The 22nd International Conference on
Offshore Mechanics & Arctic Engineering
Cancun, Mexico
8-13 June, 2003
OMAE2003-37162
DESIGN ASPECTS OF GREEN WATER LOADING ON FPSOs
Bas Buchner, MARIN
ABSTRACT
Green water is a significant problem for floating ship-type
offshore structures, which needs to be assessed in the early
design of the structure. First the present paper summarises a
new semi-empirical design evaluation procedure that can assist
in this design process. Then the practical design considerations
related to the green water problem are discussed. There are
different ways to solve the green water problem for a particular
structure at a specific location:
• Design the vessel and structures on the deck against the
predicted green water impact load levels.
• Optimise the bow shape (underwater shape and above
water bow flare).
• Increase the freeboard height such that green water is
prevented completely.
• Increase the freeboard height such that the green water
loads are reduced to acceptable levels and design for these
load levels.
• Optimise the structures on the deck to minimise the green
water impact loads.
• Use protecting breakwaters in front of critical structures on
the deck.
All these options have their advantages and disadvantages.
The semi-empirical design evaluation method and the different
design options are discussed in a case study, focussing on a
new DP FPSO concept.
INTRODUCTION
In heavy storms, the waves and ship motions can become
so large that water flows onto the deck of a ship. This problem
is generally known as ‘green water loading’. On ship-type
offshore structures green water loading can result in risk for the
ship, its crew and its sensitive equipment. Therefore, it should
be taken into account in the design of such structures.
Joaquín Lopez-Cortijo Garcia, IZAR FENE Shipyards
Recent experience in the North Sea in both the UK and
Norwegian sectors confirms the occurrence and importance of
the green water problem.
As reported by Morris, Millar and Buchner [1], from 1995
to date seventeen green water incidents have been identified on
twelve UK FPSOs. Some installations have experienced more
than one incident. Ersdal and Kvitrud [2] also report damage
incidents in the Norwegian North Sea. In January 2000 the
living quarters on the bow of the Varg FPSO was hit by green
water. This resulted in the damage of a window on the second
floor, flooding the area behind it.
In Buchner [3] methods for the evaluation of green water
on ship-type offshore structures were developed based on a
clear description of the green water physics. A semi-empirical
design evaluation method was proposed, to predict the green
water problem from the input (extreme relative wave motions)
to the output (predicted load levels). Using the results of this
research, the present paper focuses on the practical design
aspects of the green water-loading problem for the design of
FPSOs:
- Bow shape design.
- Design of green water prevention methods (raised
bulwarks, a tilted ‘canopy’).
- Choice of effective protecting structures and breakwaters.
- Structural design of these structures.
In the paper the hydrodynamic aspects will be combined
with the practical knowledge from shipbuilding experience.
In Buchner et al. [4] a case study for a typical FPSO for the
deepwater Gulf of Mexico was presented. Although in a real
design the most critical weather conditions should be
determined for a specific vessel (the maximum relative wave
motions are strongly related to the ship length for instance), the
present paper focuses on design methodology rather than the
design for a specific area. Therefore, the present paper makes
use of the 100-year Hurricane for the Gulf of Mexico, see
Table 1.
1
Copyright © 2003 by ASME
GREEN WATER EVALUATION METHODOLOGY
Although numerical prediction methods utilising
computational fluid dynamics (CFD) for green water loading
are under development, they cannot be used as practical
prediction methods at this time. Therefore a semi-empirical
method for the prediction of green water loading is used, based
on the early research presented in Buchner [6] and the
systematic model test series presented in Buchner [3]. The
method is semi-empirical, as the basis for the evaluation of the
green water problem is a linear diffraction calculation at zero
speed, coupled with results from a model test database. The
linear diffraction model makes it possible to take into account
the main characteristics of the vessel, such as its main
dimensions, underwater hull shape and weight distribution. If
necessary this linear diffraction calculation can be corrected for
the effects of current speed. The model test database provides
information that is used to define the non-linear interaction
between the fluid and the structure. Similar approaches were
presented by Hellan, Hermunstad and Stansberg [7].
Figure 1 provides a schematic of the green water
evaluation methodology. The figure provides a flow chart that
identifies the input required, the analysis to be performed, and
the output that can be obtained. The dashed lines in Figure 1
represent the input required:
• Vessel particulars with specific information on the
parameters that influence green water occurrence.
• Information from Metocean specialists to allow
development of design environmental criteria specific to
green water occurrence.
