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Dynamic Simulation as a Tool in the Design of an Offshore Gas Compression System
Copvri ghl © I F.-I. C I (jlh Triennial \I'orld Con " ress . ~Iuni c h . FRG. 19B,
DYNAMIC SIMULATION AS A TOOL IN THE DESIGN OF AN OFFSHORE GAS COMPRESSION SYSTEM K. Solemslie* and T. M. Bradley** "Cu ll/iJii.1 '. ~ ' Plfltjimll. 500 1 Bl'I'gl'll . S or"'!I\' "';' SACS Ltd. Hll llic CI'II I1'1' . ( ;rml \1'" ,,1 Road. Hrrll l/iml . :Hiddlt',I/,x. Cl-:
Abstract. Den norske stats oljeselskap a.s. (Statoil) commissioned Special Analysis & Computing Services (SACS) to perform a number of extensive dynamic simulation studies of the Statoil Gullfaks 'A' natural gas compression, re-injection and associated control systems during the initial and final design stages of the project. The simulation study addressed the behaviour and application of the control systems to the turbo compressor units with regard to the process operability and flexibility. In addition, the work also addressed system behaviour during start-up shutdown and after equipment failure. The interrelationship between the control systems and the compression units which dictate the overall process behaviour was shown and recommendations made about the process control systems to ensure satisfactory operation and provide operational flexibility. The study generated advanced knowledge of the plant behaviour for a number of design options and resulted in recommendations and information which will benefit the commissioning team and operations personnel. Keywords. Computer aided system design, control system analysis, machinery evaluation, natural gas technology, controllability.
modelling,
turbo
effect of the curvature of compressor aerodynamic performance characteristics on system controlability and operational flexibility was undertaken. The objectives of the study were to investigate two sets of manufacturers predicted performance curves and to assess if limitations on the process flexibility would be imposed by the characteristics, one set being ·shallow· the other set being relatively steep.
INTRODUCTION The Gullfaks 'A' Platform is located in block 34/10 in the Norweg ian sector of the North Sea, Southeast of the Statfjord field. It is designed to receive and process crude from its own wells and partially stabilised oil and dehydrated gas from Gullfaks 'B' Platform to specifications suitable for export or reinjection. This paper is based upon the findings of simulation studies of the gas separation, recompression, dehydration and re-injection facilities of the Statoil Gullfaks 'A' Platform. The studies performed were extensive and as such only a portion of the results have been included. The use of dynamic simulation as a mechanism for turbo machinery evaluation and process control select ion is descr ibed together with a more detailed account of the application of the control systems to the process.
A limited dynamic simulation model was used for the purposes of the investigation and subjected to a range of flow disturbances. The results of the study indicated that only small differences of operation would be brought about due to the differences in curvature. The ability of both sets of curves to provide the necessary flexibility was due to the matching of compressor s tage performance characteristics to one another. First Dynamic Simulation Studies During the detailed engineering phase of the project a high fidelity dynamic simulation model of the major topside process and instrumentation systems was built wtth, as a basis, the predicted aerodynamic performance curves of the compressors. The results of the work performed are not detailed here since they are covered, or were superceded by the final study.
OVERVIEW OF WORK PERFORMED Dynamic simulation has been used in three phases of the Gullfaks 'A' Project. compressor initial screening of curves (pre aerodynamic performance engineering phase) full scale dynamic compressor simulation based on predicted performanc
One area, independent of compressor characteristics, was only analysed during the first study and this is detailed below. The dynamic simulation model was used as a mechanism to investigate the behaviour of two alternate control schemes around the gas receiver. The control schemes proposed were a single pressure control system and a pressure flow cascade loop. The objective was to evaluate the capability of the system to deal with fluctuations in flow rates from Platform 'B', in particular those caused by ·slug flow·. The criteria used in the evaluation were:
All the studies were performed for two different design cases reflecting different schemes for developing phase two of the Gullfaks Field. Design case I with lower flow and lighter gas than design case 11. Compressor Performance Sensitivity Analysis Our ing the pre eng ineer ing phase of the Gullfaks 'A' Project a sensitivity analysis regarding the
minimisation of the rate of change of
3R l
K. So/emslie and T . \1. Bradle\ dehydrator pressure. minimisation of the absolute change of dehydrator pressure. prevention of flaring from the first stage separators. The study showed that the implementation of a properly tuned pressure to flow cacade control system combined with minimum pressure protection provided the most satisfactory method (refer to Fig. 2). Controller tuning parameters were generated for the control system by the imposition of severe, to the point of being unrealistic, process disturbances and these parameters were provided as starting values for use during initial plant start-up. Final Dynamic Simulation Studies A comprehensive simulation analysis of the Gu11faks 'A' facilities using as a basis the tested compressor performance curves was performed in the construction phase of the project. The high fidelity model previously built was updated to incorporate the revised compressor character istics and a number of minor design changes. The results of the study are discussed in the subsequent sections of this document. RECOMPRESSION STUDi Scope and Objectives The final objectives
dynamic
simulation
had
four
major
