Dynamic Analysis of a Coupled and an Uncoupled Tension Leg Platform

A Tension Leg Platform (TLP) is a floating system connected to the sea floor by a series of pretensioned cables. The cables provide horizontal and vertical stiffness to the surface vessel, ensuring stability in a wide range of sea conditions. Historically, TLP's have been used for hydrocarbon extraction below the seabed. Proposed future for TLP's include at--sea wind farms and floating islands. In this work, the focus is on the interactions between the surface platform and its tethers, and how cable model assumptions can alter the dynamic response of these systems.;To substantiate these claims, two different six degrees--of--freedom deterministic dynamic models are developed. In the uncoupled analysis, the restoring force generated by the tendons is determined by evaluating the generalized platform motion, based on fundamental kinematic principles. In the coupled model, a lumped mass system is included to model the tether dynamics. The environmental forces and moments considered to be acting on the platform are due to waves, buoyancy, and gravity. These models are utilized to conduct case studies, resulting in proposed modifications to the uncoupled tether model. The modifications increase the accuracy of the uncoupled system.;A methodology to obtain the equilibrium profile for the lumped mass cable system is also presented. Surprisingly, more effort is often needed to solve the static problem than the dynamics problem. In this work, the equilibrium cable profile is obtained with a shooting algorithm, which is more efficient than conventional methods. For the three--dimensional problem, the solution is obtained from a set of three equations, regardless of the cable discretization resolution.;Finally, an application for the coupled and uncoupled TLP models is presented. This analysis seeks an optimal tendon configuration based on minimizing the deck accelerations. The objective function represents the percentage of the population susceptible to kinetosis (motion sickness). By modifying of the tendon/platform attachment points, the population size succumbing to the effects of kinetosis can be reduced. This proposed optimization process is also shown to significantly reduce variations in tether tension.;A preliminary study is conducted to analyze the degree of discrepancy between an uncoupled TLP dynamics model in comparison to a coupled TLP dynamics model. An uncoupled TLP model ignores the intrinsic dynamics and environmental loads on the cables by treating each tether as an ideal massless spring. A coupled TLP system, in contrast, considers the effects of distributed mass along the tether. The initial investigation in this work results in metrics that can be used to forecast the discrepancy between these two systems. In general, the most noticeable differences occur in the heave and roll/pitch degrees--of--freedom, due to low tether damping. The results show that a more elaborate set of conditions, other than the platform-to-cable mass ratio, must be satisfied in order for the two models to provide similar results.