Structural Evaluation of FSO Ground Build Load-Out

7/27/2019 Structural Evaluation of FSO Ground Build Load-Out

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Structural Evaluation of FSO Ground Build Load-outY. T. Yang, H. S. Kang, B. N. Park

Offshore Basic Design & Engineering Dept, Hyundai Heavy Industries Co., LTD

ULSAN, KOREA

ABSTRACT

End of July 2002, HHI(Hyundai Heavy Industries Co., LTD)

successfully completed load-out and float-off of 300K VLCC size

AMENAM KPONO/FSO. HHI built this FSO for EPNL (ELF

Petroleum Nigeria Ltd.) using Ground build methodology. Various

systems and methodologies like-Flexi-built FPSO Hull(Fabrication on

Ground), Topside module design and erection method, Soil strength

and stability check, Load-out and float-off using multipurpose DBU

(Double Barge Unit) were combined to develop a successful Ground

build method. In this paper, we described the Numerical simulation

of FSO Structure (Ground Build) load-out using the DBU(Double

Barge Unit). A technology of Ground Build design, construction,

load-out and float-off of floating structure successfully proved by HHI

after its four years of dedicated research and development.

KEY WORDS: Ground build; Load-out; F(P)SO; Multipurpose DBU

INTRODUCTION

Floating type offshore structures showed an overall increase in market

share during the last several years in offshore construction field.

Generally, the fabrication and assembly of floating offshore structures

like F(P)SO, Rig, Jack-up etc., are carried out in the dry dock of

shipyard by stacking unit blocks sequentially from lower to upper

levels. The sequential block-stacking method can create difficulties interms of schedule and cost due to interference with overall material

flow of shipyard and due to extended use of the dry dock. In some

cases, hull fabrication and assembly are carried out in a dry dock by

block-stacking method and after that topside modules are installed by

heavy lifting crane near the quay side. This methodology has great

dependency on dock schedule and dock capacity. Hence, it is very

difficult to incorporate any design change or change in construction

process suggested by the client or contractor. In addition, by the end of

1997, orders were booked for most of the East Asian shipyards for

next 2 or 3years. So there was a need for new construction

methodology instead of conventional dry dock method.

For the success of ground build methodology of VLCC size F(P)SO,

the following specialized features were considered ;

- Ability to construct either in dock or ground

- Satisfy the ship registers regulations

- Sufficient strength under topside module loading

- Simulated Load-out & Float-off studies using F. E. Analysis

- Loadout Scheme Study, DBU Connector load Study & DBU

Connector Type Study

- Model Test, Wave induced Response Analysis and F.E. analysis

for DBU Connector

- Possibility to verify the overall structural strength by Full ship

F. E. Analysis.

NUMERICAL EVALUATION FOR OVERALL HULL

STRENGTH

General

AMENAM KPONO/FSO has an over all length of 298m, breadth of

62m, depth of 32.2m, design draft of 22m and a lightship weight ofabout 48,000ton including topside modules weight of 12,000ton.

The F. E. Analysis was performed to verify the structural integrity and

strength check of the AMENAM FSO Hull for the following

conditions for the ground build load-out :

Pre Load-out Condition (for aft part),

Side Skidding condition (for forward part),

Proceedings of The Thirteenth (2003) International Offshore and Polar Engineering Conference

Honolulu, Hawaii, USA, May 2530, 2003

Copyright 2003 by The International Society of Offshore and Polar Engineers

ISBN 1880653-605 (Set); ISSN 10986189 (Set)

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Fit-up condition (for final assembly),

Main Load-out Condition

Each condition consists of Active Jack (Jack Up) and Passive Jack

(Jack Down) case excluding fit-up condition. Soil Stability and

Strength test of HHI offshore yard also performed. In this paper we

described the pre load-out and main load-out condition only.

Pre Load-out

Although, area of HHI Offshore division yard exceeds 880,000m2, an

overall FSO length of 298m was too long to accommodate and to

fabricate in single unit. Hence, HHI has followed Pre Load-out

concept to fabricate and final Load-out.

