ABSTRACT

Motions of an ice mass prior to impact with a large, fixed offshore structure are modelled by a numerical method. Flow regimes and impact phases are defined in terms of geometric parameters. The model allows ice mass motions and impact energy to he estimated from known conditions. The method consists of a two-body diffraction solution for wave-induced oscillations, and a time-stepping procedure for drift motions. Results from several examples are presented and interpreted in terms of separation distance of the two objects. Limitations and strengths of the model, as well as areas for further developments, are described.

INTRODUCTION

Seasonal occurrence of ice off the East Coast of Canada is a concern to present drilling operations and proposed production systems. Production systems may include fixed, permanent platforms, which must resist all environment loads. Iceberg impact loads influence both the global and local designs of such platforms. Impacts are complex events dependent on many variables. The independent variables are iceberg parameters (e.g., mass, geometry, and strength), structure parameters (similar to iceberg, plus foundation properties), configuration parameters (eccentricities of the impact point) and environmental parameters (currents, waves, winds). From the independent variables, hydrodynamic interactions, arising from the increasingly close proximity of the ice mass and structure, may be derived. Such interactions directly influence load severity. The velocity and added mass of the ice mass, and its trajectory prior to impact may be significantly altered from their far-field values.

Four phases, defined by the separation of the two objects, describe the overall impact event. In the Far-Field Phase, the ice motion is unaffected by the structure. Motion may be determined solely from the ice's mass and geometry, and environmental conditions. When the two objects are close (Near-Field Phase), deviations arise from distortions in the wave, current and wind fields induced by the structure and the close proximity of the structure and the ice. Changes in the hydrodynamic characteristics of the ice mass also occur. The ice mass and the structure are sufficiently close in the Very Near-Field Phase so that the viscous effects are no longer negligible. Boundary layers around both the ice and structure may be distorted. The added mass varies markedly for this phase with proximity to the structure. Physical impact occurs in the Contact Phase. In addition to the factors described above, the load is determined by indentor geometries, material and mechanical properties of the ice mass, structure and foundation.

Work to date on iceberg impact loads has been directed mostly at the far-field and contact phases [4). The chief concern of this paper is the near field phase, in which little has been done [2, 5].

MODELLING ICE MASS MOTION NFAR A S1RUCTURE

A numerical method for describing ice- mass motions in the near-field was developed as part of a more comprehensive study [1]. Wave-induced oscil1atory motions are calculated by linear diffraction theory. The diffraction calculations are dependent on the ice mass proximity to the structure, and must be repeated at points along the drift trajectory.

This content is only available via PDF.
You can access this article if you purchase or spend a download.