OTC 17506
Selection of Trading Tankers for FPSO Conversion Projects
P. Biasotto, V. Bonniol, and P. Cambos, Bureau Veritas
Copyright 2005, Offshore Technology Conference
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Abstract
Review of single and double hull tanker designs has been
conducted to identify the relevant aspects of the hull structure
in the context of conversion into future FPSO systems. Typical
designs have been selected and structural analyzes procedures
for verification of the hull girder strength and the primary and
secondary structures have been reviewed regarding yielding,
buckling and fatigue strength.
strength. The typical defects and the main
degradation modes based on the return of experience with
tankers are also identified and a set of hull design parameters
are described in order to evaluate the hulls candidates to
conversion.
The paper provides guidelines to assist the selection of
trading tankers for conversion into FPSO. Based on a set of
hull design parameters, different hull configurations are
evaluated and the respective advantages and disadvantages
assessed in order to identify the best candidates for
conversion.
Reserve of strength and corrosion margins have to be
analyzed
for each
hull andfield
compared
the environmental
FPSO project
requirements,
including
designwith
life,
conditions and corrosion rates. Lists of typical defects and
hazards based on the return of experience with tankers are also
discussed. The paper also discuss the different designs based
on the several parameters analyzed in order to provide
guidelines to assess the hull structure condition, estimate the
repair work at conversion and the inspection effort along the
FPSO life.
FPSO conversion projects based on existing trading
tankers are still alternatives to new constructions for
developments in areas like West Africa, Brazil and South East
Asia. Nevertheless there are few tankers built before 1985 still
available for conversion. Consequently single hull tankers
built after mid 80s and double hulls built after early 90s
became natural candidates to conversion into FPSO.
Nevertheless, the decision making process regarding the
selection of such hulls faces the higher cost of refurbishment
of single hulls during the conversion and the high cost of
purchase of more recent double hulls.
Abbreviations
FPSO
MIC
Floating Production, Storage and Offloading
system
Floating, Storage and Offloading system
High Tensile Steel
Inspection, Maintenance and Repair
International Convention for the Prevention
of Pollution from Ships
Microbial Induced Corrosion
SBT
SWBM
SWSF
VLCC
VWBM
VWSF
Segregated
Ballast Tank
Still Water Bending
Moment
Still Water Shear Force
Very Large Crude Oil Carrier
Vertical Wave Bending Moment
Vertical Wave Shear Force
FSO
HTS
IMR
MARPOL
Introduction
Although we see a shift towards new-build FPSO’s, in
particular for developments in harsh environment conditions,
conversion seems to remain the basis for several projects in
areas where benign environmental conditions are predominant,
such as West Africa, Southeast Asia and Brazil.
Therefore, the possibility of fast track schedules to have an
early first oil date is also a important parameter in the decision
process for selection of the floating unit type of hull:
converted or new-build.
Data concerning VLCC tankers built between 1973 and
2004 have been reviewed in order to identify the main types of
oil tanker still operating and that may be available for FPSO
conversion projects. In-service VLCC tankers can be divided
in two main groups: single hull and double hull tankers. Single
hull designs have been in general delivered until 1995. Despite
first projects early 90’s, double hull tankers have been built
after 1995, following MARPOL resolutions 13F and 13G.
The various aspects concerning the hull design
characteristics and review of the ship’s historical information,
including operation conditions, maintenance procedures and
records of survey and inspections are reviewed based on
Bureau Veritas and the industry return of experience on oil
tankers. The aim is to identify the main parameters to be
considered during the hull selection process and the
refurbishment
refurbishme
nt work required to fit the structure for conversion.
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The return of experience from previous FPSO conversion
projects is also considered and the several steps necessary to
achieve the hull structural assessment are described in the
paper, incorporating the lessons
lessons learned from these projects.
The Candidates
Statistic data of in-service tankers built between 1973-2004
shows that around 56% are double hulls, 43% single hulls and
1% corresponding to other designs, such as double side
tankers and double bottom tankers ( figure 1).
Probably the main characteristic of single hull designs built
after 1985 is the extensive use of HTS. As there are few
tankers built between 1973-85 still available for conversion
into floating units (figure 2), single hull and double hull
tankers built after 1985 and 1995, respectively, are natural
candidates to conversion into FPSO. The decision making
process to select the hull will balance between the expected
higher cost for refurbishment of a single hull at conversion and
the higher cost to purchase a double hull tanker. As shown in
table 1, in 2004 the average cost to purchase a tanker built
between 1985-95 was in average 60 m USD against 85.5 m
USD for a vessel built between 1996-2000 ( figure 3).
Oil Tankers – Average Sale Price (m USD)
Year
1973-85
1986-95
1996-00
2001-04
2000
11.8
43.0
73.8
-
2001
12.6
35.2
79.2
82.5
2002
11.0
21.5
59.5
75.0
2003
13.0
32.8
52.0
65.4
2004
15.0
60.0
85.5
110.1
Average
Sale Price
12.3
36.3
66.1
88.3
Ships
operating
29
185
116
118
table 1: oil tanker average sale price (source: CW Kellock
& Co. Ltd.)
Three main categories of oil tankers are here defined and
their main characteristics are further described here after.
Return of experience with converted FPSO’s shows that for
each of them the structure degradation during the tanker trade
will determine the FPSO design life. The main parameters are
detail design standard, fabrication standards, operation
conditions, maintenance procedures and trading in harsh
environment.
Single Hulls (mid 70’s - 1985)
Conversion of single hull tankers built before 1985 have
been a worthwhile alternative for FPSO projects. Such vessels
are relatively cheaper and their primary and secondary
structures use to be stiffer than in hulls built after 1985, where
high tensile steel has been extensively used.
A number of different designs can be identified,
comprising American, French, German, Swedish and Japanese
standards among others. The industry has a good return of
experience with oil tankers and an extensive list of areas prone
to defects due to stress concentration and corrosion is given
[1], [2] and [3]. Nevertheless, it is well known that occurrence
of corrosion and other defects might vary according to the
shipyard design, fabrication standards, workmanship and the
operation and maintenance procedures adopted during the
trade period.
There are a number of differences and particularities
inherent to each shipyard design, but one of the most
important characteristics is the primary structure arrangement.
