Ice-induced Vibration Research Articles

Failure Analysis of Topside Facilities on Oil/Gas Platforms in the Bohai Sea

The jacket platform in the Bohai Sea oilfield is an important engineering development, offering design alternatives in this economically important region. However, ice-induced vibration in cold areas threatens the safety and operation of these platforms. On two occasions, intense ice-induced vibrations triggered the rupture of a well’s blow down pipeline, loosening the flanges on the Bohai platform, and leading to the ejection of high-pressure natural gas. Subsequent mechanical analysis of the failed parts helped define the mechanism of failure and identified the failure criteria, based on a prototype structure monitoring system. The analysis revealed that the deck’s inertial force, which resulted in ice-induced steady-state vibration, was the major cause of the accident. Three fixed cones were thus installed on the platform the following winter, effectively reducing the vibrations.

Use of an Irregular Cone to Reduce Ice-Induced Vibration of a Vertical Riser Pipe

AbstractA vertical riser pipe is often mounted on the leg of an offshore platform to transport oil and gas. However, severe ice-induced vibrations and low-cycle fatigue damage of welded joints have...

An assessment of sensitivity of the self-excited modelling approach for simulating dynamic ice-structure interactions to changes in temperature and scale effects

Empirical relationships of compressive ice strength as a function of stress rate have been utilized in models of vibrations that arise during coupled ice-structure interactions. Central to current self-excited models (SEM) is a deterministic relationship of compressive ice strength as a function of stress rate based on uniaxial compressive strength data obtained from thin, first-year sea ice. However, full-scale observations, laboratory test data and fundamental knowledge of ice material behavior suggest that any such relationship would be influenced by factors such as temperature and the scale of the interaction. In this paper, the influence of temperature and scale effects is examined and first-order estimates of their effect on modifying the assumed strength vs. stress rate relationship are presented. This approach is used to assess the sensitivity of the SEM method to changes in environmental and interaction conditions. Results suggest that accounting for changes in ice temperature and the scale of interaction considerably affect ice-induced vibration responses predicted by the model. Additional large-scale experiments, full-scale data and the development of physics-based models of ice compressive failure are needed to better account for different ice conditions and different sized structures that may be considered in design.

Physical mechanism of ice/structure interaction

ABSTRACTTo obtain the effect of velocity and structural natural frequency (structural stiffness) on ice failure, an extended dynamic Van der Pol-based single degree-of-freedom ice/structure interaction model is developed. Three basic modes of response were reproduced: intermittent crushing, frequency lock-in and continuous crushing. Further analysis on the physical mechanism of ice/structure interaction is presented on the basis of feedback mechanism and energy mechanism, respectively. Internal effect and external effect from ice and structure were both explained in the feedback branch. Based on reproduced results, energy exchanges at different configurations are computed from the energy conservation using the first law of thermodynamics. A general conclusion on the predominant type of vibration when the ice velocity increases during the interaction process is forced, self-excited and forced in each of the three modes of responses. Ice force variations also show that there is more impulse energy during the lock-in range. Moreover, ice-induced vibration demonstrates an analogy of friction-induced self-excited vibration. Finally, the similarity between strain-stress curve and Stribeck curve shows that static and kinetic friction force variations are attributed to ice force characteristic, and can be used to explain the lower effective pressure magnitude during continuous crushing than the peak pressure during intermittent crushing.

New insights about ice friction obtained from crushing-friction tests on smooth and high-roughness surfaces

Ice crushing occurs in many situations that involve a sliding frictional component such as sports involving ice-contact, ice interaction with ship hulls, and ice-on-ice sliding/crushing within glaciers and between interacting sea ice floes. Ice crushing-friction tests were conducted in the lab at −10 °C using a set of acrylic ice-crushing platens that included a flat smooth surface and a variety of high-roughness surfaces with regular arrays of small prominences. The experiments were part of Phase II tests of the Blade Runners technology for reducing ice-induced vibration. Ice was crushed against the platens where the ice movement had both a vertical and a horizontal component. High-speed imaging through the platens was used to observe the ice contact zone as it evolved during the tests. Vertical crushing rates were in the range 10–30 mm/s and the horizontal sliding rates were in the range 4.14–30 mm/s. Three types of freshwater ice were used. Friction coefficients were extraordinarily low and were proportional to the ratio of the tangential sliding rate and the normal crushing rate. For the rough surfaces all of the friction coefficient variation was determined by the fluid dynamics of a slurry that flowed through channels that developed between leeward-facing facets of the prominences and the moving ice. The slurry originated from a highly-lubricating self-generating squeeze film of ice particles and melt located between the encroaching intact ice and the surfaces.