Table 1: 100-year Hurricane for the Gulf of Mexico
Parameter
Significant wave height
Spectral peak period
JONSWAP gamma
100-year wave
12.5
13.0
3.3
Unit
metre
seconds
-
In the present study the FPSO under investigation is a DP
FPSO, which is presently under development. DP aspects of
this new concepts are discussed in various recent papers, see
Cortijo et al. [5]. Although green water from the side can be a
problem as well, the present paper focuses on the bow of the
DP FPSO.
Prior to developing and discussing the case study, the
semi-empirical design methodology to evaluate the green water
loading on various structures will be summarised. More details
can be found in Buchner [3].
The present paper is a result of a co-operation between the
authors as part of the SAFE-FLOW project. The SAFE-FLOW
project (SAFE-FLOating offshore structures under impact
loading of shipped green water and Waves) is funded by the
European Community under the ‘Competitive and Sustainable
Growth’ Programme (EU Project No.: GRD1-2000-25656). Its
objective is to develop guidance, calculation methods and risk
assessment procedures for green water and wave impact
loading appropriate to each stage of a floater project, namely:
concept development, detailed design and operation. The
project also contains a research effort to further develop and
validate the numerical simulation of the impact phenomena
under investigation (ComFLOW program).
Designer
F, p
Breakwater
F, M
Slender
structures
F, M
h
H
RAO
heading
Spectral
analysis
s
Extreme
relative
motions
Storm duration
Wave Hs,Tp,dir
Wind V and dir
Current V and dir
Shape
Position
Flare angle
Heading
calculation
Freeboard
QTFs
Heading control
RAOs
Current/wind
coefficients
Weight data
Hull shape
Linear
diffraction
Structures
h
Water height
Water velocity
H
U
s
Extreme
relative
motions
side and
stern
h
Metocean specialist
Figure 1: Overview of the semi-empirical design evaluation method
2
Copyright © 2003 by ASME
ap Is again dependent on the bow shape (full or thin) and
bowflare angle (0, 10, 30 and 50 degrees). A similar expression
can be used for the total horizontal load on the structure (F):
The following analyses are performed as represented by
the blocks in the figure:
1. Calculate relative wave motion RAOs with linear
3D-diffraction analysis with correction for possible effects
of current.
2. Calculation of the heading of the vessel with respect to the
wave direction due to the combined wind, wave drift and
current forces. This is an important parameter for the
problem of green water from the side, which increases
significantly when the angle between the waves and vessel
increases.
3. Calculate linear relative wave motions for the wave
spectra of interest resulting in the standard deviation (s) of
the relative wave motions.
4. Perform a non-linear correction of the extreme relative
wave motions, including the typical discontinuity at the
freeboard level as presented in Buchner [3] and investigate
the effects of bow flare angle and wave period. The
methodology is based on a systematic model test series
with different bow shapes (full or thin), bowflare angles
(0, 10, 30 and 50 degrees with the vertical) and spectral
peak periods (12, 14 and 16 s). The output is the expected
maximum freeboard exceedence (h).
5. Determination of water height (H) and velocity (u) on the
deck using the extreme freeboard exceedence (h) as input.
These are also based on the empirical relations for
different bow flare angles. The water height on the deck,
H, was found to be a linear function of the freeboard
exceedence at the bow according to:
H = aH ⋅ h
F = a F ⋅ h2
ap Is again dependent on the bow shape (full or thin) and
bowflare angle (0, 10, 30 and 50 degrees).
Determination of impact loading on pipes at the deck,
using the water height (H) and velocity (u) on the deck.
Evaluation of breakwaters to protect structures or
equipment if necessary.
Details of the method and parameters in the empirical
relations can be found in Buchner [3]. For the possible green
water loading from the side a similar approach is used.
The method gives the designer the possibility to evaluate
the relation between the available design parameters (dashed
lines from the top) and the resulting green water effects. This
allows the evaluation of the different options to solve the green
water problem for a specific ship-type offshore structure:
1. Design the vessel and structures on the deck against the
predicted green water impact load levels.
2. Optimise the bow shape (underwater shape and above
water bow flare).
3. Increase the freeboard height such that green water is
prevented completely.
4. Increase the freeboard height such that the green water
loads are reduced to acceptable levels and design for these
load levels.
5. Optimise the structures on the deck to minimise the green
water impact loads.
6. Use protecting breakwaters in front of critical structures on
the deck.
