Under Task 2 of the LOLEIF project the research work in the field and laboratories as well as the evaluation and analysis of obtained data and results was clustered and dived into the following 4 Subtasks:

• 2.1 - Level Ice Properties,
• 2.2 - Ridge Mechanical Properties,
• 2.3 - Ice Force Measurements at Lighthouse,
• 2.4 - Determination and Transfer of Ice Force Affecting Parameters.

which are described more detailed below.
 

Sub-task leader:	LUT
Other partners:	HSVA, LGGE

Objective

Description

• ice morphology (ice type, thickness, cracks, floe size, inhomogeneity),
• physical ice properties (internal structure, brine, air bubbles, sediment, pollution),
• mechanical properties (ultimate strength, elastic modulus, viscosity),
• fracture mechanical parameters (fracture energy, propagation rate).

Field test program

The properties of a naturally grown material such as sea ice are usually more truly described by field measurements and in situ tests than in laboratory investigations. Changes in the ice properties due to transportation and storage of samples are avoided and larger test volumes can be studied. Ice morphology in a large area around a lighthouse were studied for two years when the ice has reached its maximum extent. Ice thickness was measured manually and with an impulse radar transported by a snowmobile. The overall ice situation was mapped from a helicopter. A new field instrument for measuring fracture mechanical parameters of ice has been developed. Fracture toughness of level ice with different crack orientation were investigated with this method. Pressure meter tests in bore holes were also conducted in order to compare the strength variations at different locations. Large ice beams have been cut from the ice sheet and tested floating by the method of 4-point bending. From these tests flexural strength is obtained when fracture starts at the ice surface or the ice bottom depending on the direction of bending. Based on experiences from freshwater ice. These values were expected to be substantially lower than the tensile strength indicating stress concentrations around already existing small cracks at the upper portion of the ice cover.
 

Ice testing in the laboratory

Large ice cores were drilled out from the ice around the lighthouse. Special efforts have been made to minimise and document the influence of transportation, storage time and storage temperature. The sampled ice was well insulated and rapidly transported to ice laboratories in Europe by air. Ice strength tests generally followed international standards as given by the Ice Committee of the  International Association of Hydraulic Research. The sampled ice was classified and the contents of salt and contamination was analysed. Internal structure were described by crystal size, shape and orientation of the crystal axis.

Fracture energy and other important data on the brittle behaviour of sea ice was investigated separately by the team of LGGE. Linear fracture mechanics is one important tool to create ice load models. Fracture toughness tests were carried out on ice sampled around the measurement site. The ice was transported by air to the ice mechanics laboratory of HSVA where a sophisticated fracture toughness testing apparatus is available. The tests were executed by researchers from LGGE.
 

Deliverables / Interdependence and links with other tasks

The level ice properties from field and laboratory measurements as listed above will be distributed to all partners by the data management service (Task 3) as well as to the numerical modellers of Task 4.
 

Sub-task leader:	HUT
Other partners:	VTT, LUT, NTNU

Objectives

• ice block size,
• thickness of consolidated ice layer,
• bending, compressive and shearing strength of the consolidated layer.

Description

The present ridge keel load design is based on soil mechanics limit load analysis. It assumes simple simultaneous failure planes inside the keel and balances internal friction and cohesion shear forces with the total ridge load. In reality, failure is progressive along 3-dimensional failure surfaces which results in smaller loads. The ice rubble friction and cohesion is known through scale model tests only. In other instances ice properties exhibit strong size effects. 

Another feature in a pressure ridge is the consolidated layer. After the ridge formation loose floes start to adfreeze to each others at layers above and close to the waterline. Depending on the ambient temperatures also water filled cavities freeze. Thus, as time passes, an adfrozen consolidated layer forms. It's thickness depends on the cumulative freezing index. As it is formed of different floes at different directions and having cavities, it's strength is lower than this of the coherent parent level ice. Ridge structure interaction involves different mechanisms, consolidated layer crushing, bending and shearing, keel shearing failure inside the keel and/or along the bottom of the consolidated layer. However, nobody has measured the global strength of the ridge consolidated layer. Now novel or modified test set-ups are described by which global mechanical properties of pressure ridges are measured.

