Ice Forces Exerted on a Conical Structure in the Gulf of Bothnia

Abstract

ABSTRACT A joint Finnish consortium of industry and state research organizations has designed, constructed and instrumented a conical test structure for full-scale measurements of ice forces against a conical fixed off-shore structure. The structure and its instrumentation are described. The test cone was installed around the existing Kemi-I lighthouse in the Gulf of Bothnia. Field measurements have started. The intention is to measure the action of all typical first-year ice features, including pressure ridges, with an average sheet ice thickness of 0.8 m. Model tests with a scale ratio of 1:15 have been conducted and the results are described. Theoretical models of ice forces against a cone will be calibrated using both the model and full-scale test data. This will provide a reliable basis for extending ice force predictions to arctic conditions, where the ice is about 2 to 3 times as thick as in the Gulf of Bothnia. INTRODUCTION The advantage of a conical shape at the waterline of an off-shore structure is that the failure mode of level ice is flexural, which greatly reduces ice loads compared to crushing against a vertical structure. Both horizontal and vertical ice force components develop and the latter has a stabilizing effect, which is important in deep water applications. The underwater part of the structure can then be narrower, which also results in reduction of loads from pressure ridges. At water depths greater than 30 m the cone is regarded as the most economical choice for a fixed bottom-founded structure. The concept of the cone has been well known for more than 20 years. There are many theoretical models /1, 4, 5/ for predicting ice forces against a cone and model test results as well /3, 5, 6/. However, no full-scale field ice force data are available to evaluate theoretical models. Only tentative results have been published for a cone with a 1.5-m'waterline diameter in the St. Lawrence Seaway /2/. Theoretical models of the ice forces exerted against a cone take account of the formation of radial and circumferential cracks in the oncoming ice sheet, friction between the ice and the structure, the work required to raise broken ice floes up along the cone and for them to be pushed aside. Some models have been evaluated in model tests but due to the large scale ratios, there are many uncertainties in the prediction of ice forces for full-scale arctic off-shore structures. The objectives of this research are to acquire data for evaluating theoretical models of ice forces against a cone. The test cone around Kemi-I lighthouse in the Gulf of Bothnia will provide full-scale measurements of the ice forces against a conical structure with level ice up to 1 m thick and with first-year pressure ridges and rafted ice.