A permanent mooring is defined as a mooring that will provide station-keeping for a floating facility over a period of several years with only minimal maintenance. In most cases this will mean that the vessel will be engaged in floating production activities. The design period could be ten (10) years or more and may be in excess of twenty-five (25) years. Some on-going projects are considering 40 and 50 year design lives.
On the other hand, a temporary mooring is one that will see service for relatively short periods (weeks or months at a time), such as would be used with a MODU engaged with exploration drilling operations.
The differences between permanent and temporary moorings can be categorized as follows:
- Selection of criteria used in the design of the system
- Type and size of mooring components
- Types of mooring analyses performed
- Methods for performing installation
- Inspection and maintenance philosophy
In the US, according to accepted industry standards (API RP 2SK – Recommended Practice for Design and Analysis of Station Keeping Systems for Floating Structures), the design criteria selected for a permanent mooring system in survival conditions is recommended to be a 100-year return period storm. For a MODU a 5-year return period can be used if the vessel is to be moored away from other structures, and a 10-year storm should be used if it will be moored close to other structures. Generally, 10-year storm criteria are used most often.
Another major difference between temporary and permanent moorings involves the types of analyses performed during design. For temporary moorings, API RP 2SK recommends that strength analyses are performed to confirm that the system meets safety factor requirements to withstand 5 and 10-year storm conditions. Permanent moorings require strength analyses as well for 100-year conditions, but also require fatigue analyses to confirm that the mooring components will retain their strength for the longer 10 to 50-year design life.
The combination of more stringent design criteria and longer design life results in components of a permanent mooring system being larger and more resistant to corrosion and abrasion than a temporary mooring components – 3” chain is typically used for MODUs while 5” chain and larger is frequently used for an FPS.
Traditionally Anchor Handling Vessels (AHVs) have been used to install temporary moorings, while larger construction vessels are used to install heavier permanent mooring systems. However, in an effort to provide cost savings and added schedule flexibility during installation, InterMoor has pushed the limits of AHVs and used them to install permanent mooring components, such as for the Delta House FPS.
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