determine the operational flexibility and stability of the system verify the adequacy of the process and protective controls generate an initial set of instrument calibration and setting data to ease the commissioning of the system develop a safe and controlled method for initial start-up of the compressor To obtain these objectives the model was subjected to a variety of normal and abnormal operating conditions. These included steady state operation, changes in feed flow, compressor start up and shut down and failure of equipment. Process and Control System Description The recompression system consists of two 100% trains with individual coolers and scrubbers. Each compression train takes gas from both 50% crude separation trains as well as from the 'B' platform. Each of the two 100% gas recompression trains consists of four centrifugal compressor stages driven on a single shaft by a variable speed gas turbine. The compressors are designed to compress the total associated gas to a pressure suitable for export down the sales gas pipeline and / or for feed gas to the re-injection compressors. Gas from the third stage separator is cooled, compressed and enters the suction of the second stage recompressor at approximately five barg. The second stage discharge combines with the off gas from the second stage separators. This gas is then cooled and enters the suction of the third stage compressor at approximately seventeen barg. The third stage compressor discharge gas combines with the associated gas from the first stage separators. The mixture is cooled then dehydrated by contact with triethy1ene glycol to meet the required water dew point specification. The gas can then, depending upon the mode of operation, be combined with 'B' platform gas via the gas
receiver (refer to Fig. 2). The total gas flow enters the suction of the pipeline recompressor, its discharge gas, after cooling, being at a pressure suitable for export or suction for the reinjection system. The protective control systems comprise anti surge controls for each stage based on the "flow delta pressure" system with flow measurement located at compression suction protection of first stage compressor against vacuum by means of low pressure override on the recycle valve The process control is based on pressure control on each level of separation. Due to the low pressure in the third stage separator the pressure control valve PV-10 for this separator second stage is located between first and compressors (refer to Fig. 1). Discharge pressure from the compressor system is controlled by a control valve in the export pipeline PV-30. The gas coming from the Gullfaks 'B' platform is controlled by a pressure to flow cascade system at the outlet of the gas receiver, high pipeline recompressor suction pressure control is provided by override onto the outlet valve (PIC-25), featuring a tracking capability and a latch on the switch to pipeline recompressor suction control. Low pressure protection in the receiver is afforded by means of recycled gas from the discharge of the pipeline recompressor. Pi?e1ine recompressor suction pressure is maintained by a pressure controller, PIC-20 that generates an external set point to the speed governor. Increasing pressure at the pipeline recompressor suction is compensated for by an increase of recompression train speed which increases the forward flow capability of the compressors. Controller PIC-20 may be overridden by high pipeline recompressor discharge pressure controller PIC-35. This controller has tracking capability and a latching action on the switch to override control. Its action is required to prevent the recompression train from being accelerated to maximum speed by PIC-20 during an inba1ance between the available gas supply and demand, for example when the gas produced from the separation trains is in excess of the amount that can be forwarded by the export pipeline and/or the re- inj ect ion compressors. In such a condition the pipeline recompressor discharge pressure will increase and promote "latching" of PIC-35. Once latched, the set point of PIC-35 is equal to the set point of PIC-30. Latching results in a potentially unstable and unsatisfactory mode of control, that is, two controllers PIC-35 and PIC-30 attempting to control a common pressure to the same value. However, an inbalance in ga s supply and demand as described here can only be caused by failure in other parts of the platform or pipeline control system. The latch is therefore introduced to ensure operator type intervention to take corrective action. Recompression Operability
System
Process
Flexibility
and
The speed of the recompression train is dictated by the setpoint of pipeline r ecompressor suction and discharge pressure controllers and by the flow rate of gas across this machine. Also, at low flow rates, the location of the anti-surge control line. The first three stages of recompression ar e required to forward all gas off
Offsho re Gas Compressio n System the second and third stage separators at this dictated speed. They must therefore be able to generate sufficient pressure ratio to meet the set pipeline recompressor suction pressure. If extra pressure making capability is available at this speed the first stage recompressor discharge valve PV-10 will throttle, destroying some of this pressure making ability. However, during the course of the simulation study it was found that this extra pressure making capability was not always present in the system. During operation with design case I flow rates it was not found possible to control the system with first stage recompressor discharge throttle valve PV-10 at the speed determined by the or ig inal design pressure controller set points and with the adopted anti-surge control settings. This was because at this speed the machine did not make adequate pressure ratio and PV-10 was full open, thus third stage separator pressure was "off control". This separator pressure could therefore move between its normal operating and flare controller set points, demonstrating a potential that exists in the system for the pressure profile to slowly oscillate. When PV-10 moves "off control" the first three stages of recompression dictate the third stage separator pressure. This pressure is only permitted to rise to the controller set point at which gas is released to flare. In this circumstance if the first three stages of recompression are already operating in partial recycle, loss of forward flow to flare has no influence upon the pressure ratio making capability of these compressors. If the compressors are operating out of recycle, flaring of gas will cause the compressors to "climb" up their curves until either they make enough differential pressure to get back "on control" or go into recycle. In this condition the flare set point of third stage separator via the pressure ratio dictated by the speed of the machine determines the third stage recompressor discharge pressure maximum value. If this pressure is below the set pipeline recompressor suction pressure the first three stages of recompression "check in" and cease to forward flow. This is an irreversible or "lock in" situation. Operator intervention is then required to re-establish forward flow. This can be accomplished in a number of ways, for example, by either reducing the set pipeline recompressor suction pressure or by manually increasing the recompression train speed but essentially a speed increase is necessary. The model showed sensitivity of the process and its control systems to: changes in pipeline recompressor suction pcessure and therefore dehydrator pressure. Small changes in the set point of PIC-20 have a significant impact on recompression train speed and thus upon the ability of the first three stages of recompression to forward flow. changes of pipeline recompressor discharge pressure. Changes in the set point of PIC-30 affect recompression train speed and system operability. The system pressure profile is more sensitive to changes in pipeline recompressor suction pressure than to changes in the set point of PIC-30. changes in anti-surge control line location. The system waS found to be very sensitive to changes in the location of the control line. This affects the (maximum) pressure ratio making capability
