By Marcus Hillbom, Alleima

Subsea fasteners face one of the most demanding briefs in offshore engineering. Bolting on christmas trees, manifolds and pipelines must hold high mechanical strength for the life of the installation, resist chloride-driven localized corrosion, and do so while under cathodic protection on the seabed. That last point is where the problem lies.

Cathodic protection guards the wider structure, but overprotection drives hydrogen onto the metal surface. In high-strength materials, that hydrogen can embrittle the fastener and lead to sudden failure at loads below the design limit.

The precipitation-hardened nickel alloys used for subsea bolting today, such as Alloy 718, deliver the strength but carry a documented track record of hydrogen embrittlement failures in exactly this environment. For an operator, a failed fastener 250 m down is not a simple maintenance item; it is a costly intervention with potential safety and environmental consequences.

Developing an alternative to traditional nickel alloys

Over the past two years, the grade has also been through qualification programs with end users, OEMs and universities, which have supported end-user-specific fastener specifications and moved the material from basic performance research toward use in real subsea equipment designs.

Expanding material options from topside to subsea

For subsea fastener applications, the material is intended to provide strength levels comparable to Alloy 718 while offering resistance to hydrogen embrittlement and general corrosion. These properties are designed to help support long-term fastener performance in demanding offshore environments.

As a lower-alloyed grade than the nickel-base alternatives, it also carries a lower cost.

The corrosion performance opens a wider design question. Because the grade resists hydrogen-induced stress cracking and offers strong localized corrosion resistance, it can be specified with or without cathodic protection. This extends its use beyond the seabed into splash and tidal zones where protection may be intermittent or absent, from fixed coastal structures and offshore wind foundations to FPSO waterlines. In effect, one grade can serve a range of positions from topside to subsea that currently call for several different, more expensive materials.

Recent qualification work has supported the material's use in both topside and subsea applications, potentially simplifying material selection for engineers and equipment suppliers.

Qualification progress and next steps

Industry interest is translating into standards acceptance. Sanicro 35-HS currently holds acceptance under ASME code case 2982, a PED Particular Material Appraisal, and NACE MR0175/ISO 15156-3 and MR0103/ISO 17495-1 for sour service. Inclusion in NORSOK M630 and API 6A is in progress, with both still pending.

With qualification work continuing, Alleima said the first field installations could follow in the coming year. The company is extending the range to larger high-strength bar dimensions for components such as safety valves, packers and mechanical seals.