Structural strain responses of the jacket-type Uldolmok tidal current power plant structure under severe\ud
tidal environments were analyzed using long-term measurement data from construction to normal operation. From the\ud
measured data during construction, it was found that there were significant changes in strain responses at the steps of\ud
jacket lifting, weight-block loading, pile ejection and insertion. Strains due to permanent and tidal current loads were\ud
analyzed during removal work on one among six jacket legs, and it was found that the strains due to permanent load\ud
were much significantly changed after removal of on jacket leg. From the measurement data during normal operation, it\ud
was observed that strain responses were obviously fluctuated with M2 and M4 tidal periods and also with relatively\ud
short period of about 11 min due to the peculiar tidal characteristics in the Uldolmok strait

Lee, K.S.

Park, J.S.

Yi, J.H.

Hasanuddin University Repository

 886 
 
 
Proceedings of the 7th International Conference on Asian and Pacific Coasts  
(APAC 2013) Bali, Indonesia, September 24-26, 2013 
 
 
 
LONG-TERM STRAIN MONITORING DATA OF JACKET-TYPE OFFSHORE 
STRUCTURE FOR TIDAL CURRENT POWER GENERATION UNDER SEVERE TIDAL 
CURRENT ENVIRONMENTS 
 
J-H. Yi1,  J-S. Park1,  J. S. Park1  and  K-S. Lee
1
 
 
 
ABSTRACT: Structural strain responses of the jacket-type Uldolmok tidal current power plant structure under severe 
tidal environments were analyzed using long-term measurement data from construction to normal operation. From the 
measured data during construction, it was found that there were significant changes in strain responses at the steps of 
jacket lifting, weight-block loading, pile ejection and insertion. Strains due to permanent and tidal current loads were 
analyzed during removal work on one among six jacket legs, and it was found that the strains due to permanent load 
were much significantly changed after removal of on jacket leg. From the measurement data during normal operation, it 
was observed that strain responses were obviously fluctuated with M2 and M4 tidal periods and also with relatively 
short period of about 11 min due to the peculiar tidal characteristics in the Uldolmok strait. 
 
Keywords: Tidal current power plant jacket structure, strain long-term measurement, measurements in construction and 
operation, tidal effects. 
 
 
                                                 
1 Coastal Development and Ocean Energy Research Division, Korea Institute of Ocean Science and Technology, Sa-2-Dong, Ansan, 
Gyeonggi-Do, 426-744, SOUTH KOREA 
INTRODUCTION 
The Korean southwest coastal area has long been 
considered as a highly attractive candidate for power 
generation from tidal currents. Among the possible sites 
in the southwest coastal area, the Uldolmok strait is 
known as the most promising site owing to the 
distinctive tidal currents with very high speeds up to 4-5 
m/sec. Recently, the Uldolmok tidal current power plant 
(TCPP) was built as a pilot plant to promote research and 
development investment and speed up the 
commercialization of TCPPs (KORDI, 2011). Uldolmok 
TCPP is located between Jindo Grand Bridge and 
Byeokpa Port in the east-west direction and also between 
the towns of Jindo and Haenam in the north-south 
direction as shown in Fig. 1. Water depth at the jacket 
structure is 19 m below approximate lowest low water 
and total platform height and submerged platform height 
in water are 25.0m and 23.5m, respectively. Uldolmok 
TCPP was designed, fabricated, constructed, and 
operated according to the design guidelines for offshore 
steel jacket platform structures, in order to maintain the 
structure safely and economically under very high levels 
of tidal current loading. It is nevertheless very important 
to monitor structural responses to ensure structural 
integrity and also to establish load and response 
databases for further design of a commercialized TCPP 
farm (shown in Fig. 1 as commercialized TCPPs).  
In this study, strains of the Uldolmok TCPP under 
severe tidal environments were measured using long-
term measurement system and these data were analyzed. 
From the measured data during construction, it was 
found that there were significant changes in strain 
responses at the steps of jacket lifting, weight-block 
loading, pile ejection and insertion. Strains due to 
permanent and tidal current loads were analyzed during 
removal work on one among six jacket legs, and it was 
found that the strains due to permanent load were much 
significantly changed after removal of on jacket leg. 
From the measurement data during normal operation, it 
was observed that strain responses were obviously 
fluctuated with M2 and M4 tidal periods and also with 
relatively short period of about 11 min due to the 
peculiar tidal characteristics in the Uldolmok strait. 
 
