This paper examines the interaction of pack ice with a floating production storage and offloading unit (FPSO). The vessel maintains position using a thruster-assisted turret mooring. An ice dynamics model is used to conduct numerical simulations of the interaction of the ice cover with the vessel. The model is based on solving equations describing the conservation of mass and linear momentum together with a failure criterion for the ice cover. The vessel is modelled as a three-degrees-offreedom rigid body. The results give an evolution of the stresses and deformation of the ice cover, ice forces, and offsets of the vessel. Ice conditions and FPSO characteristics typical for the Grand Banks were used in the simulations. The results show a strong dependence of ice forces and vessel offsets on ice concentration. The roles of ice floe size and ice thickness were less significant. The conclusions indicate that for the ice conditions considered, vessel offsets would fall well within the operating capabilities of a typical Grand Banks FPSO.

This paper provides the details of ice loading events that can be used to further the understanding of pack ice driving forces in the Beaufort Sea. Several methods have been reviewed and employed including in situ stress measurements, loads on the Molikpaq offshore caisson, shoreline pile-up events, pile-ups and rubble fields on offshore shoals and relic berms, analysis of shear walls on offshore rubble fields, and analysis of deep ridge keels. Over 50 different events are identified with 33 suitable for a pack ice analysis. The data are considered in terms of both the ISO 19906 (2010) Arctic Structures Standard and pack ice pressures that can be exerted across various widths. A new approach is proposed, in which the calculated and measured values from past pack ice pressure events are used to predict limit force.

AbstractThis paper examines the response of a drillship to the action of an ice cover undergoing a gradual change of direction of motion. For effective stationkeeping, the drillship must continuously change its heading to face the oncoming ice. The simulations consider a vessel that maintains position using Thruster-Assisted Mooring (TAM). A turret mooring system resists the surge and sway movements, while the thrusters act to correct the heading. The simulations of ice deformation and drift solve equations that describe conservation of mass and momentum, and a failure criterion. The drillship is treated as a three-degrees-of-freedom (surge, sway and yaw) rigid body. The results give distributions of ice drift, deformation and stresses around the vessel as well as the forces and offsets of the drillship. The resulting peak ice forces and moments on the drillship show clear dependence on the rate of change of ice drift direction. As may be expected, ice forces and moments increase for high rates. They also increase for higher ice velocities. Analysis of the results shows that the ratio of the length of the drillship to the radius of curvature of ice drift trajectories can be used to estimate ice forces and moments in an environment of changing ice drift direction. The present work additionally included a cursory examination of the effects of ice cover conditions; namely ice thickness and the existence of ridge fragments and icebreaking tracks, which are often formed as part of ice management operations.

This paper discusses the results from an integrated research project, carried out over many years, which examined a variety of evacuation strategies for offshore structures in the Beaufort Sea. The objective of the project was to address the safety of offshore personnel working in Canada’s Arctic environment, by examining the issues surrounding emergency evacuation from an offshore structure under the diverse range of conditions in the Beaufort Sea. This was accomplished through: the development of decision flow-charts for the establishment of on-ice evacuation shelters and their associated costs and logistics; field measurements of personnel movement rates across different ice morphologies; an investigation into ice rubble stability and roughness and its influence on evacuation systems; an examination of ice management options when ice rubble is present around an offshore structure; and the presentation of generic means of evacuation and their adequacy for four “seasons” of evacuation: open water, moving pack ice, quasi-stable rubble and stable rubble/landfast ice. The project results provide 1) information related to the viability of systems for a range of realistic ice conditions, especially those involving ice rubble, 2) provide input into the development of the evacuation and rescue options and strategies for Beaufort Sea structures and 3) be used by Operators and Regulators to examine the feasibility of proposed evacuation systems for the Beaufort Sea.

