Doc #66 — Geothermal Station Maintenance Under Isolation

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RECOMMENDED ACTIONS (BY URGENCY)

First week

1. Confirm operational status of all geothermal stations. Contact Contact Energy (Wairakei, Te Mihi, Ohaaki), Mercury NZ (Kawerau, Nga Awa Purua, Rotokawa, Ngatamariki, Mokai), and any other operators.
2. Classify all geothermal station operational and maintenance staff as critical personnel (Doc #1).
3. Secure existing chemical stocks at stations: scale inhibitors, pH modifiers, corrosion inhibitors, turbine wash chemicals.

First month

4. Complete national inventory of geothermal-specific spare parts: turbine blades and nozzles, wellhead valves, control electronics, instrumentation, pump impellers, heat exchanger tubes.
5. Begin structured knowledge capture interviews with senior geothermal engineers, well operators, and reservoir engineers at all stations.
6. Assess current well condition across all fields — identify wells approaching end-of-life and wells with known casing problems.
7. Inventory all drilling rigs and associated equipment in NZ (both geothermal and petroleum-sector). Classify as strategic assets.
8. Assess chemical treatment stock levels and project depletion timelines at each station.
9. Print all station operating manuals, reservoir management plans, and maintenance schedules.

First year

10. Complete knowledge capture for all stations and wellfields.
11. Establish cross-training programmes between stations (Contact Energy staff train with Mercury NZ staff and vice versa).
12. Develop mechanical descaling procedures and schedules to reduce dependence on chemical scale inhibitors.
13. Begin trials of locally produced scale inhibitors (phosphate-based compounds from NZ rock phosphate — see Section 4.2).
14. Establish pipeline and equipment fabrication capability at regional machine shops for geothermal-specific components (Doc #91).
15. Assess all wellfield reinjection systems and optimise for long-term reservoir sustainability.
16. Develop manual/simplified control procedures for stations currently dependent on computerised control.
17. Begin geothermal direct-use feasibility assessments for greenhouse heating, industrial process heat, and district heating in the Taupo-Rotorua corridor (Doc #79).

Years 2–5

18. Transition from imported chemical inhibitors to locally produced alternatives or mechanical methods as chemical stocks deplete.
19. Expand machining and fabrication capability for turbine blade repair and manufacture (Doc #91, Doc #89).
20. Drill replacement wells as needed using NZ-based drilling capability.
21. Implement geothermal direct-use projects in the TVZ region.
22. Recruit and train next generation of geothermal engineers through polytechnic and university programmes (Doc #156, Doc #162).

Years 5–15

23. Full transition to locally maintained operations — all chemical treatments, mechanical repairs, and well interventions using NZ resources and fabrication.
24. Expand geothermal capacity if reservoir assessments support it.
25. Develop geothermal mineral extraction (silica, lithium) as potential industrial feedstock.

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ECONOMIC JUSTIFICATION

The value of geothermal baseload

NZ’s approximately 1,000 MW of geothermal capacity generates roughly 7,000–8,000 GWh per year.⁵ At a capacity factor of approximately 90–95% (among the highest of any generation technology), this represents a disproportionately large share of reliable, predictable generation.⁶

Comparison with alternatives: Replacing 1,000 MW baseload with hydro would require reserving 1,200–1,500 MW of capacity currently used for load-following, tightening the system’s ability to manage dry periods. Replacing it with wood-fired generation would require roughly 20–25 million cubic metres of wood per year — a large fraction of NZ’s total forestry harvest.⁷ Wind generation is intermittent and cannot provide the continuous, predictable output that baseload geothermal delivers without substantial storage or demand management infrastructure that NZ does not currently have at the required scale.

Maintenance investment vs. replacement cost:

Maintaining existing geothermal stations requires an estimated 150–300 person-years of labour per year across all stations — this includes operations staff, maintenance crews, well drilling teams, and engineering support.⁸ This is far less than the cost of building replacement generation of any type. Geothermal maintenance is one of the highest-return investments in the entire recovery scenario.

The breakeven is immediate. Every day that geothermal stations continue operating, NZ benefits. There is no construction period, no delayed return. The investment is in maintaining what already exists.

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1. NZ’S GEOTHERMAL STATIONS

1.1 Station inventory

All NZ geothermal power stations are located within the Taupo Volcanic Zone. The major stations are:⁹