of the recompressors in partial recycle. The most critical factor in determining the above behaviour is essentially the relationship between compressor speed and the overall pressure ratio making capability of stage one to three. This relationship is, of course, influenced by a number of other parameters besides those described above. These include the following: gas composition. Deviation from the predicted separator gas molecular weights, and in particular "lightening" of these streams will have signif icant impact upon the pressure ratio making capability of the individual compressors stages. gas physical properties. Compressor performance is, for example, very sensitive to inaccuracies in the predicted value of compressibility factors. unequal deterioration of compressor aerodynamic performance with time and therefore potential differences in pressure ratio making capability between stages. the amount of gas leakage between the inter stage diaphragm of the second and third stage recompressors. pressure losses and their possible increase with time, in particular the pressure drop through items of equipment such as heat exchangers and scrubbers. In view of the above observations and comments it was concluded that the set points of the pipeline recompressor suction and discharge pressure controllers PIC-20 and PIC-30 must be manipulated, within a defined set of constraints to enable the process to substantially improve its flexibility to forward a range of feedstocks and gas flow rates. Lowering of both these controller set points provides an additional degree of latitude in permitted perturbations from these set points in relation to the settings of overr ide controllers PIC-25 at the dehydrator and PIC-35 at the pipeline recompressor discharge. An additional real constraint is also present in the system, related to the maximum permitted continuous operating speed of the machine. Pipeline recompressor suction and discharge pressures should be set (if possible) to obtain the maximum margin of running speed below this figure consistent with maintaining PV-10 "on control". Start-up Analyses The design of the supervisory compressor control system included an automatic start-up sequence taking the compressors from minimum governor speed to full forward flow. The intention was that this could ait;o be used for automatic load transfer from one machine to the other. The dynamic model was used to simulate this start-up sequence. It was found that the transfer from full recycle to forward flow occurred within a very short period for all stages (150 rpm and 30 seconds). This resulted in "every thing happening at once" making it impossible for the operator to follow and take corrective action. After discussions between the engineers and operating personnel involved in the study it was decided to reject the completely automatic start-up. The supervisory control system was then modified to give only manual speed increase through the sensitive area. The start-up was the shown to work satisfactorily. This modification also eliminated the possibility
K. Solc mslie a nd 1'. 1\1. Bradle\ of automatic load transfer between the two machines. Based on the simulations a procedure for manual load transfer was therefore developed. Based on engineering and operating experience a "commissioning" start-up method was developed by members of the group involved with the studies. The simulation model was used to confirm the method and assist in the preparation of a separate step-by-step commissioning document. The dynamic model was able to demonstrate start-up of the compressor from rest and ensure that the proposed method provided: step-by-step method of a controlled introducing gas stream to the compression system a simple means of "packing" the pipeline via the pipeline stage of compression by using gas from the first stage separator a means of introducing third stage separator gas to the pipeline system in a controlled way whilst first stage separator gas was already being exported a means of manipulating pressure pr of He through the compression system to permit controlled introduction of second stage separator gas. RE-INJECTION COMPRESSOR STUDY Scope and Objectives The scope and objectives of this work were originally the same as those for the recompressor study. However at the start of the work suction throttle valves were introduced into the system. This added to the scope. The simulation model was used as a engineering design tool as well as a design verifier. To achieve these objectives and ver ify the design the start-up analyses were performed as an integrated part of the flexibility and operability studies. Process and Control System Description The re-injection system is installed to be used during pipeline shut downs and for modulation of gas export volumes as required by the buyers. It consists of two 50% centrifugal compressors in parallel each having a constant speed electric motor driver. The compressors have common suction coolers and scrubbers and take suction from the pipeline compressor discharge. The original proposed protective shown in Fig. 3 consisted of:
c o ntrols
as
each valves for separate anti-surge the "flow delta compressor based on pressure" system . minimum compressor suction pressure protection by modulation of well head chokes (PIC-42). pressure protection maximum discharge overr iding suction throttle valve control (PIC-44) . The process control comprised pipeline recompressor discharge pressure control acting on suction throttle valves incorporating flow balancing facilities between the compressors (PIC-40 and FIC-41). This leaves the operator with two alternative controls for the pipeline recompressor discharge pressure (PIC-30 and PIC-40). The operating mode will determine which one is used. Only one of the controllers can be in automatic at one time.
