Embedment of Steel Spiral Cases in Concrete: Lessons from a Structural Deformation Accident in China

Abstract

A spiral case structure (SCS) plays a significant role in the safe and reliable operation of a hydroelectric power plant (HPP). In an HPP with 700 MW class turbine in China, a structural deformation accident happened in the construction period causing severe loss. Based on in-situ measured data, this study focuses on two major differences of this SCS that might cause the accident: (a) the construction condition, and (b) the shape of steel spiral case (SSC). The accident is reproduced in numerical study, and the simulation results agree reasonably well with in-situ measured data. The results show that the construction condition is a main factor causing the accident, but it is not the only cause of the raising deformation. The findings reveal that the post-accident stresses of steel structures are still at a relatively low level, and it would not be a major concern. The study also shows that the SSC with non-circle sections tends to have larger deformation under internal water pressure, and the deformation of the stay ring needs more attention in the construction period. The major limitation of this study is that this study merely focuses on the construction period. If such SCSs were to be used in a wider range, a follow-up study focusing on the operation period should be considered.

1. Introduction

Hydropower plays an important role in the transition to clean power. In order to achieve the goal of net zero emissions in time, the construction of hydroelectric power plants (HPPs) has been accelerated [1,2,3]. Spiral case structure (SCS) is one of the major structures in HPP, where hydro energy is converted into electricity [4].

A SCS usually consists of steel spiral case (SSC) and mass concrete with engineered reinforcement, and it provides a circumferentially uniform water intake for a Kaplan or Francis turbine. In general, there are three types of SCSs, which are the P-type SCS, M-type SCS and N-type SCS. The P-type SCS stands for the method where SSC is embedded in concrete under temporarily water-pressurized condition, while the M-type SCS and N-type SCS are embedded under an unwatered condition. The difference between the M-type and N-type SCSs is that the N-type SCS is embedded in a natural state, while the M-type SCS has a compressible membrane covering the SSC.

In most medium and high head HPPs with storage or pumped-storage facilities, P-types SCS is always adopted for its abundant engineering practices as a well-established technology [5]. The standard construction procedure of P-type SCS is shown in Figure 1. The first-phase concrete and pedestal of the SSC are constructed previously to provide necessary support for the SSC. Then, the SSC is placed in position and sealed with the test head and test barrel. The test head is set at the inlet of the SSC, and the test barrel is set between the upper and lower stay rings. Meanwhile, the SSC is fixed to the concrete with tie rods around it to prevent the uplift of SSC during the construction of the second-phase concrete. Then, temporary internal water pressure (IWP) is specified in the SSC, and second-phase concrete is constructed with the SSC under pressurized conditions. Finally, after the set of second-phase concrete, the temporary IWP is released, the sealing devices are removed and the construction of a P-type SCS is, thus, finished.

However, at the present rate of hydropower development, the global energy pathway to net zero emissions will not be realized [6]. Considering that M-type SCSs do not need pressurization equipment in the construction period, it is believed that they can simplify the construction procedure and help reduce the span of the powerhouse [7,8,9]. Accordingly, several attempts have been made to install M-type SCSs in high head HPPs, such as all five SCSs in the Lijiaxia HPP and nine out of thirty-two SCSs in the Three Gorges HPP [10,11]. The standard construction procedure of M-type SCS is shown in Figure 2. The construction of the first-phase concrete and pedestal of the SSC is similar to that of P-type SCS. Then, after the SSC is placed in position with inner supporting rods, the top portion of it is covered with a compressible membrane. After that, the second-phase concrete is constructed, with no need for sealing devices or pressurization.

Despite the merits of M-type SCSs, there exists some concerns about them, such as the long-term mechanical properties of compressible membrane, which has been investigated during the feasibility study of the Three Gorges HPP [12,13]. Besides, for an average P-type SCS, the mass of the water inside it during pressurization can reach 1 × 10⁶ kg, which might help a lot resisting the raising displacement of the SSC during the construction of concrete. However, the current study concerning temporary IWP mainly focuses on its effect on the contact between the SSC and the concrete [14,15,16,17], and the effect of the gravity of water inside has been long neglected.