AbstractThe ability of a drillship to maintain its heading to face oncoming pack ice is crucial under situations involving changes in pack ice drift direction. The performance of a vessel employing a Thruster-Assisted Mooring (TAM) system under such conditions is examined in this paper. Numerical simulations were used to determine the stresses and deformations within the moving pack ice cover, as well as the response of a drillship with characteristics similar to published information on Stena's DrillMAX. A turret mooring system is assumed to resist surge and sway direction offsets, but provides no restoring moment to vessel's yaw. In such system, thrusters would be used to provide damping and to apply the corrective moment that controls the heading of the vessel. The pack ice cover is assumed to consist of managed floes of sizes ranging from 30 m to 50 m, and with a uniform thickness of 1 m. The ice cover moves against the vessel at a steady velocity of 0.5 m/s. Simulations start with the vessel at a heading inclined to the oncoming ice direction. The simulations predict the evolution of the distributions of ice thickness and pressures, ice forces and moments, as well as the response of the vessel. Test cases examined a range of values of the initial heading of the vessel and limits on available yaw moment that can be generated by the thrusters. The results illustrate the manner in which the vessel can correct its heading, and give the corresponding offsets, ice forces and moments.

Abstract The present work examines the loads on moored/DP-assisted drillships and their responses in managed pack ice. Numerical simulations are used to determine deformations and stresses within the ice cover and the movements of the vessel. The results give ice forces and moments on the vessel, as well as its surge, sway and yaw responses. The drillship assumed in this simulation work has a length of 230 m, a beam of 42 m, and a mass of 100,000 Mt. Simulations examine a case representing a moored vessel in a managed ice field consisting of 50 m floes that are 1 m in thickness, moving at a constant speed of 0.3 m/s. For ice moving along the surge direction, the peak surge force was approximately 2 MN. The corresponding maximum surge was 1.5 m. A parametric study further examines several aspects of ice interaction with the vessel. The results quantify various effects of ice management (the role of floe size), ice thickness and drift speed, the direction of ice movement, and stiffness of the mooring system. A case of combined mooring and thruster-assisted mooring is also examined. The additional contribution of the thrusters (or control of the DP system) is shown to somewhat reduce the offsets of the vessel. The work presents a methodology for evaluating the stationkeeping performance of vessels in managed pack ice and quantifying the role of the mooring/DP system characteristics.

Numerical simulations are carried out to examine the interaction of a managed ice cover with a drillship. Station keeping is maintained using a mooring system and Thruster-Assisted Mooring. The ice model is based on solving the momentum and constitutive equations, which determine the stresses and deformations of the ice cover. The drillship is modeled as rigid body of three degrees-of-freedom: surge, sway and yaw. A Base Case examines a managed ice cover consisting of floes with sizes ranging from 20 m to 50 m, and a uniform ice thickness of 1 m. The ice cover moves at 0.3 m/s along the surge direction. The drillship has a length of 130 m, beam of 42 m and a mass of 100,000 Mt. The resulting ice force shows peak values reaching 1.7 MN. Analysis of the ice force-time records gives probability distributions of the peak forces, the time required for the force to reach the peak (rise time to peak force), and the rate of force increase. The work includes a parametric study, which examines the effects of the size of the drillship, ice thickness variations, the existence of large ridge fragments in the ice cover, the presence of brash ice, patterns of icebreaking tracks, and thruster assistance to the mooring system.

The assessment of ice management success is important for ensuring the safety of floating structures in sea ice environments. The context is the ISO 19906 standard for arctic offshore structures, which specifies ice load requirements. General aspects of ice management systems that influence success are described and a specific example for the Beaufort Sea is provided to illustrate a way of determining managed swath width based on drift forecasting capabilities. Regulatory objectives are also discussed.

