ABS FLOATING PRODUCTION INSTALLATIONS GUIDE RULE … · 2018-12-11 · ABS TECHNICAL PAPERS 2004...

ABS TECHNICAL PAPERS 2004

ABS Floating Production Installations Guide Rule Change: FPSO Fatigue and Strength Assessment 81

Proceedings of OMAE-FPSO 2004 OMAE Specialty Symposium on FPSO Integrity

Houston, USA 2004

OMAE-FPSO 2004 - 0093

ABS FLOATING PRODUCTION INSTALLATIONS GUIDE RULE CHANGE:

FPSO FATIGUE AND STRENGTH ASSESSMENT

Ernesto Valenzuela American Bureau of Shipping

16855 Northchase Drive Houston, TX 77060, USA [email protected]

Jer-Fang Wu American Bureau of Shipping

16855 Northchase Drive Houston, TX 77060, USA

[email protected]

Haihong Sun American Bureau of Shipping


[email protected]

Tuanjie Liu American Bureau of Shipping


[email protected]

Presented at the FPSO Workshop, OMAE Specialty Symposium on FPSO Integrity (OMAE-FPSO 2004) and published in the Proceedings of OMAE-FPSO 2004, Houston, Texas, August 30 - September 2, 2004

Reprinted with the kind permission of American Society of Mechanical Engineers, New York, NY ABSTRACT

SafeHull (SH) is a system that comprises Rules and

software programs, as depicted in Figure 1. The ABS Guide for Building and Classing Floating Production Installations [2] (herein after called ABS FPI Guide) and the associated SH-FPSO software system have been in use for over 3 years in the design classification of FPSO new builds and conversion from existing tankers. This paper provides information about the new FPI Rule change (April 2004) regarding the classification process of FPSO conversions as an ongoing effort to address the needs of clients. It highlights the impact of the changes on the two typical conversion processes – the basic Ordinary Conversion and the optional SH (CS) Conversion. The paper focuses on the new requirements for fatigue assessment, i.e., the Environmental Severity Factors (ESF), and strength including loading cases that apply to SH phase A and B as a requirement for the structural review process, which is carried out by the classification society. Class requires not only the certification of the project but also future surveys of the vessel. FPSOs are also considered offshore structures,

therefore, this paper also discusses the application of the ABS Guide for Buckling and Ultimate Strength Assessment of Offshore Structures [4] applied to FPSOs and makes comparison with the ABS Rules for Building and Classing Steel Vessels [3], which uses the net scantling concept to determine buckling of structural members.

INTRODUCTION ABS introduced the SH criteria for new construction

tankers in 1993 and formalized into Rules in 1995. The next step was to extend this technology to tanker conversions already in service. ABS uses the Site-Specific Environment Assessment System (SEAS), which is based on the Environmental Severity Factors (ESFs) concept in 1997. The main function of SEAS is to assess the remaining fatigue life of the hull structures, taking into account the site-specific environment and the fatigue damage accumulated during the trading history. Based on experience gained from actual conversions and further research, this ESFs approach was refined in 1999 and SEAS was made part of the SafeHull FPSO design criteria and analysis system, which was introduced to the industry in 2000. New design criteria introduced in 2004


82 ABS Floating Production Installations Guide Rule Change: FPSO Fatigue and Strength Assessment

have been published in the latest version of the ABS FPI Guide.

The concept and details of ABS ESFs approach for extending the SafeHull tanker design rules can be found in papers by Zhao et. al.[8,9] Basically, the ESFs are a measure of severity of route-specific or site-specific wave environment relative to the unrestricted service wave environment, which is the North Atlantic environment. Two sets of ESFs are derived for the FPSO fatigue and hull strength design. The α–factors are used to adjust spectral fatigue strength performance between unrestricted ocean going tankers and the long-tern site environment. The β–factors are used primarily to adjust the dynamic component of loads that are used to assess the hull strength and fatigue capacity of connection details. Therefore, the base for both factors is the unrestricted service, which uses equal probability to account for the heading effect. The new enhancements introduced in the FPI Guide make now a distinction between spread moored and turret moored FPSOs to better account now for the wave heading effects.

Rules

The SafeHull SystemConsist of Rules &Software Programs

SafeHull Phase ASafeHull Phase B

SEASVersion 9 - 2003Version 10 -2004

TankersBulk CarriersContainer ShipsFPSOs

Rule for Buildingand Classing SteelVessels - 2004

Floating ProductionInstallation (FPI) - 2000Supplement 2 -2003Supplement 3 - 2004

Spectral-Based FatigueAnalysis for Floating

Production, Storage andOffloading (FPSO)

Systems - 2002

SafeHull FEA Guidancefor Hull StructuresGlobal 3D Analysis

(Draft - 2004)

Figure 1 Sketch of ABS SafeHull System

The SafeHull criteria are a written set of equations or requirements published in the ABS Rules and Guides. The software programs are tools that facilitate the use of the Rule requirements, which may not reflect all the requirements covered by the Rule. When applying the SH criteria, it is understood that the design approval is not limited to the execution of the SafeHull software. Often this misconception of some designers needs to be clarified.

