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RELIABILITY & OPTIMISATION OF ARTIFICAIAL LIFT SYSTEM 21 st October 2005 By Dr Sib Akhtar MSE (Consultants) Ltd Carshalton, Surrey SM5 2HW

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Presentation on theme: "RELIABILITY & OPTIMISATION OF ARTIFICAIAL LIFT SYSTEM 21 st October 2005 By Dr Sib Akhtar MSE (Consultants) Ltd Carshalton, Surrey SM5 2HW"— Presentation transcript:

1 RELIABILITY & OPTIMISATION OF ARTIFICAIAL LIFT SYSTEM 21 st October 2005 By Dr Sib Akhtar MSE (Consultants) Ltd Carshalton, Surrey SM5 2HW Tel: Effects of Extended Recycle on Gas Lift Compressors in Mature Assets

2 © MSE 2005 MSE Consultants Ltd Established UK Engineering Consultancy Specialises in Oil & Gas production facilities Process-Machinery-Controls De-bottlenecking Testing Equipment design & redesign and compressor re-wheel Modelling of oil and gas production Maintains a large database of Heavy machinery Gas Compression, gas turbines and control systems A single source of design and application information on all makes and types of heavy machinery used in Oil and Gas

3 © MSE 2005 Current Projects - 1 2nd Largest Gas (Condensate) Field in UK Expansion with New Satellite Fields New Bridge-Linked Platform Developed by AMEC AMEC Want to Optimise Compression Facilities MSE Developing New MP Compression System GASMAN Model Built to Verify Design & Performance Performance Testing and re-design options study Britannia Field AMEC/ConocoPhillipsAMEC/ConocoPhillips

4 © MSE 2005 Current Projects - 2 UKs Largest Gas Field Re-design of compressors for post-plateau production Update of existing GASMAN model Expansion to include new satellites (Bains) Optimise offshore and onshore compression Compressor vendor design audits South Morecambe Field British Gas Hydrocarbon Resources Ltd

5 © MSE 2005 Current Projects - 4 Largest new oil field in Omans southern province 100,000 bpd capacity using miscible gas injection for enhanced oil recovery Feasibility of worlds highest pressure gas injection compressors at 710 bar Design of very high pressure compressors Vendor design audit Harweel Gas Injection Compressor Study Petroleum Development of Oman/Shell

6 © MSE 2005 Current Projects - 5 Visit Lekhwair and evaluate gas lift system for enhanced oil recovery Oil production limited by gas lift compression Identify options for improved gas lift capacity Submitted proposal for further study and remedial work Lekhwair Oil Field Debottlenecking Petroleum Development of Oman/Shell

7 © MSE 2005 Current Projects - 6 ConocoPhillips BP Exploration BG Group ENI Lasmo Centrica (British Gas HRL) Identify causes of compressor performance loss Compile compressor design/selection guide Seek trends, commonalities and best practices Compressor Users Forum Joint Industry Project (JIP) – Phase III Five Operating Companies

8 © MSE 2005 Recent Projects - 2 Independent audit of gas lift compressors Design Operation Machinery reliability problems High seal failure rate Proposals for further work highlighted by audit Thistle Field Compression Study DNODNO

9 © MSE 2005 Recent Projects - 3 Re-configuration of onshore compression facilities Account for current and future compression demands Demand increases with well depletion Two-stage project to accommodate seasonal issues Measured performance degradation taken into account North Morecambe Field British Gas Hydrocarbon Resources Ltd

10 © MSE 2005 Current Projects ONGC – Heera Gas Lift Compression System Chevron – Benchamas Gas Lift Compression Lundin – Thistle PDO – Zalzala Gas Injection PDO - Saih Rawl ; Upstream LNG feed Britannia – Production Optimisation LNG - Project

11 © MSE 2005 Gas Lift System in Mature Assets Differ from newly installed systems Changes in reservoir fluids being handled ( e.g. more water and less oil and formation gas) Differences in flow capacities Changes in Process conditions ( lean out due to continuous recycling of gases over several years) Older machinery ( compressors and gas turbines) Old control systems Import Gas for start-up

12 © MSE 2005 Visual Representation of Gas Lift System

13 © MSE 2005 Typical Gas Lift Compressor for Mature Assets

14 © MSE 2005 John Crane have recommended that the seals, especially on the NDE are upgraded to the improved version of the 28AT, the 28XP. Advantages of the 28XP over the current 28AT: Polymer rings incorporated, increase operating temperature up to 600 O F, polymer rings also have a higher resistance to chemical attack Sliding Carriers, eliminates extrusion gaps through differential thermal expansion Carbide seats have shrouding to protect seal and shaft in the event of a catastrophic failure The cost of an upgraded cartridge is approximately £50,000 Gas Seals – Replacement Seal