(1)
The parameter aH is dependent on position at the deck and
the bow flare angle. The velocity of the water over the deck is
related to the square root of the water height at the fore
perpendicular H0 according to:
u = au
H0
GENERAL DESIGN CONSIDERATIONS FOR GREEN
WATER EVALUATION
The largest relative wave elevations occur when the
wavelength is equal to the ship length. The bow shape also has
a strong effect on the relative wave elevations as discussed
below:
• The effect of the buoyancy on the pitch motions - with a
thin bow the pitch motions are significantly larger than
with a full bow, which plays a role at the peak of the
relative wave Response Amplitude Operator (RAO).
• The reflection of the waves from the bow - the reflection is
the largest for full cylindrical bows and plays an important
role in the shorter waves (high frequency tail of the RAO).
An optimum FPSO bow accounts for both effects
described above - a bow that is not too full, but with sufficient
buoyancy. This typically results in an elliptical type water line.
Figure 2 compares the relative wave RAOs of FPSOs with a
thin triangular bow, an elliptical bow, and a full cylindrical
bow. The figure also shows the relative wave motion RAO of
the DP FPSO bow described in the present case study.
(2)
Determination of impact loading on structures at the deck
that interact with the green water flow. This is possible for
different structural shapes and includes a prediction of the
maximum pressure over the height of the structure. The
loading of structures on the deck has to be determined in a
number of steps:
Step a: Determination of loads on reference (squared)
structure.
Step b: Correction for distance to forward perpendicular.
Step c: Conversion to different structural shapes.
Step d: Determination of load profile and total load.
The following general expression was found for the
pressure on the structure (p):
p = a p ⋅ h2
(4)
(3)
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Copyright © 2003 by ASME
Relative wave motion (m/m)
e)
The hull design must also account for constructability
issues, i.e., a simpler structure is easier to built, inspect
and maintain.
In order to accomplish the above mentioned design
requirements of the DP-FPSO, the vessel hull has been
provided with the following features:
- Hull forms typical for new-built FPSOs, with prismatic
mid-body in way of the cargo tanks, sloped flat transom
and triangular bow. Rounded bilge of adequate radius to
accommodate bilge keels, as well as camber of suitable
height.
- The fore and aft ends have been designed to properly
accommodate the envisaged number and size of thrusters
(three forward and three aft). The triangular bow allows a
suitable arrangement for the three forward thrusters,
minimises the wind, current, wave drift forces and forward
resistance, and yields a simpler construction. In order to
provide room for installation of the offloading equipment
and flare, the hull shape aft at main deck level will be
square (full beam) in a cantilever structure of rounded
shape to prevent slamming.
- A forecastle deck of suitable height is provided to insure
acceptable green water loads in extreme design conditions.
However, this was decided after thorough evaluation of
the alternatives for mitigating the green water loading
described hereinafter. A poop deck is provided as well for
similar reasons. Bulwark of reduced height (to avoid large
induced loads on the stanchions and supporting deck) is
fitted forward and aft as required. A bow flare angle of
30 degrees has been provided above the maximum draft.
- A moonpool of diameter as specified by turret designer is
located amidships.
4
3.5
3
cyl
2.5
ell
2
tri
1.5
1
0.5
0
0
0.5
1
1.5
wave frequency (rad/s)
Figure 2: Relative wave motions RAOs for a thin
triangular, an elliptical and a full cylindrical bow
compared to the present DP FPSO bow (crosses)
Bow flare (and rake) typically reduces the amount of water
coming onto the deck as the flare pushes the water away from
the deck. However, large flare/rake angles (above 40 degrees,
say) are not recommended because they can cause significant
bow flare slamming at the hull plating. The results presented in
Buchner [3] illustrate that a flare angle of approximately
30 degrees is optimum, with the elevation of the foc’sle deck
dependent on the estimated relative wave elevations and the
philosophy for allowable green water on deck (described in
more detail later).
DP FPSO
The hull of the DP-FPSO has been designed taking the
following key aspects into consideration:
a) Environmental forces induced by waves, wind and current
should be minimised in order to keep the CAPEX/OPEX
of the Unit within reasonable limits, as well as the
emissions produced by the dual (diesel/gas) generators,
which must be continuously running in a fully DP vessel.
b) The hull configuration must allow for an adequate
arrangement of the thrusters at the fore and aft ends, to
avoid thruster-thruster and thruster hull/turret interactions.
