Hollow section steel is one of the most recognisable structural materials in the offshore world. Whether you spot it in the legs of a jacket platform, the bracing of a subsea structure, or the framework of a floating production vessel, those closed, tubular profiles are almost everywhere. But what makes them so well suited to offshore construction specifically, and how do you make sure you are sourcing the right product for your project? This article walks you through everything you need to know.
What is hollow section steel and what types are available?
Hollow section steel is exactly what the name suggests: a steel profile with a hollow interior. Unlike an I-beam or angle bar, which have open cross-sections, hollow sections form a closed shape. This gives them a distinctive set of structural and physical properties that make them highly versatile across industries.
There are three main types of hollow sections:
- Circular Hollow Sections (CHS) — round tubes, the most common form in offshore and marine applications due to their uniform strength in all directions and excellent resistance to torsion
- Rectangular Hollow Sections (RHS) — flat-sided rectangular profiles, widely used in construction and fabrication where flat connection surfaces are needed
- Square Hollow Sections (SHS) — a specific form of RHS with equal sides, often used in structural frameworks and support structures
Each type is produced through either hot-forming or cold-forming processes, and the choice between them affects the mechanical properties, tolerances, and suitability for specific environments. In offshore construction, hot-formed sections are generally preferred because they offer better toughness at low temperatures and more consistent material properties throughout the wall thickness.
Why is hollow section steel so popular in offshore construction?
The offshore environment is one of the most demanding in the world for structural materials. Structures must withstand wave loading, wind forces, salt spray, temperature variation, and in some cases, significant dynamic loads from equipment and operations. Hollow section steel handles all of these demands particularly well.
The closed cross-section gives hollow sections a high strength-to-weight ratio. A circular hollow section, for example, distributes load evenly across its entire circumference, meaning it resists both bending and torsion without requiring excessive material. For offshore platforms and jackets, where weight directly affects installation costs and structural loads, this efficiency matters enormously.
Circular hollow sections also present a smooth, continuous outer surface with no internal corners or exposed edges. This reduces the area where water, salt, and biological growth can accumulate, which simplifies maintenance and extends the service life of protective coatings. In an environment where maintenance access is limited and expensive, that is a practical advantage that engineers take seriously.
Finally, hollow sections are well suited to welded connections, which are the dominant joining method in offshore fabrication. The geometry of CHS in particular allows for complex nodal connections that distribute stress effectively, which is why you see them in jacket structures, flare towers, and crane pedestals across the offshore sector.
How does hollow section steel perform in corrosive marine environments?
Corrosion is the primary long-term threat to any steel structure in a marine environment. Hollow section steel addresses this in several ways, though it is important to understand that the base material still requires proper protection.
The closed profile of a hollow section means there is no internal cavity exposed to the external environment in the same way an open section would be. When properly sealed at the ends, the interior of the hollow section is isolated from moisture and oxygen, which significantly reduces the risk of internal corrosion. This is one reason why hollow sections are often preferred over open profiles in splash zones and submerged applications.
For the external surface, hollow sections are typically protected through a combination of surface treatment and coating systems. Common approaches include shot blasting to remove mill scale, followed by priming and multi-layer paint systems designed for offshore service. In some applications, hot-dip galvanising is used, particularly for components in the atmospheric zone. Cathodic protection systems are also applied to submerged structural elements to manage electrochemical corrosion.
The choice of steel grade also plays a role. Higher-grade steels with specific chemical compositions offer better inherent corrosion resistance, and some applications call for duplex or super duplex stainless steel hollow sections where extreme corrosion resistance is required alongside structural strength.
What’s the difference between hollow sections and open steel profiles?
Open steel profiles include familiar shapes like I-beams (HEA, HEB), angle bars, channel sections, and flat bars. These profiles have exposed internal surfaces and re-entrant corners where water and contaminants can collect. In a clean, dry industrial environment that is generally not a problem, but in offshore and marine applications it becomes a significant maintenance and durability challenge.
