SAS Linear Chain Tensioners - Offshore Technology | Oil and Gas News and Market Analysis
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SAS Linear Chain Tensioners

Winch manufacture has been a staple of SAS’ product line since 1896; however, very recently a special division has been established within the company dedicated to the development, design and manufacture of winch systems for the marine and offshore markets, predominantly for heavy-duty deepwater deployment and mooring applications.

The latest addition to our already extensive marine and offshore product line gains us entry into the highly specialised world of offshore oil floating production platforms. This latest offering, specifically aimed at spread mooring systems aboard FPSOs and FPUs of all configurations, has just emerged from a year long research and development programme using a combination of existing technology and innovative design to develop our linear chain tensioner (LCT) range of mooring equipment products.

Our official introduction and launch of this equipment took place at OTC 2012 conference in Houston where a fully functional, quarter-scale prototype of a 120mm tensioner was on display and in full operation at the booth. Later this year the prototype will also be on display during the Rio Oil & Gas and the OSEA Singapore shows.

Linear chain tensioner

The main focus for this product is placed on providing uncomplicated function, reliability in operation, ruggedness of construction, simplicity in installation and maximised performance. We offer unique features like fully corrosion-resistant critical components, modular construction, and a custom designed footprint.

We have opted to raise the standards with a unique design basic model, fixed push-type chain tensioner and support this with a range of optional styles each offering a different solution to mooring chain installation and pre-tensioning applications:

  • Push style fixed stopper tensioner: Base product type is the most compact of all models installed above deck with no equipment penetration of, or access required below, the main deck. Mounting footprint can be configured to introduce mooring loads directly into the below deck support structure such that no additional or special gussets or webs are required
  • Inverted pull style fixed stopper tensioner: This type involves equipment penetration of the mooring deck and provision of a lower access maintenance gallery but requires much lower overhead clearance and mooring deck footprint
  • Movable pull style without an integral stopper: Operates in conjunction with a group of three to six deck-mounted fixed primary stoppers for a mooring line bundle. Most cost-effective option but requires overhead gantry and hoist to reposition the tensioner between mooring lines
  • Movable push style with integral secondary stopper: For use where separate external primary stoppers are installed – such as underwater combined stopper fairlead
  • Movable horizontal style tensioner with fixed stopper and attached overboarding chute: Primarily incorporated on true ship-shape hulls or in applications where it is preferred to have mooring chain departure directly from the main deck edge
  • Stand-alone horizontal fixed stopper with integral overboarding chute: Similar to above option but used in conjunction with a rotary drum installation and pre-tensioning winch with deck edge mooring chain departure. This style requires careful consideration of the chute design to accommodate the rope to chain attachment and angular displacement of the mooring line

Underwater swivel fairlead

Vertical departure mooring chains under tension are deflected from their path down the side shell of the hull out to their seabed anchors by a chain fairlead, installed in a permanently submerged position at the bottom end of the hull and close to the bilge radius on FPSOs, pontoons or the lower spar hull on FPUs.

As a natural companion for the LCT, SAS has adapted its existing line of underwater rope fairleads for chain applications such that a complete mooring equipment package can be offered.

The fairlead is a conventional flagging style unit, bracket mounted to the hull structure and fitted with a high-tensile cast-steel wildcat. It has a rope groove cast into the core so it can handle both wire rope and chain messenger lines.

The major attribute of the SAS Fairlead is its attention to out-of-plane bending (OPB) issues on both vertical and horizontal critically loaded chain links. Special consideration has been given to the design of the wildcat chain pocket to fully support the link and selection of the hinge pin journal bearing breakaway torque under load to minimise the side load required on the wildcat to rotate the flagging arm.

The use of seawater corrosion-resistant stainless steels for exposed components, composite journal bearing material and specially formulated paint coating is employed to protect the fairlead during its permanently submerged life span with minimal maintenance requirement.
Should maintenance or inspection be required the entire unit can be dismantled and re-assembled underwater using an ROV with or without diver assistance and while maintaining tension in the mooring line.

To facilitate in-position bearing condition analysis, visual or electronic (LVDT) bearing wear indicators can be included to extend between inspection periods that require fairlead removal to deck level.

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