Moreover, the layout of the machinery spaces and the ease
of thruster overhauling is significantly affected by the
selected vessel hull forms.
c) In heavy storms (hurricanes, squall events, etc.), the vessel
behaviour against green waters and slamming occurrences
must be acceptable, insuring adequate safety of the people
onboard and sensitive equipment (fire fighting, lifesaving
equipment, etc.).
d) Layout requirements, such as: Accommodation forward
(for navigation in sail away condition after disconnection),
turret amidships (to minimise riser system dynamics and
vessel motions for turret buoy (dis)connection operations),
and offloading equipment at the stern for tandem
offloading, have to be catered for in the design.
In Figures 3 and 4 the artist impression and body plan of
the vessel are shown. The initially chosen vessel characteristics
are shown in Table 2.
Figure 3: Artist impression of the DP FPSO
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Copyright © 2003 by ASME
Pressure
Load
Moment
(H)
6.1
m
(pdeck)
75.4
kPa
(p)
241
Pa
(FX)
7234
kN
(FX)
7234
kN
REDUCE THE AMOUNT OF GREEN WATER BY
INCREASING THE FREEBOARD
In principle it is possible to increase the freeboard to such
a level that no (or a negligible amount of) green water comes
onto the deck. For the present vessel and design environmental
conditions and using the semi-empirical methodology, this
results in an extreme freeboard height of 21.0 m.
However, the question is whether such an increase is
really necessary. Equipment on the decks of ships (designed
according to normal ship rules) are designed to accept a certain
amount green water loading and therefore it is not required to
keep the deck completely dry. Although exact levels cannot be
defined, it was proposed in by Morris, Millar and Buchner [1]
to define typical susceptibility levels for green water loading:
Table 2: Main particulars of the DP FPSO
Value
214,757
260.0
46.0
28.0
20.5
(h)
8.2
m
Now a number of options for the further optimisation of
the design will be discussed:
• Reduce the amount of green water by increasing the
freeboard.
• Optimise the structure at the deck to minimise the loading.
• Protect critical equipment, using breakwaters.
• Use a sloped deck (‘canopy’).
Figure 4: Body plan of the DP FPSO
Parameter
Displacement (fully loaded)
Length between perpendiculars
Breadth
Depth
Draft (fully loaded)
Pressure
on deck
Basic
design
Water
height
Design
option
Freeboard
exceedance
Table 3: Overview of green water loads for
basic case design of the DP FPSO
Unit
Metric tonnes
m
m
m
m
Although, as explained above, an elliptical bow seems to
be the most suitable shape to prevent green waters while
keeping the drift loads within reasonable limits, the combined
effect of the triangular bow and the 30 degrees flare angle,
provides an equivalent relative wave RAO and fairly good
hydrodynamic behaviour. This can be observed in Figure 2,
where the relative wave RAOs of FPSOs with full cylindrical,
elliptical and triangular have been plotted together with the
RAO of the DP-FPSO. At higher frequencies, the behaviour is
in between the triangular and the elliptical shapes, whereas at
the peak it is between the elliptical and the full cylindrical.
0 - 3 meter freeboard exceedance:
3 - 6 meter freeboard exceedance:
6
meter and higher freeboard exceedance:
low
medium
high
In general it is found that for freeboard exceedances lower
than 3 m, the resulting loading is relatively low and can be
handled by structures designed according to existing rules. For
freeboard exceedances higher than 3 m, it is recommended to
assess the loading on sensitive equipment in detail. This
becomes critical for freeboard exceedance in excess of 6 m.
In the present paper a freeboard exceedance of 2.8 m is
assumed to the acceptable. This acceptable freeboard
exceedance requires an increase of the foc’sle deck by 7 m for
a total of 17.5 m.
With the freeboard of 17.5 m the water height on deck is
reduced to 2.1 m and the pressure on the superstructure is
reduced to 27.5 kPa.
GREEN WATER ANALYSIS AND OPTIMISATION OF
THE DP FPSO
Using the semi-empirical method described above, the
green water occurrence and loading for the basic design of the
DP FPSO was determined in the environmental conditions
from Table 1. The results are shown in Table 3 , giving the:
- Most Probable Maximum freeboard exceedance.
- Water height on deck at the fore perpendicular.
- Related pressure on the deck.
- Estimated local peak pressure on the structure.
- Total horizontal force on the structure.
- Total moment on the superstructure with respect to the
deck level.
