Velocity Effects on Conical Structure Ice Loads

2002 ◽  
Author(s):  
Dmitri G. Matskevitch
Keyword(s):  
Model Tests ◽  
Ice Load ◽  
Empirical Correction ◽  
Conical Structure ◽  
Ice Loads ◽  
Ice Velocity ◽  
Velocity Effect ◽  
Ice Failure ◽  
Bending Failure ◽  
Conical Structures

Existing design codes and most methods for ice load calculation for conical structures do not take velocity effects into account. They were developed as an upper bound estimate for the load from slow moving ice which fails in bending against the cone. Velocity effects can be ignored when the structure is designed for an area with slow ice movement, for example, the nearshore Beaufort Sea. Sakhalin structures will be exposed to ice moving at velocities up to about 1.5 m/sec. Model tests show that quasi-static methods may underestimate the ice load on a steep cone when the interaction velocity is that high. The present paper summarizes results of published model tests with conical structures that show a velocity effect. An empirical correction factor to the Ralston method is developed to account for the increase in cone load with ice velocity. The paper also discusses velocity effects on ice failure length and possible transition from bending failure to an alternative failure mode when the ice velocity is high.

On the Dynamic Interaction Between Drifting Level Ice and Moored Downward Conical Structures: A Critical Assessment of the Applicability of a Beam Model for the Ice

2011 ◽  
Author(s):  
Sjoerd F. Wille ◽  
Guido L. Kuiper ◽  
Andrei V. Metrikine
Keyword(s):  
Cross Section ◽  
Failure Process ◽  
Interaction Process ◽  
Second Phase ◽  
Floating Structure ◽  
Conical Structure ◽  
Level Ice ◽  
Ice Failure ◽  
Bending Failure ◽  
Conical Structures

Downward conical structures are believed to be an interesting concept of a floating host for oil and gas developments in deeper Arctic waters. The conical structure forces the ice to fail in bending, thereby limiting the ice loads on the structure. During the last two years, several conical structures were investigated at the Hamburg Ship Model Basin (HSVA) as part of a Joint Industry Project. This paper presents a numerical model for drifting level ice interacting with a moored downward conical structure. The goal of this development was to get insight in the key processes that are important for the interaction process between moving ice and a floating structure. The level ice is modelled as a moving Euler-Bernoulli beam, whereas the moored offshore structure is modelled as a damped mass-spring system. The ice-structure interaction process is divided into two phases. During the first phase, the ice sheet bends down due to interaction with the structure until a critical bending moment is reached at a cross-section of the beam. At this moment, the beam is assumed to fail at the critical cross-section in a perfectly brittle manner. During the second phase, a broken off block of ice is pushed further down the slope of the structure. These phases were built into one, piece-wise in time continuous model. A key result found by means of the numerical analysis of the model is that the motions of the moored floating structure do not significantly influence the bending failure process of level ice. Also the influence of the in-plane deformation and the heterogeneity of ice on the bending failure process is negligible. As a consequence, the dynamic response of the structure is for the biggest part determined by the ice failure process. Although the response of the structure can be dynamically amplified due to resonance for some particular ice velocities, no frequency locking of the ice failure onto one of the natural frequencies of the structure was observed. The model output showed qualitative agreement with the HSVA test results. It was however concluded that one-dimensional beamlike models of level ice sheets cannot accurately predict loading frequencies on downward conical moored floating structures because the ice blocks that break off are too long. Modelling level ice in two dimensions using plate theory is expected to give better results, since it takes into account the curvature of a structure and both radial and circumferential ice failure.

Influence of Structural Compliance and Slope Angle on Ice Loads and Dynamic Response of Conical Structures

2015 ◽  
Author(s):  
Gesa Ziemer ◽  
Karl-Ulrich Evers ◽  
Christian Voosen
Keyword(s):  
Natural Frequency ◽  
Model Test ◽  
Slope Angle ◽  
Offshore Structures ◽  
Model Tests ◽  
Offshore Wind Turbine ◽  
Worst Case ◽  
Ice Load ◽  
Ice Loads ◽  
Conical Structures

