Icebreaking LNG carriers fixed to the Yamal LNG project will undergo two surveys within five years. Russian Maritime Register of Shipping head of research Maxim Boyko explains why
Across the whole range of possible ship-operating conditions in ice, the ship's hull must withstand continuous movement in broken ice and in ice channels, in ramming mode and ice-compression mode.
The first three modes are mainly characterised by dynamic impacts to hull structures as ice impacts against the ship's side. This phenomenon is relevant to the bow and, when the ship moves astern, to the aft parts of the hull.
Ice compression is characterised by a high static load on the side structures. The most frequently detected damage to hull structures is a permanent deflection of side structures, which can be localised, forming the local deformations of side shell plating between stiffeners and ribs between stiffeners. Sometimes, these can be on a larger scale, involving large areas of the side structures, such as plating and stiffeners, which become plastically deformed.
When performing a survey of an ice class ship, the Register of Shipping surveyor will assess this damage to determine whether repairs are necessary. In this context, even at the ship-design stage Register of Shipping requirements consider probable damage to ship structures, which ensures that the construction remains safe, even if local separate damage occurs.
Interaction with ice can damage the paint coating of the side structures, causing corrosion of the structure. This makes it important, for safe navigation in ice, to maintain the quality of coatings, and to determine the proper corrosion and abrasion margins.
Another important point is the safety of propeller blades under ice impact. If the blade breaks on a ship in the icebreaker-led convoy it creates time and cost issues, and increases the danger of ship damage by ice.
That is why we have developed our rules, paying close attention to modern calculation technologies and the model testing of propeller blades. The Russian Maritime Register of Shipping has elaborated rules that determine the minimum dimensions of propeller blades and minimum required propulsion power based on typical load scenarios applicable to Arctic navigation.
Also relevant to the machinery is the strength of the podded propulsion units. The design ice loads on such units must be calculated properly at the design stage to ensure that the pod structure and its connection to the hull has the strength to withstand ice loads.
We have learned a lot from our experience at the Yamal project. We have gained unique feedback and data by applying ice-class rules to high-ice-class LNG carriers of non-conventional hull form and dimensions. This has helped us to expand our knowledge into non-conventional Arctic ships.
This includes the application of high-tensile steels in extremely cold ice conditions, winterisation of deck equipment and the implementation of interesting structural design solutions. Every aspect is carefully assessed to consider the safety of the design.
The result of the research has given us the opportunity to optimise the required number of drydocking surveys. An analysis of 25-plus years of data covering the annual docking of icebreakers and ice-class ships shows how the precision of the requirements for hull and propulsion systems, as defined in the Rules for the Classification and Construction of Sea-Going Ships, has ensured the long-term safe service of vessels of the given class.
Based on this analysis, since 2016 the icebreakers and ice ships meet similar requirements to dock, in survey terms, to other sea-going ships.
So instead of an annual survey in dock, icebreakers and ice-class ships now undergo two in-dock surveys of the underwater part of the hull and steerable propellers in five years. This also applies, of course, to Arctic LNG carriers.
Maxim Boyko is head of research at the Russian Maritime Register of Shipping