5
Copyright © 2003 by ASME
OPTIMISE THE STRUCTURE AT THE DECK TO MINIMISE THE LOADING
Instead of increasing the freeboard, it is also possible to
optimise the structure at the deck to minimise the green water
impact loading. The magnitude of the loads on large structures
on the deck is dependent on the shape of the structures relative
to the flow direction. Structures deflect the high velocity flow,
resulting in loads caused by the change in momentum. For
structures that deflect the flow more gradually, impact loads
are reduced significantly.
To investigate the sensitivity of the impact load on
structures on the bow for the structural shape, in Buchner [3] a
series of seven different structures was tested. The structures
can represent a protection structure around the turret for bow
turret systems, as well as an accommodation superstructure for
heading controlled FPSOs with the turret further aft. The
following structural shapes were investigated:
• Squared structure.
• Tilted structure under 30 degrees angle with vertical.
• Triangular structure with 45 degrees semi-angle.
• Triangular structure with 60 degrees semi-angle.
• Cylindrical structure.
• Squared structure with triangular support.
As an example, Photo 1 shows the tilted structure and the
triangular structure with 60 degrees semi-angle.
It is now possible to make a direct comparison between
the loading on the different structural shapes with the same
amount of green water on the deck. In Table 4 the peak
pressure and total for the same condition as the base case
design
and
the
100-year wave. Both the peak local pressure (p) and the
horizontal FX-load are compared. The pressure (p) is for a
circular force panel and presented as integrated pressure over
the total area of the panel 1.43 m2.
The loads on these structures are all significantly lower
than on the squared structure. These differences are a result of
the fact that the momentum in the x-direction is not deflected
completely (and over a longer period of time) for these
structures.
Table 4: Overview of green water loads on
different structural shapes
Structure
Squared structure
30 degrees with vertical
Triangular 45 degrees
Triangular 60 degrees
Cylindrical
Triangular support
p in kPa
241
133
63
113
192
162
FX in kN
7234
4465
2907
4407
3762
6218
PROTECT CRITICAL EQUIPMENT, USING BREAKWATERS
If it is not necessary to keep an installation on the deck
completely dry, it is possible to use a breakwater (water
deflector) in front of it. Such a structure can also be used as a
first barrier for a lighter protective structure or a superstructure.
For the design of this type of breakwaters, it is important
to keep their purpose in mind: breaking or deflecting the green
water that flows with a certain height at high velocity over the
deck, to minimise the impact on the critical structure. This
results in the following requirements:
- The breakwater should be higher than the water height on
the deck.
- The breakwater has to deflect or break the green water
flow efficiently, so that the amount of water that finally
reaches the critical structure will be minimised in amount
and/or velocity.
- The breakwater and the underlying support structure
should be strong enough to deal with the dynamic load due
to the water impact on the breakwater itself.
Photo 1: Tilted structure and triangular structure
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Copyright © 2003 by ASME
In Buchner [3] three different types of breakwaters were
evaluated:
- Traditional V-type breakwater.
- Vertical wall breakwater with its upper side tilting forward.
- Vane type breakwater.
The different breakwater shapes are shown in Photo 2 and
Figure 5. All breakwaters had a height of 4.98 m from the deck
level. The breakwaters were placed at a position of 16 m in front
of a squared structure, which was placed 30 m from the fore
perpendicular. The different breakwater shapes are shown in
Figure 5. The shaded areas in the figures indicate the parts at
which the local load is measured. Also the global load and
moment on the complete breakwater were measured.
In front of closed breakwaters, such as the traditional
V-type breakwater and vertical wall breakwater, the green water
tends to run-up vertically as soon as the water front hits the
breakwater. This fills the complete area in front of the
breakwater, which results in the rest of the water flowing over
the breakwater. This reduces the effective height of the
breakwater, see Photo 3.
To prevent this run-up as much as possible, the vane type
breakwater is a good alternative, see Beynet [8]. This open
breakwater with vertical vanes under an angle of 45 degrees,
deflects the water away from critical structures, but does not
completely block the fluid flow. This reduces the run-up in front
of the breakwater as well as the impact loading on the
breakwater itself.
Photo 2: The three breakwater types tested
Figure 5: Traditional V-type breakwater, vertical wall breakwater and vane type breakwater (from left to right)
7
Copyright © 2003 by ASME
•
To protect against the water height of 4.8 m on the deck, the
traditional breakwater needs to be 5.9 m high. The vane
type breakwater height can be limited to 5.3 m.