Model tests in ice have been conducted at the Large Ice Basin of HSVA with cylindrical and conical, compliant structures exposed to drifting level ice to investigate the influence of slope and compliance on the ice load and its breaking frequency. Main goal of the test campaign was to study the importance of structural feedback during ice-cone interaction. This is a major issue e.g. for numerical simulation of offshore structures during design phase. Four shapes were tested: 50°, 60°, 80° and 90° slope angle. The cylinder was tested in order to define the worst case scenario regarding magnitude of ice load and severity of ice-induced vibrations. Stiffness and natural frequency of the structure were chosen similar to typical values for offshore wind turbine support structures. All shapes were tested both in a compliant and fixed configuration. The breaking frequency was found to be more pronounced for the lower slope angles where the ice failed in flexural failure only, while a transition to crushing failure as observed on a cylindrical structure takes place at 80° cone angle already. This results in significantly higher ice loads on the 80° cone than on those with lower angles, but a reduced risk of severe ice-structure interaction due to the unsteady nature of the mixed mode breaking process. Although the breaking frequency is rather constant e.g. during ice impact on the 60° cone, it was not possible during the model tests to match the ice drift speed and the dynamics of the structure in a way that causes resonance. However, model test results prove that there is a risk of conical structures with low natural frequencies and low stiffness in ice plane being excited by periodic ice failure in their natural frequency, thus response amplification may take place and pose a risk to the structural integrity of conical offshore structures exposed to sea ice. This paper presents the model test setup, analysis of the results, and general findings.

Dynamic Ice Force Analysis on a Conical Structure Based on Direct Observation and Measurement

2013 ◽  
Vol 136 (1) ◽  
Author(s):  
Ning Xu ◽  
Qianjin Yue
Keyword(s):  
Video Camera ◽  
Temporal Variations ◽  
Qualitative Description ◽  
Basic Model ◽  
Broken Ice ◽  
Conical Structure ◽  
Ice Failure ◽  
Ice Force ◽  
Bending Failure ◽  
Ice Forces

In order to study dynamic ice force induced by ice-structure interaction, we adopted the most reliable method to directly measure ice force on full-scale structure. This paper mainly demonstrates the qualitative description on the basic model for dynamic ice forces based on direct measurement on the jackets with ice-breaking cone in the Bohai Sea. Temporal variations of ice force are recorded by the ice load panels, and corresponding ice failure processes on conical structures are recorded by video camera. It is found that, when an ice sheet acts on the upward narrow cone, bending failure occurs and broken ice pieces are completely cleared up by the side of the cone. The basic form of dynamic ice force in time domain is a series of impulse signals with minimum load of zero.

Dynamic Ice Forces Analysis of Conical Structure Based on Direct Measurement

2010 ◽  
Author(s):  
Ning Xu ◽  
Qianjin Yue
Keyword(s):  
Direct Measurement ◽  
Failure Process ◽  
Video Camera ◽  
Ice Load ◽  
Conical Structure ◽  
Force Variation ◽  
Ice Force ◽  
Bending Failure ◽  
Ice Forces

The dynamic ice force is produced by failure process during ice interaction with structure. The best way for describing and modeling this process is using directly measured ice force on full scale structure in situ. In this paper, the ice force variation and corresponded failure process of ice sheet were recorded by ice load panel and video camera. It is demonstrated that when ice acting on upward narrow cone and in bending failure and well clearing by side of the cone. The form of ice force history looks like impulse signal.

Assessment of Flexure Failure Models Using Loads Measured on the Conical Piers of the Confederation Bridge During 1998–2008

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Chee K. Wong ◽  
Thomas G. Brown ◽  
J. Susan Robertson
Keyword(s):  
Prince Edward Island ◽  
Monitoring Program ◽  
Physical And Mechanical Properties ◽  
Eastern Canada ◽  
Ice Load ◽  
Flexural Failure ◽  
Conical Structure ◽  
Ice Loads ◽  
Failure Models ◽  
Bridge Spans

The Confederation Bridge spans across the Northumberland Strait in Eastern Canada connecting Prince Edward Island to mainland Canada through New Brunswick. Due to the presence of ice during each winter, the bridge piers are subjected to ice loads. A comprehensive permanent monitoring program has been implemented to observe and measure the ice–structure interaction events at two piers since the start of the bridge operations in 1998. This study uses the derived ice loads on one pier, and the associated event attributes for 100 selected events. Flexural failure models are used to determine theoretical loads of the selected interaction events. It is found that the weight of the total ice rubble pile and the physical and mechanical properties of the ice sheet are the dominant parameters affecting the ice load exerted on the conical structure. A semi-empirical correlation is developed to relate the ice load with those parameters for the Confederation Bridge.