Re-injection Operability
System
Process
Flexibility
and
Initial analyses showed that the proposed control systems were not capable to handle even minor flow disturbances. This was due to the fact that there was no way that the low suction protection controller, PIC-42, could achieve its purpose independent of whether it should open or close choke valves upon low re-injection suction pressure. This can be seen from the following hypotheses. I f on loss of gas availability to the Re-injection system opening of choke valves by PIC-42 will lead to: lowering of system discharge pressure. For an essentially fixed pressure ratio at the reduced flow rate, this lower discharge pressure must therefore result in a further decrease in suction pressure. Closure of choke valves by PIC-42 will lead to: increase of discharge pressure this discharge pressure increase activates high discharge pressure override controller PIC-44 and the suction throttle valve will be closed by that controller action, leading to a further decrease in suction pressure. Further reduction of gas flow will lead to further closure of the choke valves, further discharge pressure control action etc. This is a "lock-in situation" due to the interaction of the two controllers PIC-42 and PIC-44. Additionally when PIC-44 overr ides the suction throttle valves, PIC-40 (pipeline recompressor discharge pressure control) goes "off control". Of secondary importance is the slow movement of the chokes, which is not able to provide adequate pressure protection at the suction of the compressors. To solve this problem the engineers involved proposed to introduce the action of PIC-42 onto the recycle valves via a low select mechanism. This modified control system was the basis of a large number of simulation analyses the results of which indicated that: extremely limited operational flexibility existed. A very small 'operating window' was allowed for the compressor operating point. with the developed control systems, no practical operator implemented method was available for re-injection machine start-up and establishment of forward flow. This was brought about by the simultaneous imposition of a low suction pressure limitation, due to seal oil system characteristics, and a high discharge pressure limitation dictated by the injection 'Xmas Tree' . Several ways out of these problems were evaluated. The easiest being a reduction of the minimum allowable seal oil pressure. Subsequent to a series of meetings with the compressor manufacturer a revised minimum pressure for continuous operation of 100 barg was negotiated. Consequently, this revised setting was incorporated into the simulation model and its impact upon Re-injection start-up investigated. A low suctio n pressure override controller PIC-42 set point of 120 barg was adopted as this pressure represents the predicted gas cricondenbar. Operation at the compressor suction above this pressure is, therefore, free from liquids . The impact of this revised minimum pressure was considerable, permitting satisfactory compressor start-up. The results of
the start-up sequences investigated indicate that significant increased operational flexibility is introduced into the circuit. Manipulation of the suction throttle valves by PIC-40 enables automatic single and parallel compressor operation over a range of re-injection flows from zero to compressor design rates. The final control systems adopted are as follows: anti-surge control as originally designed pipeline recompressor discharge pressure control on suction throttle valves with combined flow balance acting between compressors (PIC-40 and FIC-41). minimum compressor 5uction pressure protection by modulation of both recycle valves simultaneously (PIC-42) maximum discharge pressure protection overr iding suction throttle valve control (PIC-44) The ability of the re-injection system to cope with a wide range of flows hinges around the ability of the suction throttle valve to modulate. This valve is required to have a mechanical minimum stop to prevent "choking" of the gas recycle loop. A stop setting of 20% valve lift (corresponding to 6% of the valve maximum capacity) was found suitable in the simulation study. The mechanical stop must be reviewed during plant start-up as the setti~g depends upon the minimum operating discharge pressure of the pipeline recompressor and the character ist ics of the inject ion wells. Ideally, the setting should be such that the discharge pressure of each re-injection compressor is, at the valve minimum stop and full recycle condition, below the static well head pressure. Depending upon the characteristics of the injection wells, the injection choke valves should be operated fully open. This maximises the "operating window" of the system and also reduces the probability of activation of high re-injection compressor discharge pressure override controller PIC-44. The choice of pipeline recompressor discharge pressure control (PIC-30 and PIC-40) will be decided by operating experience. The easiest operating stategy in a combined export/injection situation would be to set a fixed export volume and let the re-injection system handle fluctuations in the gas supply. The feas ibi li ty of this strategy hinges on the ability of the suction throttle valves to remain on control, that is between full open and minimum stop. Based on the final control system and the reduced suction pressure, it was possible to develop a step-by-step start-up and commissioning procedure and to demonstrate these with the simulation model. CONCLUSIONS The use of dynamic simulation showed itself to be a very valuable engineering tool in the development of the control systems for the recompression and re-injection compressors - both as a control and modifier of existing design - and in developing new control systems. The control of a complex compression system as on Gullfaks 'A' involves so many interdependable variables that it will never be possible for the human mind to reason through all consequences of a particular disturbar.ce put to the system. By performing a comprehensive series of simulations, introducing different disturbances and even trips of the machine, the possibilities and
restrictions built into the system have been demonstrated. Where required, modifications have been made to enhance the flexibility of the control systems. The analyses have shown that the protection system for the compressors function properly. A gradual optimisation of controller tuning parameters has been obtained. This work would normally have to be performed on the equipment itself during commissioning of the systems, with the possibility of damaging the compressor or other parts of the system. Some of the disturbances will perhaps never occur, or possibly not occur for several years as they represent such severe failures. The capability of the plant and the optimum tuning of the controller parameters to handle these disturbances would then only be known after they occur the first time. The simulation analyses concerned with the normal and commissioning start-up methods have given a "proven" method of first starting up the compressors. At the time of writing, it is therefore our hope and belief that the combination of a well defined starting point for the controller tuning parameters and a tested start-up method will give a much more "painless" commissioning of the compression system than normally expected. Another main experience gained is the requirement for close cooperation between different types of specialists during the work. Compressor systems are complex "individuals", who seem to have a will of their own sometimes. A composite group of personnel consisting of mechanical, operat ional, control and process specialists must be involved. It is then possible to go through the results as they are generated, evaluate problems, and agree upon solutions. This makes the simulation work like a dynamic process. Agreed changes are implemented as they are proven required and correct, instead of waiting for the final recommendations.
