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Comparison of Turntable vs. Turret-Style Catenary Anchor Leg Mooring (CALM)

Published on 10 Jan 2018 by Brysen Lin


The turntable-style CALM buoy was originally developed by Shell Oil Company.  Various patents prevented others (without a license) from using this design.  The turret-style CALM buoy was primarily conceived as an alternative to “get around” the Shell patents. There were NO real benefits or advantages with a turret-style buoy when compared to a turntable-style buoy.

Today, all operators accept a turntable-style CALM buoy.  Not all operators accept a turret-style CALM buoy, and they tend to be favored by smaller, less experienced operators.


Turret-Style Buoys Turntable-Style Buoys
Summary
  • Developed as an alternative, since the mid-1980’s, to “get around” the Shell patents
  • Less than 25 have been installed to date
  • Marketed only by Bluewater.
  • Originally developed by Shell in the 1950's
  • Over 300 have been installed worldwide (with a majority of these still working today)
  • Marketed by SOFEC and Imodco.
Hose Wrapping

High breakout torque is required to rotate the entire buoy due to larger rotational bearing friction (due to higher loads) plus the added mass due to a square shaped buoy.  This causes wrapping of the floating hose strings around the buoy resulting in:

  • increased operating costs (to unwrap)
  • increased flexing (stress) in the first-off-the-buoy floating hose lengths, accelerated hose wear, and potential hose failures
  • increased fatigue in the buoy's overboard piping
The three row roller bearing rotates with very little rotational drag and no rotational drag from added mass.  The inherent design of a turntable-style buoy provides for free rotation (low break-out and running torques) achieved entirely by the floating hose strings as they respond to changes in direction of the surface current, wind and waves. Therefore, the floating hoses easily rotate the turntable and never wrap around the buoy.
Main Bearing Vulnerability

Main bearing under constant loading from anchor legs and continuous dynamic loading due to motions of the buoy created by wind, wave and current.   A square buoy is also subject to increased forces during survival sea states due to higher drag and wave slamming.

The overall system has a larger mass (due to the large mass of the buoy body and deckhouse, plus the large added mass from the water around the square buoy body), resulting in larger hawser loads which will reduce the life of the hawser and main bearing.

Every time a workboat, service vessel, or tanker touches any part of the buoy, the collision loads are transferred directly into the bearing.

The main bearing can not be replaced in-situ. The entire system must be removed and returned to the shipyard.

Main bearing experiences smaller loads and is only dynamically loaded when a tanker is moored.

The bearing on turntable-style buoy is conservatively designed for radial loads in excess of 350 metric tons.  The hawser loads are actually insignificant when considering bearing wear.

Collision loading is transferred directly from the buoy to the anchor legs.  The main bearing is not in the load path.  If the collision occurs directly on the boat landing, the turntable will naturally rotate, dispersing all collision loading.

The main bearing can be fully accessed, inspected and replaced in-situ.  It is enclosed inside a sealed bearing housing filled with grease.  The entire housing can be replaced so that the bearing is never exposed to seawater.

The main bearing can be fully accessed, inspected and replaced in-situ.  It is enclosed inside a sealed bearing housing filled with grease.  The entire housing can be replaced so that the bearing is never exposed to seawater.

Buoy Motions
The lower center of gravity causes higher pitch and roll motions and fatigue loading on the anchor chain, chain stoppers, piping and floating hose.
The higher center of gravity results in less fatigue to system and less stress to operating personnel.  This promotes a longer in-water service life and more pleasant working conditions.
Deck-house Arrangement

Enclosed deck-house creates a “confined space” hazard with a potential for gas accumulation.  Most modern industrial facilities (refineries, petrochemical plants, marine and offshore facilities) have a HSE system that requires a “Confined Space Entry” work permit to enter confined spaces where the potential for a hydrocarbon leak exists.  Any prudent operator of a turret-style buoy will also require this permit system.

Equipment contained in an enclosed area (where hydrocarbons may be present) require explosion proof electrical fittings, gas detection sensors, and/or forced draft ventilation.

Difficult to perform maintenance, repair or replacement activities in such confined spaces.

All equipment is naturally ventilated.  No work permits are required to board the buoy.

All equipment is designed suitable for an offshore marine environment.  No ventilation or inordinate protection is required.

Wide open and clear turntable area permits easy access for maintenance, etc.

Chain Tensioning & Re-tensioning

The chain stoppers are located underneath the buoy in a dangerous location, requiring divers to work closely below the pitching/rolling buoy.

The buoy must be locked to maintain precise heading control during tensioning and re-tensioning through the guide tube.

A turntable type design accommodates anchor leg attachments which are easily accessed by a winch located on the turntable or an external crane vessel.

Installation, anchor re-tensioning or buoy removal can be performed by surface divers at the hull perimeter.  The occupational safety hazards are therefore reduced significantly as compared to an underbuoy chain table.

Buoy Integrity
Collisions on the sides or near the corners of a square buoy will cause buckling or splitting at the corners of the square hull.
A round buoy is inherently more robust, more efficient, and more reliable (damage resistant) compared to a square buoy.   Collision loads are naturally distributed around the circumference through the reinforced skirt and shell plating.
Annular Space
Marine growth can gather between the turret and the buoy hull, causing restricted movement of the buoy.  The marine growth can not be removed without removing the buoy. The annular space can also not be inspected or repainted.
All marine growth is on external exposed surfaces and does not interfere with the function of the bearing or buoy.
Catastrophic Failure • August 1998
• Operator: Cairn Energy India Pty Limited
• Main Contractor: HHI
• Ravva Oil Field off Eastern India coast (35,000 BOPD)
• Total failure of turret-style CALM buoy after less than 2 years in service
• The Operator and an independent third-party concluded that the main bearing had failed
• First known catastrophic failure of a marine terminal




Tags: CALM Catenary Anchor Leg Mooring Turntable Turret

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