Penstock Methology-internet.pdf

8/16/2019 Penstock Methology-internet.pdf

1/8

Construction of Steel Penstocks using HT100

at Kannagawa Hydropower Plant

Ken-ichiro AOKI1 and Masayuki MINAMI1 1 Tokyo Electric Power Company, Japan

Introduction

Tokyo Electric Power Company’s Kannagawa Hydropower

Plant is located in Gunma prefecture, Japan.

For the first challenge in Japan, high tensile strength steel(HT100) with a tensile strength of 950 N/mm2 was adopted

for use in the penstocks of Kannagawa Hydropower Plant to

reduce the construction cost. To apply HT100 steel for the

penstocks, a performance verification test was conducted in

advance to make sure that HT100 steel plates and welded

joints have required performance, while the requirements for

welding procedures were determined. In the actual

construction, quality control was carried out adequately on

the basis of the determined requirements for welding

procedures and other requirements.

This paper presents the design and construction of the

penstocks of the Kannagawa Hydropower Plant. In the paper,

we primarily focus on quality control items specified inconsideration of the characteristics of high tensile strength

steel and their results.

1. Overview of Facilities

Tokyo Electric Power Company’s Kannagawa Hydropower

Plant is a pure pumped-storage power plant with an effective

head of 653 m generating a maximum output of 2,820 MW

with six motor-generators. Its unit 1, with a maximum output

of 470 MW, was put into commercial operation in December

2005.

The steel penstocks of the Kannagawa Hydropower Plant areembedded in rock mass, extending over a length of about

1,400 m with a difference in elevation of 745 m and having a

maximum design head of 1,076 m, of which the static head is

817 m, and inside diameters ranging from 2.3 m to 8.2 m.

The steel penstocks consist of upper horizontal section,

inclined shaft section, and lower horizontal section.

The inclined shaft section of the entire penstock structure is

an about 960m long steep gradient pressure conduit, with an

inclination of 48 degrees and an inside diameter of 4.6 m.

Figure 1 shows a schematic diagram of the steel penstocks of

the Kannagawa Hydropower Plant. The steel penstock is

bifurcated, at the Branch I located at the base of the headrace

surge tank, into the penstock for units 1 & 2 with a total

length of 1,397 m and the penstock for units 3 & 4 with a

total length of 1,365 m.

Tokyo Electric Power Company decided to introduce high

tensile strength steel (HT100) with a tensile strength of 950

N/mm

2

for the penstocks of its Kannagawa HydropowerPlant to reduce the construction cost. Table 1 shows the

materials used and their weight. The total weight for units 1

& 2 penstock was about 5,154 t, of which the quantity of the

HT100 steel was about 2,331 t. The total weight for units 3 &

4 penstock will be about 5,150 t, of which the quantity of the

HT100 steel will be about 1,420 t.

The construction of the penstock for units 1 & 2 was started

in April 2001, with the installation on the site completed in

July 2004. Currently, the inclined shaft and part of upper and

lower horizontal sections of the penstock for units 3 & 4 are

under construction with the progress rate being about 40% as

of May 2009; the installation of the penstock is planned to be

completed by April 2010.

Figure 1: Schematic diagram of the steel penstocks of

the Kannagawa Hydropower Plant

Conference on High Strength Steels for Hydropower Plants - Takasaki

1 - 1


2/8

TABLE 1: Materials and weights of steel

2. Design of the Penstocks

2.1 Design for Internal hydraulic pressure

In the design of penstocks embedded in rock mass, internal

hydraulic pressure is partially borne by rock mass

surrounding penstocks with the aim of reducing costs

through a reduction in the thickness of steel pipes.

In this design, the ratio of internal pressure borne by rock

mass is designed according to rock mass properties, such as

the elastic modulus and plastic deformation coefficient. The

plate thickness of the penstock is determined so that the

stress acting on the penstock as a result of the consideration

of the internal pressure borne by rock mass may not exceed

the allowable stress of the penstock. In addition, the design is

established on the safe side so that the stress acting on the

penstock may not exceed the yield point of the steel material

even if rock mass is assumed to bear no internal hydraulic

pressure.

Table 2 shows the allowable stress and yield point for the

steel material used.

