Leg Towing Analysis

7/24/2019 Leg Towing Analysis

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Leg Towing Analysis report

Prepared - Hamish Forsythe

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Contents1. Executive ummary.......................................................................................3

!. "ethod o# Analysis........................................................................................3

!.1 $riteria......................................................................................................3

!.! "odels %escription......................................................................................4

!.& 'oundary $onditions.................................................................................... 6

!.( Loading.....................................................................................................6

!.) Allowa*le tresses.....................................................................................11

!.+ ,esult..................................................................................................... 11

References.................................................................................................12

APPENDIX A............................................................................................... 13

OUTPUT DATA.........................................................................................14Model Plo!.............................................................................................1"

A##end$% & ' O(#( Plo...........................................................................1)

A##end$% * ' *+ord Pro#er$es...................................................................2,

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1. Executive SummaryThis particular is a /ac0-up rig with a triangular shaped hull with three triangular layout

truss legs positioned within it.

The purpose o# this report is to provide evidence via FE analysis that the legs provide

adeuate strength to support the structure during towing in *oth o# the #ollowing conditions2

1. #ield tow 31!).&m leg length4

!. ocean tow 311!.5+m leg length4

The results gained here are su##icient to con#irm that the leg strength will support the hull

structure and also that the rac0 choc0 rated capacity is not exceeded during any o# the towing

conditions.

6e can say that the maximum stress compared to the permissi*le stress never exceeds a ratio

o# 7.88 #or any towing condition9 which is essential as it must stay *elow 1 at all times.

2. Method of AnalysisThe Analysis #or this pro/ect was completed using the commercial FEA 3Finite Element

Analysis4 o#tware A$ Executive ).&. This so#tware is very e##icient at de#ining truss

sections. 6ithin this so#tware a three dimensional *eam-element model was de#ined via a

general leg arrangement and calculated loads 3via :;)&(##shore %rilling nit4 ,ules as well as allowa*le

stress design criteria.

;n order to 0now what the reuired stresses are9 we need to use the :;)&(


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>cean Tow - Leg strength is to *e developed to withstand a *ending moment caused *y a

1)?single amplitude roll and pitch at a 17 second period plus 1!7@ o# the gravity moment

caused *y the angle o# inclination o# the legs.

2.2 Models DescriptionThe model consists entirely o# *eam elements with the appropriate material and sectionproperties applied. These section properties include2

The dimensions

Axial area 3only #or tu*ular sections4

"oment o# inertia around and B axis

Torsional moment o# inertia 3 C B4

$ross section types supported in sacs are2

Tu*ular

6ide #lange

$ompact wide #lange

'ox

Tee

Deneral Prismatic

$hannel

Plate Dirder

Angle

$one

ti##ened 'ox

ti##ened $ylinder

The main side chords are modelled as prismatic sections as they are constructed as a split pipe

with the rac0 separating it along its local longitudinal axis. According to the leg general

arrangement9 at a height o# (787(mm a*ove 'L the chord changes its axial area and hence

we insert a di##erent prismatic section here.

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Figure 1 SACS !eamelement model

Figure 2 "ypical chord arrangement

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2.# $oundary Conditions'oundary conditions were applied at the locations on the chords where the legs were

connected to the lower and upper /ac0-case guide as well as the rac0-choc0. To simulate the

*oundary conditions and appropriate constraints on the leg #or towing conditions9 the

mem*ers connecting to the location o# the rac0-choc09 lower and upper guides were modelled

as a sti## lin0 where the end /oint on them was #ixed in all directions. This #ixidity is de#inedin A$ as 31111114 where it is constrained in all directions and all rotational axis. The

#ixidity at the end o# these mem*ers connects to the hull. Furthermore the pre-mentioned

/oints had proper end releases to simulate load trans#er *etween the leg and the /ac0 cases.

This trans#er varies depending on the euipment and the di##erence is shown *elow.

The lowerupper guides do not provide any moment restraint9 however9 the rac0 choc0 is

more complex and is a*le to constrain in *oth the vertical and lateral directions. The releases

which are applied at the ends connected to the chords are2

Local axis %x %y %G ,x ,y ,G

Lower Duide and pper Duide 1 7 1 1 1 1

,ac0 $hoc0 1 7 7 1 1 1:ote2 1 is a release in the mode and direction as shown in the mem*ers local axis. These

releases are used #or *oth the towing and storm conditions.

