Wyniki wyszukiwania - BazTech

1

Loads on an Off-Shore Structure due to an Ice Floe Impact

Staroszczyk R.

Archives of Hydro-Engineering and Environmental Mechanics

|

2007

|

Vol. 54, nr 2

77-94

EN

In the paper, the problem of dynamic impact of a floating ice sheet at an off-shore structure is considered. It is assumed that during an interaction event the dominant mechanism is the brittle fracture of ice at the ice--structure interface, that is, elastic and creep effects in ice are ignored. Since in natural conditions the edge of floating ice is usually irregular, the contact between a floe and an engineering object is imperfect. Thus, at any one time, the failure of ice occurs only in a number of small zones along a structure wall, leading to a highly irregular variation of forces exerted on the structure during the impact process. It is supposed in the analysis that the successive small-scale fracture events at the contact surface occur at random, and all these small-scale events take place independently of each other. An off-shore structure is modelled as a fixed and rigid circular cylinder with vertical walls. For an adopted geometry of the ice sheet, its initial horizontal velocity, and the variety of parameters describing the limit failure stresses in ice, the history of total loads sustained by the structure and the floe velocity variation are illustrated for a typical impact event. Furthermore, probability distributions for maximum impact forces exerted on the structure, depending on the floe size, its thickness and initial velocity, are determined.

2

Loads Exerted on a Cylindrical Structure by Floating Ice Modelled as a Viscous-Plastic Material

Staroszczyk R.

Archives of Hydro-Engineering and Environmental Mechanics

|

2006

|

Vol. 53, nr 2

105-126

EN

In this paper the problem of interaction between a coherent floating ice field and a fixed, rigid, vertically-walled circular cylinder is investigated. The ice cover, of horizontal dimensions significantly larger than the characteristic size of the structure, is assumed to be driven against the cylinder by wind drag forces. The ice is treated as a viscous-plastic material, in which the permissible stress states in the horizontal plane are bound by an elliptic yield curve. By using an associated flow rule, a constitutive law, involving two parameters defining the ice strength in compression and much smaller strength in extension, is derived in order to describe the behaviour of the material. The law predicts distinct responses during yield (occurring at high strain-rates) and during the flow when the yield condition does not apply (at lower strain-rates). The results of numerical calculations performed by a finite difference method illustrate, for chosen ice rheological parameters, the distribution of contact stresses at the ice - structure interface. Two forms of boundary conditions at the cylinder wall, free-slip and no-slip, are considered, and their effects on the horizontal loads sustained by the structure are examined. In addition, the results for the viscous-plastic rheology of ice are compared with those obtained on the assumption of a purely viscous behaviour of ice.

3

Loads exerted by floating ice on a cylindrical structure

Staroszczyk R.

Archives of Hydro-Engineering and Environmental Mechanics

|

2005

|

Vol. 52, nr 1

39-58

EN

The paper is concerned with the problem of interaction between a coherent floating ice cover and a fixed, rigid, vertically-walled circular cylinder. The ice cover, of horizontal dimensions considerably larger than the size of the structure, is assumed to be driven against the structure by wind and water current drag stresses. The floating ice cover is modelled as a plate that is subject to the action of horizontal forces and transverse bending due to the reaction of underlying water. During an interaction event, of a quasi-static character, the ice is modelled as a creeping material the behaviour of which is described by a viscous flow law with two, bulk and shear, viscosities. The viscosities change dramatically in their magnitudes during a transition from converging to diverging deformation of the material to reflect the fact that floating ice offers much less resistance to tensile rather than compressive stresses. By numerical simulations carried out by a finite difference method, the influence of the ice rheological parameters on the distribution of contact stresses at the ice - structure interface is investigated. Two types of boundary conditions at the interface, free-slip and no-slip, are considered, and their effects on the loads sustained by the structure are compared. In addition, creep buckling of the ice sheet near the structure is analysed to determine the critical time at which ice starts to fail due to exceeding its flexural strength at given loading conditions.

4

Creep buckling of wedge-shaped floating ice plate

Staroszczyk R., Hedzielski B.

Engineering Transactions

|

2004

|

Vol. 52, iss.1-2

111-130

EN

The paper is concerned with the problem of creep buckling of a floating ice plate pressin against a rigid, vertical-walled, engineering structure of a finite lenght. The plate is modelled as a truncated wedge of a semi-infinite length and constant thickness, resting on a liquid base and subjected to transverse bending due to the elastic reation of the base and in-plane axial compression due to wind and water drag forces. The ice is treated as a viscous materail, with the viscosity varying with the depth of the ice cover. The results of numerical calculations, carried out by the finite element method, show the evolution of creep buckles in the plate, and also ilustrate the behaviour of the ice cover at different levels of the in-plane axial loading, at different temperatures across the ice, and for different geometries of he wedge-shaped plate.