Offshore structures--Dynamics

This thesis presents a finite element analysis of the viscoelastic (creep) behavior of a spray ice island under its own self-weight and three levels of lateral loads. Spray ice and its use as a construction material are described in the context of other natural ice forms and the ice environment of the Beaufort Sea. The analytical results indicated that creep settlement in the absence of volumetric contraction was nominal over the course of a simulated 21-day construction period and a 79-day service life. The effects of the applied lateral loads were very localized and did not result in any appreciable deformations in the central working area of the structure. Some evidence of shear plane development and upward passive failure of the island perimeter was observed.

Technology movement toward deeper waters necessitates the control of vertically tethered systems that are used for installing, repairing, and maintaining underwater equipment. This has become an essential ingredient for the future success of the oil industry as the near-shore oil reservoirs are nearly depleted. Increased operation depths cause large oscillations and snap loadings in these longer cables. Research on this topic has been limited, and includes only top feedback control. The controllers developed in this thesis utilize top, bottom and combined top and bottom feedback. They are implemented on a discrete finite element lumped mass cable model. Comparison between PID, LQG and H infinity for all feedback combinations reveal that the Hinfinity controller with both top and bottom feedback has the best performance, while LQG has a more consistent and reliable performance for all feedback cases.