Abstract The interaction of managed ice coverswith a wide structure is examined. Numerical simulations are used to simulatethe modes of ice cover deformation and forces on the structure. Theobjective of the present work is to validate the performance of the numericalmodel using historical data recorded during operations of the Kulluk in theBeaufort Sea during the 1980s and early 1990s. This paper is the last part ofan investigation aimed at developing an approach for modeling managed iceinteraction with wide moored structures. Previous papers examined the role ofthe mooring system (Sayed and Barker, 2011) and the effects of ice coverconfinement and managed floes sizes (Sayed et al., 2012). The focus of thepresent paper is on providing quantitative comparison between predicted iceforce statistics and observations. The numerical model is based on solving equations describing theconservation of mass and linear momentum together with a plastic yieldcriterion that describes ice properties. The managed ice cover consistsof a heterogeneous ensemble of relatively small floes. The floes are assignedvaried thickness values in order to reflect field conditions. The simulationsaddress a class of interaction scenarios corresponding to confined managed ice. As the ice cover is driven within confined boundaries, pressure conditionsarise. These conditions are of interest to designers since forces areconsiderably higher than those corresponding to unconfined ice covers. Thesolution of the governing equations simulate the evolution of the distributionsof ice thickness, concentration (or coverage) and stresses. The resulting iceforces, including peak values, averages and rise-time of forces, are comparedto field measurements. The influence of ice thickness and velocity on expectedice forces is also examined. Introduction Future offshore developments in ice covered zones will likely require icemanagement support. This has generated an interest in providing reliablepredictions of the modes of interaction of managed ice covers with widestructures and the resulting forces. This paper concerns validation of anumerical approach for modeling managed ice covers interaction with structures. To date, the historical data collected during operations of the Kulluk in theBeaufort Sea during the 1980s and early 1990s provide the best availableinformation on the interaction of managed ice covers with a wide mooredstructure. The observations and measurements were analyzed by Wright (1999) and(2000). The reports of Wright provide comprehensive information on measuredforces and the corresponding modes of ice interaction with the Kulluk, icecover characteristics and ice management operations.

Numerical simulations are carried out to represent the historical data of ice interaction with the Kulluk during the 1980s. Three dimensional simulations include dynamics of the ice cover and the response of the mooring system. The results give modes of ice accumulation, clearing and ice forces. Depth-averaged simulations consider larger zones of the ice cover to examine a range of conditions that were observed during the past operation of the Kulluk in the Beaufort Sea. The simulations evaluate the effects of managed ice cover characteristics such as floe sizes and confinement. Predicted forces are compared to the historical record of measurements.

A Program of Energy Research and Development (PERD) study has recently been completed that identifies key considerations associated with the use of support vessels for in-ice personnel evacuation from offshore structures in the Canadian Beaufort Sea. As part of this work, a logic framework was developed to help with assessments of the “do-ability” of any particular support vessel evacuation approach for given scenarios, with the intent of systematically recognizing and addressing all of the important factors involved. Some of the main points made in this study are briefly summarized as follows: (1) In most in-ice situations, a direct ship-based personnel evacuation approach may well be preferred for many offshore structures; (2) The success of any ship-based Escape, Evacuation and Rescue (EER) approach is highly dependent on the capabilities and features of the support vessel(s) involved, and also on the geometry of the structure; (3) The presence of any grounded ice rubble around an offshore structure is a constraint that will typically make any ship-based EER approach impractical; and (4) Strategic and tactical assessment procedures can be developed to assess the likelihood of success of particular ship-based EER approaches for structures in ice. This paper is intended to highlight the range of considerations that were addressed in the report, and to outline some of the key aspects of the work.

Emergency evacuation from offshore structures in ice-covered waters is very challenging. Different systems are required depending upon the type of ice conditions surrounding the structure. In some cases where a stable ice sheet forms, an on-ice Evacuation Shelter can be a viable, cost-effective approach. This paper describes the conditions where this would be feasible and outlines the decision process to optimize the location of the shelter on the ice. The Evacuation Shelter that was deployed at the Paktoa C-60 drilling site is used as an example. The detailed planning requirements and implementation measures from this site are described.

For EER plans involving on-ice evacuation methods in the winter, an emergency shelter located on the adjacent ice is an important safety element for personnel working in oil or gas facilities in the Beaufort Sea. The ice surrounding a structure can vary from thin level first year ice to grounded rubble or ridges. The speed and safety of walking over the ice surface is strongly affected by the surface roughness and degree of ice rubble. Thus, a groomed trail from an oil or gas structure to an emergency shelter should be constructed that is suitable for walking. This paper discusses the methods, equipment required, construction duration and associated risks to efficiently construct and maintain the trail. The emphasis is on practical methods that can work in the wide range of ice and weather conditions that can occur in the Beaufort Sea region during the winter and spring.

