations shall be removed off site by an appropriately licensed waste contractor for suitable disposal. Emissions from emissicm point SW1 (b) shall discharge through a diffuser at a location not less than 500 meters outside the boundary of the Broadhaven Bay cSAC and at least 12.7 km offshore from the landfall location and shall be located within 100 metres of the discharge location as shown on Drawing ‘Corrib Bellanaboy Bridge Gas Terminal Treated Water Discharge Locations’ (Doc MXD:EP20080322400004, Rev. B)

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EPA Export 26-07-2013:14:38:07

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Monitoring Frequency

2nd February 2009 , . Shell E&P Ireland Ltd

Analysis/Technique

6.3 IPPC Licence Condition 6.1 3

Total Nitrogen (as N) DailyNote ’

The licensee shall, in advance of the commencement of the licensed activities, undertake a benthic community biodiversity assessment of the environs of the effluent discharge location at sea.

Standard Method

Proposed amendment:’

It is proposed that this condition be removed from the licence.

Produced water will not be discharged via the permitted outfall pipe. Treated runoff water will only be discharged via the outfall, Due to the benign nature of the surface water and the high level of treatrnent of prior to discharge the need for such monitoring is not considered necessary Surface water will remain treated to the ELVs in the licence and monitoring of the discharge will take place at the Terminal.

6.4 IPPC Licence condition 6.14

The licensee shall, in advance of the commencement of the licensed activities, submit a monitoring programme for the marine receiving waters in accordance with Schedule C.6 of this licence for agreement by the Agency.

Proposed amendment:

It is proposed to remove this Condition of the Licence.

As discussed in Section 4.9, the proposed ambient monitoring of receiving marine waters should be removed. Thlis applies for the discharge of treated produced water and treated surface water.

6.5 IPPC Licence Scheth.de C.2.2 Monitoring of Emission to Water

Emission point reference No.: SW1 (a)

Proposed amendment:

Change Monitoring Frequency from ‘Daily’ to ‘Weekly’

Comment

Daily monitoring of Nitrogen is considered unwarranted. The total N in treated produced water is expected to remain relatively constant. The major contributors to total N will be, ammonia, nitrate, nitrite and low molecular weight organic nitrogen containing molecules.

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2nd February 2009 Shefl E&P Ireland Ltd

The nitrogen will originate from the reservoir fluid and the manufacturing by-products in the corrosion inhibitor injected into the produced fluids. Both contributions will be relatively constant. In addition Nitrogen discharged into open seawater at a distance of 65km offshore does not pose an environmental risk particularly in the range of the likely concentrations. Consequently it is considered that the frequency 'of monitoring is considered not be of any real value. For this reason weekly monitoring of N is proposed.

6.6 IPPC Licence Schetlule C.6- Receiving Marine Water Monitoring

Proposed amendment:

It proposed to remove this part of Schedule C.6. As discussed previously, the monitoring of receiving marine waters is not pragmatic and is unwarranted given the very high level of treatment of water prior to discharge. Not withstanding this the produced water will be discharge into open sea in 350m water depth and will be very quickly diluted to levels that will not be discernable from background levels within a relatively short distance from the manifold. In addition to there is a 500m radius exclusion zone around the subsea installations within which fishing is prohibited for safety and integrity reasons. Monitoring will now focus on the quality of treated water leaving the site and the toxicity of produced water.

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9th March 2009 Shell E&P Ireland Ltd

MAP SHOWING LOCATION OF DISCHARGED WATER

Doc MXD:EP20080322400004, Rev. B

(1 Page attached)

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9th March 2009 Shell E&P Ireland Ltd

DETAILS OF DISCHARGES TO SURFACE WATERS

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EPA Export 26-07-2013:14:38:07

9th March 2009 Shell E&P Ireland Ltd

D I s P E F!S I o N MODELLING REPORT

Prepared by HR Wallingford

Report E,X 5927; Release 2.0; Dec. 2008

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EPA Export 26-07-2013:14:38:08

. . . , . .

