NON LINEAR ROLL JIP
PROJECT PLAN (Rev.2)
The main goal of this project is to provide the technical basis for the on the
estimation and consideration of roll motions and roll induced loads on the
design of FPSO system
An Initiative of:
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1. BACKGROUND
Although much work has already been devoted to roll motions of ships as well
as of FPSOs, these motions are still a major concern of operators in different
areas of the world and predictions still do not match the observations in the
field. Large roll motions can imply in various undesired situations going from
discomfort of the crew, downtime of process plant and degradation of hull,
appendages and lines structural integrity. As an example, in Ferreira et al [1], it
is mentioned that FPSOs operating in Campos Basin revealed the roll motions
to be the biggest concern, with observed single amplitudes of up to seventeen
degrees in the presence of swells.
The fact is that the accurate estimation of roll motions is not an easy task. The
main difficulties lie on the several non-linearity involved and for many types of
assessments we are still constrained to work with linearized models. One of the
most discussed sources of non-linearity is the damping mechanism, but there
are others, such as the coupling with mooring and risers systems. Even though
one may perform a very accurate time domain roll motion analysis in which
many sources of non-linearity have been considered, many questions arise
later: how to perform the structural fatigue assessment? Which RAO should be
provided for the riser analysis? Etc.
The issues regarding the accuracy of roll motions estimation become even
more relevant at a moment where most Class Societies accept to consider the
results from direct computations (at different manners and levels) for the
definition of the design loads for FPSO structural assessments.
It should be admitted that at this stage, there is no harmonized design practice
available and the design assumptions, as far as roll effects are concerned, lies
very much on the expertise of the designers. Therefore, the main objective of
this project is to provide design guidance for the estimation of roll motions and
consideration of roll motions effects on the various kinds of assessments
needed to ensure the integrity of the FPSO system (hull, appendages and
lines).
In order to achieve the main goal mentioned above, much data is already
available in the literature. Since the work of Froude in 1861 on wooden ships
until the present date, people have not stopped thinking about roll motions. With
respect to FPSOs, more specifically, it is worth to mention the JIP FPSO Roll,
lead by Marin, with the main objective of looking into means to mitigate / reduce
roll motions. In that project, model tests as well as full scale measurements
have been performed, in addition to a survey of different mechanisms to reduce
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roll. That project differs from this one in many aspects: this one is not aimed at
identifying mechanisms to reduce roll, but to better understand the existing
FPSOs behavior; also it is intended to consider not only the roll motions but the
effects on the structural integrity in order to define the design recommended
practices. Another very interesting JIP is the French CLAROM project called
“Roll and Current on Barges”. In this project also model tests have been
performed and it should be highlighted the relevant work made by IFREMER on
the extreme response statistics.
In spite of the extensive existing bibliographic reference, there is still need to
progress in the comprehension of roll response. Recent research work
performed by Petrobras together with COPPE indicates that the roll damping
mechanisms for FPSOs with extended bilge keels differ from the traditional ship
theory and the usual (quadratic) damping model may not be applicable for large
amplitudes of motions [2]. Petrobras has also indicated that the spread moored
FPSOs with riser balcony at a single side of the unit, may present asymmetrical
roll behavior usually not predicted during the design of the risers [3].
Furthermore, as an attempt of reducing the roll response, some recent designs
of FPSOs present considerable higher resonance period than the usual vessels.
This solution, indeed, reduces the roll motions caused by the first order loads
but reveals a new type of roll response occurring due to second order lowfrequency loads. This new phenomenon is not yet considered in the design
codes and deserves more research work.
It is also important to mention that most of the works performed very often
regard only the estimation of roll motions, and there are few works on the roll
effects for the structural assessment. With that respect, it is observed that there
is no harmonization in the Industry of the design practices and the approaches
adopted vary depending very much on the expertise of the design companies.
