CHANGE CONTROL SHEET

GAS DEHYDRATION

TABLE OF CONTENTS

Section-Page

Purpose 

Scope 

References 

Definitions 

Responsibility 

Safety and Environmental CONSIDERATIONS

Process Overview

Major Equipment Summary

System Summary

Gas Dehydration

Gas Filter Separator (MAK-2210)

Glycol Contactor (MAF-2215)

Gas/Glycol Heat Exchanger (HBG-2220)

START UP, OPERATING & SHUTDOWN PROCEDURES - Gas Dehydration system

Cold Start-up Procedure Following a Planned Shutdown.

Hot Start-up Procedure Following a Process Trip

Normal Operational Checks:

Planned Shutdown procedure

Emergency Controlled Shutdown Procedure

Alarm, Trip and PSV Settings

Valve List

Acknowledgements 

Tables

Table 1 Operating and Design Conditions for MAK-2210

Table 2 Pressure Monitoring 

Table 3 Level Monitoring 

Table 4 Temperature Monitoring 

Table 5 Operating and Design Conditions for MAF-2215

Table 6 MAF-2215 Pressure Monitoring

Table 7 MAF-2215 Level Monitoring

Table 8 Operating and Design Conditions for HBG-2220

Table 9 HBG-2220 Temperature Monitoring

Table 10 HBG-2220 Dew Point Monitoring

Table 11 Flow Monitoring 

Table 12 Cold Start-up Valve Checklist

Table 13 Cold Start-up Overrides 

Table 14 Cold Start-up Procedure Following a Planned Shutdown

Table 15 Hot Start-up Valve Checklist

Table 16 Hot Start-up Procedure Following a System Trip

Table 17 Planned Shutdown Procedure

Table 18 Emergency Controlled Shutdown Procedure

Table 19 Alarm, Trip & PSV Settings

Table 20 Valve List 

figure

Figure 1 Process Flow Diagram of the Gas Dehydration System. 

INTRODUCTION

This document is part of the Operating Manual, which covers the Gas Dehydration System. The Gas Dehydration System operations manual together with other manuals and related documents comprise the Facility Management System or FMS. The Gas Dehydration System is a controlled document maintained by MODEC INTERNATIONAL LLC, the Contract Operator of the Baobab Field. Any question concerning this document or its contents should be addressed to the Contract Operator.

Purpose

This document gives a general description of and the task lists for the operation of the Gas Dehydration System. Its purpose is to educate those who are unfamiliar with the system in how it works and the way in which the system is to be started, stopped and operated safely. This document can be used as a reference document in formulating maintenance isolation procedures.

Scope

Information contained in this document applies only to the Gas Dehydration System on the Baobab Field Development project. The information contained gives general design information, alarm settings and trip points for the various components as well as the start up, shut down and normal operating procedures for the Gas Dehydration System.

References

Refer P&IDs 1072-MI20-9020-0114-01, 1072-MI20-9020-0114-02, Gas Filter Separator Data Sheet (Doc.No.1072-2200-G21-0022),TEG Contactor Data Sheet ( Doc. No. 1072-2200-G21-0028) , Gas - Glycol Exchanger Data Sheet (Doc. No. 1072-MI20-90DS-2200/03) and Vendor P&IDs 935-P-DW-201-1/2

Definitions

See the Definitions Document of the FMS.

Responsibility

Every plant system covered by the Gas Dehydration System has been assigned to a single position title in the permanent offshore crew. The person filling the named position is jointly responsible with his off-duty relief for the integrity and the safe operation of the assigned system. In like manner it is that person's responsibility to verify the accuracy of the information contained in this document. Any confirmed inaccuracy shall be tracked by the Offshore Installation Manager (OIM) until formally corrected.

The job title of the position responsible for this system can be found in the System Responsibility Matrix in the Safety Management System (SMS) Manual (1072-MI20-15ST-0170).

Safety and Environmental CONSIDERATIONS

In the execution of the subsequent procedures all permit to work, PPE and isolation procedures required by the Baobab SMS (1072-MI20-15ST-0170) shall be followed. If any doubt exists as to the correctness of valve positions or sequence of operations then they should be discussed with the supervisor and approval sought from the Facility Manager to change the procedure. These changes shall then be captured on a permanent basis by submitting them through the required document control procedures. Specific requirements for HSE are given in each section of the procedures.

