Engineering, 2011, 3, 485-490
doi:10.4236/eng.2011.35056 Published Online May 2011 (
Copyright © 2011 SciRes. ENG
Experimental and Numerical Study of the Effect of
Surround Protection Technique on the Strain
Measurement for Offshore Jacket Platform
Yongjun Xu
Key Laboratory for Hydrodynamics and Ocean Eng ineering, Institute of Mechanics, Chinese Academy of Sciences,
Beijing, China
Received March 3, 2011; revised March 29, 2011; accepted April 6, 2011
For strain measurement on offshore jacket platform in deep water, waterproof of strain foil is always an im-
portant issue, especially, due to the high pressure in deep water. The waterproof is difficult in two places,
one is between the matrix structure and the protection structure, and another is between the lead wires and
the protection structure. The surround protection technique discussed in this paper is conventional and ideal,
and can be operative for a long time, up to five years. In this method, a metal case and tube is added on the
local position, which increases the local rigidity, but the effect on the measurement of strain is not well stud-
ied. In this paper, the effect of the surround protection technique on the strain measurement is studied by us-
ing numerical and experimental methods, and the results show that the measurement error is well in the
range permitted by engineering practice.
Keywords: Offshore Jacket Platform, Strain Measurement, Strain Foil, Surround Protect/Waterproof
1. Introduction
Electrical resistance strain gages are widely used for
stress analysis based on strain measurement for various
metal structures in hostile environment, such as pressure
vessel, submarine, deep-water vehicle, submerged pipe-
line and offshore jacket platform. Due to the high pres-
sure in deep water, the strain gages have to be isolated
from the surrounding environment to work properly.
There are two main kinds of protection/waterproof tech-
niques, one is the chemical waterproof technique for the
gage to be isolated from the environment by coating with
a certain chemical substance, and another is the me-
chanical sealing technique for the gage to be isolated
from the environment by some solid enclosure. In the
cases of chemical protections, with such as bitumen
compound [1], filled epoxy [2], self-vulcanizing rubber
electrical tape and rubber-to-metal cement [3,4], and
epoxy resin and polyisobutylene [5,6], the water-
proof/protection efficiency depends on the adhesive
strength between the protective coating materials and the
matrix, protective coating materials and the lead cable,
and the additional pressure effects are included in the
strain measured. The pressure effect was studied by sev-
eral investigators [7-9]. In the cases of mechanical seal-
ing [2,6], waterproof/protection efficiency depends on
the annular gasket between the matrix structure and the
surround metal shell, and the strain field can be signifi-
cantly changed at the location of the measurements.
Strain gages [9-14] will always be one of the best strain
measurement methods for various metal structures. Xu Y.
J. et al. [15-17] carried out an intensive study of the spot
welding flake strain g auge/sensor. In practice, it is some-
times very difficult to adhere, cure and post cure the
strain gauges on the spot, especially during the short time
limit permitted by the project. Some pre-work, such as
adhering, curing and post curing, and some preliminary
seal work, can be done in the laboratory by flake strain
gauge/sensor techno log y. Then, w e only need to weld th e
flake to the matrix in the field.
Among the various types of offshore structures, the
steel jacket platform is the most common in use, with
multi-functions for oil exploration, drilling as well as for
production. Conventionally, such platforms operate up to
Y. J. XU
a depth of about 100 - 150m. They are usually built from
tubular steel members. These structures have a very short
vibration period ranging from 2 to 8 s. Apart from the
operational loads, they also subject to environmental
loads such as wind, wave, tidal current, ice and earth-
quake loads. The safety monitoring and assessment tech-
niques of Ocean Platform Structure [18] are very impor-
tant, and the long-term strain/stress measurement is one
of the key issues for the jacket platform. The surround
protection technique [2,19] is a conventional and ideal
method, which can be used to protect/waterproof the
strain gage for a period up to five years. This protection
method adds a metal case and tube on the local position
which increases the local rigidity, but its effect on the
measurement of strain is not clear. In this paper, the ef-
fect is studied by using numerical and experimental
2. Some Protection/Waterproof Techniques
Basic Protection Requirements
Mark the area around the gauge to be covered, before
covering the gauge.