15 © MSE 2005 Project Conclusions Very high compressor discharge temperatures High Molecular weight changes cause drastic swings in compressor operation HP compressor operation stable within the central region of the head map Gas seals operating above their specification for the o-rings Reasons Poor Cooler Performance Shallow LP Compressor curve towards lower flow region of the compressor HP compressor operation stable within the central region of the head map

16 © MSE 2005 Where is the current LP Control Line? How effective is current setting able to protect the LP compressor for a sudden decrease in molecular weight Optimise control lines, and set points on both machines to give adequate protection for the swing in molecular weight observed with LP machine Increase control line further into the map for LP, and reduce HP Increase the head capacity to aid gas lift Recommendations – Control System

17 © MSE 2005 Contamination by process gas Contamination by seal gas Lube Oil Contamination Operation outside design specification Due to the high number of seal containing oil, all the areas of possible contamination have been investigated Gas Seal - Contamination

18 © MSE 2005 Protect LP Compressor from Low Molecular weight swing by increasing surge control line Small loss in head and discharge pressure due to shallow curve Recommendations - Control System – LP Surge Line

19 © MSE 2005 Increase the Head of the HP Compressor, and the overall pressure of the GLC, by decreasing surge line. Effected less by fluctuations in molecular weight, due to steeper curve Recommendations - Control System – LP Surge Line

20 © MSE 2005 ONGC – Heera Asset MSE invited by ONGC to Investigate Heera Asset Carried out a detailed investigation

21 © MSE 2005 Study Objectives Quantify existing compression system capacity Identify factors limiting existing capacity – Root Cause Analysis Compare machinery availability to typical industry averages Identify opportunities to increase production Optimise existing compression system Upgrade / replace machinery Gas turbines Compressors

22 © MSE 2005 ActivitiesActivities Design data collection Offshore testing – compressors and turbines Turbine performance analyses Compressor performance analyses System performance analyses using GASMAN Aerodynamic analyses using CENTRIF Process analyses using Hysys Tentative conclusions produced

23 © MSE 2005 Facilities Overview

24 © MSE 2005 Factors With The Potential To Limit Maximum Production Throughput Turbine performance GG compressor, combustor, power turbine Process gas compressor performance Head, efficiency Unwanted recompression of process gas (recycling) Process and control instabilities Offshore testing and subsequent analyses identifies capacity limits

25 © MSE 2005 Testing & Analysis

26 © MSE 2005 Offshore Testing All five trains tested PGC flows varied by adopting 5 out of 4 train operation and speed control Good spread of flows and turbine loads achieved Gas samples collected at various strategic points Additional design data collected Detailed analyses of test data completed at MSE

27 © MSE 2005 Turbines A, B, C: Summary of Findings All gas turbines suffering in excess of 15% power loss (between 1000kW and 3000 kW loss in power) Bleed valve malfunction suspected responsible for losses in Trains A and B Train C suffering compressor efficiency loss primarily due to IGV malfunction Lack of some instrumentation readings hindering understanding of machinery health Recommend regular performance analysis to maintain high performance and sustain higher levels of reliability

28 © MSE 2005 PGC Performance Analyses Trains A, B & C

29 © MSE 2005 Compression Train Compression Train

30 © MSE 2005 GASMAN Modelling of Compressor Trains

31 © MSE 2005 Performance Assessments – Key Methodologies Gas properties – from composition sampling taken during testing Flow meter readings corrected for mol weight effects Compressor head and efficiency maps from Factory Acceptance Test (F.A.T.) performance curves GASMAN model allows whole-train analysis Power Balancing Total gas compression power to match turbine shaft power Power balance based upon actual machinery performances All sources of mass flow considered – recycle, leakage etc

32 © MSE 2005 Head & Efficiency Curves Non-dimensional curves allow various speeds to be shown against same datum Performance datum curves taken from Manufacturers F.A.T. results

33 © MSE 2005 Train B Stage 1 Head Performance

34 © MSE 2005 Train B Stage 1 Efficiency Performance

35 © MSE 2005 Train B Stage 2 Head Performance

36 © MSE 2005 Train B Stage 2 Efficiency Performance

37 © MSE 2005 Train B Stage 3 Head Performance

38 © MSE 2005 Train B Stage 3 Efficiency Performance

39 © MSE 2005 Average Compressor Performance Losses (ref F.A.T. Curves)

