SPAR MOORING

Mooring

Introduction & overview

Mooring systems are compliant systems. They provide resistance to environmental loading by deforming and activating reaction forces. Mooring systems work as spring mechanisms where displacement of the floater from a neutral equilibrium position causes a restoring force to react to the applied loading. The tension spring effect of mooring lines derives from two mechanisms:

  • hanging catenary effect - from gravity acting vertically on the line
  • line elastic effect - from elastic stretch over the length of the line.

Mooring systems with these two mechanisms are called catenary moorings and taut moorings, respectively.

Catenary mooring

Catenary moorings are defined by standard catenary formulations, which relate the following parameters: submerged weight of the suspended lines, horizontal mooring load, line tension and line slope at fairlead. The compliance to allow for wave-induced floater motions is ensured by a combination of geometrical change and axial elasticity of the lines. The large line geometrical changes make catenary mooring systems subject to significant dynamic effects due to transverse drag load. The mooring lines in catenary mooring systems are commonly composed of steel rope and chain segments. Sometimes clump weights and buoys are used to achieve the desired line configurations.

Taut mooring

In a taut mooring system the lines are nearly straight between the anchor and fairlead. The vertical forces are taken up as anchor and vessel reactions directly. The compliance to allow for wave-induced floater motions is provided mainly by line elasticity.

The transverse geometric changes in taut mooring systems are not as large as in catenary systems, thus dynamic effects due to transverse drag loads are moderate.

Synthetic ropes have recently been proposed and used as mooring lines in a taut mooring system to provide required elasticity and low weight. Compared to steel, synthetic ropes exhibit more complex stiffness characteristics (e.g. hysteresis), which may induce important dynamic effects.