Numerical Analyses of Ice Jamming in Jacket Platform Conductor Array in Bohai Sea

Serious ice accumulating,pile-up and ice jamming occur around the conductor array of offshore jacket platforms during the winter every year in Bohai Sea,which could cause grave threats to the stability of platform structure,the safety of people and equipment,and even severer calamity.Therefore,the process of ice accumulation and ice jamming in the jacket platform area needs more concern.This study focuses on ice accumulation and jamming behaviors in the jacket platform conductor area by using a coupled two-dimensional hydro-ice dynamics model.A series of cases are conducted with different flow conditions,such as flow velocity,drifting direction and oscillatory flow.Through the simulation,the ice pile-up process is described and changes in ice-jamming thickness,ice pile-up location and ice pile-up volume are investigated.The differences in ice pile-up in the steady flow and oscillatory flow are analyzed.This study proposes a new approach to simulate the ice jamming process in the jacket platform conductor area,providing a reference for ice management on the platform.

Determination of ice jam thickness—A new approach

In winter,rivers in cold regions often experience flood disasters resulted from ice jams or ice dams.Investigations of the variation of ice jam thickness and water level during an ice jammed period are not only a practical need for ice prevention to avoid disaster and plan water resource,but also essential for the development of any mathematical model for predicting the evolution of ice jam.So far,some equations based on the energy equation have been proposed to describe the relationship between ice jam thickness and water level.However,in the derivation of these equations,the local head loss coefficient at the ice jam head and the riverbed slope factor were neglected.Obviously,those reported equations cannot be used to preciously describe the flow energy equation with ice jams and accurately calculate the ice jam thickness and water level.In the present study,a more comprehensive theoretical model for hydraulic calculation of ice jam thickness has been derived by considering important and essential factors including riverbed slope and local head loss coefficient at the ice jam head.Furthermore,based on the data collected from laboratory experiments of ice jam accumulation,the local head loss coefficient at the ice jam head has been calculated,and the empirical equation for calculating the local head loss coefficient has been established by considering flow Froude number and the ratio of ice discharge to flow discharge.The results of this study not only provide a new reference for calculating ice jam thickness and water level,but also present a theoretical basis for accurate CFD simulation of ice jams.

Ice accumulation and thickness distribution before inverted siphon

In winter, the safe operation and the water transfer efficiency of an inverted siphon are big concerns for water diversion projects in a high latitude area. This paper takes the Tanghe inverted siphon of the Middle Route of the South-to-North Water Diversion Project in China as an example, and carries out experimental studies under the real ice condition with different water levels, flow discharges and submerged depths. The ice accumulation process and the ice jam thickness distribution before the inverted siphon are tested and measured. The characteristics of the ice jam distribution and the relationship between ti /H （ratio of ice jam thickness tt to total water level H ） and Fr, （Froude number under ice jam） are analyzed. An equation for the ice jam thickness calculation before the inverted siphon is put forward. Analysis results might help the thickness prediction and the ice jam prevention of similar water diversion projects during ice period.

PROGRESS IN STUDIES ON ICE ACCUMULATION IN RIVER BENDS

River ice is an important hydraulic element in temperate and polar environments and would affect hydrodynamic conditions of rivers through changes both in the boundary conditions and the thermal regime. The river bend has been reported as the common location for the initiation of ice jams because the water flow along a river bend is markedly affected by the channel curvature. In this article, the experimental studies about the ice accumulation in a river bend are reviewed. Based on experiments conducted so far, the criteria for the formation of ice jams in the river bend, the mechanisms of the ice accumulation in the river bend and the thickness profile of the ice accumulation in the river bend are discussed. The two-equation turbulence model is used to simulate the ice accumulation under an ice cover along a river bend. A formula is proposed for describing the deformation of the ice jam bottom. Our results indicate that all simulated thickness of the ice accumulation agrees reasonably well with the measured thickness of the ice accumulation in the laboratory.

FLOATING RATE OF FRAZIL ICE PARTICLES IN FLOWING WATER IN BEND CHANNELS——A THREE-DIMENSIONAL NUMERICAL ANALYSIS

In this article, based on the theory of two-phase flow and laboratory data, a three-dimensionally model is developed to simulate the floating rates of frazil ice particles in water under covered condition. The Lagrangian trajectory method is used in the three dimensional simulation for floating rates of fxazil ice particles along a 180° bend channel. The velocity profiles in longitudinal and transverse directions, the turbulence intensity, and the residual pressure are simulated. Under the condition of gravitational similarity, the simulated floating rates of frazil ice particles in the model bend channel and the prototype bend channels are compared. Results indicate that the profiles for floating rates of ice particles for flows in the model channel are similar to those in the prototype bend channels. The simulated floating rates of ice particles are clearly higher along the convex bank than along the concave bank at each cross section. For the prototype bend channel in a large model scale, the variation of floating rate across each cross section is relatively small.