Joint efficiency is determined for the places of welding and

the sampling ratio in the nondestructive inspection, and can

be made 100% by welding in a workshop and inspection overthe entire welded length.

Tokyo Electric Power Company compared costs of

nondestructive inspection over the entire welded length with

reducing costs through a reduction in the thickness of steel

pipes assuming a 100% joint efficiency. For the penstocks

used in the Kannagawa Hydropower Plant, a reduction in the

plate thickness was adopted by assuming a 100% joint

efficiency after the inspection of entire longitudinal joint

lines.

TABLE 2: Allowable stress and yield point of steels

2.2 Design for External hydraulic pressure

To ensure safety for external hydraulic pressure, penstocks

embedded in rock mass are designed to have a safety factor

of 1.5 or more against the critical buckling pressure

calculated using the theoretical formula.

For inclined shafts of which an adequate drain slope can be

expected, Tokyo Electric Power Company adopts a design in

which drain facilities are installed to reduce external

hydraulic pressure.Figure 2 shows the design of the drain facilities of the

Kannagawa Hydropower Plant. The drains consist of both a

direct drain for draining water from the void between the

penstock and the infilling concrete and an indirect drain for

draining seepage water, which comes from the rocks, from

the void between the concrete and the rocks. Even when one

of the drains becomes ineffective, the other reduces the

external hydraulic pressure to prescribed level.

The groundwater level in the natural ground surrounding the

penstocks is assumed to be on the grand level according to

investigation. The drain facilities are designed to reduce the

groundwater potential near the penstock in the inclined shaft

section. For the penstock for units 1 & 2, the design external

hydraulic pressure was set at 40% of the hydraulic pressure

corresponding to the natural ground cover on the basis of the

two-dimensional seepage flow analysis. For the penstock for

units 3 & 4, on the other hand, with the aim of reducing plate

thickness, the design external hydraulic pressure was set at

30% of the hydraulic pressure corresponding to the natural

ground cover because a ratio of the measurement value for

the design external hydraulic pressure of the penstock for

units 1 & 2 was very small.

Penstock for units 1 & 2 Penstock for units 3 & 44)Steel

Thick

ness(mm)

Weight

(t)

Ratio

ofweight

Thick

ness(mm)

Weight

(t)

Ratio

ofweight

HT100 29-72 2331 45% 27-72 1420 28%

28-38,

621)312

28-42,

621)907

SHY685

NS-F

2002) 86%

2002) 818%

SM570 22-45 1335 26% 22-44 1541 30%

SM490B 20-52,

703)1168 23%

22-52,

703)1148 22%

SM400B ---- ---- 0% 22-26 126 2%

Total

weight

---- 5154 100% 5150 100%

1) 62-mm thick steel is for joining inlet valves.

2) 200-mm thick steel is for stiffening the branch II.

3) 70-mm thick steel is for reinforcing the branch I.

4) The penstock for units 3 & 4 is now under construction, so the values areestimated.

Steel Thickness

(mm)

Allowabl

e stress

(N/mm2)

Yield

point

(N/mm2)

Standard

HT100


3/8

Figure 2: Design of the drain facilities

2.3 Results of the Design

In designing the material and plate thickness of penstocks,

the plate thickness was decided as follows;

First, the largest plate thickness, for each unit section (with a

length of 3 m), was calculated by each candidate materialamong ;

(1) plate thickness dictated by the internal hydraulic pressure,

(2) plate thickness dictated by the external hydraulic

pressure, and (3) the smallest plate thickness dictated by the

restrictions in construction.

Then, the most cost-effective material was decided with the

costs for materials, manufacturing and installation taken into

account. In addition, while welding workability was being

given consideration to avoid an abrupt change in plate

thickness and an extreme change in the material between

adjoining unit sections, materials and design plate thickness

were adjusted adequately before determining the final design.

Figure 3 shows the results of the design of the penstock (forthe inclined shaft) of the Kannagawa Hydropower Plant. For

the penstock for units 3 & 4, the plate thickness of HT100

steel is smaller than that of the penstock for units 1 & 2 as a

result of a review of design conditions such as a change in

the design external hydraulic pressure.