40 © MSE 2005 Average Compressor Performance Losses Performance losses should be viewed in light of more reasonable performance expectations

41 © MSE 2005 Realistic Compressor Performance Expectations

42 © MSE 2005 Compressor Performance Losses Using More Realistic Expectations When more realistic performance expectation are used as a datum; Much of the efficiency losses in the three stages can be accounted for Much of the head losses in the 3 rd stage can be accounted for If API datasheets provided are representative of the as- built machines, the head and efficiency profiles generated by the machine in the field are reasonable

43 © MSE 2005 Compressor Performances - Conclusions Trains A and B are exhibiting reasonable in-service head and efficiency. It is unlikely a replacement machine for the same duty would yield significant increases in gas rates once in service Train C appears to be exhibiting higher losses in Stage 1, and may benefit from an overhaul

44 © MSE 2005 Flow & Power Reconciliation Flows are not consistent across the stages of compressor trains Later stages show additional flow Flows readings checked from flow meter DPs If additional flows are ignored, the calculated turbine shaft power cannot be achieved. When additional stage flows are included, the PGC compression powers can be reconciled with the actual turbine shaft powers.

45 © MSE 2005 Additional PGC Stage Flows

46 © MSE 2005 Potential Sources of Additional Stage Flows Anti-Surge Valves – Not likely as positions are at zero Internal leakages in compressor casings – not always detected by flow meters. Flows would be too large to reconcile heads and efficiencies. Cascade system between Knock Out Drums

47 © MSE 2005 KOD Cascade System

48 © MSE 2005 Potential Leakage Paths via KOD Cascades

49 © MSE 2005 Offshore Test Performed Possible to eliminate cascade recycle via manual block valve at entry to next KOD in cascade. Measurements taken on Trains A and B with cascade system open Cascade isolation valves shut stage by stage and traces of stage flows recorded. Train A turbine power maintained at constant level (via EGT control) Forward flows recorded

50 © MSE 2005 Cascade Test – Flow Traces Train A

51 © MSE 2005 Cascade Test – Train A Flows At Same Power

52 © MSE 2005 Cascade Testing – Practical Considerations Build up of liquid in final KOD was observed to be rapid – caution during additional testing Liquid level control valves may be over-sized – rapid draining of liquid after reinstating cascade was observed Possible that liquid control valve set-points set to values that stabilise the system through permanent recycling (valve open)

53 © MSE 2005 Cascade – Liquid Stability Simulations indicate that phase equilibrium is highly sensitive – small changes in temperature can produce quantities of liquid Crux point is around 40 o C, all coolers operate above this area Production of liquids may be related to JT affect across drain valves

54 © MSE 2005 Cascade – Conclusions Current liquid level control of KOD is problematic Liquid level control valve settings are allowing gas to re-circulate around compression stages – this wastes power and limits maximum rates Recirculation reduces rates by 6 to 7 kSm 3 /hr For the same power, rates could be increased by 16% if recirculation of gas was removed

55 © MSE 2005 Cascade – Recommendations Recommend that valve sizes be investigated to more effectively regulate rate of liquid drain and prevent formation of gas leakage path Two new trains appear to achieve liquid level control effectively – liquid levels maintained at around 15%, thus preventing gas recycle loop and unstable operation A better understanding of liquid stability and its relation to temperature and pressure would be beneficial

56 © MSE 2005 ConclusionsConclusions Gas Lift Compression System for mature assets were analysed by testing and careful analysis Model analysis necessary to verify test performance of gas compressor and drivers Compressor power and engine power balance achieved Turbine ISO power of 9.2 MW reduced to 7.6 MW when corrected for high ambient temperature 30 C Gas path analysis identified additional power losses from bleed valves and Inlet Guide Vanes settings. These losses are fully recovered by maintenance activities

57 © MSE 2005 ConclusionsConclusions Gas Compressors performance was found reasonable when compared with industry norms Flow discrepancies found between compressor were investigated and analysed This lead to the discovery of gas recycling through the KOD liquid flow lines. This was later verified with a simple test procedure This helped to with an immediate increase in gas delivered by the compressors GASMAN software helped to identify problems with gas turbines and flow recycle in compressors. This was verified by test carried out in the field where mass flow through compressor stages were carefully measured and problem rectified


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