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OGP Marine Risks Workshop Proceedings

OGP

. FPSO Design & Conversion PracticeTaco Terpstra, Gusto Engineering

Foresight in

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FPSO DESIGN AND CONVERSION PRACTICE

OGP Conference

London, March 17, 2003

T. Terpstra

IHC Gusto Engineering BV

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FPSO: ship or offshore structure?

Consequences in design procedure

Applicability tanker gained experience

Restrictions

When apply offshore standards and how combine?

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International Association of Oil & Gas Producers

OGP

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Contents

Design procedure and approach Offshore and shipbuilding approach

FPSO hull design procedure

Specific design issues Hull design features

Design loads

Hull / topsides interface

Materials

Hull fatigue aspects

Corrosion allowance

Specific conversion issues Steel renewal criteria

Novelty in FPSO conversion

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Shipbuilding and offshore approach

Offshore approach

Strong focus on design reliability

Extensive adoption of direct calculation principles

Use of multi-disciplinary design teams

Shipbuilding approach

Focus on production process, keen on costs

FPSO part of shipyards mass production, less flexibility during

8 -12 week dry dock assembly time

Implications of building offshore structures not always clearly

realised

Clear division between disciplines

FPSO Design & Conversion Practice - Taco Terpstra, Gusto Engineering ()

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FPSO Hull Design Process

Concept Design- Global FEA

Detailed Design

- production drawings

- welding details to further develop

out for tender,

consultant further

develops design

Operator

Shipyard

Designconsultant

Confirm with shipyard critical structural details

Operational phase

- Hull condition monitoring and survey program

- Newbuilding supervision and site management

68weeks

Basic Design-Detailed Finite Element Analysis

-Detailed Fatigue Analysis

-identification of critical structural welds

16-20

weeks

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Concept Design Overall dimension

Tank arrangement

Global hull structure

Global finite element analysis

Analytical Tools:

a) Rules based calculations

b) FEA, 3-tank model

Global structural analysisobjective:

a) Examine strength

b) Determine hull flexibility

c) Accurate steel weight

estimate


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Basic Design Mooring system integration

Riser system integration

Detailed transverse strength

analysis

Local strength and fatigue

analysis Identification of critical welds

Topsides / hull interface plan

Class Approval

Local structural analysis

objective:

a) Fatigue performance of

stiffener connections

b) Transverse web end

connections

c) Riser integration structure

CourtesybyBureau

Veritas

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Hull Design Features

New build designs tend to excessive hull proportions:

L/B = 5.1 (Tanker > 5.5)

L/D = 9.2 (Tanker > 11.0)

CB

= 0.97 (Tanker < 0.9)

Excessive hull proportions lead to:

High still water loads, ample buoyancy at the ends /

large topsides weights amidships

Increased freeboard for smaller ship length

Hull size shall match shipyard dock capacity


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Tank Arrangement Newbuild FPSO

Machinery space

ballast in side tanks

3CT ballast tank reduce SWBM

Disadvantage is empty tank in fully

laden condition

Machinery space

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Buoyancy Curves

New Build and Conversion FPSO

0

0,2

0,4

0,6

0,8

1

1,2

0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1

Length of Vessel x/L [-]

Buoyancy[-]

New build FPSO

Conversion FPSO

Ample buoyancy

induce high SWBM


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Still Water Bending Moments New Build FPSO

-12000000

-10000000

-8000000

-6000000

-4000000

-2000000

0

2000000

4000000

6000000

8000000

0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00

Lengthof Vessel x/L [-]

BendingMome

nt[kNm]

Hogging Rules Design

Sagging Rules Design

Hogging Actual

Sagging Actual

Hump/hollow due to

ballast tank

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Design Loads

Still water loads exceed tanker standard design values

SWBM and SWSF for newbuild exceeds tanker design

values by 95%

SWSF for conversion exceeds design values by 77%

Normal tanker practice not directly applicable

Concentrated topsides loads

Single deck module Fz = 130Te

Double deck module Fz = 2000Te

Concentrated loads may result in local overturning

moments at supports, ship structure not designed for


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Hull / Topsides Interface

Difference in dimensioning of plate sides:

Shipbuilding moulded plate side

Offshore center of plate

Offshore and shipbuilding standards conflict in weld

design for topsides supports

FP or PP weld details to be applied according to offshore

standards

Shipbuilding use fillet welds, throat thickness 0.17*t

Special weld details at specific locations disrupt

shipbuilding production

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Midship Section 2.2 mbls storage