Measure and record the gauge grid resistance and the
resistance-to-ground after removal of soldering flux
but before applying coating.
Mix two parts of materials thoroughly and take care
of unmixed materials on the container surfaces.
‘Wet’ the surface with a small amount of materials
before the full protection procedure.
Do not smooth the protective covering but have a
high build-up at the edge.
Avoid sharp corners at the coating edge, as break-
down can begin at such places.
Remove, where practicable, possible breakdown
points from the structure surface (e.g. terminals).
Use pressure-tested cables and anchor them, both
inside and outside the coating area, before coating.
Degrease all cable cores in contact with the coating
and ensure separate entry into the coating (which ap-
plies also to ribbon cables).
Make the cable/coating interface as long as possible.
2.1. Short-Term Protection Technique
When a relatively short-term protection is required, up to
6 months in wet conditions, the technique illustrated in
Figure 1 proves to be satisfactory. Provided that the
above basic protection requirements are observed, this
technique will provide adequate gauge protection at
pressures up to 2500lbf/in2 (1757673kgf/m2).
2.2. Over-Lap Pr otecti on Tec hnique
Over-lap protection technique differs fundamentally
from the ‘short-term’ protection technique in that it is a
‘double barrier’ technique, although both barriers can be
of the same material, as illustrated in Figure 2. The ca-
ble/coating interface is double with respect to the
‘short-term’ protection technique, to increase the life of
the protection structure. Using the ‘over-lap’ protection
technique, an adequate gauge protection in wet condi-
tions for up to 12 months can be expected.
A ‘three barriers’ over-lap protection technique [14]
was proven to be successful in the strain measurement in
sea water of 10 meters deep for Chengbei-25A jacket
platforms for about 6 months.
2.3. Long-Term Technique
Neither the short-term protection technique nor the
over-lap protection technique provides really substantial
mechanical protection in addition to the protection
against moisture. In case where the mechanical protec-
tion is necessary (e.g. for oil rigs, jacket platform), an
overall metal surround and cable entry can be used. The
surround technique is illustrated in Figure 3, and this
technique could adequate gauge protection in deep water
for up to 5 years.
A surround protection technique [19] was proven to be
successful in the strain measurement in sea water of 40
meters deep for W11-4 jacket platform (as Figure 4
shows) for about 3 years in south sea of China.
3. Experimental Study on the Effect of
Surround Protection Technique
3.1. Specifications and Geometrical Data of
Prototype Surround Case
A steel surround protection case is employed in this
Figure 1. Short-term pr ote c tion tec hnique .
Figure 2. Over-lap protection tec hnique .
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Y. J. XU487
study as illustrated in Figure 5(a), and the corresponding
geometric dimensions are shown in Figure 5(b).
3.2. Specifications and Installation of Test
It is well known that the stress/strain state indicates the
local extension-compression characteristics of the off-
shore jacket platform. A steel plate sample of 0.2 m long,
0.1 m width and 0.01 m thick is employed, and six bolt
shanks (M6) are welded to match with the six holes on
the surround case. An annular gasket (3 mm) is closely
pressed between the case and the sample by screwing six
nuts as illustrated in Figure 6(a). The tension-compres-
sion tests are carried out in LETRY(10T) as shown in
Figure 6(b) .
3.3. Experimental Results
After several pre-loading and unloading processes, we
measure the strains at the center point by loading and
unloading with stress t
of 8 Mpa, 10 Mpa, 12 Mpa
and 14 Mpa. A group of average strains based on two
loading and unloading processes are obtained and listed
in Table 1, and the corresponding stress-strain curves of
protection/waterproof and no-protection/no-waterproof
are shown in Figure 7. The strain measurement of the
Figure 3. Surround protection tec hnique .
