Hydrodynamic Analysis for a Logistical HUB

• Main Author: Løken, Erik
• Format: Others
• Language: English
• Published: Norges teknisk-naturvitenskapelige universitet, Institutt for marin teknikk 2012
• Subjects: ntnudaim:7608; MTMART Marin teknikk; Marin hydrodynamikk
• Online Access: http://urn.kb.se/resolve?urn=urn:nbn:no:ntnu:diva-18595

The conventional way of transporting personnel from shore to offshore platforms is done by helicopter. For large distances of transportation this results in high costs due to the limitations in the maximum number of persons each helicopter can transport per trip and due to the high prices on helicopter fuel. In this thesis is an alternative solution to this conventional transportation proposed by the utilization of a logistical HUB. The concept is based ferries doing the transportation of personnel from shore to the HUB and helicopters doing the remaining, relatively short, transfer from the HUB to the respective platforms. The HUB evaluated is based on the characteristic Sevan 650 design, having a cylindrical shape with diameter D=78m in the waterline.The models that have been analyzed in this thesis were modeled in GeniE, the hydrodynamic analyses were done in Wadam and the post processing was performed using Postresp.Models of a platform with a single tunnel cut out with varying tunnel length L have been evaluated, and the motions of these models as well as the surface elevation inside the tunnel were studied in detail. It was recorded a two peaked response in heave for the models with tunnel lengths ranging from 30m<L<40m. This unexpected behavior was found to be due to diverging values for the added mass in heave for increasing tunnel lengths for models with intact tunnel bottoms. This was adjusted for by removing the bottom of the tunnel to add damping to the system, resulting in the usual one peaked response in heave being retrieved. The reason the two peaked response in heave occurred was concluded to be due to Wadam neglecting viscous effects including the viscous damping. This leads to the system having little or no damping and the added mass to diverge towards negative infinity giving unphysical motion representations.Since critical situations for the platform-ferry interaction will occur during loading and unloading of personnel from the ferry to the platform and during entry of a ferry into a tunnel, the wave pattern inside and on the immediate outside of this tunnel have been studied and evaluated. A total of 4 different designs for the layout of the tunnels have been proposed and evaluated to find the design that results in the least surface elevation inside the tunnel and at the tunnel entrance. A window of acceptance for the incoming wave headings were established with the intention of minimizing the surface elevation. The designs were also evaluated regarding their ability to resist large motion for a variety of incoming wave periods. It was concluded that a three tunnel solution with the tunnels being shifted 120 degrees relative each other would result in the smallest platform motions for wave periods smaller than 18s. A design consisting of 4 tunnels, where three of the tunnels are shifted 30 degrees relative each other and the last tunnel being located opposite of these three would result in the smallest surface elevations. An operability study was done for all 4 designs proposed based on the elevations inside and at the tunnel entrance. It was found that the 4 tunnel design described above would result in the largest operability for the platform. It was also found that this 4 tunnel solution would be unstable in roll due to an unsatisfactory low transverse metacentric height. This low metacentric height could be adjusted for by installing a vertical wall in the waterline in the transverse direction of the tunnels orientation.