Station	Capacity (MW)	Operator	Commissioned	Technology	Key notes
Wairakei	~160	Contact Energy	1958	Steam turbines (single & double flash)	NZ’s oldest, extensively upgraded. Some original 1950s equipment still in service. Binary units added.
Te Mihi	~166	Contact Energy	2014	Triple-flash steam	Modern, replaced some older Wairakei capacity. Shares the Wairakei geothermal field.
Ohaaki	~65	Contact Energy	1989	Single flash	Reservoir pressure declined significantly; output well below original 116 MW design.
Kawerau	~170	Mercury NZ (various)	Multiple stages	Binary + steam	Multiple plants on a large field. Also supplies process heat to Norske Skog paper mill and other industrial users.
Nga Awa Purua	~140	Mercury NZ	2010	Triple-flash	Contains one of the world’s largest single-shaft geothermal turbines (a ~147 MW Fuji Electric unit).
Ngatamariki	~82	Mercury NZ	2013	Binary (ORC)	Uses organic Rankine cycle with pentane working fluid.
Rotokawa	~34 (base)	Mercury NZ (joint venture with Tauhara North No. 2 Trust)	1997 (expanded)	Flash + binary	The Rotokawa field also hosts Nga Awa Purua. Combined field output ~174 MW.
Mokai	~100	Mercury NZ (with Tuaropaki Trust)	1999 (expanded)	Flash + binary	Significant Maori ownership through Tuaropaki Trust. Stages added 2005, 2007.

Total installed geothermal capacity: approximately 1,000–1,060 MW.¹⁰

1.2 Technology types

Understanding the technology type matters because it determines the maintenance profile:

Flash steam plants (Wairakei, Te Mihi, Nga Awa Purua, Ohaaki, parts of Kawerau, Rotokawa, Mokai): Hot geothermal fluid (typically 230–320°C) is depressurised (“flashed”) in a separator vessel. The resulting steam drives a conventional steam turbine. The remaining brine (liquid) is reinjected. Flash plants expose turbine blades and steam pathways directly to geothermal steam containing H2S, silica, and other minerals. Scaling and corrosion in steam turbines are primary maintenance concerns.

Binary (organic Rankine cycle) plants (Ngatamariki, parts of Kawerau, Mokai, and others): Geothermal fluid passes through a heat exchanger, transferring heat to a secondary working fluid (typically pentane) that vapourises and drives a turbine.¹¹ The geothermal fluid never contacts the turbine, reducing scaling and corrosion exposure. However, binary plants introduce a different dependency: the organic working fluid is an imported consumable, and heat exchangers must resist brine corrosion.

Mixed plants: Most larger installations combine flash and binary systems. Flash units handle high-temperature steam; binary units capture additional energy from the separated brine before reinjection.

1.3 What makes geothermal maintenance different from hydro

Hydro stations (Doc #65) work with relatively clean water. Geothermal stations work with fluids that are actively hostile to equipment: temperatures of 150–320°C, dissolved silica (hundreds of ppm), H2S (toxic, corrosive), CO2 (acidic), and chloride salts. Steam flashing causes rapid pressure and temperature changes that trigger mineral precipitation. Unlike hydro, geothermal wells produce continuously under reservoir pressure — shutting in a well changes wellbore conditions and can itself cause scaling and thermal shock problems.

The result: geothermal maintenance is more materials-intensive, more chemically dependent, and more time-sensitive than hydro maintenance. A hydro turbine neglected for a few years accumulates repairable cavitation damage. A geothermal pipeline neglected for months can scale shut.

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2. CRITICAL MAINTENANCE CHALLENGES

2.1 Silica scaling

The problem: As geothermal fluid rises from the reservoir and depressurises, dissolved silica (SiO2) becomes supersaturated and precipitates as amorphous silica scale on every surface it contacts — pipelines, separators, turbine nozzles, heat exchangers, reinjection wells. Scaling rates vary by field but can be substantial: in some NZ fields, unmanaged scaling can reduce pipeline internal diameter by 1–5 mm per month depending on fluid chemistry and temperature conditions.¹²

Scale is not a cosmetic problem. It reduces flow capacity, impairs heat transfer in binary plant heat exchangers, erodes turbine performance by altering blade geometry, and can block reinjection wells — which threatens reservoir management (Section 5).

Current management: Chemical inhibition (imported polycarboxylate or phosphonate compounds that delay precipitation¹³), pH modification (acid or caustic to keep silica in solution), mechanical descaling (high-pressure water jetting, pigging, manual chipping — labour-intensive but no imports required), turbine washing, and design measures (operating temperatures/pressures to minimise supersaturation, clarifiers/crystallisers).

Depletion concern: Chemical scale inhibitors are imported with no current NZ production. Consumption is tens to hundreds of tonnes per year across all stations.¹⁴ Existing stocks are estimated to last 6–18 months at normal dosing (assumption). Reduced dosing with increased mechanical descaling can extend this.

2.2 Corrosion from H2S and acidic fluids

The problem: Hydrogen sulphide (H2S) is present in virtually all NZ geothermal fluids, typically at concentrations of tens to hundreds of parts per million.¹⁵ H2S attacks carbon steel, copper, silver, and many common alloys through sulphide stress cracking and general corrosion. It is also extremely toxic — lethal at concentrations above approximately 500–1,000 ppm in air depending on exposure duration¹⁶, and NZ geothermal stations already manage H2S exposure as a routine occupational health concern.

In addition to H2S, geothermal brines may be acidic (pH 3–6 in some NZ fields) due to dissolved CO2 and other species. Combined acid and H2S attack is particularly aggressive.

