Estimation of Loads Due to Simultaneous Occurrence of Waves and Iceberg Impacts

ABSTRACT A procedure for estimating simultaneous wave and iceberg loading is developed based on Monte Carlo techniques. Iceberg impact loading is determined using an established energy balance method. Associated wave loads are introduced to the method using a wave height distribution, weighted by the monthly occurrence of icebergs, and prudent simplifying assumptions as to iceberg/structure/ wave interactions. Estimates of the return period loads are made for a large diameter gravity base structure, and for the example selected, the combined loads increase modestly compared to the iceberg impact only loads INTRODUCTION Offshore design practice often assumes the simultaneous occurrence of extreme environmental events. For conventional structures in familiar offshore areas, the assumption has no great impact on design. However, for other applications, joint occurrence of extremes leads to unrealistic and overly conservative load estimates. Such an application is a fixed large diameter gravity base structure (GBS) in an iceberg environment. Methods of computing zero-wave iceberg impact loads are addressed by various authors (ex. [1]). In such methods, the wave load simultaneous with the impact load is ignored (Le. the zero-wave load case). Neglecting any simultaneous wave load is an unconservative assumption, and is usually avoided. However, the simultaneous occurrence of the extreme wave event and the extreme iceberg impact event is a very rare event. To produce reasonable first estimates of, a Monte Carlo model is adapted to estimate rationally the total combined wave and iceberg load. The Monte Carlo model and the description are a first order approximation to the iceberg impact and wave load phenomenon. Full and detailed modeling of an impact event is complex and the present state-of-the-art capabilities and input accuracy do not presently justify increased modeling accuracy. Sufficient and reasonable conservatism is included in the simple method described below to ensure that results represent an upper bound estimate. As an example, a GBS with typical North Atlantic arctic environmental and iceberg values are input into the Monte Carlo model. ICEBERG IMPACT MODEL Iceberg impacts are complex events dependent on many variables. Typically the iceberg impact models are based on a zero-wave assumption (i.e., wave loads are ignored) and use an energy balance method in the computation. An example of one iceberg impact model is given in [1] which is used in this work. The energy based iceberg impact model equates the far field kinetic energy of the iceberg to the energy expended in crushing ice. The modeling equation is (MATHEMATICAL EQUATION AVAILABLE IN FULL PAPER) where Cm is the added mass, m is the iceberg mass, u is the far field iceberg speed, 0 is the strength of the iceberg ice and V is the volume of the crushed ice. Equation (1) is used to predict the volume of crushed ice, and from the volume, the projected contact area (A) can be computed. The force (F) on the structure is calculated as