Icebergs can pose risks to platforms in arctic and subarctic regions. These risks require careful consideration during design, and as well during operations. Platforms must be designed to withstand potential impacts from icebergs, or to disconnect and move offsite to avoid impacts. ISO 19906 allows use of ice management to mitigate iceberg and sea-ice actions. In the case of icebergs, management may include detection, monitoring, towing, disconnection and evacuation. Threat assessment is also a critical input to the iceberg management decision-making process. For example, given one or more detected icebergs and available information on the iceberg and environment characteristics, what is the probability of exceeding platform design ice actions? Based on the threat assessment, better decisions can be made regarding which iceberg to manage, whether more information should be acquired, and whether shut-down or evacuation is needed.
This paper describes a new tool developed to estimate the distribution of iceberg impact actions from an encroaching iceberg given concurrent metocean conditions, conditional on impact. The tool can be used in a number of ways depending on the information available to the user. It can be used to assess the threat from a single iceberg or can be used to compare actions from multiple icebergs in the region, or for the same iceberg but with changing weather conditions. The iceberg load assessment tool is demonstrated for several example cases on the Grand Banks, showing the benefit of improved iceberg characterization obtained through rapid iceberg profiling.
Approaches are presented in this paper for estimating the global mooring loads and response of a semisubmersible drilling rig, as a result of pack ice loading. The focus is on loading events from pack ice conditions relevant to the Grand Banks, where the pack ice typically consists of small floes and limited concentrations. The current practice for semi-submersible drilling operations on the Grand Banks is to avoid contact with pack ice by disconnecting and moving off-station in the event of an ice incursion. From a global loads perspective this may be unnecessary, given that the typical pack ice is of low severity and mooring loads may well be within acceptable limits. To be able to operate in pack ice while moored, operators need to demonstrate that the moored semisubmersible will have sufficient structural and mooring capacity to withstand the ice loads. Some existing semi-submersible hulls have ice strengthening in place as specified by a classification society, with associated allowable operating criteria in terms of ice conditions. These operating criteria are to ensure sufficient structural capacity given the ice conditions. No standardized approaches are currently available to quantify global pack ice loads and associated offsets for moored semi-submersibles, which are needed to assess the required mooring capacity. The objective of this paper is to address this gap and present approaches that can assist in specifying allowable operating criteria for station-keeping in pack ice.
Design of offshore structures for arctic and subarctic regions requires consideration of wave, wind and ice actions. If individual actions are not mutually exclusive, then combined actions also need consideration. ISO 19906 recommends that, when possible, extreme level combined actions should be determined based on the joint probability distribution of the actions. As an alternative, ISO 19906 provides a framework where a user can determine principal and companion extreme actions independently, and sum these with calibrated combination factors applied. While the combination factors in ISO 19906 were calibrated over a range of conditions and platforms, site-specific information is not taken into account when applying the method. In this paper, a procedure is presented for determining extreme level combined actions for sea ice and waves based on site-specific sea ice and wave information, accounting for the joint probability distribution of the actions. The procedure is demonstrated for an example fixed structure on the Grand Banks off Canada's east coast. The results are compared with extreme actions determined using the ISO 19906 combination factors.
The current practice for protecting wellheads and associated subsea facilities from icebergs on the Grand Banks is an Excavated Drill Centre (EDC), which is simply an excavation in the seabed in which wellheads and associated facilities are placed. Free-floating icebergs simply drift over an EDC, with the exception of those that roll as they pass over an EDC and increase draft sufficiently to enter. The risk from gouging icebergs entering an EDC is a function of the clearance between the surrounding undisturbed seabed and the top of the facilities in the EDC, and the distribution of gouging iceberg keel penetration depths. A field program conducted in Bonavista Bay in 2015 was used to estimate iceberg rolling rates, and an analysis of high resolution iceberg profile data collected in 2012 was used to determine the associated distribution of iceberg draft changes that occur due to rolling, and thus the rate at which iceberg keels penetrate an EDC due to rolling events. Modeled iceberg grounding rates and iceberg scour data from the Jeanne d'Arc were used to estimate the rate at which gouging icebergs enter EDCs. Iceberg gouge data from the Jeanne d'Arc and a dynamic time-step iceberg simulation using the 2012 iceberg profile data were used to determine the impact rate for facilities in the EDC as a function of the distance between the midline and the top of the facilities (clearance). The analysis addresses some of the conservatisms in the current approach, allowing for reduced EDC excavation depths.