Results of Field Monitoring on Ice Actions on Conical Structures

2011 ◽  
Vol 133 (4) ◽  
Author(s):  
Ning Xu ◽  
Qianjin Yue ◽  
Yan Qu ◽  
Xiangjun Bi ◽  
Andrew Palmer
Keyword(s):  
Bohai Sea ◽  
Field Work ◽  
Interaction Process ◽  
Structure Interaction ◽  
Interaction Processes ◽  
Ice Conditions ◽  
Conical Structure ◽  
Ice Loads ◽  
Ice Forces ◽  
Conical Structures

Ice-structure interaction plays a central part in determining ice loads and ice-induced vibrations. This is a controversial research issue, and many factors make the problem more complicated. The authors have been monitoring several ice resistant structures in the Bohai Sea for 20 years and have measured ice forces and simultaneously observed ice-structure interaction processes. This paper describes typical physical ice sheet–conical structure interaction processes, field data, and theoretical explanations for different ice conditions and structure dimensions. The conclusions are more widely applicable, and we relate them to field work on ice resistant conical structures in other ice-covered regions. Further work will quantify ice loads on conical structures once the interaction process is understood.

Main Factors of Ice Sheet-Conical Structure Interaction Process Based on Field Monitoring

2009 ◽  
Author(s):  
Ning Xu ◽  
Yan Qu ◽  
Qianjin Yue ◽  
Xiangjun Bi ◽  
Andrew Clennel Palmer
Keyword(s):  
Ice Sheet ◽  
Field Work ◽  
Interaction Process ◽  
Structure Interaction ◽  
Interaction Processes ◽  
Ice Conditions ◽  
Conical Structure ◽  
Ice Loads ◽  
Ice Forces ◽  
Conical Structures

Ice-structure interaction plays a central part in determining ice loads and ice-induced vibrations. This is a controversial research issue, and many factors make the problem more complicated. The authors have been monitoring several ice resistant structures in the Bohai Sea for twenty years, and have measured ice forces and simultaneously observed ice-structure interaction processes. This paper describes typical physical ice sheet-conical structure interaction processes, field data and theoretical explanations, for different ice conditions and structure dimensions. The conclusions are more widely applicable, and we relate them to field work on ice-resistant conical structures in other ice-covered regions. Further work will quantify ice loads on conical structures once the interaction process is understood.

Auto-determination of ice forces on arctic structures

1987 ◽  
Vol 14 (4) ◽  
pp. 571-580
Author(s):  
T. G. Brown ◽  
M. S. Cheung
Keyword(s):  
Sea Ice ◽  
Structural Characteristics ◽  
Full Description ◽  
Ice Load ◽  
Primary Determinant ◽  
Ice Loads ◽  
Local Pressures ◽  
Ice Failure ◽  
Ice Pressure

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.

Model Testing of Ice Barriers Used for Reduction of Design Ice Loads

2003 ◽  
Author(s):  
Peter Jochmann ◽  
Karl-Ulrich Evers ◽  
Walter L. Kuehnlein
Keyword(s):  
Bending Strength ◽  
Model Tests ◽  
Individual Model ◽  
Mean Values ◽  
Drilling Rig ◽  
Design Load ◽  
Ice Load ◽  
Ice Drift ◽  
Ice Loads ◽  
Ice Model

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.

Advanced Weathervaning in Ice

2011 ◽  
Author(s):  
William Hidding ◽  
Guillaume Bonnaffoux ◽  
Mamoun Naciri
Keyword(s):  
Rapid Change ◽  
Ice Sheet ◽  
Model Tests ◽  
The Arctic ◽  
Mooring System ◽  
Sudden Increase ◽  
Time Step ◽  
Ice Loads ◽  
Level Ice ◽  
Drift Direction

The reported presence of one third of remaining fossil reserves in the Arctic has sparked a lot of interest from energy companies. This has raised the necessity of developing specific engineering tools to design safely and accurately arctic-compliant offshore structures. The mooring system design of a turret-moored vessel in ice-infested waters is a clear example of such a key engineering tool. In the arctic region, a turret-moored vessel shall be designed to face many ice features: level ice, ice ridges or even icebergs. Regarding specifically level ice, a turret-moored vessel will tend to align her heading (to weather vane) with the ice sheet drift direction in order to decrease the mooring loads applied by this ice sheet. For a vessel already embedded in an ice sheet, a rapid change in the ice drift direction will suddenly increase the ice loads before the weathervaning occurs. This sudden increase in mooring loads may be a governing event for the turret-mooring system and should therefore be understood and simulated properly to ensure a safe design. The paper presents ADWICE (Advanced Weathervaning in ICE), an engineering tool dedicated to the calculation of the weathervaning of ship-shaped vessels in level ice. In ADWICE, the ice load formulation relies on the Croasdale model. Ice loads are calculated and applied to the vessel quasi-statically at each time step. The software also updates the hull waterline contour at each time step in order to calculate precisely the locations of contact between the hull and the ice sheet. Model tests of a turret-moored vessel have been performed in an ice basin. Validation of the simulated response is performed by comparison with model tests results in terms of weathervaning time, maximum mooring loads, and vessel motions.

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