same vessel. Relevant modifications could imply the updating of the input
data of the following category.
4. Hydrostatic Data
These data could change, in principle, each time the mooring or riser system
is redefined. For the thesis design procedures the following data will be
assumed:
Full load Ballast
„ Draught (T) 22.4 m 8.49 m
„ Longitudinal metacentric height 385.5 m 828.0 m
„ Transverse metacentric height 7.92 m 17.59 m
„ Longitudinal location of the centre of 166.77 m 179.22 m
floatation (from AP)
„ Waterplane area 16881 m2 15395 m2
The reported values do not include the vertical component of the loading
from mooring lines and risers, which should be added on the basis of relevant
design. The load contribution from mooring lines and risers can generally be
neglected in the preliminary analyses. Therefore above data can be kept
constant.
5. Offloading System
No Design Situation relevant to an Offloading Condition is defined for the
system performance assessment (see section 7.3). The definition of the
offloading system can, hence, be disregarded.
6. Allowable Loads from the Mooring System
A conservative estimate of the maximum allowable vertical load from the
mooring system has been determined [45] as follows:
T
all
= 37.2 MN
A possible exceedence of the above limit during the analyses should be
considered just a warning relevant to a possible insufficient hull strength that,
hence, could need a more direct structural analysis. It is worth noting, in this
concern, that the hull structure is likely to allow an applied load 50% higher
than the value above defined.
7. Service Life
A service life of 15 years is to be ensured for the mooring system.
4. Riser System Description
1. General
The design of the riser system is outside the scope of the work, but some
information are necessary for the mooring system design. These aspects can
be considered as design data for the riser system. The design type ("free
hanging" or "lazy wave") will be defined on the basis of site and production
requirements; it is not considered here as a design datum. The following data
are derived from Tecnomare documentation.
2. Locations of subsea completions
Two subsea completions are to be considered as riser starting points at the
positions defined in Figure 17.
3. System composition
In addition to production risers, a number of service lines run from seabed to
the vessel turret. Generally they serve for water or gas injection, gas lift,
control umbilicals, etc.. Two production risers are to be considered for each
subsea manifold. Very near to each pair of production risers, a gas injection
flowline is envisaged to run toward terminations close to the subsea
production manifolds. In addition, two water injection flowline are to be
considered. Their terminations are rather far from the subsea manifolds,
however, at least in their hanging part, they run close to the production risers.
Finally, three umbilicals depart from the turret: two running aside of the
water injection flowlines and one with the gas injection flowline directed to
Abo North. Entanglements with production risers and mooring lines are to be
avoided and interfaces with the risers itself are to be provided (if necessary).
4. Internal diameters
The minimum internal diameters of the risers are defined in the light of production
requirements. The minimum internal diameter of all the production flowlines is 8".
5. Mooring design
1. Introduction
The present section reports the results of the preliminary design of the
mooring system for the Abo FPSO obtained using the OptMoor procedure
with the goal being to achieve a global mooring system material cost saving.
As already explained in chapter 6, the optimisation process concerns only the
mooring line configuration with the exception of the structures and fittings
such as turret, fairleads, etc. For this reason the results are to be read with
particular care. Also the buoys, where necessary, may be introduced in the
line make-up but they are not included in the optimisation process.
2. Design Data Summary
The geographical data of the oil field, the corresponding environmental
conditions and the main characteristics of the vessel are reported in section
9.2 and 9.3. Proper diffraction calculations have been executed in order to
calculate the hydrodynamic coefficients of the vessel; these values have been
stored in the OptMoor program data file. In addition to the above
information, specific data, necessary to perform the optimisation procedure
are:
Max offset in intact condition = 60 m (12 % of water depth)
Max offset in damaged condition = 75 m (15 % of water depth)
Line angle at anchor point in damaged condition = 0 deg
Max line stretch = 0.005%
Usage factor in intact condition = 43 %
Usage factor in damaged condition = 67 %
Drag coefficient for mooring ropes = 1.2
Drag coefficient for chains = 1.2
In the solutions with chain in the middle of the mooring line, an additional
constraint has been imposed in order to avoid the top rope touching the sea
bottom. It is not possible to impose a heave motion constraint since the
envisaged mooring typologies do not allow its value to be limited.
1. Design data comments
In general terms the maximum vessel offset is to be defined in order to allow
safe operations of the riser system. The actual requirement on the maximum
offset depends on the specific riser design, which in a pre-dimensioning
phase is not yet available. In general, for preliminary mooring system design,
a reference value for maximum offset is in the order of 10 ψ 15 % of the water
depth. Definite requirements on this parameter will come out from a global
assessment of both the riser and mooring systems to be carried out
contemporaneously in a further design phase.
The usage factor values are in agreement with the Design Criteria. The value
of 1.2 for the rope drag coefficient is an average value, it takes into account
the external surface condition and the rope diameter. It is slightly different
from the one specified in the Design Technical Specifications. However, this
parameter is not fundamental to the preliminary design of a mooring system.
