ADVANCED PROCESS CONTROL AT NORTH WEST SHELF LNG PLANT LE CONTROLE DE PROCEDE AVANCE DE L'USINE GNL DE NORTH WEST SHELF

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1 ADVANCED PROCESS CONTROL AT NORTH WEST SHELF LNG PLANT LE CONTROLE DE PROCEDE AVANCE DE L'USINE GNL DE NORTH WEST SHELF Derek Hodges Senior Process Control Engineer Woodside Energy Ltd Karratha, Western Australia, 6714 ABSTRACT Advanced Process Control (APC) has developed and matured over the last two decades, mainly in the refining and petrochemical industries. It is used to tightly control quality and to maximise production rates and yields of existing facilities and is most effective where capital investments, plant complexity and productions rates are high. Properly applied multi-variable predictive control technology projects have demonstrated high utilisation and benefits at low project risk. To-date, LNG plants have not applied APC to the same extent, despite meeting the above criteria, to which attractive marginal economics can be added. As operator of the North West Shelf LNG plant producing LNG, Domestic gas, LPG and condensate, Woodside started in 1995 an APC implementation program for all the four main production units. This paper will not focus on detailed APC design, but will put APC in the wider context of plant reliability and optimisation and demonstrate the results with plant data. Experience tells us that APC design and implementation is a vehicle to gain good detailed understanding of the economic drivers of the plant and automatically consolidates this learning in the APC applications. After that, the applications provide an excellent tool to further optimise production and reliability in a safe and consistent manner. APC on the NWS LNG plant has therefore triggered and captured many spin offs in ideas and improvements. The paper will also address the importance of only regarding APC as the Pinnacle of structural improvements, on top of good and reliable systems. The implementation of APC on the NWS LNG plant went hand in hand with major investments in compressor surge control, safeguarding and distributed control systems and a complete new layout and design of the CCR. RESUME Le contrôle de procédé avancé ( APC) a été développé pendant les deux dernières décennies, principalement dans le raffinage et la pétrochimie. Il permet de contrôler étroitement la qualité, augmenter les taux de production et les rapports d'usines existantes et il est particulièrement efficace lorsque les coûts d'investissement, la complexité et les niveaux de production sont élevés. Les technologies de contrôle multi variable correctement appliquées ont démontré leur capacité et leur avantage assorti d'un faible risque. A ce jour, les usines de GNL n'ont pas appliqué l' APC avec une telle ampleur, PS2-2.1

2 bien que les critères précédents soient réunis, auxquels des avantages économiques marginaux peuvent être ajoutés. En tant qu'opérateur de l'usine GNL de North West Shelf qui produit du GNL, du gaz naturel pour le marché domestique, des G.P.L. et de condensât, WOODSIDE a commencé en 1995 un programme d'installation d'apc pour les quatre unités de productions principales. Cette présentation ne se concentre pas sur la conception détaillée de l' APC, mais le place dans un contexte plus large de fiabilité et d'optimisation et démontre les résultats à l'aide des données directes de l'usine. L'expérience prouve que la conception et l'installation de l' APC est un moyen d'atteindre une bonne compréhension détaillée des paramètres économiques de l'usine et de consolider automatiquement cette apprentissage dans les applications de l' APC. Ensuite, ces applications fournisse un excellent outil pour poursuivre l'optimisation de la production et de la fiabilité d'une façon sûre et logique. L' APC de l'usine NWS a par conséquent déclenché et eu un effet d'entraînement dans les idées et les améliorations. Cette présentation insiste aussi sur l'importance de considérer l' APC comme le "pinacle" d'un ensemble d'améliorations structurelles, au sommet de systèmes efficaces et sûrs. L'installation de l' APC s'est accompagné d'investissements majeurs dans le contrôle anti-pompage des compresseurs, dans un contrôle commande distribué et sauvegardé, et dans une nouvelle conception et implantation de la salle de contrôle. PS2-2.2