During the construction period of a 700 MW class turbine set with M-type SCS in China, a structural deformation accident happened with the maximum raising deformation of 45 mm of the SSC, and it took over four months to remove about 1200 m³ concrete before reconstruction, causing severe economic loss [18].

However, the mechanism behind the accident is understudied and much uncertainty still exists about the construction of such SCSs so far. Moreover, much of the current literature on SCSs focuses more on its performance during operating period under ideal conditions, while unexpected factors concerning the construction procedure is rarely studied. Accordingly, the aim of this paper is to investigate the cause of the accident, as well as several factors concerning this accident. It is hoped that this work will contribute to a deeper understanding of embedment of SSCs in concrete and provide some lessons from this accident to prevent similar accidents in the future.

This paper is organized as follows. Section 2 presents the project profile and an overview of the accident. Section 3 gives an introduction of the numerical model. Section 4 presents the simulation results and discussion. The conclusions are drawn in Section 5.

2. Xiluodu HPP and the Structural Deformation Accident

2.1. Project Profile of the Xiluodu HPP

The Xiluodu HPP is located on the Jinsha River, Southwest China. As one of the largest HPPs in the world, the Xiluodu HPP has eighteen pump turbine sets, and each set has a capacity of 770 MW, totaling to a capacity of 13,860 MW. Pictures of the Xiluodu HPP are shown in Figure 3.

Compared with other SCSs in most medium and high head HPPs, there exists two major differences in the No.9 M-type SCS in the left underground powerhouse of the Xiluodu HPP. First, most SCSs with such high water head are P-type SCSs, which means that they are constructed under pressurized condition. Since the No.9 SCS is M-type SCS, it is constructed in an unwatered condition, and it is rare for HPPs of this scale. If the construction had gone on as planned, this would have shortened the construction time. The second difference is the shape of the SSC sections. Conventionally, all sections of SSC are designed to be circle in shape, which benefits the flow pattern in it. However, in order to consume more water in a limited horizontal range, most sections of the No.9 SSC are quasi ellipses. Figure 4 shows the plain layout and typical section of the SSC, the sections between section 1 and 2 are circular, and other sections are three-centered compound curves.

The diameter of the inlet of the SSC is 7.2 m. The maximum and minimum liner thicknesses of the SSC are 74 mm and 29 mm, respectively. The MDP of the SSC is 2.87 MPa, which is equal to the bursting pressure of maximum headwater plus water hammer under load rejection condition.

2.2. Overview of the Structural Deformation Accident

The construction of mass concrete of the SCS is divided into four layers, and each layer is divided into four quadrants, as shown Figure 5. The construction order is: (a) the first and third quadrants of the first layer, (b) the second and fourth quadrants of the first layer, (c) the second layer, (d) the third layer and (e) the fourth layer.

During the construction of the second quadrant of the first layer, severe structural raising deformation happened, with the maximum deformation of the SSC at about 45 mm, and the stay ring at 23.7 mm. Figure 6 shows the in-field scene after the accident. The deformations at several typical points are marked in Figure 6a. The dotted circle represents the position of the upper stay ring, and the yellow mark shows the position and value of the maximum deformation of the SSC and the upper stay ring.

By the time of the accident, the concrete of the first and third quadrants of the first layer has set, and the following phenomena were observed:

(1)

Structural inclination happened in the SSC and stay ring. The maximum raising deformation of the stay ring is 23.7 mm, which is observed in the second quadrant. The minimum raising deformation of the stay ring is 0.5 mm in the fourth quadrant.

(2)

Several tie rods have got detached from the SSC, and distorting damage of tie rods was observed in the second quadrant.

(3)

Structural distortion of inner supporting rods was observed in the second quadrant.

(4)

Except for raising deformation, elliptical deformation was also observed in the upper flange, as well as the stay ring.