Current management: Materials selection (carbon steel for low severity, stainless 316L/duplex for moderate, specialty alloys for the most aggressive environments¹⁷), protective coatings (epoxy, FRP, rubber linings), cathodic protection, chemical inhibitors, and corrosion monitoring (coupons, ultrasonic thickness measurement).

Import dependencies: Stainless steel and specialty alloys are entirely imported. NZ Steel (Doc #89) produces carbon steel from ironsand but not stainless — the chromium, nickel, and molybdenum are not mined in NZ. Existing stainless stocks represent a finite inventory. FRP materials are also imported.

2.3 Turbine blade erosion and fouling

In flash plants, turbine blades are exposed to steam carrying silica particles and brine droplets, causing erosion and scale deposition. Blades are manufactured from specialty stainless steels or titanium by OEMs (Fuji Electric, Mitsubishi, Toshiba)¹⁸ and are imported. Overhauls every 3–7 years include inspection, repair, or replacement. Binary plants avoid this problem (clean working fluid).

Import dependency: Replacement blades are among the most demanding fabrication tasks. Local fabrication is possible in principle (Doc #91) but requires appropriate alloy stock and tight dimensional tolerances. Locally fabricated blades from carbon steel with hard-face overlay would have shorter service life than OEM parts — estimated at 30–60% of OEM blade life (see Section 4.5) — meaning more frequent overhauls and somewhat reduced turbine efficiency.

2.4 Well maintenance and drilling

Geothermal wells have finite lives — typically 10–30 years — due to casing corrosion, wellbore scaling, thermal cycling damage, and reservoir changes.¹⁹ They require periodic workover (cleaning, casing repair, stimulation), replacement drilling (1,000–3,000+ metres depth), and wellhead maintenance (valves, expansion spools, flow measurement).

NZ’s drilling capability is a genuine strength. Several contractors operate geothermal and petroleum rigs in NZ, with experienced crews.²⁰ The petroleum sector (Taranaki) provides additional rig availability and cross-trained personnel.

Import dependencies for drilling: Drill bits (tungsten carbide — imported), drilling mud chemicals (bentonite, barite, polymers — mostly imported, though NZ has some bentonite deposits²¹), casing pipe (some producible from NZ Steel, though current casing is speciality pipe), and cementing chemicals (Portland cement is NZ-produced, but specialty well cements use imported additives).

2.5 Control electronics and SCADA

The same challenge as hydro (Doc #65) and the grid (Doc #67): modern control systems depend on electronic components with finite life that NZ cannot manufacture. Geothermal stations are less amenable to full manual reversion than hydro — chemical dosing and gas extraction (removing CO2 and H2S from the condenser) benefit from automated monitoring — but the core generation process can operate under simplified control. Strategy: maintain existing systems, stockpile spares, document all configurations in print, prepare for progressive simplification.

2.6 Cooling system maintenance

Most NZ geothermal stations use evaporative cooling towers. These require structural maintenance (timber, concrete, plastic fill — NZ-producible materials, Doc #97), water treatment (currently imported biocides and antiscalants — chlorination from local salt electrolysis is a feasible substitute, Doc #48), and fan/pump maintenance. Air-cooled condensers at some stations (notably Ngatamariki) avoid water treatment but depend on large fan arrays and finned heat exchanger surfaces.

2.7 Binary plant working fluid

Binary plants use pentane (isopentane or n-pentane) — flammable, volatile, not produced in NZ. The working fluid circulates in a closed loop with typical annual losses of 1–3% of charge under good maintenance.²²²³ At 2% annual loss (typical of well-maintained seals), a plant loses ~20% of charge in 10 years — significant but manageable if makeup fluid can be sourced. At 5% (degraded seals or a leak event — an assumption, as degradation rates under reduced maintenance are uncertain), meaningful capacity loss occurs within 3–5 years.

Potential NZ sources: Marsden Point refinery (closed 2022) could potentially be partially reactivated.²⁴ Taranaki gas condensate is another possibility. Both pathways are genuine but uncertain.

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3. CONSUMABLE DEPLETION TIMELINES

The following estimates assume competent centralised management and rationing. Without coordinated management, timelines compress significantly.

