Random Ice Load Analysis On Offshore Structures Based On Field Tests

This thesis presents an analysis to the random ice load on the offshore structures in the ice infested area based on several field tests results.Several extensive full scale monitoring projects have been carried out to investigate ice load and ice induced vibration on the offshore structures in ice infested area. Both the vertical structures and conical structures have been monitored in the measurements. The vertical structures include GCP platform, MDP mooring pole in JZ9-3 oil filed and Norstromsgrund light house in Sweden. The conical structures are 3 platforms in JZ20-2 oil filed. In the measurements, the ice loads were directly recorded by load panels;dynamic responses of the structures were recorded by the accelerometers;ice thickness, velocity and interaction between the ice and structures were recorded by video camera as well as in-situ observation. Large amount of high quality full scale ice load data have been collected in the tests.It was found in the tests that commonly occurred vibration on vertical structures is random vibration when the ice sheets drift fast and crushing against the structure. Scarcely but heaviest vibration is self-excited vibration caused by slowly moving ice. A load function in time domain based on the fractal Weierstrass function and an auto spectral model for crushing ice force similar to the gust wind speed spectrum were derived through the analysis to ice load data. A new formula for the largest crushing ice load on vertical structures is also developed based on the size effect found in the test data. The method agrees well with the result obtained by the theoretical analysis using fracture mechanics.It was found in the tests that ice loads on the narrow ice breaking cones are periodic. Vibration could be induced by the periodic ice load if the structure is slender. A random ice load model is derived from the load panel data. The ice forces are simplified as series of triangular pulses with random periods and magnitudes. Determination of the random ice force magnitudes and periods also discussed in the paper. An auto spectral density model is derived from the spectral analysis to the ice load data. The load spectrum is between narrow and wide band and can be simulated by Neumann spectrum. Detemination of the spectrum parameters is was also discussed in the thesis by studying relationship between the spectrum parameters and ice force parameters. The advantage of the ice breaking cone for the structures was also discussed in the thesis by studying related ice-cone interaction problems.The thesis also presented application of the ice load model in the ice induced vibration preparing analysis, ice induced fatigue analysis and dynamic optimization analysis considering the ice induced vibration.