This paper also illustrates the application of the ABS Guide for Buckling and Ultimate Strength Assessment of Offshore Structures to determine the buckling of stiffened

panels and compares the results to the one obtained using the ABS Rules for Building and Classing Steel Vessels.

PHASE A AND PHASE B REVIEW The SafeHull Phase A criteria contains SafeHull

loads, global and local scantling requirements. The scantling review includes strength evaluation for site-specific environment and fatigue assessment taking into account previous trading history as a tanker. SH/FPSO Phase B is a verification of the initial scantlings through a total strength finite element stress analysis for a set of standard load cases with defined SafeHull loads. This FEM strength analysis is only required for classification for FPSO seeking SH(CS) notation or new build. The detailed Phase A and Phase B review procedures are described by Tam et. al.[6]

FATIGUE ASSESSMENT ENHANCEMENTS Fatigue is one of the important elements in the

classification of the FPSO hull structure, and it is even more so for FPSO vessels converted from existing tankers. For FPSO conversions, the estimated remaining fatigue lives of welded joints and critical structural details must be accurately assessed, especially for areas where high strength steel is used. The minimum acceptable target life for converted FPSO is the greatest of the on-site design life of the FPSO, the time to the next Special Survey, or five years.

The Definition of Environmental Severity Factor (ESF) α Factors in SEAS

The first step in the SH/FPSO Phase A review is to establish the ESFs for the route, transit and site environments. With the trading route history data and the FPSO site environment inputted, the SEAS program module is executed to determine the α factors for fatigue damage.

The general concept of this type of ESF is to compare fatigue damage resulting from different environmental conditions. The alpha type ESFs are obtained for six regions of the hull structure. This set of six structural locations with related alpha factors is compared with the base case for each environment. The six structural locations are:

• Longitudinal on deck. • Longitudinal on bottom plate. • Longitudinal on side shell. • Longitudinal on longitudinal bulkhead. • Longitudinal on inner bottom. • Longitudinal on centerline bulkhead. The ESF α factor is defined in SEAS as:

S

U

DD

=α (1)

where



DU = fatigue damage based on the base environment for the hull structural regions, which is also called ABS base-1, and corresponds to the North Atlantic environment.

DS = fatigue damage based on the considered environment of historical routes, historical sites or intended site, for the hull structural regions.

New SEAS Spectral Fatigue Approach The SafeHull simplified fatigue approach had been

calibrated with tanker fatigue data. In order to take advantage of this proven approach and to utilize the spectral fatigue analysis, a combined approach is used to calculate the fatigue life for an FPSO at a specific site. This approach employs the concept of ESF factors. The spectral fatigue approach can be calibrated against the simplified approach based on the Unrestricted Service environment as:

Prob.) Equal curve, SN(curve) SN(

prob.) Equal curve, SN(SpectralU

SimplifiedU

Lf

L

⋅= (2)

where L is fatigue life, U represents the Unrestricted Service (base) environment, and f (SN curve) is the calibration factor between the simplified fatigue approach and the spectral fatigue approach. In equation (2) the fatigue life refers to unrestricted service but the same calibration factor is applicable to environments other than Unrestricted Service.

The SN curve is fixed to F2 curve in SEAS spectral

fatigue analysis. The difference between F2 curve and other structural details that use other SN curves is corrected by the S-N curve correction factors that are presented in Figure 2. In the current SEAS approach, equal probability wave heading is always used to calculate fatigue damage in the Unrestricted Service Environment. Therefore, the remaining fatigue life LR for site-specific environment can be expressed as:

heading) requried E,(Curve)-E(

Curve)-E(SpectralSite

R

Lf

L

⋅

= (3)

and the ESF α factor in Equation 1 can be written as:

)prob.equal,()headingrequired,(

2

2

FLFL

SpectralU

SpectralSite=α (4)

In the new spectral fatigue approach, the remaining

fatigue life is expressed by the following formula:

heading) requried E,(

heading) required E,(heading) required E,(

Curve)-E( )(

SpectralSite

SpectralSEASU

SpectralSite

R

L

LL

L

⋅

= (5)

The new approach does not use calibration on the spectral fatigue method, because the purpose is not to cancel the simplified fatigue result, but to use it adjusted by a factor that represents the relative severity of the

environments. The fatigue life values calculated for Site and Unrestricted Service environments are consistent with the vessel headings. This means that for any structural detail, if head sea is used for Site, the same heading is also used for Unrestricted Service.

Comparison between alpha factors obtained from current SEAS approach and new approach is shown in Figure 2.

SN F2 curve was used in this study. The environment for this study is a bimodal sea state with Bretschneider spectrum for local wind generated waves and JONSWAP spectrum for swells. The cosine4 wave spreading is used for swell, and cosine2 spreading for waves. As shown in the figure, the current SEAS approach and the new approach give the same trend for the equal probability case. However, the SEAS current approach predicts more conservative results than the new approach does for oblique, head and following seas.

