Project co-ordination and management

Comprises all management and co-ordination measures necessary to execute the project including linking with and reporting to the European Commission.

WP leader:
HSVA

Contributors:
All project
partners

End user workshop

Within a workshop of 2 days duration consensus on priorities dedicated to the various tasks in the project were established together with end-users. Furthermore, relevant data and information were exchanged among the project teams.

WP leader:
NTNU

Contributors:
All project
partners

Load Spectra

Objectives:

• To provide an understanding of environmental processes which lead to various kind of ice actions on offshore structures and thereby endanger the ultimate and fatigue capacity of structures;
• to provide input to future ice loading codes.

Sea ice dynamic models are available for ice-infested sea areas but a link between these models and ice load prediction models is missing at present. Such a link is needed in particular while forecasting the fatigue life of a hydrocarbon production platform. Also, existing probabilistic models for the ultimate loading conditions need detailed information on the encounter frequency of different kind of ice features at the site of the structure.

The fatigue-driven loads consist mainly of the higher amplitude loads with probabilities of exceeding in the range of 10% or less. Their frequency of occurrence depends on random combinations of site, wind, current, temperature as well as on structure's geometry and compliance.

A semi-empirical method will be derived for an evaluation of a fatigue load spectrum at a given site of an offshore structure. The method exploits a sea ice dynamic model for a climatic analysis of ice velocity spectra as well as a probabilistic ice load model and simplified equations for the dynamic magnification due to the structure. Data collected on several structures in two semi-enclosed seas - the Baltic Sea and the Bohai Bay will be used for calibration. More calibration data acquired on the lighthouse will become available during this project.

WP leader:
VTT

Contributors:
HSVA,
LUT,
NTNU

Ice - Structure Contact Processes

Objective:

• To provide a physical explanation for the load magnifying effects in ice- structure interaction.

Work description and applied instrumentation:

The ice interaction with a compliant structure is a very complex process, which is not fully understood at present. Furthermore, a single event of ice - structure interaction may involve both ductile and brittle ice failure mechanisms. Earlier studies have considered these two failure modes mostly as separate phenomena. A third unclear problem relates to steady state vibrations induced by ice. It was common belief among that this problem can be solved by installing an upward breaking or downward breaking cone at the waterline of the structure. New full scale data from the Bohai Bay shows convincingly that these solutions are not always sufficient.

A numerical model was derived in the earlier funded EU project LOLEIF to explain the first two phenomena explained above. The model considers the confining interfacial effects due to crushed ice and velocity dependent friction. Laboratory tests will be made to clarify the input parameters of this model. Further basic studies are made on the role of temperature, creep and the propagation of horizontal cracks at the edge of intact ice. Video records and other documentation of full scale tests are used to understand the ice failure processes as a function of velocity and stress state. This is of particular importance for the development of prediction models.

The third problem will be studied by developing a mechanical model for the dynamic interaction between an ice field and a conical structure. The model will be calibrated using old medium-scale data from the Baltic Sea as well as full scale data from the Bohai Bay.

WP leader:
VTT

Contributors:
HSVA,
LUT,
LGGE

Measurement of Ice Forces and
Ice Force Effecting Parameters

Objectives:

• To provide a set of full scale data collected during two winter seasons;
• to deliver input data for calibration of existing ice load prediction models and for better understanding the ice structure interaction process.

Problems tackled and brief description of work:

The measurements were carried out at Lighthouse Norstrømsgrund an existing research infrastructure located in the most northern part of the Baltic Sea about 30 nm south east of Luleå. The lighthouse is outfitted with 9 rigid ice force sensing panels covering 60% of the area interacting with ice. The aim is to determine the total load acting on the structure as well as the load distribution. The measuring system has the capacity to show the nature of non-simultaneous failure during crushing events. One of this 9 panels of each having 1.2 x 1.6 m in size, is divided into 8 segments with a size of 0.5 x 0.4 m. This segmentation provided information on the local pressure and ice pressure distribution on a structure.

Simultaneous information on the dynamic response of the structure was provided by 4 accelerometers and 4 inclinometers installed at two different levels of the lighthouse. For the interpretation of the ice loads the ice thickness was determined by two different and independent methods. One will measured the distance between the sea bottom and the ice cover by use of an echo sounder system. The second - an electromagnetic (EM) induction measurement system - determined the ice thickness from above the ice surface. The EM measurements were carried by a subcontractor of HSVA who has used this method several times in arctic and antarctic regions. The velocity of the drifting ice was determined from video recordings by use of image processing software. Information on extreme ice features like pressure ridges and hummocks were given from a laser scanning device, a submerged echo sounder and different a video observation system.

While the laser scanner provided data of the ice surface the echo sounder delivers data of the subsurface of the ice features. The video observation system consisted of 4 video cameras, a time lapsed and a real time recorder and a control monitor. This system delivered information on the ice situation in the vicinity of the lighthouse as well as on the overall ice situation. Both information are needed for data interpretation.

When the ice winters began (usually in February a minimum of two persons lived on the lighthouse and operated the data acquisition and video observation systems.

WP leader:
HSVA

Organisation
and logistics
on site:
LUT

Participants
in site
investigations:
HSVA,
VTT,
NTNU,
LUT,
HYDROMOD,
CU

plus the
STRICE advisors
R. S. Ferderking
and D. Sodhi

Determination of Ice Mechanical Properties
Part I: Field Tests

Objectives:

• To measure mechanical properties of the moving sea ice in the vicinity of the instrument fitted lighthouse;
• to provide relevant input data for calibration of numerical ice load models.