Fig. 1 Yard Layout during Fabrication

In Pre Load-out concept, the construction of AMENAM KPONO/FSO

has been carried out in two parts. After fabrication, the aft part has

been moved to quayside, cantilevered out into sea for about 60m to

facilitate final assembling for Load-out. For construction purpose,

AMENAM FSO was divided into 160m aft-part and 138m forward

part including SBM(Single Buoy Mooring). Forward and aft parts

including topside modules were constructed by block-stacking

method.

Model geometry and Analysis

The 3-D F.E. model used for this analysis represents the full aft part

hull structure (from A.P. to Fr. 31+1350mm), based on the basic key

plan and structural drawings. Two cases are considered for analysis.

Case 1 : Pre load-out Active Jack condition

The weight of aft part for load out was about 27,000ton and

fifty-two(52) m x 4 active shoes were installed at longitudinal

bulkhead and side shell of aft part for moving to quayside fromfabrication area. During the pre load-out active jack condition, the

weight of the aft part is evenly distributed along skid-way foundation

and no stress concentration was found. (Refer Fig. 2)

Fig. 2 The after part deflection during the active jack condition

Case 2 : After Pre load-out Passive Jack condition

After the pre load-out, web frame No. 12 is located at end of

quay-wall and about 60.0m hull structures overhang from the

quay-wall, 18 fabrication supports are located under each frame

from Fr. 12 to Fr. 14 and 4 supports are located under each frame

from Fr. 15 to Fr. 30. For the structural integrity of the AMENAM

FSO hull at above mentioned over hanging position for 2-months

approximately, before main load-out, HHI executed some case

studies to verify the supporting system using sensitivity analysis

method.

The edge part near quayside has to take various concentrated loads

due to soil settlement, overhang of structure, environment loads

etc, when the aft part is in cantilevered position.

Fig. 3 The after part overview at near the quay-wall

To consider the yard settlement, deformation of temporary supports

and to relax load concentration near the quay wall, HHI has carried

out some case studies as follows :

HHI used three types of timber stiffness at each support location as

given below to consider possible support deformation. The minimum

timber dimension was 700 mm (L) x 700 mm (B) x 280 mm (T). For

the timber stiffness calculation, elastic modulus used is 66N/mm2 ~

16100N/mm2. (1) Base Case: timber direction perpendicular to grain,

(2) Alternate -1: timber direction parallel to grain and (3) Alternate 2:

Elastic modulus value was extracted from the test results of HHI.

(1) Base case : K = 6.00E+08N/m (E=16100N/mm2/20)

(2) Alternative 1 : K = 1.21E+10N/m (E=16100N/mm2)

(3) Alternative 2 : K = 4.95E+07N/m (E=66N/mm2)

(A) First HHI applied enforced deformation of 5mm, 10mm, 15mm

and 20mm in successive cases at Frame no. 12 of AMENAM FSO

hull to relax a load concentration near the quay wall.

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In the above cases support reactions and maximum stresses are within

limit. However, soil stability between quayside and pile foundation

below frame no 12 to frame no 15 is need to be checked.

0

1000

2000

3000

4000

5000

6000

7000

Frame No.

Reaction(Ton)

5mm

10mm

15mm

20mm

Fig. 4 Case study result for Case A

(B) Second HHI applied 0 20 mm relative deformation between

Frame no. 12 to Frame no. 31 of AMENAM FSO Hull for all possible

alternative cases to consider yard settlement.

In the above cases support reactions and maximum stresses are within

limit.

0

1000

2000

3000

4000

5000

6000

7000

8000

Frame No.

Reaction(Ton)

5mm

10mm

15mm

20mm

Fig. 5 Case study result for Case C

(C & D) After investigating the results from above cases (A) and

(B) to avoid soil stability problem HHI removed temporary

support below Frame no. 13 and applied the enforced deformation

in successive cases as per Case A and Case B respectively.

All of the above cases are combined with support location

optimization to check stress concentration in FSO hull due to

enforced deformation condition, yard displacements condition and

support deformation condition.

-90.0

-70.0

-50.0

-30.0

-10.0

10.0

Web frame No.