 
Fig. 1. Location of Uldolmok TCPP 
 
 
 
 
Long-Term Strain Monitoring Data of Jacket-Type Offshore Structure for Tidal Current Power Generation  
 
887 
 
CONSTRUCTION AND MEASUREMENT 
 
Main Construction Phases 
Figure 2 shows the main construction stages for 
Uldolmok TCPP; (1) lifting both of the jacket and upper 
deck structures, (2) installing the jacket structure at the 
power plant site, (3) loading a weight block of 480 ton 
for increasing the stability against sliding and 
overturning under severe tidal current, (4) drilling seabed 
ground with a reverse circulation drilling (RCD) 
equipment, (5) inserting inner piles and pouring grout-
concrete for combining jacket legs and inner piles, and 
(6) fabricating the house structure and power plant 
facilities and finally connecting the jacket structure with 
land-side office by truss-type catwalk (KORDI 2011). 
 
 
Fig. 2. Main Construction Stages 
 
Long-Term Measurement System 
The Uldolmok TCPP measurement system tracks 
three categories of data: (1) power-related data including 
rotational speed and torque at the side of turbine; (2) 
structural responses including strain, acceleration, and 
dynamic tilts; and (3) environmental data including tidal 
current and temperature, as shown in Fig. 3. This study 
focuses solely on the strain data, which were used to 
evaluate the tidal load levels using strain data under 
severe tidal environments. 
Eight (8) strain gauges were installed at the bottom of 
jacket legs (DL-18m) based on the design drawing and 
structural analysis results as shown in Fig. 4. The gauges 
were installed inside of the structure with specialized 
protecting cases for protecting the gauges and cables 
from the tidal current loadings and also from unexpected 
impacts by small and/or medium scale floating objects. 
Considering the main tidal current directions, i.e. east 
and west directions, six (6) strain gauges (i.e. GS-W-E, 
SG-NW-E, SG-SW-E, SG-NE-W, SG-SE-W, and SG-E-
W) were attached for measuring yy , while two (2) strain 
gauges were attached to measuring xx . The portable 
handy-type data loggers (i.e. TC31k) with batteries were 
used to measure strain responses every 30 minutes 
during construction works, and the static data logger 
with an external power was utilized to measure the strain 
every 1 minute  after completion of construction work. 
 
 
 
Fig. 3 Layout of Uldolmok TCPP measurement system 
 
 
Fig. 4. Installation Points for Strain Gauges 
 
 
ANALYSIS OF STRAIN DATA 
 
Measurement Data at Lifting Phase 
Strain gauges instrumented after construction are 
generally able to measure relative strains based on the 
strain under permanent loading and/or initial loading 
conditions. Strain gauges can be also instrumented 
during construction work, for example, strain gauges can 
be embedded with rebar in concrete structure cases. 
However, the accurate strain measurement cannot be 
guaranteed even in such a case, because the initial 
loading condition cannot be ideally zero. Therefore it is 
more practical to install the sensors after construction 
work and the initial strain level under permanent loading 
can be numerically evaluated and added into the 
measured strain data in most cases. 
In this study, strain gauges were installed when the 
jacket structure was laid in the land-side fabrication yard 
near the installation site. The jacket structure was first 
stand, lifted and temporally free hanging by a floating 
crane. The numerical simulation could be more 
accurately carried out because the supporting conditions 
became clearer than the free-standing condition. When 
the structure was hanging by a floating crane, the 
connecting points became roller supports. The jacket 
 J.-H. Yi, et al. 
888 
 
