Wind turbine towers are being planned in ice covered regions subject to pressure ridges (e.g. the Great Lakes). Conical collars are often employed to reduce ice loads from level ice and their associated dynamics. For level ice, downward breaking cones have some advantages. It is not clear if this is the case for pressure ridges. This paper presents an improved method for ridge loads on wind turbines with downward breaking cones and makes comparisons with upward breaking cones.
First year pressure ridges can be formidable ice features and usually control design ice loads in the sub-Arctic. Important components of a ridge creating ice loads are the consolidated layer at the surface (which is considered as solid ice) and the ridge keel below consisting of ice rubble, but much thicker. The load due to the consolidated layer is usually derived as if it is thick level ice. On a cone, methods for level ice assume it can be idealized as a plate on an elastic foundation (the water) and equations have been developed for upward and downward breaking cones. But for a ridge on a downward cone, to break the consolidated layer downwards requires it to be pushed into the keel rubble below. This will have a different foundation modulus than water buoyancy. A method is developed to account for this difference. The method uses an iterative approach to determine the point of breaking of the consolidated layer (and associated load) accounting for the ridge geometry, keel rubble shear strength, the flexural strength of the consolidated layer and the buoyancy forces. The keel loads on the vertical shaft below the conical collar are based on the method currently in ISO 19906 (2010) but modified to add the effect of additional rubble in the keel from breaking the consolidated layer downwards.
In examples, it is shown that the breaking force can be about twice that of breaking the consolidated layer without the keel present. This might be seen as a disadvantage for downward breaking cones vs upward breaking. However, it is also shown that the clearing forces on an upward cone are higher; which tends to balance out the lower breaking force. Example loads are given on typical wind turbine bases due to typical ridges. Upward and downward breaking configurations are compared.
The paper provides new methods for ice loads due to ridges acting on wind turbine structures not currently covered by existing methods.
During July 2018, an expedition was carried out offshore northern Newfoundland to central Labrador to profile, track, and forecast the drift of icebergs. One of the central goals of the drift modelling work was to test potential improvements in iceberg drift forecast accuracy up to 24 hours when measured iceberg profiles are used as opposed to estimated iceberg draft and mass. During the expedition, 14 icebergs were profiled using a rapid iceberg profiling system which uses a multibeam for the underwater portion of the iceberg and a LiDAR for the freeboard. The 14 icebergs were tracked on the vessel marine radar, and their drift was forecast using a physical model which time integrates the momentum balance of the forces acting on the iceberg. The iceberg profiles were three-dimensional point clouds which provided a highly accurate representation of the iceberg dimensions and shape, and from which a volume and mass could be readily calculated. The point cloud was projected into a two-dimensional plane from 16 perspective angles and averaged into a single projection of iceberg keel and freeboard against which the currents and winds were forced in the drift model, respectively. Average results for the forecast iceberg position versus observed at 24 hours show approximately a nearly 3 km or 18% improvement when iceberg profiles are incorporated into the drift model as opposed to using estimated iceberg draft, shape, and mass. The drift model will become part of an integrated ice profiling, forecasting, and management system for oil and gas exploration and drilling operations on the Grand Banks offshore Newfoundland.
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.
Up to present, the annual iceberg contact frequency for short subsea flowline systems designed for offshore Newfoundland and Labrador has been less than the target reliability level. For longer flowlines, iceberg contact rates will be higher and the consequence of such contacts must be considered. It is possible, for example, that the pipe gets pushed into the seabed with acceptable damage to the pipe and/or localized ice failure takes place. If it can be demonstrated that a pipe could survive some impacts, it might be possible to avoid costly protection strategies such as trenching or rock berms. This paper describes physical tests conducted as part of a preliminary investigation to assess the consequence of a free-floating iceberg interacting with a flowline placed on the seafloor. Two scenarios were considered in this testing program. The first focused on understanding the local iceberg failure processes and the second evaluated the transverse flowline motion when a free-floating keel snags a flexible pipe laid on the seabed.
Offshore Newfoundland and Labrador, Canada, development costs associated with iceberg protection pose significant challenges in terms of project execution and economics for marginal field subsea tie-backs. The current standard practice is to assume that if an iceberg makes contact with a subsea flowline, the flowline is dragged indefinitely imparting significant load to the connections at each end. To isolate flowlines from downstream and upstream assets, weak links are installed in the flowline that are designed to separate once a specified level of tension is reached. This prevents damage to wellheads and other subsea equipment and eliminates the possibility of uncontrolled hydrocarbon release. However, the weak links are very costly and possess inherent risk of failure, which can lead to an uncontrolled release of hydrocarbons. This paper addresses the requirement of weak links by analyzing the flowline tensions transmitted due to iceberg-flowline-soil interaction events.
