Design Guidance for Lifting Systems - McDermott - updated Sept2011

Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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
DHL
MBL
SKL
SWL
Dynamic Amplication Factor
Dynamic Hook Load
Minimum Breaking Load
Skew Load Factor
Safe Working Load
1
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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
<100
1.30
100 - 1000
1.20
1000 - 2500
1.15
>2500
1.10
* Applicable also for lifts from deck of derrick barge
Onshore*
1.15
1.10
1.05
1.05
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
2
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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:
Resolved padeye load=
=
(vertical 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.
3
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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)
sin (sling angle)
λW x CF x DAF x SKL + SLW
=
sinθ
Sling force =
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)
4
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
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=1pin diameter rope diameter
This is outlined in the following table.
Pin diameter/Rope diameter
Bending Efficiency Factor
<0.8
Not
advised
0.8
0.44
0.9
0.47
1.0
0.50
1.5
0.59
2.0
0.65
3.0
0.71
4.0
0.75
5.0
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.
5
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
1.15
Sling Manufacturing Tolerances
The manufacture of slings and grommets should be to a recognized code or standard,
International Standard ISO 2408 and for cable laid slings Guidance Notes PM20.
The length of the slings or grommets should normally be within tolerances of plus or
minus 0.25% of the nominal length. During measurement a tension load of 2.5 – 5.0%
of the minimum breaking load should be applied. A matching set of slings will satisfy
the above criteria.
1.16
Consequence Factors
The following consequence factors (CNF) shall be further applied to the structure,
including the lift points and their attachments into the structure:
Lift points including spreaders
Attachments of lift points to structure
Members directly supporting or framing into the lift
points
Other structural members
Offshore
1.35
1.35
1.15
Onshore
1.15
1.15
1.10
1.00
1.00
The above factors shall be applied to the forces used for component design which are
based on the calculated lift point loads after consideration of all the factors shown in
Sections 1.1 through 4.11.
Lift point design load=Resolved padeye load x consequnce factor
λW x CF x DAF x SKL x CNF
=
sinθ
Examples:
For a 3000 tonne module lifted offshore
CF = 1.1
DAF = 1.1
SKL =1.25
CNF= 1.35
λW
Lift point design load =
x 1.1 x 1.1 x 1.25 x 1.35
sinθ
λW
=
x 2.04
sinθ
For a 1000 tonne module lifted offshore
6
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
CF = 1.1
DAF = 1.2
SKL =1.25
CNF= 1.35
λW
x 1.1 x 1.2 x 1.25 x 1.35
sinθ
λW
=
x 2.23
sinθ
Lift point design load =
NOTE: Recommended sling angle θ ≥ 60 degrees.
2.
Sling Selection
Level of Package
The majority of erectors prefer level or nearly level lifts (rule of thumb, maximum outof- level 2.5o across a diagonal). A package will lift such that it will rotate until its
Centre of Gravity (C.G.) is beneath the hook of the crane. Knowing the length of
slings, vertical position of the package C.G., and package geometry, by simple
triangulation, the out of level from one corner to another can be determined.
If equal length slings are used, the slings will align to a point equidistant from the
lifting eye positions. Note this must not be confused with the geometric centre of the
package or the Centre of Gravity. The angle between sling and top of package must
not be less than 45o.
If the lift is unacceptably out of level, then the sling lengths can be modified to bring
the hook close to or directly above the C.G. so that the package lifts level. This can be
accomplished by specifying individual lengths for each sling. It is difficult to fabricate
slings all of the same length. The resulting inequality will produce an uneven
distribution of load. Slings are normally purchased as matching sets, so that they can
be reused on other projects.
If out of level must be adjusted, this should be accomplished by using pairs of equal
length slings which will in effect reduce out of level in one direction only. The length
of slings can be adjusted by means of link plates or shackle to shackle connection.
(Shackle to shackle connection only to be used after discussion with the erector).