Consumable	Current source	Estimated managed life	NZ substitute pathway
Scale inhibitors (polycarboxylates, phosphonates)	Imported speciality chemicals	6–18 months at normal dosing	Mechanical descaling; NZ rock phosphate-derived phosphates (partial substitute); pH management with NZ-produced acid/caustic
Corrosion inhibitors	Imported	6–18 months	Cathodic protection (zinc anodes from NZ sources); material selection; accept higher corrosion rates with monitoring
Stainless steel pipe/plate (316L, duplex)	Imported	Finite stock — years to a decade depending on existing inventory and rate of replacement	Carbon steel with increased wall thickness and corrosion allowance; FRP lining from existing stock; eventually carbon steel with sacrificial replacement strategy
Specialty alloys (Inconel, Hastelloy, titanium)	Imported	Very limited stock	No NZ substitute. Restrict use to most critical applications. Repair and refurbish rather than replace.
Turbine blades/nozzles	Imported (OEM manufactured)	5–15 years depending on erosion rates and spares inventory	Local fabrication from best available alloy stock (Doc #91). Performance and life will be shorter than OEM parts.
Drill bits (tungsten carbide)	Imported	Depends on drilling activity — stock for perhaps 5–20 wells’ worth at current inventory levels (estimate)	Resharpening and reconditioning extends life. No NZ tungsten carbide production.
Drilling chemicals (bentonite, barite, polymers)	Imported	1–3 years of stock depending on activity level	NZ bentonite deposits exist (Canterbury) but are of variable quality.25 Some NZ-sourced clay substitutes possible. Polymer mud additives have no NZ substitute.
Binary working fluid (pentane)	Imported	5–15 years at low loss rates	Potential from Taranaki gas condensate or refinery reactivation — uncertain.
Elastomeric seals and gaskets	Imported	Progressive failure over 3–10 years depending on application	Leather seals, machined PTFE (from existing stock), compressed fibre gaskets. Accept increased leakage in some applications.
Control electronics	Imported	5–15 years depending on component and environment	Cannibalization, simplified analogue controls, manual reversion where possible. No NZ substitute.
H2S safety equipment (gas detectors, respirators)	Imported	Detectors: 2–5 years (sensor element life). Respirator filters: 1–3 years of stock.	Wind direction awareness, natural ventilation, manual monitoring (wet chemistry H2S tests). Increased safety risk.

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4. LOCAL SUBSTITUTION PATHWAYS

4.1 Mechanical descaling as primary scale management

As chemical stocks deplete, scale management shifts to mechanical methods — a proven approach that predates modern chemical inhibition. Methods: high-pressure water jetting (pumps available from NZ industrial cleaning companies, maintainable locally), pigging (pushing mechanical scrapers through pipelines — fabricable from NZ steel), manual chipping and grinding, and turbine washing (online water injection and offline washing during shutdowns).

Performance gap: Mechanical descaling is more labour-intensive and requires more frequent shutdowns. Expect perhaps 5–15% reduction in annual capacity factor and a larger maintenance workforce dedicated to descaling. This is a real cost but far better than losing the stations.

4.2 Locally produced scale inhibitors

Existing stocks of phosphate-based fertiliser products could be redirected for scale inhibition.²⁶ Simple polyphosphate dosing is more realistic near-term than synthesising commercial phosphonate formulations (which would require phosphate rock + sulfuric acid + organic chemistry — Doc #113). Polyphosphate dosing is estimated to provide 30–60% of the scale-inhibition effectiveness of commercial formulations, though this range is a rough estimate based on general phosphate chemistry; actual performance will vary by field and fluid composition, requiring station-specific field testing.²⁷ Combined with increased mechanical descaling, this may maintain operations at reduced but acceptable capacity.

4.3 Acid and caustic production

Hydrochloric acid and caustic soda are producible from salt via the chlor-alkali process.²⁸ The dependency chain: NZ coastal salt (evaporated or mined), electric power (available from the grid), and electrolytic cells (requiring electrode fabrication from titanium or graphite — titanium is imported, graphite may be available from existing industrial stocks). Constructing a chlor-alkali plant is a significant industrial project requiring acid-resistant containment vessels, membrane or diaphragm separators, and brine purification capability — realistically a Year 2–4 timeline (Phase 2–3).

Sulfuric acid is potentially producible from NZ geothermal sulfur sources (natural deposits at TVZ volcanic sites) or from pyrite, but requires acid-resistant containment (Doc #113).

In the interim, existing acid and caustic stocks in chemical distributor warehouses should be requisitioned and allocated to geothermal stations as a strategic priority.

4.4 Carbon steel as a stainless steel substitute

Carbon steel with increased wall thickness can substitute for stainless steel in many pipeline and vessel applications, accepting a finite service life (5–15 years vs. 20–40+ for stainless) and planned replacement cycle. This is how Wairakei originally operated in 1958.²⁹ More frequent replacement requires more steel (NZ Steel — Doc #89), more welding (Doc #94), and more planned outages — a manageable degradation. NZ Steel at Glenbrook produces carbon steel plate, and NZ pipe fabrication workshops can roll and weld plate into pipe sections.³⁰

4.5 Turbine blade repair and fabrication

Turbine blade repair through weld overlay — building up eroded or damaged blade surfaces with hard-facing alloy — is a technique already practised in NZ. The welding consumables for hard-facing (typically cobalt or nickel-based alloys such as Stellite³¹) are imported, but existing stocks in welding supply warehouses plus stocks held by the geothermal companies themselves should last for multiple repair cycles — the exact number depends on inventory levels that should be established through the spares census (Recommended Action #4).

Local fabrication of replacement blades requires: appropriate alloy stock (realistically carbon steel with hard-face overlay — inferior to OEM alloys), multi-axis or skilled manual machining with OEM drawings or careful measurement of existing blades (Doc #91), and high-quality surface finish (affects both aerodynamic performance and erosion resistance).

