Choosing the right hollow section steel for a marine project is not always straightforward. The environment is demanding, the loads are real, and the wrong choice in grade or wall thickness can have serious consequences. Whether you are outfitting a vessel, building an offshore structure, or working on a port facility, understanding how hollow sections work and what to look for makes the difference between a reliable structure and an expensive problem.
This guide walks you through the key decisions, from steel grade and section type to certifications and wall thickness, so you can source with confidence.
What is hollow section steel and where is it used in marine projects?
Hollow section steel is structural steel formed into a closed cross-section, typically square, rectangular, or circular. Because the material is distributed around the perimeter of the section rather than concentrated in a web and flanges, hollow sections offer excellent resistance to both bending and torsion. This makes them particularly well suited to applications where loads come from multiple directions.
In marine and offshore projects, you will find hollow sections used across a wide range of applications:
- Structural frames and supports on vessels and offshore platforms
- Crane booms and lifting structures
- Handrails, guardrails, and safety barriers
- Deck structures and subframes
- Gangways, access platforms, and ladders
- Pipe supports and equipment frames in industrial marine environments
The combination of structural efficiency, clean appearance, and relatively low surface area for corrosion protection makes hollow section steel a practical and cost-effective choice across these applications.
What are the different types of hollow sections available?
There are three main types of hollow sections, each with its own geometry and best-fit applications.
Circular Hollow Sections (CHS) are round tubes with uniform strength in all directions. They are the most common choice for columns, bracing, and any application where loads or forces are not concentrated in one plane. CHS sections also have the smallest surface area relative to their cross-section, which helps when applying protective coatings.
Rectangular Hollow Sections (RHS) have a rectangular cross-section, which makes them easier to connect to other structural members using flat welds or bolted connections. They are widely used in frames, supports, and fabricated structures where a flat face is useful.
Square Hollow Sections (SHS) are a special case of RHS where all four sides are equal. They offer symmetrical strength in both axes and are often used where a neat, uniform appearance matters alongside structural performance.
For most marine structural applications, CHS and RHS are the most frequently specified, though the right choice always depends on the loads involved and how the section connects to the rest of the structure.
What steel grades are used for hollow sections in marine environments?
Steel grade selection is one of the most critical decisions when specifying hollow section steel for marine use. The marine environment is aggressive: saltwater, humidity, temperature cycling, and mechanical stress all place demands on the material.
The most commonly used grades for hollow sections in marine and offshore applications include:
- S355J2H — A high-strength structural steel with good weldability and impact toughness at low temperatures. This is one of the most widely specified grades for offshore and marine structural hollow sections in Europe.
- S235JRH — A lower-strength general structural grade suitable for less demanding applications such as handrails, light framing, and secondary structures.
- ASTM A500 — The American standard for cold-formed hollow sections, commonly specified for projects following US codes, including offshore platforms in the Gulf of Mexico.
- ASTM A53 / A106 — Often used for circular hollow sections in piping and structural applications under American standards.
- Stainless steel grades (316L, 304) — Used where corrosion resistance is the primary concern, particularly for handrails, exposed fittings, and coastal structures where galvanic corrosion is a risk.
For offshore applications, classification societies such as Lloyd’s Register, DNV, and Bureau Veritas will often specify the grade requirements directly. Always check the applicable standard and classification rules before finalising your specification. You can explore the full range of steel products available from Marine Steel to find the right grade for your project.
What’s the difference between hot-finished and cold-formed hollow sections?
This is a distinction that matters both structurally and commercially, and it is worth understanding before you specify.
Hot-finished hollow sections are produced by forming the steel at high temperature, which relieves residual stresses in the material. The result is a section with more uniform mechanical properties, tighter dimensional tolerances, and better performance in structural calculations. Hot-finished sections are typically specified for load-bearing applications in marine and offshore structures, particularly where fatigue resistance matters.
Cold-formed hollow sections are produced at room temperature, which means higher residual stresses remain in the material. They are generally less expensive and are suitable for lighter structural applications, secondary structures, and non-critical uses. Cold-formed sections tend to have sharper corner radii, which affects both appearance and weld preparation.
For primary structural members in a marine project, hot-finished sections are almost always the better choice. The additional cost is justified by the improved material properties and the reduced risk of issues during inspection and certification.
How do you choose the right wall thickness for a marine hollow section?
Wall thickness selection is driven by several factors working together: the structural loads the section must carry, the corrosion allowance required for the service environment, the welding requirements, and any minimum thickness rules set by the applicable classification society or design standard.
In a marine environment, corrosion is a significant factor. Even with protective coatings or cathodic protection, it is common practice to add a corrosion allowance on top of the structurally required wall thickness. The size of this allowance depends on the exposure conditions, the protection system used, and the intended service life of the structure.
As a practical starting point, most structural engineers working on marine projects will specify a minimum wall thickness of 6 mm for primary structural members, even when the structural calculation alone might permit something thinner. For sections exposed to splash zones or direct seawater immersion, the corrosion allowance is typically higher.
Always work with your structural engineer or classification surveyor to confirm wall thickness requirements. Getting this wrong is not just a structural risk. It is also a certification risk that can delay your project significantly.
What certifications and documentation should hollow section steel have for marine use?
Documentation is not a formality in marine procurement. Classification societies, project engineers, and port authorities will require evidence that the steel meets the specified standard, and missing certificates can hold up a project just as surely as a missing component.
For hollow section steel used in marine and offshore applications, you should expect and request the following:
- Mill certificates (EN 10204 3.1 or 3.2) — These confirm that the material was tested and meets the specified standard. A 3.1 certificate is issued by the manufacturer; a 3.2 certificate is countersigned by an independent inspection body or classification society.
- Classification society approval — For offshore and vessel applications, the steel may need to be approved by the relevant class society (DNV, Lloyd’s, Bureau Veritas, ABS, etc.).
- Heat and cast numbers — Traceability from the certificate to the physical material is essential for quality control and inspection purposes.
- Dimensional inspection reports — Confirm that the section dimensions are within the tolerances specified by the standard.
When sourcing hollow sections for a marine project, always specify the certificate level you require at the point of enquiry. A supplier who understands the marine sector will ask for this information proactively, not as an afterthought.
How Marine Steel helps you source the right hollow sections
Sourcing structural steel for a marine project involves more than finding the right product. It means having a supplier who understands the environment, the documentation requirements, and the time pressure you are working under. That is exactly what we are built for.
- Broad stock of hollow sections in multiple grades, dimensions, and wall thicknesses
- Mill certificates and class-approved material available
- Custom tube processing including cutting to size, bending, drilling, and rolling
- Locations in Rotterdam and Houston to serve maritime and offshore clients globally
- 15+ years of experience working with ship chandlers, offshore operators, and industrial buyers
- One point of contact who thinks along with you, not just fulfils an order
You do not need to explain the basics twice. Tell us what your project requires, and we will make sure you get the right material, with the right documentation, delivered without delay. Get in touch with our team to discuss your hollow section requirements today.