structure was eventually being under tension-only state 
by self-weight. If strain gauges were installed at the 
utmost bottom of jacket legs, then strains must be 
measured as zero when the structure was hanging. But 
strain gauges were installed in just lower parts of jacket 
legs due to interferences with many horizontal members; 
hence somewhat extent of tensile strain can be expected 
at the hanging condition. Figure 5(a) shows the 
measured stain data during lifting operation by a floating 
crane, and it can be observed that the strains were 
significantly increased under hanging condition. In other 
words, the members became under tension-governing 
state. After the structure was temporally fixed on the 
barge ship for transportation, the strains were reduced 
again. It is also noteworthy that the strains were not 
perfectly recovered after fixing on the barge ship 
because of the different supporting condition at the 
bottom of jacket legs; hence, free standing condition 
before lifting and fixed condition by strut tie after fixed 
on the barge ship. 
The numerical model was indirectly investigated by 
comparing the differences in measured and analyzed 
strains under free standing and hanging conditions. For 
the modeling of the jacket structure under free standing 
condition, six (6) points at the end of jacket legs were 
considered as hinge supporters, while the top of four 
jacket legs (i.e. A1, A2, B1 and B2) were considered as 
roller supporters because the crane cables cannot resist 
the horizontal forces as shown in Fig. 6.  
Fig. 7 shows the numerically evaluated strains under 
both of free standing and hanging cases. In the cases of 
four (4) jacket legs (i.e. A1, A2, B1 and B2), the 
compressive strains at the free standing case were 
evaluated as about -80   and those were changed as 
about 20   at the hanging case after lifting operation, 
i.e. strain levels were totally increased as amount of 
about 100  . In the cases of C1 and C2 jacket legs (i.e. 
SG-W-E and SG-E-W), the strain levels were evaluated 
as relatively lightly changed than other cases, which is 
because the upper deck and load block weights were not 
directly transferred through these two jacket legs,. The 
measured and analyzed results were compared in Fig. 8. 
In the cases of strain differences in SG-SE-W and SG-
NE-W, the differences were almost identical as about 
100  , however the differences for SG-E-W and SG-
WE-S are quite big, which is caused from the problem 
related to supporting condition, i.e. the bottom parts of 
jacket legs, in the case of free standing condition. In the 
case of general steel structures like Uldolmok TCPP, the 
geometric and material properties can be reasonably 
obtained by design drawings, therefore these errors 
originated from the modeling errors on assumed 
supporting conditions, and these discrepancies can be 
overcome by numerical model updating using by 
measured responses. 
 
18:00:00 21:00:00 00:00:00
-200
-150
-100
-50
0
Time (hh:mm:ss)
S
tr
a
in
 ( 

)
 
 
SG-SE-W
SG-NE-W
SG-E-W
SG-NE-S
SG-W-E
SG-NW-E
SG-SW-E
SG-NW-S
 
Fig. 5. Measured strain during lifting operation 
 
 
Fig. 7. Supporting conditions for free-standing and 
hanging cases 
 
W-E SW-E NW-E SE-W NE-W E-W NW-S EW-S
-120
-80
-40
0
40
80
Sensor ID
S
tr
a
in
 ( 

)
 
 
Lifting
Free Standing
 
Fig. 7. Numerically evaluated strains for free-standing 
and hanging cases 
 
W-E SW-E NW-E SE-W NE-W E-W NW-S EW-S
0
40
80
120
160
Sensor ID
S
tr
a
in
 ( 

)
 
 
Numerical
Experimental
 
Fig. 8. Comparison of measured and analyzed strain 
changes from free-standing to hanging cases 
 
 
 
 
Long-Term Strain Monitoring Data of Jacket-Type Offshore Structure for Tidal Current Power Generation  
 
889 
 
Measured Strain during Removal of a Jacket Leg 
The Uldolmok TCPP was structurally modified by 
removing one of six jacket legs (i.e. C2 jacket leg) to 
enhance the quality of incident tidal current such as 
turbulent intensity from January 1 to March 25, 2010. 
Fig. 9 shows the measured strain from February 1 to 
April 30, 2010 during removal work, and it can be 
obviously observed that the strains were significantly 
changed in the cases of SG-SE-W and SG-NE-W which 
were measured at the jacket legs relatively nearer to the 
removed jacket leg, C2. And it can also observed that the 
mean level by permanent load were changed a lot while 
the amplitudes were not severely changed because the 
tidal current loading level, which is a major live load, is 
almost same for the cases before and after removing one 
jacket leg. 
 
Strain during Normal Operation 
Fig. 10 shows the measured strain with tidal height 
and tidal current speed for three days from September 5 
to 7, 2010. The measured strains for about 12 hour per 
day (from 5am to 11am, from 5pm to 11pm) were 
obviously fluctuated, while the other 12 hours’ responses 
(from 11am to 5pm and from 11pm to 5am) were slowly 
and consistently changed without severe fluctuation. 
These observations were compared with tidal data for the 
same period, which showed that the periods with the 
smaller fluctuations were ebb tide conditions, and the 
others were flood tides. Therefore, the fluctuations were 
larger in flood tides, and smaller in ebb tides. These 
fluctuations were originated from severe fluctuation in 
tidal current speed and also relatively higher tidal speed 
as shown in the graph in the middle of Fig. 10. The 
maximum speed was up to about 1.6 m/sec in the flood 
tide while that is just up to about 1.0 m/sec in the ebb 
tide. It is caused that the main stream rapidly passes 
away along the center line after the Jindo Grand Bridge 
which can be observed from the Uldolmok Strait. It can 
be also observed that the strain is still deviated from 
mean levels even in still water conditions, and it means 
the tidal current speed is not zero along the water depth. 
 