D

eflection(mm)

Original

Add 15mm(Base)

Add 15mm(Alt1)

Fig. 6 Hull Deflection Curve for After Pre Load-out

Results

From the analysis results for all the cases, maximum stresses in

hull and support reaction values are acceptable and under the

allowable limits. HHI has selected following case for actual work

according to the feasibility of construction.

- Enforced deformation at Frame no. 12 15mm

- Yard settlement(relative) 20 mm(max)

- Temporary support below Frame no. 13 Removed

- Timber stiffness considered Base case value

Fig. 7 Support arrangement for after the pre load-out condition

The maximum deflection of AMENAM FSO after part is 46.0mm

during the pre load-out active jack.

The maximum Von-Mises stress of plates is 162 N/mm2 at side shell

in jack passive (after the pre load-out) condition. All plates are within

the allowable stress limits during pre load-out condition.

The maximum value of buckling unity check is 0.78 in jack passive

condition at side shell plate. All plates, stiffeners, girders buckling

stresses within the allowable limits.

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Soil strength and stability check for ground build

In scheme of pre load-out and final load-out of FSO structure, the

main foundation components are skid way located on mat foundations,

skid way located on piles (near the quay-wall), quay-wall with an

arrangement of link beam and onshore bridge for load-out. The most

critical operation from geo-technical point of view was pre load-out,

where part of aft structure was cantilevered on the quay wall for about60 m length. The entire hull was made to rest on fabrication support.

The main challenge was to find-out an optimum support arrangement

to satisfy quay-wall stability, bearing capacity, settlement and

associated stress in the FSO hull.

Table 1. Soil Strength and Stability results

Foundation stability and bearing capacity were checked for various

stages of construction and it has been found that HHI yard has a

sufficient bearing capacity of more than 150ton/m(Table 1).

Settlement estimations were also made to optimize support

arrangements and to keep the stresses in the FSO structure within

allowable limits.

Table 2. Soil Strength and Stability summary

Sliding Overturning Bearing Capacity

During Pre-Loadout (-) 11.0 2.5 2.6 4.5 5.0 3.8 4.2

After Pre-Loadout (-) 11.0 5.6 5.9 7.9 8.7 2.5 2.6

During Loadout (-) 11.0 3.7 3.9 6.1 6.6 3.7 3.9

Rock pad Stability 1.4

Factor of SafetyLoad Condition

Elevation

(m)

Load-out using multi-purpose DBU

Overall length of AMENAM FSO is 298m and the estimated lightship

weight is 48,000 Metric ton. HHI had selected 51,000 Metric ton for the

design of load-out in order to have sufficient engineering margin

including environmental effects. AMENAM FSO has two inner skin

bulkheads and side shell. These bulkheads were supported by

symmetrical 124 m active shoes system during the normal load-out

operation. From the 3D analysis results, longitudinal bulkhead and

side-shell share 82% and 18% load respectively, under the active shoe

up-lifting force.

To keep no Heeling condition for DBU, HHI implemented a special load

sharing plan. According to the Load sharing plan Longitudinal bulkhead

and Side shell of FSO has taken the 65% and 35% of uplifting force

respectively, especially from 124m active shoes during the Load-out.

Model geometry

The AMENAM FSO full ship 3-D F.E. model consists of eight (8) superelements and detailed block divisions as shown in the Fig.8 below.

The model composed of 184,229 nodes and 319,578 plate & beam

elements. The AMENAM FSO hull structure is modeled by means of

three or four node shell and two node beam elements. Steel panels are

modeled by plate & shell elements with appropriate thickness, while

girders, webs and longitudinal stiffeners are modeled by T-beam with

appropriate geometrical properties (flange and web thickness) and

eccentricity. Plate element sizes in the model are as same as the

longitudinal spacing of hull structure (800mm ~ 930mm).

Fig. 8 The 3-D model and boundary condition of AMENAM FSO

Basic Model Load

Load-out weight, Yard settlements and Barge deflections due to still

water and wave induced bending moments are the main loads arises

during the load-out operation. The active jack stroke 250mm was

sufficient to accommodate Hull deformation and Yard settlement. Fornormal operation, yard settlement effect has not been considered as

load-out design load.