• The horizontal load on the traditional breakwater is
15938 kN (moment with respect to the deck level
53651 kNm), whereas the horizontal load on the vane type
breakwater is only 5938 kN (moment 17337 kNm).
It can be concluded that the vane-type breakwater is an
interesting concept, because it combines its protective features
with an open access to the deck and low loads on the structure
itself. The traditional breakwater has a larger horizontal load
than that on the structure to be protected itself, due to its larger
width. This load also needs to be supported by the underlying
deck structure.
Photo 3: Run-up of green water in front of the
traditional breakwater
Due to the run up problem, the breakwaters should be
higher than the water height expected at the breakwater location.
Based on the results presented in Buchner [3], a safety factor is
defined for the required breakwater height, see Table 5. The
water height on the deck has to be multiplied by this factor to
find the required breakwater height.
Table 5: Safety factors for the breakwater height
Breakwater
Traditional V
Vertical wall
Vane
Safety factor
1.21
1.17
1.10
USE OF A SLOPED DECK (‘CANOPY’)
Another interesting green water protection concept is the
sloped deck (or canopy), a principle already applied on
container vessels. It proved to be very effective during the Joint
Industry Project ‘F(P)SO Green Water Loading’, see Buchner
and van Ballegoijen [9].
With a sloped deck the green water flowing onto the deck is
decelerated by the sloped deck. The water also stays at a high
level at the moment that it is on the sloped deck. It does not
flow away over the bulwark, as is the case for a normal deck. As
a result of this, the flow of additional water onto the deck, which
is dependent on the water level difference on the deck and
outside the deck, is minimised. The result is a minimal thin
sheet of water on the sloped deck with hardly any impact on the
structure.
In Buchner and van Ballegoijen [9] comparisons were made
in regular waves using a full elliptical tanker bow with a
traditional flat deck and the sloped deck, see Figure 6. The slope
was only 10 degrees with the horizontal.
Table 5 confirms that the vane type breakwater is the most
effective in the prevention of run-up. The remaining water
height behind the breakwater and the loading on the structure
behind the breakwater (which is partially due to the flow from
the sides behind the breakwater) are almost equal for all three
breakwater types.
Another point of concern is the peak load on the breakwater
itself at the moment of impact. This load is important for the
design of the breakwater structure itself, but also on the
underlying support structure below the deck. The method to
determine these loads is described in detail in Buchner [3].
For the present case study two breakwater types were
studied: the traditional V-type breakwater and the vane type
breakwater. Evaluation of the two breakwater types results in
the following comparison:
Figure 6: Sloped deck (or canopy) on the bow deck
8
Copyright © 2003 by ASME
Photo 4: Comparison between the green water loads with flat deck (left) and with sloped deck (right)
Photo 4 shows a visual comparison between two stages in
the flow of the green water on the horizontal (above) and sloped
(below) deck.
Table 6 summarises the result of this comparison, showing
the water height and pressure on the deck, as well as the load on
a squared structure with its front 30 m from the fore
perpendicular.
Basic design
Sloped deck
-
(H) (p deck) (p)
(FX)
3.3 m 42 kPa 92 kPa 3080 kN
1.0 m 23 kPa 16 kPa 928 kN
Pressure
on squared
structure
Load on
squared
structure
(h)
9m
8.5 m
Pressure
on deck
Water
height
Design option
Freeboard
exceedance
Table 6: Comparison between the green water loads
with flat deck and with sloped deck
-
With similar freeboard exceedances (9.0 and 8.5 m), the
water height on the deck is reduced significantly (with a
factor 3).
- The loading on the structure on the deck is even more
reduced: the peak pressure with a factor of almost 6. The
total horizontal load is a factor 3 lower.
Although the amount of measurements with the sloped deck
is limited, in Figure 7 a comparison is made between the regular
wave test results with the sloped deck and the trends presented
in Buchner [3] for a horizontal deck.
Table 7 summarises the coefficients in Expressions (1)-(4)
for the sloped deck (using a least fit) and the horizontal deck.
Again significantly reduced heights and pressures are identified
from these results.
Table 7: Comparison between coefficients in the
Expressions (1)-(4) with the flat deck
and with sloped deck
Horizontal deck
The following can be concluded from the photos and table:
The relative wave motions (and freeboard exceedance) are
affected because the amount of green water on the deck of
the ship affects the ship motions and because the flow onto
the deck influences the relative wave motions around the
ship as well.
aH
0.56
0.10
ap
2.02
0.28
aF
9
Sloped deck
93.4
15.6
Copyright © 2003 by ASME
a.
