Dynamisk bevegelse av stigerør indusert av isbelastning på en bøye

Abstract

To protect sensitive equipment from the inhospitable environment, offshore installations in arctic regions often depend on placing this equipment on the seabed. Installations on the surface will be subjected to environmental loads such as floating ice. A specific design of the surface structure can therefore be advantageous. A conically shaped buoy is an example of such a structure. Its sloping side has proved to deflect ice in a favorable manner. Ice is a very complex material and its properties is governed by many factors. In contact with a structure it can fail in several ways, and proper knowledge about the mechanisms that govern this is crucial in the calculation of ice loads. A review of the failing mechanisms of ice is given in this thesis. Special consideration is given to the failing mechanism governing the contact between an ice raft and a sloping structure. Ice loads on a conically shaped buoy can result in periodic motion of the buoy if it is anchored to the seabed. A riser connecting the sub sea installation with the buoy will be affected by this motion. To verify if the “riser – buoy” concept is a feasible solution for arctic environments, a thorough study of the periodic motion of the buoy and the resulting dynamic motion of the riser is given in this thesis. Both analytical methods and specialized computational tools are applied in this study. The dynamic motion of the riser will cause stresses varying both along the length of the riser and in time. The maximum values of these stresses will give an initial indication to whether the riser can sustain this motion. The maximum stresses are therefore compared to a given stress value expressing the largest allowable stress in the riser. In the analytical model, the riser is described as a continuous system, and several simplifications are made in proportion to the actual riser system, especially with respect to geometry. Modal analysis is used to describe the dynamic riser motion in this model. The actual riser system is modeled using RIFLEX, a specialized computational tool. The two models allow for an investigation of the possible degree of approximation between the dynamic motion of a rather complex system and a more simple model.