Morten Bjerkås

DIMB Consult AS

This paper presents some of the experience with Swedish lighthouses constructed in the time period 1930-1974 with regards to design ice loads. Sweden erected altogether 59 caisson-type lighthouses in the time period 1930 to 1992. Most of the structures have sustained well the annual ice drift whereas few of the structures have been displaced and/or...

This paper provides an overview of ice actions on Norströmsgrund lighthouse within the LOLEIF/STRICE projects in 2000 to 2003. Ice actions are quantified through weekly peak panel loads and weekly load intensities. Environmental conditions are quantified by an estimated level-ice thickness together with wind speed and wind directions. Annual ice th...

In the FATICE project we have addressed the fatigue damage on fixed offshore structures exposed to drifting ice. This is an important challenge in the development of energy production from offshore wind in the Baltic and involves at least five element: a) define ice statistics, b) predict the structural response (ice-structure interaction simulatio...

In ice infested regions drift ice can cause serious damage to offshore structures and ships. The origin of the drifting ice is the winds and the currents. However, it is known that the Gulf of Bothnia does not have strong currents; therefore, it is the wind conditions and wind force that drive large ice sheets. In this study, we analyze the met-oce...

Fatigue caused by ice-induced vibrations (IIV) is an important design consideration for offshore structures exposed to drifting sea ice. The occurrence of IIV is promoted by, but not limited to, certain combinations of ice thickness and ice drift speed, which makes them fundamental input parameters for structure fatigue life estimation. To that end...

A method is proposed to estimate when harsh ice induced vibrations could be expected at an offshore site in an ice-infested region. The method aims to come up with specific estimates based on input parameters that are easily accessible. Therefore, the proposed method uses measured air temperatures, wind speeds, wind directions and ice concentration...

A method is proposed to estimate when harsh ice induced vibrations could be expected at an offshore site in an ice-infested region. The method aims to come up with specific estimates based on input parameters that are easily accessible. Therefore, the proposed method uses measured air temperatures, wind speeds, wind directions and ice concentration...

The signature and occurrence of frequency lock-in (FLI) vibrations of full-scale offshore structures are not well understood. Although several structures have experienced FLI, limited amounts of time histories of the responses alongside measured met-ocean data are available in the literature. This paper presents an analysis of 61 measured events of...

The sea ice interaction with a structure may cause system changes and affect the feasibility of common approaches to track modal parameters. In this paper, a covariance-driven stochastic subspace identification method is used to identify the natural frequencies from 190 time series of ice actions against a lighthouse structure. The results are sort...

Based on the incentive from the Ministry of Transport and Communications in Norway of replacing ferry crossings with permanent fixed crossings, there is a need for new crossing technology for the deepest and widest fjords. The present paper presents a concept consisting of a floating pontoon bridge with a submerged floating tunnel at mid-span, the...

A method is proposed to estimate the number of vibration cycles caused by frequency locked-in ice loads on a bottom fixed structure. Studies of the onset of frequency lock-in have been performed and based on that, different onset criteria are studied. It is shown in this work that most existing onset criteria significantly overestimate the vulnerab...

A full scale event with frequency locked-in ice loads is presented in this work. Data collected at the Norströmsgrund lighthouse in Gulf of Bothnia during the STRICE project consists of several events with lock-in like ice loads. The 180 seconds long event presented in this work occurred 30 March 2003 and was one of the harshest during the entire m...

The present paper describes a novel concept of a self-installing offshore wind turbine. A concept for combined installation of the substructure and turbine in one single operation without the need of expensive installation vessels is described. The stability of the concept during transport and installation is obtained by two structurally connected...

Previous research has shown that the force between ice and a structure is very far from uniformly distributed in space and highly variable in time. A detailed reexamination of field measurements on nine load panels on the Nörstromsgrund lighthouse in the northern Baltic displays the same non-uniformity. Before ice-induced vibration begins, the forc...

A new mooring concept applicable for floating bridges is presented. An idea adopted from the tension leg platform concept is used. The basic idea is to obtain side anchoring of long floating bridges at crossing sites with deep water (> 500 m). The concept is constructed with two separated pipelines, installed across the fjord. An artificial seabed...

Norströmsgrund lighthouse full-scale data from the 2002/03 winter show that ice-induced vibrations of a vertical-sided offshore structure are occurring more frequently from mid-March on, rather than earlier in the winter. Meteorological data are used to calculate the ice growth and it is found that the occurrence of ice-induced vibrations may corre...

Steel jacket support structures for offshore wind have an increasing popularity for water depths larger than 20 m. Traditionally, support structures such as monopiles and gravity based foundation have dominated at shallow water. Offshore wind industry deals gradually with larger water depths, and light weight foundations as steel jackets are believ...

Norströmsgrund lighthouse full-scale data from the winter 2002/2003 shows that ice induced vibrations of a vertical sided offshore structure is occurring more frequently from mid-March and later, than earlier in the winter. Metrological data is used to calculate the ice growth and it is found that the occurrence of ice induced vibrations may corres...