Generally two main types of structural arrangement are
noticed:
Longitudinal ring stiffener system comprising deep
•
girders within center and side tanks, supporting the
transverse bulkheads horizontally stiffened. There are no
horizontal stringers.
•
Horizontal system comprising in general four stringers
within center and side tanks, supporting the transverse
bulkheads vertically stiffened. In general a longitudinal
ring is provided in way of the center line girder.
Fatigue was clearly not the main concern in the design of
connections of primary and secondary elements for such type
of vessels, but in some way, it was apparently compensated for
the stiffer structural arrangement, larger corrosion margins and
less use of HTS.
During this period, oil tankers were built worldwide in
different shipyards, some of them having poor fabrication
quality.
Single Hulls (1985 - 1995)
The structure design of such ships is optimized by means
of finite element calculations and the extensive use of HTS to
reduce the weight of steel. The hull structure design has used
ST355, ST315 and ST235 steel types in different manners, in
particular in the side shell plates, stiffeners and web frames.
Some of them are built only using HTS in the cargo area.
The use of HTS, in particular in the side shell area around
the neutral axis, was found to be one of the main causes of
problems associated to this type of vessel. Fatigue cracks of
side shell longitudinal connections to transverse primary
structures are probably the most typical defect of such designs,
as fatigue strength of side shell longitudinals and corrosion
margins are affected by the less stiffer structural panels at
bottom
side shell
areas
areas.
.
As and
a general
rule,
hulls
built with ST355 should be
specially regarded as such vessels have an extensive record of
defects and likely have been reinforced with additional
brackets after construction and before the second Class
Renewal Survey.
Another important parameter is the use of asymmetric
profiles, i.e. angle stiffeners in the side shell and bottom
panels. The use of angles at those locations having high
probability of failure should be specially considered when
evaluating hulls candidate to conversion, in particular when
associated with use of steel ST315 and ST355.
Double Hulls (1996 - towards)
Despite double hulls are here considered from 1996, a
number of design alternatives have been proposed in the early
90’s. But the double hull milestone is 1998/99, when new
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3
builds experimented a sharp increasing and typical Japanese
and Korean shipyards have been consolidated as the main
references on double hull designs.
In the same way as for single hulls, the industry [4] has
published guidelines listing the areas prone to defects due to
stress concentration and corrosion. But again the
th e occurrence of
corrosion and defects might vary according to the shipyard
design, fabrication standards, workmanship and the operation
and maintenance procedures adopted during the trade period.
Nevertheless, such designs profit of improvements on fatigue
some have experimented structural anomalies and defects,
independently of they are newbuild or converted units. Despite
they may be presented in a different degree of intensity,
corrosion wastage and defects due to fatigue are the major
threats for both types of hull.
Deterioration processes are initiated during the tanker trade
period (approximately 45 FPSO’s operating were built before
1985, 4 between 1985-95 and 1 after 1995) and will be present
along the FPSO’s service life.
A number of FPSO’s have been converted on the basis of
assessment methodologies, design of connection details and
fabrication standards over the past 10 years.
Differently of the second category of single hull tanker
designs, ST355 is almost no longer used and typical steel
distribution combines ST315 at bottom and upper deck areas
with ST235 around the neutral axis areas. The latest one with
in general a percentage not less than 30% of the total cross
section.
Double hull designs have the number of structural
connections increased up to 15% due to the inner bottom and
side longitudinal bulkhead stiffener connections to transverse
floors and web frames, where despite the easier access to
inspection might increase the inspection effort of critical joints
along the FPSO service.
Double hull tankers have an optimized hull design, in
general less than 5% margin in comparison with the class
the Classification Society rules requirements, unless additional
requirements were specified by operators. It means that
inspections at conversion have been carried out within the
scope of Class Renewal Survey based on a 5-year cycle, where
substantial corrosion of structural components is verified in
accordance with applied rules. Surveys are carried out in other
to identify anomalies and defects, and necessary repairs are
performed in order to bring the structure to as-built
configuration.
The problem with such approach is that the durability of
the hull structure is not properly assessed against the required
service life for the FPSO, in addition safety margin of critical
components is not evaluated. As a consequence, these units
might arrive at the first and second class cycle having
substantial corrosion of the structural components of the
structure, increasing the risk of buckling and fatigue.
requirements, in particular at lower areas of the transverse
section.
Asymmetric profiles as angle stiffeners are again one of
the most important aspects regarding fatigue strength for side
shell longitudinal end connections. The use of angles, in
particular at those locations having h
high
igh probability of failure
should be specially considered when evaluating hulls
candidate to conversion.
Vessels built after 1997 have similar structural
arrangement, one of the most significant differences between
such designs is probably the position of the cross-ties, fitted
either within the center cargo tanks or within the side cargo
tanks. It is noticed that for the first one ( figure 4.a), the side
shell and the lower hopper structures use to have higher
deflections. It means that special attention should be paid to
side and inner shell longitudinal stiffeners connections,
horizontal girders and hopper connections with the inner
bottom and inner hull structures.
Consequences may be divers, comprising low and majors
events in case proper mitigation actions are not taken:
For designs where the cross-ties are fitted within the side
cargo tanks (figure 4.b), side shell structure deflections are
decreased and stress levels are expected to be reduced. On the
other hand, longitudinal bulkheads are expected to have
deflections increased, in particular at alternate loading
conditions where center tanks are full filled and side tanks are
empty. Therefore, special attention should be paid to the inner
longitudinal bulkhead connections and in way of horizontal
stringers.
Return of Experience on FPSO’s
There are approximately 97 FPSO’s and 74 FSO’s in
operation or available worldwide [5]. Over 60% are
conversion projects.
Over the past five years, 33 FPSO’s have been delivered
and most of them after 1996. Return of experience shows that
•
Reduction of storage capacity,
•
non planned shutdowns for repairs,
•
dry-docking for major repairs,
•
Structural failure
Leak of oil, consequently risk of pollution and
explosion,
Most common anomalies found in converted FPSO’s are
the following:
•
•
Excessive pitting of horizontal structures
•
Substantial corrosion (grooving)
•
Knife edging
Cracks due to fatigue
Cracks due to fatigue result from a combination of several
parameters. The trading history of the vessel, site specific
conditions (for harsh environments) and high stress ranges
experienced during the loading and offloading cycle in the
FPSO service are the main reasons. However, poor detail
designs, low standard of workmanship and corrosion of
welded joints accelerating the occurrence and increasing
significantly the number of defects, where some of the most
common are the following:
•
•
transverse brackets at transverse ordinary frames
•
end of horizontal stringers
•
end of cross ties in way of the side shell and
longitudinal bulkhead web frames
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end of longitudinal stiffeners in way of bulkhead
penetrations and ordinary web
web frames.