Basic Design
For the basic design, as indicated earlier, it has been derived
a freeboard exceedance of 8.2 m, and a pressure on the front
bulkhead of 241 kPa. Although the structure can be designed for
this pressure, there are other considerations (safety of personnel,
impact on sensitive equipment, ...) that require remedial actions
to be implemented to take the amount of embarked water down
to acceptable levels. The deck and superstructure front need to
be reinforced with respect to Ruled design pressures (e.g.,
178 kPa at the front according to DnV Rules).
Horizontal load on structure (FX) in kN
Impact pressure (p) in kPa
Water height (H) in m
4
2
0
2
4
6
Exceedance of freeboard (h) in m
8
10
0
2
4
6
Exceedance of freeboard (h) in m
8
10
0
2
4
6
Exceedance of freeboard (h) in m
8
10
50
2000
1000
Figure 7: The relation between the freeboard
exceedance (h) and the water heights on deck
(above), local peak pressures (middle) and total
horizontal load (bottom) for the flat deck (dashed line)
and with sloped deck (solid line)
Finally it can be concluded that the canopy type deck can
be constructed much lighter than the normal deck structure: the
water heights and pressures on the deck are much smaller. Due
to the protecting canopy structure the underlying deck structure
can be constructed lighter as well.
DESIGN CONSIDERATIONS FOR DP FPSO
In this section, the different alternatives proposed to
mitigate the effect of the green water loading are assessed from
a design standpoint, with a special focus of the DP FPSO
design.
b. Freeboard Increase
As shown before, if the freeboard is increased from 10.5 m
to 17.5 m, this results in a freeboard exceedance of 2.8 m
(which is acceptable as it falls within the susceptibility level
considered ‘low’), and a pressure on the superstructure of
27.5 kPa. The resulting pressure does not require any additional
reinforcement to the forecastle deck or superstructure over that
required by Class Rules (about 42 kPa according to DnV Rules).
Although there is a significant impact on the vessel design
and fabrication (for the present case study the estimated weight
of the forecastle (7.5 m above main deck) is of approximately
500 tonnes, however, the following advantages can be highlighted:
- The amount of embarked water is very limited and
therefore it is not necessary to make additional
reinforcements or to further protect sensitive equipment.
The risk for personnel vanishes.
- There is a large space available in the forecastle that can be
used to accommodate equipment, stores, etc. If accounted
for at the conceptual stage of the Project, other spaces can
be relocated in this area.
- The forecastle can be integrated typically with the bow
structure (side shell, accommodation decks, transverse
frames, etc.). The scantlings of elements below the
forecastle deck can be reduced. Whenever the helideck is
located over the forecastle deck, it provides adequate
support and reduces the height of the supporting structure.
c.
Shape of Structure on Deck
The design of the front bulkhead of a exposed structure on
deck using one of the geometries outlined earlier (triangular,
cylindrical, tilted, ...), bear basically problems related to the
underdeck reinforcement and layout optimisation, especially for
superstructures. For protective structures of turrets or other
equipment, it may be a convenient solution.
The lack of alignment between the front bulkhead and the
deck structure deals to more complicated connections. Also, in
the case of an accommodation block, shapes different from the
square makes the layout of spaces more difficult to arrange and
less rational.
Additionally, if this alternative is the only applied, the risk
for personnel and sensitive equipment is still unacceptable.
d. Use of Breakwaters
From the comparison between the traditional and the vane
breakwaters, the following favourable conclusions can be drawn
for the later:
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Copyright © 2003 by ASME
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It is more efficient and the loads induced lower (as
mentioned earlier). This leads to a shorter and lighter
structure.
- It is easier to support on the deck as it can be properly
aligned with existing transverse girders. Thus, necessary
reinforcement is reduced compared to the traditional
breakwater.
- It provides an easier arrangement on deck and the
interference with other equipment is less than that in a
traditional breakwater.
Besides the above advantages of the vane type breakwater,
this alternative needs to be carefully evaluated, as there is still a
significant water height on deck, which may impair the safety of
personnel or other sensitive equipment. The dimensions of the
required vane type breakwater are in any case quite large (say
30 m wide and 5.3 m high).
