Noriaki Oka

Axial and Lateral Coupled Response of a Deepsea Riser for Scientific Drilling

Axial and lateral coupled response of deepsea drilling risers in hang-off condition was investigated. Lateral oscillation of riser has a possibility of being excited by axial tension fluctuation. The lateral response can be described by Mathieu’s equation of parametric oscillation. Numerical analysis method was formulated and verified by comparisons with experimental results. Stability conditions were clarified using the numerical method. Stability of the 4,000m riser for Scientific Drilling was also investigated and guideline for design of ultra deepwater risers was proposed.Copyright

Estimation of wind loads on VLFS of semisubmersible type

Abstract This paper presents the results of investigation of wind lift force on VLFS of semisubmersible type. In the present study, wind–wave tank experiments on the scale model were also conducted in regular progressive waves. Lift forces and wind pressure were measured as a function of column diameter, unit number, superstructure and wave condition, and divided into time-averaged mean and fluctuating components. The importance of lift force, especially at the leading edge of deck, was verified in the experimental results. The results showed that fluctuation in lift force which was caused by interaction between the wind and wave was significantly large in typhoon conditions. Measured lift forces were linearly related to fluctuation of wind velocity. A lift force model is further proposed for the estimation of lift forces on VLFS of semisubmersible type and the effectiveness of this model was confirmed by the present experimental data.