- Vertical gravity Load

The structural coded weight was generated automatically by SESAM

program. Weight of non-coded misc. items e.g. external turrets, living

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quarters, topsides modules, etc weights were calculated based on weight

control reports and inputted as separate loads. Contingency of 10% was

considered to account for stiffeners, paint and welds.

1) Hull structural dead loads (BLC 1)

2) Topsides module loads -Include equipments (BLC 2)

3) Turret & suspended loads (BLC 3)

4) Living quarters and heli-deck loads (BLC 4)

- Uplifting force from Active shoe System

The active shoe system is able to distribute load effectively and evenly

on the load-out beams. To make good stability during the load-out

operation, different up-lifting forces and locations are calculated at each

active jack groups.

5) Active jack up-lifting force (BLC 5)

- Deformation for boundary displacements

For the structural integrity at jack-down condition, the Hull boundary

displacement was extracted from active jack-up condition and combined

with DBU deformation at 10m, 60m and 100m progress of skidding on

DBU during the main load-out.

6) Hull deformation (BLC 6)

7) DBU deformation(BLC 7-9)

Load Combinations and boundary conditions

During the main load-out, boundary conditions considered are as shown

in Fig. 8 and active jack was located on web frame No. 18 to 42 of each

side shell and longitudinal bulkhead.

For the consideration of barge deformation during the main load-out,

HHI considered combination of hull deformation (BLC6) & barge

deformation (BLC7 ~ 9) applied at each load-out step.

1) Load combination 1 (LCB 1)

Combined vertical gravity loading and up-lifting force for jack active

condition (Load-out initial condition)

LCB 1 = BLC1 + BLC2 + BLC3 + BLC4 + BLC5


Combined jack active condition and barge and/or hull deformation after

progressed 10m on board of the barge

LCB 2 = BLC1 + BLC2 + BLC3 + BLC4 + BLC6 + BLC7



progressed 60m on board of the barge




progressed on board of the barge


Results

During the load-out, the maximum deflection of AMENAM FSO was

340mm between A.P. to F.P. but within the active shoe support the

deflection was less than 100mm.

The maximum Von-Mises stress in plate elements was 214 N/mm2 at

side shell during 100m progressed on board condition. Stress contours

near the longitudinal center of gravity location at initial condition areshown in Fig. 9 ~ 12. All plates were within the allowable stress

limits during main load-out condition.

The yield and buckling checks have been carried out using PLATE

WORK module of SESAM programs for plate elements and based on

the criteria of DNV classification Notes No.30.1, respectively. The

maximum value of buckling unity check is 0.88 at side shell plate after

100m progressed on board in barge condition. For all plates, stiffeners,

girders buckling stress are within the allowable limits.

Fig. 9 Load-out Initial condition X-axis normal stress contour

(Near the L.C.G Location: -161N/mm2~ 180N/mm2)

Fig. 10 Load-out Initial condition Y-axis normal stress contour


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Fig. 11 Load-out Initial condition shear stress contour


Fig. 12 Load-out Initial condition Von mises stress contour


NUMERICAL SIMULATION FOR MULTI PURPOSE

DBU (Double Barge Unit)

HHI designed two submersible barges for multipurpose activities likeload-out, tow and float-off. Each of two barges can submerge to 25.5

m with several sizes of portable casing tank and separate control system.

Two barges can work separately for small cargo or combine together for

heavy cargo. In case of AMENAM project, the two semi-submersible

barges (HDB-1011&1012) were combined with the help of connector to

make a single unit called DBU.

FWD

FWD

Fig. 13 Configuration of Double Barge Unit

Load-out Scheme Study

Due to the long length, huge weight of AMENAM FSO, loadout

direction was studied on the point of view of relative motion, DBU

connector load and mooring force at several cases. After the several

scheme case studies, longitudinal direction with connected two barges

was selected. Case study for connector type was also performed based

on the motions and loads expected at connectors under all the schemes.