In summary, some reasons to propose this JIP are listed below:
♦
♦
♦
It is necessary to elaborate harmonized practices and common
knowledge for the estimation of roll motions and consideration of roll
effects onto structural assessments;
Even though recent work indicates that the commonly adopted quadratic
damping model for roll is no longer valid for high amplitudes of motions of
FPSOs with barge shape or extended bilge keels, all the latest JIPs still
employed that model. This may be under conservative for extreme
responses.
It is important to understand the coupling between FPSO and risers
system in order to improve reliability of those structures.
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♦
♦
♦
There is a lack of projects about second order roll motions of FPSOs.
There is a lack of projects going beyond the estimation of roll motions up
to the effects of roll on the structural assessments of the FPSO system
(hull, bilge keel and risers).
There is a lack of guidance about performance of model tests (type and
number of repetitions) in order to get good statistical results, especially at
low amplitude levels (relevant for fatigue assessments).
2. OBJECTIVE
To provide the technical basis for the estimation and consideration of roll
motions and roll induced loads on the design of FPSO system.
3. MAIN TECHNICAL ISSUES
The following technical issues have been identified as important to be explored
during this project:
3.1 Roll damping estimation
For resonant response like roll, one very important parameter is damping. As
roll is a major concern for operation of FPSOs in areas with severe
environmental conditions such as offshore Brazil, the most efficient way of
increasing the damping and reducing the roll has been demonstrated to be the
use of extended bilge keels. However, recent studies indicated that the roll
damping mechanisms of FPSOs with extended bilge keels may differ from the
established ship theory, especially with respect to the evolution of damping with
the roll amplitude as can be observed in Figure 1 extracted from [2] below. In
that Figure it can be observed that the damping increases linearly with the roll
angle as expected from the ship theory. For the FPSO with extended bilge keel,
however, the damping presents a “plateau” at high roll amplitudes.
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Figure 1 – Example of decay tests results for FPSO without bilge keels and with extended one
(1.5m) [2]
Thus, it is necessary to enhance the comprehension of roll damping
mechanisms of FPSOs with extended bilge keels by analyzing the vortex
shedding patterns, which dominates the damping in this case. This can be done
through model tests and numerical computations (CFD).
Another related problem is the estimation of loads acting on the bilge keels.
Although these structural elements, until now, have not been considered as
critical for the integrity of the unit, its damage can lead to a change on roll
behavior which may represent a serious threat.
Furthermore, very few model tests campaigns have been performed for FPSOs
with high resonance periods. The damping modeling at those periods of
response involves large uncertainties and needs to be investigated.
3.2 Roll motions computation
As roll is a non linear process, ideally its estimation should be based on time
domain computations. However, this approach is considered time consuming
and the frequency domain is the approach actually adopted. Frequency domain,
however, involves issues associated to the linearization method applied to the
damping.
In the case of spread moored FPSOs with large number of risers attached to a
riser balcony in a single side of the unit, the coupling between the floater and
the lines can lead to asymmetrical roll behavior. In Ferreira et al [3], it is
indicated that part of the effects of the lines could be captured in frequency
domain, but to get the full action of the risers a fully coupled time domain
analysis would be recommended.
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Figure 2 – Comparison between full scale roll measurements with frequency domain computation
with equivalent linear damping equal to 8% of critical damping [3].
For low-frequency roll motions computation, one more complexity arises from
the computation of the second order roll moment. Due to the difficulties in
calculating the complete second order loads (QTFs in frequency domain), there
are several formulations in the literature which are adapted to the different
systems, depending basically on two parâmetros: water depth and resonance
frequency of the system. In [4], a review of those formulations has been
executed and the summary of the results are illustrated in below:
Figure 3 – Applicability of different formulations for second order loads computation
It is important to remark that not all the hydrodynamic software have all the
formulations implemented, and it is important to benchmark practices and tools
for the second order roll moment computation. Furthermore, although the
horizontal loads have often been found in reasonable agreement comparing to
experiments, the vertical loads have been less studied. In [5], it is indicated that
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the consideration of the variation of low-frequency loads with regard to the
mean position of the body may bring an improvement to the motion simulation
model. Nevertheless, very few works have been done after as second order
vertical loads have not been a main concern until now. Still with respect to the
loads prediction for second order roll computations, the hydrodynamic coupling
between heave and roll should be better investigated. This would require the
computation of the loads in time domain instead of in frequency domain. In any
case, it should be stressed that, as the process is very non linear, the
estimation of extreme second order roll motions should be performed in time
domain.
