Research On Ice Force Of Conical Offshore Structures

Ice force is the dominant enviromental force for offshore structures in cold regions. The progress of ice force research is quite slow maily due to the lack of knowledge on sea ice fracture. The reasonal mechanical explanation and generally accepted function have not been found yet. Sea ice crushing failure occurs on traditional vertical structures and induces the most harmful ice force, resulting in the concept of conical offshore structure, so as to reduce the ice force magnitude. The principal idea of sloping or conical structure is to change the ice failure mode from crushing to bending. Research on peak ice force on conical structures has been carried on since 1970's, whereas there was limited work on dynamic ice force. The team of Dalian University of Technology conducted field measurements on coincal jacket structures, on which distinct dynamic problems under ice action were observed in the Bohai Sea. The ice force models developed by the statistics of full scale test data have been widely used on the offshore structures in this sea area, however limitation still exists when applied to various structure types in other sea areas.Because of the complex sea ice failure process when acting on cone, only the real tests could obtain the accurate and comprehensive information. Based on the directly measured ice force and ice failure process, the mechanism and the model of ice force on conical structure are developed. Based on the field and lab test on cones of different sizes, the mechanism of the main factors of dynamic ice force (period, amplitude and time-varing characteristics) were revealed. The criterion of dynamic ice force and the definition of narrow/wide cone were proposed, which indicates the applicable range of the conclusions from tests. The vibration mitigation effect was validated based on the comparison of ice force and structure response obtained from full scale tests.1. Full scale tests on two jackets in the Bohai Sea and lab tests in Hamburg Ship Model Basin (HSVA) were conducted.Full scale tests could provide the most accurate information, including sea ice failure process and ice force. Ice force directly measured from the load panels installed on JZ20-2MUQ platform contains the time-varing characteristics of ice force. The synchronously observed ice-cone interaction process was the mechanism of dynamic ice force. In order to avoid the limitation that the research is conducted on a specific structure, the full scale tests were conducted on the largest conical jacket structure JZ20-2NW platform, and the global ice forces were measured by the fiber strain sensors installed on underwater structural components. The basic conclusions on sea ice failure and ice force on conical structures were obtained from full scale tests.The lab tests on cone model were carried out, in order to investigate the influence of cone parameters to sea ice failure and ice force. In the lab tests the sea ice failure could be measured in more detail, including the upwards and downwards cones, and thus the conclusions from full scale tests could be proved.2. Combined with the directly measured sea ice failure mode, the ice force period and amplitude were statistically analyzed, and the calculation model of dynamic ice force of conical structure was developed based on the physical mechanism.The mechanism for dynamic ice force is the ice sheet's continual bending failure on cone. Ice force period is the duration of sea ice failure, which was determined by the sea ice broken length and ice speed. Ice force amplitude was determined by the energy release with sea ice internal cracks formation and propagation, which also determine the sea ice failure modes.Combined with the mechanical analysis of sea ice fracture, mainly two kinds of ice failure modes are determined:wedge typed and plate typed bending failure. The primary influencing parameters on sea ice failure modes are discussed in the range of various parameters during tests, especially the size of cone. Based on the analysis of sea ice failure, which was the basis when considering the applicable range of the conclusions, the caclulation model for ice force period and amplitude are obtained by statistics of measured data in tests.3. The basic form and criterion of dynamic ice force, the definition of narrow and wide conical structures were developed based on the ice-cone interaction process.Ice-cone interaction process is the mechanism for time-varying ice force. Combined with the directly measured ice froce from load panels, the basic form of dyanmic ice force was developed:periodical ice force composed of triangle force (loading and unloading) and total unloading. The total unloading corresponds to the broken ice total clearing in front of the cone, which could ensure the subsequent intact ice sheet directly acting on cone. Only in this occasion, the sea ice continual bending failure and dynamic ice force arise. The definition of narrow and wide cone was brought up:if the broken ice could be totally cleared, the cone is called narrow cone; whereas if the broken ice could not cleared but pile up in front of the cone, the intact sea ice could not directly act on cone, the cone is called wide cone. The dynamic ice force calculation model on narrow cone was founded and validated by real measured structure vibrations.4. The vibration mitigation effect by adding cone was evaluated based on the full scale measurement.The purpose of adding cone on vertical offshore strucuture was to reduce the ice force magnitude and ice induced strong vibration. The feasibility and effect of adding cone were proved by compare of directly measured ice force and structure response on structures with and without cone. The locked-in ice force and steady state vibration caused by ice crushing failure on vertical structures could be avoided basically. The ice force is obviously reduced based on the directly measured ice force from field tests, and the compare results from structure vibration measured before and after adding cone showed the quantitative effect.This paper investigates the mechanism and calculation model of dynamic ice force of conical structures, and also the effect of adding cone on vertical offshore structure. The dynamic ice force function was founded based on the sea ice failure when acting on cone. The conclusions are quite helpful for the vibration prediction, safety evaluation and optimized design of offshore structure in cold regions.