S-N Curve Effect on ESF α Factors

The α values calculated by SEAS are based on the F2 SN curve. However, the SN curve applicable to an individual structural detail is not always an F2 curve. For different SN curves, there are adjustment factors in the current SafeHull-Phase A as shown in Table 1. The factors given in the table are applied to α factors calculated in the SEAS program.

Table 1 S-N curve adjustment factors for α-factors

Curve

C3 (applied to α’s of intended and historical sites)

C2 (applied to α’sof historical

routes) D 1.10 1.10 E 1.10 1.05 F 1.05 1.025 F2 1.0 1.0 G 0.925 0.975 W 0.85 0.95

The current SafeHull for FPSO system does not

consider the B and C detail classifications. With these curve adjustment factors, the remaining

fatigue life can be finally written as:

⎟⎟⎠

⎞⎜⎜⎝

⎛−−= ∑∑

==

SR N

j j

jN

i i

iSHR C

SC

SLCL

1 3,21 2,2133 αα

αα (6)

where: NR ,NS represent the numbers of historical routes and

sites, respectively Si, Sj represent the service year of the vessel on their

i-th route and j-th site α1 is the ESF fatigue calibration factor between

spectral fatigue and SafeHull fatigue α2i is the ESF fatigue factor between the I-th

historical route and the ABS Base-2 wave data α2j is the ESF fatigue factor between the j-th

historical site and the ABS Base-1 wave data



S-N curve correction factor C3 for Deck

0.000

1.000

2.000

3.000

4.000

5.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C 3 (D

eck)

New approach

Factor used in SEAS

S-N curve correction factor C3 for bottom

0.000

1.000

2.000

3.000

4.000

5.000

6.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C3 (

botto

m) New approach

Factor used in SEAS

S-N curve correction factor C3 for side shell

0.000

1.000

2.000

3.000

4.000

5.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C3 (

side

she

ll)

New approach

Factor used in SEAS

S-N curve correction factor C3 for LBHD

0.000

1.000

2.000

3.000

4.000

5.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C3 (

LBH

D)

New approach

Factor used in SEAS

S-N curve correction factor C3 for Inner bottom

0.000

1.000

2.000

3.000

4.000

5.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C3 (

INB

M)

New approach

Factor used in SEAS

S-N curve correction factor C3 for Centerline Bulkhead

0.000

1.000

2.000

3.000

4.000

Bcurve

Ccurve

Dcurve

Ecurve

Fcurve

F2curve

Gcurve

Wcurve

C3 (

CLB

H)

New approach

Factor used in SEAS

Figure 2 SN curve correction factors for different SN curves



Figure 3 shows the comparison results of a study that has been conducted for the SN curve correction factors using the new approach. It is noticed that the correction factors are very close between the current SEAS approach and the new approach for SN curves of F, F2, G, and W type.

For SN curves of D and E type, the current SEAS factors are more conservative than the results obtained for the new approach. In addition, the new approach gives the correction factors for SN curves of B and C type, which are not included in the current version of SEAS.

Longitudinals stiifners on bottom plate

0.000

5.000

10.000

15.000

20.000

25.000

head sea Following sea Equalprobability

Oblique

Alp

ha3_

botto

m

New approachSEAS current approach

Longitudinals stiffners on longitudinal bulkhead

0.000

5.000

10.000

15.000

20.000

25.000

head sea Follow ing sea Equal probability Oblique

Alp

ha3_

stbd

_she

ll

New approach

SEAS current approach

Longitudinals stiffners on side shell

0.000

5.000

10.000

15.000

20.000

25.000

head sea Follow ing sea Equal probability Oblique

Alp

ha3_

port

_she

llNew approach

SEAS current approach

Longitudinals stiffners on centerline bulkhead

0.000

5.000

10.000

15.000

20.000

25.000


Oblique

Alp

ha3_

stbd

_she

ll


Longitudinals stiffners on inner bottom

0.000

5.000

10.000

15.000

20.000

25.000


Oblique

Alp

ha3_

stbd

_she

ll


Figure 3 Comparison of alpha factors obtained from current SEAS approach and new approach

Wave Characterization

A new update in the SEAS program is the bimodal

wave application. A sea state that comprises coupled contributions from both seas and swells is better represented by a sea spectrum in a double peak form, such as Ochi-Hubble 6 parameter spectrum. The current version of SEAS (V10) only accommodates two single-peak wave spectra (Bretschneider and JONSWAP). An

alternative approach is proposed to consider the effect of a bi-modal sea state. The essence of this approach is to assume that waves and swells can be separated by decomposing a given wave scatter diagram into two independent sections: one on the high frequency side representing waves, and another representing swells. Each section, which is modeled by a single peak spectrum, can be viewed as a new scatter diagram associated with an occurrence probability. The inverse of the calculated

Longitudinals on deck

0.000

5.000

10.000

15.000

20.000

25.000


Oblique

Alp

ha3_

deck




α−factors represent the damages for waves and swells. These two separated wave scatter diagrams are to be combined to obtain the final damage, as per Equation (7).