Performance gap: Locally fabricated blades may have 30–60% of the service life of OEM blades (rough estimate), meaning more frequent overhauls and lower average efficiency. This is a manageable degradation — the stations continue to generate power at somewhat reduced output.

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5. WELLFIELD MANAGEMENT

5.1 The reservoir is the foundation

All surface plant maintenance is worthless if the reservoir declines below useful output. NZ’s fields have been under active management for decades, and reservoir engineering is among the strongest areas of NZ’s geothermal expertise. Key principles:³²

• Reinjection: Returning used fluid to the reservoir maintains pressure and extends field life. All NZ’s modern stations practise reinjection. It is essential.
• Monitoring: Continuous measurement of well output, reservoir pressure, and surface features (subsidence, changes in natural hot springs). Used to track reservoir health and adjust production.
• Production limits: Overproduction leads to declining output, reservoir cooling, and potentially irreversible damage. Ohaaki is the cautionary example — output declined from 116 MW design to ~65 MW due to early overproduction.³³
• Well spacing: Locating production and reinjection wells to maximise heat extraction without short-circuiting (cold reinjected fluid reaching production wells prematurely).

5.2 Reservoir management under isolation

Monitoring can continue with basic instruments (pressure gauges, temperature loggers, flow meters) that are more robust than complex SCADA. The critical capability is reservoir engineering expertise to interpret the data — NZ has this.

Reinjection system maintenance is directly linked to reservoir sustainability. If reinjection wells fail and cannot be worked over or replaced, the station must either discharge waste fluid to surface waterways (environmental damage) or reduce production. This makes drilling/workover capability (Section 2.4) essential.

Conservative production strategy: Under isolation, the incentive to maximise short-term output is intense. But overproduction that damages the reservoir trades short-term gain for long-term loss. This document recommends that reservoir management decisions remain with qualified reservoir engineers, not with political authorities responding to immediate demand.

5.3 Field-specific notes

Wairakei-Tauhara: World’s longest-operating field. Well-managed in recent decades. The Tauhara extension was under development pre-event and may offer additional capacity.³⁴ Kawerau: Large, mature field with dual electricity/industrial heat role. Multiple operators require coordination. Ngatamariki and Rotokawa: Young fields, good reservoir condition — should sustain current output for decades. Mokai: Conservative management under Tuaropaki Trust. Ohaaki: Already depleted; low priority for additional investment unless reservoir recovery is demonstrated.

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6. GEOTHERMAL DIRECT-USE APPLICATIONS

6.1 Why direct use matters under isolation

Using geothermal heat directly — for space heating, greenhouses, industrial process heat, aquaculture, drying — is thermodynamically more efficient than converting to electricity first. Iceland uses geothermal heat for ~90% of space heating and extensive greenhouse agriculture.³⁵ NZ has comparable resource quality but has barely developed direct-use outside Kawerau’s industrial heat and small-scale Rotorua projects. Expanding direct use in the TVZ is a high-return opportunity under isolation.

6.2 Priority direct-use applications

Greenhouse heating (Doc #79): The most compelling direct-use application under nuclear winter. Geothermal heat can maintain greenhouse temperatures sufficient for crop production — including crops that cannot survive outdoors under severe cooling. The Taupo-Rotorua corridor has abundant low-to-medium temperature geothermal resources suitable for greenhouse heating. Construction of geothermally heated greenhouses should begin in Phase 1 and expand through Phases 2–3.

Key requirements: greenhouse structures (timber frame, glass or plastic — Doc #164), piped hot water distribution from existing wells or new shallow wells, and agricultural expertise for greenhouse production. Wells producing 80–120°C fluid suitable for greenhouse heating are shallower (typically 200–1,000 m vs. 1,000–3,000+ m for power station wells) and less technically demanding to drill, though they still require drilling rigs, casing, and wellhead equipment.³⁶

District heating in Rotorua: Rotorua already has limited bore-based geothermal heating. A coordinated district heating scheme would reduce electricity demand for heating and improve quality of life. The technology is well-proven internationally (Iceland, France, Turkey).³⁷ The barrier is infrastructure construction.

Industrial process heat at Kawerau: Already supplies process heat to the Norske Skog paper mill and other industrial users. Under isolation, any industrial processes requiring heat (drying, sterilisation, food processing, timber drying) should be co-located with Kawerau or other suitable geothermal sources where possible.

Aquaculture and drying: Warm geothermal water can support aquaculture otherwise impossible under nuclear winter cooling. Geothermal heat at 60–100°C is well-suited for large-scale food drying and dehydration (Doc #78).

6.3 What direct use requires

Direct-use development requires shallow well drilling (less demanding than power station wells but still requiring rigs and casing), insulated pipeline construction (carbon steel pipe from NZ Steel with locally fabricated insulation — Doc #89), heat exchangers (for low-temperature applications below ~120°C, carbon steel or copper tube-and-shell designs are fabricable from NZ materials, though with shorter service life than stainless steel units — see Section 4.4), and pumps (Doc #91). NZ geothermal consultancies have direct-use design experience. The main constraint is construction capacity under recovery conditions.