The assumption that an iceberg drags a flowline indefinetly imparting significant tension on the end connections can be challenged. This paper seeks to estimate the tension loads developed in an untrenched flexible flowline due to interaction with free-floating and gouging icebergs. Large deformation finite element analysis is utilized to simulate the iceberg-flowline-soil interaction scenario. The iceberg keel is idealized with shape and dimensions based on analysis of recent iceberg profiles. A sensitivity study is conducted to assess the influence of keel size, gouge width and depth on flowline tension developed throughout the flowline resting on very dense sand. The sand constitutive behavior is modelled using a user subroutine accounting for the effects of mean effective stress and relative density on the soil strength and volumetric response.
The ice-flowline-soil interaction mechanisms are detailed for free-floating and gouging interaction events. During interaction with free-floating icebergs, the flowline is typically depressed into the seabed, and the keel rides over the flowline. The gouging interaction scenario simulates the complex interaction between the frontal soil mound developed during the gouging process and the untrenched flowline.
This paper provides new insight into the iceberg-flowline-soil interaction scenario that has not been examined previously. Based on the analysis results presented, an alternative strategy to mitigate tension transfer to downstream and upstream assets is discussed.
In prior work to define an improved hydrodynamic approach to flutter calculations, Centrale Nantes, Bureau Veritas Marine & Offshore and Farr Yacht design investigated the possibility of defining a linearized unsteady hydrodynamic model using a fluid response database coming from a series of 2D unsteady RANSE computations. The approach is compared to the Theodorsen theory. The linearized hydrodynamic model was used in a strip theory model for frequency domain flutter analysis. In this latest work, the IMOCA 2006 keel which has been used previously in frequential domain flutter calculation is also analyzed using an alternative and more accurate solution, featuring a fully coupled FSI modal approach with CFD.
As described in the literature, the results present a large impact of the unsteadiness on the phase and module for both lift and moment with a fairly good match compared to Theodorsen theory. The implementation of the results on a frequency domain flutter analysis tool reduces the critical speed for the studied model. Thus the results are closer to the 3D modal CFD approach which gave an lower critical speed.
This paper aims to numerically simulate the loading process when a moored ship is intruded by an ice ridge. Ice force caused by ice keel is calculated based on suggestions from ISO while the ice force due to consolidated layer is taken as level ice and simulated with circumferential crack method. The equation of motion is solved at each time step. A case study is given to show main features during the moored ship and ice ridge interaction. The result shows that the present numerical simulation is promising to be used in the design for moored structures in ice ridge.
In the Arctic, there exist many different types of features such as pure level ice, brash ice, ice rubble and ridges, ridge fields and icebergs, all with different structural and mechanical properties and behavior. For ships and offshore structures, first year ice ridge is a key consideration due to the extreme ice loads acting on the structures. It is crucial to determine the design load levels for offshore structures in ice-infested waters, can also bring a threat to shipping and navigation activities.
Typically, an ice ridge is formed when ice sheets are compressed against each other due to environmental factors, such as wind, current in the sea, thermal expansion etc. From geometry aspect of ice ridge, it is composed of three parts: sail, consolidated layer and keel. The above water part, called the sail, has pores filled with air and snow. The underwater part, called the keel, has pores filled with water and air pockets can exist. The ridge keel is further separated into an upper refrozen layer called the consolidated layer and a lower unconsolidated part. The consolidated layer grows through the ridge lifetime as a function of the surrounding meteorological and oceanographic conditions, air and water temperature, snow depth and the velocity of the wind, and surrounding currents are of principal importance. There was a wide variation in the shapes of the first-year sea ice ridges (Timco & Burden, 1997).
By developing general constitutive laws for ice ridge, Heinonen (2004) and Serré (2011) used finite element software to simulate the ice ridge load. At present, moored ships are often used to oil exploration and exploitation in ice-infested waters. For example, starting in the mid-1970s to the late 1980s, Dome Petroleum deployed floating drill-ships named Canmar during the summer months. In some water, the ice ridge action should be taken into consideration. A sketch of the moored ship in ice ridge is shown in Figure 1.