DYNAMIC SIMULATION AS A TOOL IN THE DESIGN OF AN OFFSHORE GAS COMPRESSION SYSTEM K. Solemslie* and T. M. Bradley** "Cu ll/iJii.1 '. ~ ' Plfltjimll. 500 1 Bl'I'gl'll . S or"'!I\' "';' SACS Ltd. Hll llic CI'II I1'1' . ( ;rml \1'" ,,1 Road. Hrrll l/iml . :Hiddlt',I/,x. Cl-:
Abstract. Den norske stats oljeselskap a.s. (Statoil) commissioned Special Analysis & Computing Services (SACS) to perform a number of extensive dynamic simulation studies of the Statoil Gullfaks 'A' natural gas compression, re-injection and associated control systems during the initial and final design stages of the project. The simulation study addressed the behaviour and application of the control systems to the turbo compressor units with regard to the process operability and flexibility. In addition, the work also addressed system behaviour during start-up shutdown and after equipment failure. The interrelationship between the control systems and the compression units which dictate the overall process behaviour was shown and recommendations made about the process control systems to ensure satisfactory operation and provide operational flexibility. The study generated advanced knowledge of the plant behaviour for a number of design options and resulted in recommendations and information which will benefit the commissioning team and operations personnel. Keywords. Computer aided system design, control system analysis, machinery evaluation, natural gas technology, controllability.
modelling,
turbo
effect of the curvature of compressor aerodynamic performance characteristics on system controlability and operational flexibility was undertaken. The objectives of the study were to investigate two sets of manufacturers predicted performance curves and to assess if limitations on the process flexibility would be imposed by the characteristics, one set being ·shallow· the other set being relatively steep.
INTRODUCTION The Gullfaks 'A' Platform is located in block 34/10 in the Norweg ian sector of the North Sea, Southeast of the Statfjord field. It is designed to receive and process crude from its own wells and partially stabilised oil and dehydrated gas from Gullfaks 'B' Platform to specifications suitable for export or reinjection. This paper is based upon the findings of simulation studies of the gas separation, recompression, dehydration and re-injection facilities of the Statoil Gullfaks 'A' Platform. The studies performed were extensive and as such only a portion of the results have been included. The use of dynamic simulation as a mechanism for turbo machinery evaluation and process control select ion is descr ibed together with a more detailed account of the application of the control systems to the process.
A limited dynamic simulation model was used for the purposes of the investigation and subjected to a range of flow disturbances. The results of the study indicated that only small differences of operation would be brought about due to the differences in curvature. The ability of both sets of curves to provide the necessary flexibility was due to the matching of compressor s tage performance characteristics to one another. First Dynamic Simulation Studies During the detailed engineering phase of the project a high fidelity dynamic simulation model of the major topside process and instrumentation systems was built wtth, as a basis, the predicted aerodynamic performance curves of the compressors. The results of the work performed are not detailed here since they are covered, or were superceded by the final study.
OVERVIEW OF WORK PERFORMED Dynamic simulation has been used in three phases of the Gullfaks 'A' Project. compressor initial screening of curves (pre aerodynamic performance engineering phase) full scale dynamic compressor simulation based on predicted performanc
One area, independent of compressor characteristics, was only analysed during the first study and this is detailed below. The dynamic simulation model was used as a mechanism to investigate the behaviour of two alternate control schemes around the gas receiver. The control schemes proposed were a single pressure control system and a pressure flow cascade loop. The objective was to evaluate the capability of the system to deal with fluctuations in flow rates from Platform 'B', in particular those caused by ·slug flow·. The criteria used in the evaluation were:
All the studies were performed for two different design cases reflecting different schemes for developing phase two of the Gullfaks Field. Design case I with lower flow and lighter gas than design case 11. Compressor Performance Sensitivity Analysis Our ing the pre eng ineer ing phase of the Gullfaks 'A' Project a sensitivity analysis regarding the
minimisation of the rate of change of
3R l
K. So/emslie and T . \1. Bradle\ dehydrator pressure. minimisation of the absolute change of dehydrator pressure. prevention of flaring from the first stage separators. The study showed that the implementation of a properly tuned pressure to flow cacade control system combined with minimum pressure protection provided the most satisfactory method (refer to Fig. 2). Controller tuning parameters were generated for the control system by the imposition of severe, to the point of being unrealistic, process disturbances and these parameters were provided as starting values for use during initial plant start-up. Final Dynamic Simulation Studies A comprehensive simulation analysis of the Gu11faks 'A' facilities using as a basis the tested compressor performance curves was performed in the construction phase of the project. The high fidelity model previously built was updated to incorporate the revised compressor character istics and a number of minor design changes. The results of the study are discussed in the subsequent sections of this document. RECOMPRESSION STUDi Scope and Objectives The final objectives
dynamic
simulation
had
four
major