3.3 Statistics of extreme roll motions
After the roll motions are computed, one may be interested in estimating what
would be the most probable extreme roll motion for a given period (e.g. 100
years). This poses several questions:
♦ If the assumption of Rayleigh distribution of probabilities reasonable (on
safe side);
♦
In case of time domain simulation, how long it should last or how many
seeds should be considered to get a converged MPMV.
♦
Which law should be used to compute the extreme value. Is Weibull
reasonable?
3.4 Structural assessment
Finally, it should be understood that the estimation of roll motions is not the end
of the story. Roll motions induce loads that are dominant for many structural
elements dimensioning.
The design of FPSOs still relies very much on the loads prescribed for ships by
the Class Societies, although some Class Societies begin to accept the results
from direct computations for the correction of the prescriptive loads based on
the fact that this type of structure will operate in a unique location along its life.
The correction is often made on the envelope of loads (extreme) only and the
way the load parameters are combined between them (through load
combination factors) usually remain as for the ship.
Ideally, for a non-linear process, a time domain structural analysis would be
recommended. However, at this stage, the computational costs are
unaffordable and this is not a practical solution. But some simplification can be
made considering that the structure response may be assumed linear although
load is not. This assumption can give rise to many different approaches for
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structural analysis evolving from simplest to more complex ones. These
different approaches should be benchmarked in order to assess the impact of
each.
3.5 Model tests
Model testing also involves some technical issues to be clarified in order to give
valuable results to the design. As model tests are expensive, the definition of
the model tests campaign should be dealt with carefully.
For the roll damping estimation, the following questions need to be answered:
♦
Which type of model tests can give more reliable value of damping:
decay tests in calm water, forced roll in calm water, forced roll in waves?
♦
How different is the damping in waves comparing to the damping in calm
water?
♦
How many tests should be performed in order to be able to draw some
statistics on the damping coefficients? 3, 5, 10, 20?
Another complexity lies on the consideration of the lines effects in very deep
water, in which the system must be truncated.
And, finally it is desirable to compare the second order vertical moments from
experiments with computations. However, in deep waters, the second order
loads are much lower than the first order. Thus, there are some issues about
the errors on measurements.
The project is also intended to benchmark model testing practices and come up
with the best practices when roll response is concerned.
4. PROJECT ORGANIZATION
The project is organized in work packages that group related subjects. Each
work package is coordinated by SINTEF or Bureau Veritas, depending on each
company involvement in the related subject. Bureau Veritas will also be in
charge of the global management.
4.1 WP0 - Project management (Leader : BV; Executing Party: BV)
This work package is dedicated to the global management of the project,
including the financial management and administrative coordination of the
teams involved. This WP is in charge of:
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♦
Establishing the quality procedures to be applied in the project in order to
ensure that the communication among the various companies involved
will be efficient and the good standard for the project products.
♦
Providing a tool for disseminating information of the project;
♦
Organizing meetings and preparing the minutes of meetings;
♦
Elaborate the progress reports to the members;
♦
Collect and check the expenses of the project;
♦
Make sure that the project is following the expected schedule;
4.2 WP1 – Review of state-of-the-art and state-of-the-practices
(Leader: BV)
This WP will be coordinated by Bureau Veritas. This WP is subdivided into two
tasks:
a) WP1.A – First order roll motions
For first order roll motions, much work is available and should be gathered
for the benefit of the project. Regarding the tools employed for the first order
roll estimation, It is not expected large deviation in results obtained by
different companies if they work with the same premises. Thus, this task
focuses in benchmarking premises and practices rather than simulation
tools. The following work is to be done:
♦ Review the previous work available in public domain or proprietary
results made available to the JIP by the company that owns the
results.
