A floating storage regasification unit, a floating LNG producer and an LNG carrier have one thing in common beyond the cargo: every one of them runs thousands of metres of small-to-medium bore pipe carrying seawater at 3-32 °C through coolers, ballast lines, fire & wash systems, and the regas / liquefaction heat exchangers. In a topsides module, a turret or a hull, those lines see velocity, sand, sulfides, occasional chlorination, vibration and constant thermal cycling. Pick the wrong alloy, and the line fails at the welded joint in 3-7 years. Pick the right one — almost always a 90/10 or 70/30 copper nickel alloy — and the same line is still in service in year 30. The choice is not about a material catalogue; it is about a small set of standards, a velocity envelope, and a galvanic-isolation discipline that has to be specified before the RFQ goes out.

EZ Steel Industrial has been producing copper nickel alloy pipe, copper nickel flanges, and companion pipe fittings and gasket stud bolt and nut kits from its Changsha base since 1994 — 500+ employees, 480,000+ t/y capacity across 8 product categories, with project book that includes petrochemical facilities, marine ship-building and power-plant / aerospace supply. This guide is the field-tested summary of how a 2026 buyer should specify, cross-check, and source a Cu-Ni piping package for a floating LNG asset.

1. Why 90/10 and 70/30 Cu-Ni Dominate Offshore LNG Seawater

Three things decide a seawater-pipe alloy: corrosion in chloride, resistance to sulfide attack, and tolerance of the velocity / sand / vibration regime. 90/10 (UNS C70600 / C70620) and 70/30 (UNS C71500 / C71520) are the two alloys that hit all three targets at a price the shipyard can live with. Pure copper is too soft for piping, aluminium brass fails in sulfide service, and titanium is over-specified for the bulk of the cooling / ballast network. The Cu-Ni family gets its corrosion resistance from a thin, self-healing surface oxide film that re-forms within hours if it is mechanically damaged — that is the engineering reason a 30-year-old Cu-Ni cooler bundle can be inspected and returned to service, while a 304 stainless cooler in the same seawater service has to be replaced.

The four UNS numbers a buyer actually writes in the RFQ

• C70600 — 90/10, the general-purpose seawater alloy. Seamless (B466) and welded (B467) pipe, fittings (B467), tube (B111).
• C70620 — modified 90/10 with lower lead (≤ 0.02 %) for improved weldability. Use when the welding procedure is GTAW / GMAW with full-penetration butt welds and the inspector will hold you to ASME B31.3.
• C71500 — 70/30, the higher-velocity / higher-temperature / longer-life alloy. Selected for the seawater inlet sections, regas-module cooling, and any pipework downstream of a chlorination dosing point.
• C71520 — modified 70/30 (Pb ≤ 0.02 %), for welded high-integrity service.

2. The Standards That Cover a Cu-Ni Seawater Piping System

The buyer-side mistake is to write "Cu-Ni" in the RFQ and expect a single standard to cover pipe, fittings, flanges and the joint. There is no single standard. The Cu-Ni package in an FSRU / FLNG / LNG carrier typically spans 4-6 standard documents, and the way they map to each component is the first thing to lock before the mill quotes.

3. The Velocity Envelope — The Single Most-Important Design Number

Velocity decides whether a Cu-Ni line runs for 30 years or fails at the welded elbow in year 5. The EEMUA 234 limits are not "soft" engineering guidance — they are the basis for class society approval, and the inspector will check them. The two limits that matter most:

The visual representation of the operating envelope, as a buyer should keep it on the datasheet, is:

4. FSRU vs FLNG vs LNG Carrier — Where the Cu-Ni Sits in Each

The Cu-Ni scope is similar across the three asset types, but the proportion of pipe, fittings, flanges and valves shifts with the operating profile. The table below is what the engineering and procurement teams should be looking at together before the RFQ goes out.

The engineering insight that ties the three together: in every case, the sea-chest, the overboard discharge and the chlorination dosing point are the highest-velocity / highest-erosion parts of the system, and the alloy there is the design driver. If you cannot inspect a pipe once the module is in service, that pipe should be 70/30 rather than 90/10 — even at the 1.4-1.8× unit-cost premium.

5. The Bolted Joint — Where Cu-Ni Systems Actually Leak

The failure mode in a Cu-Ni seawater system is almost never the pipe itself. It is the joint: a galvanic cell between the Cu-Ni flange and a carbon-steel bolt, a gasket that has absorbed chloride and swollen, or a weld that was made without a backing-gas purge and now has porosity on the seawater side. The bolted-joint stack is therefore the section of the package that deserves the most specification time, not the pipe wall.

5.1 Galvanic isolation

When Cu-Ni flanges are bolted to a steel pipe (or a steel flange), the galvanic series puts the steel at risk. The standard fix is to use Cu-Ni stud bolts and nuts (per ASTM B151, matched to the flange UNS number), or — for budget-critical sections — to use carbon-steel bolting with a CSA-class gasketing kit (a phenolic gasket with an EPDM ｉｎｓｅｒｔ that breaks the galvanic path). EZ Steel ships copper nickel flanges with matching Cu-Ni stud-bolt kits in the same heat to avoid this risk by default. The buyer can also specify a 1.5-3 mm thick "isolation gasket" (phenolic + EPDM) as a third option for budget-driven packages.