3 ADVANCED PROCESS CONTROL AT NORTH WEST SHELF LNG PLANT INTRODUCTION Advanced Process Control (APC) has developed and matured over the last two decades, mainly in the refining and petrochemical industries. It is used to tightly control quality and to maximise production rates and yields of existing facilities and is most effective where capital investments, plant complexity and productions rates are high. Properly applied multi-variable predictive control technology projects have demonstrated high utilisation and benefits at low project risk. As operator of the North West Shelf LNG plant producing LNG, Domestic gas, LPG and condensate, Woodside saw in 1995 an opportunity to utilise this technology to increase revenues. However, it was simultaneously recognised that APC is not something that can be just bolted on and left to make money. To be successful, it needs to be fully integrated with the overall instrumentation, control systems, plant understanding and operations, and would be better considered as icing on the cake, to be obtained when the fundamentals are correct. A 5-year strategic plan was generated to encompass all aspects associated with the holistic improvement of the controls systems at the NWSV plant. This included: Management commitment to the plan, with guaranteed resourcing. Upgrade to DCS systems Upgrade to Safeguarding systems Control room upgrade Repairs, corrections and re-tuning of the base layer DCS controls Replacement of the Domgas and LNG compressor anti-surge controls with new state-of-the-art control systems Implementation of APC on the major production units. It was recognised from the beginning that a phased approach was necessary, because of limited resources. The following table shows the actual and planned implementation dates of the various phases of the overall project. Three separate phases were planned for funding purposes, and implemented within agreed schedules. The final upgrade of safeguarding systems is linked to major shutdowns of LNG trains, and will be completed over the next two years. Actual Phase Planned 1996 Master plan agreed with JVPs DCS Upgrades commenced Stabiliser APC (5 units) Fractionation APC (2 trains) /98 Compressor Control Upgrades (19 compressors) 1997/ /99 Central Control Room Upgrade /99 LNG APC (3 trains) /2003 Upgrading safeguarding systems 1999/ Domgas APC LPG Integration 2000 PS2-2.3

4 To minimise complexity of implementation and to minimise on-going support requirements for all APC projects, it was recognised that a standard platform would be needed. Woodside chose to standardise on Honeywell s RMPCT software for modelpredictive multi-variable control, and developed all the APC projects using additional technical support from Honeywell HiSpec Solutions working in conjunction with inhouse engineers. This has proven to be a very successful technique, by maximising the benefits of local knowledge and the skills of the consultant in their particular product to minimise the implementation time, while still achieving a high quality transfer of technical knowledge into the plant for on-going support. BACKGROUND In 1995, the NWSV plant production was limited both by LNG Fractionation and Stabiliser capacity. Stabilisers were recognised as the first key area of opportunity, since incremental condensate was immediately marketable. The offshore and on-shore facilities are operated together as a tightly integrated joint facility, with dry gas from Rankin, condensate and LPG-rich gas from Goodwyn, and LPG-rich gas from the oil production facility on the Cossack-Pioneer blended into a common trunkline to maximise utilisation of the onshore facility. Onshore liquids processing capacity was a bottleneck, restricting condensate production from Goodwyn. LPG GAS CONDENSATE WANAEA/COSSACK NORTH RANKIN A GOODWYN A TRUNKLINE- 12 HOUR DEADTIME STAB DOMGAS LNG FRAC ONSHORE PLANT PS2-2.4

5 STABILISER APC In the development of the Stabiliser APC it had been recognised that there was an existing problem with the controls systems on the stabilisers, and this was rectified via a separate project in At the same time, the basic controls were completely re-tuned, and together this improved operation considerably, increasing production by about 4%. The APC was then designed and implemented on this stabilised operation. The Stabiliser APC design encompassed feed maximisation and tighter control of specifications by using product quality estimators, column pressure reduction and better control over the column heat duty. An additional benefit was anticipated in improved compressor reliability from the more stable operation. The APC was commissioned in 4Q 1996, and the unit s capacity was increased by an additional 3%. Pressure in the stabiliser units was dropped by more than 10 percent, and production records were set in 1997, and broken again in 1998 before a decline in offshore liquids production saw onshore capacity exceed offshore supply capability in This did not mean an end to benefits, however. The improved reliability and operability of the stabilisers under APC meant vastly reduced recovery times after a plant trip or interruption to offshore supply, and on-going stability of product quality. Because the LPG-rich stabiliser overheads streams from the different units are directed to different parts of the plant, having spare capacity allowed first manual and then finally automatic management of LPG handling by varying the rates of different stabilisers in later APC developments. Monthly Condensate Production Production Records APC Commissioned PS2-2.5

