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Code check DNV

Code check DNV#

Riggings are checked against the DNVGL-ST-N001 chapter 16

The following sections follow DNV chapters 16.2, 16.3 and 16.4 respectively to document how the code is implemented in DAVE.

Load factors#

The purpose of this section is to calculate the loads occuring on the rigging items. These are later compared against the allowable loads.

DAVE uses a physical model to accurately calculate the forces occurring in the rigging system components.

The governing loads are determined by applying a number of variations on the model, representing the uncertainties. Together this results in a set of load-cases. The magnitude of the variations depends on the level of uncertainty and is user input.

Weight contingencies and CoG#

The user shall define an CoG envelope and maximum weight to account for contingencies and inaccuracies.

Typically the Upper Bound Design Weight [5.6.2.2] shall be used in the DAVE model.

This results in a number of load-cases each of which is analyzed separately.

Tilt#

Section

Requirement

DAVE

16.2.2.2

For lifting operations carried out where the Centre of Gravity of the lifted object is above the lift points, care should be taken to ensure that the stability of the lifting arrangement is considered in the design. This is of particular concern where spreader bars or spreader frames are used as part of the lift system. Stability should be demonstrated for these conditions allowing for both vertical and horizontal offsets in the position of the Centre of Gravity.

checks and reports extreme lifted object inclinations

16.2.3.1

Object tilt due to CoG position and/or imposed horizontal loads (see [16.2.6.16] for possible causes of horizontal loads) will influence the sling load distribution for most rigging configurations. The effect of tilt should be considered in the load calculations where relevant

Physics fully included

16.2.3.2

The rigging geometry shall normally be configured so that the maximum tilt of the structure does not exceed 2° for level lifts, however see [16.2.3.4] for lifts at a known tilt. The sling angle should normally be as described in [16.3.4]. Where calculated maximum tilt is less than 2°, it is normally not necessary to consider related effects in the sling load calculations.

Tilt is checked and reported

16.2.3.3

Variable sling elongation, sling length and lift point fabrication tolerances could increase object tilt. Where lifting points are located below the vertical CoG of the object, forces in the most utilised slings will tend to increase due to sling elongation; in this case a suitable factor should be determined

Physics fully included

16.2.3.5

Where long slings are used and there are small distances between the lift points, the effect of the sling tolerance on new build slings is to be checked to ensure that excessive tilts are not introduced into the lifted structure.

Physics fully included

2-Hook lifts#

The requirements of mis-alignment between the two hooks depend on the type of operation. Typical values for both cranes on the same vessel are default. Values may be changed by user if required.

default

16.2.4.1

relative horizontal offset between cranes

0

16.2.4.2

hook elevation difference

+/- 1.0m

16.2.4.5

A yaw factor of at least 1.05 shall be applied

1.05

Dynamic amplification factors#

DAF is user-supplied. It is implemented as an increase of gravity. As such it acts on the lifted object as well as rigging and spreaderbars.

Guidance for applicable DAF factors is given in tables 16-1 and 16-2 of DNVGL-ST-N001.

Note: DAVE can do frequency domain analysis to aid in determination of an applicable DAF, but this is out of the scope of this document.

Skew load factor#

Skew loads are additional loading caused by rigging fabrication tolerances, fabrication tolerances of the lifted structure and other uncertainties with respect to asymmetry and associated force distribution in the rigging arrangement. The skew load factor (SKL) is a load distribution factor based on:

  • rigging length manufacturing tolerances,

  • sling/grommet measurement tolerances over measuring pins,

  • rigging arrangement and geometry,

  • fabrication tolerances for lift points,

  • sling/grommet elongation,

  • crane hook geometry,

  • Deflections of lifted object (see [16.8.6]).*

DAVE covers the skew load by varying the lengths of the rigging items and transforming those into separate load-cases. The magnitude of this variation is user-defined per rigging-system. A SKL of 1.0 is used in combination with the governing load-case. This in is line with 16.2.6.9 “the actual skew factor may be determined using a more detailed analysis allowing for actual rigging properties, extreme tolerances for new build rigging and hook rotation.”.

Guidance on the magnitude of the variations is as follows:

  • between slings of a matched pair: 0.5d

  • general length tolerance: 1.5d

The stiffness of the sling/grommet is available in the model.

Special loads#

Special loads (tuggers, etc) shall be modelled in the DAVE model if applicable. For example using Force nodes.

Sling load distribution#

A 45% / 55% load distribution is applied for slings/grommets running over an intermediate item such as a hook or grommet protector. This is applied automatically.

Option for the user to override to any different distribution on an sling-by-sling basis to cover:

  • 49:51% for “greased sheave on a trunnion”. [16.2.8.1]

  • 32.5:67.5% for “Where upending a structure requires the doubled sling or grommet to slide over a trunnion or crane hook” [16.2.8.2]

  • “Where slings are used in any more than a double configuration e.g. doubled-doubled or grommets are used doubled, calculations to justify the arrangement shall be documented.” [16.2.8.3] . This may be the case for grommets doubled over grommet protectors.

