Optimum breakwater safety levels based on life-cycle cost optimization
Burcharth, Hans Falk; Sørensen, John Dalsgaard; Kim, Seung-Woo
Publication date:
2016
Document Version
Publisher's PDF, also known as Version of record Link to publication from Aalborg University
Citation for published version (APA):
Burcharth, H. F., Sørensen, J. D., & Kim, S-W. (2016). Optimum breakwater safety levels based on life-cycle cost optimization. Department of Civil Engineering, Aalborg University. DCE Technical reports No. 204
General rights
Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.
- Users may download and print one copy of any publication from the public portal for the purpose of private study or research.
- You may not further distribute the material or use it for any profit-making activity or commercial gain - You may freely distribute the URL identifying the publication in the public portal -
Take down policy
If you believe that this document breaches copyright please contact us at vbn@aub.aau.dk providing details, and we will remove access to the work immediately and investigate your claim.
Downloaded from vbn.aau.dk on: July 15, 2022
1 Appendix A1 Background note containing assumptions and formulae applied in
optimizations analyses of rock and cube armoured rubble mound breakwaters 1. Objective
To identify the optimum cost safety levels for rubble mound breakwater armored by rock and Cubes in shallow, moderate and deep water.
The study comprises the influence of the following parameters on the minimum cost safety level:
- Real interest rate
- Service lifetime of the breakwater
- Downtime costs due to malfunction of the breakwaters - Repair policy
- Damage accumulation
2. Procedure in numerical simulations for identification of minimum cost safety levels 1) Select type of breakwater
2) Design geometries of the structure corresponding to the chosen H
sTvalue (determi nistic design is sufficient)
3) For each structure geometry calculate construction costs 4) Define repair policy and related cost of repair
5) Define down-time costs related to damage levels 6) Define a model for accumulation of damage
7) For each structure geometry use stochastic models for wave climate and structure re sponse (damage) in Monte Carlo simulation of occurrence of damages within servic e life time (uncertainties included)
8) Calculate for each structure geometry the total capitalized costs for each simulation.
Calculate the mean value and the related safety levels corresponding to defined desi gn limit states
9) Identify the structure safety level corresponding to the minimum total costs
2 Shallow water cross section: h 1 . 5 H
s 2 . 7 D
nDeep water cross section: h 1 . 5 H
s 2 . 7 D
nFig. 1. Shallow and deep water cross sections
Volume per meter for shallow water conditions
] ) 3
( ) 3 ( [ 2 / 1 ] 4 1
2 5
. 0
[
n n 12 n 2 c n 2 c n n 2n
armour
a D a D D n D n R D R D aD
V
] 3 1
) 3
( 4 1
) 2 3 5 . 0
[(
12 22 21
b D b a D n D R D aD n D n
V
filter
n
n
n
c
n
n
n) 1 ( ) ) 3 ( (
2
2 2 221
2
V c D c D h b D n
b
V
filter c
filter
n
n
n
n n
n
core
n n D h b c D a b n D
V 0 . 5 (
1
2)
2 4 ( ( 3 ) ) ( ) 1
22where R
c D ( a b c ) D
n, front slop 1 : n
1, back slop 1 : n
2, armour, first filter,
and second filter layers heights are a D
n, b D
n, c D
n, respectively. When D
n 1 . 75 m then
filter 2 is omitted, i.e. filter 2 is substituted by core material. The total volume of the core
is then, V
filter2 V
core3 ] ) 3
( ) 3 ( [ 2 / 1 ] 4 1
2 5
. 0
[
n n 1 n 2 c n c n nn
armour
a D a D D n D n R D R D aD
V
] 3 1
) 3
( 4 1
) 2 3 5 . 0
[(
12 22 21
b D b a D n D R D aD n D n
V
filter
n
n
n
c
n
n
nn n s
shallow filter
filter
V h H b c D c D
V
2
2 1 . 5 [ 1 . 5 ( 1 . 7 ) ] l l n D
b V
V
core
coreshallow [ ( 12 . 4 )
n ( 1
1) 0 . 5 ]
where R
c 1 . 5 H
s, l h 1 . 5 H
s ( 1 . 7 b c ) D
nand h R
c ( a b c ) D
nin calculation of V
coreshallowFilter 1 (Quarry rock)
Mass
FM
AM
AM
AM 0 . 1
~ 7
1
15 Mass density,
s 2 . 65 t / m
33 / 1 1
50
s F n
D M
Filter 2 (Quarry rock)
Mass M
F2 0 . 1 M
F1 0 . 01 M
AMass density,
s 2 . 65 t / m
33 / 1 2
50
s F n
D M
Free board R
CR
Cis determined such that the transmitted wave height due to overtopping in a sea with return period equal to structure life time T
Lis 0.50 m.