6 FRACTIONATION APC Developed concurrently with the Stabiliser APC project, the Fractionation APC was more complex, and presented considerably more challenges in implementation. Again, the simplicity of marketing the product into existing sales systems meant that this was a key opportunity, but in contrast to the stabilisers, which are at the front of the on-shore operation, and can be manipulated fairly readily using the slug-catcher capacity, the fractionation units (two parallel trains) were at the back end of the operation. This makes them subject to much greater disturbances, including variation in feed rate and quality, as well as the ambient temperature changes that affect the entire site. The typical operation of the Fractionation units prior to introduction of APC was characterised by short periods of poor operation with off-spec production interspersed with longer periods at much higher than required purity to either blend back the off-spec product or meet the required purity for making refrigerant grade propane. The cost of running for extended periods with higher purity targets is a penalty in production capability, since reflux rates and column loadings are all higher. This was recognised as a substantial opportunity for capacity improvement. The normal steps associated with developing a model-based control system were followed. The basic controls were evaluated for performance and as with the Stabilisers, were completely re-tuned. This provided a substantial increase in the stability of operation. The APC development then commenced, with the design phase followed by plant step-tests, development of empirical models, and then validation, training and commissioning. Since the Fractionation trains are quite similar to those found in refineries, the design was fairly straightforward, but because there is little hold-up or capacity in the overall system, the step-tests proved to be very difficult. Key aspects, such as rich and lean extremes in feed quality and rate are dictated by offshore operations, and not easily manipulated, especially in the clean steps required for good model development. The variation in operational modes, such as when making refrigerationgrade propane or liquid ethane for LNG mixed refrigerant also needed to be managed, and these extremes were initially not well catered for. The result was an APC that was difficult to operate and did not perform well under certain conditions. As a result of this, a subsequent follow-up visit with different personnel was arranged. They were able to identify and overcome the key non-linearities introduced by these high-purity modes, and following some small but significant design changes and extensive re-tuning by in-house engineers, the application was made robust. Once this was achieved, the benefits were considerable. By being able to consistently make the required high purity grades when required, the time taken operating in these modes was vastly reduced, with virtually no wastage waiting for product to meet specification. This then released time to operate with much more relaxed specifications, and the consequent drop in reflux allowed the tower pressures to be considerably reduced. This allowed a substantial increase in capacity, which was absorbed by increases in the amount of LPG injected offshore. Overall production rate increased by up to 40%. An additional simple, but very valuable enhancement optionally forced one train to be automatically pushed to its limits, rather than balancing the load equally. Since the performance of the two trains was essentially the same at a given set of conditions, (although varying considerably under different feed and ambient conditions), this allowed the available capacity to be quickly measured, as the difference between the unloaded train and the one loaded to capacity. It also identified the limiting constraint. From this, PS2-2.6

7 the offshore mix could then be adjusted to minimise the spare capacity, while still allowing a sufficient margin for operability. Operation with the APC identified the key bottlenecks within the fractionation units, and separate projects were established to remove them by re-traying of towers. These projects were commissioned this year, and further increased production capability. In this way, the APC worked in conjunction with process operation and engineering, providing a stable baseline for analysing operation and hence facilitating additional enhancements. Having stable operation under APC control allows the engineering staff to conclusively demonstrate where bottlenecks exist, and for what period of time, and hence provide solid justifications for improvement projects. Even when not loaded to capacity, the Fractionation APC still provides vastly improved rejection of disturbances caused by upstream changes, and excellent product quality control NWS LPG Comparison to Saudi Specification - Butane Pentanes Plus in Butane Pentanes Plus (vol%) APC Implemented 0.00 Pentanes Plus (vol% ) Saudi Spec Max GPA Spec PS2-2.7

8 LPG (C3/C4) Production Fractionation APC first implemented LNG APC With the Stabiliser and Fractionation APC s proven operational and commercial successes, Woodside commenced work on the LNG APC. This had previously been identified as the greatest potential source of revenue, but also the most technically challenging, hence the order of planning. The LNG train components are fairly typical, with GE Frame 5 gas turbines driving mixed refrigerant and propane compressors, using APCI s main cryogenic heat exchangers, and totally air-cooled. The combination of air cooling with gas turbines means the process is subject to continual disturbances from the variation in ambient temperature, which is regularly 8-10 degrees C on a daily diurnal. Additional variations of 5-10 degrees C also occur, caused by heat interaction across LNG trains, depending on variations in wind direction and strength. Normal operation was to run at maximum possible capacity. The baseline operation prior to commencement of the APC program was by operatorset flows of mixed refrigerant, with LNG production automatically adjusted to maintain MCHE outlet temperature. Since maximum capacity was continually varying, the operator would be continually adjusting MR flows on the three trains, either generally up, to take advantage of additional capacity as temperatures cooled, or down to stay ahead of reducing capacity as temperatures increased. Because there was always a lag in this process, capacity was inevitably less than a theoretical maximum, by a margin estimated at 2-4%. Wind shifts in particular caused difficulties, as a train capacity could be reduced by 20% in less than 30 minutes. (Often, another train s capacity would increase, but taking advantage of this was always secondary to ensuring the reduction in capacity was managed to avoid a train trip.) The compressor anti-surge controls were also not adequate. Sudden changes to the process could induce trips generated by the anti-surge controls, although the plant instrumentation was not always able to identify whether it had been a real surge event. Subsequent trips on surge during re-start of the operation were also not uncommon. PS2-2.8

