crane pedestral design

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Engineering & Construction Sector

DESIGN REFERENCE

OFFSHORE STRUCTURESCRANE PEDESTAL DESIGN

DR 352 Rev 0 L MAY 1991

John Brown Engineers & Constructors Limited20 Eastbourne Terrace, London W2 6LE



DESIGN REFERENCE DR 352/0 L

OFFSHORE STRUCTURESCRANE PEDESTAL DESIGN

CONTENTS

1. OBJECTIVE

2. DEFINITION

3. DESIGN SPECIFICATION3.1 CRANE SPECIFICATION3.2 DUTY FACTOR3.3 CRANE UTILISATION3.4 STATE OF LOADING3.5 LIFT-IMPACT FACTOR3.6 API-RP2A RECOMMENDATIONS

4. MAXIMUM LOADING FOR DESIGN4.1 LLOYDS'S METHOD4.2 AN INDEPENDENT METHOD4.3 COMPARISON OF THE METHODS4.4 RECOMMENDATION

5. FATIGUE DESIGN5.1 CONVENTIONAL FATIGUE ANALYSIS5.2 COMMENTARY5.3 CRANE VIBRATIONS

6. ACCIDENTAL LOAD

7. REFERENCES

FIGURES AND TABLES

REV 0 ISSUED MAY 1991

PREPARED BY: OFFSHORE STRUCTURES

APPROVED BY: ...........................K. LOGENDRA,ASSOCIATE DIRECTOROFFSHORE STRUCTURES

AUTHORISED BY: ................... H. THIRKELL,DIRECTOR OF ENGINEERING




OFFSHORE STRUCTURES PAGE 1 OF 17CRANE PEDESTAL DESIGN

1 OBJECTIVE

The objective of the Design Practice is to give detailed guidance on the static and fatiguedesign of the pedestal structures of offshore platform cranes. An offshore crane is subjectto significant shockloading which should be addressed in an adequate manner in the designof the crane pedestal.

2 DEFINITION

The crane pedestal is, in general, a vertical tubular structure spanning at least two topsidedeck levels (see Fig. 1-3). In some cases this tubular is also used for diesel storage. However, the safety aspect of diesel storage must be addressed as part of a Formal SafetyAssessment. The slew ring of the revolving crane is mounted onto the top of the tubularwith or without a transition cone called the pedestal adoption.

The definitions of the symbols used in this Design Practice are given immediately aftertheir introduction in design equations.

3 DESIGN SPECIFICATION

For the design of the crane pedestal two loading conditions will have to be considered: themaximum loading and the fatigue loading.

No specific guidance on crane pedestals is given in the DEn Guidance Notes (Ref.1)despite the fact that a series of crane accidents occurred in the early 80's. The section onthe design of the crane supporting structure can make effective use of API-RP2A (Ref. 2)as will be discussed in Section 4.2. It is also worth noting that some interesting field dataon offshore crane behaviour are reported in Ref. 3.

It is common practice in John Brown to use a combination of the Lloyd's Code on LiftingAppliances in a Marine Environment, Chapter 3, Sect. 3 on offshore cranes (Ref. 4), andthe British Standard on rules for the design of cranes (Ref. 5). This Design Practice willreview the adequacy of this method and will bring it in line with the Guidance Notes andAPI-RP2A (Ref. 1 and 2) as applied to other parts of the topsides and substructure design.

3.1 CRANE SPECIFICATION

The following crane specific information is required:

- a crane capacity curve (see Fig. 4)- seastate dependent crane derating coefficients- the weight of the boom and the hook

In some cases (e.g. Bruce) there is also an accidental load requirement which stipulates thatthe pedestal must be stronger than the crane strength. In that case the vendor supplied datashould also contain a crane failure envelope.





3.2 DUTY FACTOR

According to Lloyd's Register it is logical and reasonable to reflect the harsh duty of anoffshore pedestal crane by a

Duty Factor (DF) = 1.2

This factor is to be applied to the lift-load only and in combination with Lloyd's dynamicamplification factors (see Ref 4 Ch. 3 Sect 2.3.1).

3.3 CRANE UTILISATION

According to BS 2573 (Ref. 5) the crane operating life is reflected by two parameters (seeTables 1 and 2):

- the Class of Utilisation (U1-U9)- the State of Loading (Q1-Q4)

Historically the following utilisations have been used:

U3 for unmanned installations (N = 125,000 cycles)U5 for drilling/production platform (N = 500,000 cycles)

The class can be revised based on Client supplied data.

3.4 STATE OF LOADING

The previous sections only addressed the general working environment and the number oflifts in the course of the crane useful operating life. The aim of the parameter Q in BS 2573is to reflect the average severity of the loading as a percentage of the maximum craneloading for the crane pedestal fatigue analysis.