♦
Compare Class Societies approaches regarding the consideration of
roll for FPSOs.
♦
Review the state-of-practice in the Industry collecting the contribution
of the participants and through review of literature or based on
experience of Class Societies;
b) WP1.B – Second order roll motions
For second order roll motions, much less work is available and there is still a
need to benchmark both practices and background of tools for computation
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of second order vertical loads. Furthermore, as already mentioned, second
order roll motion is not contemplated in the current Rules of Class Societies.
4.3 WP2– Damping estimation (Leader : SINTEF ; Executing
Parties : SINTEF, UFRJ / LOC)
This work package will deal with the damping estimation basically through two
different approaches: experiments and numerical simulations. It will be
coordinated by SINTEF. This WP is subdivided into two tasks which are
detailed bellow.
a) WP2.A – Model tests at LOC
LOC is a waves and current laboratory at the Federal University of Rio de
Janeiro. The installation will be used to perform tests using a typical section
of a FPSO in the same way as presented in [2].
Figure 4 – Picture of LOC with a FPSO section being tested. Extracted from [2]
The laboratory is equipped with PIV device enabling the monitoring of fluid
kinematics during the tests. The laboratory is equipped with PIV device
enabling the monitoring of fluid kinematics during the tests. The tests
campaign will involve the following:
♦
Roll decay tests;
♦
Roll decay with fixed center of rotation;
♦
Forced oscillation in calm water;
♦
Forced oscillation in waves;
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A matrix of experiments will be developed based on the variation of the
following parameters:
♦
Dimension of the bilge keel (0.8m, 1.2m and 1.8m)
♦
Loading condition (3 drafts);
♦
Initial roll angle (3)
♦
Inertia and GM (full scale natural period: 15s, 30s, 40s)
♦
Riser balcony (1)
The bilge keel of 1.8m will be instrumented with strain gauge.
b) WP2.B – CFD Simulations
The objective of this task is to perform CFD simulations to further assess the
effects of the FPSO appendages (bilge keels and riser balcony) on vortex
dynamics and viscous roll damping. This task builds up on the results of
WP2.A, as the same FPSO model will be employed in the CFD simulations.
The following steps will be followed:
♦
Simulations at the model scale: a group of model tests from WP2.A
will be selected and reproduced with the CFD simulations for the
validation of the CFD model in model scale. An extended matrix of
numerical simulations will be elaborated and further investigation of
the vortex dynamics and roll damping will be performed.
♦
Simulations at full scale simulations: Based on the results and
validation from the previous step, the CFD model will be extrapolated
to full scale in order to assess the vortex dynamics and roll damping
that one may observe in reality. Particular attention will be given to
simulations in irregular sea states, and results from this step will be
used as input for the motions and structural assessments.
4.4 WP3 – Model tests at LabOceano (Leader: SINTEF; Executing
Parties: SINTEF and LabOceano)
This work package will be coordinated by SINTEF.
The model tests that would have been previously performed at LOC are
intended to give further insight on the damping mechanisms for FPSOs, which
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have specific appendages comparing to a ship (extended bilge keels and riser
balcony). However, only a section of the FPSO hull will be represented and the
scale will be ultra reduced. It is important at a later stage, after numerical
analysis have been done, to perform model tests of the complete system. This
will be done at LabOceano / COPPE. Figure 5 below presents a picture of the
basin.
Figure 5 – Picture of LabOceano basin
The campaign will include tests with the FPSO in two loading conditions in order
to vary the roll resonant period having the vessel responding at first order range
or second order range.
For the lower resonant period, the campaign will include:
♦
Decay tests in calm water;
♦
Tests in regular waves;
♦
Tests in irregular waves;
♦
Tests in directional sea (with spreading);
♦
Tests with the mooring and risers systems connected;
The bilge keel will be instrumented with strain gauge.
For the higher resonant period (at second order range), the campaign will
include:
♦
Decay tests in calm water;
♦
Tests in regular waves;
♦
Tests in irregular waves;
♦
Tests in bi-chromatic waves.