Minimum R
Cis 1 . 50 m t
A t
F1 t
F2 1 . 50 m 1 . 963 H due to construction road on top of core.
Case 1, rock armour: D
n50 0.312 H
s, s
om=0.03, s
op=0.02
Wave transmission formula by van der Meer and d’Angremond(1991) for Rock armoured
Low-crested, submerged, and reef breakwaters
4 structures)
where 5 . 42 0 . 0323 0 . 0017 0 . 51
84 . 1
50 50
n n
s
op
D
B D
s H b
,
,
1.071 2.217 ( ) , min for 0.02 max
0.831 2.217 ( ), min for 0.04
L
L
T
s s t C op
C T
s s t C op
H H m R s
R
H H m R s
.
Case 2 and 3, cube armour: D
n50 0 . 28 H
s, s
om=0.025, s
op=0.02(~0.016) d’Angremond et al.(1996) suggested this formula for B / H
s,i 8
) 1
( 64
. 0 40
.
0
0.5031 . 0
, ,
e
opH B H
C R
i s i
s c t
( 0 . 075 C
t 0 . 8 )
where
5 . 0 tan , /
tan
s
op
0 . 432 , for 0 . 04
02 . 0 for
, 502 . 0 40
. 0
, ,
,
op op i
s c i
s t s
t
s
s H
R H
C H
Therefore,
, ,
1.26 2.50 ( ) , min for 0.02
max
1.08 2.50 ( ), min for 0.04
L
L
T
s s t C op
C T
s s t C op
H H m R s
R
H H m R s
Note that the freeboards R
cin all Cases are determined by the set minimum level of +1.5 m for the top of the core material to be used as construction road.
Limit state and repair policy
Repairs are assumed to take place immediately after the damage limit for repair is exceed.
5 Damage levels S (rock) N
od(cubes) Estimated D Repair policy
Initial 2 0 2 % No repair
Serviceability (minor damage,
only to armor)
5 0.8 5 % Repair armor
Repairable (major damage, armor + filter 1)
8 2.0 15 %
Repair armor + filter 1 Ultimate
(failure)
13 3.0 30 %
Repair armor + filter 1 and 2 * D is the relative number of displaced units (US Army, 2006)
Linear regression is applied to evaluate the damage levels between serviceability damage level and ultimate damage level. These equations are the relationship between S or N
odand D .
) 999 . 0 ( 1031 . 0 0311 .
0
S R
D
) 99 . 0 ( 0511 . 0 1126
.
0
0
N R
D
d4 6 8 10 12 14
S (damage) 0
0.1 0.2 0.3 0.4 0.5
Estimated D
Each damage level
D = 0.0311 S - 0.1031 (R = 0.999)
6
0.5 1 1.5 2 2.5 3
N0d (damage) 0
0.1 0.2 0.3 0.4
Estimated D
Each damage level
D = 0.1126 Nod - 0.0511 (R = 0.99)
Fig. 3. The relationship between D and N
odfor cubes Costs of repair
D = 5 %
Cost of repair of minor damage, C
RI ( 1 K ) D C
I,armorR ,
in which C
I,armoris the initial construction cost of the main armor layer, R 3 . 0 is a factor signifying high cost of repair, and K 0 . 3 is a factor signifying mobilization costs. The chosen values of R and K are estimates, but can vary considerably from case to case.
