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Technical Information

CALM Buoy and Hawser Off-take Systems

Key

  • 1. CALM/FPSO Chain Chafe
  • 2. Hawser Shackle
  • 3. Mooring Hawser
  • 4. Shuttle Tanker Chafe Chain
  • 5. Chain Support Buoy Arrangement
  • 6. Pick-Up Buoy Rope
  • 7. Pick-Up Buoy Arrangement

Pick Up / Messenger Rope

The pick-up rope is connected to the ship end of the chafe chain and typically consists of 150 metres of floating rope complete with an eye at each end. (See page 12 for further details).

The rope can vary in length from 120-180 metres, and in diameter from 64-80mm. At some terminals where the pick-up rope is not kept connected to the chafe chain when the berth is unoccupied, differing arrangements may be employed to facilitate connection / disconnection of the pick-up rope.

Chafe Chains

Mooring hawsers should terminate with a chafe chain at its shipboard end. The recommended 76mm size has been established based on the diameter of the material forming the common stud links. (See page 15 for further details).

Selection of the appropriate chain should take into account the SPM mooring arrangement, SWL required and the properties of the chain grade selected. Typically chafe chains are approx. 8 metres in length, composed of 76mm stud link chain.

If Chain-Trough Buoys are used the length of the chain may have to be increased. Each chain should terminate at the shipboard end with an oblong plate for connecting the chain to the pickup rope bow shackle.

Weak Links

Weak links, if fitted, should be selected such that the recommended bow chain stopper, chafe chain, hawser or connection to the SPM do not constitute the weakest yield strength of MBL component of the entire system.

Weak links, if fitted, should be designed, manufactured and tested under a certification scheme. Bridon can provide guidance on inspection criteria.

CALM Buoy to FPSO Side

Mooring Hawsers

When selecting hawsers, it is critical to account for not only strength but also energy absorption and fatigue performance (OCIMF Thousand Cycle Load Level).

Further information is available in the OCIMF 2000 ‘Guidelines for the Purchasing and Testing of SPM Hawsers’.

The NWBS (New Wet Break Strength), energy absorption and fatigue performance of hawsers deteriorate during service under the influence of factors such as cyclic load history, hawser type, construction, environmental conditions, damage and stowage arrangements between use. These factors should be taken into account when determining the appropriate hawser for the mooring system and hawser retirement criteria.

It is recommended that terminal operators develop hawser retirement procedures based on a combination of manufacturer’s recommendations, their own experience at the location and the results of hawser testing.

Service data gained using hawser load monitoring equipment can be valuable in this respect.

Support Buoys

When the berth is unoccupied, each chafe chain may be supported by flotation devices. One method is to use a swivel-ended type support buoy that is connected by a short length of chain to the end link of the chafe chain, adjacent to the hawser.

Another method is to use a through-type chain support buoy. Support buoys should have reserve buoyancy equivalent to at least 20% of the weight in air of the material to be supported.