Shut down overrides for this system can be invoked only when the system is continuously monitored. Proper procedures and systems should be followed while testing the trips. Where overrides are put in place for testing, these shall be removed at the end of testing so as not to inhibit shutdown functions.

Process Overview

Major Equipment Summary

The main items of equipment within the Gas Dehydration facilities are as follows:

Gas Filter Separator MAK-2210

Glycol Contactor MAF-2215

Gas/Glycol Heat Exchanger HBG-2220

System Summary

The gas dehydration unit is required to condition the gas to meet the following specifications.

Maximum Water Content : 5 lbs./MMSCF at 15.5 C, 1.0 barg

The moisture must be removed to meet the export gas/water dew point specification. The compressed gas from the 2^nd Stage discharge of the Booster Gas Compressors enters the bottom of the Gas Filter Separator (MAK-2210) to remove the bulk of the water contained in the gas. The gas then proceeds to the Glycol Contactor (MAF-2215) which contains structured packing. This packing is designed to promote contact between the gas and the TEG. In the Contactor, gas enters the vessel at the bottom section of the vessel, it is contacted with the lean glycol (contains very little water) which enters at the top, the gas is then in counter-current flow to the TEG. Most of the water is removed from the gas into the glycol. This is because the water has a greater affinity for the glycol than the gas.

The dry gas exits the top of the Contactor passing through the Gas/Glycol Heat Exchanger (HBG-2220) to increase the temperature of the gas and then on to the 3^rd stage BGC. The rich TEG (contains more water) exits at the base of the Contactor and is sent to the TEG (Triethylene glycol) Regeneration Unit for removal of the water and for reuse. See Figure 1 for a diagram of the dehydration system.

Figure Process Flow Diagram of the Gas Dehydration System.

Gas Dehydration

The gas dehydration system function is to remove water in the gas from the 2^nd stage of the booster gas compressors. The water content must be lowered to less than 5 lbs./MMSCF, producing gas suitable for sales / export or for the gas lift system.

Gas Filter Separator (MAK-2210)

The inlet separator removes liquid water and heavy hydrocarbons from the compressed gas. Several factors need to be considered when operating a gas filter separator. The liquid level should be regulated or checked regularly so that plugs or upsets do not result in carryover. This is achieved with a high-high-liquid-level shutdown system and level control valves 10M-LV-2210-1 and 10M-LV-2210-2.

Gas from the 2^nd stage of the booster compressors enters the bottom section of the separator where the fluids are removed due to the different densities of the gas/liquids being handled. The liquids collected flow back to the 2^nd stage suction scrubber of the booster compressors under level control. The gas in the separator moves up the vessel through the baffles to the fibre filter cartridges where particulate and additional liquids are coalesced / removed under a second level control system for the top of the vessel. The gas flow is from the top of the vessel to the Glycol Contactor (MAF-2215).

If the Gas Filter Separator is poorly operated, contaminants may be carried over into the contactor, resulting in the following problems in downstream equipments:

Free liquid water may enter the absorber and overload the glycol in the absorber, which may prevent the gas from being dried to pipeline specifications.

Hydrocarbon contamination of the glycol may cause foaming.

Heavy hydrocarbons may foul the heat exchange surfaces in the reboiler, resulting in poor heat transfer, localized thermal degradation of the glycol, inadequate glycol regeneration etc.

Sodium chloride and calcium chloride may enter the system. Sodium chloride often precipitates in the reboiler; calcium chloride precipitates in the coldest portions of the system such as the absorber. Salt contamination may ultimately necessitate replacement of the glycol.

Equipment Description and Control

The Gas Filter Separator (MAK-2210) is a vertical vessel made of carbon steel with an outer diameter of 610 mm and T/T height of 3050 mm.

Table Operating and Design Conditions for MAK-2210

Pressure Monitoring, Control and ESD Instruments:

The vessel is provided with the following pressure/differential pressure transmitter to alert the operator to blockages over the filters within the separator. The differential pressure high alarm will alert the operator when filter element cleaning or replacement is required.