Nonlinear model parameter identification for ice-induced vibrations

The problem of level ice interacting with compliant structures is addressed, where the ice loads can depend on the dynamical behavior of the structures. We are interested in a special type of ice-induced vibration, known as frequency lock-in, and characterized by having the dominant frequency of the ice forces near a natural frequency of the structure. It is shown that accurate estimates of the model parameters for the well-known Määttänen’s model for ice-induced vibrations can be obtained from measurements of the structural vibrations and the ice velocity. Määttänen’s model uses a state-dependent piecewise nonlinear function for the ice crushing strength, which leads to nonlinear negative damping in the equations of motion of the considered structure. The identification is achieved by means of an Unscented Kalman Filter using simulated noisy measurements of the structural behavior.

Seismic Performance and Ice-Induced Vibration Control of Offshore Platform Structures Based on the ISO-PFD-SMA Brace System

Pall-typed frictional damper (PFD) has higher capacity of energy dissipation, whereas shape memory alloy (SMA) has excellent superelastic performance. Therefore, combining PFD and SMA together as a brace system has a great prospect in vibration control of structures. This paper investigates the performance of offshore platform with three structural configurations including the SMA brace system, the ISO-SMA (where ISO stands for isolation) brace system, and the ISO-PFD-SMA brace system, which are subjected to seismic and ice-induced excitations. In this study, PFD-SMA brace system is installed on the isolation layer of jacket platform, which is under earthquake excitations and ice loading. Then, the reduction of vibration is evaluated by using ANSYS program. The results show that the PFD-SMA brace system is useful in reducing the seismic response and ice-induced response of offshore platform structures; meanwhile, it also demonstrates excellent energy dissipation and hysteretic behavior.

Classification criterion of narrow/wide ice-resistant conical structures based on direct measurements

Ice-induced structural vibration generally decreases with an increase in structural width at the waterline. Definitions of wide/narrow ice-resistant conical structures, according to ice-induced vibration, are directly related to structure width, sea ice parameters, and clearing modes of broken ice. This paper proposes three clearing modes for broken ice against conical structures: complete clearing, temporary ice pile up, and ice pile up. In this paper, sea ice clearing modes and the formation requirements of dynamic ice force are analyzed to explore criteria determining wide/narrow ice-resistant conical structures. According to the direct measurement data of typical prototype structures, primary quantitative criterion of the ratio of a cone width at waterline (D) to sea ice thickness (h) is proposed. If the ratio is less than 30 (narrow conical structure), broken ice is completely cleared and a dynamic ice force is produced; however, if the ratio is larger than 50 (wide conical structure), the front stacking of broken ice or dynamic ice force will not occur.

锦州9-3油田CEPD-SLPW平台间栈桥冰激振动分析

平台间的栈桥作为海洋平台上的重要子结构，具备与整体平台结构完全不同的固有动力特性。本文针对锦州9-3油田CEPD与SLPW平台之间的栈桥及其附属的重要管线的工程设计，考虑平台在冰激振动事件中，平台间栈桥及其附属的重要管线上的振动响应，进行有限元分析计算，并根据计算结果进行相应的分析评价。计算发现栈桥作为平台子结构在继承了平台的冰激振动后，又由于结构特有的动力特征将平台整体传递的振动转化为其特有的振动形态，尤其是在垂向的振动水平明显放大。根据计算结果进行的相关评估见下文。 As an important substructure of the offshore platform, the bridge connecting two platforms pos-sesses a completely different inherent dynamic nature compared with the whole structure. Based on the engineering design of the bridge and pipelines connecting CEPD and SLPW platform in Jinzhou 9-3 oilfield, an analysis on the dynamic response of the bridge structure induced by sea ice is reached in this paper by using finite element method. According to calculation result, bridge, as a platform substructure, not only inherits the ice-induced vibration of the platform, but also translates vibration from the platform into its unique form due to the dynamic characteristics of structure. Especially the vertical vibration level is significantly enlarged. The relevant evaluation is discussed afterwards according to the computing result.