0111 C

SwellsSeasfinal

⋅⎟⎟⎠

⎞⎜⎜⎝

⎛+=

ααα (7)

where: C0 is the time overlap factor for seas and swells

occurring at the same time.

SEAS has also implemented the Ochi-Hubble double peaked spectrum. This allows using the scatter diagrams without decomposition, and allows to use a different heading for swells and waves for each cell in the scatter diagram in the same SEAS run.

Remaining Fatigue Life Calculation New updates for remaining fatigue life calculation for

transverse members have been implemented into SafeHull-FPSO phase B. A rapid fatigue calculation scheme for connection details in way of transverse members has been developed for remaining fatigue life calculations for transverse members in phase B. The calculation scheme includes the following steps: • Make two-phase B runs for β = 1 and β at intended

site. • The α3 at intended site is then computed based on

two FE results. • Assume the α2 (historical site) is close to α3

(intended site). • The α2 can be calculated separately.

The detailed procedure of fatigue strength assessment

for FPSO is shown in Figure 4.

Get element thickness and maternal property data for each element from NASTRAN output data file.

Compute permissible stresses using simplified method for each plate thickness

Get element stress range data for each element from SAFEHULL Phase B output data files for unrestricted sea and site cases.

Input SN curve type, Weibull shape parameter, number of stress cycles, stress concentration factor (SCF) and thickness correction data for unrestricted sea and site cases.

Compute α3 factor for each element based on F2 SN curve using simplified method.

Input number of historical route and site, number of service years for each historical route and site and their associated α2 factors.

Compute remaining fatigue life for each element.

Print fatigue life and fatigue check result for each element.

Figure 4 Flow chart for remaining fatigue life calculation for transverse members in Phase B



SEAS STRENGTH ASSESSMENT ENHANCEMENTS The Definition of Environmental Severity Factor (ESF) β Factors

Environment Severity Factor (β) is a severity

measure of the intended environment relative to the base environment in SH in terms of extreme loads, or it is sometimes called the ratio between the design case and the base case. It is defined in SEAS as

U

S

LL

=β (8)

where LS = the most probable extreme load based on site-

specific environment with 100-year return period for the dynamic load parameters, this is the ‘design case’.

LU = the most probable extreme load based on unrestricted service environment with 20-year return period for the dynamic load parameters, this is the ‘base case’.

A β value of 1.0 corresponds to the unrestricted condition of a seagoing tanker. A value of β greater than 1.0 indicates a more severe wave induced load environment than the unrestricted case, and vice versa.

Approaches There are two approaches to calculate base case load

LU. Approach 1 (current SEAS approach): Equal

probability wave heading is used to calculate LU. Approach 2: New enhancements introduce in the

FPI Guide. The values calculated for LS and LU are to be consistent with vessel headings. This means that for any dynamic load parameter, if head sea is used for example on the calculation of LS, the same heading is to be used for the calculation of LU.

For each dynamic load parameter, the severity

measure β is to account for the vessel headings as follows:

• For the intended site, the worst case between head seas, following seas and equal probability

• If no rosette information regarding the environment directionality is available for the transit condition, the worst case between head seas and equal probability between head seas and oblique seas (60 degrees from the bow), with oblique seas coming either from the port or the starboard side

There are 13 β factors corresponding to the 13 dynamic load parameters specified in ABS FPI Guide. The applications of these β factors are given in Appendix 2 of the FPI Guide.

As mentioned before, the Beta values are a direct function of the environmentally induced loads at the installation site compared to the unrestricted service environment that is the basis of the Rules. The Beta values can also address other differences and factors between the design basis of a sea going vessel and a moored vessel. These include:

i) Different design basis return periods for environmental loads (20 vs. 100 years).

ii) Major effects of mooring system on predicted vessel load effects (including weathervane type behavior of a turret moored system).

iii) Different assumed wave energy spreading characterization between the open ocean and a site-specific situation.

iv) Different basis of extreme design storm characterization (e.g., long-term winter storm vs. hurricane dominated characterization).

v) Different assumed wave spectra between the open ocean and a site-specific situation.

The Loading Condition Used to Determine ESF β In practice, the hull is loaded over a large range of

tank loading patterns and external drafts. To determine the β factors for tankers, it is assumed that the hull loading representing a condition at, or near, the full load case is most appropriate and compatible with the SafeHull criteria of the ABS Rules for Building and Classing Steel Vessels.

For FPSOs, local loads become important as they are affected by individual tank filling levels and vessel drafts. The user can input actual loading patterns to reflect these cases, which can be different from those specified for a tanker in the ABS Rules for Building and Classing Steel Vessels.

The ability to input user defined local loading cases can be especially important for an FPSO, which in comparison to a tanker may have reduced local scantlings due to calmer sea conditions. Then otherwise secondary load cases, such as those representing tank testing and the FPSO’s transit voyage to the installation site may become significant, even controlling some aspects of the structural design for FPSOs.