The damage cumulated during the trading history of the
vessel uses to be the most significant parcel of the total
cumulated damage comprising tanker and FPSO phases, in
particular for ships operating in severe environment such as
North Sea and North Atlantic areas. Cumulated fatigue
damage due to wave cycles for an FPSO operating in benign
environments such as West Africa and Brazil (Campos Basin)
are in general less significant when compared with a harsher
environment. For this one the fatigue damage due to sea load
may be in the same order of that one observed during the
tanker trade period.
The fatigue damage caused by loading and unloading
cycles during FPSO operations can be also estimated and
cumulated with the damage due to wave cycles, but in general
the number of cycles associated to the shuttle tanker arrival is
low, consequently the cumulated damage is in general
negligible.
On the other hand, the effect of the loading and unloading
cycle due to differential head on the oil tight bulkheads and
side shell panels may lead to high stress ranges, specially in
poor detail designs. Crack propagation analysis shows that the
loading sequence may accelerate the growth of cracks initiated
during the tanker trade period [6].
•
Hull Selection
Evaluation of VLCC’s candidates for conversion into
FPSO’s may include a significant number of parameters
concerning the characteristics of these vessels. Obviously the
first step is to evaluate their adequacy regarding the basic
requirements of the FPSO conversion project, which are
associated with the field development. Storage capacity and
the cargo tanks arrangement (number of tanks, subdivision,
etc) are some of these parameters.
Certainly one of the most important and difficult aspects to
be evaluated is the effective service life of the unit, which
shall meet the required service life established based on the
field estimated life.
Once several candidates may be available, attention should
be paid to the strength verification of the overall hull and its
structural components. Structure safety factors should be
assessed taking into account the hull condition at the end of
the FPSO service life.
Engineering assessment of the overall and local hull
structure strength against yielding and buckling criteria has
been quite successful for the majority of the FPSO projects.
However, experience shows that an accurate assessment of the
main deterioration modes of these components may not be an
easy task, in particular concerning corrosion and fatigue
deterioration.
Fatigue is related with cycle loads (due to wave and cargo
loading-offloading), the structure stiffness, structural detail
design, shipyard fabrication standards, workmanship and
corrosion rates. The hull condition is linked to operation and
maintenance aspects in such way that the condition of a
specific vessel might not be the same as for a similar ship
operated by different owners and in different trades.
Therefore, the FPSO design life will be directly affected by
the fatigue damage and corrosion wastage cumulated during
the previous tanker phase.
Ideally, design review and assessment of the hull condition
should be supported by a detailed inspection of the entire hull
structure before select the vessel to be converted. A hot spot
map of the hull structure, derived from a detailed hull structure
assessment based on finite element analysis, is also useful to
evaluate the different hulls, however it may not be available
during the selction process. Therefore, the various aspects
concerning the hull design characteristics may need be
reviewed in a more qualitative way.
The hull design review may comprise a preliminary
analysis of the existing as-built drawings of the vessel. Based
on the return of experience, a number of parameters can be
defined and a checklist elaborated for a quick review of the
candidate vessels.
For long term projects, where the unit should stay on site
during 10, 15 or even 25 years, this would not be enough to
ensure the integrity of the unit along the intended service life.
A complete hull structure assessment should be available in
the early stage of the project and preferable before the
refurbishment program is defined.
As a minimum scope, the tanker selection process should
include for each vessel the review of the following data:
•
As-built drawings: look for detail designs prone to
fatigue, percentage of HTS, corrosion additions, etc.
•
UTM Reports and survey and inspection records to
identify corroded areas and typical defects: look for
typical defects due to fatigue, high stressed areas and
buckling. Check for substantial
substantial corroded areas.
•
repair specifications in order to evaluate:
- extension of scantling renewal due to substantial
corrosion along the tanker service
- mitigation of defects, including repairs, modifications
and strengthening undertaken on board
•
Operation and maintenance practices (type of oil,
temperature, washing, corrosion protection, etc)
•
Review of tanker trading history: tankers operating in
severe weather are prone to have fatigue related
problems
Details Design
Critical areas within the tank structure can be defined as
locations that, by reason of stress concentration, alignment or
discontinuity, need particular attention for what regards the
construction, the design and the survey.
In general these locations can be divided in two main
groups:
•
connections of the longitudinal ordinary stiffeners with
transverse primary supporting members (transverse
bulkheads and web frames)
frames)
•
connections of primary supporting members
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5
Ordinary stiffener connection with transverse supporting
structures
It is through these connections that the loads are
transmitted from the secondary to the primary structure
members.
These details are subjected to high cyclic loading through
the ship’s life and they constitute one of the most subjected to
fatigue potential problem areas. Repair of such details may
affect the tanker refurbishment schedule, as the work required
to repair, renew and strengthen these details is much more
The risk of cracks with bending mode is limited with this
design but it remains a probability of cracks due to shear
mode. Nevertheless, tolerances should be strict in such way
that production and construction are delicate processes.
significant than the required weigh of steel.
The following parameters are considered the most
important ones regarding fatigue strength of such connections:
important parameters regarding fatigue strength of such
connections:
Angle connection (figure 7):
The most general type of welded joint is the angle
connection found mainly in the following structures:
•
the location and the number of brackets (on one or both
sides of the transverse primary member),
member),
•
the shape (soft toes or not) and the size of the brackets,
•
the longitudinal stiffener profile (symmetrical or not).
•
the misalignment of the webs of longitudinal ordinary
stiffeners
•
the use of HTS in the side shell plate, stiffeners and
web frames
the details of slots and collar plates
The different types of ordinary stiffener connections with
the transverse web stiffeners, and how the change of type of
connection may increase the fatigue strength of the detail by
reducing the stress concentration factor is illustrated in figure
5.