Figure 3: The design of the penstock (for the inclined shaft)

Penstock for units1&2 Penstock for units 3&4

①SM400

②SM490

③SM570

④SHY685

⑤HT100


1 - 3


4/8

3. Procedure of the penstock installation

Figure 4 shows an overview of the penstock installation of

the Kannagawa Hydropower Plant.

The penstocks were constructed by cutting steel plates, preparing edges and bending the plates in manufacturer’s

plant. And then, due to transportation limits, these were

constructed by transporting the half-pipe parts to the site,

welding the parts into 3 m-long unit pipes and then into 15

m-long unit pipes in the temporary workshop at the site, and

installing in the inclined shaft and welding the unit pipes.

The installation of steel penstock in the inclined shaft was a

critical path of the construction schedule. Since the adoption

of HT100 steel allowed one to reduce the penstock weight in

the inclined shaft section, the length of an installation unit

pipe was increased from 12 m (four 3-m long unit sections),

the longest in the past, to 15 m (five 3-m long unit sections),

thereby reducing the number of times of installation in theinclined shaft and hence reducing the time necessary for

installation by 20%.

In the inclined shaft, a construction method that was based on

the installation of two unit pipes taken as one work cycle was

adopted. Consequently installing steel a unit pipe and filling

concrete helped shorten the construction period. The

installation speed of HT100 steel sections including concrete

filling was 11 days/30 m. The mean installation speed of the

entire inclined shaft was 10 days/30 m.

Regarding the weld processes adopted for the penstocks at

the site, the SAW(submerged arc welding) method was

adopted for straight pipes and the SMAW(shielded metal arc

welding) method for bend pipes at the temporary workshop,

while the method of automatic MAG(metal active gas)

welding from one side of the inner surface was adopted for

the inclined shaft, with the SMAW method adopted for other

on-site welding operations.

4. Quality Control for the Penstocks

4.1. Overview of Quality Control for the Penstocks

To apply HT100 steel for the penstocks, Tokyo Electric

Power Company primarily focused on the prevention of

brittle fracture, while quality control items and criteria forsteel plates and welded joints were determined, especially in

consideration of toughness and weldability.

Figure 5 shows the penstocks installation procedure and

important inspection items of quality control for the

penstocks of the Kannagawa Hydropower Plant. Tokyo

Electric Power Company’s approach to quality control is

fundamentally based on both the checking of all records of

inspection by the fabricator and witness sampling inspection.

In addition to this, specifically for important inspection

items, the witness inspection of all articles is predominantly

used. In the following, the important items and results of

quality control are described using the pieces of data

obtained during the construction on the penstock for units 1

& 2 for examples.

Place

Material making

Longitudinal welding

Material Inspection

Before painting Inspection

Half pipe manufacturing

Assembly

（ transportation ）

Inspection

Nondestructive Inspection

Groove Inspection

Visual Inspection

Receiving&Inspection

Groove Inspection

Visual Inspection

Groove Inspection

After painting Inspection


Steel penstock installation procedure

Temporaryworkshop

Circumferential welding

Completion

Asssembly

Steel mill

Unit pipe of 3m fabricating

Painting

Installation

Circumferential welding

Workshop of fabricator

Inclinedshaft

Dimension Inspection

Half pipe Inspection


Visual Inspection

Unit pipe of 15m fabricating

Assembly

Figure 5: Important inspection items of quality control

for each procedure

Figure 4: Overview of the penstock installation


1 - 4


5/8

4.2 HT100 Steel Plates and Welding Materials

Table 3 and Figure 6 show the quality control criteria and test

results for HT100 steel plates. Generally tensile strength tests

are conducted using C-direction of the steel plate. However,

the preliminary performance verification test revealed thatthe L-direction strength bearing hydraulic pressure tended to

be smaller than the strength in the C-direction in HT100

steel. In the actually executed work, for this reason, the

tensile strength test was conducted both in the L- and C-

directions. Since the impact test also showed that the

toughness near the middle of the plate thickness tended to

be inferior to that at the 1/4-thickness position, the toughness

was checked at positions of 1/4-thickness and 1/2-thickness

in the actual construction.

Table 4 shows the mechanical properties of HT100 deposited

metal and the control standard for the quantity of hydrogen.The specifications of the deposited metal were characterized

by tensile strength for which slightly softer nature than the

base metal was required in consideration of the balance with

the toughness and weldability and by the requirement for the

quantity of hydrogen.