Table Pressure Monitoring

Process variable from 10M-PIT-2210-15 is used for activation of high, low and low low-pressure alarms. Low low pressure alarm on 10M-PIT-2210-15 will cause bypass of gas dehydration unit (refer to cause and effect matrix). PDAH-2210-14 will alert the operator that there is some blockage in the filter i.e. some cleaning operation is required for the filter in the gas filter separator. PDAH-2210-18 will be used at the start up time when XV-2210-17 will be used for initial pressurization of the system. After equalization of pressure PDAH-2210-18 will disappear and the system to be lined up by opening XV-2210-16.

Level Monitoring, Control and ESD Instruments:

The vessel is provided with the following produced liquid level sight glass, control, alarm and trip instrumentation to aid in monitoring the interface:

Table Level Monitoring

Both 10M-LT-2210-1 and 10M-LT-2210-2 are level transmitters located on the top and bottom sections of the vessel respectively. They modulate control valves 10M-LV-2210-1 and 10M-LV-2210-2, via the respective controllers, to direct the liquid flow out of the vessel to the second stage suction scrubber of the booster gas compressors. Both of these control loops have high and low level alarms used to alert the operator of an upset condition.

The process variables from 10M-LT-2210-3 and 10M-LT-2210-4 are used in the gas separator the process shutdown logic system for activation of both high high and low low-level alarms respectively. High high on 10M-LT-2210-3 will result in by pass of gas dehydration unit, as per the Cause & Effects matrix. Activation of the low low on both the transmitters will result in the closing of the shut down valve 10M-SDV-2210-11, stopping the liquid flow to the second stage booster gas compressors suction scrubber. Refer to section 6.0 for the alarm trip settings.

Temperature Monitoring, Control and ESD Instruments:

The vessel is provided with the following local temperature indicator used to determine the temperature of the gas out into the contactor:

Table Temperature Monitoring

Process variable from temperature transmitter 10M-TT-2210-8 will activate for temperature alarm low and high in the gas outlet line of gas filter separator and it is also used for differential temperature indication and alarm between glycol and gas inlet to contactor on TDI-2210-8.

Over-Pressure Protection:

Over-pressure protection of the vessel is provided by pressure safety valve 10M-PSV-2210-9 set at 63.8 barg, relieving to the HP flare header. Only one PSV is required to meet the relief capacity of the separator. A PSV set at 29.3 barg, 10M-PSV-2210-12, has been provided for over pressure protection of liquid outlet line of the separator.

Glycol Contactor (MAF-2215)

The Glycol contactor is a packed column where lean glycol is pumped to the top. Gas (from the Gas Filter Separator MAK-2210) rises from the bottom in countercurrent flow with lean glycol. The moisture content in the gas is absorbed by the hygroscopic glycol flowing down from the top. Moisture free gas from the top is directed to Gas/Glycol Heat Exchanger (HBG-2220) for heating, and is sufficiently analyzed for moisture content using the online water Dew Point analyzer AT-2220-5 and sent to the 3^rd stage suction scrubber of booster gas compressor system for further compression and thereafter to the sales gas meters or gas lift system.

The contactor is vertical to allow proper glycol flow with sufficient gas/liquid contact and operates at the pressure of the incoming gas (approximately 57 barg). A vane mist eliminator at the top of the contactor will reduce glycol losses by preventing glycol from being carried out with the dry gas.

Equipment Description and Control

Glycol Contactor (MAF-2215) is a vertical Carbon Steel vessel with an inner diameter of 1067 mm and height (T/T) 8000-mm. The contactor has the following internal features:

Structured Packing

Wire Mesh Mist Eliminator

Glycol Distributor.

Table Operating and Design Conditions for MAF-2215

Pressure Monitoring, Control and ESD Instruments:

The vessel is provided with the following differential pressure transmitter to alert the operator to blockages over the structured packing within the contactor. The high alarm will alert the operator when cleaning of the structured packing is required, or the gas velocity across the packing is too high.