The estimated weight of the traditional breakwater and
underdeck reinforcement is of 50.0 tonnes, whereas for the vane
type breakwater is if about 60.0 tonnes. Although the height of
the vane type breakwater and the applied loads are lesser than in
the traditional breakwater, the special configuration of the
former yields a somewhat higher steel weight.
e.
Use of a Sloped Deck (‘Canopy’)
The sloped is a smart solution to minimise green water
loading on equipment and superstructures, as discussed earlier.
Its efficiency (for a slope of 10 degrees) can be considered
equivalent to that of raising the forecastle deck 7.0 m. As can be
observed, the resulting impact pressures on the front bulkhead
are quite similar. Nevertheless, there are several drawbacks
(related to the sloped configuration of the deck) in this
alternative for application to FPSOs that should be mentioned:
- From a structural point of view, the connection area
(‘wedge’ type) of the main deck to the ‘canopy’ deck is
complicated because of the small angle between both decks
(10 degrees).
- Mooring and other deck equipment is more difficult to
arrange in this option than in a conventional horizontal
deck. Operating and safety aspects are better resolved in the
later.
- Support for the helideck supporting structure is more
difficult in this alternative and the height of the supporting
pillars significantly higher than in the ‘forecastle’ option.
- The space below the ‘canopy’ deck can be hardly used.
- Even though the steel weight of this alternative is much
lesser than that of a 7.0 m forecastle (estimated weight
180.0 tonnes), the higher production rate (hours/tonne) and
reduced efficiency associated to the items outlined above,
lead to similar ‘as-built’ costs for both alternatives.
- It is more complicated to integrate the sloped deck with the
vessel bow than the forecastle. In both alternatives, the
scantlings of structural elements below the deck can be
reduced.
Thus, for FPSO applications, it looks that to raise the
forecastle deck up to an adequate height that reduces the
freeboard exceedance down to a ‘low’ susceptibility level, is the
most overall suitable option. This is confirmed by the fact that it
is the alternative most frequently used in FPSOs. However, for
applications different from the case study investigated in this
paper, there may be other alternatives more convenient.
CONCLUSIONS
In the present paper the practical design considerations
related to the green water problem are discussed. The following
can be concluded:
• Green water is a significant problem for floating offshore
structures, which needs to be assessed in the early design of
the structure.
• The developed semi-empirical design evaluation procedure
can assist in this process.
• There are different ways to solve the green water problem
for a particular structure at a specific location. They all
have their advantages and disadvantages. It is the authors’
opinion that those solutions focussed on minimising the
freeboard exceedance (increase freeboard or sloped deck)
should be used as the primary choice. The other options are
more suitable for protecting personnel or sensitive
equipment, and to minimise green water loading which, in
turn, can not be completely avoided.
REFERENCES
[1] Morris, W.D.M., Millar, J. and Buchner, B., 2000, “Green
Water Susceptibility of North Sea FPSO/FSUs,” 15th
Conference on Floating Production Systems (FPS),
London.
[2] Ersdal, G. and Kvitrud, A., 2000, “Green water on
Norwegian Production Ships,” ISOPE2000, Seattle.
[3] Buchner, B., 2002, “Green Water on Ship-type Offshore
Structures,” PhD-thesis Delft University of Technology,
2002.
[4] Buchner, B., Voogt, A.J., Duggal, A.S. and Heyl, C.N.,
2002, “Green Water Evaluation for FPSOs in the GoM,”
OTC, Houston.
[5] Cortijo, J.L, Lago, F. and Mendez, A., 2002/2003, “New
FPSO-Based Concepts for the Deep Water Challenge,” 17th
Annual Conference on Floating Production Systems
(December 2002, London) and Petrotech 2003 Conference
(January 2003, New Delhi).
[6] Buchner, B., 1995, “The Impact of Green Water on FPSO
Design,” OTC paper 7698, OTC 1995, Houston.
[7] Hellan, Ø., Hermunstad, O.A. and Stansberg, C.T., 2001,
“Design Tool for Green Sea, Wave Impact, and Structural
Response on Bow and Deck Structures,” OTC paper 13213,
OTC 2001, Houston.
[8] Beynet, P., 1994, “Personal communication on vane type
breakwater.”
[9] Buchner, B. and Ballegoijen, G. van, 1997, Joint Industry
Project: “F(P)SO Green Water Loading,” A, B and C,
MARIN report No. 13644-1-ZT, Wageningen.
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Copyright © 2003 by ASME