The position of the center of torsion of angle profiles
(shifted off the profile) induces additional local stress in the
profile, differently T profiles have their center of torsion in
way of web plate (plane of symmetry). In general T profiles
should be used in areas subject to high local pressure, i.e.
upper zones of side shell, longitudinal bulkheads and inner
hull.
The type of scallop is another important parameter
regarding fatigue of longitudinal stiffener end connections
(figure 6). The main aspects are the following:
•
Scallop with or without collar plate:
The classic connection is the scallop connection, a
relatively large cut-out of the primary member leaving only
one side welding possible for the web of the secondary
stiffener against the primary member.
To provide welding on other side, a collar plate may be
fitted. This collar plate is not in the plan of the web of the
primary member, which could lead to possible problems of
cracks. Nevertheless, this is less critical than the profiled slot
form the fabrication point of view.
Profiled slot:
The method is to cut out of the transverse member a
section, which is infinitesimally larger than the web plate of
the secondary stiffener and a larger cut–out for the flange of
the longitudinal. This gives the possibility to weld from the
both side the connection between the web of the stiffener and
the plate of primary member allowing a good transmission of
the stresses.
Connections of primary supporting members
The most critical types of joint are the welded angles and
cruciform joints that are subjected to high magnitudes of
tensile stresses.
The following parameters are considered the most
•
double bottom in way of transverse bulkheads with
lower stool,
•
the double bottom in way of hopper tanks,
•
the lower part of transverse bulkheads in way of the
lower stool (if any),
•
the lower part of inner side in way of hopper tanks.
Cruciform joint (figure 8)
The cruciform connection is a particular case of angle
connection. Indeed, the angle between the plates is now a right
angle. The cruciform connections may be found in:
•
the double bottom in way of transverse bulkheads
without lower stool,
•
the double bottom in way of the inner side when there
are no hopper tanks,
•
the double bottom in way of longitudinal bulkheads.
Return of Experience on Oil Tankers
Corrosion protection system
In general in service single hull tankers are found having
moderate corrosion rate and thickness diminution. However
there are some cases where excessive pitting in the bottom and
top tank in cargo tank plating have been reported due to
microbial attack in areas where coating protection is not
provided.
Cargo tanks of typical single hull tankers may not have
been provided with corrosion protection systems, i.e. coating
and anodes. Residual water from oil cargo causes grooving
and pitting corrosion in the upper surface of horizontal
structures like stringers and bottom plating at the aft end of the
cargo tank.
The normal corrosion rate in cargo tanks uncoated areas of
double hull tankers is expected to be moderate based on the
previous experience with SBT tankers, where horizontal
structures like stringers and bottom plating would be areas to
be subjected to special attention. However, cases of
accelerated corrosion rate in cargo tanks have been reported.
In addition, there has also been an increase in the pitting
corrosion rate in cargo tank bottom plating.
Double hull designs can be found having cargo tanks partly
painted with epoxy coating in the under deck (3 meter below)
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OTC 17506
and eventually the tank top plating (inner bottom), varying
depending on owner requirements. But areas where coating
fails may have accelerated corrosion, in particular in the deck
head areas subject to increased deflections and stress levels
due to the use of HTS.
Accelerated corrosion in cargo tanks may be due to
microbial attack from bacteria in the cargo oil. Cargo
temperatures in double hull tankers can be found up to 20 °C
higher than in single hulls due to the insulation provided by
the inner hull. These higher temperatures would provide the
necessary conditions to the microbes remain active longer and
produce corrosive acidic compounds increasing the risk of
MIC.
Higher temperatures mean that the humidity is higher,
increasing the amount of water vapour in the air space above
the ballast and cargo tanks. Therefore, coating in ballast tanks
bottom shell remains continuously wet, having mud settled in
particular at aft locations due to the ship trim. This
accumulation of mud could also generate a higher risk of MIC.
Corrosion prevention systems are likely provided in ballast
tanks. In order to prevent accelerated corrosion of the under
deck and the tank bottom areas, a number of operators require
painting of these areas also in
in cargo tanks.
The total surface area to be coated and maintained in
double hulls can be up to three times larger when compared to
single hull tankers. Consequently, the maintenance of coating
systems is one of the most important aspects regarding the hull
structure condition.
The following main aspects should be considered in order
to evaluate the durability of the coating system of cargo and
ballast tanks:
•
Coating specification
•
Coating preparation and application
Workmanship during construction (grinding of sharp
edges for instance)
Double hull tankers have structural flexing increased
compared to single hull designs which leads to higher cracking
potential in particular in the deck, inner shell and inner bottom
areas.
•
Fatigue of longitudinal stiffeners connections
Connections in way of transverse bulkheads are, in
general, fitted with double brackets, however those
connections in way of ordinary frames are often fitted with
single brackets. In cases where HTS is extensively used,
backing brackets might be necessary be fitted in way of
ordinary frames along the hull length at conversion into FPSO.
As a general rule, the following parameters were found to
be the most significant ones regarding fatigue strength of side
shell longitudinal connections and increase fatigue life:
•
Double brackets reduce stress concentration factors.
•
Type of steel: ST235 construction leads to stiffer
structures around the neutral axis. In the contrary
ST315 and ST355 lead to less stiff structures.
•
Shaped brackets and flat bars reduce stress
concentration factor. However fatigue life drops
quickly where flat bars are fitted.
Flexural (figure 9) and shear (figure 10) are the two most
common failure modes regarding fatigue cracks at such type
of connections.
The most common defects in single hull designs are
attributed to the extensive use of HTS, in particular in the side
shell construction. Some tankers built between 1985-95 had
longitudinal stiffener connections locally renewed and
provided with backing brackets around ten years service
service life in
order to increase fatigue strength.
In a general way, utilization of HTS in double hull
designs results in the increasing of deflections and stress
levels, affecting negatively fatigue life of structural
connections and the effective life time of coating systems.
Double hull tankers built early 90’s were subject to a
number of typical defects due to poor design details, specially
at side shell longitudinal connections to primary transverse
structures and connections of hopper to inner bottom and side
longitudinal bulkhead. Significant improvements on design
detail and workmanship have been achieved and implemented
in the designs built late 90’s.
It is remarkable that a number of double hull tankers
operating in severe environments like North Sea and North
Atlantic have reported damages at side shell and bottom
•
longitudinal stiffeners in way of transverse primary structure.