ICE JAMS IN A SMALL RIVER AND THE HEC-RAS MODELING

This paper describes a model of a 3.06km long river reach between two small reservoirs under both open flow and ice covering conditions for different operational settings of the stoplogs in the downstream reservoir. The HEC-RAS model developed by the Hydrological Engineering Center of US Army Corps of Engineers was used to compare different approaches in terms of flow velocity, water level and the Froude number. The impacts of heavily vegetated main channel and floodplain on ice accumulations were investigated. And it is shown that this vegetation plays a significant role in the formation of river ice jam during winter period and thus the vegetated channel has strong influence on ice flooding. In addition, the paper explores the impact bouh of the operation of the stoplogs during the winter period and the presence of the downstream dam on the accumulation of ice jam along this river reach.

Simulations of ice jam thickness distribution in the transverse direction

River ice often forms in the cold regions of northern hemisphere which can lead to ice jams （or ice dams）. Water level can be significantly raised due to ice jams. As a consequence, disastrous ice flooding may be resulted, such as the ice jam flooding in the Nechako River in Prince George in winter 2007-2008. In the present study, the equations describing the ice jam thickness in the transverse direction are derived. The impact of the secondary vortex is considered while the cohesive force within ice cubes is neglected in the model. The relationship between the parameter β and the total water depth is established based on the assumption that all other variables except the velocities are kept constant on the same cross section. By using the parameter β and the developed equations, the ice jam thickness in the transverse direction can be predicted. The developed model is used to simulate the ice jam thickness in the transverse direction at the Hequ Reach of the Yellow River in China. The simulated ice jam thicknesses agree well with the field measurements on different cross sections.

Impact of bridge pier on the stability of ice jam

River ice jam is one of the most important issues in rivers in cold regions during winter time. With the extra solid boundary due to the ice cover, the flow condition under ice-covered conditions is completely different from that of a open channel flow. The presence of bridge piers will further change the velocity field around the bridge piers. As a consequence, the formation and the accumulation of ice jams in the vicinity of the bridge pier will be affected. On the other side, the formation of an ice jam around the piers can cause extra turbulence to reduce the stability of a river bridge. The present study focuses on the stress analysis of the ice jam in the vicinity of a bridge pier. By developing a governing equation for describing the equilibrium state of an ice jam, the stability of the ice jam around bridge piers is analyzed and determined. As seen from the field data in literature, the stability estimations of an ice jam around bridge piers determined by the present method agree well with the field observations. Therefore, the proposed approach can be used for the prediction of the formation of ice jams around bridge piers.

Impacts of bridge piers on water level during ice jammed period in bend channel——An experimental study

Experiments are carried out in an ＂S-shaped＂ flume in the laboratory under both open flow and ice-jammed conditions to study the impacts of bridge piers in a bend channel on the variation of the water level. The variations of the water level under the ice jammed condition with bridge piers are compared to those without bridge piers in an 180° bend channel. Results indicate that the bridge piers in the S-shaped channel have obvious impacts on the ice accumulation and the water level. The increment of the water level with the presence of the bridge piers is less than that without the bridge piers in the channel. Different arrangements of the bridge piers result in different increments of the water level. When one bridge pier is installed in the straight section of the channel（between 2 bends） and another one at the bend apex（for a convex bank）, the increment of the water level during the equilibrium ice jammed period is between that with a single bridge pier located in the straight section of the bend channel and that with a single bridge pier located at the bend apex. It is also shown that the increment of the water level during the equilibrium ice jammed period increases with the increase of the average thickness of the ice jams.

Simulation of ice accumulation around bridge piers during river breakup periods using a discrete element model

During river breakup periods in winter or early spring, a large amount of ice floes of various sizes will be transported by flowing water. The presence of bridge piers in water alters velocity field. As a consequence, the transport process of ice floes around bridge piers will be influenced. It often causes the ice floe congestion around bridge piers. In the present study, the influences of the ice floe size and coverage ratio of ice floating on water surface on ice floe congestion have been examined. The influence of bridge pier layout on ice jam development has been assessed. Results showed that during river breakup period, the ice floe transport capacity around bridge pier decreased. With the presence of bridge piers, the critical coverage ratio of ice floating on water surface for the ice floe congestion depends on the ratio of the side length of ice floe to the pier spacing distance (a / Ln) and ice floe-length Froude number associated with the ice floe length (Fra). A discrete element model (DEM) has been developed to simulate the ice floe accumulating process around bridge piers. Simulation results agree well with those of experiments.