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Report EX 5927 Release 3.0 January 2009

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Corrib Offshore Gas Field Development Produced Water Disoersion Assessment

Project Report title Client

Document Information

Corrib Offshore Gas Field Development Produced Water Dispersion Assessment RSK Environment Ltd

Project Manager Project Director

Mr Matthew Wood Dr Chris Mead

0 HR Wallingford Limited

HR Wallingford accepts no liability for the use by third parties of results or methods presented in this report. The Company also stresses that various sections of this report rely on data supplied by or drawn from third pariy sources. HR Wallingford accepts no liability for loss or damage suffered by the client or thirdparties as a result of errors or inaccuracies in such thirdpariy data

.. EX 5927 11 R. 3.0

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Summary

Corrrib Offshore Gas Field Development

Produced Water Dispersion Assessment

Report EX 5927 December 2008

RSK Environment Ltd (RSK) is working with Shell on the Corrib Offshore Gas Field Development. A discharge of produced water is proposed, in around 350m water depth, and RSK required a study to assess how quickly discharge constituent (for example, metals) concentrations are likely to reduce to near-background values.

It is understood that discharge will take place approximately 2m above the seabed, possibly beneath a protection cover. The maximum discharge flow rate is to be 65m3/day. Under normal operating conditions the discharge will be positively-buoyant; that is, the density of the discharge will be lower than that of the ambient seawater at the discharge point.

An assessment of the dispersion of the produced water has been undertaken. The results of mixing zone modelling indicate that, for the range of conditions assessed, predicted concentrations of each of the discharge constituents can be reduced to within 10% of background values up to 300m downstream, to within 5% of background values up to 500m downstream, and to within 1% of background values up to 1600m downstream of the discharge location. It should be noted, however, that for many of the discharge constituents, significantly smaller mixing zones are required to reduce concentrations to values approaching background concentrations.

The results of the present modelling have been obtained assuming that the produced water is discharged vertically into an unbounded seawater environment, and it should be considered when using the results in this report that dilution rates may vary if the discharge is located under a protection cover.

EX 5927 iii R. 3.0

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

iv R. 3.0 EX 5921

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Contents Title page Document Information Summary Contents

1

ii iii V

1 . Introduction ...................................................................................................................... 1

Scope of work ........................................................................................................ 1 1.3 Report structure ................................................................................................... 1

1.1 Background ........................................................................................................... 1 1.2

2 . Data assessment ............................................................................................................... 1 2.1 Ambient conditions .............................................................................................. 1

Currents .................................................................................................. 1 2.1.2 Water depth ............................................................................................ 3 2.1 . 1

2.2 Discharge characteristics ..................................................................................... 3 2.2.1 Produc. ed water characteristics ............................................................... 3 2.2.2 Outfall configuration .............................................................................. 3

3 . Methodology .................................................................................................................... 4

4 . Dispersion assessment ............................. 1 ........................................................................ 5 General behaviour of the discharge plume ..................................... ; .................... '5 Mixing zones ................. : ..................................................................................... 5 Discussion of the modelling simplifications ........................................................ 7

Potential for build-up of the discharge ................................................... 8 Presence of a protection cover ............................................................... 8

4.1 4.2 4.3

4.3.1 Ambient currents .................................................................................... 7 4.3.2 4.3.3

5 . Conclusions and recommendations .................................................................................. 9

6 . References ...................................................................................................................... 10

Tables Table 2.1 Table 2.2 Table 4.1 Table 4.2a Dilutions required to reduce discharge concentrations to within certain

Table 4.2b Distances required to reduce discharge concentrations to within certain

Year-long near-bed current observations (adapted from Reference 1) ...................... 2 Normal operation discharge characteristics ............................................................... 3 Distance to minimum dilutions of 1O:l and 100.1. for different current speeds ....... 6

percentages of amtiient seawater concentrations ....................................................... 7

percentages of ambient seawater concentrations .............................................. : ........ 7

Figures Figure 4.1 a Predicted trajectory for ambient current speed of 0.05m/s Figure 4.1 b Predicted trajectory for ambient current speed of O.l5m/s Figure 4.lc Predicted trajectory for ambient current speed of 0.25mls Figure 4.ld Predicted trajectory for ambient current speed of 0.35mls Figure 4.2 Variation of arsenic concentration with distance downstream

EX 5927 V R . 3 0

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Corrib Offshore Gas Field Development Produced Water Disperslon Assessment

vi R. 3.0 EX 5927

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

I. Introduction 1.1 BACKGROUND

This report describes work commissioned by RSK Environment Ltd (RSK) in November 2008.

RSK is working with Shell on the Corrib Offshore Gas Field Development. A discharge of produced water is proposed, in around 350m water depth, and RSK required a study to assess how quickly discharge constituent (for example, metals) concentrations are likely to reduce to near-background values.

It is understood that discharge will take place approximately 2m above the seabed, possibly beneath a protection cover. The maximum discharge flow rate is to be 65m3/day. Under normal operating conditions the discharge will be positively-buoyant; that is, the density of the discharge will be lower than that of the ambient seawater at the discharge point.