This paper presents an assessment of the viability of locating evacuation shelters on landfast ice or rubble fields surrounding offshore structures in the Beaufort Sea. For platforms surrounded by stable landfast ice and/or rubble, on-ice evacuation to an evacuation shelter (ES) may be identified as part of an EER plan during the Beaufort Sea's winter season. There are a number of factors that must be recognized when considering the deployment of an ES on the ice cover around Beaufort Sea structures; for example, the stability and morphology of the ice, the location and number of routes to shelters on the ice and limitations caused of the nature of the emergency. Additionally, issues surrounding personnel access off of a platform and egress to the ES must be examined, along with key concerns about the viability of using an ES soon after freeze-up and during break-up. This paper provides some representative examples of full scale data to illustrate key points about the viability of on-ice ES deployments in winter, on a scenario basis, as a function of water depth. The information that is presented here was assembled through a combination of field experience, literature reviews, and discussions with operating personnel who are directly involved with this type of problem area.

An existing pressure vs. area curve for determining local ice pressures on offshore structures and ship hulls was included in the CSA S-47.1 and API RP 2N standards for offshore structure design. The data comprising the original curve was based on multi-year ice obtained from medium scale impact tests, ship ramming tests, and ice load panels on the Molikpaq structure. The curve subsequently has been adapted to warmer climates. where multi-year ice is absent. In adapting the pressure vs. area curve to new requirements for application in other codes and practices, it was observed that the ship impact tests may be too conservative for the local ice pressure curve. Also, further analysis of relevant data from the Molikpaq measurements in the Beaufort Sea has been obtained. Thus the pressure-area curve has been-updated to reflect the new data, and the ship impact data removed. The local ice pressures that were used in the original Molikpaq design have been included and there is very good agreement between the design pressures and those derived from a database not available at the time. The new relationships that have been developed for local ice pressure versus area are presented.

In the Canadian Beaufort Sea, the design ice load can often govern the cost of gravity based (GBS) oil or gas production structures. By installing suitable ice barriers around the structure, the design ice load for the GBS and hence its cost may be reduced. The cost of three candidate GBS were calculated as a function of design ice load and soil strength. Similarly the cost of the ice barrier was estimated as a function of barrier type, design ice load and soil strength. It was found that for some of the production structures on cohesive soils, the overall cost of the GBS plus the cost of the barrier was less than designing the GBS for the full ice load.

A new standard is being developed for the design of oil and gas production facilities in ice-covered and cold regions – ISO 19906, Arctic Structures. While the standard is due to be published in 2009, considerable effort has already gone into its development. In the present paper, some of the rationale that went into the development of requirements for floating structures in this standard is described. It is emphasized that this paper represents the views of the authors – the material is not sanctioned by the International Standards Organization (ISO).

A field program was carried out in the Canadian Beaufort Sea to obtain quantitative details of the time required to traverse a variety of ice rubble conditions, typical of those that may surround an offshore structure in landfast ice. The results indicate that the time required to cross rubble fields increases rapidly with increasing rubble severity, determined by factors such as rubble height, ice block size and snow depth. The quantitative findings have implications for emergency evacuation from an offshore structure when an on-ice Evacuation Shelter is identified in the EER strategy for the structure. The results and analysis show that preparing and maintaining a suitable egress path across ice rubble is essential for safe evacuation from an offshore structure surrounded by landfast ice and a grounded rubble field. The implications for an offshore structure in a variety of ice conditions are presented.

Effective design of an emergency evacuation system from an offshore structure in sea ice conditions must take into account all aspects of the anticipated ice conditions at the site. Evacuation systems are typically comprised of several different components or subsystems. These subsystems allow evacuees a choice in response to the emergency incident onboard the installation, as well as to environmental conditions off the installation. This paper discusses four possible evacuation means—preferred, primary, secondary and tertiary. Of these four techniques, only the preferred means of evacuation is not strongly influenced by the ice regime around the structure. This paper examines these ice regimes and quantifies the size of the broken ice region around a structure for different ice conditions. A large number of parameters affect the size of this broken ice zone including the general ice regime, the existence of a grounded rubble field, the failure mode of the ice, the ice thickness, and the ice roughness. Typical damage zone extent is extremely variable and can be in the order of 25 m from the structure for ice thickness of 1.5 m. This has significant consequences for evacuation systems deployed from the structure.