In designing an oil and gas platform for offshore arctic and subarctic regions, operators may need to consider potential iceberg impacts when determining optimal structure configuration and ice strengthening requirements. Ice strengthening requirements will depend on the frequency of impacts, the sizes and shapes of icebergs impacting, the impact velocities as determined by the response of icebergs to currents and waves and the strength of the ice. Global ice strength will influence overall design, and local ice strength will influence local structural design. Failure of ice in crushing is a complex process involving mechanisms such as spalling, pressure melting and recrystallization, which are very difficult to model. As a practical approach, global force is often modeled as the product of nominal contact area times global crushing pressure, with global crushing pressure estimated based on full scale measurements. During iceberg impacts, contact area increases with penetration, with the maximum area influenced strongly by the initial kinetic energy of the iceberg, and to a lesser extent by driving forces during the impact. Ice strength, as observed during field measurements, has a significant random variance, both in time during an interaction, and from interaction to interaction. This variance is especially important when designing for iceberg impact loads in regions such the Grand Banks off Canada's east coast where load events are very infrequent, on the order of once every 10 years given ice management. While ice strength data for sea ice loads is often presented in terms of upper limit strengths based on the assumption that there are large numbers of interactions per year, a probabilistic approach that explicitly considers the frequency of events is more appropriate.
In this paper, emphasis is given to global ice strength as relates to the total force on a structure, rather than local ice pressure as relates to local design for fixed structural areas on the platform. A strong scaling effect is observed in which the average global strength of ice decreases as the nominal area of contact increases. There is a lack of observed ice strength data for interactions involving failure of iceberg ice at large contact areas; a consequence of which is that there is not consensus in industry regarding the most appropriate strength model to use. While ISO 19906 presents a probabilistic model that accounts for variance in ice strength as contact area increases, with random coefficients to account for the variance between impacts, use of a minimum pressure cut-off for large areas is suggested due to the lack of ice strength data for large contact areas. ISO 19906 does not give guidance on the selection of the cut-off. A review of relevant data is presented here and different models for the minimum pressure cut-off considered, with example calculations presented.
Icebergs can pose a risk to offshore oil and gas structures in arctic and sub-arctic regions of the world. The Iceberg Load Software (ILS) was developed to determine design loads on structures following the spirit of ISO 19906:2010, helping designers better understand the impact forces and moments the structures must be designed to withstand. The ILS is a fully probabilistic model which accounts for the range of iceberg shapes, sizes and strengths, and environmental conditions expected at the platform location. The model is applicable to fixed structures such as a gravity based structure (GBS), as well as floating structures such as a floating production, storage and offloading (FPSO) vessel. Users can incorporate the effectiveness of iceberg detection, physical management, and disconnection (where applicable for floating platforms) in mitigating the risk of impact with an iceberg.
The input relationships and distributions used to characterize the iceberg population are based on measured data typically collected in the region. These data include everything from basic measurements such as iceberg length, width or sail height to the more detailed shape information in the form of complete three dimensional iceberg profiles. In 2012, a major field program was carried out (
The objective of this study was to use the high resolution iceberg profiles to update models characterizing iceberg shape in the ILS. These includes models for area-penetration, contact location and impact eccentricity. In addition, relationships correlating iceberg draft and mass to waterline length were updated using the new profiles. Example simulations were performed for a generic structure using the ILS to demonstrate the influence of the updated models, distributions and relationships on the output design forces and moments.
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.18.104.22.168.2:"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.
Huang, Yujian (CARD, C-CORE) | Qiu, Wei (Department of Ocean and Naval Architectural Engineering, Memorial University of Newfoundland) | Ralph, Freeman (CARD, C-CORE) | Fuglem, Mark (Ice Engineering, C-CORE)
Global ice loads at different locations on a ship during rams into ice are a function of ship motions and added mass in addition to the failure mode and strength of the ice. In the literature, various analytical added mass models have been used for ship-ice interactions, which could lead to significant differences in the prediction of global ice loads on ships. In this work, an improved added mass model has been developed based on numerical results and existing analytical models. Added mass coefficients of three ice-going ships, CCGS Amundsen, CCGS Louis S. St-Laurent and MVArctic, were estimated using four analytical added mass models. It was found that the differences in added mass coefficients predicted by these models are significant and enhancements can be made. A body-exact numerical simulation tool based on the potentialflow theory, MAPS0, has been used to compute the added mass coefficients of the three vessels in the frequency domain and the results were used for the development of the improved added mass model. The improvement in the load predictions has been demonstrated by applying the new added mass model to the three vessels.