Test procedures were planned and the needed hardware was purchased during the year before actual testing.
 

Internal structure of the ridges 

A few ice ridges are selected for the study and their geometrical characteristics were recorded by conventional methods. The thickness of the consolidated layer was measured and compared with a subsequent model prediction based on the cumulative freezing index. The internal structure of the consolidated layers of the ridges was studied by using a large-diameter core drill and a pressure meter. In addition a novel technique, scanning radar tomography, was used in order to get information on the texture of porosity in the keel as well as the amount, size and shape of the macro-scale defects which exist inside the consolidated layer. The results of two separate techniques were checked against each other and then used to evaluate various internal strength parameters of the ridge in medium and in large scale. The capabilities and accuracy of scanning radar tomography has been earlier tested in geotechnical surveys. An extension to study the internal structure of pressure ridges would mean a major breakthrough in getting accurate data on ridge morphology. These preliminary tests aimed on achieving information on the internal porosity as well as the amount and size of the macro-scale defects inside the consolidated layer. Two separate techniques were used to evaluate various internal strength parameters of the ridge on medium and large scales.
 

Ridge keel friction and cohesion

The keel shape was measured by core drilling and by sonar. The sail of the ridge was smoothened. The parent ice sheet and consolidated layer has been cut by chain saws into loose floes. Keel bottom displacement transducers were installed as well. An instrument fitted plate, a wedge with small angle from horizon, has been manufactured and installed on the bow of a ballasted barge. Both horizontal and vertical loads from the plate to the barge were measured by load cells and strain gauges. The barge was pushed slowly by an icebreaker so that the wedge evenly presses downwards the now completely loose keel. The loads on the loading plate and deformations of the keel bottom and top surface was measured simultaneously. The top view was also recorded by a time lap video. Force displacement data were used to calculate the internal friction and cohesion in the keel. Such tests have been repeated for different pre-cut ridges. For comparison, similar tests were conducted for virgin ridges which include the failure loads of consolidated layer together with keel loads.

The uncertainties of measurements were small. The maximum size of test ridges was estimated to give good margin for capacity of the pushing and measurement system. Barges and ice breaker pushing has been used earlier in ice testing. The ridges were selected according to the parent sheet ice thickness. Both horizontal and vertical loads, and the shape of the ridge have been measured with good accuracy. The uncertainty in this method is how well the shape of the failure surface can be captured. The top shape recordings were straightforward, the bottom ones required a grid of keel bottom displacement transducers.
 

Consolidated layer friction and cohesion to underlying keel

A section of the consolidated layer was cut free from edges. and it's shape, thickness, porosity, and weight have been measured. It was pushed by the edge of the instrumented plate to slide horizontally above the keel. The friction and cohesion is then likely different from those inside the keel. This information is needed as the ridge failure surface may proceed along the bottom of the consolidated layer.

The uncertainty of these measurements depends on how well the thickness and porosity of the consolidated layer can be measured. It is also be necessary to determine at which level the shearing occurred during the test. Adequate accuracy has been achieved by core drilling and dismantling the consolidated layer after the measurement. 
 

Consolidated layer compressive strength

The consolidated layer includes cavities and cracks that influence the global strength of this part of a ridge. This effect has been studied earlier both in laboratory and in medium-scale field conditions. The existing data were analysed to evaluate the integrated strength of the consolidated layer.
 

Consolidated layer bending and shearing strength

Beam like rectangular cross sections were cut from the consolidated layer and tested in bending. The dimensions of these cross sections have to be at least two times larger than the parent ice sheet thickness. With a short cantilever beam it is possible to have shearing failure before bending failure. Hence beams of different length were tested.

The uncertainties in these measurement come from errors measuring consolidated layer thickness and buoyancy load distribution. After beam has broken it's buoyancy level changes are related to the buoyancy loads. Upwards bending is easier because it does not require to deform the ridge keel. This is the natural direction for an upward breaking cone but opposite to that of a downward breaking cone or ship bow. However, it is expected to get more reliable data than the non-existent data presently.
 