This report presents the inethodology and results of the assessment undertaken.

1.2 SCOPE OF WORK

Modelling was required to assess the dispersion of produced water for the proposed “normal” discharge scenario,.with a maximum flow rate of 65m3/day. The concentration of each discharge constituent was provided by RSK (see Chapter 2).

1.3 REPORT STRUCTURE

Chapter 2 of this report describes the findings of a review of data supplied by RSK. Chapter 3 introduces the modelling tools used in this study, and the results of the produced water dispersion assessment are presented in Chapter 4. The conclusions of the study are stated in Chapter 5.

2. Data assessment 2.1 AMBIENT CONDITIONS

2. I. I Currents RSK provided HR Wallingford with the results of a field study for the Corrib development (Reference 1). Current data is presented as a frequency table for different speeds and directions over a year of observations. Currents were measured at 1 Om above the seabed, but Reference 1 takes the values as valid at 1 m, without reduction, to allow a small margin of safety for structural desigdstability issues. This data is summarised in Table 2.1.

EX 5927 1 R. 3.0

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. .- . .. . . . .. ~~

Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Observed current direction Observed current speed (m/s)

000<022.5

022.5<045 ~~

045<067.5

0904 12.5

~~

112.5435

135457.5

157.5<180

180<202.5

202.5<225

225<247.5

247.5<270

270<292.5

292.5<3 15

315<337.5

337.5<360

Total occurrence number I Yo [ 42.4 I 52 I 5.4 I 0.2 1 100

In the table above, “number” denotes the number of observations for each combination o speed and direction.

’ current

The data indicate that near-bed current speeds at the discharge site are below 0.2m/s for more than 90% of the time. For the year during which sampling was undertaken, maximum observed currents were less than 0.4m/s.

Reference I states that theoretically-derived spring tide current speeds at the site (from tidal harmonic constituents) are around 0.15m/s, but the basis for this statement is not known, nor whether this comment refers to peak speeds. However, this is consistent with the data observed, given that the currents at the site are likely to result from a combination of physical processes, including oceanic currents, storm surges, and tidal flows.

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

The currents at the discharge point appear not to have strong directional bias such as one would expect if tidal effects dominated the prevailing flows. However, there does appear to be a slight tendency for northeastward currents to dominate. This is confirmed by Table 3.4a of Reference 1 (not reproduced here) which presents the current data scatter.

2.1.2 Water depth According to Reference 1, the water depth at the site is approximately 330m. At the nearest observation site to the discharge location, the mean spring and neap tidal ranges were found to be 3.Im and 1.4m respectively. These represent less than 1% change in total water depth through the tidal cycle, which can be ignored for the purposes of the discharge dispersion calculations.

2.2 DISCHARGE CHARACTERISTICS

2.2.1 Produced wafer characteristics The parameters and constituent concentrations for the produced water discharge under normal operation are shown in Table 2.2. Discharge concentrations are as defined in the IPPC Licence granted in 2007 to Shell for discharge of produced water offshore of Erris Head. Background seawater concentrations, supplied by RSK, were derived from water quality monitoring undertaken in 2008 offshore of Erris Head. Under normal operating conditions the discharge will be positively-buoyant, that is, it will have a density lower than that of the ambient seawater.

Table 2.2 Normal operation discharge characteristics Table 2.2 Normal operation discharge characteristics

2.2.2 Outfall configuration Several design drawings for the outfall configuration were provided by Shell. It is understood that:

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Produced water will be pumped to the site in two separate cores of an umbilical, one 19mm diameter and the other 25.4mm diameter. At the release site, the cores will be bent upwards (but not necessarily exactly to the vertical), away from the umbilical. The produced water will leave the cores approximately 2m above the bed. It is not known how far apart the two cores will be at the point of release (for example, whether they will discharge directly next to each other). A fibreglass protection cover, approximate dimensions: 17m x 12m x 4m, may be placed on the bed at the site of the discharge, so that the pipes will discharge into the space between the protection cover and the manifold. The protection cover would have seven access apertures/windows (one fore, three port, three starboard), each with approximate dimensions: 1.5m x lm. Therefore, whilst the discharge may initially be approximately vertical, the release into the open water outside any protection cover may in fact be lateral through the access apertures.

I

3.