NEW FPSO RAO DATABASE AND USER DEFINED RAOS

SH/FPSO system contains a reference RAO database of global motions and load responses of typical tankers, which are used by the SEAS ESF program module for determining the ESFs. This database is has been built from seakeeping analyses of five tankers with lengths between 150 m to 350 m. A major assumption in the analysis is that the vessels are taken as free floating, and no topside structure has been considered. For FPSOs, the mooring system and topside structure can affect certain global motions and loads considerably[9]. The mooring/riser system applies restraints on the global motions of the FPSO, which can no longer be considered as free floating and the topside structure adds weight to



the hull, affecting the FPSO’s hydrostatic characteristics and thus the motion-induced dynamic inertia response. When the system RAOs is used, the results may not be conservative, as each FPSO can have quite different and unique turret position mooring and topside arrangements. The water depth at the installation site is also another factor that can have an influence on the RAOs. SEAS has added an RAO database based on 3 FPSOs with lengths between 320 m and 350 m, besides the database based on tankers. The FPSO database includes the effect of the topsides mass distribution. In addition, ABS encourages designers and shipyards to perform their own seakeeping analysis to obtain the FPSO RAOs. When using user’s RAOs, the RAOs of thirteen (13) DLPs are to be calculated and inputted into the SEAS ESF program module.

PHASE B BUCKLING AND ULTIMATE STRENGTH ASSESSMENT

Buckling and ultimate strength assessments for the hull and topsides structures of an FPSO system are to be performed incorporating Phase B stress analysis. ABS current practice for buckling and ultimate strength assessment for the hull structures of an FPSO system should be based on Ship Rules [3] meanwhile the criteria in the ABS Guide for Buckling and Ultimate Strength Assessment for Offshore Structures (hereafter called Offshore Guide)[4] are to be followed for the topsides structures.

The buckling and ultimate strength criteria between ABS Ship Rules and the Offshore Guide are different. The criteria from Ship Rules are based on the concept of ‘net scantling’, which is the Rule minimum design thickness of as-built ships . The β -factors are introduced to consider site-specific environmental conditions. The stresses obtained from SafeHull with the load condition modified by the β -factors are substituted into the buckling and ultimate strength criteria from Ship Rules to assess the buckling and ultimate strength of plate and stiffened panels. The material reduction factor is to be taken into consideration to define the permissible stresses in the criteria.

The buckling and ultimate strength criteria in the ABS Offshore Guide are based on the gross thickness, which is the Rule minimum thickness plus the Rule corrosion margin or minimum as-gauged value, whichever is smaller. Load conditions representing the operation or most unfavorable loads are to be considered. In the ABS FPI Guide, the primary loading conditions are the ‘Normal Operations’ and ‘Severe Storm’. The former is defined as the operating environmental loading combined with dead and maximum live loads appropriate to the function and operations whereas the latter is the design environmental loading combined with dead and live loads appropriate to the function and operations during the design condition. The different allowable utilization factors to be used to calculate the permissible stresses are specified depending on the structural component geometry, loading type and failure modes [4].

Both, the ABS Ship Rules and the Offshore Guide contain the buckling and ultimate strength assessment for plates and stiffened panels. In order to establish the difference between them, a comparison study is made for a converted FPSO hull. The main characteristics of the FPSO are given in Table 2.

Table 2 Main Characteristics of the Converted FPSO

Length overall, L 334.0 m Length between perpendiculars, LBP 320.0 m Breadth molded, B 54.5 m Depth molded, D 27.0 m Design draft, T 21.4 m Block coefficient, CB 0.83 Steel grade Mild

To determine the global response of the hull

structures, a 3-hold FE model of the hull structure within 0.4L was created. The total loads of the 18 SafeHull standard load cases were automatically generated and applied to the 3-hold model by SafeHull system. The von Mises stress contour and deflection to Load Case No. 2 is illustrated graphically in Figure 5.

Figure 5 von Mises Stress Contour and Deflection to Load Case No. 2

Figure 6 provides the comparison of buckling and

ultimate strength assessment based on Ship Rules and the Offshore Guide. Some observations from the comparison study are given below:

• The ratio of plate buckling for the deck panels and bottom plates is very stable, which lies between 0.83 and 0.85.

• The small slenderness ratio (β = 1.15) of bottom panels and biaxial stresses (σx,σy) located in the shaded region as shown in Figure 7 cause plate ultimate strength difference relatively large compared with others.

• The buckling and ultimate strength ratios are significantly affected by the ratio of nominal design corrosion value to nominal design thickness. Therefore, they are distributed more widely for side shell and longitudinal bulkhead panels because the plate thickness and stiffener dimensions of side shell and longitudinal



bulkhead vary frequently along the vertical direction.

• The effect of nominal corrosion design value on tripping of angle-bar stiffeners is more significant than flat bar and T-bar stiffeners. The tripping ratio of angle-bar stiffeners shifts toward 1.0. Some of them are even greater than 1.0.