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7. STATION-SPECIFIC PRIORITIES AND VULNERABILITIES

7.1 Wairakei and Te Mihi

Priority: Very high. Combined ~326 MW. Wairakei (1958, extensively upgraded) has the advantage of institutional memory of operating with carbon steel and without modern chemical treatment. Te Mihi (2014) is modern — less near-term maintenance but more dependent on OEM parts. Key vulnerability: shared field and fluid supply network.

7.2 Kawerau

Priority: Very high. ~170 MW plus industrial heat supply — the dual electricity/heat role makes it uniquely valuable. Key vulnerability: multiple operators on one field require a single field management authority.

7.3 Nga Awa Purua

Priority: High. ~140 MW from a single large Fuji Electric turbine (~147 MW rated).³⁸ Key vulnerability: capacity concentrated in one turbine. Catastrophic failure could mean extended outage. Fuji Electric documentation must be secured in print.

7.4 Ngatamariki

Priority: High. ~82 MW binary. Key vulnerability: pentane working fluid dependence (Section 2.7) and OEM-specific ORC equipment. Ormat documentation must be secured.

7.5 Mokai, Rotokawa, and Ohaaki

Mokai (~100 MW): Significant Maori ownership through Tuaropaki Trust integrates kaitiakitanga principles into resource management — an approach that aligns with the conservative, long-term reservoir strategy recommended here. Tuaropaki Trust’s governance role should be maintained.

Rotokawa (~34 MW base plant, separate from Nga Awa Purua on the same field): Joint venture with Tauhara North No. 2 Trust. Moderate-high priority.

Ohaaki (~65 MW, reduced from 116 MW design capacity due to reservoir decline): Demonstrates the consequences of suboptimal reservoir management. A cautionary example that informs conservative management of all other fields. Lower priority for new investment — maintain at current reduced output if maintenance cost is proportionate, but do not invest heavily to restore lost capacity.

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8. WORKFORCE

8.1 NZ’s geothermal expertise — a genuine strength

NZ has a substantial base of geothermal expertise developed over six decades of commercial operation, spanning: the University of Auckland’s geothermal programme (training engineers since 1979), GNS Science (Te Pu Ao) with extensive research at Wairakei and Lower Hutt, engineering consultancies (AECOM, Jacobs, and specialist firms) with international project experience, operator in-house teams at Contact Energy and Mercury NZ, drilling contractors, and the NZ Geothermal Association (NZGA).

This expertise is not replaceable from a textbook. It represents decades of accumulated field experience with NZ-specific conditions. Knowledge capture (Recommended Action #5) should prioritise senior personnel at GNS Science and the university programme as well as station operators.

8.2 Workforce numbers

Precise workforce numbers are not publicly available in disaggregated form. An estimate based on the scale of operations: NZ’s geothermal industry employs in the range of 500–1,500 people directly and indirectly, across power station operations, drilling, engineering consultancy, research, and support services.³⁹ The skills census (Doc #8) should establish the actual figure. What matters most is not the total number but the distribution of specific skills — how many experienced reservoir engineers, how many well drillers, how many turbine maintenance specialists, how many control system engineers.

8.3 Training the next generation

Geothermal engineering spans mechanical, chemical, geological, reservoir, electrical, and drilling disciplines. The University of Auckland’s geothermal programme (Doc #162) should be expanded as a national priority, with close integration between academic training and on-station apprenticeship. Cross-training between stations (Recommended Action #11) serves both knowledge resilience and workforce development.

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9. CRITICAL UNCERTAINTIES

Uncertainty	Impact	Mitigation
Silica scaling rates without chemical inhibitors	Determines frequency and intensity of mechanical descaling. May force capacity reduction if scaling outpaces mechanical removal.	Begin trials immediately of reduced-chemical and chemical-free operation. Station-specific — each field’s chemistry is different.
Corrosion rates with carbon steel substitution	Determines replacement frequency and steel consumption. If corrosion is faster than expected, maintenance burden increases.	Monitor aggressively (ultrasonic thickness, corrosion coupons). Build replacement schedule into planning.
Turbine blade life with local fabrication	Determines overhaul frequency and turbine availability.	Start blade repair early while OEM spares are available. Gather data on locally fabricated blade life.
Binary plant working fluid availability	Determines binary plant longevity. If pentane cannot be locally sourced, binary plants decline over 10–15 years.	Minimise losses (seal maintenance), explore Taranaki gas condensate as source. Consider converting some binary capacity to flash if reservoir conditions allow.
Drilling capability under isolation	Determines whether replacement wells can be drilled as existing wells fail.	Protect drilling equipment and crews. Inventory and ration drill bits and drilling chemicals.
Reservoir long-term behaviour	Reservoirs respond to decades of production. NZ’s reservoirs are generally well-managed, but behaviour over 30–50 year horizons is inherently uncertain.	Maintain conservative production rates. Continue reservoir monitoring. Preserve reservoir engineering expertise.
Control system lifespan	Determines when manual/simplified control is needed.	Same strategy as hydro: stockpile, document, prepare for simplification.
Ohaaki reservoir recovery	Whether Ohaaki’s reservoir partially recovers if production is reduced.	Monitor. If recovery occurs, it provides bonus capacity. Don’t invest heavily on the assumption it will.
Nuclear winter effects on cooling tower performance	Colder ambient temperatures may actually improve cooling tower performance (better heat rejection). But changed precipitation and water availability could affect makeup water supply.	Monitor. Net effect is likely slightly positive for cooling performance.
Institutional coordination between operators	Contact Energy and Mercury NZ currently operate independently. Under crisis conditions, coordinated management of the national geothermal resource is essential.	Establish a national geothermal coordination body early, with participation from both companies, GNS Science, iwi partners, and government.