 
Fig. 10 Measured strain data from September 5 to 7, 
2010. (circles denote times at still water) 
 
The changes in measured strains were analyzed in the 
frequency domain to identify the main frequency 
components. Figs. 11 and 12 show the frequency 
components of the measured strain data for two 
frequency ranges, i.e. 0.5-5 cycles/min for Fig. 11 and 
0.04-0.2 cycles/min for Fig. 12. From Fig. 11, it can be 
observed that strains were fluctuated with constant 
cycles of 0.001343 cycles/min (for the case of the first 
and main peak) and 0.002686 cycles/min (for the case of 
the second and lower peak), which corresponded to cycle 
lengths of 12.412 hour and 6.206 hour, respectively. The 
value of 12.412 hour is almost identical to the tidal M2 
component of 12.42 hour, and the value of 6.206 hour is 
also very close to the M4 period of 6.21 hour. The 
measured strains were therefore strongly related to the 
 
02/28/10 03/07/10 03/14/10 03/21/10
-200
-150
-100
-50
0
SG-SE-W
Dates (MM/DD/YY)
S
tr
a
in
 (

)
02/28/10 03/07/10 03/14/10 03/21/10
0
50
100
150
200
SG-NE-W
Dates (MM/DD/YY)
S
tr
a
in
 (

)
02/28/10 03/07/10 03/14/10 03/21/10
0
50
100
150
200
SG-NW-E
Dates (MM/DD/YY)
S
tr
a
in
 (

)
02/28/10 03/07/10 03/14/10 03/21/10
-200
-150
-100
-50
0
SG-SW-E
Dates (MM/DD/YY)
S
tr
a
in
 (

)
02/28/10 03/07/10 03/14/10 03/21/10
-100
-50
0
50
100
SG-NE-S
Dates (MM/DD/YY)
S
tr
a
in
 (

)
 
Fig. 10. Measured strain data during removal of one jacket leg (C2) 
 J.-H. Yi, et al. 
890 
 
tidal changes as expected. From Fig. 12, it can be known 
that the short period of fluctuation was existed with 
11.25 min of period, which is originated from a peculiar 
characteristics of Uldolmok strait. 
 
0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
x 10
-3
0
0.5
1
1.5
2
2.5
3
x 10
5
Frequency (min
-1
)
A
m
p
li
tu
d
e
f=1.343x10-3 min-1  T=12.412 hours
f=2.686x10-3 min-1  T=6.206 hours
 
Fig. 11. Frequcny component of measured strains: 0.5-5 
cycles/min 
 
0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2
0
5
10
15
20
25
30
35
40
Frequency (min
-1
)
A
m
p
li
tu
d
e
f=0.889 min-1  T=11.25 min
 
Fig. 12. Frequcny component of measured strains: 0.04-
0.2 cycles/min 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
CONCLUSIONS 
In this study, structural strain responses of the jacket-
type Uldolmok TCPP were measured under severe tidal 
environment and analyzed using measured data from 
fabrication and installation to normal operation. The 
following conclusions are made; (1) there were 
significant changes in strain responses at the steps of 
jacket lifting, block loading, pile ejection and insertion, 
(2) the strains due to permanent load were much 
significantly changed after one jacket leg removal, (3) 
strain were fluctuated with M2 and M4 tidal periods and 
also relatively short period of about 11 min due to the 
peculiar tidal characteristics in the Uldolmok strait. 
 
ACKNOWLEDGEMENTS 
 This research was a part of the project titled 
“Development of active-controlled tidal stream 
generation technology” funded by the Ministry of Land, 
Transport and Maritime Affairs, Korea (20110171). 
 
REFERENCES 
Korea Ocean Research and Development Institute 
(KORDI). Development of utilization technique for 
tide and tidal current energy. Final Report submitted 
to the Ministry of Land, Transport and Maritime 
Affairs, 2011. 


Long-term starin monitoring data of jacket-type offshore structure for tidal current power generation under severe tidal current environments

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Long-term starin monitoring data of jacket-type offshore structure for tidal current power generation under severe tidal current environments

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