A new way of simulating ice actions onto structures with use of finite element (FE) method has been applied in recent years. The method involves an extension of the conventional finite element method to account for fracture with use of cohesive elements and has already been applied to several other ice action simulations. This paper describes a ref...

A case study on the lighthouse Norströmsgrund in the Gulf of Bothnia has been performed. Design ice load for the lighthouse is compared to existing code recommendations. It was found that the lighthouse was designed for a load level 110 % higher than what is proposed by the recently issued ISO/DIS 19906 design code. By the fact that the structure h...

Pipe bend pressure-moment capacity curves have been derived based on full scale pipe bend tests and numerical simulations. The present capacity curves show that pipe bends on Ormen Lange spoolpieces are, according to three different criteria, well within capacity limits, while design codes indicated that utilization of the bends was exceeded. FE mo...

Ice actions to the Norströmsgrund lighthouse are simulated by means of the computational cohesive element model. The numerical model is developed in the framework of finite elements. Fracture of the ice sheet is accounted for by the cohesive elements placed at internal finite element mesh boundaries in order to track traction versus separation. One...

Ice actions to the Norströmsgrund lighthouse are simulated by means of the computational cohesive element model. The numerical model is developed in the framework of finite elements. Fracture of the ice sheet is accounted for by the cohesive elements placed at internal finite element mesh boundaries in order to track traction versus separation. One...

This paper gives an introduction to how Continuous Wavelet Transform (CWT) can be applied in analyzing ice load time series to gain more information about the distribution of power on different frequencies in the time domain. The aim of the study was to localize short intervals of intermittent crushing in initially long time series. Intermittent cr...

This paper gives a review of selected public available full scale measured ice forces to fixed structures. Altogether 31 data sources are collected where the highest ice load, a current ice thickness and a structural width is reported. The main emphasis of this study is to formulate an engineering tool to predict upper bound ice forces to structure...

Ice actions on offshore structures are one of the main concerns for engineering activities in cold areas with ice-infested waters. This thesis deals with three aspects of ice actions, namely design ice loads from level ice, dynamic ice actions of resonant character, and the actions caused by ridged ice. Ice pressures on full-scale structures have b...

A wavelet cross-correlation analysis was applied to ice loads and corresponding acceleration signals measured at the Norströmsgrund lighthouse in the Gulf of Bothnia, Sweden. It was shown that the excitations of the lighthouse by actions from ice floes are influenced by structural responses with dominant excitation frequencies in the same range as...

This paper presents an interpretation of full-scale data from the lighthouse Norströmsgrund in the Gulf of Bothnia. From analyses of load and response patterns, periods with ice crushing were decided to be either intermittent or continuously crushing. Details of the initialization of intermittent crushing were studied and two different types of tri...

A medium scale test program of ice - structure interaction has been conducted on Svalbard, Norway. Ice sheets were naturally grown from sea water and pushed against a circular structure with a diameter of 0.665 m. Four experiments were conducted, two in 2003 (test 1 and 2) and two in 2004 (test 3 and 4). In 2004, the ice sheets were 0.10 m and 0.16...

This paper addresses the prediction of global design ice loads on offshore structures related to the structural width. Fourteen events from full-scale measurements on the lighthouse Norströmsgrund are analysed to find out how the effective width should be taken into account for estimation of ice forces. Simultaneous failure is found during ductile...

Part II of this paper addresses the ice loads from interactions between first-year ice ridges and a lighthouse in brackish water. Altogether 33 relatively small ice ridges that interacted with the Norströmsgrund Lighthouse were selected and studied in detail. The most significant loads from ice ridges seem to be concentrated in the upper part of th...

This paper addresses the interaction between first-year ice ridges and an instrumented lighthouse in brackish water. Altogether 33 relatively small ice ridges were selected and studied in detail. From 12 of the ice ridges, a thickness of the consolidated layer was assumed. The deepest keel found is 3.34 m and the highest sail is 1.65 m. From 12 of...

Ice has been sampled from first-year sea ice ridges and level ice both in the Van Mijen fjord in Svalbard and in the North-western Barents Sea. In-situ drop ball test and un-confined compression tests in the laboratory were conducted on horizontal and vertical samples of ice from the consolidated layer, the rubble and the level ice. The salinity, d...

This paper describes a cylindrical structure, Norströmsgrund in a marine environment installed with 9 load measuring panels. The load measuring system covers 162 of the perimeter of the cylinder. With uni-directional ice drift against the structure about 180 of the structure perimeter may be affected by ice. Thus the estimated global load will depe...

Field and laboratory experiments on cylindrical specimens oriented in the plane of the ice cover and perpendicular to it are reported as well as spatial distribution of temperatures and salin-ity in the ice. Nominal strain rate was constant at 10 −3 s −1 for all the tests, both load and displacements were monitored continuously. The strength of ver...

The paper is a review of the scaling problem related to ice loads on offshore structures. Three different scaling laws are derived, one from the strength of materials, one from the theories of linear elastic fracture mechanics and one from a quasibrittle material approach. Then a theory that claims no size effect but a drift speed effect is present...