The State of Loading of offshore cranes is best reflected by

Q2 - moderate state of loading

In the fatigue analysis the state of loading is incorporated by a parameter Kp which is calledthe load spectrum factor. Its value is dependent on the state of loading and the value of Kpassociated with Q2 is

Kp = 0.63

It is a multiplication factor for the total pedestal bending moment. Following Section2.3.2.1 of BS 2573 it could be derived independently based on Client supplied data but it isquestionable if these additional calculations would effectively improve the accuracy of thefatigue analysis.





Note 1: The recommended value of m = 3 in the expression for Kp (see Sect. 2.3.2.1 inRef. 5) corresponds to the slope of the DEn-SN curve in a log-log scale.

Note 2: According to the expressions in Ref.5 a utilisation of 10, 60, 30% of the lifts at100, 60, 40% of the crane capacity corresponds with a Kp = 0.63 equal to Kp forthe Q2 state of loading.

3.5 LIFT IMPACT FACTOR

The most onerous loading condition for an offshore crane will be experienced during theoffloading of a supply boat. Since the hook-speed of the main hoist will be low incomparison with the supply boat heave motion there will be a significant impact on thecrane when the load comes free from the supply boat for the first time.

This impact factor can also be called dynamic amplification factor (DAF); it is a randomvariable because it will depend on the actual heave motion of the supply boat at the point oflift-off. The specific values for the DAF in a crane analysis can be derived from theequations in Ref 4, Ch. 3 Sect. 3.3.2; some specific numbers are given in Sect. 4.1.

The DAF can also be found from computer simulations or from field measurements andcan be as high as 3.0.

3.6 API-RP2A RECOMMENDATIONS

API-RP2A Sect. 7.3.1 summarised the guidance on the crane supporting structures asfollows:

"7.3.1 Static Design. The supporting structure should be designed for the deadload of the crane plus a minimum of 2.0 times the static rated load as defined inAPI Spec. 2C and the stresses compared to the Par. 3.1.1 allowables with noincrease."

In the light of the discussion and analysis of Chapter 4 this design condition is significantlylighter than the recommendation of this DR. Therefore the API recommendations shouldnot be used for crane pedestal design except for Gulf of Mexico type environmentalconditions.

4 MAXIMUM LOADING FOR DESIGN

The maximum loading governing the design of the crane pedestal will be in accordance tothe Lloyds's code on Lifting Appliances (Ref. 4), Ch. 3, Sect. 2.16.2. More specifically thegoverning load condition will be Case 2 for the crane in operating mode with wind. Inparticular the maximum moment in the pedestal will be governing. For this case the effectsof the horizontal loads on the crane boom (which is so important for the crane design itself)can be ignored.

4.1 LLOYD'S METHOD





According to Ref. 4, Ch. 3 the design load for the pedestal will require the incorporation ofa duty factor (DF) and a dynamic amplification factor (DAF) on the hook load.

More specifically the following values can be obtained from Ref. 4, Chapter 3:

Duty Factor (DF) = 1.2 (Sect. 2.3.1)DAF (for Hs = 1.6m) = 1.61 (Sect. 3.3.2)DAF (for Hs = 3.9m) = 2.07 (Sect. 3.3.2)

Using these factors together with the vendor supplied data and after inclusion of the staticmoment due to the weight of the hook and the boom the most unfavourable moment for thedesign of the crane pedestal is:

Mmax = Mhook + Mboom + DF * DAF * Mhookload

The maximum stress as a result of this moment is to be compared with the allowablebending stress pa:

pb = 0.57 py (Sect. 5.3.2)

4.2 AN INDEPENDENT METHOD

It was noted in Sect. 3.2 that the maximum DAF on the hook load can be as high as 3.0. This value will be used in the independent method.

Secondly the pedestal is a thin-walled tubular structure which should be designed inaccordance to API-RP2A Section 3.2.3. The one-third increase in allowable stressesshould not be applied to the crane pedestal. For py = 340 MPa and D/t = 75 the allowable

bending stress according to API-RP2A is:

pb = 0.65 py

For values of D/t and py different from D/t = 75 and py = 340 MPa the equation for pb in

API-RP2A Sect. 3.2.3 should be used.

This value of pbis to be combined with a dynamic amplification factor for the hookload

equal to

DAF = 3.0

Finally pb is to be obtained from the following equations for Mmax





Mmax = Mhook + Mboom + DAF * Mhookload

4.3 COMPARISON OF THE METHODS

Two comparisons between the two methods will be made base on the static part and thedynamic part of the crane loading.