Environmental Loads
Different to other Class societies, Bureau Veritas ([7], [8])
apply loads depending on the operational conditions of the
floating unit. There are variations in the overall severity of the
climate in a given area, additionally different vessel
dimensions, shape and load distributions need be taken into
account. Therefore each project should be provided with
calculation of hydrodynamic loads and vessel motions in the
frequency domain, using the 3D diffraction-radiation method
and taking account of site water depth.
Site specific conditions need be taken into account in the
hull structure assessment of the FPSO, including metocean
data of the site and the mooring conditions, in order to
properly define the sea loads on the structure. Such data
include the wave directions, the wave spectrum and the
relative headings.
The main wave load parameters derived from the direct
hydrodynamic models and used in the structural anlyzes are
the following:
•
Global Hull girder loads (Wave Bending Moment and
Wave Shear Force)
•
Relative wave elevation, that is indicative of variation
of pressure on side shell
•
Vessel accelerations
Symmetric profiles avoid lateral bending of
longitudinal stiffeners. On the other way angle profiles
Hydrodynamic analysis
The outputs of the hydrodynamic analysis determine the
design load parameters to be applied to the structural model in
order to assess the scantlings of the structure. The
lead to lateral bending.
hydrodynamic model takes into account the unit hull forms,
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the lightweight distribution (including structure weight,
topside weight, turret weight, etc.), the loading conditions and
the connections with the seabed. At least three loading
conditions are in general analyzed, comprising the full range
of the FPSO loading conditions:
•
Minimum draught
•
Intermediate draught
Maximum draught
FPSO’s are generally moored by two different types of
systems that influence the calculation conditions of the
hydrodynamic analysis:
•
7
and dynamic loading applied to the hull structure.
SWBM and SWSF will be driven by the maximum values
of VWBM and VWSF (including safety margins) and the
calculated total hull girder capacity, taking into consideration
the corrosion wastage along the tanker trading service.
It should be noted that the direct analysis of wave induced
shear force will result in a distribution slightly different to the
one given by the typical rule distribution. Despite VWSF
distribution has two peak values at approximately L/3 from the
hull aft and fore ends, the hull capacity needs be checked
•
Spread moored design: the unit is maintained in a
constant position independent of the sea and current
heading, while the environmental loads have a
predominant direction. Generally, the mooring lines are
connected to the main deck at the fore and aft ends.
•
Turret moored design: the unit is free to weathervane
and has a natural tendency to orientate in the direction
of the most severe environment component. The
mooring lines are connected to the turret, generally
located in the fore part of the unit.
along the length of the unit and in particular at the position of
transverse bulkheads, where the total shear force has its
maximum value. Oil tankers often have local reinforcements
in the longitudinal bulkhead and side shell plating, over one to
two web spacing, aft and fore of transverse bulkheads, to
allow for peak values of the total shear force.
Considering the wave load parameter results for a unit
moored offshore Angola for instance, the site specific values
for VWBM and VWSF are approximatelly 0.3 and 0.5 of the
North Atlantic reference values, leading to an increasing over
the minimum Rule value of 0.65. Nevertheless, the allowable
values would still be increased up to 50% for SWBM and up
to 35% for SWSF.
Wave load parameters
The hydrodynamic analysis results in the following
Hull Structure Assessment
Assessment of the FPSO hull structure should verify the
parameters valid for the offshore unit at the intended site and
with precision of each value along the length of the unit:
adequacy of the tanker hull with the project specification, it
means the strength of global and local structures, considering
storage capacity, topsides additional weight and specific
environmental loads. Furthermore, the hull condition at the
end of the unit intended service life should be also assessed to
ensure the hull structure integrity along this period.
The refurbishment work necessary to fit the hull for
conversion is not the most expensive cost regarding the whole
conversion, however it plays an important role in the
conversion schedule, as delays caused by the works in the hull
might delay the unit commissioning. Repair and strengthening
works prior or at conversion will also affect the unit along its
service life.
Due to time constraints, extensive structural analyses may
not be feasible before inspections carried out prior to the
refurbishment work. However, always when possible it is
•
wave induced bending moment,
•
wave induced shear force,
•
total accelerations in all directions, at the center of
gravity of each compartment and at relevant positions
in topside areas,
•
relative wave elevation,
sea pressure on the shell diagrams.
The values of the parameters may be modified to include
safety margins or adjusted to rule values when applicable.
Depending on the site, the results of the hydrodynamic study
may be higher or lower than the rule standard values.
In the case when site-specific loads are considered, the results
•
of
the hydrodynamic
calculation
mayapplied
be directly
in the
structural
analysis, depending
on the
safetyinput
margins
or
class rules, if the case.
Hull Girder Capacity
The hull girder total capacity needs to be checked along
the length of the unit.
Maximum permissible still water bending moment
(SWBM) and shear force (SWSF) are derived from the wave
bending moment (VWBM) and the wave shear force (VWSF)
values obtained from the hydrodynamic analysis results.
Calculations show that in general SWBM and SWSF could
be increased for benign environmental conditions, such as in
Angola and in Nigeria for instance, but varying depending on
the ship characteristics
characteristics..
The additional reserve of strength due to less severe
environments should not be fully credited to the allowable
SWBM and SWSF to assure the normal ratio between static
recommended to have first calculations and results of the hull
structure assessment available in an early stage. This would be
helpful to define the scope of the inspections and consequently
to specify the required steel renewals and modifications.
Hull structural assessment is a multi-step analysis
procedure, where global coarse mesh analysis are used
followed by local fine mesh analyses at critical locations,
selected based on the coarse mesh results and on the return of
experience.
The structural analysis is based on the design load
parameters given by the
the hydrodynamic analysis.
Differently of newbuild hulls, structural assessment of
converted FPSO’s requires two phases to be considered:
tanker and FPSO, as an FPSO presents specific characteristics
when compared with trading tankers. The main characteristics
of the FPSO are the following:
8
OTC 17506
•
The unit is fixed on a specific site, generating specific
loads.
•
The unit is permanently moored (no dry dock is
planned for inspection and maintenance).
maintenance).
•
Additional loads generated by the mooring system,
risers and topsides.
Continuously loading and unloading on site (constant
variation of draught), differently of tanker which are
generally operated in full and ballast conditions.
