27 March 2026 – The International Association of Classification Societies (IACS) has published a new recommendation, Rec.197 that complements the existing requirements of IACS Unified Requirement W11, currently addressing hull structural steels up to 100 mm thickness.
With the increasing scale of modern ships and offshore structures, the use of thicker steel plates has become more prevalent in critical structural regions (e.g. deck stringers, sheer strakes, hatch side coamings and other highly stressed structural members). The Recommendation therefore provides harmonised guidance for steels with thicknesses greater than 100 mm and up to 200 mm, reflecting both evolving industry practice and the accumulated experience of IACS Member Societies.
Scope of application
The Recommendation applies to:
- normal and higher strength hull structural steels;
- steel plates and wide flats;
- thickness range greater than 100 mm and up to 200 mm.
For thicknesses exceeding 200 mm, individual assessment by the Classification Society remains required.
Harmonised technical framework for thick structural steels
The Recommendation establishes a harmonised technical framework within IACS covering the main requirements applicable to thick hull structural steels, including:
- manufacturing processes and approval of steelmaking facilities;
- acceptable delivery conditions (normalized, controlled rolled, thermo-mechanically controlled processed (TMCP), and quenched and tempered steels);
- chemical composition and carbon equivalent limits;
- mechanical properties and impact toughness requirements.
Toughness requirements and brittle fracture considerations
A key technical aspect of the Recommendation concerns the specification of impact toughness requirements for thick hull structural steels.
As the thickness of structural steel increases, the potential consequences of brittle fracture become more significant for hull structural integrity. The minimum impact energy requirements have therefore been established on the basis of fracture-mechanics-based considerations linking structural thickness, design temperature and material toughness.
The resulting toughness criteria were further evaluated through comparison with recognised fracture mechanics assessment methodologies, confirming the technical consistency of the adopted requirements.
Testing methodology
The Recommendation also clarifies the testing and sampling arrangements applicable to thick steel products, including:
- tensile testing with specimens located at approximately quarter thickness;
- impact testing generally performed at quarter thickness locations;
- the possibility, subject to agreement with the Classification Society, to use impact specimens taken at mid-thickness with appropriate acceptance criteria.
Relevance for industry
This Recommendation is expected to be of particular interest to:
- ship designers and shipbuilders involved in large vessels and heavily loaded hull structures;
- steel manufacturers producing thick plate products for marine structural applications;
- projects involving large structural members or high design stresses in primary hull structures.
By providing harmonised guidance across IACS Member Societies, the Recommendation supports the safe and consistent application of thick hull structural steels in modern ship construction.
Ajay Kumar, IACS GPG Chair, commented
“The increased scale of modern ships and offshore structures has resulted in greater use of thicker steel plates, and the publication of Rec. 197 is an important step forward in providing valuable guidance for ship designers, builders, and steel manufacturers. Drawing on the collective expertise and experience of IACS Member Societies, this new recommendation will help ensuring a harmonized technical framework for hull structural steels up to 200mm, which in turn will enhance safety.”
Ends/
Contact: Robert Ashdown, IACS Secretary General
E: robertashdown@iacs.org.uk T: +44 (0) 20 7976 0660
International Association of Classification Societies
Secretariat 4 Matthew Parker Street, London, England SW1H 9NP
E: secretariat@iacs.org.uk
T: +44 (0)20 7976 0660
Notes to Editors:
- Dedicated to safe ships and clean seas, the International Association of Classification Societies (IACS) makes a unique contribution to maritime safety and regulation through technical support, compliance verification and research and development. More than 90% of the world’s cargo carrying tonnage is covered by the classification design, construction and through-life compliance Rules and standards set by the twelve Member Societies of IACS.
- More information about IACS can be found by visiting iacs.org.uk and in our Annual Review available online at http://www.iacs.org.uk/about/iacs-annual-review/