[Posted on March 12th 2014 by Bill Stewart]
As any marine organization can tell you, installing mooring systems is more than anchoring a single CALM buoy or setting a fixed tower on the sea bed. It is critical to know how ships of different weights and sizes will react in the given location under different circumstances. Doing a mooring analysis is critical to determine the best mooring option and its placement.
Take tanker loading and offloading as an example. Takers are extremely sensitive to wave direction in an open water situation. In most cases, spread mooring would not work due to this movement. Tankers often use single point moorings that allow them to move around the single point based on the direction of the waves and movement of the water flow.
Determining what kind of single point mooring is appropriate to a specific application requires careful analysis. Here are a few mooring options:
- Fixed towers with a slew ring work well in some single point applications. It works best in shallower areas where wave action is not heavy. At greater depths or in areas with heavy wave action, the tower can begin to bend. These towers are also susceptible to damage from even minor tanker bumping.
- A catenary anchor leg mooring (CALM) are not as sensitive to water depth and wave action as a fixed tower mooring is. It is able to handle some bumping from tankers without sustaining extensive damage. Major tanker impacts though can cause damage that can put the CALM out of commission for a good length of time and cost a lot to fix. The mechanical components are also subject to extreme weather which requires ongoing, sometimes expensive, maintenance.
- A SALM (single anchor leg mooring) solves the problem of expensive damage that CALMs can experience due to major tanker impact. They do this by placing the mechanical systems in an underwater arrangement at a depth below a ship’s keel. The simple surface buoy is easy to replace if its hit by a tanker. The biggest issue with the SALM is the maintenance difficulty and cost.
- An Articulated Loading Column (ALC) is a mix of a fixed tower and a SALM. The tower holds the swivel and hoses above the likely point of impact with a ship. There is an articulated joint below water attached to a concrete base embedded in the bottom. This joint helps the mooring move in heavy wave action.
Determining which mooring application is right for a particular situation requires the ability to model the many factors that influence the ship and the mooring. A mooring analysis tool is essential in this process. It allows for quick and thorough modeling of a number of scenarios. Here at Stewart we use Orcaflex when we do a mooring analysis. This software package is the best marine engineering software package on the market today.