9 The master-plan generated scopes for four separate aspects of the LNG instrumentation upgrade. They consisted of replacement of the anti-surge controls on all the major compressors (5 per train) with state of the art CCC controls; upgrading of the central control room, including migration of the DCS systems to new generation Honeywell equipment; replacement of the obsolete shutdown systems (instrument protective systems); and Installation of APC on three LNG trains, with a planned target of 1% increased production, and a stretch target of 3%. As with the previous APC projects, the first step was a rigorous evaluation of the basic regulatory control system. Most controllers were re-tuned, and overall stability increased. However, it was clear from preliminary analysis that data gathering for the APC could only commence once the compressor upgrades were completed, as the existing plant compressor controls would be too unstable to allow step-testing. The 15 major compressors on the three LNG trains were successfully cut over to CCC controls progressively, commencing with LNG3 in May The benefits were immediately apparent, with a significant drop in train trips associated with surge protection. This provided increased production, mainly from increased availability factors, and better recovery after upsets. It also provided the stable operating platform necessary to develop the LNG APC. Three separate APC s were generated for each train, managing the Sulfinol Unit (CO2 removal), Scrub column and Liquefaction unit combined, and a third controller to manage Mixed Refrigerant make-up. This last controller was identified in the preliminary analysis as necessary to stabilise the baseline of operation and enable further optimisation to take place. With continually varying MR composition, and varying capacity due to ambient temperature changes, it was exceedingly difficult to determine whether a plant operational change had had a real, reproducible positive benefit, or that conditions had simply changed. This had been recognised as a problem for many years, but without solution. The Dehydration unit is mostly driven by sequence logic, and was excluded from the APC design. The APC s were progressively implemented, with Sulfinol APC in June 1998, MR Make-up December 1999, and the Liquefaction APC, which controlled the operation around the main cryogenic exchanger in February/March The benefits were immediately obvious, in the APC s ability to handle constraints not seen by the basic control systems. In particular, the first application commissioned was on a unit that was operating with a feed tube leak in the main exchanger. This was causing cold temperatures in the MR vapour exit from the bottom of the warm bundle, which is a plant constraint due to metallurgy limitations in the first mixed refrigerant compressor. The operators were managing the temperature constraint by manually limiting the rate, but because of its tendency to suddenly drop sharply toward a trip limit, they sensibly tended to run with a healthy margin of safety, and hence at a slightly lower production rate. This temperature had been identified in the APC design as a potential constraint, and the APC was able to maximise production subject to this limit, as shown in the following plot. PS2-2.9

10 LNG2 - Managing cold temperatures Feed Flow MR Exit Temperature alarm limit Actual MR vapour exit temperature. Note APC limit progressively lowered APC commissioned here Operating Limit : -45 degrees C Jan Jan Jan-99 5-Feb Feb Feb-99 Once confidence had been gained by operations that the APC could indeed control the throughput while honouring the constraints, it was possible to lower the margin between the APC operating limit, and the actual plant constraint, to further increase rate. Since the benefit of the APC was always going to be small (a few percent at most, unlike the major gains seen in Stabilisers and Fractionation), and the production capability varies continually as a function of ambient temperature, it was necessary to use statistical techniques to measure the benefit. This is most easily seen as a plot of the number of days operating at different percentages of the technical maximum capability. The following plot contains data from three successive years of operation, and shows the progressive effects from baseline (no improvements), the addition of the CCC control systems and the associated improvement in reliability, and then the addition of the LNG APC. This is a classic APC improvement profile, showing a two-fold benefit. The actual plant capacity has not been increased, but the existing capacity has been more effectively utilised. There is an increase in typical daily production rates, as shown by the shift in the histogram peak by 2-3% to a higher percentage of the technical maximum, and also an improvement in the consistency of operation, as demonstrated by the narrower spread of operation. The number of days at % of technical maximum capacity increased four-fold. The combination of the increased production with more consistent operation gave a potential yearly production benefit from the APC of 1.9%. The actual benefit realised is less than this, since the operation is not always at maximum rate, being occasionally limited by tankage, shipping or offshore constraints. Nonetheless, the benefit easily exceeded the project cost in the first year. The Mixed Refrigerant composition has also been stabilised. This gives further opportunity for optimisation, since the desired MR composition can be set as a control target, and the APC will deliver the result within a day or so, and then hold it steady at the new composition. Plant tests to optimise the MR composition have not yet been performed, but it is anticipated that small improvements can be made, particularly with seasonal changes PS2-2.10

