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RELIABILITY & OPTIMISATION OF ARTIFICAIAL LIFT SYSTEM. Effects of Extended Recycle on Gas Lift Compressors in Mature Assets. 21 st October 2005 By Dr Sib Akhtar MSE (Consultants) Ltd Carshalton, Surrey SM5 2HW Tel: 020 8773 4500. MSE Consultants Ltd.

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    1. RELIABILITY & OPTIMISATION OF ARTIFICAIAL LIFT SYSTEM Effects of Extended Recycle on Gas Lift Compressors in Mature Assets 21st October 2005 By Dr Sib Akhtar MSE (Consultants) Ltd Carshalton, Surrey SM5 2HW Tel: 020 8773 4500

    2. MSE Consultants Ltd • Established UK Engineering Consultancy - 1988 • 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. Current Projects - 1 Britannia Field AMEC/ConocoPhillips • 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

    4. Current Projects - 2 South Morecambe Field British Gas Hydrocarbon Resources Ltd • UK’s 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

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

    6. Current Projects - 5 Lekhwair Oil Field Debottlenecking Petroleum Development of Oman/Shell • 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

    7. Current Projects - 6 Joint Industry Project (JIP) – Phase III Five Operating Companies • 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

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

    9. Recent Projects - 3 North Morecambe Field British Gas Hydrocarbon Resources Ltd • 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

    10. 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. 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. Visual Representation of Gas Lift System

    13. Typical Gas Lift Compressor for Mature Assets

    14. Gas Seals – Replacement Seal 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 600OF, 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

    15. 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. Recommendations – Control System 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

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

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

    19. Recommendations - Control System – LP Surge Line • 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

    20. ONGC – Heera Asset • MSE invited by ONGC to InvestigateHeera Asset • Carried out a detailed investigation

    21. 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. Activities • 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. Facilities Overview

    24. Factors With The Potential To LimitMaximum 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. Testing & Analysis

    26. 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. 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. PGC Performance AnalysesTrains A, B & C

    29. Compression Train

    30. GASMAN™ Modelling of Compressor Trains

    31. 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. 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. Train B Stage 1 Head Performance

    34. Train B Stage 1 Efficiency Performance

    35. Train B Stage 2 Head Performance

    36. Train B Stage 2 Efficiency Performance

    37. Train B Stage 3 Head Performance

    38. Train B Stage 3 Efficiency Performance

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

    40. Average Compressor Performance Losses • Performance losses should be viewed in light of more reasonable performance expectations

    41. Realistic Compressor Performance Expectations

    42. Compressor Performance LossesUsing 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 3rd 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. 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. Flow & Power Reconciliation • Flows are not consistent across the stages of compressor trains • Later stages show additional flow • Flows readings checked from flow meter DP’s • 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. Additional PGC Stage Flows

    46. 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. KOD Cascade System

    48. Potential Leakage Paths via KOD Cascades

    49. Offshore Test Performed 27-6-05 • 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. Cascade Test – Flow Traces Train A