Winches

Winches provide a way of modelling tension-controlled or speed-controlled winches. They connect two or more points in the model by a winch wire, fed from a winch inertia (typically representing a winch drum) which is driven by a winch drive (typically representing the hydraulics that drive the drum).

As well as connecting its two end points, the winch wire may, optionally, pass via intermediate points, in which case it does so as if passing over a small frictionless pulley at each point. The wire tension on either side of each intermediate point is then applied to that point; if the point is offset on the object involved then this gives rise to an applied moment.

Two types of winch are available.

Simple winches

Simple winches are perfect, in that we assume that the winch inertia is negligible and the winch drive is perfect, so that it always exactly achieves the requested constant tension or constant speed. No data are therefore required for winch inertia or winch drive.

Detailed winches

Detailed winches include modelling of the performance of the winch drive system – its deadband, stiffness, inertia, damping and drag. Consequently they require more data and are harder to set up.

We recommend using simple winches unless you know the characteristics of the winch drive system and believe that its performance significantly differs from the constant tension or speed ideal. In particular, simple winches are appropriate

• at the early design stage, when the type of winch to be used has not yet been decided
• if the duty is such that the winch drive will give near to perfect constant tension or constant speed performance
• if the winch drive data are not available.

Winch control

OrcaFlex winches allow quite complex offshore operations to be modelled. The winch drive can be operated in a number of different control modes which fall into two broad classes:

Length-controlled

For modelling constant speed winches. The winch wire is paid out or hauled in at a specified (and not necessarily constant) velocity.

Tension-controlled

Tension-controlled winches are usually hydraulic devices whose performance deviates quite seriously from the target tension ideal. OrcaFlex therefore provides facilities for modelling winch deadband, damping and drag forces (proportional to velocity and velocity squared respectively), and winch stiffness effects such as those caused by hydraulic accumulators.