The phenomena above showed that severe deformation happened to steel structures under the effect of external loads in the construction period. It should be noted that the stay ring serves as a significant component in the load-bearing of turbine set. Therefore, considering the need for safe and reliable operation of turbine set, the accident makes it unable to meet the standard of structural deformation control. In order to better understand the cause of the deformation, this accident is numerically reproduced in this study based on in-situ measured data. Meanwhile, the effect of two major differences in this SCS is analyzed and compared. It is hoped that this work will contribute to a deeper understanding of these innovations in high-head SCSs, thus preventing similar accidents in the future.

3. Numerical Simulation

3.1. Numerical Model

The numerical study is based on the ABAQUS software. Figure 7a shows the FE model of the SCS. The concrete is modeled with C3D8 solid elements. The SSC, the test head, the stay ring and the flange are modeled with S4 shell elements. The tie rods and the inner supporting rods are modeled with T3D2 truss elements. The typical element edge length is 0.2–0.8 m.

The role of the inner supporting rods is to help maintain the original shape of the SSC in the construction period, and it is fixed to the internal surface of the SSC. Accordingly, the inner supporting rods are connected to the SSC with shared nodes. The role of the tie rods is to prevent the SSC from uplift in the construction period, and it is fixed to the external surface of the SSC. Therefore, the tie rods are connected to the SSC with shared nodes.

Figure 7b shows the surfaces of contact pairs. Thirteen sets of contact pairs are defined at the interfaces between the SSC and concrete structures. The surfaces of pedestals are defined as slave surfaces of different surface sets, and the surface of the SSC is defined as a shared master surface of all surface sets. The sliding formulation is defined as small sliding. The classical coulomb friction model is adopted to simulate the interaction between the SSC and concrete structures. The friction coefficient is defined as 0.25 with overall consideration of the authors’ previous work [17], as well as the research of Rabbat and Russell [19], Baltay and Gjelsvik [20], Zhang [21], Hu [22] and Zhang [23].

3.2. Material Parameters

By the time of the structural deformation accident, the construction of the concrete has just begun. Therefore, the role of the second-phrase concrete was merely to provide necessary support for the SSC, and no further function was expected of the concrete at this stage. Moreover, the in-situ condition indicated that structural failure was only observed at the rods.

Accordingly, the pedestal concrete is described as linear-elastic, while the steel shells and the rods are described as elastic–perfectly plastic. The elastic modulus of concrete is taken as 25,500 MPa, the Poisson’s ratio as 0.167 and the mass density as 2500 kg/m³. The elastic modulus of the steel shells and the rods is taken as 206,000 MPa, the Poisson’s ratio is taken as 0.3 and the mass density is taken as 7850 kg/m³. The yield strength of the steel shells and the rods are taken as 490 MPa and 275 MPa, respectively.

3.3. Load Analysis and Boundary Conditions

The accident happened at the beginning of the construction of second-phase concrete. Considering the complex structure of the SCS, 24 grouting and exhaust holes were designed on the stay ring. The diameter of the holes was 29 mm, and six of them were within the second quadrant. The accident happened during the construction of concrete in the second quadrant, and the in-situ condition showed that five out of the six holes were blocked. What is more, the concrete in the accident area was high-flowing concrete. Based on the detailed accident conditions stated above, experts on the site believed that the direct cause of this accident was poor quality control of the concrete construction. Under continuous pumping of concrete, the lack of ventilation triggered a boost in the pressure on the SSC, leading to raising deformation [18]. Since the concrete of the first and third quadrants of the first layer has set by the time of the accident, the pressure from external load can only exist in in the second quadrant. Accordingly, the range of the external load is thus determined, and it is in accord with the construction arrangements.

Given that the external load was unexpected, there was not any direct record of its specific value. Fortunately, the range of the external load was explicit, and the authors managed to acquire the equivalent value through back analysis. With the value of the external load at 1.8 MPa, the FE results encouragingly agree well with the in-situ measured data in terms of the raising displacement. Thus, the range and value of the load is shown in Figure 8.