D = 15 %
Cost of repair of major damage, C
R2 ( C
I,armor C
I,filter1 K C
I,armor) D R , where C
I,filter1is the initial construction cost of filter 1.
D = 30 %
Cost of repair after a failure, C
R3 ( C
I,armor C
I,filter1 C
I,filter2 K C
I,armor) D R , where
2 ,filter
C
Iis the initial construction cost of filter 2.
Downtime costs
When D 15 % is added downtime costs given as 200,000 EUR/day in 3 months. The
relative short duration of 3 months is justified only for outer breakwaters with no berths
7 Structure length
Calculations performed for a structure length of 1 km and damage is assumed to take place over the whole length of the breakwater.
Stability formulae
Rock armour (plunging wave,
m
mc, P 0 . 4 , N
z 1000 , tan 0 . 5 )
6 . 2
0.2 0.18 0.1 0.25tan
0.5
z om
n s
s
S P N s
D
N H Van der Meer (1988a)
Cube armour
0.3 0.1
4 . 0
0 . 1 7
.
6
omz od n
s
s
s
N N D
N H Van der Meer (1988b)
This formula is valid for the slope of structure 1:1.5 so the formula has been modified by Hudson equation analogy to cover the slope of structure 1:2.
0.3 0.1
4 . 3 0
/ 1
0 . 1 7
. 5 6
. 1
2
omz od n
s
s
s
N N D
N H
Damage accumulation model
The damage was accumulated until to be damaged the serviceability, repair, or ultimate
limit state. If the damage was occurred, the structures were repaired immediately. We
performed the two cases which are with and without considering damage accumulation
model. Regardless of the damage accumulation, the number of waves in one storm was
generated in 1000 waves. The damage that is less than a damage of serviceability limit
state is neglected in case of no damage accumulation. After that, we can only explain the
damage accumulation model. There are several damage accumulation models. Now, the
modified Melby and Kobayashi’s (1998) model was decided in this calculation.
8
50 n
om
This equation can be used to calculate the damage level S due to the incident waves with constant H
sstarting from S 0 at t 0 ( N
z 0 ). To calculate the cumulative damage level in real situations of H
sand N
z, the damage level S
iwas expressed as
,0.51
5 . 0 , 5
50 5 . 25 0
. 18 0 . 0
,
1
6 . 2 tan
zi zin om
i s i
i
N N
D s
P S H
S
where S
i1= known damage level at
, 1
z iz
N
N . We assumed that each storm was generated in the N
z 1000 .
For the Cubes, the relative damage level N
od,ican expressed as
,0.751
75 . 0 , 5 . 2 1
. 0 0 3
/ 1
, 0 ,
0
6 . 7
0 . 2 1
5 . 1
1
z i z im n s
d
d
N N
D s H N
N
i
i i
4. Formulation of total cost functions
The optimum design is determined using the optimization problem formulated assuming no rebuilding in case of failure. No benefits, costs related to loss of life and cost of
decommissioning at the end of service lifetime are included.
TLt
F t F
R R
R R
T
C T C
IT C T P t C T P t C T P t r
1
( 1 )
) 1 ( ) ( ) ( ) ( ) ( ) ( )
( ) (
min
1 1 2 2where
T return period used for deterministic design T
Ldesign life time
) (T
C
Iinitial costs (building costs) )
1
( T
C
Rcost of repair for minor damage
9 )
2
( T
C
Rcost of repair for major damage )
2
( T
P
Rprobability of major damage in year t )
( T
C
Fcost of failure including downtime costs )
(T
P
Fprobability of failure in year t r real rate of interest
5. Characteristics of design variables in stochastic model Rock armour, slope 1:2
The Van der Meer formula (1988a) is used. The limit state equation is written:
5 . 0 5
50 5 . 25 0
. 18 0 .