RSK requested that, at the present time, the effects of any protection cover on discharge dispersion should not be included in‘the calculations. For the purposes of modelling, a vertical discharge was assumed, into a range of ambient current speeds representative of the likely currents outside an:y protection cover. A single-outlet discharge was assumed (this is probably more conservative, in terms of dilution, than assuming that the discharges from the two cores occur separately). In order to ensure that the release was as “hydraulically similar” as possible to the actual discharge, it was assumed that the cross- sectional area of the discharge outlet was equal to the sum of the individual cross- sectional areas of the two umbilical cores (this gave a single discharge outlet diameter of 3 1.7mm). The resulting discharge exit velocity was around 1 m/s, which is typical of the expected pipe exit velocities, but is likely to be very much larger than the velocities at which the diluted discharge would flow through the access apertures.

This approach will b’e discussed further in Chapter 4.

Methodology Based on the natural hydrodynamic conditions at the site discussed in Chapter 2, an assessment of the likely dilution of the produced water discharge plume was made using the CORMIX expert system (Reference 2).

CORMIX is an international1:y accepted software system for the analysis, prediction and design of aqueous toxic or conventional pollutant discharges into diverse water bodies. It incorporates an expert system that uses the characteristics of the discharge (flow rate and configuration) and of the receiving water (depth, width, current speed, etc) to determine a class for the discharge jet. It then calculates the centre-line trajectory and dilution rate of the jet to the edge of the near-field area.

CORMIX also has some capability for estimating the mid- and far-field dispersion of the effluent, which has been utilised here. However, it must be appreciated that CORMIX cannot represent detail such as spatially varying bathymetry or current patterns; it provides approximations for uniform environments.

CORMIX has three sub-systems:

CORMIXI , for submerged single-port diffuser discharges

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0

0

As it had been decided (see above) to represent the release as a vertical point source, CORMIX 1 was selected for the present modelling.

CORMIX2, for submerged multi-port diffuser discharges CORMIX3, for buoyant surface discharges.

Data on the strength of the ambient currents in the vicinity of the site was selected from the information described in Chapter 2. CORh4IX modelling was undertaken for ambient currents at the mid-points of the data bin ranges: 0.05m/s, 0.1 5m/s, 0.25m/s and 0.35m/s. As the currents closer to the bed are likely to be weaker than those higher up in the water column, the implications of this assumption will be discussed in Chapter 4. As the discharge is assumed to make an angle of 90" with the sea bed, the dilution of the discharge is independent of the direction of the ambient current.

The water depth at the site is around 330m, and varies by less than 1% during the tidal cycle. At such depths, mixing will not be significantly influenced by water level variations due to the tide or waves, and therefore an invariant water depth of 330m was assumed for the purpose:: of modelling.

The CORMIX predictions were used to estimate potential mixing zones for the following:

0

0

To reduce constituent concentrations to 10% and 1 % of their values at the release point' To reduce constituent concentrations to within IO%, 5% and 1% of the ambient values.

4. Dispersion assessment 4.1 GENERAL BEHAVIOUR OF THE DISCHARGE PLUME

Discharge of the produced water vertically into an unbounded ambient environment results in the formation of a positively-buoyant plume. The turbulent plume rises from the release point entraining ambient seawater along its boundaries. The rise of the plume is at first due to its initial vertical momentum, but eventually, as momentum dissipates to the environment, the plume continues to rise under its own buoyancy. Predicted trajectories for the discharge plume are shown in Figures 4.1 a, 4.1 b, 4. I C and 4.1 d for ambient current speeds of 0.05m/s, 0.15m/s, 0.25m/s and 0.35m/s respectively. For relatively strong ambient currents, the plume is more rapidly deflected to the horizontal than at lower current speeds, which results in a narrower and less diluted discharge.

4.2 MIXING ZONES

Table 4.1 shows the distances required to reduce concentrations of constituents in the discharge to 10% and I % of their original values (that is, dilutions of 10:l and 1OO:l respectively). It can be seen that for the range of current speeds tested, dilutions of 100: 1 are generally achieved within a 5m three-dimensional radius of the discharge point. \

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Corrib Offshore Gas Field Development & 7 HR Wallingford Produced Water Dispersion Assessment

Table 4.1 Distance to minimum dilutions of 1O:l and 100:1, for different current speeds

, Ambient Percentage Distance (m) to minimum dilution Current occurrence I 1OO:l

Tables 4.2a and 4.2b present the dilutions and distances required to reduce concentrations of constituents in the discharge to within lo%, 5% and 1 % of the ambient seawater values. Clearly, for constituents whose discharge concentration is close to the seawater value (e.g. suspended solids), relatively small dilutions are required. For constituents at much larger concentrations than ambient (e.g. hydrocarbons, and certain metals), dilutions of several orders of magnitude are required.