Using the equations in Sect. 4.1 and 4.2 and by setting the hookload equal to zero a directcomparison can be made between the allowable static bending stresses.

The Lloyd's method (stat) pb = 0.57 pyThe alternative method (stat) pb = 0.65 py

Using the equations in Sect. 4.1 and 4.2 and by setting the boom weight and the hookweight equal to zero a direct comparison can be made between the allowable dynamicbending stress in the pedestal due to the hook-load. By dividing the maximum allowablebending stresses by the Duty Factor and DAF the following values are obtained.

The Lloyd's method (dyn) pb = 0.23 pyThe alternative method (dyn) pb = 0.22 py

(These numbers are found as follows:

0.23=0.57/(1.2 x 2.07);0.22 = 0.65/3.0)

4.4 RECOMMENDATION

The alternative method incorporates a well recognised model to accommodate the bendingstrength reduction for thin-walled tubulars.

In addition the allowable static stress in the alternative method has been found to be 10%higher (using industry accepted practices) than the Lloyd's method.

Therefore it is recommended to apply the alternative method of Section 4.2 for the ultimatedesign of pedestals for offshore cranes.

5 FATIGUE

The assessment of the fatigue strength of crane components should address the followingload histories:

- the lift-off of a load from a supply boat- the dynamics in the crane system as a result of the impact forces during lift off- the setting-down of the load on the platform





From a review of methods and consequences it is concluded that the BS 2573/Lloyd'sprocedure considering each lift-off and setting-down as a fatigue cycle is governing for thecrane pedestal. This procedure will be further addressed in Sect. 4.1. It is noted that theAPI-RP2A recommendation on crane pedestal fatigue should not be used for North Seaconditions; it may form a simple basis for light cranes on platforms operating in Gulf ofMexico conditions.

The crane vibrations will be discussed in Sect. 5.3. They are the result of impact forcesduring lift-off from a supply boat and forms a governing fatigue loading on many cranecomponents. For the crane pedestal its inclusion leads to a small correction which can bedisregarded within the accuracy of its overall fatigue analysis.

5.1 CONVENTIONAL CRANE FATIGUE ANALYSIS

From the crane specific data the following is required for the fatigue analysis.

- Class of utilisation (U3 - U5) representing the number of lifts during the lifetimeof the crane. (See Sect. 3.3)

- State of loading as reflected in Q2 = 2 and Kp = 0.63. (See Sect. 3.4)

The maximum stresses in the crane pedestal will directly depend on the bending momentand we are specifically interested in the maximum positive and negative bending moment.

5.1.1 The Maximum and Minimum Moment for Fatigue

The lift-off moment to be considered for the fatigue analysis is :

Mmax = Kp * (Mhook + Mboom + DF x DAF x Mhookload)

This moment should be checked for the following conditions and seastates:

(a) - maximum reach associated to maximum load(b) - maximum load at the maximum associated reach

(i) - seastate 2-3 (Hs = 1.6m) with a DAF = 1.61(ii) - seastate 4-6 (Hs = 3.9m) with a DAF = 2.07

This leads to four cases (a-i, a-ii, b-i, b-ii).

The opposite sign to the lift-off moment will occur during placing of the load on theplatform. This moment is similar to the lift-off moment with one exception that theoperation is seastate independent which can be reflected by a DAF = 1.0 or :

Mmin = Kp * (Mhook + Mboom + DF x Mhookload)

Comment 1: If the crane is optimally designed using Lloyd's recommendation for the DAF





then the maximum moment in the crane pedestal will be independent of the seastate.

Comment 2: Due to the effect of the boom weight on the pedestal moment it is expectedthat the maximum pedestal moment will be concurrent with the maximum reach.

Comment 3: Since the minimum pedestal moment (occurring while placing the load on theplatform) is seastate independent it can be demonstrated that the minimum pedestalmoment reaches its (absolute) extreme value for the highest load (i.e lowest seastate) andfor the maximum reach.

These three comments should be verified for the fatigue analysis. If confirmed then theyform the immediate basis for obtaining the maximum and minimum moment to be used inthe fatigue analysis of the crane pedestal as follows:

Mmax = Kp * (Wh + 0.5 Wb + DF x DAF * Wl) *rmaxMmin = Kp * (Wh + 0.5 Wb + DF x Wl) *rmax

where

Kp = 0.63 (see Sect. 3.3)

Wh = weight of the hook

Wb = weight of the boom

Wl = weight of the hookload

DF = 1.2 (see Sect. 3.2)DAF = 1.61 (see Sect. 4.1)rmax = maximum reach

5.1.2 The Allowable Stress

The maximum and minimum moments also give the corresponding R-value which isdefined as:

R = Mmin./Mmax. = min. stress/max. stress

For crane pedestals the class F welding detail should be used. The corresponding tablefrom BS 2573 for class-F welding details is copied as Table 3 in this DR.

