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

  2. Local

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

image-20230927161655649

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:

image-20231013143457796

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.

image-20231013170027674

image-20231013170006083

image-20231013165933796

image-20231013170117078

image-20231013170129032

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