Laboratory experiments to study ice-induced vibrations of scaled model structures during their interaction with level ice at different ice velocities

A simplified model of a typical bottom-founded structure was forced through ice sheets in a laboratory experiment to study ice-induced vibrations. The ice forces exerted on the structure are identified in conjunction with the response of the entire structure using a joint input-state estimation algorithm. Novel insights into ice-induced vibration phenomena are obtained by comparing, on different time scales, measured and estimated response quantities and forces/pressures. First, the identified forces, ice velocities and time-frequency maps of the measured responses are presented for a series of ice-induced vibration tests. It is shown that the ice forces excite more than one mode of the structure and that the transition ice velocity at which the vibrations shift from the first to the second mode increases with reduced foundation stiffness and reduced superstructure mass. Second, a detailed analysis of the interaction between the structure and the ice edge is performed on a smaller time scale by comparing the locally measured pressures at the ice-structure interface to the identified structural responses and forces. It is shown that structural vibrations at a frequency higher than the dominant vibration frequency cause cyclic loading of the ice edge during intermittent crushing. These vibrations led to an increasing loading rate prior to ice failure. During an event that shows the tendencies of frequency lock-in vibrations, the structural response was dominated by a single vibration frequency.

Ice-induced non-structure vibration reduction of jacket platforms with isolation cone system

The jacket platforms are strong enough to resist extreme ice forces in Bohai Sea, China. However, ice-induced vibration acceleration has been observed and caused non-structure failure such as human discomfort and gas leakage due to flange loosing of pipes in winters. As a method of reducing ice-induced vibrations, this paper proposed an isolation cone system composed of cones, springs and dampers, which is located on the pile of jacket platforms at sea level. In order to examine the ice-induced vibration acceleration reduction effectiveness of the isolation cone system for jacket platforms, a numerical simulation is conducted to JZ20-2MUQ jacket platform as an example. It is shown from the numerical study and observed data that the isolation cone system is effective in reducing the ice-induced vibration acceleration responses and avoiding non-structure failure of jacket platform structures.

Vibration Reduction of Bucket Foundation Platform with Fixed Ice-Breaking Cone in the Bohai Sea

AbstractThe bucket foundation platform is an important engineering design alternative in oil/gas field developments in the Bohai Sea because of its economic and structural characteristics. Severe ice-induced vibrations were observed on the Jinzhou 9-3E (JZ9-3E) bucket foundation platform in the Bohai Sea attributable to the ice action against the riser. A fixed ice-breaking cone was installed on the riser to reduce ice-induced vibrations. The ice-induced vibration response of the platform was observed and recorded. The observed data showed that the fixed ice-breaking cone can effectively reduce ice-induced vibrations.

Physical Mechanism of Ice-Induced Self-Excited Vibration

AbstractSteady amplitude vibration on cylinder structures caused by dynamic ice loading has been observed during the measurements on prototype structures. Further analysis indicates that it belongs to self-excited vibration, in which the vibration amplitude stays constant and the frequency of the dynamic ice load is locked by the frequency of the structural vibration. Based on the conclusions from ice mechanics, this study proposes a physical mechanism to describe the ice-induced self-excitation process: during the loading phase within a single vibration cycle, the compressive strain rate in the ice sheet close to the structure enters a ductile-brittle transition range, and thereby the steady formation of micro cracks maintains the resistance of the ice sheet as the ice load steadily increases; when the density of cracks in the ice reaches a critical level, cracks coalesce and collapse, resulting in the unloading phase. In this way structure vibration dominates the time-variation of the ice load. The prop...

Research about the Dynamic Ice Response of Concrete Structure in Environment with Freezing-Thawing

The several familiar ice-structure interaction models were established according to collision dynamics theory. The numerical simulations of the ice-structure interactions were implemented by using the explicit dynamic analysis software ANSYS/LS-DYNA, according to the concrete reduction model and bi-axial strength criterion established by indoor test after freezing-thawing damage. The simulated ice force and structure displacement response were presented using ice sheet with a certain constant speed moving to the structure. What more, the effect of super bridge system is taken into consider in simulation models. The simulated results show that the ice forces applied on structure are insensitive to alteration by ice velocity, and the circular structure and the rectangular one are unlikely to generate ice-induced vibration because of the effect of super bridge system.

Numerical Simulation of Sea Ice Compressional Strength with Discrete Element Model

The compressional strength of sea ice is a key parameter to determine the interaction between ice cover and offshore structure. In this study, the discrete element model (DEM) with particle bonding function is adopted to model the sea ice compressional strength. The bonding strength is set as a function of the ice temperature and ice salinity, and their influences on sea ice compressional strength are observed. The simulated results are compared well with the physical experimental data. With the improvement of this DEM, the ice load and ice-induced vibration of offshore structure can be simulated.