Using the R-value calculated above and the number of cycles in accordance to the class ofutilisation (U3 - U5) the corresponding maximum allowable fatigue (tension orcompressive) stress can be read directly from Table 3.

The maximum stress together with the maximum pedestal moment determines the sectionmodules or (if the diameter is specified) the material thickness of the pedestal.





5.2 COMMENTARY

Fatigue in the UK sector of the North Sea is in general addressed using the DEn GuidanceNotes (Ref. 1) and therefore it is useful to make some comparative remarks on theprocedure of Sect. 5.1. The only difference will be in the selection of the SN curve; allother aspects (the number of cycles, the F curve, the Kp - value) are found to be identicalusing the information contained in the Guidance Notes.

The two differences between Ref. 1 and BS 2573 (Ref. 5) in the fatigue curves are:

- DEn do not recognise the R-dependency- for R = -1 (fatigue with a zero mean) the allowable stress is different.

This is reflected in the following data for the maximum stress-amplitude in MPa at U5(500,000 cycles). In this table, for completeness, the API-RP2A fatigue allowables havebeen included as well.

Stress in MPa BS 2570 DEnGN API-X API-X'

R = -1.0R = -0.7

6271

5464

6981

5767

This difference between BS 2573 and the Guidance Notes is significant from a design pointof view. But it should be noted that their difference is equivalent to doubling the estimatedfailure rate, (failure = through thickness crack) from 2% to 4% in the lifetime of the crane. Because of the ease of inspection of a crane pedestal as compared with underwater parts ofthe structure it is recommended to apply the slightly less conservative BS 2573 data.

5.3 CRANE VIBRATIONS

The impact due to supply-boat offloading will result in vibrations in the crane system andthese vibration gradually reduce in magnitude until the equilibrium condition is reached. The amplitude of the fatigue loading is governed by (DAF - 1.0) * hook-load and the totalstress range is twice this amplitude; secondly the number of active cycles will depend onthe system damping

In general the damping of structural systems is small; for example a value of 2% of criticaldamping seems realistic implying that in 5 cycles the amplitude of the oscillations isreduced to 50%. It can then be demonstrated that for these conditions (with this damping)using Ref. 1 information the fatigue damage in the crane pedestal due to crane dynamicscan be ignored. This is contrary to the findings of Ref. 3.

It should be noted, though, that crane vibrations form an important aspect in the design ofthe crane boom and other components in the pedestal crane.

6 ACCIDENTAL LOAD





For the design of the Bruce pedestal crane the following accidental load scenario wasstipulated.

"The crane pedestal, and supporting structure, will be analysed to demonstrate aminimum factor of safety against collapse of 1.5. The applied loading for thiscase shall be determined from crane failure envelopes supplied by the cranevendor. For this case, allowable stresses will be limited to the yield stress of thepedestal material."

The accidental load scenario may well be governing for the design of the crane pedestal andits supporting steel work. The main reason for this scenario is the occurrence of accidentslike the hook snatching the supply boat causing pedestal failure and fatalities in the early80's.

Without further data the following two changes are recommended for incorporation in theabove description.

a) safety against collapse to be reduced from 1.5 to 1.3b) yield stress to be replaced by allowable stress with a one-third increase.

Pt. 1 can be further reduced after review of the vendor data supporting the crane failureenvelope.

Pt. 2 is a reflection of API-RP2A on thin walled tubulars in line with the comments in Sect.4.2.

7 REFERENCES

a) Department of Energy; Offshore Installations: Guidance on Design,Construction and Certification (Fourth Edition) 1990.

b) API-RP2A API Recommended Practice for Planning, Designing andConstruction Fixed Offshore Platforms 1989 (18th Edition).

c) Shauschausen, J. and Gran, S. Supply Boat Motions; Dynamic Response andFatigue of Offshore Cranes OTC 3795, 1980.

d) Code for Lifting Appliances in a Marine Environment, Lloyd's Register ofShipping, Jan. 1987.

e) BS 2573 Rules for the Design of Cranes Part 1: Specification for theClassification Stress Calculations and Design Criteria for Structures 1983.





FIGURE 1. A TYPICAL CRANE PEDESTAL SUPPORT

STANDARDS\DR\DR352\FIG-1.WPG





FIGURE 2a. PRIMARY STEEL ARBROATH DECK






FIGURE 2b. PRIMARY STEEL ARBROATH DECK






FIGURE 3. DETAILS OF THE CRANE PEDESTAL






FIGURE 4. CRANE LOAD CAPACITY CURVE


crane pedestral design

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