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4.5 WP4 – Effect of riser balcony and lines on the roll motions
(Leader: SINTEF; Executing Parties: SINTEF and BV)
This work package will be coordinated by SINTEF with the objective of
assessing the coupling effects between the mooring and risers systems and the
FPSO on the roll motions. At the end it is envisaged to assess both:
♦
The effects of the balcony and the asymmetric configuration of the riser
lines on the response;
♦
The effects of the asymmetric roll motions on the structural response of
the risers
Numerical simulations in frequency domain and time domain will be performed
for the global FPSO system (vessel+mooring lines+risers). A dataset from an
existing FPSO will be used, which will be provided by Petrobras.
State-of-the-art methods will be employed. Different approaches shall be
explored to represent the impact of the risers configuration asymmetry on the
roll response. Hydrodynamic and structural analyses will include several
experiments taking into account different combinations of parameters, such as
sea states, presence/ absence of risers, influence of the balcony, coupled vs.
decoupled simulations in frequency or time domain, etc. A systematic analysis
will be used to assess how the roll motion is affected by the balcony, and how
the riser structure is affected by the asymmetric roll.
Full scale measurements will also be employed as far as practicable. However,
some difficulties are expected on the evaluation of full scale data due to
uncertainties on the directionality of the sea which can significantly affect the roll
response.
4.6 WP5 – Numerical benchmark of second order roll computations
(Leader: BV; Executing Parties: BV and SINTEF)
This work package will be coordinated by Bureau Veritas with the aim of
comparing computations of second order roll motions performed by different
companies using different software. At least Bureau Veritas, SINTEF and
Petrobras will perform analyses independently and blindly using different
hydrodynamic tools. Other companies are also invited to contribute to this
benchmark. The main idea is to identify discrepancy on results due to methods
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or tools with the final goal of harmonizing the approaches in order to reach
comparable results among the participants.
4.7 WP6 – Structural assessments (Leader: BV; Executing Parties:
BV and SINTEF)
This work package will be coordinated by Bureau Veritas with the aim of
assessing the impact of the non-linearity in roll through different methods going
from simple to more complex ones.
As the complete non-linear hydro-structural problem is very complex to solve
and in terms of computation time it is unfeasible for the time being, it is
necessary to make some simplifications. In this project, we will still consider that
the structural response remains linear although the loads are not. Therefore, the
stresses due to a combination of different load effects can be obtained by a
linear combination of the stresses under unitary loads. Based on this
assumption Bureau Veritas proposes a new method to be developed within this
WP, which would be able to account for non linear effects on extreme and
fatigue analysis.
In this new method, the structural response will be pre-computed for unitary
values of a few load parameters (ex. Transverse acceleration induced by the
roll angle), focusing on details that are governed by these load parameters. The
loads would be computed in time domain and the pre-computed stresses would
be combined linearly. A 2D structural model will be employed.
The new method proposed will be considered as the most refined one and
compared to simpler methods: prescriptive (Rules), EDW and spectral. The idea
at the end is to check which would be the simplest method that could provide
reasonable results.
Furthermore, SINTEF will assess bilge keel fatigue life from non-linear roll
simulations. Pressure fields from the full-scale CFD simulations will be used as
input for the structural simulations. Finite element models will be made for three
bilge keel geometries, and the fatigue life of welds will be assessed using a
deterministic approach. The fatigue lives obtained with this approach will be
compared with current recommended practice for spectral stochastic fatigue
analysis. CAD drawings for the bilge keel geometries and stochastic fatigue
analysis shall be provided by the project consortium.
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4.8 WP7 – Guidelines (Leader: BV; Executing Party: BV)
This work package will be coordinated by Bureau Veritas with the aim of
elaborating a document providing the technical basis for the roll motions
estimation and consideration of roll motion effects for structural assessments.