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10. CROSS-REFERENCES

• Doc #1 — Stockpile Strategy: requisition of chemical stocks, spare parts, drilling consumables
• Doc #8 — Skills Census: geothermal workforce, equipment, and spares inventory
• Doc #34 — Lubricant Production: turbine and pump bearings, valve mechanisms
• Doc #65 — Hydroelectric Maintenance: sister document; shared challenges (control systems, lubrication, workforce)
• Doc #67 — Transpower Grid: grid integration, dispatch, baseload role
• Doc #69 — Transformer Maintenance: station transformers share grid-wide challenges
• Doc #79 — Geothermal Greenhouses: direct-use food production
• Doc #79 — Greenhouse Construction: methods applicable to geothermally heated greenhouses
• Doc #89 — NZ Steel Glenbrook: carbon steel for replacement pipework and vessels
• Doc #91 — Machine Shop Operations: fabrication of turbine components, valve parts, fittings
• Doc #94 — Welding Consumables: geothermal pipework and vessel welding
• Doc #113 — Sulfuric Acid: prerequisite for some chemical treatment pathways
• Doc #157 — Trade Training: geothermal operations workforce pipeline
• Doc #162 — University Reorientation: geothermal research and education

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FOOTNOTES

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1. MBIE, Energy in New Zealand 2023. Geothermal generation was 7,943 GWh in 2022 (~18% of total) from ~1,047 MW installed capacity. https://www.mbie.govt.nz/building-and-energy/energy-and-n...↩︎

2. Bibby, H.M. et al., “Geothermal fields of the Taupo Volcanic Zone, New Zealand,” Journal of Volcanology and Geothermal Research, 1995. The TVZ extends ~300 km from Mt Ruapehu to Whakaari/White Island and contains NZ’s entire high-temperature geothermal resource.↩︎

3. Bixley, P.F. and Clotworthy, A.W., “Evolution of the Wairakei geothermal reservoir,” Geothermics, 2009. Wairakei was the first geothermal station outside Italy and Iceland (1958).↩︎

4. NZ-trained geothermal engineers have worked on projects in over 60 countries. See: NZGA; University of Auckland School of Environment.↩︎

5. MBIE, Energy in New Zealand 2023. Geothermal generation was 7,943 GWh in 2022 (~18% of total) from ~1,047 MW installed capacity. https://www.mbie.govt.nz/building-and-energy/energy-and-n...↩︎

6. Electricity Authority EMI database. https://www.emi.ea.govt.nz/ — Geothermal capacity factors of 90–95% compare to 40–55% for hydro and 30–45% for wind.↩︎

7. MPI, Forestry and Wood Processing Data. NZ harvest ~35 million m³/year. Replacing ~8,000 GWh geothermal at 30% thermal efficiency would require ~20–25 million m³ of wood per year. Order-of-magnitude estimate.↩︎

8. Estimate from Contact Energy and Mercury NZ annual reports, scaled to include contractors and support services. Actual figure should be established through the skills census (Doc #8).↩︎

9. Station data from Contact Energy and Mercury NZ annual reports, Electricity Authority EMI database, and NZGA publications. Capacity figures are approximate nameplate ratings. Ownership structures simplified.↩︎

10. MBIE, Energy in New Zealand 2023. Geothermal generation was 7,943 GWh in 2022 (~18% of total) from ~1,047 MW installed capacity. https://www.mbie.govt.nz/building-and-energy/energy-and-n...↩︎

11. DiPippo, R., Geothermal Power Plants, 4th ed., 2016. Binary plants use organic working fluids (lower boiling point than water) to generate power from lower-temperature resources without exposing the turbine to geothermal fluid.↩︎

12. Mroczek, E.K. et al., “Silica scaling in cooled siliceous waters,” Geothermics, 2015; Brown, K., “Mineral scaling in geothermal power production,” UN University Geothermal Training Programme, 2013.↩︎

13. Mroczek, E.K. et al., “Silica scaling in cooled siliceous waters,” Geothermics, 2015; Brown, K., “Mineral scaling in geothermal power production,” UN University Geothermal Training Programme, 2013.↩︎

14. Chemical consumption figures are commercially sensitive. The estimate of tens to hundreds of tonnes/year across all stations is based on general industry practice. Actual figures should be established from operator records.↩︎

