DNV is the classification for close to half the world’s fleet of very large ore carriers. This experience has provided unique insights to the demanding structural requirements on these giants of the sea. Owners also recognize that about 37% of the trading routes of these ships is in a harsh environment.

Such ships are subject to buckling on the deck and at the bow. Cracks potentially appear in side longitudinals, longitudinal coaming brackets and also in bottom longitudinals. The forward bottom is also subject to indentation and slamming.

Another area where DNV has invested extensive research is in wave induced hull girder vibrations, namely springing and whipping. Springing is caused by resonance while whipping is caused by impact loads such as bow wave impact. From full scale measurements, we have seen that heading into harsh weather in ballast condition has the biggest effect. The vibration level, and its consequence on fatigue and ultimate loading, has been further documented through extensive model testing performed in cooperation with Norwegian University of Science and Technology.

The waves causing springing may be small and distance between waves might be in the order of 1/10 of the ship ength and only a couple of meters high. For a 300m ship these waves appear as ripples. Detectable whipping, on the other hand, occurs in high sea states, and may be counteracted by reducing the speed and changing the ship’s eading. To date, current classification societies’ rules do not reflect whipping and springing as a contributor to fatigue damage or extreme loading.

DNV is, however, producing a guideline for taking into account the additional fatigue effect of wave induced hull girder vibrations based on the experience gained from full scale measurements and model testing on blunt ships DNV is also performing full scale measurements of the phenomenon on other ship types, such as container carriers and gas carriers, in order to modify the guideline so it may be applied to other ship types. Hence, DNV has the tools and experience to counteract these challenges.

For owners, the minimum notation with IACS Unified requirements applicable to Ore Carriers (S1A & S21) and Enhanced survey programme would be +1A1 Ore Carrier ESP ES(O), while an enhanced notation would be +1A1 Ore Carrier ESP ES(O) NAUTICUS(Newbuilding) BC-B IB(+) NAUT-OC E0 TMON BIS BWM-E(s) OPP-F.

Nauticus Newbuilding is a supplementary to the main class and on a voluntary basis. Essentially it describes an
extended calculation procedure for the verification of the hull structure: It particularly focuses on FEM procedure within the midship area for determination of main scantlings and also extended fatigue requirements for end structures of longitudinals.

For further articles from DNV Greater China, please download the PDF available under "downloads" in the right column.