  • Fibre slings [16.2.8.5]

Note that 16.2.8.4 “a lift rigging includes two parallel slings (i.e. two slings connected between the same lift points)” is covered by DAVE.

Derivation of hook, lift point and rigging loads [16.3]#

Hook loads#

The Static Hook Load (SHL) and Dynamic Hook Load (DHS = SHL x DAF) are calculated by DAVE for each of the load-cases.

This means that all variations, including CoG envelopes, tilt, hook elevation for multi-hook lifts, are included.

The calculated hookload includes the weight of all applicable rigging items [16.3.2.6]

16.3.2.7 - “For lifting operations involving pivoting and/or upending manoeuvres” it is advised to use the DAVE timeline feature to find the governing cases of the operation and then preform an analysis on those steps.

16.3.2.8 - The crane capacity is user supplied, ref [16.7.3] and refers to the allowable hook load including dynamics and excluding the weight of the crane block itself. It is checked by DAVE against the DHL.

Liftpoint loads#

Governing liftpoint loads are obtained for the governing over all the load-cases. This means that:

  • cog envelopes [16.3.3.1],

  • applicable effects and factors including DAF [16.3.3.2],

  • elevation effects and hook and geometry [16.3.3.3]

are fully included.

Effects of friction on the liftpoint loads [16.3.3.4] are not calculated by DAVE and should, if applicable, be accounted for separately

If LiftPoint nodes are used (typical for padeyes and trunnions) then the full liftpoint loads (force, force components and moments) are reported at the basis of the liftpoint. Otherwise the loads and moment at the center of the connection point (the center of padeye hole or center of the circular trunnion surface) are used.

Reported values are:

  • minimum and maximum load [kN]

  • minimum and maximum load components in all three directions (relative to liftpoint reference frame) [kN]

  • minimum and maximum angles [deg] in vertical and sidelead directions.

Sling loads#

The sling design load \(F_{SD}\) is calculated by DAVE for each of the load-cases. Where applicable the user-defined load-distribution factor is applied to increase the force.

All other effects [16.3.4.1, 16.3.4.2, 16.3.4.3, 16.3.4.5] are intrinsically included with the rare exception of an accurate representation of the self-weight of a sling/grommet if it is loaded with less than 10 times its self-weight (for example catenaries).

Allowable loads#

The allowable load on a rigging component (resistance) is checked against the occuring loads (loads). The fraction between the two results in a unity-check (UC) which should be below 1.00

Slings/grommets#

DNVGL-ST-N001 section 16.4

The unity-check of the grommet/sling is calculated as

\(UC = F_{sd} / F_{allowed} = {F_{sd} \over {MBL / \gamma_{sf}}} < 1.0\)

Where \(\gamma_{sf}\) is the nominal safety factor is the product of the partial safety factors summarized below:

partial safety factor

default

\(\gamma_f\)

lifting factor

1.3
may be reduced to 1.2 if skew-loads are accounted for accurately and dynamics < statics

1.2

\(\gamma_c\)

consequence factor

1.3 (can be lowered if consequences of failure are negligible)

1.3

\(\gamma_m\)

material factor

steel >= 1.5
(as)-new steel: >= 1.35
Polyester: 1.65
HPME and aramid: 2.0
Other fibre materials: 2.5

1.5

\(\gamma_f \cdot \gamma_c \cdot \gamma_m\)

combination

should be at least 2.3

(2.34)

\(\gamma_r\)

reduction factor

the largest of \(\gamma_s\) and \(\gamma_b\)

( \(\gamma_s\) )

termination factor

Cable laid slings

1.33

Otherwise: Grommets, Fiber (incorporated in MBL),Swage fittings

1.00

( \(\gamma_b\) )

bending factor

\(1 \over {{1-{0.5 \over {sqrt(D/d)}}}}\)

calculated automatically (note)

\(\gamma_w\)

wear and application factor

steel: 1.0 ; >=1.1 for re-use without thorough inspection

1.00

note on bending factor:

  • For synthetic slings and grommets the bending factors is considered to be included in the MBL. Bending factor is not applied if the material type name contains “HPME”.

Shackles#

The allowable load on a shackle is the lowest of the following:

  • WLL x DAF

  • Shackle MBL / 3.0

  • Documented proof load value for shackle.

Both MBL and WLL are defined in DAVEs standard shackle database. So no additional user input is required as long as standard shackles are used.

If a non-standard shackle is used then it needs to be added to the database.

Notes:

  • For GP shackles the SF = 5.0 so WLL x DAF will be governing.

  • For Crosby the design factor (design factor: ratio between nominal or minimum breaking strength and rated load of the rigging hardware, ASME B30) depends on the size.

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