Plate thickness 50mm

n=41 n=12

Element C P S Ceq PCM C P S Ceq PCM

Maximum 0.11 0.008 0.001 0.572 0.273 0.12 0.006 0.001 0.603 0.311

Minimum 0.09 0.001 0.001 0.533 0.254 0.11 0.002 0.000 0.592 0.288

Maximum

content

0.14 0.01 0.005 0.59 0.29 0.14 0.01 0.005 0.62 0.33

TABLE 3: The quality control criteria and test results for HT100 steel plates

Ceq=C+Mn/6+Si/24+Ni/40+Cr/5+Mo/4+V/14

PCM=C+Si/30+Mn/20+Cu/20+Ni/60+Cr/20+Mo/15+V/10+5B

Figure 6: The quality control criteria and test results for HT100 steel plates


1 - 5


6/8

TABLE 4: Mechanical properties of HT100 deposited metal

and the control standard for the quantity of hydrogen.

4.3 Welding Procedure Control

Table 5 shows the requirements for welding procedures. Therequirements for welding procedures for HT100 steel were

specified on the basis of the results of the preliminary weld

cracking test. The results of the weld cracking test showed

the following findings:

a. In single pass welding, the preheat temperature for

preventing weld cracking was similar or higher in a U-groove

weld cracking test than in a y-groove weld cracking test.

Thus both tests should be conducted to determine preheat

temperature for HT100.

b. A multi-layer weld cracking test showed that postheating

(150°C for 2 hours) was highly effective for preventing

cracks. In SHY685NS-F, postheating could be omitted whena nondestructive test was conducted at over 48 hours after the

completion of welding operation. However, postheating was

indispensable for preventing cracks for HT100 when preheat

temperature was similar to that for SHY685NS-F.; and

c. Use of welding materials for SHY685NS-F in single passwelding by SMAW and MAG enabled preheat temperature

for preventing cracks to be lowered. Thus, use of welding

materials for SHY685NS-F in SMAW and MAG for root

pass welding and tack welding was effective for preventing

cracks.

For SM570 and SM490B, a special specification of weld

crack sensitivity PCM against weld cracking of not exceeding

0.2% was also applied, and preheating was omitted except on

cold days.

Table 6 shows the track records of heat input control for

HT100 welded joints. Compared with the control standard of

45 kJ/cm, the average heat input in the actual construction isabout 34 kJ/cm in the SAW and SMAW methods and about

16 kJ/cm in the automatic MAG method applied in the

inclined shaft.

In controlling the welding atmosphere, the water vapor

pressure was calculated from the results of measurements of

the ambient temperature and humidity, and the relationship

between the values obtained and the control standard was

used in order to limit the time for which the welding

materials were left alone. Figure 7 shows the track records of

the control of water vapor pressure at the temporary

workshop.

Welding method SMAW SAW MAG (Reference)

HT100

steel plate

Yield strength(N/mm2)

≧ 785 ≧ 785 ≧ 785 ≧ 885

Tensile strength

(N/mm2)

930-1000 930-1000 930-1050 950-1130

Elongation (%) ≧ 12 ≧ 12 ≧ 12 ≧ 12

Charpy impact

absorption energy

(J)

≧ 47

(at –10℃)

≧ 47

(at -10℃)

≧ 47

(at -10℃)

≧ 47

(at -55℃)

Ductile fracture

percentage (%)≧ 50 ≧ 50 ≧ 50 ≧ 50

Hydrogen content

(mL/100g)≦ 6 ≦ 3 ≦ 2 ----

Minimum preheating

temperature (°C)

Steel Thickness

(mm)

SMAW

SAW

MAG

Interpass

temperature

Limit heat input

(kJ/cm)

Postheating

conditions

Not

exceeding

50

100 80HT100 and

SHY685NS-F

Over 50 125 100

At least the

preheating

temperature

and not

exceeding

230°C

The mean not

exceeding 45, and

the maximum not

exceeding 50

At least 2

hours at over

150°C

Notexceeding

25

---- Not exceeding 60 ----SM570

Over 25 ---- Not exceeding 80 ----

SM490B ---- ---- ---- ----

SM400B ----

No preheating

---- ---- ----

a. For SMAW and MAG for HT100, welding material for SHY685NS-F was used for root pass and tack welding.

