Mooring System Optimisation for Floating Production Systems

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.