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SAFEICE Informe resumido

Project ID: 506247
Financiado con arreglo a: FP6-SUSTDEV
País: Finland

Final Report Summary - SAFEICE (Increasing the safety of icebound shipping)

The SAFEICE project aim was to create a scientific basis for ice class rules (ship hull strength) and for placing requirements on ice classes. The main purposes in the SAFEICE project was to develop semi-empirical methods based on measurements to determine the ice loads on ship hull, to find relationship between operational conditions and ice load, to develop ship-ice interaction models to assess the design ice loads on ship hull, to develop methods to estimate ultimate strength of shell plating and frames and to develop methods to analyse ice damages. The target was to decrease the risk involved in winter navigation. Baltic Sea, Okhostk Sea and Canadian waters are used as validation areas for ice load predictions.

The aims were achieved by compiling a database of earlier information on ice loads and ice pressures. This is a collection of full scale ice load data measured on board ships of various types sailing in different sea areas. Ice load data sets were used in validation of deterministic ice load models. The ice loading process has a stochastic nature. The stochasticity of ice loads influences the design ice load value. In the SAFEICE project, probability based methods in ice load evaluation were developed and validated with measured data.

The strategic objectives addressed, in the order of priority, have been:
1. decrease the environmental and material risks to shipping in ice covered waters by creating a unified basis for winter navigation system for first year ice conditions including the methods to get the required ice class;
2. develop semi-empirical methods based on measurements and advanced theoretical models to determine the ice loads on ship hull and relate these to the operational scenarios and the ice conditions;
3. develop ship-ice interaction models and stochastic models to assess the design loads on ship hull. The outcome is a description of the ice load versus ice and operational parameters;
4. create a framework to develop design codes and regulations for plastic design basis for icebound ships.

The project was structured into 10 work packages (WPs), as follows:

WP 1: Management
The first 12 months period aimed to have a good start for the project by organising kick off meeting, review the work plan, prepare the consortium agreement and establish a project web page.

WP 2: Compilation of data on ice loads on ship hulls
The overall objective was to bring together all earlier ice load and damage data and develop a common storage and analysis framework so the results of various test programmes can be combined and compared on a rational basis. Physical model tests to determine area factors for ice loading. The scope of the work package was increased to include more extensive data on ship damage due to impact with ice.

WP 3: Analysis of data
The overall objectives for the WP 3 were to:
- develop a picture of ice pressure distribution on ship hulls,
- develop an understanding on local ice pressure on ship hulls as a function of area,
- identify major lacks in existing field data on ice load on ship hulls, and
- describe relationship of measured ice loads with ice and operational conditions.

WP 4: Characterisation of the operative environment
The objectives of WP 4 were: develop a picture of ice conditions, terminology and description, develop a picture of applying traffic control and requirements for shipping, describe the use of icebreakers to assist the winter navigation and describe the relationship of traffic restrictions or requirements for an ice class and ice conditions.

WP 5: Risk analysis
The objectives of WP5 were:
1. development of probabilistic based methods which relate exposure time in ice to ice loads;
2. determination of risk level, calculation of the load carrying capacity distribution of structural members and estimation of the probability that the ice load exceeds the capacity somewhere along ship hull;
3. simulation of different ice loading scenarios using, for example, bootstrap and Monte Carlo simulation techniques. Main target is to estimate maximum load of various loading scenarios;
4. application of local ice pressure and contact area relationship to ice load calculation;
5. development of a numerical procedure for time-domain stochastic simulation of ship motion in broken ice and ice loading of the hull.

WP 6: Load modelling
The objectives of WP 6 were: to obtain a better picture of the overall spatial distribution of ice loads on ship hulls and of the dependence of ice loads on ship speed, ice thickness and strength in level ice conditions; to reach a sufficient level of accuracy in the computation of ice loads, so that these can used as input in structural analysis; calculated results were validated with measured ice load data from different ships and design ice loading scenario was selected in deterministic way.

WP 7: Load prediction
The objective of WP7 was to develop a formulation for design ice loads on ships. This formulation should be based on the ice conditions on the operation area of the vessel. The work further contained a reliability analysis verifying the predicted ice loads and an application to ship ice rules.

WP 8: Structural response and damage analysis
The aim of WP 8 was to develop scenarios for structural response including damage under ice loading. Assess especially the potential rupture of different ship structures. Analysis of ultimate load carrying capacity of ship shell structures was also be conducted.

WP 9: Design methods
The objective of this WP was to make a synthesis of the results in the WP 7 and 8. The synthesis was a design procedure which could be applied in amending or making ice class rules or designing ships starting from encountered ice conditions.

WP10: Field trials
The objectives of WP 10 were:
1. to collect new ice load data from different ship types in different ice conditions; Baltic, Japan and Canada;
2. to apply and test alternative solutions of strain and stress measurement instrumentation;
3. to obtain ice properties measurements simultaneously with load measurements;
4. to collect targeted ice load data for validation of developed ice load calculation routines.

The foregoing synthesis provided a possible approach to the structural design of ice class ships wherein a given risk level sets the design point, since this is a design procedure which can be applied in developing ice class rules or designing ships, starting from encountered ice conditions. A methodology was demonstrated, however, it was found that between the ice condition data available and the associated ice loads, it was difficult to find sufficient separation in the data to test the method. Separation based on maximum ice thickness produced the best results of the methods tried.

The results of the simulations indicate that simulation can be used as a prediction method for maximum ice loads on a ship, at least on short term voyages and at the bow of the ship. The simulated loads on bow of both reference ships are the same magnitude than the measured loads. The results of mid and aft ship did not fit the measured loads that well. This can be because the contact between ice and ship's side is not that clear as it is assumed on the calculation method. On thin ice and compressive ice when the contact is clearer the simulated loads were the same magnitude as the measured loads. In addition, a numerical procedure for time-domain stochastic simulation of ship motion in broken ice and ice loading of the hull were developed.

Although, short term ice loads on the ship hull can be computed or measured with model tests in level ice, it does not appear yet to be possible to compute an extreme long term ice load. If extreme values of the input parameters related to ice properties are used in the computations, the result is of course an extreme ice load. The problem is that there is no sufficient knowledge about statistical distribution of the ice parameters used as input. Therefore also the exceedence probability of such a computed extreme load is not known. For this reason, the extreme loads need still today to be predicted based on long term measurements, i.e. long-term statistics on the actual ice loads.

Compressive ice as source of iceloads on ship hull was also studied applying geophysical scale ice dynamics models with different set ups and parameterizations. Results obtained, show well variability of compressive stresses and relative estimates, quantative estimates of compressive ice loads on ship hulls still need further investigations.

Altogether the various contributions from different research organizations participating in SAFEICE and using different methods has given us a more complete picture of the ice load distributions on ship hulls than anybody ever had before.

In general, the load-deflection behaviour of plating and framing can be used to identify the load resulting in a collapse limit state (ULS) while the load-permanent set behaviour can be used to identify the load resulting in a serviceability limit state (SLS). Although both ULS and SLS limit states (and partial safety factors) would be used explicitly in a true limit state design, it is here investigated if energy-based analysis methods based on rigid plastic collapse can adequately address both limit state criteria. Finally, although there are three possible types of structural failure, namely plastic deformation, instability and fracture, only the first type is covered herein.

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