Width approx. 5m

Long. BHDs to line up with

topside supports


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Tank loading deformation

Two adjacent tanks, one LBHD

Tank loading deformation

Three adjacent tanks, two LBHDs

Difference in deformation to be

addressed in conceptual design

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Regions with enhanced scantlings (transverse web frame)

In order to support topsides modules, enhanced scantlings of upper

strakes of BHDs and transverse web frames to cope with high

concentrated loads

Use of local plate inserts to be avoided, since this would disrupt the

yards production process

t = 12.5mm

Increase to t = 15mm

minimum

Increase upper plate

strake BHD t > 22mm

sides

hell

long.

BHD lo

ng

.BHD

main deck


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Support stools on main deck Side view support stool

FP WELD

PP WELD

SUPPORT

LOADSFz

My

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Flexible Supports Process Module: 4-point-support

Elastomeric pads account for hull

flexibility and are rotational free,

less bending moment introduced in

hull structure


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Tubular supports

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Materials

Difference in material selection procedure

Shipbuilding based on zones and class, I - V

Offshore based on function and criticality

FPSO hull material selection

Main hull structure based on shipbuilding standards

Special structures based on offshore approach

Topsides supports

Turret/mooring integration

Offshore and ship selection criteria can be combined in atransparent selection table

Special requirements

Reserve on material qualities, use of Z-quality forenhanced flexibility

Higher material grades on owner request for improvedfracture toughness


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Hull Fatigue Aspects

Safety factors shall be increased to values between

2 < FS < 10 depending on criticality & inspectability,

deviation from tanker rules where FS = 1.0

Other details like topside supports and mooring & riserintegrations shall also be verified

Note that not included in the analysis are: Local corrosion which has a large impact on cracking

(void side tanks coated or dehumidified)

Workmanship (alignment and weld quality) which haslarge impact on cracking

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Hull Fatigue Details Side Shell

TSCF as guideline for details

avoid these details

transv.web

transv.web

side

shell

add backing bracket


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Engineering Approach

FPSO hull according to CN 30.7

First screening based on World Wide data, FS = 1.0

World Wide FS = 1.0 corresponds to FS 3.0 for benign

environment

Limited experience analysis software

Use of different S-N curves for ship and offshore

structures

Basic S-N curves for ship structures, excluding SCF

Series of S-N curves for offshore structures

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Corrosion Allowance

Tanker experience: nominal Rule corrosion rate

Rules state nominal corrosion rate 1.0, 1.5 and 2.0 basedon 20-year design life

Analysis software use these default corrosion values

Ship rule allowances may not be sufficient

Consistency required between Rule design values andmeasured / TSCF values

Apply corrosion design allowances on plating andstiffeners based on (in-house) FPSO measured dataand/or TSCF

Corrosion allowance to be based on offshore design life,not on ship repair life


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Steel Renewal Criteria

Inspection and survey of tanker hull is still often based onmarine practice

Marine practice refers to a 5-year regular special survey

Consequently it is common practice that often hullstructural details are approved with 5-year inspectionperiod as reference

No special survey for FPSOs applicable, special survey isequivalent to FPSO design life

Solution is to provide conversion steel renewal criteriato be derived from fatigue analysis

Check on plate thickness and section modulus

Proportionality of existing stiffeners is limiting factor

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Steel Renewal Criteria (case study)

Severe pitting corrosion instiffener flange

Initially approved based on 5-year survey experience

Repair in yard: $3/kg

Repair offshore: $30/kg


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Novelty in FPSO Conversion

Conversion of a suezmax tanker to equivalent VLCC sizeFPSO

Production capacity is 100,000 bbls, installed on smalldeck area

Congested deck area due to module supports, cable traysupports and spread mooring arrangement

To overcome these problems sponsons are added to: Increase deck space area

Support of process modules

Increase of deadweight

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Novelty in FPSO Conversion


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Conclusions

FPSO to be considered as an offshore unit, ship design

gained experience forms the basis

Enhanced level of detail in concept and basic design

Differences in tanker and FPSO design

Hull design

Loads

Structural design criteria

Ship and offshore approach can be combined for effective

design

Tanker Structure Coorporative Forum (TSCF) shall be

more frequently applied in FPSO design

Foresight in

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FPSO DESIGN AND CONVERSION PRACTICE

OGP Conference

London, March 17, 2003

T. Terpstra

IHC Gusto Engineering BV


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fpso con03

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