Out-of-plane Loading on Padeye
The sling and lifting eye should be aligned in the same plane and a careful check may
be required on out of plane bending of the lifting eye if pairs of different length slings
are used or the lifting eyes point to their (not package) geometric centre, or the
dimension of the crane hook is large. Large eyes should be designed for out of plane
bending.
7
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
8
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
2.3
Practical Fabricated Sling Lengths
In order to design the rigging system, and estimate the weight of the slings, it is
necessary to know the minimum practical fabricated length of the slings, which is
governed by the lengths of the eye, the splice and the body between the termination of
the splices. For hand spliced cable laid sleeve, their lengths in terms of norminal sling
diameter, are given below. Note that two values are given for each part, normal
minimum lengths are represented by the lower values but the higher values are
currently used by manufacturers for large diameter slings (D greater than 12 inches).
Lower
Higher
Eye
Splice
Min.
Body
Splice
Eye
12D
16D
27D
30D
15D
15D
27D
30D
12D
16D
Min. Total
Length of
Sling
93D
107D
9
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
3.
Shackle Selection
The calculated sling load as defined in Section 1.9 will define the required shackle safe
working load. The calculated safe working load must be referred to standard Green Pin
or McKissick (Crosby) or other manufacturers tables from which will be selected the
shacle to be used for the lift.
Example: calculated sling load 275 tonne, use 300 tonne SWL shackle.
Careful reference should also be given to Section 1.12 bend radius of sling which may
dictate a larger shackle than that selected from load consideration alone.
10
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
11
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
4.
Structural Calculations
Load Cases and Structural Modelling
Structural calculations, based on load factors discussed in previous sections, shall
include adequate load cases to justify the structure. For example, for an indeterminate,
4-point lift, the following load cases should normally be considered.
§
§
§
§
§
Base case, using lift weight, resolved to the lift points, but with no skew load factor
Lift weight, with skew load factor applied to one diagonal
Lift weight, with skew load factor applied to the other diagonal
Lift weight with 75 mm tolerance applied to one diagonal set of slings
Lift weight with 75 mm tolerance applied to the other diagonal set of slings
In all cases, the correct sling angle and line of action, and any offset or torsional loading
imposed by the slings shall be considered.
Structure
The overall structure shall be analysed for the loadings indicated in Section 4.1
The primary supporting members shall be analysed using the most severe loading
resulting from Section 4.1, with a design factor of 1.15 applied (see Section 1.16)
Lift Points
An analysis of the lift point and attachments to the structure shall be performed using
the most severe load resulting from Section 4.1, and a design factor of 1.35 (see Section
1.16). A 5% side load should also be applied, as should any torsional load resulting
from the 55:45 2-part sling loading.
Where the lift point forms a structural node, then the calculations shall also include the
loads imposed by the members framing into it.
Spreader Bars or Frames
Spreader bars or frames, if used, should be similarly treated with load cases as above,
and consequence factors in accordance with Section 1.16. The effect of fabricator
tolerance in set up should be included.
5.
Lift Eye (Padeye or Trunnion) Design
5.1
Positioning of Lifting Eyes
Lifting eyes should be positioned where possible at or near the main nodes of the
structure so that load can be distributed into the main framing and thus hopefully reduce
or eliminate additional reinforcement for the lift phase.
12
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
The lifting eyes should be located as symmetrically as possible about the Centre of
Gravity position. The distance between lifting eyes should not be less than half the
dimension of the total module to avoid a significant sling force increase by a small
offset of the Centre of Gravity.
5.2
Permissible Stresses
The lifting eye will be checked against AISC permissible stresses.
increase will be allowed.
Tension
Compression
Bending
Shear
Combined
Bearing
Hertz Stress (bearing stress at pin hole)
Averaged tension at section through pin hole
No one-third
0.6Fy
0.6Fy
0.6Fy
0.4Fy
0.75Fy
0.9Fy
2.0Fy
0.45Fy
For through thickness loadings, the allowable stress should be reduced to 0.2 Fy, where
Fy is the minimum yield stress of the material.