Table MAF-2215 Pressure Monitoring

Level Monitoring, Control and ESD Instruments:

The vessel is provided with the following glycol level sight glass, control, alarm and trip instrumentation:

Table MAF-2215 Level Monitoring

10M-LT-2215-4 is a level transmitter located at the base of the vessel. It modulates control valve 10M-LV-2215-4, via controller 10M-LIC-2215-4, to direct the rich glycol flow out of the vessel to TEG regeneration unit. Both high and low level alarms are installed to alert the operator to of the upset conditions within the vessel.

The process variable from 10M-LT-2215-2 is used in the process shutdown logic system for activation of both high high and low low-level alarms. Activation of the high high will result in a package shutdown, where the lean glycol pumps will be stopped and bypass of the gas dehydration system will be done. The low low alarm will result in the closure of shutdown valve 10M-SDV-2215-1, as per cause and effect matrix.

Refer to section 6.0 for the alarm trip settings.

Over-Pressure Protection:

Over-pressure protection of the vessel is provided by pressure safety valve on gas inlet line10M-PSV-2215-6 set at 63.78 barg, relieving to the HP flare header. Only one PSV is required to meet the relief capacity of the contactor.

Gas/Glycol Heat Exchanger (HBG-2220)

This is a shell and tube type heat exchanger using the exiting compressed and dehydrated gas to further cool the lean glycol, being pumped from the regeneration package, before entering the contactor. The temperature of the lean glycol leaving the gas/glycol heat exchanger should be about 5 to 10ºC higher than the dry product gas temperature to prevent hydrocarbons from condensing in the contactor. A minimum lean glycol temperature of 27ºC should be maintained to avoid excessive absorption of natural gas components.

Equipment Description and Control

Gas/Glycol Heat Exchanger (HBG-2220) is a shell and tube heat exchanger constructed from carbon steel.

Table Operating and Design Conditions for HBG-2220

Temperature Monitoring, Control and ESD Instruments:

The heat exchanger is provided with the following temperature, control, alarm and trip instrumentation:

Table HBG-2220 Temperature Monitoring

The outlet line stream temperature for both the shell and tube are monitored in the control room through the use of the transmitters and indicators. It is important to monitor the temperatures for all inlets and outlets of the heat exchanger to determine when the heat transfer has decreased and cleaning is required. As previously mentioned it is also important to monitor the lean glycol temperature entering the contactor to ensure there is minimal absorption of contaminants. Differential temperature indicator provided between the lean glycol inlet to the contactor and the gas inlet to the contactor to ensure that there are sufficient temperature differences between these two streams to prevent hydrocarbons from condensing in the contactor.

Dew Point Monitoring

The moisture content of the gas must be less than 5 lbs/MMSCF to be suitable for export. A moisture dew point analyzer and indicator have been installed on the tube outlet of the heat exchanger to determine the dew point. A high alarm is installed on the indicator to alert the operator to a high water content. The following instrumentation is available.

Table HBG-2220 Dew Point Monitoring

Flow Monitoring

In the gas/glycol heat exchanger (HBG-2220) the lean TEG inlet line to HBG-2220 provided with a flow element FE-2215-10 to monitor the TEG inlet flow to the glycol contactor through gas/glycol heat exchanger.

Table Flow Monitoring

START UP, OPERATING & SHUTDOWN PROCEDURES - Gas Dehydration system

Cold Start-up Procedure Following a Planned Shutdown.

The following procedure details the steps to be undertaken to start-up the Gas Dehydration system following maintenance activities that may have involved piping intrusion works on part / all of the systems / subsystems.

This start-up procedure assumes that the system has been purged of all oxygen with an inert gas and that no explosive mixture exists within the system. Failure to do so may expose the Facility / personnel to unnecessary risks.

The following conditions are assumed to exist in executing this procedure: -

No Permits to Work are in force on any part of the system that may prohibit, or hinder, the safe start-up of the equipment.

The Fire and Gas Detection System, ESD Systems and control facilities within the CCR are operational including all alarms, trips and controllers.

All vents, purge points and manual sample points are closed and blanked, or capped off, where necessary.

Ancillary system requirements

The following ancillary systems must be available for operation prior to commencement of this procedure: -

Power is available to the skid

The instrument air system must be in operation.

The flash and booster gas compressors must be ready for operation.