• When SafeHull standard loads are applied, the Offshore Guide is generally more conservative than the Ship Rules.

• ABS will use the Offshore Guide to determine buckling of stiffened panel after a Rule Change Notice (RCN) is conducted to modify the FPI Guide.

Figure 6 Comparison between Ship Rules and Offshore Guide

0%

20%

40%

60%

80%

100%

0.7 0.8 0.9 1 1.1 1.2

Ratio = Ship Rules/Offshore Guide

Perc

enta

ge

PLATE BUCKLING (Mean=0.8358 COV=0.29%)PLATE ULTIMATE STRENGTH (mEAN=0.8259 COV=0.94%)STIFFNER BEAM-COLUMN BUCKLING (Mean=0.8313 COV=0.41%)STIFFNER TRIPPING (Mean=0.8462 COV=0.12%)

Number of Panels = 672Load Cases =18

0%

20%

40%

60%

80%

100%

0.6 0.7 0.8 0.9 1.0 1.1


Perc

enta

ge

PLATE BUCKLING (Mean=0.8409 COV=0.69%)PLATE ULTIMATE STRENGTH (Mean=0.7852 COV=6.30%)STIFFNER BEAM-COLUMN BUCKLING (Mean=0.8417 COV=0.28%)STIFFNER TRIPPING (Mean=0.8507 COV=0.14%)

Number of Panels =576Load Cases =16

0%

10%

20%

30%

40%

50%

0.7 0.8 0.9 1.0 1.1 1.2


Perc

enta

ge



0%

10%

20%

30%

40%

50%

60%

70%

0.7 0.8 0.9 1.0 1.1 1.2


Perc

enta

ge





0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0

Ship Rules

Offshore Guide

l /s =5.65β=1.15

τ/τ0=0.50

OPTIONAL NOTATIONS FOR FPSO INCLUDING FATIGUE LIFE NOTATIONS

For a new-build ship-shaped FPI (i.e., FPSO, FPS or FSO) hull structure, the SafeHull criteria are meant to provide a minimum design fatigue life of 20 years. Where a vessel’s design calls for a fatigue life in excess of the minimum design fatigue life of 20 years, the optional class notation FL (year) will be assigned at the request of the applicant. This optional notation is eligible provided the excess design fatigue life is verified to be in compliance with the criteria in the Rules for Building and Classing Steel Vessels. Only one design fatigue life value is published for the entire structural system. Where differing design fatigue life values are intended for different structural elements within the vessel, the (year) refers to the least of the varying target lives. The ‘design fatigue life’ refers to the target value set by the applicant, not the value calculated in the analysis. The notation FL (year) denotes that the design fatigue life assessed according to the Rules for Building and Classing Steel Vessels is greater than the minimum design fatigue life of 20 years. The (year) refers to the fatigue life equal to 25 years or more (in 5-year increments), as specified by the applicant. The fatigue life will be identified in the Record by the notation FL (year); e.g., FL(30) if the minimum design fatigue life assessed is 30 years.

When an existing tanker is converted to an FPSO, FPS or FSO, in the process referred to as an ‘ordinary’ conversion, and the FPSO, FPS or FSO is classed under the provisions of Chapter 4, Section 4 of the ABS FPI Guide, the expected minimum remaining fatigue life of the structure is to be documented by recording its value in the Record. The RFL notation will be followed by the value of the expected minimum remaining fatigue life in years, the year of maturation of fatigue life and the specific site of installation. For example, RFL(15), 2018 in Gulf of Mexico indicates that the expected minimum remaining fatigue life of the structure is 15 years, which will be matured in the year 2018 for conditions

anticipated in Gulf of Mexico. The RFL (Year), Year in (site of Installation) notation as applied to ‘ordinary conversions’ is mandatory. Notice that a new build FPSO relocated to another location cannot be considered anymore as a new build. Therefore, for this case still the RFL(year) Year in (site of installation) applies.

SURVEY REQUIREMENT FOR AN “ORDINARY CONVERSION” Survey Record Review

Ordinary conversions refer to vessel conversions that have been previously classed. Prior to structural review, the survey reports for the vessel that are related to structural fractures, repairs, or modifications during its service history should be obtained. The service records should be reviewed to identify whether fractures have occurred in the hull structure and/or whether the structure details have been modified during its service life. In addition, it should be noted whether major structural modifications have taken place that will affect the scantling review and analysis.

A Floating Installation is expected to remain moored on site during the entire service life, and it is therefore without the ready access to the repair and maintenance facilities used by a ship. The drydocking survey and special survey of the hull are to be carried out in accordance with the requirements of ABS Rule Requirements for Survey after Construction. All modifications to the vessel are to be carried out in accordance with ABS approved drawings and follow the IACS Shipbuilding and Repair Quality Standard requirements. All renewed material should be replaced by steel of the same or higher grade and to the approved design scantling or greater. Workmanship is to be carried out in accordance with the ABS Guide for Shipbuilding and Repair Quality Standard for Hull Structures.

