Final Design

System Overview

The final design decisions were made after several iterations to the original design choice. Chains were still the primary component of the system, connecting the ship to the dry dock walls on both sides in a symmetrical fashion. To improve stability in the system, contact points were utilized for multiple chain extensions. Figure 1 below displays the series of chains connected to different points on the ship.

Figure 1: Top-View Diagram of Chain System

Many naval ships, such as the LCS 1 Freedom used for analysis, possess various attachment locations welded into place along the edges. These attachments are normally double bitts. Since bitts were decided to be the most secure mechanism for securing lines, this is convenient for the design.

Chain System Primary Component: Tension System

One of the primary design models for this project relates to the specific implementation of the chains in the system. They needed to extend from the concrete wall to the ship in an efficient and modifiable manner. The chain mechanism designed for this system consists of multiple components: the rail, the tension system, the static upper chock, and the mobile lower chock. A profile view diagram of the mechanism is displayed in Figure 2 to the right.

The chain begins in the tension system. This component is mean to secure one end of the chain so that tension can be modified. The chain is then fed through the upper chock and downward towards the lower chock. The upper chock remains stationary throughout the process. It is meant to provide guidance over the edge of the cement wall. The chain then feeds through this lower chock and extends towards the double bitt attachments welded to the ship. Here, it is tightly secured to the ship.

Adjusting for Different Ships

The angle at which the chain extends to the ship is an important factor. It relates directly to the height of the ship with respect to the point of rotation at the bottom of the keel. The design of this mechanism accounts for the diversity in ship dimensions. The lower chock is mobile in the vertical direction. It slides along the rail and is anchored into the concrete at the appropriate position. This allows the angle of the chain to be adjusted, which ultimately requires less force in tension to secure the chain. Each of these rail systems is spaced about 7.62 m (25 ft) apart from each other along the dock. These can be spaced closely because of their low profile. While there can be numerous rail systems along the wall, they do not take up much space, so there is still much room for maintenance operations.

Figure 2: Profile View of Chain Tension System

Secondary Design Component: Spring-Dampener

In the chances that a rail system component does fail, the team design a secondary rail system. It essentially operates the same, but the tension system component is replaced with a spring-dampener mechanism. A profile view can be seen in Figure 3 to the left. This is internally designed to allow the appropriate adjustment of the tension levels in the chain, should the primary rail system fail. If this were implemented, it would only be required every 15.24 m (50 ft) along the dock, alternating every other rail component

Figure 3: Profile View of Spring-Dampener Secondary Component

Final Evaluation of Performance

From the various vibrational tests, it was determined that there is a specific chain tension for any given vessel that will prevent ship movement and chain nodal oscillation. As was evident from the catenary analysis, this point is approached as the slack in the chains approaches zero. However, due to the real chains’ diameter and weight, this tautness cannot be reached. In addition, trying to apply these results would put the ship under constant unnecessary tension. After reducing the tension of the chains and running the simulations, the model was observed to have minimal angular rotation about the pivot point. At the same time, chain nodal oscillation and jerking was observed. Based on the results of the tests conducted on the scaled model, the chain system would be able to support an earthquake of 6.9 on the moment magnitude scale with minimal ship displacement. However, it was suggested to the sponsor that additional testing should be performed on the chains and their behaviors under real earthquake conditions.