There are currently two types of chain in common use within the marine industry. Studlink chain, which is the most popular, is used by the shipping and oil industries. Studless chain, which has no studs, is generally used in special mooring applications such as permanent moorings for FPSO’s. Open link chain is typically used for marine moorings.
       
      Stud link chain, PDF data sheet                         Studless chain, PDF data sheet                                               

Chain size is generally expressed as the diameter of the steel at the bending area. This can mean that steel bars of 78-79mm may be used to manufacture chain of 76mm diameter. Chain can be fitted with open end links to enable
shackle connections to be made. These end links are normally forged to the chain using an intermediate link also known as an enlarged link. These links are larger than the diameter of the chain to take into account the differing
radii and the reduced strength of the links due the end link being studless. Chain strengths are expressed as grades followed by a number. The letter used varies with countries but the strength of the chain remains the same.
The United Kingdom uses “U”, France and Spain uses “Q” and the Scandinavian countries use “K”. The number relates to the type and hence the strength of the steel. U1 grade is mild steel, U2 is a high tensile steel and U3
is a special heat treated steel. These grades are normally only used within the shipping industry as the oil industry demands even greater strengths for the chain used. The original grade designed for the offshore industry was ORQ
(Oil Rig Quality). Although this chain is still in use it has been superseded by new grades such as Rig Quality 3 and Rig Quality 4. These grades were introduced by the classification societies in order to standardise quality. The
same grades also apply to the joining links that may be used with the chain. Tables showing the various strengths of chain are shown overleaf. The offshore industry dictates that chain must be periodically inspected for wear and defects. The level of inspection and the intervals of these surveys are laid down by the classification authorities. Viking Moorings carries out such inspections in line with relevant classification society requirements.