Therefore, specific assessment of the FPSO hull structure
needs be undertaken taking into consideration such
particularities. The following limit states are verified for the
hull structure components:
•
Yielding
Buckling
Ultimate
Strength
the external sea pressure is calculated at the full load draught
when the side tanks are considered empty (cargo loading
conditions), and at the light ballast draught when the side
tanks are considered full (ballast loading conditions).
Strength of the primary structure
The first objective of the finite element analysis is to
determine the stress distribution in the primary supporting
members. It allows verifying that the scantlings comply with
the yielding and buckling criteria.
Two levels of meshes are generally necessary to assess the
strength of the structure. The first step is the global coarse
mesh model (figure 11). The 3D coarse mesh model allows
verify the overall behavior of the primary structure. There are
two main approaches:
•
Fatigue
Hull Girder
Plating
Ordinary
stiffeners
Primary
supporting
Structural
details
Hull girder strength
The first step of the hull structure assessment is in general
the verification of the global hull girder strength. A yielding
check is performed, comparing the bending moment applied to
the structure does not exceed the bending moment capacity
provided by the actual hull scantling configuration. The
bending moment applied to the structure is given by the
maximum allowable still water bending moment derived from
the assessment of the loading conditions of the unit (operation,
transport, maintenance, etc) and VWBM derived from the
wave load parameters analysis.
It should also be checked that the bending moment applied
to the structure is lower than the ultimate bending moment
capacity of the hull girder, taking into account a safety factor
increased compared to ships.
Local strength of plating and ordinary stiffeners
The second step is the verification of the scantlings of the
plating and the ordinary stiffeners. These elements are
assessed through a 2D section model loaded with the design
load parameters defined by the hydrodynamic analysis. A
yielding check and a buckling check are carried out for
stiffeners and plates.
In all cases, local loads are calculated for the expected
most severe conditions. Each element is analyzed considering
the compartments as being alternately full or empty, for the
purpose of maximizing the loads induced on that element by
the cargo carried. Similarly, for the elements of the outer shell,
The simpler and faster one is based on a three-cargo
tank model, where the beam theory is used to balance
the model and obtain the desired bending moment and
shear force distribution in the mid tank area. VWBM,
VWSF, accelerations and relative wave elevations
derived from hydrodunamic analysis are used.
Complete ship model including the entire hull structure
over the unit length, from aft to fore ends can be used.
Obviously it is a more time consuming approach. On
the other end, accelerations and wave pressure
distribution derived from hydrodynamic analysis are
applied along the hull structure model in order to
equilibrate it and obtain the correct bending moment
and shear force distribution along the hull.
Fine meshes are also performed to get more accurate stress
levels in specific locations (figure 12).
Detailed Stress analyses are in general performed for the
following typical primary structures:
•
•
horizontal stringers in way of typical oiltight bulkhead
•
horizontal stringers in way of typical swash bulkhead
•
typical transverse ring
•
typical first transverse ring aft and forward oiltight and
swash bulkheads
•
Longitudinal girders in way of oiltight bulkhead and
swash bulkhead
FPSO specific areas (topside supports, turret structure,
hull connections and other hull attachments).
Main scenarios
The tanker and the FPSO phases need be distinguished in
structural assessment of the converted FPSO to proper
consider the effects of previous corrosion and cumulated
fatigue damage during the trade, but also the floating unit
design characteristics and operation conditions.
The following scenarios are therefore usually studied:
(a) As-built tanker at construction date
(b) Actual condition (at conversion date)
(c) End of FPSO service life: derived based on the estimated
corrosion wastage cumulated during the FPSO phase.
Scenario (a) is mainly used to assess the fatigue damage
cumulated during the tanker phase. It may be also used for a
•
first screening of the candidate vessel in order to identify the
OTC 17506
9
most loaded areas to be further investigated during inspections
carried out prior to the hull refurbishment program.
program.
In scenario (b), the existing 2D and 3D finite element
models (initially modeled based on the as-built configuration)
are updated in order to take into account the FPSO actual
configuration, new loading conditions, site specific
environmental loads, structural alterations and topsides added
at conversion. Derived from the latest gauging reports and/or
estimated based on typical oil tanker corrosion rates [2], the
wastage cumulated during the tanker trade period can be
Environmental loads for fatigue assessment of the tanker
may be derived based on the analysis of the trading route. In
cases where the trade data is not available, standard Bureau
Veritas Rules wave load parameters for a typical World Wide
trade tanker can be used.
In general, tanker analyses are carried out based on the
typical loading conditions: full load and ballast loading
conditions leading to maximum sagging and hogging load
cases. For the FPSO, review of the operational loading
conditions comprising the loading/unloading sequences should
calculated and the hull overall scantlings are updated.
Renewals foreseen during the refurbishment may be also taken
into account if already decided. The FPSO hull structure
strength will be verified in accordance with the several limit
states above described.
Scenario (c) can also be studied, incorporating the
corrosion wastage expected during the FPSO phase, the
expected hull structure condition of the unit at the end of its
service life is assessed. The reserve of strength of the hull
structure components could be estimated and for those
elements found critical, the alternatives to mitigate the
deterioration effects along the life of the unit could also be
identified, such as:
be carried out to determine the representative loading
conditions to be considered in the analysis.
•
Additional renewal
conversion
•
Specify corrosion protection system to avoid corrosion
wastage at critical locations.
•
and
strengthening
work
at
Improve the IMR plan to ensure the hull structure
integrity
Fatigue
The efficiency of the structural connections subjected to
high cyclic stresses needs be checked with respect to possible
fatigue related problems.
Fatigue assessment can be divided into main groups:
•
connections of the longitudinal ordinary stiffeners with
transverse primary supporting members (transverse
bulkheads and web frames)
frames)
connections of primary supporting members
The first group can be assessed by BV program
•
VeriSTAR/MARS
where
of details is used
to evaluate
the fatigue strength
of library
end connections
of longitudinal
stiffeners by mean of fatigue deterministic approach. 2D
analysis is therefore carried out in order to evaluate the
strength of side shell and bottom longitudinal connections
along the cargo region due to fatigue flexural mode in way of
each oil tight bulkheads and in way of ordinary frames.