b. For tack welding of HT100 and SHY685NS-F, welding materials of a strength level that was one rank lower than that for the

actual welding.

c. Postheating was omitted for SHY685NS-F if nondestructive test was conducted 48 hours after welding or later.

d. The preheating temperature for tack welding was at least 25°C higher than the minimum preheating temperature stated above.

e. In SMAW and SAW, the preheating temperature was at least 25°C higher than the minimum preheating temperature stated above

when the partial hydraulic vapor pressure was over 25.5 mmHg.

f. For SM570 and SM490B, steels with PCM of not exceeding 0.2% was used and no preheating was conducted provided that the

temperature was preheated to 40°C when the air temperature was 5°C or below.

g. When welding was suspended, postheating was preformed using the conditions stated above or the temperature was retained until

welding was restarted.

TABLE 5: The requirements for welding procedures


1 - 6


7/8

4.4 Offset and Angular Distortion of Welded zones

From the viewpoint of preventing brittle fracture, checking

the shape of welded zones inducing the stress concentration

is one of the most important control items. In the actual

construction, quality control was conducted on undercuts,

overlaps, reinforcement weld heights, offsets, angular

distortions and so forth. Among these, the offset and the

angular distortion of a longitudinal joint were put to

quantitative control. Figure 8 shows the track records of

control of offsets and angular distortions of welded zones ofHT100.

4.5 Nondestructive Inspection

The Japanese standards require that welded zones in the main

pressure bearing parts of a penstock be put to nondestructive

inspection after welding operation as needs arise, and that the

methods of nondestructive inspection be based on the

radiographic test or ultrasonic test as a rule. Table 7 shows

examples of the criteria (for lineal flaws) for the radiographic

test and ultrasonic test.

(1) Nondestructive test on the penstock for units 1 & 2The nondestructive inspection on welded zones of the

penstock for units 1 & 2 of the Kannagawa Hydropower

Plant was conducted according to the following policies in

consideration of the track records of inspection on penstocks:

a. The main pressure bearing parts should be put to the

radiographic inspection as a rule; and

b. Branch pipe shells and reinforcement beam connections to

which the application of the radiographic inspection was

difficult due to structural problems should be put to the

ultrasonic test.

Pass maximum heat input

(kJ/cm)

Average heat input (kJ/cm)Place of welding Welding

method

Number

of joints

Max. Min. Average Max. Min. Average

Temporary

workshop

SAW 365 48.4 30.9 39.1 39.8 27.7 32.8

Temporary

workshop

SMAW 20 48.4 37.9 45.0 41.1 27.2 36.3

Field SMAW 87 49.1 26.6 39.9 38.9 19.9 32.0

Inclined shaft MAG 25 37.6 24.6 30.1 18.5 13.1 16.4

Control standard Not exceeding 50 Not exceeding 45

Thickness

t (mm)

RT (for linear flaws) UT

Not exceeding 50 Not exceeding t/3 and

16 mm

Over 50 Not exceeding t/4 and

12 mm

Not exceeding

t/2 and 30 mm

0.0

1.0

2.0

3.0

4.0

5.0

6.0

20 30 40 50 60 70 80

Plate thickness(mm)

O f f s e t r a t

i o

f o r t

h i c k n e s s

( % )

Control Standard ≦5%

≦3mm

0.0

0.5

1.0

1.5

2.0

2.5

3.0

20 30 40 50 60 70 80

Plate thickness(mm)

A n g u

l a r

d i s t o r t

i o n

( °

)Control Standard ≦2.5°

TABLE 6: The track records of heat input control for HT100 welded joints

Figure 7: The track records of the control of water vapor pressure at the temporary workshop

Figure 8: The track records of control of offsets andangular distortions of welded zones of HT100

TABLE 7: Examples of the criteria (for lineal flaws) for

the radiographic test and ultrasonic test

0

10

20

30

40

2001/11/1 2002/1/31 2002/5/2 2002/8/2 2002/11/1 2003/1/31 2003/5/3 2003/8/2 2003/11/1 2004/2/1 2004/5/2 2004/8/1

Date

W a t e r v a p o r

p r e s s u r e ( m m H g )

Contorol Standard：25.5mmHg


1 - 7


8/8

Table 8 shows the rules for the time to conduct

nondestructive tests on the penstock for units 1 & 2. In

consideration of the occurrence of delayed cracking in high

tensile strength steels having performance equal or superior

to that of SHY685NS-F, the time to conduct nondestructivetest was specified.