Work description and applied methods

During the periods when ice force measurements took place, ice was be sampled and tested whenever the ice situation allowed. Ice cores were analysed at the lighthouse to provide in situ temperature, salinity and density profiles of the ice cover. Besides this fundamental work additional ice was sampled and deep frozen for later shipment and testing. The internal structure of the ice was studied on thin and thick sections where crystal size, shape and orientation of the crystal axis was determined. The use of image analyses was utilised and a series of mechanical tests at different size, loading rates and temperatures were carried out on certain ice types.

Furthermore, fracture mechanical tests were performed directly on sea ice at the area. Large blocks of ice were cut from the ice cover and fractured floating on the water. The energy needed to open a crack and the crack propagation speed was measured.

To better understanding the fracture mechanism of ice, small scale fracture toughness test were carried out at the site. For these tests a special designed test frame was used. To fulfil the requirements of such type of tests, wide range of cross head speed and small crack opening displacement values have to be controlled. An older analogue control unit has been replaced by a digital one. New laser sensors with high resolution and a high speed video camera system was applied to study and visualise the fracture behaviour of the ice in the field.

WP leader:
HSVA

Participant:
LUT

Determination of Ice Mechanical Properties
Part II: Laboratory Tests

Objectives:

• To determine mechanical ice properties as input parameters for the data evaluation in WP 5;
• to better understand the ice - structure contact process as input for WP 3;
• and to correlate new data with existing ones as well as with predictions in WP 6.

Work description and applied methods

As noted in WP 3, ice - structure interaction involves both ductile and brittle behaviour of ice, as well as ice crushing. Therefore, a physical understanding of the ice - structure contact processes or a correct evaluation of new field data require determination of mechanical properties of ice taken from the field as well as a series of experiments on artificial ice to develop an experimentally based modelling of the behaviour of damaged ice.

The investigations focus on tensile strength of level ice as well as on mechanical behaviour of damaged ice both by applying laboratory and physical model tests.

WP leader:
LGGE

Participant:
LUT.

Ridge Full-Scale Measurements

Objectives:

• To supplement full scale data on ice ridges;
• to apply new methods for ridge full scale measurements of ice mechanical parameters of ridges;
• to better determine failure processes of ridged ice in particular dedicated to improved understanding of the role of the keels of ice ridges.

Work description and applied methods

A lot of experimental data of ridge mechanical properties were measured in the earlier EU funded project LOLEIF by using a ridge loading rig. A connection between the global strength parameters, cohesion and friction, of the ridge keel was found. This data gives also knowledge about the failure type and the location of the failureplane in rubble. More data with a different type of the internal stress state in the ridge keel are required for definite determination of both cohesive and frictional strength. Extra data are also needed to study accurately the ridge keel internal structure effects (for example effects of the ice block size and shape).

A new insight of the internal structure of the ridge keel consolidated part was provided by aforementioned measurements. Several rafted layers, (up to six of 20 cm thickness each) were found in ridge fields in the Gulf of Bothnia (Finland). New tests are needed to investigate the effects of the rafted layers in the ridge keel.

A new test type was applied to determine the consolidated layer bending strength. Two long parallel leads, locatged near to each other, are cut through the consolidated layer. A beam of the consolidated layer with ice rubble underneath between the leads was then loaded vertically - a bending test with fixed ends.

Another test type was applied by loading the beam like a cantilever, when a lead is cut also to one end of the test beam. Because the failure mode between these test types is different the consolidated layer bending strength both at the bottom and at the top of the layer can be determined. This is the first effort to measure the consolidated layer mechanical properties.

By these tests important data about the consolidated layer contribution to the strength of the keel were gathered. When a punch test is done for the same ridge, the strength of the consolidated layer can be determined definitely.

WP leader:
HUT

Data Processing and Evaluation of Results

The acquired field data will be processed, evaluated and compiled by the different teams which conducted the field and laboratory measurements.

Results will be presented event related, as time series and by other suitable means and will facilitate as input for subsequent evaluation and modelling work.

WP leader:
HSVA

Participants:
LUT
LGGE


NTNU
for data from
WP 4.1,
WP 4.2 and
WP 4.3

HUT
for data from
WP 4.4

Correlation of Existing and New Data with Predictions

The purpose of this work package is to correlate relevant existing data with new data and ice load prediction methods (both pure theoretical and empirical methods).

Emphasis is made to identify ice failure modes with respect to velocity and state of stress. This will provide relevant information for prediction models and the basis for recommendations towards new codes.

WP leader:
NTNU

Participants:
CU
HSVA
HUT
VTT

Recommendations towards Codes

A one day workshop for interested parties to set requirements and guidelines for the European ice load design code will be organised and conducted. Data from the LOLEIF and STRICE projects will be used to evaluate existing codes with reference to the full-scale measurements.

Based on this guidelines for an European ice load design code will be proposed.

WP leader:
HUT

Participants:
HSVA
NTNU
VTT

Data and Information Management

This work package combines the applicable work of management and handling of data and information acquired and produced during the project in compliance with respective guidelines and requirements for RTD projects conducted under FP5 as well as with applicable data management procedures and standards.

Furthermore, the work package accomplishes set-up, operating and updating of the project's web site for the duration of the project.

Finally at the end of the project, all relevant project data and information will be merged and assembled in comprehensive collections.

WP leader:
HYDROMOD

Participants:
HSVA
NTNU

Reporting and Dissemination of Results

All produced information and results will be compiled in a final report which will be the basis for a workshop organised by the co-ordinator for discussing the results with other experts and end-users.

By this workshop and also through the STRICE web page and presentation on international conferences the results of the STRICE project will be disseminated and offered for exploitation.

WP leader:
HSVA

Participants:
CU
HSVA
HUT
HYDROMOD
LGGE
LUT
VTT