determine the operational flexibility and stability of the system verify the adequacy of the process and protective controls generate an initial set of instrument calibration and setting data to ease the commissioning of the system develop a safe and controlled method for initial start-up of the compressor To obtain these objectives the model was subjected to a variety of normal and abnormal operating conditions. These included steady state operation, changes in feed flow, compressor start up and shut down and failure of equipment. Process and Control System Description The recompression system consists of two 100% trains with individual coolers and scrubbers. Each compression train takes gas from both 50% crude separation trains as well as from the 'B' platform. Each of the two 100% gas recompression trains consists of four centrifugal compressor stages driven on a single shaft by a variable speed gas turbine. The compressors are designed to compress the total associated gas to a pressure suitable for export down the sales gas pipeline and / or for feed gas to the re-injection compressors. Gas from the third stage separator is cooled, compressed and enters the suction of the second stage recompressor at approximately five barg. The second stage discharge combines with the off gas from the second stage separators. This gas is then cooled and enters the suction of the third stage compressor at approximately seventeen barg. The third stage compressor discharge gas combines with the associated gas from the first stage separators. The mixture is cooled then dehydrated by contact with triethy1ene glycol to meet the required water dew point specification. The gas can then, depending upon the mode of operation, be combined with 'B' platform gas via the gas
receiver (refer to Fig. 2). The total gas flow enters the suction of the pipeline recompressor, its discharge gas, after cooling, being at a pressure suitable for export or suction for the reinjection system. The protective control systems comprise anti surge controls for each stage based on the "flow delta pressure" system with flow measurement located at compression suction protection of first stage compressor against vacuum by means of low pressure override on the recycle valve The process control is based on pressure control on each level of separation. Due to the low pressure in the third stage separator the pressure control valve PV-10 for this separator second stage is located between first and compressors (refer to Fig. 1). Discharge pressure from the compressor system is controlled by a control valve in the export pipeline PV-30. The gas coming from the Gullfaks 'B' platform is controlled by a pressure to flow cascade system at the outlet of the gas receiver, high pipeline recompressor suction pressure control is provided by override onto the outlet valve (PIC-25), featuring a tracking capability and a latch on the switch to pipeline recompressor suction control. Low pressure protection in the receiver is afforded by means of recycled gas from the discharge of the pipeline recompressor. Pi?e1ine recompressor suction pressure is maintained by a pressure controller, PIC-20 that generates an external set point to the speed governor. Increasing pressure at the pipeline recompressor suction is compensated for by an increase of recompression train speed which increases the forward flow capability of the compressors. Controller PIC-20 may be overridden by high pipeline recompressor discharge pressure controller PIC-35. This controller has tracking capability and a latching action on the switch to override control. Its action is required to prevent the recompression train from being accelerated to maximum speed by PIC-20 during an inba1ance between the available gas supply and demand, for example when the gas produced from the separation trains is in excess of the amount that can be forwarded by the export pipeline and/or the re- inj ect ion compressors. In such a condition the pipeline recompressor discharge pressure will increase and promote "latching" of PIC-35. Once latched, the set point of PIC-35 is equal to the set point of PIC-30. Latching results in a potentially unstable and unsatisfactory mode of control, that is, two controllers PIC-35 and PIC-30 attempting to control a common pressure to the same value. However, an inbalance in ga s supply and demand as described here can only be caused by failure in other parts of the platform or pipeline control system. The latch is therefore introduced to ensure operator type intervention to take corrective action. Recompression Operability
System
Process
Flexibility
and
The speed of the recompression train is dictated by the setpoint of pipeline r ecompressor suction and discharge pressure controllers and by the flow rate of gas across this machine. Also, at low flow rates, the location of the anti-surge control line. The first three stages of recompression ar e required to forward all gas off
Offsho re Gas Compressio n System the second and third stage separators at this dictated speed. They must therefore be able to generate sufficient pressure ratio to meet the set pipeline recompressor suction pressure. If extra pressure making capability is available at this speed the first stage recompressor discharge valve PV-10 will throttle, destroying some of this pressure making ability. However, during the course of the simulation study it was found that this extra pressure making capability was not always present in the system. During operation with design case I flow rates it was not found possible to control the system with first stage recompressor discharge throttle valve PV-10 at the speed determined by the or ig inal design pressure controller set points and with the adopted anti-surge control settings. This was because at this speed the machine did not make adequate pressure ratio and PV-10 was full open, thus third stage separator pressure was "off control". This separator pressure could therefore move between its normal operating and flare controller set points, demonstrating a potential that exists in the system for the pressure profile to slowly oscillate. When PV-10 moves "off control" the first three stages of recompression dictate the third stage separator pressure. This pressure is only permitted to rise to the controller set point at which gas is released to flare. In this circumstance if the first three stages of recompression are already operating in partial recycle, loss of forward flow to flare has no influence upon the pressure ratio making capability of these compressors. If the compressors are operating out of recycle, flaring of gas will cause the compressors to "climb" up their curves until either they make enough differential pressure to get back "on control" or go into recycle. In this condition the flare set point of third stage separator via the pressure ratio dictated by the speed of the machine determines the third stage recompressor discharge pressure maximum value. If this pressure is below the set pipeline recompressor suction pressure the first three stages of recompression "check in" and cease to forward flow. This is an irreversible or "lock in" situation. Operator intervention is then required to re-establish forward flow. This can be accomplished in a number of ways, for example, by either reducing the set pipeline recompressor suction pressure or by manually increasing the recompression train speed but essentially a speed increase is necessary. The model showed sensitivity of the process and its control systems to: changes in pipeline recompressor suction pcessure and therefore dehydrator pressure. Small changes in the set point of PIC-20 have a significant impact on recompression train speed and thus upon the ability of the first three stages of recompression to forward flow. changes of pipeline recompressor discharge pressure. Changes in the set point of PIC-30 affect recompression train speed and system operability. The system pressure profile is more sensitive to changes in pipeline recompressor suction pressure than to changes in the set point of PIC-30. changes in anti-surge control line location. The system waS found to be very sensitive to changes in the location of the control line. This affects the (maximum) pressure ratio making capability