Tn the modelling, settling of suspended substances has been neglected, as CORMIX cannot take this physical process into account. In terms of dilutions calculated within the water column, this is considered to be a conservative approach.

It can be seen that as current speeds increase, the required mixing zone lengths also increase. This is due to the increased level of deflection of the produced water discharge plume by the ambient currents, which results in a narrower and more concentrated plume. For the range of conditions assessed, predicted concentrations of each of the discharge constituents are reduced to within 10% of background values up to 300m downstream, to within 5% of background values up to 500m downstream, and to within 1% of background values up to 1600m downstream of the discharge location. It should be noted, however, that significantly smaller mixing zones are required for many of the discharge constituents.

Figure 4.2 also shows the variation in concentration of one of the discharge constituents, arsenic, with distance downstream for different ambient current speeds.

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Constituent

Table 4.2a Dilutions required to reduce discharge concentrations to within certain percentages of ambient seawater concentrations

Distance (m) required to reduce discharge concentrations to within certain Dercentages of ambient seawater values, for different ambient current speeds

Sus ended solids 0.1 0.2 0.2 0.3 Total Nitro en 4.9 9.1 13 17 ,--1?-- Hydrocarbons 66 143 - 213 - 277 -

IXKTXFI 0.25 I 0.35 j 0.05 I 0.15 I 0.25 I 0.35

0.2 0.4 0.5 0.6 8.1 15 23 30 109 - 242 - 361 - 469 -

I m/s I m/s I m/s I m/s I m/s I m/s I m/s I m/s n H 1 6.6 13 18 24 1 11 21 .31 41

.. .

Nickel Zinc

49 105 40 85

0.5 1.0 1.3 1.8 I 0.9 1.8 2.5 3.2 - Cadmium Arsenic Mercu

Chromium

25 15 7.3 13 2.0 21 --

157 126 37 22 11 18 2.7 y&

6.5 24 11 3.6 1.8 41 18

178 143 42 25 12 21 3.4 35 -

266 213 62 37 18 31 4.8 53

345 277 82 48 24 41 6.3 69 -

4.3

4.3.7 Ambient currents

DISCUSSION OF THE MODELLING SlMPLlFlCATll

1 Yo 0.05 I 0.15 I 0.25 1 0.35 m/s I m/s I m/s I m/s 34 71 105 137

0.8 1.5 2.1 2.7 25 52 77 100 392 820 1219 1582

3.4 6.3 261 602 207 485 66 143 40 84

'21 42 34 71 6.1 12 56 121

NS

9.1 898 722 213 125 62 105 17

180

12 1164 93 7 277 163 81 137 22 234 -

Dispersion modelling was undertaken for representative ambient current speeds at the mid-points of the data bin ranges of the observed currents (taken from Reference 1). As stated in Chapter 2, currents were measured at 10m above the seabed, but Reference 1 takes the values as valid at lm, without reduction, to allow a small margin of safety for

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I

Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

structural desigdstability issues. Currents closer to the bed are likely to be weaker than those higher up in the water column, due to friction at the seabed.

The predictions above show xhat weaker ambient currents may result in shorter mixing zones, due to the resulting wider discharge plumes. Therefore, the actual mixing zones required, for an unbounded vertical discharge, are likely to be smaller than those presented above.

4.3.2 Potentia/ for build-up of tbe discharge The formulation of the COFU4IX software assumes steady-state conditions and therefore the software cannot represent detail such as any build-up of discharge constituents which may occur near the outlet. Such build-up is usually associated with tidally-reversing environments, particularly where ambient currents are weak, which can result in partially-diluted effluent remaining in the vicinity of the discharge site. This can reduce the potential for initial dilution of a discharge. However, the analysis presented in Chapter 2 indicated that currents at the discharge point appear not to have significant directional bias such as one would expect if tidal effects dominated the prevailing flows. There does appear to be a slight tendency for northeastward currents to dominate, which would tend to carry the produced water away from the discharge point. The combination of these factors makes it unlikely that the discharge constituents would build up around the site, for a vertical unbounded discharge.

4.3.3 Presence of a protection cover This study assumed that no protection cover would be in place at the discharge site. The effects of this assumption, if a cover was to be put in place, are considered here.