Scheme 1 3 Barge Systems: (HDB1011, Space Barge, HDB1012)

Load out of FSO in Transverse direction of barges

Scheme 2 2 Barge Systems: (HDB1011, HDB1012)

Load out of FSO in Longitudinal direction of barges

To verify the structural integrity of the schemes, HHI performed

numerical simulations and model tests for the loadout process.

Maximum displacements, accelerations and sectional forces were

obtained and compared.

From numerical simulations and model test results it was found that the

connector loads and maneuverability in load out/ towing operations

within the channel were better in scheme 2 compared with scheme 1.

The synchronizing of ballasting operations was also simpler in scheme 2.

And mooring in scheme 2 was found to be more practical than scheme 1.

Generally Scheme 1 seems to be the best in motion response point of

view and scheme 2 seems to be the most advantageous in the aspect of

design load and operation.

< Transverse Direction : Scheme1>

Fig. 14 Model Test for Loadout Scheme

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Table 3 Maximum Accelerations Model Test

HS Surge Sway Heave Roll Pitch Yaw

m m/s2

m/s2

m/s2

deg/s2

deg/s2

deg/s2

Scheme 1 1 0.106 0.103 0.354 0.179 0.283 0.020

Scheme 2 1 0.129 0.195 0.625 0.779 0.254 0.057

Acceler ation

Table 4 Maximum Loads Numerical Simulation

HS Fx Fy Fz Mx My Mz

m Kn Kn Kn Kn.m Kn.m Kn.m

Scheme 1 1 4156 18367 34707 378417 836535 356655Scheme 2 1 2913 17387 13280 87225 389019 266910

Sectional

Loads

Connector Design

Loads at connector were mainly function of difference of stability

properties of two barges, accuracy of ballasting and dynamic force from

waves. Loads were studied at each operation with respect to the static

forces and wave induced dynamic forces.

At loadout and float off stage, relative heeling moment induced by

ballast control error and skid reaction change was governing factor of

connector load. However, at towing condition, due to minor changes at

ballast and skid reactions, dynamic forces by waves were governing

factor comparing with static loads.

Following design conditions were considered for DBU connector design

a) Ballast Control Error

Following criteria had been considered, based on the Ballasting Pump

Capacity, to calculate the resulting moment.

Ballast Error Time 10 min., Ballast = 680 t Resulting Moment due to Ballast Error = 5,365 t.m

Fig. 15 Ballast Error

b) Skid Reaction Change

While AMENAM FSO is being skidded on DBU skid way, reaction

forces could be changed by 1) difference of vertical acceleration

component due to wave and also 2) difference in deballasting operation

between two barges. According to the model test result, 2.75% of

vertical acceleration was expected. Hence, 5% of load was applied in

order to take care of this change in skid reaction. Resulting heeling

moment from possible deviation of reaction was considered forconnector design.

Skid Reaction Change Range = 5%, Reaction Change = 1,300 t Resulting Skid Reaction moment = 24,947 t.m

Fig. 16 Skid Reaction Change

c) Wave force

Other load coming on the connector is dynamic load due wave force.

Various characteristic responses has been calculated for Load-out, Tow

and Float-off conditions for wave loading using WADAM program

released by DNV.

Table 5 Environmental condition for wave response analysis

Hs Pe ri od (Tz ) He ad in g

Meter Meter Sec. Deg.

LOADOUT8.36 1 4.5 ~ 6 0 ~180

TOWING 8.36 2.5 4.5 ~ 6 0 ~180

FLOAT OFF 20.4 1 4.5 0 ~180

WaveDBU Draf t

Characteristic Responses considered are Longitudinal Shear Force (Fx),

Split Force (Fy), Vertical Shear Force (Fz), Relative Heeling Moment

(Mx), Relative Trim Moment (My) and Yawing Moment (Mz).Table 6 is

showing the Maximum Characteristic Responses for the above

operation environment condition.

Table 6. Environmental Force

As a result of studies of the connector loads during load out, towing and

float off operation based on the ballasting plan, operation simulation,

model test and wave response analysis, relative heeling moment and

vertical shear force were found major factors controlling the design of

DBU with connector.