11 Days per annum at a given percent of tech max Progressive Improvement in Approach to Technical Maximum Capacity Baseline With CCC controls With CCC plus APC Percent of Technical Maximum Capacity DOMGAS AND LPG INTEGRATION APC S With LNG APC commissioned, the last major processing units to have APC implemented were the Domestic Gas (Domgas) trains. The realisable benefit here was in maximising the recovery of saleable LPG from the Domgas feed, since the actual Domgas production rate is set by market demand. This would be achieved by maximising the LPG extraction capability of the Domgas trains, while still delivering the same final Domgas flow. It was estimated that the LPG recovery could be increased by 4%, or 60 tonnes per day, for an estimated project cost of A$700K. The project was in design phase in 1Q1999, the step tests were performed in 3Q1999, and the project was commissioned in 1Q2000 for a final budget of approximately A$500K. The project benefits have exceeded initial estimates, with LPG recovery improved by more than 10%. In addition to the revenues from LPG recovery, the Domgas also provides the improved stability of operation and rejection of disturbances from upstream in the process, most notably variation in feed composition due to changes in offshore operation. Being a model-predictive controller, the Domgas APC has the ability to predict when Domgas heating value specifications are likely to be exceeded due to a significant change in feed composition. In this scenario, it will give up on optimising the C3/C4 recovery, and preserve the quality of the saleable domestic gas, until the process is again capable of supporting increased LPG recovery. The LPG extraction in the Domgas units is more efficient than in the Scrub Column in the LNG trains, so an opportunity also existed to manage the site-wide LPG optimisation. This is achieved via the LPG integration APC. This ties together the Stabiliser APC, the Domgas APC, and the LNG trains, by preferentially sending flow to specified process units to maximise overall LPG recovery. The following block diagram illustrates the interconnection, where LPG-rich gas from the stabilisers can be directed either to Domgas, or to LNG via the Trunkline Onshore Terminal (TOT). Provided stabiliser capacity exists, the rates on different stabilisers can be adjusted to maximise the PS2-2.11

12 flow of LPG-rich feed into Domgas (and the Domgas APC), until that process is at a maximum, as determined by the APC limits. The remaining rich feed gas will be directed to LNG via the TOT. Domgas APC Domgas / Stabiliser Integration Overview TOT LNG TOT Domgas Domgas Lean Gas Slugcatcher 1,2 3 4,5 Stabilisers Rich Gas Liquids In addition, the LPG management APC also manages the level in the slugcatchers, by ramping the Stabilisers rate (via the Stabiliser APC) to keep the levels within specified limits. The level changes reflect the transient imbalances between the feed usage within the plant and the gas and liquids arriving from the trunkline. A predictive controller provides a distinct advantage here, since there is a large dead-time (several hours) between changes to operations offshore and the consequences arriving onshore, because of the length of the trunkline. This is a highly complex phenomenon and the APC models do not predict it perfectly, but do sufficiently well to still give good control of the level. CONCLUSIONS At the NWSV LNG plant, APC has proven to be a very cost-effective way of optimising the plant operations. For an overall investment of less than A$5 million, project payback times have all been less than six months, with some just a few weeks. While some of these benefits were simply from re-tuning the basic control systems, this was achieved in a holistic manner as part of a structured approach to improving the overall control systems. Additional intangible benefits have followed from a better understanding of the key process operations, and particularly the limitations. In several cases, this has led to separate debottlenecking projects. Because the APC consistently operates within the allowable operating envelope, the number of process alarms has also been significantly reduced, as have plant trips across all the process units. Again, this is obviously not solely due to APC, but rather as a consequence of a structured approach to improving the instrumentation and control systems, including the safeguarding systems. Another significant benefit that is hard to quantify is the improved speed of detection of abnormal operations. Since the APC provides a very consistent operation, deterioration in performance due to some hardware fault is more readily detected, and hence can be corrected. PS2-2.12

13 Overall, each APC project has delivered or exceeded in all respects the required benefits defined in the project specifications. They have also delivered significant intangible benefits, in improved understanding of the plant processes, reduced workload and stress on operators (particularly during the summer months), more stable operation with consequent reduced stress on equipment, smoother rate changes, and considerably more stable operation at minimum rates. PS2-2.13