Probabilistic Assessment of Multi-Year Sea Ice Loads on Upward Sloping Arctic Structures

The drilling rig Sunkar, owned and operated by Parker Drilling under contract to Agip KCO, was the first drilling rig in the North Caspian Sea. In this area sea ice may occur between November and April. The original ice protection concept of the drilling rig was based on the fact that ice loads were partly taken and/or reduced by two rows of heavy piles. As an alternative concept ice model tests for Sunkar with installed ice barriers (Ice Rubble Generators) around the rig were carried out in the Large Ice Model Basin of HSVA in order to establish the design ice loads and to prove that the design forces can be reduced significantly by using these ice barriers. The test series were carried out in 1.3 m thick level ice with a bending strength of 770 kPa. Ice drift angle, ice drift speed, spacing between the ice barriers, as well as the angle of the ice barriers were varied. The design water level simulated in the model tests was about 4 m. As maximum measured ice load values are a result of coincidental ice failure occurrences these values are much more scattered than the mean values of ice model tests. Even if an individual model test could be repeated exactly, i.e. exactly within measurable limits, the maximum load would be different. Therefore the design load needs to be obtained by using a sophisticated statistical approach. To establish the design load an extreme value distribution, a Gumbel-Probability-Distribution (GPD) for each individual model run has been applied. The ice model tests have shown that a significant ice force reduction can be achieved if the drilling rig Sunkar is protected with ice barriers. The reduction of the maximum horizontal global ice load amounts to approx. 63% when Sunkar is protected by ice barriers. The ice barriers initiate ice rubble and areas of rafted ice as well as ice accumulation between the barriers, which lead to ice bridging with a spacing of 60 to 80 m between the ice barriers. As a final result it was found that the stability of Sunkar will be sufficient under any angle of drifting ice if ice barriers are installed.

Ice loads, due to large absolute values and diversity, are most important for the calculation of Arctic marine and river structures. The cost of the structure being designed may change several times with a change in the design ice load; therefore, determining the ice load of a given probability is an important task. Some ice formations can significantly change the value of the ice load, but they are not present in the normative documents and standards. Ice collars can be described as thicker ice around heat-conducting structures, which can lead to an increase in loads. However, there is not any method for calculation of ice collar influence on the loads present in the current documents. Ice collar is the reason for thicker ice at the structure-ice contact surface, which lead to an increase in the contact area and can lead to a change in the mechanism of ice failure. In some cases, the presence of an ice collar leads to an increase in the horizontal ice load from the level ice on the structure. The method of discrete elements is considered to solve the problem of the influence of the ice collar on the ice loads. This method allows a time-dependent description of the entire process of destruction of the ice collar. Two-dimension settings are used in vertical and horizontal planes. The relative ice load depending on the thickness of the ice and freezing-degree-days are obtained.

Ice-capable jackups can offer mobility and efficiency while drilling in shallow, ice-infested waters. However, the industry has limited experience operating jackups in ice environments, and technology gaps and uncertainties remain when assessing ice loads on multi-legged jackup units. The Graphic Processing Unit (GPU)-based Discrete Element Method (DEM) can be applied to investigate the degree of ice effects on multi-legged jackup units. In previous applications, the DEM method was used to simulate ice interaction with structures, but computations were limited to simple interactions. Introducing a GPU platform enables a substantial increase in the number of elements characterized so that the interaction of multi-legs and ice can be modeled with greater efficiency. For example, level ice modeled with 3-D bonded spherical elements can address the buoyancy, current effects and drag force encountered in harsh environments. Additionally, the parallel bonding approach and de-bonding criteria are used to model level ice freezing and breakage. In this investigation, the global ice loads on a jackup unit's single leg are analyzed to determine the effects of ice thickness, ice strengths and drifting velocities. The estimated ice loads are validated against full-scale data acquired from one application in Bohai Bay. Further, the mechanism of the interaction between the multiple legs and level ice is analyzed using the large-scale GPU-based simulation, the effects of sheltering and jamming are investigated and the ice loads simultaneously acting on the multiple legs are estimated to determine the total shear force and overturning moment.

This paper describes a variety of programs specifically designed for the determination of sea ice and iceberg loads on Arctic offshore and nearshore structures. As any ice load is a function of the interaction between ice feature and structure, the design of arctic structures is very much an interactive process. Many other factors determining the overall loads and local pressures are functions jointly of ice feature and structural characteristics. For example, the ice strain rate which is a primary determinant of ice strength and failure behaviour may be determined from ice velocity and structure size.The paper details the development of a number of programs directed at the evaluation of quasi-static ice loads, dynamic ice loads, and corresponding local pressures between ice and structure. Examples are provided of the use of the various programs, including the data required and the type of outputs resulting.As a number of the programs incorporate quite extensive theoretical developments or, in one case, a large number of discrete interactions, full description of each program is beyond the scope of this paper. The reader is directed to the listed references for full developments of the various programs and algorithms. Key words: sea ice, iceberg, global ice load, local ice pressure, finite element, ice/structure interaction, probabilistic analysis, ice failure mode.

