Effect of Skew on Lifting Analysis

Effect of Skew on Lifting Analysis

1 Abstract

Many jackets and topsides are installed by lifting. Lifting analysis needs to be carried out to

check the integrity of structure during onshore and/or offshore lifting. Lifting should be

carried out as per API RP2A requirements, lifting weight should be multiplied by factors to

account for dynamic amplification, change of center of gravity and other factors which will

affect lifting operations. API factors for lifting analysis can be used if fabrication tolerances

of slings do not exceed the allowable values as per API RP2A. Otherwise skew should be

considered in lifting analysis. Some codes such as DNV accept use of additional factors to

consider skew effect.

To investigate skew effect on lifting analysis, three different topsides were analyzed to

evaluate different parameters that may contribute to the effect of skew on lifting analysis.

Also, to evaluate whether applying a factor to account for skew effect in lifting operations

will result in design which is conservative or not. The analysis will be carried out assuming

one pair of opposite slings are carrying 75% of loads while the other pair are carrying 25% of

loads. This can be achieved by applying temperature gradient to increase (or decrease)

lengths of slings. Weights of decks are varying from 800 ton to 1500 ton, lifting analysis

were carried out for three cases:

• Normal case without skew.

• First skew case where two opposite slings will carry 75% of load and other slings will

carry 25% of load.

• Second skew case where two opposite slings will carry 25% of load and other slings

will carry 75% of load (opposite to first skew case).

The analysis shows that the following points have effect on the results of the analysis:

• Variance in slings lengths

• Angles of slings

• Location of center of gravity

• Lifting arrangements

• Modeling of fabrication tolerances

Based on the results of the analysis, the following points were concluded:

• Appling a factor to consider skew will save a lot of analysis time but this will not be

on the conservative side for topsides with a considerable shift in center of gravity.

• Variance in slings lengths and angles of slings have an important effect on results of

lifting analysis thus selection of lifting arrangement is of great importance to avoid

overstresses and failure in members / structures and to reduce effect of skew.

2 Structural Analysis

Skew effect on lifting analysis should be considered if fabrication tolerances as per

API requirements cannot be achieved. The structure to be analyzed assuming one pair of

opposite slings is carrying 75% of loads while the other pair is carrying 25% of loads. This can

be achieved by applying temperature gradient.

Another way to analyze skew but with less accuracy is to connect the slings to two

lifting points, each opposite slings will be connected to a point. One point only will be

restrained vertically and the other joint which is not restrained vertically will be subjected to

a vertical force upward equal to 75% of topsides weights. This will force slings connected to

this joint to carry 75% from topsides weight and other two slings will carry 25% only from

topsides weight. The analysis will be repeated again by swapping joint conditions to check

other case of skew. It should be noted that results of this simulation are not accurate since

this method is not simulating the actual behavior of structure during lifting.

To investigate skew effect on lifting analysis, three different topsides were analyzed to

evaluate different parameters that may contribute to the effect of skew on lifting analysis.

Also, to evaluate whether applying a factor to account for skew effect in lifting operation

will result in design which is conservative or not. The analysis will be carried out assuming

one pair of opposite slings are carrying 75% of loads while the other pair are carrying 25% of

loads. This can be achieved by applying temperature gradient to increase (or decrease)

lengths of slings. Weights of decks are varying from 800 ton to 1500 ton, lifting analysis

were carried out for three cases:

• Normal case without skew using API factors of 2 and 1.35 for members connected to

lifting joints and other members.

• First skew case where two opposite slings will carry 75% of load and other slings will

carry 25% of load. Dynamic amplification factor (DAF), consequence factor (COF) and

center of gravity shift factor (COG) will be applied (1.2, 1.15 & 1.03 based on Noble

Denton criteria).

• Second skew case where two opposite slings will carry 25% of load and other slings

will carry 75% of load (opposite to first skew case). Dynamic amplification factor

(DAF), consequence factor (COF) and center of gravity shift factor (COG) will be

applied (1.2, 1.15 & 1.03 based on Noble Denton criteria).

The results of the analysis of skew cases will be compared to the results of analysis using API

factors.

The weights and lifting configurations for the three structures to be analyzed are shown on

the next pages.

2.1 Weight and Slings Angles for First Topsides

� Topsides weight: 857 ton

� Sling A1 angle 55.6 degrees


� Sling B1 angle 57.3 degrees


Graphs for topsides configuration and center of gravity are shown below.

2.2 Weight and Slings Angles for Second Topsides






Graphs for topsides configuration and center of gravity are shown below

2.3 Weight and Slings Angles for Third Topsides






Graphs for topsides configuration and center of gravity are shown below

3 Analysis Results

3.1 Analysis Results for First Topsides

Forces in slings for three different cases based on the carried out analysis (normal case

without skew - first skew case - second skew case) are shown in next graphs.

Maximum unity check ratios for joints and members for lifting analysis using API factors of 2

and 1.35 are compared with similar results of skew analysis in the next table:

Element check

API factors Skew analysis

Factor 2.00 Factor 1.35 Case 1 Case 2

Member 0.78 0.70 1.42 1.15

Joint 0.46 0.32 0.50 0.48

High interaction ratios were obtained for skew case, applying a factor instead of carrying out

the analysis will not be on the conservative side here.

3.2 Analysis Results for Second Topsides





Element check



Member 0.80 0.80 2.74 1.30

Joint 0.11 0.49 0.70 0.69

High interaction ratios were obtained for skew case, applying a factor instead of carrying out

the analysis will not be on the safe side here.

3.3 Analysis Results for Third Topsides





Element check



Member 0.76 0.38 0.70 0.72

Joint 0.37 0.25 0.38 0.42

Interaction ratios within the allowable limits were obtained for skew case, applying a factor

instead of carrying out the analysis will be on the safe side here.

4 Discussion and conclusions

The analysis shows that the following points have influence on skew effect on lifting

analysis:

• Variance in slings lengths

Slings with high variance in lengths will result in more skew effect on lifting analysis.

• Angles of slings

Increasing of sling angle will result in less skew effect on lifting analysis.

• Location of center of gravity

Symmetric or almost symmetric center of gravity will result in less skew effect on

lifting analysis.

• Lifting arrangements

Lifting arrangements have great effect on lifting analysis and skew effect, skew effect

can be reduced (or increased) based on the selected lifting arrangements.

• Modeling of fabrication tolerances

Modeling of fabrication tolerances will affect the analysis and results, fabrication

tolerances should be modeled based on slings lengths as per API requirements.

4.1 Conclusions

� Applying a factor to consider skew instead of modeling fabrication tolarances will

save a lot of analysis time but this will not be on the conservative side espically for

topsides with a considerable shift in the center of gravity. However, consideroing of

skew by modeling fabrication tolarances for such topsides can result in very high

interaction ratios and very conservative design.

� Variance in slings lengths, angles of slings have a clear effect on results of lifting

analysis and skew effect on lifting analysis.

� Selection of lifting arrangment is important, skew effect is less for some lifting

arangment such as spreader bar. Selection of lifting arrangement is of great

importance to avoid overstresses and failure in members / structures.

� Skew have a considerable effect on lifting analysis and should be always considered

to check the structural integrity under different skew cases.

Documents

Effect of Skew on Lifting Analysis