3D finite element analysis is needed to capture the local
stresses in non-standard details for use in the fatigue
calculation. Therefore, very fine mesh models ( figure 13) are
used, having element size between once and twice the
thickness of the structural element.
Assessment of detail connections shall take into account
both tanker and FPSO phases. Fatigue damage taking into
account wave cycles and loading/unloading cycles are
cumulated with the damage associated to the tanker phase in
order to estimate the remaining fatigue life at conversion.
Deterministic fatigue strength assessment:
The deterministic methodology has been further developed
for the FPSO’s by introducing several draughts and loading
cases over the expected lifetime of the floating unit. The
calculation method applied today by Bureau Veritas has been
calibrated against spectral fatigue strength assessment and the
methodology is applied in-house for screening of the structural
details.
The results of the deterministic approach are in general
expected to be more conservative than those given by the
spectral one.
Spectral fatigue strength assessment:
In order to assess more precisely the fatigue damage of the
structural details a spectral fatigue strength assessment may be
carried out. The spectral analysis is to be carried out in the
following three steps:
•
hydrodynamic analysis
•
structural analysis
calculation of the fatigue damage
The hydrodynamic analysis determines the loads and the
resulting motions generated by the environmental loads; the
site-specific environmental
environmental loads are thus taken into account.
The loads obtained through the hydrodynamic calculation
are applied to the structural model. The structural model
provides the RAO’s of stresses at location of interest, within
the model. The fatigue damage is then calculated based on
statistics of stress ranges.
•
At least three draughts (and associated loading conditions)
5 headings and 25 frequencies are to be taken into
in to account, but
may be adapted depending on the type of mooring.
The acceptable damage depends on the location, the
accessibility for inspection, maintenance and repair, and on the
consequences of failure.
Loading unloading fatigue assessment:
Fatigue damage cumulated during the FPSO phase due to
wave and loading/offloading cycles is also assessed.
For the calculation of the stress range due to the loading
unloading, the wave at a return period not less than one day is
to be taken into account.
The damage ratio of loading unloading is to be combined
with the one due to the wave effect.
10
OTC 17506
Considerations about welding of structural members
To evaluate the standard and quality of welding on
seagoing tankers with respect to possible conversion to
Floating Storage Unit, a comparison has been made with a
new-built FPSO and a tanker.
In general, welding leg length and scantlings of the FPSO,
with the exception of interface structures with Offshore
equipment and topsides, are often made on the basis of Ship's
Rule requirements.
Due to welding of interface structures and FPSO-owner's
structural analysis, there are a number of parameters that can
be proper assessed
assessed only by means of inspections.
special requirements for particular areas (additional thickness,
fatigue life etc.), welding leg length of FPSO seems to be
more significant. Possible difference is coming from the more
comprehensive loading booklet giving higher values of hull
girder still water bending moment and shear forces.
defined in order to determine the total steel renewal required at
conversion. The expected corrosion during the FPSO service
life is to be taken into account. Therefore the renewal
thickness at conversion is defined as follows:
Deck Transverse Capacity and Topsides Modules
Integration
Hull structure elements of oil tankers built after 1985 are in
general more optimized than in previous designs, particularly
the 70’s tankers often used
used in previous conversion projects. In
addition, double hull tankers have in general higher stress
levels than single hull tankers. Therefore top tank structure
strength and their integration with the topsides modules need
to be carefully investigated in order to identify needs for
additional strengthening.
Topside modules should be supported in way of transverse
and longitudinal bulkheads and deck transverses. Finite
element models, as described above, are used to verify
strength of such structures and specify eventual additional
strengthening of the under deck structure. Top down analysis
using VeriSTAR Hull is an efficient way to perform such
verifications taking into consideration local loads induced by
topsides, but also the hull behavior due to sea loads and
internal liquid loads. Global loads induced by the hull girder
bending are also taken into account.
account.
Refurbishment Program
Refurbishment work of the oil tanker hull may be
undertaken before or at conversion, depending on the FPSO
project strategy. The scope of work should be defined based
on the hull structure condition assessment and on detailed
inspection
of internal
and external part of the hull, supported
by
finite element
analysis.
The refurbishment specification is expected to contain at
least the following main contents:
Outcomes from the hull structural assessment should
provide
•
Plates and stiffeners to be renewed and/or strengthened
based on the FPSO hull structure verification
•
Plates and stiffeners to be renewed and/or strengthened
based on the future corrosion
corrosion study
Detail connections to be renewed, modified and/or
strengthened based on fatigue analysis, taking into
consideration the cumulated damage during the tanker
trade and FPSO phases
Despite information incorporated in the previous survey
•
and inspection reports may be taken into consideration in the
•
Pitting on tank bottom plating, stiffeners and other
horizontal members.
•
Brackets and web stiffeners may present heavily
corroded.
Plate renewal
Renewal thickness of plates and stiffeners needs to be
T
Re newal
= TRe quired +
LF
FPSO
×t
Where:
TRenewal = renewal thickness at conversion
TRequired = required thickness derived from rules or by mean of
hull structure calculations as described above.
LFFPSO = required FPSO service life
t = yearly corrosion rate for the studied element
According to class rules, substantial corrosion margin is
defined as 75% of the allowable corrosion margin. Therefore,
those elements found during the FPSO service life having
substantial corrosion will be subject to further inspections,
more extensively and more often. Therefore, in order to avoid
the increase the scope of inspections during the unit service
life, the renewal thickness at conversion should also include
sufficient margin to avoid substantial corrosion of such
elements found critical in the hull condition assessment.
Fatigue
Return of experience shows that the most common actions
in order to address fatigue cracks are the following:
•
fitting of backing brackets on side shell and bottom
longitudinal stiffeners in way of transverse bulkheads
and ordinary web frames
changes of toes of horizontal girders, longitudinal
girder ends and web frames joints
Inspections should confirm whether the critical fatigue
details are the same as indicated in the as-built drawings.
Previous repairs as indicated in the tanker repair specifications
and survey records should be verified as well.
Fatigue analysis is in general undertaken by means of finite
element calculations, in general to determine the cause of the
damage and test the alternatives of repair configuration
obtained by improving of the detail design and reducing of
stress concentrations.
•
OTC 17506
11
Conclusions
The selection of vessels for conversion into FPSO will
probably face two main types of design: single hull tankers
built between 1985-95 and double hulls built after 1996.