5.3
Attachment to Structure
The lifting eye main plate will be checked in a manner similar to that expounded in text
books for typical beam to beam connections (see AISC and Steel Designer’s manual for
typical examples), checking effects of shear and bending. Maximum shear stress rather
than average shear stress shall be used. The design of lifting eye involving transfer of
load “through the thickness” of plates should be avoided. If unavoidable, the allowable
stress will be reduced to conform to Section 5.2.
In the following sections, a simple padeye design is illustrated with associated checks
highlighted. The geometric parameters in the figure are:
P = design sling load
Pc= cheek plate load
D = diameter of lifting eye hole
d = diameter of shackle pin
T = thickness of main plate
t = thickness of cheek plate
R = Radius of main plate
r = radius of cheek plate
W = jaw width of shackle
a = thickness of main plate + cheek plates = T + 2t
13
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
5.4
Hole Size
The hole size will be larger than the shackle pin or link plate to be used depending upon
pin size. Use 3mm for diameter up to 50 mm, and 6mm up to 120 mm diameters, and
4% for larger diameters up to 12 mm maximum.
D = d + (3 to 6 mm)
14
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
5.5
Bearing Stresses
Bearing stresses will be calculated by dividing the design lifting eye load by the pin
diameter multiplied by total thickness of cheek plate and lifting eye main plate. If the
clearance of the pin is greater than the limits specified in 5.4, the stresses should be
checked according to the Hertz formula.
Bearing stress = f p =
5.6
P
D ( T+2t )
Cheek Plate Selection
Cheek plates should be selected to provide a nominal 12 mm (0.5”) clearance between
cheek plate and jaw opening of shackle (i.e. 0.25” each side). Cheek plates should not
exceed main plate thickness. Centralisers should be added where appropriate to satisfy
these requirements.
The load carried by a cheek plate should be proportioned directly according to total
cheek plate plus lifting eye plate thickness. The circumferential weld between cheek
plate and lifting eye plate will be sized by dividing the proportioned load by half the
circumference of the cheek plate.
Where the width of the main plate and check plates < 0.8 of the shackle jaw opening,
the capacity of the shackle should be de-rated (to 50% capacity for mid-span point
load).
W = a + 12 mm
Load in cheek plate = Pc =
Pt
T+2t
Stress in cheek plate weld=
5.7
Pc
πr
Pull-out Shear
Pull out shear should be checked by dividing the design lifting eye load by twice the
total cross sectional area of metal between hole and outer edge of lifting eye including
cheek plate.
Pull out shear stress = fps = (P/2) / {2t (r-0.5D) + T(R-0.5D)}
5.8
Tension Failure
The lifting eye should be checked against tension failure in any plane within or outside
the cheek plate area.
tension stress = f t =
P
failure length x T
15
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
5.9
Side Load
Lifting eye design should allow for a side load of magnitude 5% of the design lifting
eye load acting at the centre of the pin hole. If there is a horizontal component of sling
load acting normal to the main padeye plate, then this is to be added, plus 5%.
5.10
Edge Distance
The minimum distance from the centre of any hole to the edge of the lifting eye plate
shall be 1.25D, where D is the hole diameter, or 75 mm between edge of hole to edge of
lifting eye plate, whichever is larger.
R ≥ 1.25D
5.11
Combined Stresses
A von Mises yield criterion should be used to check against the combined effects of
coincident shear, axial load and bending
f c = f x2 +f y2 -f x f y +3f s2
where fx and fy are the algebraic sum of the axial and bending stresses in the x and y
planes respectively, and fs is the algebraic sum of the coincident shear stresses due to
torsion and/or bending.
16
Selected sections from McDermott In-house Design Guide for Lift Systems – Y.S. Choo – Apr04
Project Examples for plate trunnions and padeyes
Placement of doubled slings onto plate trunnion for deck structure
Two examples of padeye with main plate slotted into the circular deck member
17