Also9 %I direction and , I rotation in the a*ove ta*le.

Figure # Fixidity% 111111% at end of connecting chords

2.& 'oadingLoad are applied to the model through speci#ic com*inations o# *asic load cases. Each *asic

load case is derived #rom a speci#ic #orce applied to the structure and these are as #ollows2

1. Leg dead weightJ uni#ormly distri*uted load in the vertical direction on each chord. K0:m

!. ;nclined weightJ uni#ormly distri*uted load in the lateral direction on each chord K0:m.

The two *asic load-cases separate the longitudinal 3x4 and transverse 3y4 load vectors.

&. Lateral inertial loadJ ni#ormly increasing load in the lateral direction on each chord. The

load is derived *y setting the starting distri*ution amplitude to 7K0:m at the *ase o# the

model and to a maximum value at the top o# the leg. Two *asic load cases separate the

longitudinal 3x4 and transverse 3y4 load vectors. K0:m

(. =ertical inertial loadJ uni#ormly distri*uted load in the vertical direction on each chord.

This simulates the uneual downward #rom the di##ering vertical acceleration at each chord.

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6e apply each o# the a*ove loads to *oth a #ore and a#t leg9 thus giving us a total o# eight

*asic load cases.

The derivation o# load amplitude and position9 and their method o# application are explained

in the #ollowing sections.

The weight o# the leg9 the depth o# the spud can9 the #ull length o# the leg and the appliedweight perm is incredi*ly important to determining the overall vertical stresses on the

structure as this directly has an e##ect on all the stresses.

Leg 6eight excluding can )+&8.(5 0:

Full leg length 1!).& m

pud can height (.)8! m

Leg 6eight (+.870:

m

Leg weight9 as vertical load on each chord 1).)80:

m

dra#t (.)8! m

Also the calculation o# the *asic load cases are shown in the #igure *elow2

Load case direction description1 3all chords4 x Longitudinal load due to the pitch

! 3all chords4 y Transversal load due to roll

& 3chord 14 -G =ertical load due to pitch or roll

( 3chord !4 -G =ertical load due to pitch or roll

) 3chord &4 -G =ertical load due to pitch or roll

+ 3all chords4 x Longitudinal load due to inclined weight

8 3all chords4 y Transversal load due to inclined weight

5 3all chords4 -G =ertical load due to deadweight

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The com*ined load cases are separate #or two conditions2 #ield towing and

ocean towing. Each condition as mentioned *e#ore has di##erent

reuirements in terms o# inertial acceleration and natural rollpitch period.

The accelerations are *ased upon Figures ( and )2

Figure ( )*(#&+,ac-up ules parameters for calculation of inertial accelerations and to/ing loadings

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Figure 4.2 - Totalcombined load

Figure 4." - Combinedinertial and inclinedlateral load - a!!lied on

ure 4 - $erticalding%!lied on all


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Figure 0 ,ac-up general arrangement% sho/s yi and xi for the ac-up

The acceleration is calculated at the top o# the leg and is applied to each appropriate inertial

load case as the load #actor. The vertical loads per chord are also calculated and each chords

*asic load case is appropriately #actored.

The method #or calculation o# all vertical and inertial induced loads are shown *elow.

Diven that we 0now the A' reuirements #or transit towing conditions9 we must *e a*le to

apply these in order to calculate the overall loading #orces.

6e must 0now the #ollowing parameters2

1. angular acceleration #or the pitch Mp and the roll Mr. these are o*tained #rom the #ollowing

#ormula2

R=AR (2

TR)2

P=A

P(2

TP

)2

6here A is the roll single amplitude o# the unit as de#ined in the A' ">% ,ules 3+

degrees #or #ield tow9 1) degrees #or ocean tow4. when we apply these values in the a*ove

euation we should convert these values to radians. Also9 T is the natural period o# the

motion9 which we 0now to *e 11s #or #ield tow and 17s #or ocean tow #or either pitch or roll.

A#ter we have #ound the a*ove values9 we can use the *elow euations to #ind2

a4 the horiGontal load distri*ution under roll and pitch motion

*4 the glo*al #orces in upright 3only loads induced *y pitch motion4 and inclined position

3only loads induced *y roll motion4 #or the vertical #orce induced *y the leg to the unit

structure under rollpitch motion at upper guide level.