The return of experience from previous conversion projects
shows that both designs are prone to defects due to fatigue and
corrosion wastage. Despite the particularities inherent to each
hull design, corrosion and fatigue deterioration cumulated
along the trade period will affect the FPSO hull structure
integrity and performance along its service life.
References
Thought that FPSO conversion projects requires fast track
schedules, a detailed hull structure assessment by means of
finite element analysis may not be available at the selection of
the vessel to be converted. Therefore, the various aspects
concerning the hull design characteristics, ship’s historical
information, including operation conditions, maintenance
procedures and records of survey and inspections should be
reviewed at least in qualitative way during the hull selection
process.
In the same way that the hull structure may vary in
function of the shipyard design and fabrication standards, the
hull condition may change depending on the trade, operation
conditions and maintenance procedures. Therefore a complete
hull structure assessment by means of finite element analysis
and consideration of the tanker trade phase degradation modes
seems the most efficient way to proper assess the FPSO hull
Co-operative Forum, Witherby & Co. Ltd.
[4] Guidelines for the Inspection and Maintenance of Double
Hull Tankers - Tanker Structure Co-operative Forum,
Witherby & Co. Ltd., 1995.
structure design and screening of the critical areas and
components of the hull. Therefore, as far as possible, first
results should be available early to assist the selection of the
hull and the specification of the refurbishment program.
program.
Even if the hull is not the expensive part of the production
project, as a large investment is made on the topsides and
subsea equipment, special attention needs to be paid to the hull
effective design life. Safety margins should be estimated and
the necessary procedures to ensure the hull structure integrity
should be incorporated in the unit’s
un it’s IMR plan.
Offloading Surface
Surface Units – NR 497, Octobe
Octoberr 2004 – Bureau
Veritas.
Acknowledgement
The authors wish to thank Bureau Veritas for permission to
publish this paper. The views expressed are those of the
authors and do not necessarily reflect those of Bureau Veritas.
[1] Guidance Manual for the Inspection and Condition
Assessment of Tanker Structures – Tanker Structure Cooperative Forum, Witherby & Co. Ltd., 1986.
[2] Condition Evaluation and Maintenance of Tanker
Structures - Tanker Structure Co-operative Forum, Witherby
& Co. Ltd., 1992.
[3] Guidance Manual for Tanker Structures – Tanker Structure
[5] Floating Production Systems, assessment of the outlook for
FPSO vessels, production semis, TLPs and spars –
International Maritime Associates, Inc.. New Orleans, USA
March 2004.
[6] Otegui J., Orsini M.: Converted FPSO’s. Making a
Worthwhile Conversion – DOT 2004.
[7] Arselin E., Cambos P., Frorup U.: New FPSO Rules Base
don Return of Experience. – OMAE – FPSO’04 – 0091.
Houston, USA 2004.
[8] Rule Note on Hull Structure of Production, Storage and
[9] Rules for the Classification of Steel Ships – November
2004. Bureau Veritas.
12
OTC 17506
Hull Type 1973-2004
0.9%
36.0%
56.4%
6.7%
SH 73-84
SH 85-95
DH 96-04
Others
source: Lloyd's Register
Figure 1: hull type of VLCC’s built between 1973-2004.
Ships in Service - build 1973-2004
45
500000
40
450000
400000
35
350000
30
300000
25
y
t
Q
250000
20
200000
15
150000
10
100000
5
50000
0
0
1973
1976
1979
1982
1985
1988
1991
1994
1997
2000
Year
source: Lloyd's Re
Register
gister
Figure 2: oil tankers operating per year of build and average deadweight.
2003
t
w
D
e
g
ra
e
v
A
OTC 17506
13
1973-85
Ship Sales 2000-04 (USD)
> 250,000 dwt
1986-95
1996-00
120.0
2001-04
100.0
80.0
$
m
(
e
g
a
r
e
v
a
60.0
40.0
20.0
0.0
2000
2001
2002
2003
year
2004
source: CW Kellock & Co Ltd
Figure 3: average sale price of VLCC’s between 2000-04.
STRESS
STRESS COMPONENTS
LOAD CASE
LOAD CASE
PANEL SIGVM
1. 13E
13E+
+ 02
1. 06E+
+0
02
9. 87E
87E+
+ 01
9. 12E+
+0
01
B
8. 46E
46E+
+ 01
A
9
PANEL SIGVM
7. 05E
05E+
+ 01
7. 60E+
+0
01
6. 08E+
+0
01
5. 64E
64E+
+ 01
4. 56E+
+0
01
4. 23E
23E+
+ 01
3. 04E+
+0
01
2. 82E
82E+
+ 01
Y
Y
1. 52E+
+0
01
Y
Y
X
X
1. 41E
41E+
+ 01
Z
Z
X
X
0. 00E
00E+
+ 00
D I SPLACEMENT S ;
LOAD CASE
0. 00E+
+0
00
Z
Z
9:
PART I AL CARGO LC8 , HEAD , S , D , TROUGH , VBM
-1. 52E
E+
+01
D I SP
SP L ACEMENT S ;
LOAD CASE
9:
P ART
ART I AL CARG
CARGO L C8, HE A
AD,
D, S , D , T ROUGH, V BM
Figure 4: double hulls typical structural arrangement arrangement
Transverse
element
Stiffener
flange
Stiffener
web
Longitudinal
plating
Flat bar: Detail when
low stress area
One backing bracket:
improved design
Figure 5: detail of longitudina
longitudinall stiffener connections.
Two backing brackets:
reinforced design
14
OTC 17506
no collar plate
collar plate
full slot
Figure 6: scallops of longitudinal
longitudinal stiffener connections.
Figure 7: angle connection
Figure 8: cruciform joint
Shear introduced in the web of primary
members by longitudinal stiffeners under local
pressure: ‘local shear’
shear’
Shear mo
Shea
mode:
Typical crack at
transverse web frame
Figure 9: shear mode induced crack
Shear in the web of primary member itself due to
global displacements: ‘global shear’
OTC 17506
15
F lexur
lexur al m
mo
ode:
Typical crack at transverse web
frame
Figure 10: flexural mode
induced crack
Figure 12: fine mesh model – detailed stress analysis
Figure 11: Coarse mesh model
Figure 13: very fine mesh model – fatigue analysis