The way in which the discharge is introduced to the sea will be critical to its dilution following release. Factors such as the discharge orientation relative to the prevailing flows (inside or outside any protection cover), the effective cross-sectional area of the openings through which the discharge is introduced to the open waters, and the velocities of the discharge itself and of the seawater at the point of release, will all affect the way in which dilution of the discharge by ambient seawater initially takes place. The addition of a protection cover would transform the above relatively simple open-ended pipe discharge scenario into a more complex situation, to which the methods presented in this study are not fully appropriate. .

Detailed analysis of potential flow patterns within a protection cover is beyond the scope of the present study. However, it is likely that flows inside a protection cover, which would interact with the discharge at its point of release, would be relatively complex, and would be different to the prevailing ambient currents at the site outside a protection cover.

As modelling has been undertaken for the produced water as a point release discharging vertically, into an unbounded environment, the model predictions are likely to be different to the actual behaviour when the discharge arrangements are implemented if a protection cover is used.

In view of the above, whilst it is not possible to make recommendations as to whether options for positioning the discharge outside a protection cover should be considered, it can be said that the modelling tools used for this study would be likely to predict the

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5.

discharge’s dilution more accurately for a discharge outside a protection cover than for a discharge inside a cover.

Initial tests carried out for unbounded discharges of both neutrally-buoyant and positively-buoyant produced water indicate that buoyant effluent is likely to mix and dilute more readily than a discharge with neutral buoyancy, due to the additional entrainment that occurs during the rising phase of the plume. Therefore, the confinement of the discharge within a protection cover, which is likely to inhibit the rise of the plume, may place a significant restriction on the potential dilution. Assessment of the potential reduction in dilution is beyond the capabilities of the current modelling approach.

Depending on the exact structure of any protection cover, produced water may build up underneath the lid before emerging from the side apertures. The effects of this on dilution cannot be accurately determined as part of the present analysis. Whilst the discharge might be introduced into the open water over a wider area (due to the division between the different apertures, which may afford extra dilution to the discharge), there may also be a decrease in dilution due to the reduction of momentum-induced jet mixing.

Conclusions and recommendations An assessment of the dispersion of produced water from a proposed release point at Corrib has been undertaken. The results of mixing zone modelling indicate that, for the range of conditions assessed, predicted concentrations of each of the discharge constituents can be reduced to within 10% of background values up to 300m downstream, to within 5% of background values up to 500m downstream, and to within 1% of background values up to 1600m downstream of the discharge location. It should be noted, however, thal for many of the discharge constituents, significantly smaller mixing zones are required to reduce concentrations to values approaching background concentrations.

The results of the present modelling have been obtained assuming that the produced water is discharged vertically into an unbounded seawater environment, and it should be considered when using the results in this report that dilution rates may vary if the discharge is located under a protection cover.

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, Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

6. References I . Corrib Field Development - Metocean Criteria Corrib 'Field and Sealine Route,

Metoc Report No. 978, October 2000

2. Doneker, R. L., and Jirka, G. H. (2007), "CORMIX User Manual. A hydrodynamic mixing zone model and decision support system for pollutant discharges into surface water.", Office of Science and Technology, U.S. Environmental Protection Agency, Washington, DC 20460

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Figures

,

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.. . . .

!--

& W i d Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

*bt Hiul#UG -- &lo

I t k d a l \ c o - r * \ b . I C b W M t ~ = l p d

I I I I I I I I I I I I 1 1 1 1 1 1 1 1 1 1 1 1 1 l 1 I I I I 1 1 1 1 1 1 1 1 1 1 1

1 1 1 1 1 1 1 1 1 1 1 1

Figure 4.la Predicted trajectory for ambient current speed of 0.05m/s

Figure 4.lb Predicted trajectory for ambient current speed of 0.15m/s

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

COrfO vr,Ued -dD - OJ3ll% 1ceor(b.r, l l u l m 81IdUaI

I

25m - I - - - - - ---

I I I I I I I I I i I I I I I I I I I I I

I - !

Figure 4.lc Predicted trajectory for ambient current speed of 0.25m/s

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Corrib Offshore Gas Field Development Produced Water Dispersion Assessment

Arsensic concentration vs. distance downstream (normal operating conditions)

0 10 20 30 40 50 60 70 so 90 IO0

0.1

Y - P 0

'I $

f 0.01 E g 1 E z 3

a M - - E

0.001 Distance downstream (m)

Ambient current speed (ds): -0.05 -0.15 - 0.25 -0.35

Discharge concentration

O.OSmgil _---_

Ambient

0.001 mdl _ _ _ _ _ _ _ concentration

Figure 4.2 Variation of arsenic concentration with distance downstream

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