The fuel gas supply system must be operational.

Flare and separation facilities are operational.

Glycol regeneration system must be ready for operation

Cold Start-up Valve Checklist

Refer to the P&ID 's identified as pertaining to the relevant system whilst carrying out this procedure.

The system operator should check the system valves are in the positions detailed below.

Where valves are found to be out of position, the operator must satisfy himself it is safe to move the valve before doing so. This may involve referencing to an Isolation Permit or relevant Permit to Work.

Table Cold Start-up Valve Checklist

Cold Start-up Overrides

During the cold Start-up of the Gas Dehydration system the following inputs to the shutdown system may require to be overridden at the Shutdown Panel in the Central Control Room until steady operating conditions are established:

Table Cold Start-up Overrides

Cold Start-up Procedure

Note: This procedure assumes that the system has had all oxygen removed and is under nitrogen blanket, which has been confirmed with manual measurements. Do not proceed with this procedure if this is not the case due to explosion hazards and degradation of the TEG in the presence of oxygen.

Table Cold Start-up Procedure Following a Planned Shutdown

Hot Start-up Procedure Following a Process Trip

The following hot start-up procedure details the steps to be taken to re-start the Gas Dehydration system facilities following a system trip.

The procedure assumes all the facilities listed below have been operating normally but have shutdown as a result of a system trip or by manual initiation.

The reason for the shutdown has been investigated and rectified and it is considered safe to re-start the system.

The procedure assumes that the system is still O₂ free and that an explosive mixture has not accumulated within the system / piping.

The levels, pressures and temperatures within the Gas Dehydration system are at or near normal conditions.

The following conditions are assumed to exist in executing this procedure: -

No Permits to Work are in force on any part of the system that may prohibit, or hinder, the safe start-up of the equipment.

The Fire and Gas Detection System, ESD Systems and control facilities within the CCR are operational including all alarms, trips and controllers.

It is assumed that in a short duration shutdown it is unlikely that spades or blinds would be moved, or drain and vent valves opened unless required to take corrective action on the cause of the shutdown. It is therefore unnecessary to recheck these items other than if they have been disturbed.

All vents, purge points and manual sample points are closed and blanked, or capped off, where necessary.

Communications have been established between all the personnel involved with the start-up procedure.

Safety/Briefing Meetings have been held with all directly concerned parties outlining the workscope and procedures.

The following Utility and Ancillary Systems are available:

Electrical Power Distribution

Instrument Air

Flash and boost gas compressors

Fuel gas system

Table Hot Start-up Valve Checklist

Hot Start-up Procedure

During the Hot Start-up of the Gas Dehydration System there should be no requirement to have any input over-ride to the to the shutdown system. Note 1 - Dependant upon the pressure remaining within the system, there may be the requirement to over-ride the TEG circulation pumps PALL 2325A/B-2 to enable a pump re-start. This will depend on the length of time that the pumps are shutdown, and at what trip point this switch is set at.

Note-2: This procedure assumes that the temperature of the system has not dropped significantly. It also assumes that the pressure in the system has not been lost through the opening of the blow down valve. This valve will open on an ESD and PSD in the event that this occurs the system will have to be repressurized following the cold start-up procedure.

Table Hot Start-up Procedure Following a System Trip

Normal Operational Checks:

The normal operational conditions, together with alarm and trip set points for each item in the Gas Dehydration are provided in section 6.0.

On a regular basis the field operator should conduct the following routine checks:

Regularly line walk the system to visually inspect for leaks and noise from rotating equipment and other defects in the equipment such as leaking control valves and filter housings etc.- particular attention should be paid to any abnormal noise coming from the pumps.

Check all levels, pressures and temperatures in the process system.

Periodically conduct cross checks between the field instruments and the respective CCR controller indications to determine the accuracy of the instruments. Remedial action should be taken when significant discrepancies are noted.

Monitor the differential pressure on the Gas Filter Separator (MAK-2210) on a regular basis, high differential pressure indicates that the filter elements are plugged and will require changeup. Open the by-pass line around the vessel if filter changeup is required whist the dehydration system is still online, if this is permissible under the Isolation policy. If not, then the dehydration system will require shutdown / depressurization / isolation to change the filter elements.