Also a drag coefficient of 1.2 for the chain is considered an average value. It
takes into account the shape, the orientation of the chain links and the overall
dimension. For the evaluation of the drag force, the equivalent diameter of
the chain corresponds to the double of the actual diameter. For the chain, the
drag coefficient is also slightly different from the one specified in the Design
Technical Specifications.
3. Mooring configurations selected for the preliminary design
As already widely explained, the optimisation procedure only considers the
geometrical characteristics of the single line, namely the diameter and length
of each line segment and the line pretension. Other mooring parameters, like
the line composition, presence of buoy and clump weight, total number of
lines and the mooring pattern, are not changed within the process.
Consequently a complete optimisation design should be done by performing
several optimisation processes corresponding to different selection of these
parameters. For this case, different typologies of the mooring system have
been analysed, i.e.:
ξ Bow-mounted turret mooring system with conventional catenary lines
equally spaced.
ξ As above, with the addition of intermediate buoys.
ξ Spread mooring system.
ξ Spread mooring system with buoys.
For each of the above configuration, the line make-up has been optimised in
terms of weight and the pretension as well as the sizing of each segment
required at this scope were found out. In order to have a more comprehensive
view of the mooring optimisation problem, the selected mooring typologies
were studied by also varying the total number of lines, in order to find the
number of lines minimising the global mass of the mooring system.
Therefore, each mooring typology has generated a number of solutions
characterised by a different number of lines and submitted to the optimisation
procedure. It is to be noted that the line composition in terms of segment
typology, given as input to the OptMoor and not changed by the procedure
itself, has been selected among those normally used in the offshore mooring
applications. In the following table, the mooring solutions studied and
representing the optimum as concerns the number of lines are shown.
Solution
number
Number
of lines
Typology Pattern Line composition
1 6 Turret system Symmetrical Chain + rope + chain
2 6 Turret system Symmetrical Chain+rope+buoy+rope+chain
3 8 Spread system 30° - 60° Chain + rope + chain
4 8 Spread system 30° - 60° Chain+rope+buoy+rope+chain
Table 23 - Mooring solutions studied
4. Mooring optimisation results
For each of the four configurations the OptMoor process has been applied
and the corresponding results are reported in the following: Table 24 and
Table 25 show the optimum overall geometric characteristics and mass, while
Table 26 and Table 27 report the main performances of the mooring systems.
Line composition
Resting
Anchor => => => Vessel
Pretension
1
Chain ORQ
Ι = 50 mm
L = 730 m
Six strand
Ι = 53.8 mm
L = 834 m
Chain ORQ
Ι = 50 mm
L = 25 m
201. kN
2
Chain ORQ
Ι = 50 mm
L = 718 m
Six strand
Ι = 55.5 mm
L = 585 m
Buoy
N.B.= 64 kN
W= 2.2 t
Six strand
Ι = 54.9 mm
L = 250 m
Chain ORQ
Ι = 50 mm
L = 25 m
92. kN
3
Chain ORQ
Ι = 63.7 mm
L = 955 m
Six strand
Ι = 83.7 mm
L = 681 m
Chain ORQ
Ι = 56.2 mm
L = 25 m
323. kN
4
Chain ORQ
Ι = 72.3 mm
L = 750 m
Six strand
Ι = 71.6 mm
L = 538 m
Buoy
N.B.=106kN
W= 3.6 t
Six strand
Ι = 84.6 mm
L = 250 m
Chain ORQ
Ι = 57.5 mm
L = 25 m
142. kN
Table 24 - Line composition and related horizontal pretension of the mooring
solutions
Solution
number
Total line
length [m]
Total line
mass [Mg]
Total mooring
mass [Mg]
1 1589 50.5 304.
2 1578 50.5
(*)
303.
(*)
3 1661 104.5 836.
4 1563 104.9
(*)
840.
(*)
(*) Buoys are not included
Table 25 - Mooring solutions mass
For each solution the anchor loads, the maximum offset and the maximum
usage factors are listed for both intact and damaged conditions. Both co-
linear and non-co-linear environmental conditions (see section 3.8) were
analysed and, for each mooring solution, the worst situation is reported in the
tables.
Solution
number
Horizontal
anchor loading
[kN]
Vertical anchor
loading [kN]
Maximum
offset [m]
Maximum
usage factor
1 699. 0. 40.8 43.0%
2 751. 0. 60.0 43.0%
3 807. 0. 40.9 41.7%
4 743. 0. 46.3 31.1%
Table 26 - Mooring performance for "intact condition"
Solution
number
Horizontal
anchor loading
[kN]
Vertical anchor
loading [kN]
Maximum
offset [m]
Maximum
usage factor
1 865. 0. 48.9 52.4%
2 817. 0. 68.2 46.6%
3 1705. 0. 75.0 61.8%
4 1678. 0. 75.0 67.0%
Table 27 - Mooring performance for "damaged condition"
The optimisation concerning the variation of the number of lines, whose
results have led to the above optimum solutions for the selected four
typology of mooring configurations, shows that there is a number of lines
minimising the mass of the global mooring system. In the following table, the
trend of global mass variation with respect to the number of lines for the
considered mooring typologies is shown.