of the recompressors in partial recycle. The most critical factor in determining the above behaviour is essentially the relationship between compressor speed and the overall pressure ratio making capability of stage one to three. This relationship is, of course, influenced by a number of other parameters besides those described above. These include the following: gas composition. Deviation from the predicted separator gas molecular weights, and in particular "lightening" of these streams will have signif icant impact upon the pressure ratio making capability of the individual compressors stages. gas physical properties. Compressor performance is, for example, very sensitive to inaccuracies in the predicted value of compressibility factors. unequal deterioration of compressor aerodynamic performance with time and therefore potential differences in pressure ratio making capability between stages. the amount of gas leakage between the inter stage diaphragm of the second and third stage recompressors. pressure losses and their possible increase with time, in particular the pressure drop through items of equipment such as heat exchangers and scrubbers. In view of the above observations and comments it was concluded that the set points of the pipeline recompressor suction and discharge pressure controllers PIC-20 and PIC-30 must be manipulated, within a defined set of constraints to enable the process to substantially improve its flexibility to forward a range of feedstocks and gas flow rates. Lowering of both these controller set points provides an additional degree of latitude in permitted perturbations from these set points in relation to the settings of overr ide controllers PIC-25 at the dehydrator and PIC-35 at the pipeline recompressor discharge. An additional real constraint is also present in the system, related to the maximum permitted continuous operating speed of the machine. Pipeline recompressor suction and discharge pressures should be set (if possible) to obtain the maximum margin of running speed below this figure consistent with maintaining PV-10 "on control". Start-up Analyses The design of the supervisory compressor control system included an automatic start-up sequence taking the compressors from minimum governor speed to full forward flow. The intention was that this could ait;o be used for automatic load transfer from one machine to the other. The dynamic model was used to simulate this start-up sequence. It was found that the transfer from full recycle to forward flow occurred within a very short period for all stages (150 rpm and 30 seconds). This resulted in "every thing happening at once" making it impossible for the operator to follow and take corrective action. After discussions between the engineers and operating personnel involved in the study it was decided to reject the completely automatic start-up. The supervisory control system was then modified to give only manual speed increase through the sensitive area. The start-up was the shown to work satisfactorily. This modification also eliminated the possibility
K. Solc mslie a nd 1'. 1\1. Bradle\ of automatic load transfer between the two machines. Based on the simulations a procedure for manual load transfer was therefore developed. Based on engineering and operating experience a "commissioning" start-up method was developed by members of the group involved with the studies. The simulation model was used to confirm the method and assist in the preparation of a separate step-by-step commissioning document. The dynamic model was able to demonstrate start-up of the compressor from rest and ensure that the proposed method provided: step-by-step method of a controlled introducing gas stream to the compression system a simple means of "packing" the pipeline via the pipeline stage of compression by using gas from the first stage separator a means of introducing third stage separator gas to the pipeline system in a controlled way whilst first stage separator gas was already being exported a means of manipulating pressure pr of He through the compression system to permit controlled introduction of second stage separator gas. RE-INJECTION COMPRESSOR STUDY Scope and Objectives The scope and objectives of this work were originally the same as those for the recompressor study. However at the start of the work suction throttle valves were introduced into the system. This added to the scope. The simulation model was used as a engineering design tool as well as a design verifier. To achieve these objectives and ver ify the design the start-up analyses were performed as an integrated part of the flexibility and operability studies. Process and Control System Description The re-injection system is installed to be used during pipeline shut downs and for modulation of gas export volumes as required by the buyers. It consists of two 50% centrifugal compressors in parallel each having a constant speed electric motor driver. The compressors have common suction coolers and scrubbers and take suction from the pipeline compressor discharge. The original proposed protective shown in Fig. 3 consisted of:
c o ntrols
as
each valves for separate anti-surge the "flow delta compressor based on pressure" system . minimum compressor suction pressure protection by modulation of well head chokes (PIC-42). pressure protection maximum discharge overr iding suction throttle valve control (PIC-44) . The process control comprised pipeline recompressor discharge pressure control acting on suction throttle valves incorporating flow balancing facilities between the compressors (PIC-40 and FIC-41). This leaves the operator with two alternative controls for the pipeline recompressor discharge pressure (PIC-30 and PIC-40). The operating mode will determine which one is used. Only one of the controllers can be in automatic at one time.