5.2 Gasket selection

Compressed non-asbestos fibre (CNAF) gaskets with nitrile or EPDM binder are the default for Cu-Ni class 150 / PN 10 service. Spiral-wound gaskets with graphite or PTFE filler and a 304 / 316 inner ring are used for Class 300 / PN 25 and above. PTFE gaskets are chemically clean and resist chlorination, but they creep under sustained load and need re-torqueing on the first hot cycle. EZ Steel ships matching gasket stud bolt and nut kits for every Cu-Ni flange shipment, with the gasket and bolting material traceable to the same QA plan.

5.3 Welding the joint

Cu-Ni welds need backing-gas (typically 99.99 % argon at 15-25 cfh) on the inside of the pipe to prevent internal oxidation. The filler metal is AWS A5.6 / A5.7M ECuNi (for 90/10) or the matching 70/30 filler (for C71500). The weld procedure should be qualified per ASME IX or EN ISO 15614, and the inspector should require dye-penetrant or radiographic NDT on every joint — not just on a sampling basis. A weld that looks clean from the outside but has internal oxide will start pitting within the first 18 months of seawater service.

6. The Project-Bundle: Why Mills-Direct Matters More for Cu-Ni Than for Most Other Alloys

The reason the project-bundle model matters more for Cu-Ni than for carbon steel is the documentation chain. Each component — pipe, fitting, flange, stud bolt, gasket — has to be traceable to the same heat (or to a heat with the same chemistry / mechanical envelope). A stockist-sourced package typically has 3-4 different heat numbers, 3-4 different MTC chains, and a class surveyor who will raise a non-conformity if the chemistries do not match. A mill-direct package has one heat, one MTC chain, one QA team, and one set of EN 10204 3.1 (or 3.2 for class-witnessed) certificates. EZ Steel's 8-category, 480,000+ t/y Changsha operation is built around this model — copper nickel alloy pipe, pipe fittings, copper nickel flanges, pipe flanges in carbon / stainless / alloy, gasket stud bolt and nut, and industrial valves — all from the same production system with one project manager and one MTC chain.

What the buyer actually saves with a mill-direct Cu-Ni package

• 3-5 weeks critical path on a typical 8-12 week fabrication window, by collapsing separate supplier inspections into one visit.
• 5-10 % material cost by removing the stockist margin on Cu-Ni fittings and flanges (the most stockist-sensitive part of the package).
• One MTC chain — EN 10204 3.1 for the bulk of the package, 3.2 for class-witnessed (DNV / Lloyd's / BV / ABS) sections, with no document reconciliation between suppliers.
• One freight booking — typically 12-20 % logistics saving on a 20-40 t Cu-Ni package, and one customs entry.

7. Four 2026 Trends a Cu-Ni Buyer Should Know

8. RFQ Checklist for a Cu-Ni Seawater Package (12 items)

1. Alloy UNS number — C70600 / C70620 for 90/10, C71500 / C71520 for 70/30. State the modified (low-Pb) variant for welded service.
2. Standard documents — pipe ASTM B466 (seamless) or B467 (welded); fittings ASME B16.9 (BW) or B16.11 (SW); flanges ASME B16.5 (Class 150 / 300); EEMUA 234 compliance required for the package; class-society reference (DNV / Lloyd's / BV / ABS) named explicitly.
3. Size range and schedule — NPS or DN, schedule / wall, length type (single random, double random, cut-to-length, U-bend for cooler headers).
4. Velocity envelope — nominal and peak (m/s). State the design velocity class; this is what the inspector will check.
5. Joint discipline — Cu-Ni flanges with Cu-Ni bolting, OR Cu-Ni flanges with carbon-steel bolting and isolation gaskets. State the choice; do not leave it to the mill.
6. Gasket type — CNAF / nitrile, spiral-wound, PTFE, with the relevant ASME / EN dimensional standard.
7. Weld procedure — ASME IX or EN ISO 15614 qualified, with ECuNi filler, backing-gas on the inside, and NDT scope (PT or RT, 100 % for class service).
8. MTC — EN 10204 3.1 for the bulk of the package, 3.2 for class-witnessed sections. Digital MTC acceptable if the class society accepts.
9. NDT scope — hydrostatic 1.5× design pressure (or eddy-current per B467 alternative), 100 % welds, PMI on the flange / fitting heat numbers.
10. Companion hardware — pipe fittings, copper nickel flanges, gasket stud bolt and nut, and industrial valves with Cu-Ni body or matched trim, all bundled to the same MTC chain.
11. Packaging & shipping — wooden crate, end caps, desiccant, container vs break-bulk, class-society marking (DNV 2.7-1 / EN 12079 for offshore containers).
12. Mill-direct scope — single supplier for pipe + fittings + flanges + bolting + gaskets + valves, with one project manager and one MTC chain.

9. Five Buyer Questions, Answered