Offshore drills and safety valves: high load joints in hostile places

Offshore drills and safety valves ask for everything at once: high force in a small envelope, corrosion resistance, temperature capability and documentation. Disc springs for oil and gas duty are specified from the environment inwards, using corrosion resistant stainless grades, X22 CrMoV 12-1 to 500 degrees C, or Inconel X718 and X750 to 600 degrees C.

On a rig, a component is judged less on unit price than on what it costs to reach it again. That reverses the normal specification instinct. You do not choose the cheapest material that works. You choose the one that removes an intervention from the schedule.

Start with the environment, not the load

For corrosion resistant disc springs the options are the chromium nickel spring steels X10 CrNi 18-8, available up to around 3.0 mm thickness, and the precipitation hardened X7 CrNiAl 17-7, available up to roughly 2.5 mm. Both become magnetic through cold forming. On the washer side, X5 CrNiMo 17-12-2 austenitic stainless gives excellent corrosion resistance with lower magnetisability.

Where heat and corrosion arrive together, Inconel disc springs in X718 or X750 are the answer, with excellent corrosion resistance and capability up to 600 degrees C. X718 is virtually cobalt free, which is why it is often used in nuclear reactor technology. Where the duty is hot but dry, X22 CrMoV 12-1 is effective to 500 degrees C, with the clear caveat that it is not a corrosion resistant grade.

Design for creep, not just for load

At elevated temperature, a combination of temperature, time and tension can cause creeping, which shows up as a loss of installation height or a loss of spring force. On a safety critical valve that is not a wear item you inspect, it is a design input. Allow for it, or move the material up.

Safety valves: force that has to be right the first time

A safety valve spring sits still for long periods and then has to act exactly once, correctly. Three disc spring properties make that dependable: high load from a short installation length, a linear or progressive characteristic depending on the stack, and no setting under static loads once preset. A part that has held a set force for years is still holding the force you specified.

Offshore drills: shock plus scale

Drilling duty combines heavy dynamic load with limited space, which is exactly where high load in a small space earns its keep. Parallel stacks raise force at the same deflection, series stacks absorb movement at the same force, and combined arrangements produce the characteristic curve the assembly actually needs. Guidance matters more as loads climb: a guide element to prevent lateral movement, inside guidance preferred, and guides hardened to at least 60 HRC with a minimum case depth of 0.8 mm where possible.

The bolted joints around it

Flanges and structural connections on this kind of plant are the definition of a vibration exposed joint that nobody wants to revisit. SCHNORR safety washers lock through friction and mechanical locking, maintained pretension force after 2,000 cycles in Junker testing, and outperformed split lock, detent edged, ribbed and spring washers. Where the surface is hardened or the hole is elongated, type VS is the correct specification.

Documentation is part of the specification

Certifications and brochures are published for download, and CAD drawings are available for layout and drawing packs. Beyond the standard range, SCHNORR develops tailor made solutions that go further than DIN or EN, including special sizes, which is often what an offshore envelope requires.

• SCHNORR components are used in offshore drills and safety valves.
• X10 CrNi 18-8 is available up to about 3.0 mm and X7 CrNiAl 17-7 up to roughly 2.5 mm. Both become magnetic through cold forming.
• X5 CrNiMo 17-12-2 austenitic stainless offers excellent corrosion resistance with lower magnetisability.
• Inconel X718 and X750 give excellent corrosion resistance up to 600 degrees C. X718 is virtually cobalt free and often used in nuclear reactor technology.
• X22 CrMoV 12-1 is effective to 500 degrees C but is not corrosion resistant.
• Creep from temperature, time and tension can cause loss of installation height or spring force.
• Type VS safety washers are recommended for hardened surfaces and are applicable to elongated holes.
• Certifications, brochures and CAD drawings are available for download.