Consolidated layer evolution

Fresh ridges were monitored and while ageing, their consolidated layer thickness was measured together with environmental data. This was repeated with ridges of different parent ice sheet thickness and different ambient temperature histories. Radar tomography as explained above was applied as well. The internal structure of the ridges was used to investigate the extent of consolidation and to locate partly consolidated areas as well. The data ere used to refine and validate the thermodynamic numerical model of NTNU for consolidated layer evolution.
 

Deliverables

Raw and analysed data on mechanical properties of ridge material were transmitted by  to the partners who required the information for their numerical modelling work (HUT, LUT, NTNU and VTT). The analysed data were compiled within the final report and the final data collection.
 

Interdependence and links with other tasks

HUT was responsible for planning the tests, conducting the field measurements and the data analysis. The field measurements have been carried out in close co-operation with NTNU and VTT. LUT gathered data on ice physical properties of level ice at the measurement close to the lighthouse where the ice force measurements in Subtask 2.3 were conducted. HUT organised the field logistics and hardware. All partners involved have a long lasting experience in field research operations in northern and arctic regions including utilising of icebreakers to exert loads on ice and structures.

The results of these investigations were needed for Subtask 4.3 to develop and validate theoretical numerical models for simulating ridge - structure interactions and to predict resulting ice loads from ridges.

Objectives

Description

During the summer 1998 a steel shield has been mounted to the 5.8 m diameter vertical shaft of the lighthouse to which 10 ice force panels were mounted later on. Eight panels were mounted in the upper level, side by side half around the south-western surface of the lighthouse. They face in the direction of the predominant ice movement. The panels, each having a load surface of 200 cm (length) by 80 cm (width), work according to the electrical strain gauge load cell principle. A description of the conceptual panel design is given below.

In addition to the ice force measurements information was obtained on ice drift, ice velocity, ice thickness, ridge ice profile and the overall ice situation. The winter season 1997/1998 was used to install one pilot ice force measuring panel at the lighthouse in order to secure it's and the entire measuring systems' quality and reliability. The main ice force measurement campaign was conducted in the winter and spring seasons of 1999 and 2000.

The sub-task leader (HSVA) has formerly used tri-axial load cells in ice impact studies on icebreaker hulls several times over the last 12 years. They were made available free of charge for the full scale measurements and have been used as measuring device for the panels. These load cells allow the determination of friction forces. The load cells or measuring bolts provided the fixed support of the load surface against the base plate of the panel. The inside of the panel is sealed by a special rubber sealing profile in order to prevent water to penetrate into the panel interior, i.e. to prevent the formation of ice inside the panel which would cause errors in force measurements. 

The loads of fragmented ice blocks of pressure ridges has been determined in the lower row of panel arrangement. These forces of the unconsolidated part of ridges were supposed to be small. 
 

Static and dynamic calibration tests

Each single load measuring bolt and load cell as well as the complete ice force panels has been checked in a comprehensive testing program which HSVA in co-operation with LUT has carried out in the laboratory. The load has been applied in several steps up to the design load in normal direction at various locations on the front plate (e.g. close to the edges in order to prove that the response of the panel is independent of the point of load application). 
 

Data acquisition and evaluation

The signals originating from the ice sensing panels were processed by means of DC - Amplifiers which was supplied to the analog-digital converter of the data collection system (DAS). The essential part of the DAS is a personal computer which included several analog-digital converters to handle up to 128 measuring channels. Connected to the PC is a protocol printer and a digital audio tape drive for data storage. A radio link was installed to transmit selected measured data to the base control station in order to get rough information about the ice loads on the lighthouse.
 

Data Acquisition Software

The used data acquisition software was able to reduce the measuring data online (Dynamic Data Reduction) at a digitised frequency of at least 50 Hz per channel and stores the values in binary format on one of the hard discs.

