Differences between permanent and temporary moorings

Permanent vs. temporary moorings

A permanent mooring is defined as a mooring system 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 floating vessel will be engaged in energy production activities. The design period could be ten years or more and may be more than 25 years. Some ongoing projects are considering 40 – 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). A Mobile Offshore Drilling Unit (MODU) engaged with exploration drilling operations uses temporary moorings. 

Outside the service period, the differences between permanent and temporary moorings can also be categorised 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

Mooring standards and criteria

In the US, accepted industry standards are 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 five-year return period can be used if the vessel is to be moored away from other structures. 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.

Although an international standard^[1] is currently being developed, permanent moorings for Floating Offshore Wind are designed to 50-year return period metocean conditions, which is different than the 100-year return period design requirement for oil and gas permanent moorings.

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 for 100-year conditions. They 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 temporary mooring components. MODUs typically use 3” (76 mm) mooring chain while 6” (152 mm) mooring chain is frequently used for a floating production storage (FPS). For floating wind turbines, the mooring chain is significantly larger and heavier at 8.6” (220 mm) in diameter. 

Traditionally, Anchor Handling Vessels (AHVs) have been used to install temporary moorings. 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, Acteon’s Moorings and Anchors business line, has pushed the limits of AHVs and used them to install permanent mooring components, such as for the Delta House FPS.

[1] The current DNV standards are used: DNV-RU-OU-0512 Floating Wind Installations; DNV-ST-0119 Floating Wind Turbine Structures; DNV-SE-0422 Certification of floating wind turbines; DNV-RP-0286 Coupled analysis of floating wind turbines.