Measurement and Analysis of Ice Forces on Cook Inlet structures

Abstract

ABSTRACT The paper presents results of ice force measurements made in Cook Inlet, Alaska, over a number of years. Data include measurements made on a specially instrumented test pile as well as strain gauge records from a number of different platforms. Ice force values are interpreted within the framework of a conceptual model corresponding to visual observations of the behavior of ice impinging on structures. Forces are related to ice conditions and related meteorological influences. Correlations are made between ice thicknesses measured in Cook Inlet and published ice growth formulas. Such correlation provides a basis for prediction of expected extreme thicknesses on the basis of historical weather records. Consideration is also given to the effect of ice temperature upon ice forces on structures. Force records show details of the dynamic response of structures to ice loading. Conclusions drawn from study of the recorded dynamics are summarized. From extensive observations of ice conditions in the Cook Inlet area, a variety of ice phenomena are described and analyzed in relation to their potential effect upon structures. With all such phenomena considered, the paper develops predictions of maximum ice loading to be anticipated. INTRODUCTION Subsequent to drilling of the first discovery wells in 1962, the upper segment of Cook Inlet, Alaska has been the site of an intensive offshore oil development activity. The most notable feature of the offshore development in this area has been the design and construction of 14 large offshore structures capable of resisting the extreme environment. Although the region is subarctic, Cook Inlet can develop a substantial ice cover. The potential destructiveness of the ice is magnified by a tide range of more than 20 feet and the accompanying 6-knot current. In the absence of design information applicable to the conditions of Cook Inlet, an extensive effort was devoted to a study of the region and to determination of ice forces. This effort was primarily executed by two groups of oil companies, Pan American Petroleum Corporation being operator for one of these groups. The in-field work involved extensive reconnaissance testing of ice as well as the direct measurement of forces on several types of structures installed in the Inlet. A part of this activity has been described (Refs. 1, 2 and 3), but results are presented only in a general way. The purpose of the present paper is to summarize specifically the results of ice force measurements and to relate these force measurements to ice conditions. In addition, attention is devoted to study of the dynamic response of structures to the action of sea ice floes. NATURE OF COOK INLET ICE Most of the ice cover which forms on Cook Inlet has properties similar to ice Observed in other parts of the world. In standard ice observation terms, the seasonal ice cover on Cook Inlet is thin, first-year ice. The floes of the ice cover vary from medium to large floes, individual floes more than one-quarter mile across being common.