0
tan
2 .
6
om n zs
H
N
D s
P Z
H S X
g
s
where the parameters are describes in Table 2.
Cubes, slope 1:2
The van der Meer formula is used, but modified to slope 1:2. The limit state equation is written:
75 . 0 5 . 2 1
. 0 0 3
/ 1
0
6 . 7
0 . 2 1
5 . 1 1
z m
n s H
d
N
D s H X N Z
g
s
where the parameters are describes in Table 3.
10
Variables Description Distribution
value deviation
S critical damage level see Table 1
H
sannual maximum significant wave
height Weibull Various
Hs
X model uncertainty wave height Normal 1 0.1
Z model uncertainty Normal 1 0.0645
model parameter Normal 1.57 0.06
N
zNumber of waves in one storm 1000
s
omwave steepness Normal 0.030 0.006
D
narmor size Normal 0 . 35 H
sTCOV=0.05
T
H
sdesign wave height with return period T years
armor density 2.65 ton / m
311
Variables Description Distribution
value deviation
N
odcritical damage level See Table 1
H
sannual maximum significant wave
height Weibull Various
Hs
X model uncertainty wave height Normal 1 0.1
Z model uncertainty Normal 1 0.1
model parameter Normal 1.33 0.03
N
zNumber of waves in one storm 1000
s
omwave steepness Normal 0.025 0.005
D
narmor size Normal 0 . 28 H
sTCOV=0.01
T
H
sdesign wave height with return period T years
armor density 2.40 ton / m
36. Case studies Table 4. Case study data
Case Water depth
Armor density
Wave climate
Stability formula
Built-in unit prices
core/filter 2/filter
1/armor in EURO/ m
31 10 m 2.65 t / m
3Follonica van der Meer (1988a) 10/ 16/ 20/ 40
2 15 m 2.40 t / m
3Follonica van der Meer (1988b) 10/ 16/ 20/ 40
3 30 m 2.40 t / m
3Sines van der Meer (1988b) 5/ 10/ 25/ 35
12 h
55 .
0 .
Table 5. Distribution parameters for H
s- data samples (PIANC, 1992)
Site
Total number Average
number per year Weibull Exp.
N H
s'Follonica 46 5.94 1.14 0.58 2.69
Sines 15 1.25 1.78 2.53 7.10
Weibull distributed annual maximum wave height
'
exp 1 )
(
sH
sH
sH F
T -year maximum wave height
T s
s T
s T
s
H H H
F H
F
'
exp 1 ) ( )]
( [
Deterministic design Rock armour
57 . 1 , 2 cot , 1000 ,
4 .
0
N
z
P ,
) (
4 84
. 5
5
25 . 0 50
c m m om
n s
s D
S H
25 . 0 25
. 2 0
. 0 50
706 . 7 84
. 5
4
om omn
s
s s
D
H
50
0.312
n s
D H ( s
om 0.03 )
13 relationship between the significant wave height and diameter of rock armour is not
changed due to the conservative deterministic design.
0.312
T( 0.030)
n s om
D H s
where T is the return period of wave height. The return period is used from 5 years to 1000 years (i.e. 5, 10, 25, 50, 100, 200, 400, 500, 1000) in the cost optimization.
Cube armour
33 . 1 , 2 cot ,
1000
N
z,
8 . 7 0
. 6
0 . 2 1
5 . 1
75 . 0 5 . 2 1
. 0 0 3
/ 1
0
zm n s
d
N
D s H N
0.4
0.1 0.1
0.3
1.33 1.1(6.7 0.8 1) 2.592 1000
s
om om
n
H s s
D
) 025 . 0 ( 27 .
0
s omn
H s
D
Mass density of sea water and concrete armour unis ranges from 1.03 to 1.025 and from 2.3 to 2.4 respectively. Therefore, in this calculation of cost optimization for cubes armour unit, the relationship between the significant wave height and diameter of cubes is expressed as
) 025 . 0 ( 28 .
0
sT omn