What needs to be emphasized is that in this new update, new bottom plate pitting repair specifications are recommended. The specifications apply to pitting found in both ballast and cargo tank bottom plating. There are four main approaches used for dealing with severe bottom pitting.

(a) Partially Crop and Renew Affected Bottom Plating. Partial cropping and renewal is primarily a matter of proper welding technique, selection of an adequately sized plate insert and appropriate non-destructive examination (NDE) of repaired areas.

(b) Clean Pitted Areas and Cover with Special Coating. Cleaning out and covering with special coating without use of filler or weld build-up need only be limited by the maximum allowable depth of the pits (or allowable minimum remaining thickness of the bottom plating) permitted from a strength or pollution risk standpoint. The allowable loss of bottom cross-sectional area must also be considered.

(c) Clean Pitted Areas and Fill with Plastic Compound Use of plastic compound filler, such as epoxy, can be considered similar to (b) because no strength credit is given to the filler material.

Figure 7 Ultimate Strength Interaction Curves of Bottom Plate Panels



(d) Fill by welding. Filling with welding warrants consideration that is more serious.

Suggested welding practices for bottom plating are noted below.

For pitting up to 15% of bottom plating thickness, no immediate remedial action is necessary. However, if the surrounding tank bottom is specially coated, corrosion progress in the pitted areas may be very rapid due to the area ratio effect of protected versus non-protected surfaces, therefore, as applicable, the coating is to be repaired.

Scattered pitting up to 33% (1/3t) of bottom plating thickness may be filled with epoxy or other suitable protective compounds, provided the loss of area at any transverse section of the strake in question does not exceed 10%. Any areas that have been repaired by this method must be “mapped” and noted for close-up survey in the Survey and Inspection Plan required by ABS FPI Guide.

Pitting of any depth may be welded provided there is at least 6 mm (0.25in.) remaining original plating thickness at the bottom of the cavity and there is at least 75 mm (3 in.) between adjacent pit welding areas. The maximum nominal diameter of any pit repaired by welding may not exceed 300 mm (12 in.).

The new requirements for the welding of pits are also specified in the new update and listed below.

(a) Pit Welding. It is recommended that pit welding in bottom plating be built up at least 3 mm (0.125 in.) above the level of the surrounding plating and then ground flush. This is mandatory for higher-strength steels grades D and E, for very small areas (less than 75 mm (3 in.) in diameter and for such welding done afloat.

(b) Surface Preparation. Pitted areas are to be thoroughly cleaned of rust, oil and cargo residues prior to welding.

(c) Filler Metal. When welding, the filler metal grade appropriate to the pitted base metal and preheating, if applicable, are to be employed.

(d) Welding While A Vessel Afloat. For welding below the waterline of a vessel afloat, properly dried low hydrogen electrodes are to be used. The pitted areas against water backing are to be preheated sufficiently to drive off any moisture that might be present. The preheat is to cover at least 102 mm (4 in.) of the material surrounding the welding or four times the material thickness, whichever is greater.

(e) Layer of Welding Metal. A layer of weld metal is to be deposited along a spiral path to the bottom center of the pitted excavation. The slag is to be completely removed and the next successive layer is to be similarly deposited to build up the excavation at least 3 mm (0.125 in.) above the level of the surrounding plating.

(f) Extensive Pit Repairs. For extensive pit repairs, i.e. greater than 20% intensity, of steel grades D, E and higher strength steel, welding against water backing is not recommended.

(g) Nondestructive Examination. All welds to pitted areas in bottom plating are to be subject to nondestructive examination, with particular attention to boundaries of the

welded areas and at intersections of the welded areas and existing structural welding. Also, for welds of higher-strength steels, the NDE method is to be suitable for detecting sub-surface defects.

(h) Coating. In order to reduce the likelihood of possible galvanic attack at the boundaries of built-up areas, coating over the area with a compound such as epoxy/glass flake should be considered. Also, where the pitting is in small areas of coating breakdown, it is essential to restore the coating intact in order to avoid the possible rapid corrosion of small bare areas in large protected areas (area ratio effect).

(i) Doublers. Fitting of doublers over pits is not considered a satisfactory repair. Cleaning and using non-strength contributory fillings to fit doublers is considered a temporary solution subjected to surveyors approval.

NEW UPDATES IN ABS SAFEHULL-FPSO V10 RULE MINIMUM REQUIREMENTS

The ABS SafeHull FPSO approach uses the Environmental Severity Factor (ESF) concept to adjust the tanker unrestricted service criteria of the ABS Steel Vessel Rules. To maintain some degree of minimum safety standards, the FPI Guide has set minimum requirements for certain key design parameters. It is worthwhile to mention them here.

