Friction calculations

DNVGL-ST-N001 prescribes that friction needs to be considered:

“When appropriate, allowances for special loads should be made in the derivation of loads on the lifted structure, lift points and rigging system. Examples of special loads include tugger line loads, guide loads, wind loads, hydrostatic loads, hydrodynamic loads, suction loads, friction loads etc.” [16.2.7.1]

Section 16.2.8 then continues to describe the effect of friction on a doubled slings or grommets.

DAVE’s rigging module provides two options to incorporate friction effects in rigging systems.

1. Global

   • further reading: incluincluding_friction_in_loadcases and friction in cables

2. Local

   • further reading: Local_friction_calcualation

The global option enforces friction on a connection and then re-solves the complete system:

The local option only applies a factor on the load in the item that encounters the friction. In the example above that would mean that a load-increase factor is applied on the grommet, but not on the shackle.

When to use what

Use global if you can, use local if you must.

The global option is clearly more advanced and physically correct. If it works then it is definitely the preferred option. However there are plenty of situations where the global option either becomes too computationally expensive (too many load-cases) or results in situations that are physically not possible. A common example of when global friction can not be used when there is insufficient resistance or stability in a system to absorb the friction forces:

Comparison

Two doubled grommets

Grommet #1 using global friction

Grommet #2 using local friction

The resulting UC ins grommet #1 is slightly higher because the global method considers the actual turn (which is slightly larger than 180 degrees) while the local method just uses a factor of 1.1 regardless of the turn.

How to define

Which method to use can be defined per sling or grommet. This is done in the rigging analysis section of the report.

For local there are the following options:

- Default (for 45%/55%)
- Custom factor (for other configurations)

For global the friction factor can be defined for each of the connections

Scripting

Friction configuration can be defined for a RiggingAnalysisSection. It is defined as a dictionary where the key is the name of the SlingGrommet node and the value defines how friction is to be modelled. Value can be:

Value	Meaning
None	default, use Global
-1	use default local configuration
single number >= 0	use local configuration with given number as custom factor
tuple, list	global using custom coefficients