Design Guidance for Lifting Systems - McDermott - updated Sept2011

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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Design Guidance for Lifting Systems – Extracted from McDermott In-house Guide
This file summarises relevant information for the rigging arrangement, sling load calculation,
shackle selection and proportioning of simple padeye. More details can be found in other
specifications and recommendations, such as API RP2A, Offshore 93 papers.
The figure below shows the overall lifting arrangement for a deck module to be installed onto
the jacket structure. As in many project examples, it should be remembered that the engineer
needs to ensure that there is no weak link in the lift system.
The following sections summarise the associated considerations.
Symbols
DAF Dynamic Amplication Factor
DHL Dynamic Hook Load
MBL Minimum Breaking Load
SKL Skew Load Factor
SWL Safe Working Load

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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1. Loads
1.1 General
The lifting system shall be designed for actual load to be lifted modified by appropriate
load factors. The various components of the system shall be checked against the safe
working load and/or allowable stresses for the particular materials used.
1.2 Weight Contigency Factor
Weights of items to be lifted should be calculated using a recognized weight control
procedure. For an un-weighed structure, a weight contingency factor (CF) of 1.1 shall
be applied to the calculated weight (W).
Gross Weight = calculated weight x contingency factor = W x CF
The weight should be considered to act at the most unfavorable centre of gravity
position.
Where it is desired to reduce the contingency factor by using the results of weighing the
structure, the weighing procedure and results shall be submitted for review. A
weighing factor of 1.03 shall be applied to the weighed weight.
Gross Weight = Weighed weight x weighing factor
1.3 Dynamic Amplification Factor
For lifts by a single vessel, the following dynamic amplification factors (DAF) shall be
applied.
Gross Weight (tonne)
Offshore
Onshore*
<100
1.30
1.15
100 - 1000
1.20
1.10
1000 - 2500
1.15
1.05
>2500
1.10
1.05
* Applicable also for lifts from deck of derrick barge
Lift weight = gross weight x DAF = W x CF x DAF
For offshore lifts by 2 or more vessels, lift weight as computed above shall be
multiplied by a further DAF of 1.1.
1.4 Hook Load
In general, when considering the loading on a padeye or the structure, the lift weight as
defined above should be used. Loads in slings, and the total loading on the crane
should be based on hook load, where:
Hook Load = Lift weight + rigging weight = W x CF x DAF + RW

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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Rigging weight includes all items between the padeyes and the crane hook, including
slings, shackles and spreaders as appropriate.
1.5 Padeye Resolved Lift Weight
The padeye resolved lift weight (PRLW) is the vertical load at each padeye, taking into
account lift weight and centre of gravity only.
Where allowable centre of gravity position is specified as a cruciform or other
geometric shape, then the most conservative centre of gravity position within the
allowable area should be taken until the position can be determined with confidence.
Padeye resolved lift weight (PRLW)
= Resolved lift weight for CoG (λW) x CF x DAF
= λW x CF x DAF
1.6 Skew Load Factor (SKL)
Skew load is the extra loading in slings caused by effect of inaccurate sling lengths and
other uncertainties with respect to force distribution in the slinging arrangement.
For indeterminate 4-sling lifts using matched slings, a skew load factor (SKL) of 1.25
shall be applied to each diagonally opposite pair of lift points in turn. For determinate
lifts, the SKL may be taken to be 1.0 for matched slings.
The skew load factor of 1.25 is representative of a 3” variation between shortest and
longest sling in accordance with API RP2A. If the fabrication and sling tolerances
exceed these limits a detailed analysis taking into account these tolerances should be
performed.
Vertical padeye load = padeye resolved lift weight x SKL
= λW x CF x DAF x SKL
1.7 Resolved Padeye Load
The resolved padeye load is the vertical padeye load divided by the sine of the sling
angle:
(vertical padeye load)
Resolved padeye load= sin (sling angle)
λW x CF x DAF x SKL
sinθ
=
where sling angle is angle between sling and horizontal plane prior to lifting and
rotation of the structure.

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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1.8 Lateral Padeye Load
Provided the lift point is correctly orientated with the sling direction, then a horizontal
force equal to 5% of resolved padeye load shall be applied, acting through the
centerline and along the axis of the pin-hole or trunnion.
If the lift point is not correctly orientated with the sling direction, then the computed
force acting along the axis of the pin-hole or trunnon plus 5% of the resolved padeye
load shall be used.
1.9 Sling Force
The sling force is the vertical padeye load plus the sling weight (per sling) divided by
the sine of the sling angle.
(vertical padeye load+sling weight)
Sling force = sin (sling angle)
λW x CF x DAF x SKL + SLW
sinθ
=
1.10 Skew Load factor (SKL) for Multi-Hook Lifts
For multi-hook lifts, the total skew load factor should be the SKL calculated in
accordance with Clause 1.6, which accounts for fabrication tolerances, multiplied by
the yaw effort factor and the tilt effect factor.
The yaw effect factor of 1.05 should be used to account for increased sling loading due
to rotation of the object about a vertical axis.
The tilt effect should account for the increased sling loading caused by rotation of the
object about a horizontal axis and the effect of longitudinal deviation of the hooks from
their theoretical positions. The tilt effect factor may be calculated for a tilt of 3 degrees
when the cranes are on the same vessel, and for a tilt of 5 degree when the cranes are on
separate vessels (longitudinal deviation of the hooks included).
1.11 2-Part (doubled) Sling Factor
Where a 2-part sling (doubled sling) passes over, round or through a shackle, trunnion,
padear or crane hook, other than at a termination, the total sling force shall be
distributed into each part in the ratio of 55:45. The application of the 55/45 distribution
will cause a moment which must be considered in the design of the lift point. This ratio
is also applicable to slings attached to sister plates and clevis plates in lifting aids.
Sling load = Sling force x 0.55 (for 2-part slings)

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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1.12 Bending Efficiency Factor
Where any rope is bent round a shackle, trunnion, padear or crane hook, the break load
shall be assumed to be the calculated rope breaking load multiplied by a bending
efficiency factor.
0.5
Bending efficiency factor=1- pin diameter rope diameter
This is outlined in the following table.
Pin diameter/Rope diameter
<0.8
0.8
0.9
1.0
1.5
2.0
3.0
4.0
5.0
Bending Efficiency Factor
Not
advised
0.44
0.47
0.50
0.59
0.65
0.71
0.75
0.78
Sling breaking load = rope breaking load x bending efficiency factor
It should be noted that termination and bending factors should not both be applied, the
one which results in the lower value of breaking load will govern and should be used.
1.13 Grommets
Grommets require special consideration to ensure that the rope breaking load and
bending efficiency have been correctly taken into account.
The core of a grommet should be discounted when computing breaking load. The
breaking load of each part of a grommet is therefore usually taken as 6 times the unit
rope breaking load, after spinning reduction.
Grommet BL = 6 x rope breaking load
Typically a grommet will be used with one end over the crane hook, and the other end
connected to a padeye by a shackle. The bending efficiency factors at each end may
differ, and the more severe value should be taken. The total breaking load of the
grommet used in this manner is:
2 x 6 x (lower bending efficiency factor) x rope breaking load
1.14 Safety Factors
The minimum safety factor to calculate breaking load in a sling or grommet, after
! Resolution of load based on centre of gravity position and sling angle
! Consideration of the factors shown in Sections 1.1 to 1.11 as appropriate, and
! Consideration of the governing efficiency factor from Section 1.12
Shall be not less than 3.0 for new slings, and not less than 4.0 for older slings.