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OMAE-FPSO 2004-0093

1
Copyright © 2004 by ASME
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
evalenzuela@eagle.org

Jer-Fang Wu
American Bureau of Shipping
16855 Northchase Drive
Houston, TX 77060, USA
jwu@eagle.org
Haihong Sun
American Bureau of Shipping
16855 Northchase Drive
Houston, TX 77060, USA
hsun@eagle.org
Tuanjie Liu
American Bureau of Shipping
16855 Northchase Drive
Houston, TX 77060, USA
tliu@eagle.org


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 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 factors
are used to adjust spectral fatigue strength performance between
unrestricted ocean going tankers and the long-tern site
environment. The 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.
2
Copyright © 2004 by ASME
Rules
The SafeHull System
Consist of Rules &
Software Programs
SafeHull Phase A
SafeHull Phase B
SEAS
Version 9 - 2003
Version 10 -2004
Tankers
Bulk Carriers
Container Ships
FPSOs
Rule for Building
and Classing Steel
Vessels - 2004
Floating Production
Installation (FPI) - 2000
Supplement 2 -2003
Supplement 3 - 2004
Spectral-Based Fatigue
Analysis for Floating
Production, Storage and
Offloading (FPSO)
Systems - 2002
SafeHull FEA Guidance
for Hull Structures
Global 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) 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

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 factor is defined in SEAS as:

S
U
D
D
=



(1)

where
D
U
= 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.
D
S
= 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
(
Spectral
U
Simplified
U
L
f
L =
(2)
where L is fatigue life, U represents the Unrestricted
Service (base) environment, and f (SN curve) is the calibration
factor