15. Ellis, A.J. and Mahon, W.A.J., Chemistry and Geothermal Systems, 1977. H2S concentrations vary between fields; Wairakei and Rotokawa are known for relatively high H2S.↩︎

16. NIOSH Pocket Guide to Chemical Hazards: Hydrogen Sulfide. Immediately dangerous to life or health (IDLH) concentration is 100 ppm; concentrations of 500–1,000 ppm can cause rapid loss of consciousness and death depending on exposure duration. https://www.cdc.gov/niosh/npg/npgd0337.html↩︎

17. Lichti, K.A., “Corrosion chemistry of some New Zealand geothermal environments,” Proceedings World Geothermal Congress, 2005.↩︎

18. The Fuji Electric turbine at Nga Awa Purua was at commissioning the world’s largest single-cylinder geothermal turbine. NZ station maintenance is typically under long-term OEM service agreements.↩︎

19. Hole, H., “Geothermal well design — casing and wellhead,” UN University Geothermal Training Programme, 2008. Some Wairakei wells have served 50+ years; others require replacement within 10–15 years.↩︎

20. Several drilling contractors operate in NZ with geothermal experience. Taranaki petroleum drilling provides additional rig availability and cross-trained crews. Exact rig count should be established via asset census (Doc #8).↩︎

21. GNS Science mineral resources database. NZ bentonite deposits (Canterbury) are of variable quality and undeveloped for drilling applications.↩︎

22. Typical ORC module of 20–40 MW may contain 50–200 tonnes of working fluid. Not routinely published; estimated from plant thermal capacity and cycle design.↩︎

23. Branchini, L. et al., “Systematic comparison of ORC configurations,” Energy, 2013. Annual losses of 1–3% of charge under good conditions.↩︎

24. Marsden Point refinery closed as a refinery March 2022 (converted to import terminal). Reactivation theoretically possible but would require significant effort and feedstock.↩︎

25. GNS Science mineral resources database. NZ bentonite deposits (Canterbury) are of variable quality and undeveloped for drilling applications.↩︎

26. NZ superphosphate industry imports rock phosphate. Existing phosphate stocks are limited relative to agricultural demand (Doc #80); diversion to geothermal use requires careful allocation.↩︎

27. Polyphosphate scale inhibition effectiveness varies with fluid chemistry, temperature, and silica concentration. Commercial phosphonate formulations are engineered for specific conditions; simple polyphosphate dosing provides partial but less targeted inhibition. The 30–60% range is estimated from general phosphate chemistry literature; station-specific field trials are required to establish actual performance.↩︎

28. The chlor-alkali process requires salt (NZ coastal production), electric power (available), and electrolytic cells. Well-understood chemistry, but constructing a plant is a significant industrial project.↩︎

29. Wairakei’s original (1958) surface infrastructure used carbon steel extensively, providing a benchmark for expected corrosion rates in NZ geothermal fluid chemistries.↩︎

30. NZ Steel product range: hot-rolled coil, plate, structural sections. No stainless, alloy, or heavy-wall specialist pipe. See Doc #89.↩︎

31. Stellite is a family of cobalt-chromium alloys manufactured by Kennametal Inc. (formerly Deloro Stellite). Used extensively for hard-facing in geothermal, steam turbine, and valve applications due to resistance to erosion, corrosion, and high-temperature wear.↩︎

32. Grant, M.A. and Bixley, P.F., Geothermal Reservoir Engineering, 2nd ed., Academic Press, 2011.↩︎

33. Clotworthy, A., “Ohaaki geothermal system: reservoir response to 30 years of production,” Proceedings World Geothermal Congress, 2005. Decline attributed to production exceeding natural recharge plus reinjection.↩︎

34. Contact Energy’s Tauhara Stage 2 (~250 MW binary cycle) was under construction in the early 2020s. If operational or near-operational at the time of the event, it would significantly expand NZ’s geothermal capacity. If incomplete, the partially built infrastructure and drilled wells are a resource that could potentially be finished using NZ capability.↩︎

35. Orkustofnun (National Energy Authority of Iceland). Iceland uses geothermal for ~90% of space heating plus extensive greenhouse agriculture. NZ’s resource is comparable but almost exclusively developed for electricity.↩︎

36. DiPippo, R., Geothermal Power Plants, 4th ed., 2016. Direct-use wells for greenhouse heating typically target shallow, lower-temperature reservoirs (80–120°C at 200–1,000 m depth), compared to power station wells targeting 230–320°C resources at 1,000–3,000+ m.↩︎

37. Lund, J.W. and Toth, A.N., “Direct utilization of geothermal energy 2020 worldwide review,” Proceedings World Geothermal Congress, 2020. France operates the Paris Basin district heating system (one of the world’s largest); Turkey is the world’s largest user of geothermal district heating by installed capacity.↩︎

38. The Fuji Electric turbine at Nga Awa Purua: ~147 MW single shaft, at commissioning (2010) the world’s largest single geothermal turbine.↩︎

39. NZGA has reported ~3,500 people in the broader geothermal sector. The number directly in power station operations is a subset. Should be verified via skills census.↩︎