For local ice loads, and based on numerical simulation, this paper presents a method for calculating fatigue damage to a ship's structure as it navigates in ridged ice fields. A semi-empirical method is introduced to develop the numerical model of ship-ice interaction in level ice and consolidated layer in ice ridges. Rankine's plasticity model is applied to calculate ice loads in ridge keels. A Weibull model is useful to describe ice load peaks. The structural fatigue stress is found using structural beam theory. According to the ice thickness distribution and a proper S-N curve, fatigue damage can be estimated based on the Palmgren-Miner rule. An example of fatigue damage calculation is presented. The calculated fatigue damage in ridged ice is much greater than that in level ice because of the ice ridge effects.
Fatigue damage can be important for ships operating in the harsh environment of ice-covered waters. Damage can entail oil leakage or even catastrophic failure, threatening overall structural safety. Nevertheless, research into fatigue damage caused by ice action has not been developed well compared with wave action. To date, most studies of fatigue damage caused by ice-induced loads have been conducted using field measurements. Zhang and Bridges (2011) introduced deterministic fatigue assessment using the Ship Right FDA ICE Procedure, as proposed by Lloyd's Register, to assess fatigue damage to a ship's structure induced by ice loads. Suyuthi et al. (2013) derived closed form expressions of fatigue damage for several statistical models of stress amplitude. However, the field measurements are usually quite limited and incomplete. For that reason, it is difficult to evaluate fatigue damage correctly and to provide guidance for the design of new structural components or new ship routes. Compared to field measurements, the ice conditions and ship hull can be easily varied in a numerical simulation, which is useful to complement the lack of ice load data in some regions, or to predict the fatigue life for new structures when only ice condition data are needed. The numerical method seems promising to evaluate fatigue damage.
In order to create capability for analyzing course instabilities of sailing yachts in waves, the authors have set up a mathematical model comprised of two major components. The first is an aerodynamic model focused on the calculation of the forces on the sails, taking into account the variation of their shape under wind flow. As the sails are very thin, their shape is adapted according to the locally developing pressures. Thus, the flying shape of a sail in real sailing conditions differs from its design shape and it should be considered as initially unknown. The authors have tackled the fluid-structure interaction problem of the sails using a 3D approach, where the aerodynamic component of the model involves the application of the steady form of the lifting surface theory, in order to obtain the force and moment coefficients. The deformed shape of each sail is obtained using a shell finite element formulation. The other component of the presented mathematical model refers to the hydrodynamic part and it is focused on handling the motions of the hull, with her appendages, in water. The hydrodynamic part is comprised of sub-models for hull reaction, hydrostatic and wave forces. A potential flow boundary element method is applied for the calculation of the side forces and added masses of the hull and the appendages. The calculated side forces are then incorporated into an approximate scheme for identifying the hull reaction terms. The wave excitation involves the calculation of Froude - Krylov forces while radiation terms are found using a strip theory formulation.
In northern regions, ice forces, or actions, must be considered in the design of structures such as light piers, bridge piers, and offshore platforms. Estimates of ice forces in Canadian waters are usually obtained by consulting design standards such as those developed by the International Organization for Standardization (ISO) and the Canadian Standards Association (CSA). These design standards draw on available analytical formulae. Field measurements are available from several sources that suggest reasonable agreement with analytical results for simple cases involving wide structures.
One of the remaining uncertainties in estimating design loads, however, is the contribution of force imposed below the waterline due to unconsolidated keels of ice ridges. Only cursory guidance is provided by the standards associations and their analytical design equations. Close inspection of those formulae show that force estimates can become excessive in situations where the expected keel depth is great compared to the designed structure width. Such scenarios would be expected in offshore oil and gas operations where drilling risers, jack-up legs, and even jacket structures may be exposed to ice ridges.
The present work examines available approaches for evaluating ridge keel forces, including passive pressure calculations. The processes of ice rubble failure and the corresponding stress distributions are considered in the context of classical soil mechanics applied in geotechnical engineering. Design standards are also used to calculate ice forces for a range of ridge keel properties, keel geometries, and structure design widths. Field measurements from the Norströmsgrund lighthouse and the offshore Molikpag caisson are then examined and compared to the forces obtained using these approaches.
The authors conclude that the shape factor adopted in ISO 19906 plays an important role in calculations considering narrow structures and deep keels. It is also shown that the sensitivity of ridge keel load calculation to geometric factors varies considerably with structure width. Furthermore, an absence of real world data from ridge keel interactions with very narrow structures precludes validation of present models in these situations and should be the focus of data collection and model refinement.