4.9 WP8 – Full scale measurements (Optional) (Leader: SINTEF;
Executing Party: SINTEF)
This work package will be coordinated by SINTEF with the overall goal of
enhancing the comprehension of the non-linearity involved in the roll motions of
moored FPSOs. Large discrepancies are observed between the roll predictions
from early design phase and the actual full scale roll motion. The objective here
is to investigate the sources of non-linearity that can be responsible for those
discrepancies. In particular, we will explore the impact of the coupling between
the mooring and risers system and the vessel on the vessel roll motions.
5. DELIVERABLES
The JIP will produce the following deliverables:
♦
Reports with review of the state-of-art methods related to first order roll
motions prediction and associated structural assessments and for
second order roll motions;
♦
Model tests reports;
♦
Reports with description of methods and results of numerical analyses;
♦
Guidelines for roll motions estimation and consideration of roll effects on
the structural assessments
6. SCHEDULE
The Non linear ROLL JIP will run for 2 years. The kick of meeting will be held in
Rio de Janeiro in Brazil as soon as minimum number of participants is reached
(expected for March 2013).
The work packages will be performed according to the following schedule,
which is given in months. The months marked in red correspond to
disbursement events by participants. They have been chosen in way of
coinciding with the end of major tasks such as model tests campaigns.
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M1
M2
M3
M4
M5
M6
M7
M8
M9
M10
M11
M12
M13
M14
M15
M16
M17
M18
M19
M20
M21
M22
M23
M24
WP0
WP1
WP2.A
WP2.B
WP3
WP4
WP5
WP6
WP7
WP8
7. BUDGET AND PARTICIPATION FEES
The following budget distribution is based on the scope of work as described in
item 4 above. However the final scope and budgets may be rearranged together
with the JIP participants during the kick-off meeting.
Cost
(k R$)
TOTAL
Activity
WP0
Global Management
115
WP1
Review of state-of-the-art and state-of-the-practices
71
WP2.A Model tests at LOC
545
WP2.B CFD Simulations
251
WP3
Effect of riser balcony and lines on roll motions
104
WP4
Numerical Benchmark of second order roll
310
WP5
Model tests at LabOceano
694
WP6
Structural Assessments
272
WP7
WP8
Total
Guidelines
Full scale measurements
84
Optional
2446
The budget to cover the complete scope of work described, with exception of
the full scale measurements, amounts in R$ 2 446k (Two million, four hundred
and forty six thousands Brazilian Reais), which is equivalent to 907 Keuros (rate
at 28/11/2012).
The following participation fees are based on a number of 10 to 15 participants:
♦
Oil companies: 250k BRZ (~93 Keuros at 28/11/2012)
♦
Others: 180k BRZ (~67 Keuros)
The following distribution of disbursement along the years is expected:
♦
2013: 40% (2 disbursements)
♦
2014: 45% (2 disbursements)
♦
2015: 15% (1 disbursement)
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For some oil companies based in Brazil it is also interesting to know the way the
costs are distributed between companies and research institutes. This
information is found in the graph below:
REFERENCES
[1] Marcos D. Ferreira et al – Hydrodynamic Aspects of the New Build FPSOBR
– 2nd International Workshop on Applied Offshore Hydrodynamics – Rio de
Janeiro, Brazil 2005
[2] Allan C. de Oliveira et al – The Influence of Vortex Formation on the
Damping of FPSOs with Large Width Bilge Keels – OMAE 2012 – Rio de
Janeiro, Brazil
[3] Marcos D. Ferreira et al – Asymmetric FPSO Roll Response due to the
Influence of Lines Arrangement – OMAE 2012 – Rio de Janeiro, Brazil
[4] Guillaume de-Hauteclocque et al – Review of Approximations to Evaluate
Second Order Low-Frequency Load - OMAE 2012
[5] Chen X.B & Molin B. – Numerical Prediction of Semi-Submersible NonLinear Motions in Irregular Waves – Fifth International Conference on Numerical
Ship Hydrodynamics (1990)
CONTACT
For more information, please contact Bureau Veritas:
Flávia Rezende (Bureau Veritas)
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Tel: +55 21 2206 9436
e-mail: [email protected]
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