Different configurations of connectors, such as hinged type and fixed

type, were analyzed to meet the required criteria and the configuration

of connector shown below was selected to cater the requirement of

required strength for heeling moment. Additionally one bottom

connector was also provided to take care of compression forces due totug boat during operation. To keep the bottom connector in compression,

suitable ballast plan were developed also.

t t t t.m t.m t.m

LOAD OUT 104 1,030 826 2,681 9,603 11,613

TOWING 353 2,574 2,066 6,702 32,327 40,433FLOAT OFF 103 480 294 9,161 8,978 27,824

MY MZFX FY

Wave Induced Force s (Maximum)

FZ MX

BALLAST

ERROR

!"!"!"!"

CASING CASINGAMENAM FSO

HDB 1012 HO

CASING CASING

BALLAST

ERROR

HDB 1011 HO

20.0%30.0% 30.0% 20.0%

SKID REACTION

CASING CASING

20.0%30.0% 30.0% 20.0%

SKID REACTION

HDB 1012 HO

CASING CASING

HDB 1011 HO

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HDB

1012 HOHDB

1011HO

UPPER PARTRIGID CONNECTOR FRAME

LOWER PARTRIGID CONNECTOR FRAME

Fig. 17 Rigid Frame Connector

FEM Analysis for connectors

For the above connector configuration FE analysis has been carried out

to find the capacity of connector..

Fig. 18 F.E. Model for Float Off Operation

The results showed that the Capacity of connector is as follows;

Against Bending Moment : 40,000 tm

Against Shear force : 23,500 t

As it is difficult to control and monitor the dynamic loads due to wave,

the operation limit has been finalized to keep the maximum heeling

static moment less than 30,000tm. The static B.M. and S.F. has been

monitored at control room with the help of Load Master Computer

(LMC), by keeping a margin of about 10,000 tm for dynamic load.

CONCLUSION

- The Ground Build Load-out of VLCC size AMENAM F(P)SO had

been verified by F. E. Analysis for various condition and executed

successfully. The maximum stresses and deflection of the hull structure

during the pre load-out and main load-out is well below the allowable

value, almost 60% of the extreme design condition. The Yield and

Buckling checks have also been checked as per the DNV classification.

- The concept of using DBU (Double Barge Unit) for load-out was

also a challenging move which proved successful and cost effective.

However, constant monitoring, precise load sharing plan, accurate

ballast control, proper mooring arrangement at various stages of on

board condition is very essential for success of project.

- With the use of Multi Barge unit Ground Build method can eliminate

all the constraints with conventional dry dock construction like capacity,

schedule, cost, incorporation of changes, etc. and concluded that Flexi

hull can be used as an efficient construction method.

- Ground Building study for irregular type Ultra Heavy Floating

Structure construction using multi barge unit is the next step in Floating

Offshore Structure construction Industry. Also, the use ofSynchronized Multi barge unit for removal and/or shifting of Fixed

Structure can be added scope.

REFERENCES

BV, September, 1998,Rules and Regulations for the Classification of

Ships

DnV, January, 1996, Rules for Planning and Execution of Marine

Operations

Hyundai Heavy Industries, May, 2002," Ballasting and Stability analysis

for Load-out of AMENAM FSO"

Hyundai Heavy Industries, May, 2002," Foundation Analysis & Design

report for Load-out of AMENAM FSO"

Hyundai Heavy Industries, June, 2002," Hazop for Load-out & Float-off

of AMENAM FSO"

Hyundai Heavy Industries, January 8, 2000,"Load-out Procedure for

RBS-8D Semi-Submersible Drilling Rig"

Hyundai Heavy Industries, May, 2002,"Operation Manual for Load-out

of AMENAM FSO"

Hyundai Heavy Industries, September, 2001," Outline method statement

for Load-out & Float-off of AMENAM FSO"

Hyundai Heavy Industries, May, 2002,"Strength analysis of Connector

for HDB 1011/1012"

KAIST, October 30, 1985,"Development of Design Technology of

Offshore Platforms for Offshore Oil Production

Noble Denton Co. Ltd, Guidelines for Marine Operations

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Structural Evaluation of FSO Ground Build Load-Out

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