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a i4 HoriGontal Load distri*ution under roll motion2

Fir=p i(1.2gsin (AR)+Rzi)

6hereJ

pi 2 weight9 in tonnes9 o# an elementary length o# leg li. 3We can find this via dividing the full

weight of the leg, by the length of the leg4.

Gi 2 %istance9 in m9 measured as shown in Fig. (.

* i4 And9 the total vertical #orce F=P9 in 0:9 induced *y the leg under roll is o*tained via2

1.2gcos (AR )+Ry iFVR=Pleg

6hereJ

Pleg 2 Total weight o# the leg9 in tonnes

yi 2 %istance9 in m9 as shown in Fig. (.

imilarly #or the pitch motion2

a ii4 HoriGontal Load distri*ution under pitch motion2

Fip=pi(1.2 gsin (AP )+Pzi)

* ii4 The vertical #orce F=Pcan *e #ound #rom2

1.2gcos (P )+Px iFVP=Pleg

6here2

xi 2 %istance9 in m9 measured as shown in Fig. (.

The com*ined load cases are grouped into two part2

1. Field Transit

!. >cean transit

There are reversals o# lateral loading in the longitudinal direction only since the legs are

symmetrical in the transverse direction. The com*ined load cases are shown in #igure 8.

;t should also *e mentioned that i# we apply the #ull horiGontal #orces #or the whole structure

to each leg9 we can set the load condition #actor eual to 1& seeing as there is three chords.

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Figure & - Load cae name and direction

Figure ' - Calculation o( leg loading (or all condition

2.( Allo/a!le StressesAllowa*le stresses are as per the A;$ manual o# teel construction A% codes. 'ecause the

applied loads are instantaneous maximums and are cyclic in nature we can use the 1.&&&&

#actor #or the allow stress modi#ier 3A">%4. The chord and rac0s are +N7"Pa9 the internal

*races are !(7"PA. The #ollowing general rules #or permissi*le 3i.e. $OI1.74 stresses are

#ollowed2 FIFyF..

where2

Fy I speci#ied minimum yield point or yield strength.

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F.. I #actor o# sa#ety

For com*ined loadings F.. I 1.!) #or axial or *ending stresses9 F.. I 1.55 #or shear stress.

ield stress Allowa*le shear

stress

Allowa*le

axial*ending stress

$hord +N7.7 &+8.7 ))!.7

;nternal *racing !(7.7 1!8.8 1N!.7

HoriGontaldiagonal

*racing

&+7.7 1N1.) !55.7

A$ unit chec0s #or A;$ also account #or other stress limiters such as un*raced length and

allowa*le compression loads.

2.0 esult;d "ax $

HoriGontal *races H71H7&H7! 7.)57.787.7+

-*race 717& 7.8N7.81

$hord $71$7! 7.&87.+(

There#ore9 the legs satis#y code reuirements.

Re(erenceA&/ R(les for 0($ld$ng and class$ng Mo0$le Os+ore Dr$ll$ng Un$s 2,12

NI"34Jac(# R(les /I

AN/AI/* 36,1,

/A*/ E%ec($5e ".3

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A))E*+I, A

INPUT I7E AND OUTPUT DATA

T)T +ATA

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Model )lot/

Figure A" - Member 0rou! - $ertical 1belo 43&34 AL5

Figure A2 - Member grou! - 6ori7ontal 1elo 43&34 AL5

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Figure A8 - Member 0rou! - $ertical 1abo9e 43&34 AL5

Figure A4 - Member grou! - 6ori7ontal 1abo9e 43&34 AL5

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Figure A: - ;oint Fi


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Figure " - Ma< C Ratio

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,.3) 8*,19

,.64 8*,29

,.1 8P719

,."-

8:,19

,.,6

,.)1

8;,39

,.) 8;,19

,.,) 8:,39


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Figure 2 - =ort Load cae (or eac# member grou!

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ORA

OP 8:,39

OP 8*,19

OR

ORA

OPA 8:,19

ORA

OP 8P719


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A!!endi< C - C#ord )ro!ertie

-Shear area 843.24 cm^2

Z-Shear area 175.43 cm^2

Area 1002.21897 cm^2

Ip 285794.9612 cm^4

I-y 130080.0393 cm^4

I-z 155714.9219 cm^4

-Shear area 731.9405338 cm^2

Z-Shear area 175.4265338 cm^2

Area 907.230006 cm^2

Ip 267885.4311 cm^4

I-y 125039.8784 cm^4

I-z 142845.5527 cm^4

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