Freeman Ralph

Probabilistic Methodology for Design of Arctic Ships

With the increased demand for hydrocarbon and mineral resources, as well as tourism, marine transportation in the arctic continues to increase. The region is harsh and fragile, making safety and environmental protection paramount. A key concern is how to estimate extreme design events that first and foremost satisfy safety and then economy.A rational approach to design of arctic ships based on probabilistic methods is detailed in this paper, including global impact forces and local panel design. Using a probabilistic approach, extreme design events can be identified by combining annual, seasonal and regional variability in environmental conditions with model uncertainty, and integrating these directly in the design methodology. Extreme design loads are estimated based on the annual number of interaction events, and the design strategy - target exceedence criteria established based on general public safety. The approach also provides a comprehensive basis for the selection of an appropriate ice class given certain operational requirements (e.g. an icebreaker for facility protection, or suitability of a cruise liner, having minimal ice class, to operate through a particular season). Otherwise, design for extremes is largely based on observational experience and judgment from one or more experts and such experience only reflects a relatively short span of natural occurrence. Neither is it appropriate to arbitrarily establish the most extreme condition imaginable.For extremal analysis, a parent distribution of global impact forces is first developed either through numerical ship-ice simulations or directly from measured full scale ship ram trial data. Using the parent distribution, and the expected annual number of ram events, a new design distribution representing the maximum of n annual force events is developed. Based on the global analysis, mean penetration and duration can be estimated which, when combined with number of interactions per year, provides a measure of exposure, a key input for local design analysis. A rational local pressure design model is presented that is derived based on measured ship ram data. Peak pressures through the full ram duration are considered and not just realizations at some arbitrary point of maximum force. A local scale effect is measured where pressures on smaller areas (i.e. <10m2) increase considerably above the global scale effect. A design pressure area curve based on a design strategy (e.g. 100 yr return period) is produced. A hypothetical design illustration is provided for a ship along a particular route including estimates of global forces and local design pressures. A linear trend in forces based on logarithmic trend in number of ramming events is observed. This illustrates that linking the design forces and pressures directly with expected number of interaction events is most reasonable and appropriate.Vertical impact forces estimated using the Polar Class rules are compared with estimates using the rational probabilistic approach and measured full scale MV Arctic data. The analysis illustrates how measured forces and expected exposure can be used for design and classification, as well as calibration. Preliminary results indicate that the higher PC1 and PC2 class forces seem rather conservative and a large gap exists between PC2 and PC3. Further calibration is needed. Introducing different design strategies (e.g. elastic-plastic and fully plastic) for corresponding load levels (e.g. 10−2extreme and 10−4abnormal) should be considered, allowing the designer to better understand the performance of his design.Copyright

Design of a Shipboard Local Load Measurement System to Collect Managed Ice Load Data

Abstract Due to a lack of data, currently (and justifiably) conservative ice load assumptions are made in rig assessments allowing only very small floe sizes to contact non-Polar classed drilling r ...

Probabilistic Design Load Method for the Northern Sea Route

A probabilistic design method allows us to link statistical data from the operational area of the vessel with design loads providing the availability for more precise safety level assessment, which is important to ensure safe and sustainable ship transit in ice covered waters. Statistical design methods are well used for open water using spectral analysis. Wave induced loads are estimated by linking statistical load parameters to the sea state parameters. Statistical methods to estimate ice-induced loads are also available, however, current Polar Class rules are not considering probabilistic methods for determining ice-induced loads. This paper shows how developed probabilistic methods can be used for the design of ice going ships, especially for ships operating along the Northern Sea Route (NSR). The method presented in this paper will combine available data from full-scale measurements performed in the Arctic with ice conditions defined using historical data from satellite sources. The full-scale measurements are used to develop the parent distribution, which forms the basis for the extreme load prediction based on the number of excepted interactions along the NSR. Satellite data from history will be used to model ice conditions, e.g. ice type and ice concentration, along the route.Copyright

Structural Response of Ice-Going Ships Using a Probabilistic Design Load Method

Ships operating in ice-covered waters experience intense loads from ice features, particularly multiyear ice. Therefore, their structures have to be able to withstand these loads, making structural design paramount. Current formulations of ice class rules do not fully account for the probabilistic nature of ice loads, i.e. scale effects for local ice pressures captured in full-scale measurements. Furthermore, ice class rules do not consider route-specific ice conditions when calculating the design load, i.e. the exposure of the vessel to ice crushing determined by the number and duration of rams. An approach to arctic ship design based on probabilistic methods was developed by Jordaan and co-workers in 1993 and is described in this paper. The approach is used to estimate extreme design loads based on the annual interaction events and the design strategy (target exceedence criteria). The objective of this paper is to select an appropriate ice class for a vessel navigating along the northern sea route, and to compare the design requirements with those determined using the probabilistic approach based on measured data and expected exposure. Local hull pressures have been measured using the USCGC Polar Sea for a range of ice conditions including first year and multi-year ice. Impact conditions similar to those expected along the Northern Sea route were selected and corresponding pressurearea parameters used for input into the probabilistic approach discussed above. This paper will compare the design and response of an exemplary stiffened panel using the described approach to requirements given in Finnish Swedish Ice Class Rules. A case study structure will be analyzed using Finite Element Method for a chosen exposure scenario and target safety level.Copyright