Numerous field studies show that mature first-year (FY) ridges consist of a solid ice consolidated layer (CL) below which is a keel of ice rubble. The CL is generally thicker than the ambient level ice. With the decrease in multi-year ice in the Arctic, FY ridges may become the controlling ice feature at many Arctic locations. In sub-Arctic regions ridges are smaller but they also usually control design ice loads. For vertical structures, ISO 19906 (Standard on Arctic Offshore Structures) suggests calculating the load due to crushing of the CL and adding this to the load to fail the keel. In ISO a formula is provided for the load to fail the keel on a vertical-face which assumes that the keel material has Mohr-Coulomb properties. No specific algorithms are given in ISO for FY ridge loads on sloping structures. The work described in this paper addresses this gap.
The study investigated three bounding methods and compared them with the measured loads due to FY ridges on Confederation Bridge (which has piers with upward breaking cones). The three bounding methods are: Model A; which assumes the CL breaks in bending and rides up as level ice and the keel load is calculated assuming a "dead wedge" is created on the slope which converts the slope into a vertical face against which the keel fails. This model can make use of the methods in ISO 19906 for calculating these two components and can be considered to be the implied "current ISO approach". Model B assumes that the CL layer fails in bending as "level ice" on an elastic foundation and rides up the slope with the accompaniment of additional ice rubble scooped-up from the sail and keel of the ridge. Model C assumes that the FY ridge can be idealized into an equivalent "solid ice" beam using composite beam theory. Then the beam on elastic foundation method, as used for solid ridges, is used to estimate breaking and clearing loads.
These various approaches are reviewed and the derived loads are compared to failure modes and measured loads from Confederation Bridge for selected events involving FY ridges. Based on these comparisons a hybrid of Models B and C is recommended and the paper gives the details of how to apply this method.
When used for example structures, the new model gives loads which are 40 – 50% lower than the current approach implied in ISO 19906. The method can be adapted to downward sloping structures.
Croasdale, Ken (KRCA) | Brown, Tom (U of Calgary) | Wong, Chee (U of Calgary) | Shrestha, Noorma (CARD) | Li, George (Shell International) | Spring, Walt (Bear Ice Technology) | Fuglem, Mark (C-CORE) | Thijssen, Jan (C-CORE)
In ISO19906 (2010) (Arctic Offshore Structures) specific algorithms are provided for level ice loads on sloping structures; they are based on the separate work of Ralston and Croasdale. These methods were developed decades ago and comparisons with full scale data, especially from Confederation Bridge, suggest that certain idealizations can be improved; more importantly that they may be over-predicting the measured loads. For these reasons it was decided to critically review the existing Croasdale et al algorithm (as specified in ISO) and update it based on learnings from Confederation Bridge, other experience and new ideas. During the study, over 50 ice interaction events at Confederation Bridge were chosen as geometrically similar to thick ice acting on an Arctic structure. The interaction process and relevant parameters (such as ride-up height) were documented in detail and the measured loads compared with predictions for each event.
In ISO 19906 (2010), there are no algorithms provided for calculating loads on sloping structures due to interaction with multi-year (MY) ridges; only references are provided for a range of methods; to quote from Clause A.126.96.36.199.2:"Multi-year ridge actions against conical structures can be estimated using a variety of methods [Croasdale, 1980)], [Nordgren and Winker (1989)], [Wang (1984)]." A study was undertaken to revisit the theories for breaking and ride-up of MY ridges and if possible to improve them. A new simplified method for long ridges has been developed which includes secondary failures associated with the hinge pieces which are successively broken as the ridge is pushed higher prior to rotation of the broken pieces around the structure. For wide ridges, failure across their width has also been quantified and this mechanism can lower ridge loads compared to prior methods. The new method also recognizes the loads associated with the clearing of level ice fragments ahead of the ridge. The key findings have been incorporated into a methodology which is described by relatively simple equations and these are provided in the paper. Example calculations and sensitivities are provided.
Multi-year (MY) ridge and level ice interactions with sloping and conical structures involve complex ice feature shapes and ice failure mechanisms. The limited available field data makes calibration of associated load models difficult. To account for associated randomness and uncertainty, models may tend to be on the conservative side.
New deterministic algorithms were recently developed to calculate loads more accurately for interactions of MY level ice and MY ridges with an upward sloping structure. This paper presents the application of these recently developed formulations in a probabilistic framework using SILS. SILS is a Monte-Carlo type simulator developed by C-CORE following the general procedures outlined in ISO 19906. Ice and metocean input parameters are defined by the user either as a fixed value (e.g. friction coefficients) or a random distribution (e.g. ice drift speed, floe size). The yearly encounter frequency is first estimated for these ice features for the site of interest. The ice loads are then determined for each of simulated interaction event occurring over the model timespan, using the deterministic load formulations. By simulating a large number of years of ice interactions, design ice loads can be determined that correspond to various low annual probability of exceedances.
This paper demonstrates how complex loading scenarios, modelled in terms of idealized deterministic models, can be incorporated within a Monte-Carlo framework to provide design level loads. During the model implementation and analysis of results, significant improvements were identified and implemented in the deterministic model, resulting in a more robust model and better design estimates. The results provide valuable insights regarding model inputs and behaviour corresponding to the extreme design ice loads. An example of a full probabilistic analysis is presented in the paper to illustrate the models. Here the probabilistic framework of SILS is used to assess a Base Case scenario and a number of sensitivity cases using different environmental inputs and model parameters.
Punch tests were carried out on manufactured freshwater ice rubble keels during the Pipeline Ice Risk and Mitigation (PIRAM) and Development of Ice Ridge Keel Strength (DIRKS) projects. The majority of the tests were carried out while the keel was still in contact with the soil tray, as would be the condition for a grounded sea ice ridge or stamukha. Results showed that peak ice pressures on the platen ranged from 61 kPa to 427 kPa, with an average value of 192 kPa and a standard deviation of 131 kPa. A conservative value for the bearing capacity of grounded freshwater ice rubble can be approximated to be 550 kPa (calculated by taking the mean plus three standard deviations).
The continuously increasing demand for energy has pushed hydrocarbon exploration into arctic and northern environments. While industry is evolving in this relatively new area of expertise, knowledge gaps remain in the engineering of both pipelines and structures to withstand ice loading. Subsea pipelines are often at risk of being damaged by gouging ice features, such as icebergs or sea-ice ridges. This occurs when an ice feature drifts into shallow waters and contacts the seabed, producing long narrow gouges or scours that can be meters deep, tens of meters wide and hundreds of meters long.
Ice gouging mechanisms are very different, depending on the ice type. Icebergs are solid bodies that do not experience significant failure while gouging the seabed. Ice ridges consist of an assembly of ice pieces (ice rubble) that are typically bonded together to form a competent matrix and, as such, can deform though shear, tension and compression. To date much of the research on ice gouging has focused on icebergs. This is not only because icebergs produce the deepest gouges but also due to the complexities involved with modelling both the ice rubble and soil deformation, and the interaction between both materials. Understanding these processes is, however, vital for development of subsea infrastructure in regions where sea ice ridges are the dominant ice hazard (e.g. Beaufort Sea, North Caspian Sea and Offshore Sakhalin).