Acceleration-oriented design optimization of ice-resistant jacket platforms in the Bohai Gulf

The Bohai Gulf in China is home to a marginal oil field that freezes during winters. The two key aims of the designers are to design both economical and ice-resistant offshore oil platforms. Originally, the jacket platforms were designed to withstand the extreme static ice force. However, severe ice-induced vibrations have been observed during the winters. Researchers have systematically studied the ice-induced vibration and the related approach to ice-resistant jacket platform design. As a part of these studies, this paper proposed an acceleration-oriented design optimization of ice-resistant jacket platforms in the Bohai Gulf. This approach focused on the dynamic performance of the jacket platforms in terms of the deck acceleration and employed the structural optimization technique to achieve an economical and rational design. The proposed design optimization accounts for the uncertainty of the ice environments and the variability of the ice-resistant structure during the optimization procedure by introducing a failure probability-based approach. This is used to determine the short-term dynamic ice case for dynamic analysis of the ice-resistant jacket platform. Finally, an example of a real ice-resistant jacket platform demonstrates the applicability and feasibility of the proposed method. This can help designers achieve a design that satisfies dynamic performance criteria while maintaining an acceptable weight.

A Dynamic Model for Ice-Induced Vibration of Structures

A dynamic model for the ice-induced vibration (IIV) of structures is developed in the present study. Ice properties have been taken into account, such as the discrete failure, the dependence of the crushing strength on the ice velocity, and the randomness of ice failure. The most important prediction of the model is to capture the resonant frequency lock-in, which is analog to that in the vortex-induced vibration. Based on the model, the mechanism of resonant IIV is discussed. It is found that the dependence of the ice crushing strength on the ice velocity plays an important role in the resonant frequency lock-in of IIV. In addition, an intermittent stochastic resonant vibration is simulated from the model. These predictions are supported by the laboratory and field observations reported. The present model is more productive than the previous models of IIV.

Experimental study on mitigation of ice-induced vibration for offshore platforms with a tuned mass damper

Experimental studies have been conducted on the tuned mass damper (TMD) – a passive control device. These studies investigate the feasibility that the TMD can mitigate the vibrations induced by ice forces on offshore steel jacket platforms. A TMD device was designed and manufactured with respect to an offshore oil platform. Large-scale testing is expensive and the requirements for model similarity in model-scale testing are hard to meet. Thus, an experimental technique that focuses on the control device itself and emulates the behaviour of the structure with a simulator is presented. This novel experimental system consists of a virtual part and an actual part called the semi-experimental testing system. The virtual parts include scaled external load and a scaled simplified platform model. The actual parts include the TMD device and a moving table, which is controlled by a hydraulic actuator. By introducing the interaction between the ice sheet and the structure, the performance of the device is investigated and reasons of effectiveness reduction are explained. The results show that an optimally tuned TMD can reduce the amplified response significantly under ice load action. Finally, the application possibilities of vibration absorption using TMD to other arctic offshore structures are discussed.

Estimating Soil Liquefaction in Ice-Induced Vibration of Bucket Foundation

The two suction foundation platforms installed in the Bohai Sea have vertical narrow columns passing through the water level. The continuous ice crushing on the columns and thereby the dynamic ice-structure interaction may usually happen during the ice season resulting in sustained violent vibrations of the structures. The paper first introduces the ice-induced vibration analysis of the suction foundation platform by using the self-excited vibration theory. Then the maximum dynamic shear stresses in the soil induced by ice are obtained from the analyses. By comparing the dynamic stresses to the cyclic strength of soil, which can be determined according to soil characteristics and features of the dynamic loading, the potential soil liquefaction is finally assessed.

TLD Technique for Reducing Ice-Induced Vibration on Platforms

Tuned liquid damper (TLD) containers are designed to provide their sloshing forces in opposite directions against the inertia loads of the vibrating structure and thus to reduce the vibration. The sloshing forces that result from liquid motion may be determined either analytically or experimentally. The efficiency of the TLD in vibration reduction then can be estimated by attaching the TLD subsystem onto the dynamic equation of the vibrating structure. This paper describes investigations of the TLD system for an existing Bohai production jacket, which has routinely been excited by drifting ice during the winter seasons. In addition to a theoretical demonstration, a scaled model test and a full-scale field experiment were performed in this study. The technical strategy and main results of the investigation are presented in this paper.