Re-injection Operability
System
Process
Flexibility
and
Initial analyses showed that the proposed control systems were not capable to handle even minor flow disturbances. This was due to the fact that there was no way that the low suction protection controller, PIC-42, could achieve its purpose independent of whether it should open or close choke valves upon low re-injection suction pressure. This can be seen from the following hypotheses. I f on loss of gas availability to the Re-injection system opening of choke valves by PIC-42 will lead to: lowering of system discharge pressure. For an essentially fixed pressure ratio at the reduced flow rate, this lower discharge pressure must therefore result in a further decrease in suction pressure. Closure of choke valves by PIC-42 will lead to: increase of discharge pressure this discharge pressure increase activates high discharge pressure override controller PIC-44 and the suction throttle valve will be closed by that controller action, leading to a further decrease in suction pressure. Further reduction of gas flow will lead to further closure of the choke valves, further discharge pressure control action etc. This is a "lock-in situation" due to the interaction of the two controllers PIC-42 and PIC-44. Additionally when PIC-44 overr ides the suction throttle valves, PIC-40 (pipeline recompressor discharge pressure control) goes "off control". Of secondary importance is the slow movement of the chokes, which is not able to provide adequate pressure protection at the suction of the compressors. To solve this problem the engineers involved proposed to introduce the action of PIC-42 onto the recycle valves via a low select mechanism. This modified control system was the basis of a large number of simulation analyses the results of which indicated that: extremely limited operational flexibility existed. A very small 'operating window' was allowed for the compressor operating point. with the developed control systems, no practical operator implemented method was available for re-injection machine start-up and establishment of forward flow. This was brought about by the simultaneous imposition of a low suction pressure limitation, due to seal oil system characteristics, and a high discharge pressure limitation dictated by the injection 'Xmas Tree' . Several ways out of these problems were evaluated. The easiest being a reduction of the minimum allowable seal oil pressure. Subsequent to a series of meetings with the compressor manufacturer a revised minimum pressure for continuous operation of 100 barg was negotiated. Consequently, this revised setting was incorporated into the simulation model and its impact upon Re-injection start-up investigated. A low suctio n pressure override controller PIC-42 set point of 120 barg was adopted as this pressure represents the predicted gas cricondenbar. Operation at the compressor suction above this pressure is, therefore, free from liquids . The impact of this revised minimum pressure was considerable, permitting satisfactory compressor start-up. The results of
the start-up sequences investigated indicate that significant increased operational flexibility is introduced into the circuit. Manipulation of the suction throttle valves by PIC-40 enables automatic single and parallel compressor operation over a range of re-injection flows from zero to compressor design rates. The final control systems adopted are as follows: anti-surge control as originally designed pipeline recompressor discharge pressure control on suction throttle valves with combined flow balance acting between compressors (PIC-40 and FIC-41). minimum compressor 5uction pressure protection by modulation of both recycle valves simultaneously (PIC-42) maximum discharge pressure protection overr iding suction throttle valve control (PIC-44) The ability of the re-injection system to cope with a wide range of flows hinges around the ability of the suction throttle valve to modulate. This valve is required to have a mechanical minimum stop to prevent "choking" of the gas recycle loop. A stop setting of 20% valve lift (corresponding to 6% of the valve maximum capacity) was found suitable in the simulation study. The mechanical stop must be reviewed during plant start-up as the setti~g depends upon the minimum operating discharge pressure of the pipeline recompressor and the character ist ics of the inject ion wells. Ideally, the setting should be such that the discharge pressure of each re-injection compressor is, at the valve minimum stop and full recycle condition, below the static well head pressure. Depending upon the characteristics of the injection wells, the injection choke valves should be operated fully open. This maximises the "operating window" of the system and also reduces the probability of activation of high re-injection compressor discharge pressure override controller PIC-44. The choice of pipeline recompressor discharge pressure control (PIC-30 and PIC-40) will be decided by operating experience. The easiest operating stategy in a combined export/injection situation would be to set a fixed export volume and let the re-injection system handle fluctuations in the gas supply. The feas ibi li ty of this strategy hinges on the ability of the suction throttle valves to remain on control, that is between full open and minimum stop. Based on the final control system and the reduced suction pressure, it was possible to develop a step-by-step start-up and commissioning procedure and to demonstrate these with the simulation model. CONCLUSIONS The use of dynamic simulation showed itself to be a very valuable engineering tool in the development of the control systems for the recompression and re-injection compressors - both as a control and modifier of existing design - and in developing new control systems. The control of a complex compression system as on Gullfaks 'A' involves so many interdependable variables that it will never be possible for the human mind to reason through all consequences of a particular disturbar.ce put to the system. By performing a comprehensive series of simulations, introducing different disturbances and even trips of the machine, the possibilities and
restrictions built into the system have been demonstrated. Where required, modifications have been made to enhance the flexibility of the control systems. The analyses have shown that the protection system for the compressors function properly. A gradual optimisation of controller tuning parameters has been obtained. This work would normally have to be performed on the equipment itself during commissioning of the systems, with the possibility of damaging the compressor or other parts of the system. Some of the disturbances will perhaps never occur, or possibly not occur for several years as they represent such severe failures. The capability of the plant and the optimum tuning of the controller parameters to handle these disturbances would then only be known after they occur the first time. The simulation analyses concerned with the normal and commissioning start-up methods have given a "proven" method of first starting up the compressors. At the time of writing, it is therefore our hope and belief that the combination of a well defined starting point for the controller tuning parameters and a tested start-up method will give a much more "painless" commissioning of the compression system than normally expected. Another main experience gained is the requirement for close cooperation between different types of specialists during the work. Compressor systems are complex "individuals", who seem to have a will of their own sometimes. A composite group of personnel consisting of mechanical, operat ional, control and process specialists must be involved. It is then possible to go through the results as they are generated, evaluate problems, and agree upon solutions. This makes the simulation work like a dynamic process. Agreed changes are implemented as they are proven required and correct, instead of waiting for the final recommendations.