Mooring
Typology
Number of
lines
Mass of line
(Mg)
Global Mass
(Mg)
5 66.7 334
6 50.5 303
7 48.9 342
1
8 44.8 359
5 62.6 313
6 50.5 303
7 46.4 325
2
8 41.8 334
8 104.5 836.3 3
12 67.0 804.0
4 8 105.0 840.0
Table 28 - Comparison between different mooring solutions
The spread system 30°-60° pattern (solutions number 3 and 4) with 8 lines is
illustrated in Figure 18 while the turret mooring system, with 6 lines arranged
in a symmetrical pattern (lines equally spaced by a 60° angle), is represented
on Figure 19.
Figure 18 - Spread mooring system 30°-60° pattern
Figure 19 - Turret mooring system. Symmetrical pattern (lines equally spaced
60°)
The layout of the line of solution 1 and 3 are represented on Figure 20 and
Figure 21 respectively. The lines are represented in the neutral configuration,
corresponding to the non-co-linear environmental condition. The
dimensioning configuration is different for the two mooring systems: the
intact condition for the turret mooring system and the damaged condition for
the spread one.
Figure 20 - Line configuration for solution number 1. Non-co-linear
environment.
Intact condition
Figure 21 - Line configuration for solution number 3. Non-co-linear
environment. Damaged condition
5. Conclusions
The mooring optimisation for the case study of Abo field gives the following main results:
ξ Among the two mooring systems analysed, which are the turret and the spread systems, it
is noticed that the most effective in terms of mass reduction and, hence, reduction of costs
with respect to mooring components, is the turret system. In fact, the global mass of the
mooring lines when the turret system is adopted is on the order of one-third of the spread
system (300 t for the turret compared to more than 800 t for the spread).
ξ On the basis of the environmental data of the ABO field and the imposed constraints, such
as the maximum offset and others, the adoption of buoys does not imply cost advantage. In
fact, from Table 25 it comes out that the total mass of the mooring lines with or without the
adoption of intermediate buoys is practically the same. This happens for both turret and
spread moorings. Hence, no cost reduction due to line sizing can be envisaged, while
expensive items such as buoys, will lead to an increase in the total cost of the mooring
system.
ξ The optimisation has also shown that there is a number of lines minimising the global
mass. This come out to be 6 for turret system, while for spread mooring system no
significant reduction of the global mass is obtained by increasing the line number from 8 to
12.
ξ On the basis of the results previously reported, the optimum solution for the ABO field is a
turret system with 6 lines equally spaced and having the following main characteristics.
Line composition
Anchor => Vessel
Chain ORQ
Ι = 50 mm
L = 730 m
Six strand
Ι = 54 mm
L = 834 m
Chain ORQ
Ι = 50 mm
L = 25 m
Table 29 - ABO field optimum solution
ξ In conclusion, it is to be remarked that the above results should not be the only thing
driving the final selection of the mooring system. This selection should be done taking into
account other aspects not considered in the present study, such as the cost of the turret
structure and relevant mechanical equipment, the cost of other mooring fittings even more
expensive, the installation costs, the operative costs related to the different methods of
offloading, etc.
4. CONCLUSIONS
The presented strategy for automatic optimisation of mooring system design is feasible and
appears to be a promising way of improving the anchor system design process. The potential
and performance of the optimisation process depends to a large extent on the capability of
the mooring analysis program, in fact it is necessary to include both low frequency and wave
frequency dynamics. In some problems the diameter and length variables may interact so
that the iso-cost surface becomes nearly parallel with the safety factor constraint surface. In
such cases it may be useful to fix one of these variables in order to obtain both a well-
defined optimum points and to improve the convergence velocity towards these points. The
ability of the search algorithm to find feasible solutions from unfeasible starting points is a
useful feature that can also eliminate a lot of trial and error efforts in the design process.
However, the procedure is not fail-safe and due to the Iterative Optimisation Procedure (see
section 6.7), the search may not converge to the best solution. In fact, the main drawback of
the IOP is that the optimisation algorithm may return a line geometry that causes the static
or dynamic analysis to fail. This requires an experienced user, who knows both the
fundamentals and main limitations of the moored floating body analysis program. To
improve the capability of the analysis program one should also incorporate automatic
calculation and updating of low frequency damping parameters affected by the mooring line
modification. In addition, frequency domain analysis can still be improved with respect to
the modelling of nonlinearities, three dimensional static and dynamic mooring line analysis,
etc. Improved performance modelling, supplementing the present line cost model with other
cost items representing buoyancy modules, clump weights, anchors, installation and
maintenance costs, will also contribute to making the automatic optimisation programs a
more interesting tools for the mooring system designer.
Ricevi informazioni sui nostri servizi, sulle offerte e non perdere news e consigli su università e lavoro.