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 20208, "Sakhalin-1: Technology Development for Frontier Arctic Projects," by J.M. Hamilton, ExxonMobil Upstream Re - search, and J.E. Jones, ExxonMobil Development Company, originally prepared for the 2009 Offshore Technology Conference, Houston, 4–7 May. The paper has not been peer reviewed. As the oil and gas industry pushes into frontier Arctic areas, new technologies and design criteria will be required to ensure safety and cost effectiveness of increasingly challenging developments. Sakhalin-1 provides a good example of a development in a frontier subarctic area where location-specific design criteria had to be developed and innovative-technology solutions were required to deal with demanding environmental conditions including sea ice, large breaking waves, and earthquakes. Introduction First production from the Exxon Neftegas Limited (ENL) Sakhalin-1 Phase-1 development of Chayvo field began in October 2005, marking the culmination of nearly 10 years of design-criteria and technology development. The development concept consists of an offshore drilling platform, an onshore drilling and treating facility at Chayvo, an oil-export pipeline that runs east/west across the island and Tatar Strait to mainland Russia, and a storage and tanker export terminal at DeKastri. The oil is exported by ice-strengthened tankers south through the Tatar Strait, which is partially covered by up to medium first-year ice (70 to 120 cm) in the winter. The offshore drilling platform, Orlan, is approximately 10 km offshore in approximately 15-m water depth. The platform annually is exposed to approximately 4000 km of drifting first-year pack ice up to 1.5-m (level ice) thickness, which originates in the Sea of Okhotsk north of the island and drifts south along the eastern coast. The controlling ice load features are pressure ridges, with keels extending to depths greater than 30 m (2000-year design value). The design ice load on the 80-m-wide, vertical-sided structure is approximately 385 MN. Design wave loads are generated by late-summer and fall storms that can produce breaking waves at the platform. Global wave loads are on the order of the ice loads, and wave run-up and locally high slamming pressures represent design challenges. Sakhalin Island seismicity is moderate to high, with the active Piltun fault system running north/south on the island as close as 10 km from onshore facilities and 20 km from the offshore platform. The 1995 Neftegorsk earthquake, which registered moment magnitude 7.1, had its epicenter approximately 50 km from the Sakhalin-1 facilities. All facilities must be designed for earthquakes, and the export pipeline, which crosses multiple faults, must be able to withstand several meters of fault displacement.

Ice loads on the design waterline (DWL) of several vessels were computed with the program Venice to obtain ice load distributions on ship hulls advancing straight ahead in level ice. The purpose is to obtain the spatial distribution of ice loads on the ship hulls in level ice, including the information on how the ice loads vary as functions of ship speed or ice thickness. For one ship hull the computed ice loads were compared with ice load distributions measured in model scale. The computed distributions were also compared with the design line load distributions used in the Finnish-Swedish ice rules and in the Polar Class rules. Some clear differences were found.

Numerical simulation on an icebreaking vessel with fixed drift angles in level ice

Existing national and international standards for determination of level ice and ice ridge loads on sloping offshore structures recommend different methods for the analysis. The objective of this paper is to review the codes and standards recommendations regarding ice-sloping structures interaction process and highlight the differences between them. Development of offshore hydrocarbon fields in the Eastern Barents Sea is foreseen to take place in the near future while developments already take place in the Pechora Sea and offshore Sakhalin as well as in the Northern Caspian Sea. One of the most difficult issues facing the designer of offshore structures for these areas is how to design for loads from level ice and ice ridges. The ice load considerations will have a major effect on the form and cost of these structures. It is known that different designers use very different ice load estimates (Shkhinek et al., 1994). The standards recommend different methods for determination of the global ice loads on both cone-shaped and sloping rectangular structures. For determination of the global ice loads on these types of structures, it is obvious that the ice-structure interaction process must be identified. Rubble effects must be included in the analysis. The ice-structure interaction process for these geometries depends on many factors, such as; the ice thickness, ice strength, ice-structure friction coefficient, ice velocity, width of the structure and slope angle of the structure. The methods for determination of ice loads recommended by the different standards are very much influenced by local ice conditions and the parameters listed above are given different importance in the different standards. The differences in loads calculated by using the different standards and their validity for the ice-structure interaction process have been investigated and example calculations are presented to show these differences. It is thought that the paper may be of interest for those preparing the new ISO standard (ISO 19906) on Arctic Offshore Structures.