In the application of the criteria, no individual value of a strength β factor is to be less than 0.5 and no minimum required values of any net scantlings is to be less than 85% of the value obtained for the unrestricted service condition. In particularly, ABS require that the minimum hull girder section modulus of an FPSO shall not less than 85% of that required for unrestricted service. This 85% limit is imposed to prevent excessive loss of hull girder strength due to corrosion as a vessel ages. The new FPI Rule Change (ABS FPI Guide, April 2004 version) has released the 0.5 beta limit considering that now it is a requirement to verify the FPSO transit condition as well as the tank test condition at the site with an associated 1-year storm environment. Therefore, by imposing the 85% limit on the hull girder section modulus, it has been verified that the 0.5 limit on the beta factors were redundant.

Therefore, based on the ABS recent experiences on FPSO conversion projects, the following enhancements have been introduced in the SH/FPSO criteria and software: • Calculate loads and required Rule minimum scantling

for transit condition, which now is included in the FPI Rule Change. This Rule Change also adds minimum heading requirements for spread and, or turret moored FPSOs.

• Calculates fatigue life using as-built scantling (Tanker) and Rule reassigned / Gauged for FPSO on-site service;

• Provide program flexibly to add special load cases for transit, flooding, offloading and tank inspection, etc. in Phase B finite element analysis.



Usually, it is much harder for users to provide the wave conditions that an oil tanker has experienced on route records in past services. SafeHull SEAS provides users the convenience to obtain the approximate wave conditions from the global wave database, which is called ABSWAVE. For the assessment of pre-existing fatigue damages of an FPSO conversion, the wave conditions are retrieved from the ABSWAVE database along the routes that a vessel serviced in the past.

In current version of SafeHull SEAS program, the number of canned routes has increased from six to 25. Table 3 shows the 25 canned routes.

Table 3. Twenty-five canned routes in SH-FPSO V10

1. Dover Strait to New York

2. Los Angles to Yokohama

3. Valdez to Los Angles 4. Yokohama to Persian

Gulf 5. Gulf of Mexico to

Persian Gulf 6. Dover Strait to Persian

Gulf 7. Dover Strait to Gulf of

Mexico 8. Europe to Persian Gulf 9. Med Sea to Persian

Gulf 10. Persian Gulf to Med

Sea 11. Europe to Med Sea 12. Gulf of Mexico to

Persian Gulf

13. Persian Gulf to Gulf of Mexico

14. Persian Gulf to Red Sea

15. Red Sea to Persian Gulf

16. Persian Gulf service 17. Red Sea service 18. Med Sea service 19. Gulf of Mexico to Med

Sea 20. Med Sea to Gulf of

Mexico 21. Africa to North

America 22. Persian Gulf to

Singapore 23. Singapore to Persian

Gulf 24. Persian Gulf to Korea 25. Korea to Persian Gulf

CONCLUSIONS This paper introduced enhancements to FPSO fatigue

and strength assessment procedures that have been included in the ABS FPI Guide rule change. The enhancements include new spectral fatigue approach and bimodal wave application for fatigue assessment and new buckling and ultimate strength criteria. In addition, the new ABS FPI Guide introduced optional notations for FPSO including fatigue life notations and survey requirement for an “ordinary conversion”. These new updates represent part of our continuous effort to better serve our clients as well as the offshore industry.

ACKNOWLEDGMENTS The authors wish to thank their colleagues who

provided comments. The views expressed in this paper are those of the authors and are not necessary those of the American Bureau of Shipping.

REFERENCES [1] ABS, 1998, Hull Thickness Measurement (Gauging

Surveys), American Bureau of Shipping [2] ABS, 2003, Rules for Building and Classing Steel

Vessels, American Bureau of Shipping. [3] ABS, 2004, Guide for Building and Classing

Floating Production Installation, American Bureau of Shipping.

[4] ABS, 2004, Guide for Buckling and Ultimate Strength Assessment for Offshore Structures, American Bureau of Shipping.

[5] Tam, G and Wu, J.F., 2002. A Rational Approach for the Evaluation of Fatigue Strength of FPSO Structures, The 14th OSEA International Oil & Gas Industry Exhibition and Conference

[6] Tam, G and Huang, W, 2004. SafeHull Application of Existing Tankers to FPSO. OMAE2004-51017, 23rd Conference on Offshore and Arctic Engineering, Vancouver, Canada.

[7] Wang, Ge, 2003, A Study of Allowable Reduced Hull Girder Section Modulus of Ship-Shaped FPSOs, RD Report 2003-06, American Bureau of Shipping

[8] Zhao, C., Bai, Y. and Shin Y., 2001, Extreme Response and Fatigue Damages for FPSO Structural Analysis, Proceedings of the 11th ISOPE Conference, Vol. 1, pp.301

[9] Zhao, C., Wu, J.F, Huang, K, and Shin Y., 2002, Environmental Severity Factors and their Application in FPSO Hull Design, OTC 14231, Offshore Technology Conference

ABS FLOATING PRODUCTION INSTALLATIONS GUIDE RULE … · 2018-12-11 · ABS TECHNICAL PAPERS 2004...

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Transcript of ABS FLOATING PRODUCTION INSTALLATIONS GUIDE RULE … · 2018-12-11 · ABS TECHNICAL PAPERS 2004...