In addition, statistical data of the measured channels such as minimum, maximum and mean values as well as the standard deviation were printed out immediately after a measurement cycle was finished in order to prove the correct operation of the measuring system. 
 

Data Evaluation

Data evaluation was done on personal computers and workstations which are connected in a local area network. Several data evaluation programs were used to compute the raw data and to present the results in tables and graphs on laser printers.
 

Deliverables

Raw and analysed data of ice forces of various ice conditions against the 10 load measuring panels were distributed to all partners in the project. More condensed data results were be provided to the members of the Advisory Committee and by the Interim and Final Reports.
 

Interdependence and links with other tasks

HSVA co-operates with all partners of the project since the ice force measurements are the central research topic, the information of which is required by all other partners for their tasks. Special co-operation was organised with LUT who was directly involved in the full scale measurements as well.
 

Sub-task leader:	HSVA
Other partners:	HUT, LUT

 

Objectives

Description

Determination of ice thickness and profiling of surface and subsurface geometry of ice formations

Another key parameter influencing the load on a marine structure is the thickness of the ice cover. Therefore a short range underwater sonar system consisting of a PC based sonar processor and an electronic scanning sonar heads was installed. This system was able not only to determine the thickness of a level ice cover but also the underwater profile of ice formations like ridges and rubble fields passing the structure. The surface profile of the passing level ice cover and other ice features were determined by using a distance laser which is fixed stationary at the lighthouse. 
 

Ice drift velocity

The ice drift velocity (i.e. speed and direction) is one of the main parameters affecting the total load on a marine structure and thus was recorded simultaneously with the ice forces. Laser-Doppler-Systems were used to measure the ice drift velocity. As these systems pick up the speed in the radial direction only two speedometer were installed to determine the ice drift's speed and direction correctly.
 

Meteorological and hydrological data

The parameters described above depending mainly on meteorological and hydrological data such as air and water temperature, wind speed and direction and the current speed and direction. A meteorological station and a current meter were installed and the measured data have been logged. 
 

Documentation of the overall ice scenario

A video camera system consisting of three video cameras and long play video recorders was installed to document the overall ice scenario in the nearby area of the lighthouse (also in periods when there is no occupation by a measuring team). This system was controlled via a bi-directional radio link operated from LUT. Further, the overall ice situation was mapped from helicopters and for larger area of the Baltic Sea from satellite images. 
 

Radio link to shore base

The transfer of video images as well as compressed data from the lighthouse to the shore base was important to support decisions on mobilisation of the field campaigns and to have a remote control for the data collection system if weather conditions prevent measurement teams from entering the lighthouse. For this purpose a video radio link system with bi-directional telemetry was purchased and installed. The modular universal deployable system consists of a field station and a control station. The system components of the field station are a video camera (colour) with wide / zoom optics, watertight and heated housing, power supply plus a sensor module for environmental parameter (wind speed, salinity, CTD etc.). The control station was installed on the shore base at  LUT. The components of the control station are a colour video monitor, a video recorder plus a PC based control module.
 

Co-operation

The beginning and duration of each measuring campaign depended on the ice conditions. Actual lighthouse occupation for six weeks during one winter / spring period appeared to be a reasonable estimate. The ice property measuring teams of HUT, LUT, NTNU and VTT started working as soon as the ice conditions on site allowed safe work on the ice. This sub-task required intensive co-operation with partners involved in the field measurements in preparation and execution of the measurements as well as for data transfer.
 

Data distribution

Raw and analysed data were transmitted regularly to the other partners.
 

Deliverables and dissemination

The ice force effecting parameters such as ice thickness, ridge profile, ice drift velocity, meteorological data as well as video pictures on the ice - structure interaction and the overall ice scenario were transferred to all other partners as soon as the data have been obtained and analysed. By a more comprehensive way these data were also compiled for the final report.
 

Interdependence and links with other tasks

Installation of measuring devices and the measurements was carried out by HSVA with the assistance of LUT. The latter provided together with HUT  the logistics inclusive facilities to establish the radio link. Linkage with all other partners and Task 4 was done via the data management service (Task 3).