Despite the success of previous studies, there are still no reliable and accurate methods for determining the ice load on marine hydraulic structures (HS), taking into account the mechanical properties of ice and the characteristics of the ice situation. The object of the field study was the closed Sharapov Shar Bay in the Kara Sea, the ice situation of which is characterized by the presence of a stable land fast ice with a flat ice field. The subject of the study was the ice load in closed bays that have a stable land fast ice throughout the entire period of maximum ice impacts. The conditions of the bay provide viscoplastic deformation of the ice, and a stable land fast ice will deform around the HS in a state of prolonged creep without accumulation of cracks and damage. A method for determining ice loads has been developed, taking into account the features of a stable land fast ice. Ice loads on the HS should be determined on the basis of initial data on the nature of the ice situation in the HS construction area for the period of maximum ice loads. When determining the global load from flat ice on an extended structure with a vertical front wall for the conditions of Sharapov Shar Bay in April, when the maximum load from a stable land fast ice is observed, it is recommended to take the value of the correction factor kv =0.055. The calculated ice load on the HS should be taken as the maximum of all loads for the considered phase of the ice regime. Provided that the viscoplastic deformations of ice are taken into account, the calculated load on the HS is the load when breaking the soldered ice, exceeding the value that occurs during the viscoplastic operation of ice in conditions of stable land fast ice. The practical implementation of the proposed method is possible only after the relevant methodological requirements are included in the current regulatory documents. The use of current regulatory requirements in determining the calculated ice load does not allow to justify the use of gravity HS for the conditions of the Sharapov Shar Bay. And the proposed approach to the assessment of the ice load, taking into account the peculiarities of the phases of the ice regime and the transients between them, allowed us to justify the possibility of using a gravitational HS.

Future projections of atmospheric icing in Norway

A comparison of the ice loads proposed by IACS and ice loads with a non-constant distribution over the load patch (von Bock und Polach, 2006) indicated that a plastic design approach might cause plastic deformation of the ship hull. Quinton (2012) stated that the calculation methods used for the unified requirements (IACS, 2011) for static glancing impacts differ from dynamic loads. However, the research is based on laboratory experiments and would require additional validation against full scale measurements for better proof. Riska et al. (2002) stated that earlier findings on the nature of ice loads from full scale measurements have already been incorporated into the Finnish-Swedish ice class rules. In addition, the measured ice loads have been compared with loads derived from existing rules and a modification of the rules was suggested based on those measurements (Kujala et al., 2007). It is stated by Riska that the shortcomings of the rule formulations are known, but there is a need for new full scale data for additional development (Kujala et al., 2007, WP7.4).

Melt ponds are usually modeled for light transfer as horizontally infinite water layers on level ice, and the albedo of floe is determined by a linear combination (LC) of melt pond and bare ice albedos weighted by their areal coverages. However, this method does not reflect the actual conditions because ice floes have a limited size. In the present study, an idealized two‐dimensional Monte Carlo (MC) model was employed to investigate the influence of melt ponds and floe size on the apparent optical properties (AOPs) of summer sea ice. The results showed that the albedo and vertical light transmittance of large floes mainly depend on the melt pond fraction and ice thickness, which is consistent to previous results. However, also the floe size plays an important role in the AOPs of small floes. Two parameters were proposed to present the accuracy of the LC method for small floes with lower sea ice concentration: the ratios of sea ice albedo and transmittance determined by the LC (αline, Tline) to the values in the MC model (α, T), Kα = αline/α, and KT = Tline/T, respectively. Due to the lateral transmittance, Kα, KT ≥ 1 and asymptotically approach 1 with floe size increasing to infinity. To reduce the biases in albedo and transmittance due to floe size, new parameterization formulas were provided for Kα and KT with the distance into the marginal ice zone and in different melting stages. The results have potential to be implemented into future sea ice models to correct the AOPs of small sea ice floes obtained via the LC method.

P>Sea ice monitoring is an important field of scientific research and relevant to operational applications. One of the major engineering challenges in undertaking production developments in Arctic offshore regions is the frequent presence of extreme ice features that pose a hazard to facilities and surrounding subsea infrastructure. The information on extreme ice features (i.e., ridges, icebergs etc.) is important from the standpoint of potential ice load levels on fixed structures and ice scouring of seafloor facilities. Satellite observation has been shown to be useful for extracting and characterizing ice regimes. Sea ice can be monitored using satellite imagery acquired by different types of sensors: microwave radiometer, optical instrument and synthetic aperture radar (SAR). The outputs of sea ice monitoring may include various ice parameters such as edge, thickness, concentration, classification, iceberg detection, and ice statistics. This paper describes application of high and low resolution SAR imagery for sea ice monitoring and to resolve local features and extend the statistical baseline to larger regions because extreme ice features may be invisible or ambiguous with other ice features in these data. The use of higher resolution imagery allows for easier detection of ice features and provides sufficient spatial detail necessary for detecting ice features from sea ice, identification and estimation of size and geometry of ice floes and icebergs. It was demonstrated that SAR sensors with multiple resolution lead to a better understanding of ice conditions including ice edge, concentration, floe statistics, and other ice features such as icebergs. A technique based on SAR interferometry was used for identification of iceberg in sea ice as well as for extracting iceberg topography.

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.

In this study, a numerical method was developed based on peridynamics to determine the ice loads for a ship navigating in level ice. Convergence analysis of threedimensional ice specimen with tensile and compression loading... | Find, read and cite all the research you need on Tech Science Press