TEG CONDITION - Take a sample of rich and lean glycol once a week and conduct the following testing:

o        Examine for finely divided precipitate. This normally contains iron products of corrosion (FeS and Fe₃O₄). This can lead to substantial operating problems within pumps and reboilers if allowed to accumulate to any degree. Good filtration practices should adequately cope with this problem.

o        Two-phase separation for condensate contamination. Possible causes are incorrect operation of production separator, glycol flash drum or carbon filter.

o        Check for thermal degradation. Usually can be determined by visual inspection of the TEG. If the sample appears darker and smells of burnt sugar it is an indication of degradation occurring. This can be the result of excessive reboiler temperatures.

o        Check the TEG for purity - i.e. Lean TEG purity should be ~99% with Reboiler temps of ~204 deg C.

o        Check the TEG pH. The pH should be in the range of 6.0 to 6.2 for rich glycol and between 7.6 to 8.0 for lean glycol. Add M.E.A / T.E.A as required for pH control. High pH can tend the glycol to foam and be carried out the overhead line with the gas phase, resulting in excessive TEG losses. IF pH control chemical is added, it should be added slowly by means of a small dosing pump if possible whilst the TEG is circulating as opposed to a slug dose of the pH buffer.

Duty pump selection is to be reviewed weekly or as required by the Operations Supervisor.

Routine checks should be performed to collect level, pressure, pump deliverability and TEG composition data in order to ensure optimum operation of the glycol circulation pumps and pumping capacity is maintained. Regularly check the in-line suction strainers on the TEG circulation pumps for plugging. Low delivery pressure / rates can be caused by the suction strainer being fouled.

Check the glycol circulation pumps, pumping elements for leakage. Maintain the pump / splined drive coupling frame space for the correct lubrication level on a routine basis.

CONTACTOR - The primary operating problems associated with the contactor are as follows:
Insufficient Dehydration - Resulting in out of spec gas, in this case the lean TEG flow must be reviewed.
Foaming - Resulting in loss of TEG with the vapor stream. High gas velocities and chemical contamination can result in this scenario.
Calcium Chloride - Dissolves in the liquid at higher temperatures and precipitates in the cooler areas leading to plugging of the packing, resulting in foaming and glycol losses.

Monitor the Differential pressure across the Contactor as indicated on PDT 2215-5, high differential indicates plugging / fouling of the structured packing section on the vessel. This will require further investigation and possible removal cleaning / replacement of the contactor packing.

Regularly check the temperature difference between the gas inlet to the contactor and the lean glycol inlet to the contactor through TDI-2210-8 to ensure that glycol inlet temperature 5°C-10°C higher than that of gas inlet temperature. This will prevent any hydrocarbon condensation from the gas in the glycol contactor.

Planned Shutdown procedure

The following procedure details the steps to be taken to conduct a planned shutdown of the Gas Dehydration System as part of an overall shutdown of the FPSO process facilities in preparation for inspection / maintenance activities.

The following conditions are assumed to exist in executing this procedure: -

Safety / Briefing Meetings have been held with all directly concerned parties outlining the workscope and procedures.

Communications have been established between all persons involved in this procedure.

o        No operations or Permits to Work are in force that may prohibit the shutdown of the system.

Note: When shutting down the gas dehydration system the flash gas and boost compressors should be shut down at the same time, and gas will be sent to the HP Flare. This will leave the dehydration system pressurized and the glycol circulating through the regeneration system.

Table Planned Shutdown Procedure

Emergency Controlled Shutdown Procedure

The following procedure details the steps to be taken to conduct an emergency controlled shutdown of the Gas Dehydration system.

Plant Status -The assumption is that an upset condition has occurred on the process or downstream facilities detected by the automatic shutdown systems, that requires the immediate controlled shutdown of the Gas Dehydration system by the ESD system. There should be no cause for intervention by operators.

Table 18 Emergency Controlled Shutdown Procedure

Alarm, Trip and PSV Settings

Table Alarm, Trip & PSV Settings

Valve List

Table 20 Valve List

Acknowledgements

With my signature I do hereby acknowledge that I have read and understood the preceding document.