Structurally, open profiles are excellent in bending along their strong axis, which is why I-beams dominate in building construction where vertical loads are the primary concern. Hollow sections, by contrast, perform more evenly under multi-directional loading, torsion, and compression. This makes them the natural choice for columns, bracing members, and three-dimensional space frames where loads come from multiple directions simultaneously.
From a fabrication standpoint, hollow sections require more precise cutting and fitting for welded connections, particularly at nodal joints where multiple members meet. However, modern CNC cutting equipment handles this well, and the structural advantages generally outweigh the additional fabrication complexity in offshore applications.
What steel grades are used for hollow sections in offshore applications?
Grade selection for offshore hollow sections depends on the specific application, the design temperature, and the applicable design code. Several grades are commonly specified:
- S355 — the workhorse grade for structural offshore applications, offering a good balance of strength, weldability, and toughness. Sub-grades such as S355J2H and S355NH are specifically designed for hollow sections and include toughness requirements at sub-zero temperatures.
- S420 and S460 — higher strength grades used where weight reduction is a priority, such as in large jacket structures or topside frameworks. These require more careful welding procedures.
- API 2H and API 2W — American Petroleum Institute grades commonly specified for offshore structural tubulars, particularly in the US Gulf of Mexico and projects following API standards.
- Duplex and super duplex stainless steel — used in highly corrosive environments or where process fluids demand superior corrosion resistance alongside structural performance.
For projects following ASTM standards, grades such as ASTM A500 and ASTM A501 are relevant for structural hollow sections, while ASTM A53 and ASTM A106 cover structural and pressure pipe applications that overlap with hollow section use in offshore piping systems. Certifications and material test reports are standard requirements in offshore procurement, so always confirm documentation availability when sourcing.
How do you source the right hollow sections for an offshore project?
Sourcing hollow sections for an offshore project is rarely straightforward. Specifications can be complex, lead times are often tight, and mistakes in grade, wall thickness, or certification can cause costly delays or, worse, structural problems down the line. A few practical steps help get it right:
Start with a clear specification that includes the steel grade and sub-grade, the outside diameter and wall thickness, the applicable standard (EN, ASTM, API), the required testing and certification, and any surface treatment requirements. If you are not sure which specification applies, working with a supplier who understands offshore requirements and can advise on grade selection is far more efficient than trying to work it out independently.
Stock availability matters enormously when a project is on a tight schedule. A vessel modification or platform repair cannot wait weeks for a special order. Choosing a supplier with broad stock across multiple dimensions and grades reduces the risk of delays and eliminates the need to source from multiple parties. You can explore our full range of steel products and hollow sections to get an idea of what is available from stock.
Also consider whether you need any processing alongside the raw material. Hollow sections for offshore use often require cutting to length, end preparation for welding, blasting and priming, or even rolling and bending for curved structural members. Having these services available from the same source as the material saves time and coordination effort on a project where both are in short supply.
How Marine Steel helps with hollow section steel for offshore projects
We supply hollow section steel to clients in the offshore, maritime, construction, and industrial sectors from our warehouses in Rotterdam and Houston. With over 15 years of experience in the industry, we know what offshore projects demand, and we stock accordingly.
Here is what we offer for hollow section procurement:
- Circular, rectangular, and square hollow sections across a wide range of dimensions and grades
- ASTM, EN, and API certified material with full documentation and material test reports
- In-house tube processing including cutting to size, blasting, priming, and galvanising
- Custom dimensions and special grades available on request
- Fast quotation and delivery from stock in Rotterdam and Houston
- One point of contact who thinks along with your specification and helps you get it right the first time
We understand that offshore procurement is time-sensitive and that errors in material specification are not an option. Whether you need a standard grade from stock or a custom processed section for a specific application, we are here to help you source it efficiently and correctly. Contact us directly for a fast quotation or to discuss your project requirements with our team.
