Substantion of the energy approach to parameter determination of the cyclic ice force against offshore structures

Определение величины силового воздействия ледовых образований на морское ледостойкое основание производится по расчетной формуле, описывающей принятую модель разрушения льда. Лед в контактной зоне ледового поля с сооружением, в зависимости от его температуры и скорости взаимодействия, может разрушаться в широком диапазоне механизмов его разрушения – от вязкого до хрупкого. Здесь представлена разработка модели упруго-хрупкого разрушения льда, учитывающая разрушение кромки ледового поля всеми возможными типами трещин, делящими локальную область массива льда в зоне контакта с сооружением на клиновидные или трапециевидные фрагменты. Предложено все процессы разрушения льда, имеющие различную природу, рассматривать как единообразный, но комплексный процесс, математическое описание которого во времени можно выполнить с использованием универсального энергетического критерия разрушения. Приводятся обоснования закономерности такого подхода, основанные на феноменологическом анализе явления циклического разрушения льда в контактной зоне, на результатах численного моделирования картины развития разрушений в массиве льда у кромки ледовой плиты на контакте с опорой сооружения, а также на данных физического моделирования изучаемого процесса. В качестве универсального критерия в работе рассматривается предельное значение накопленной в единице объема льда перед сооружением потенциальной энергии упругого сжатия – удельной упругой энергии механического разрушения морского льда.

Numerical Simulation of the Ice Breaking Process for Hovercraft

A hovercraft can adapt to an ice area, open water, land and other environments, owing to its unique hull structure. It also plays an important role in transporting supplies, rescuing people, breaking ice and conducting other tasks. Ice load prediction is very important for structural safety and navigation of a polar ship, especially in design of air cushion icebreakers or ice breaking platforms. In this paper, based on a simplified circumferential icebreaking pattern, the icebreaking force of the hovercraft operating on the ice sheet at low speed is simulated in a numerical way. Numerical analysis of the icebreaking process with different ice thicknesses and bending strengths are performed. The numerical results are compared with model test data in a time domain for three operating cases. By analyzing the average ice force, the errors between numerical simulation results and model test measurements are less than 30%. The present study is significant for the preliminary design of new icebreaking hovercraft and it assists the operation possibility for existing hovercraft.

Risk assessment of sea ice disasters on fixed jacket platforms in Liaodong Bay

Sea ice disasters seriously threaten the structural safety of oil platforms in the Bohai Sea. Therefore, it is necessary to carry out a risk assessment of sea ice disasters on oil platforms in the Bohai Sea. In this study, a risk assessment of sea ice disasters on fixed jacket platforms in Liaodong Bay, in the Bohai Sea, was performed in five steps. Firstly, the formation mechanisms of sea ice disasters were analyzed and the sources and modes of sea ice risks were summarized. Secondly, according to the calculation formulas of extreme ice force, dynamic ice force, and accumulated force, several ice indices such as thickness, motion, strength, period, and concentration were proposed as the hazard indices, and corresponding values were then assigned to the proposed indices based on ice conditions in the Bohai Sea. Thirdly, based on four structural failure modes – structural overturning by extreme ice force (Mode 1), structural fracture failure caused by dynamic ice force (Mode 2), the damage of facilities caused by dynamic ice force (Mode 3), and structural function failure caused by accumulated ice (Mode 4) – the structural vulnerability index, overturning index, dynamic index, ice-induced vibration index, and function index were proposed, and corresponding values were assigned to the structural vulnerability index of fixed jacket platforms in Liaodong Bay. Fourthly, the weight of each risk index was determined according to previously recorded sea ice disasters and accidents, and the sea ice risk was then calculated with the weighted synthetic index method. Finally, with the above index system and risk assessment methods, the risk assessment of sea ice disasters on 10 jacket platforms in three sea areas in Liaodong Bay was carried out. The analysis results showed that efficient sea ice prevention strategies could largely mitigate the sea ice-induced vibration-related risks of jacket platforms in Liaodong Bay. If steady-state vibration occurs (usually in front of the vertical legged structure) or the structural fundamental frequency is high, the structural vulnerability is significantly increased and the calculated risk levels are high. The sea ice risk assessment method can be applied in the design, operation, and management of other engineering structures in sea ice areas.

Evaluation of the relative contribution of meteorological and oceanic forces to the drift of ice islands offshore Newfoundland

On 29 April 2015, four beacons were deployed onto an ice island in the Strait of Belle Isle to record positional data. The ice island later broke up into many fragments, four of which were tracked by the beacons. The relative influences of wind drag, current drag, Coriolis force, sea surface height gradient and sea-ice force on the drift of the tracked ice island fragments were analyzed. Using atmospheric and oceanic model outputs, the sea-ice force was calculated as the residual of the fragments' net forces and the sum of all other forces. This was compared against the force obtained through ice concentration-dependent relationships when sea ice was present. The sea-ice forces calculated from the residual approach and concentration-dependent relationships were significant only when sea ice was present at medium-high concentrations in the vicinity of the ice island fragments. The forces from ocean currents and sea surface tilt contributed the most to the drift of the ice island fragments. Wind, however, played a minimal role in the total force governing the drift of the four ice island fragments, and Coriolis force was significant when the fragments were drifting at higher speeds.

Analysis of Offshore Structures Based on Response Spectrum of Ice Force

With the development of large-scale offshore projects, sea ice is a potential threat to the safety of offshore structures. The main forms of damage to bottom-fixed offshore structures under sea ice are crushing failure and bending failure. Referred to as the concept of seismic response spectrums, the design response spectrum of offshore structures induced by the crushing and bending ice failure is presented. Selecting the Bohai Sea in China as an example, the sea areas were divided into different ice zones due to the different sea ice parameters. Based on the crushing and bending failure power spectral densities of ice force, a large amount of ice force time-history samples are firstly generated for each ice zone. The time-history of the maximum responses of a series of single degree of freedom systems with different natural frequencies under the ice force are calculated and subsequently, a response spectrum curve is obtained. Finally, by fitting all the response spectrum curves from different samples, the design response spectrum is generated for each ice zone. The ice force influence coefficients for crushing and bending failure are obtained, which can be used to estimate the stochastic sea ice force acting on a structure conveniently in a static way. A comparison of the proposed response spectrum method with the Monte Carlo method by a numerical example shows good agreement.