Ice research aims to cut uncertainty in design and operation
Wendy Laursen explains that ice design is a case of brute force, but not ignorance, as classification societies turn research into safety for ships operating in the Arctic.
Ice towing tank tests often show a large scatter in measured data and low repeatability so Lloyd’s Register of Shipping has been tackling the problem with an on-going research program aimed at improving confidence in new designs. Sailing through ice requires increased structural strength and modified hull lines to cater for ice loads and also changes in resistance imposed by brash ice acting on both hull and propulsor. In the past these modifications were based on best practice assumptions and ice towing tank tests results. However, wide variation in mechanical properties makes sea ice incredibly difficult, perhaps impossible, to scale in model tests. Most ice towing tanks use refrigerated water with additives to simulate sea ice characteristics. Depending on the duration of the tests the model ice properties will change significantly.
Modern numerical methods, especially the explicit finite element method (FEM), could be applied to most ship-ice and propeller-ice interaction topics to bypass the scaling problems. The drawback of using only FEM models in ship-ice and propeller-ice interaction, though, is that the contribution of fluid forces prior to impact is neglected, for example, for ice particles in brash ice channels. This is too crude an assumption. In reality, the trajectory that ice particles follow before impacting on the hull or propeller is largely influenced by the flow conditions.
Lloyd’s Register has recently developed a computational fluid dynamics (CFD) based ice interaction capability which has been verified for its ability to track ice blocks of various sizes, impacting upon and flowing past the hull of a ship. The path each ice particle takes along the hull was successfully recorded and used as input for modelling along with the history of contact forces. Furthermore, a methodology has been developed to generate complex three-dimensional shapes that can realistically interact with any hull form and this has led to the development of a fully coupled fluid structure interaction solution.
Further validation of the methodology is expected to be achieved through a link with a joint industry project involving Ålands Landskapsregering, ILS Oy, Rolls Royce, BLRT Group and LR which undertook full scale measurements on a new icebreaking ferry for service in the Ålands Islands in the Finnish Archipelago. The project represents the first full scale measurements in ice on a ship fitted with controllable pitch azimuth thrusters, with a complete measurement system for the hull structure, engineering systems and underwater observations.
Germanischer Lloyd (GL) is also using advances in the computer optimisation of hull forms in the design of ice class vessels. GL Maritime Software and GL’s FutureShip units offer modelling systems that use CFD techniques and massively parallel processing to calculate hull optimisation for both free water sailing and heavy ice conditions. Various factors, strength, capacity, or energy efficiency for example, can be prioritised in the generation of thousands of hull forms, as a simulation of the effect of different operating conditions on each shape allows the designer to select a vessel tailor made for the desired operational model.
Operating in some of the most sensitive ecosystems anywhere, ice class vessels in development will have to be designed with a mind to the newly introduced energy efficiency design index (EEDI). Ice classed vessels that will be subject to the requirements of the regulations will have their EEDI assessed and a correction applied to the calculation to account for the specific design elements of the vessel.
Alongside the continued development of GL’s own ice class rules, there are a number of projects which are in development. The guideline Arctic Shipping will focus on the legal aspects of operation in Artic regions and is soon to be published. In the key area of winterisation, the design aspect that allows a vessel to operate safely and covers such things as heating of ballast and access routes, ice radar, crew protection, and machinery requirements, GL’s rules are in the final stages of preparation and will soon be published. Compliance with the requirements of chapter 13 of the OCIMF (Oil Companies International Marine Forum) SIRE (ship inspection report programme) 2011 will soon be certifiable.
The inclusion in the OCIMF questionnaire of winter navigation and winter navigation training questions in the vetting inspections on tankers sailing in ice-covered waters also mean that attention to crew development is more important for vessels operating in ice covered waters. As the demand for vessels increases, so too will the demand for crew; and crew inexperience has been identified as a significant cause of ice damage to vessels. GL Academy, as part of the Icetrain consortium, offers a course which fulfils the requirements of for training in IMO resolution 1024(26), Guidelines for Ships Operating in Polar Waters, and STCW (standards of training, certification and watchkeeping for seafarers) 2012, Annex 2.
This course provides practical instructions and the information needed to operate safely in harsh winter conditions, especially in the Baltic Sea, through a combination of theoretical presentations and special training on the bridge simulator. The course focuses on: ship ice interaction, ship design for ice operation, icebreaker operation, management and legislative issues, ship handling and manoeuvring in ice, cargo handling and ballast water operation, ice information services, working and occupational health in cold winter conditions, and life saving and rescue operations in cold winter conditions.
In the coming 2012 release of DNV’s Nauticus Machinery, support will be provided for the revised IACS polar class rules and the Finnish-Swedish ice class rules for Northern Baltic. The calculations are based on the methods outlined in DNV’s classification note 51.1 Ice strengthening of propulsion machinery which came into force in 2011, as well as corresponding DNV rules.
DNV Software’s Nauticus Machinery is a set of calculation tools intended for strength assessment of rotating machinery components and systems. The software package includes analysis tools for shaft alignment, torsional vibrations, gear rating and shaft fatigue. Nauticus Gear Rating is intended for cylindrical and bevel gears. The calculation procedures cover gear rating as limited by contact stresses, tooth root stresses and scuffing resistance. Shaft Fatigue is a rule based calculation tool for validation of fatigue life and safety factors in steel shafts according to IACS and DNV rules.
The existing Nauticus Shaft Fatigue and Nauticus Gear Rating tools will be upgraded to calculate the load carrying capacity (ultimate and fatigue strength) according to the ‘cumulative damage ratio’ approach. Although the underlying calculation methods requires time-domain simulation to find the maximum ice torque loads, the software tools also include some estimation methods requiring less effort. In general the calculation method is based on fatigue tests (SN-data) and estimation of cumulative damage ratio (Palmgren – Miner’s Rule).
The Russian Maritime Register of Shipping (RS) continues to expand its large cargo vessel work with a range of measures for design and operational safety with on-going research into the icebreaking capabilities of large ships, the need for icebreaker support in heavy ice and the use of LNG as a fuel. Considering the wide scope of the work, RS is striving to set up close co-operations with international partners with mutual interests.
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