Table 9 shows the track records of nondestructive tests on the

penstock for units 1 & 2. Nondestructive tests over entire

lines were conducted on longitudinal joints bearing hydraulic

pressure with the aim of rationalizing the design by means of

setting the joint efficiency at 100%. For branch pipes with

complicated structures that require a high welding skill,

nondestructive tests were conducted on entire weld lines. For

HT100 steel circumferential joints, entire line inspection was

conducted at the beginning of welding operations, and the

rate of inspection was reduced to about 10% after the quality

of welding had been verified. These inspections allowed one

to make sure that all welded joints satisfied the criteria.

(2) Nondestructive test on the penstock for units 3 & 4

With the aim of increasing the work efficiency and

decreasing construction costs, the ultrasonic test based on an

automatic ultrasonic tester was adopted for the

nondestructive inspection on welded zones in the

construction of the penstock for units 3 & 4 of the

Kannagawa Hydropower Plant. Before introducing an

automatic ultrasonic tester, tests were conducted on

artificially flawed test pieces to verify the performance of the

tester.

Figure 9 shows how the ultrasonic test is carried out. With

the tester introduced this time, flaws detecting operation is

carried out from both sides on one face of a welded zone

simultaneously. First, flaws are searched for coarsely, square by square, at intervals of 5 mm, and if reflected echoes are

detected, flaw detection is carried out finely at intervals of 1

mm again. All items of obtained data are recorded in the

form of electronic data, and the judgment of data on flaws is

carried out automatically.

Rules for the time to conduct nondestructive inspection for

the penstock for units 3 & 4 (Table 8) and the rate of

execution of nondestructive inspection on longitudinal joints

(conducted over entire lines) are the same as those for the

penstock for units 1 & 2.

Figure 9: Automatic ultrasonic test equipment

Conclusion

First challenge of introducing HT100 steel with tensile

strength of 950 N/mm2 to penstocks of a hydropower plant in

Japan was started at Kannagawa Hydropower Plant.

Especially in case of utilizing new materials, quality

management is the key issue to complete the project without

fatal defects since many unexpected phenomena might

happen and counter measures should be taken immediately

for completion of the project.

In Kannagawa, therefore, quality management system was

established by close cooperation between Tokyo Electric

Power Company as the owner, engineer and fabricator with

support of steel mill. At the any stage of the construction,

quality control was been made jointly.

Tokyo Electric Power Company’s approach to quality control

is based on deciding control items and criteria, carrying out

the witness inspections, and analyzing their results. Thequality control was conducted from the viewpoint of not only

judging the results for control criteria but also improving

control criteria and our design.

As the result of these efforts, the construction of the penstock

for units 1 & 2 completed successfully and no irregular stress

and strain has been observed by measuring system since

starting operation of unit 1 in December 2005.

The construction of the penstock for units 3 & 4 is now on

going, and the effort for quality management will be also

continuing until completing the construction with achieving

targeted quality successfully.

Steel Place of

welding

Time of testing Postheating

conditions

HT100 All places At least 24 hours

after the completion

of welding

150°C for at least

2 hours

Inclined shaft At least 24 hours


of welding

150°C for at least

2 hours

SHY685

NS-F

Temporary

workshop

At least 48 hours


of welding

None

SM570,

SM490B

SM400B

All place No rules None

HT100General penstock body

Longitudinal joint Circumferential joint

Branch II

Entire length

1034.737(m)

57% of the welded length

1560.779(m)

Entire length

129.906(m)

JIS standard materialsGeneral penstock body

Longitudinal joint Circumferential joint

Branch I

Entire length

2090.011(m)

12% of the welded length

670.173(m)

Entire length

281.031(m)

TABLE 8: Rules for the time to conduct nondestructive tests

TABLE 9: The track records of nondestructive tests


1 - 8

Penstock Methology-internet.pdf

Documents

Transcript of Penstock Methology-internet.pdf