Figure 4. W11-4 jacket platform
surround protection case is listed in Table 1, with the
average relative error of about 2.54%.
4. Numerical Study on the Effect of
Surround Protection Technique
4.1. Finite Element Model
After the nuts are bucked up and the annular gasket is
enough pressed, and the thickness of the annular gasket
is 2 mm. The main relative error of strain measurements
is due to the local structure rigidity of the surround pro-
tection case. Two uttermost distribution cases of the six
nut shanks are discussed in this paper as shown in Fig-
ure 8. The nut shanks and surround protection case are
treated as a one-piece construction and the effects of the
six nuts are ignored.
A commercial finite element analysis code ANSYS is
used for the stress analysis. As shown in Figure 9, a
quarter of the model due to its symmetry is used in the
finite element analysis. The corresponding finite element
meshing is also shown in Figure 9.
4.2. Numerical results
The modulus of elasticity of the case steel is
2.07 11Ee
and its Poisson’s ratio is 0.3
, and the
Figure 5. Prototy pe of sur round protection case.
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Y. J. XU
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Table 1. Experimental tension results.
) waterproof
) Error (%)
8 39.5 38.5 2.53
10 49 47.75 2.55
12 59 57.5 2.54
14 68.75 67 2.55
Figure 7. Corresponding stress-strain curves of experimen-
tal results.
modulus of elasticity of the rubber is and its
Poisson’s ratio is 7.84 6Ee
. It is very difficult to simu-
late the annular gasket accurately, especially, the effect
of tightening and the equivalent modulus of the annular
gasket. We assume that the equivalent modulus of elas-
ticity of the tightened annular gasket is enhanced by one
order of magnitude by tightening hard (namely,
7.84 7
) and its equivalent Poisson’s ratio remains
to be 0.46
. The numerical results obtained are listed
in Table 2, and the corresponding stress-strain curves of
protection/waterproof and no-protection/no-waterproof
are shown in Figure 10. The relative errors of the strain
measurement of the surround protection case are also
listed in Table 2 , with the average relative error of about
1.333% for style 1 and the average relative error of about
Figure 6. Surround protection tec hnique .
Table 2. Numerical results.
Style 1 Style 2
) waterproof (
)Error (%) waterproof (
) Error (%)
2 9.662 9.533 1.335 9.529 1.377
4 19.324 19.066 1.335 19.057 1.382
6 28.986 28.599 1.335 28.586 1.380
8 38.647 38.133 1.330 38.115 1.377
10 48.309 47.666 1.331 47.643 1.379
12 57.971 57.199 1.332 57.172 1.378
14 67.633 66.732 1.332 66.701 1.378
Y. J. XU
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Figure 8. Two different installation styles.
Figure 9. Finite element model and its corresponding
Figure 10. Corresponding stress-strain curves of numerical
1.379% for style 2. So the average relative error is in the
range of 1.333% - 1.379%.
5. Conclusions
In this paper, experimental tests and FEM simulations
were carried out to study the effect of the surround pro-
tection technique on strain measurements on offshore
jacket platform. From the experimental and numerical
results, the following conclusions may be drawn.
The average relative error of the strain measurement
due to the surround protection case is about 2.54%, as
shown by experimental results.
The FEM calculation shows that the average relative
error of the strain measurement due to the surround
protection case is in the range of 1.333% - 1.379%.
All experimental and numerical results show that the
average relative error is in the range permitted by en-
gineering practice. The error is a one-sided error and
error-correction can be easily done by selecting a
correction factor.
All experimental and numerical results show that the
surround protection technique is an ideal protection
method of strain measurement for offshore jacket
All these investigations provide a theoretical basis for
practical applications of the surround protection tech-
nique for the strain measurement on offshore jacket
6. Acknowledgment
This work was supported by the Knowledge Innovation
Program of the Chinese Academy of Sciences (Grant No.
KJCX2-SW-L03), and I would like to express my appre-
ciation to Ms. Changzhen Kan for her support in the fin-
ishing of the experimental testing.
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