According to the construction procedure of a powerhouse, the mass concrete below the altitude of 354.60 m has already set, as is shown in Figure 5. Meanwhile, the tie rods are fixed to the same altitude during the construction of SCS. Therefore, the mass concrete below the altitude of 354.60 m is considered as a fixed bottom boundary for the model, and displacement constraints are applied to the bottom of the FE model.

3.4. Validation of the Numerical Model

In this subsection, a mesh sensitivity analysis is performed, and the reliability of the current numerical model is validated as well.

Based on the authors’ previous work, structural analysis of SCS can be roughly considered as an axisymmetric problem [8,24,25]. Meanwhile, considering the mechanical behavior of SCS, the mesh size of the SSC is the key dominant factor that decides whether the mesh can yield a proper response of the FEA. Therefore, a mesh sensitivity analysis is performed with an additional model created for comparison. The medium and fine meshes have element edge lengths no greater than 0.8 m and 0.4 m, respectively.

Figure 9 compares the radial displacement, circumferential stress and stress in flow direction of a typical section with different meshes under IWP. The position of the representative points of the section is shown in Figure 4. The comparison shows that the FEA results are hardly distinguishable from each other at most points. Meanwhile, it should be noted that the difference of stress in flow direction at the two points near the stay ring is relatively larger. Nevertheless, the comparison gives a good indication that both the medium and fine meshes can yield accurate FEA results in terms of the displacement and stress of a SCS. In the following analysis, consequently, the current model with medium element size (<0.8 m) is employed.

In the construction period, dial indicators were used to monitor the deformation of steel structures. The arrangement of dial indicators is shown in Figure 10. Twenty-four sets of dial indicators were applied at the stay ring to measure the radial displacement of the upper stay ring, as well as the vertical displacement of the upper stay ring and the upper flange. Compared with the displacement data obtained in the field, no substantial differences are found in the FEA-reproduced results, and most results are similar in terms of their displacement values and distributions. A more detailed comparison and analysis is presented in Section 4.1. Consequently, it is believed strongly that the current numerical model would lend itself well for use by this study.

4. Results and Discussion

4.1. Effect of Construction Condition

In this subsection, the raising deformation of the SSC is numerically reproduced and compared with the in-situ measured data. Meanwhile, in order to investigate the effect of construction condition on this issue, a hypothetical case is also considered in which the temporary IWP is specified equal to 1.60 MPa, according to Chinese design code [26]. Hereafter, the terms unwatered case and pressurized case will be used to refer to the two cases for brevity. The only difference between these two cases is the construction condition. The unwatered case simulates the construction procedure of M-type SCSs, where sealing devices and pressurization is not needed. The pressurized case simulates the construction procedure of P-type SCSs.

Figure 11 shows the raising deformation of the upper stay ring. Overall, although there is some inconsistency between the FEA results and the in-situ measured data, no substantial difference is found in their distributions and ranges. The peak value of the FEA results is 27.58 mm, which is slightly higher than the in-situ measured data. Accordingly, it is reasonable to believe that the assumed load condition can well represent the actual situation. The raising deformation values of more than one third of the notes in the unwatered case are over 10 mm, which makes it impossible for the turbine set to operate in a stable and reliable state.

As for the pressurized case, the peak value is 10.26 mm, which is 37% of that in unwatered case. Except for the node with peak value, the raising deformation values of other nodes are below 10 mm, which benefits the structural stability greatly. The distributions of the raising deformation of two cases are similar, while the values of the pressurized case are much lower than that of the unwatered case.

Figure 12 shows the vertical displacement of the stay ring. The effect of the external load is quite obvious in both cases. The maximum vertical displacement in the unwatered case is 31.10 mm, while that in the pressurized case is 12.95 mm. Meanwhile, negative vertical displacement is observed in the pressurized case, and the maximum negative vertical displacement is 5.14 mm. Accordingly, the temporary IWP in the construction period does help control the raising deformation of the stay ring.

The results indicate that the temporary IWP not only helps create an initial gap between the SSC and the concrete, but also plays an important role in controlling the reliability of the construction procedure.

Moreover, the pressurized case also indicates that construction condition is not the only factor that caused the raising deformation accident, and the effect of external load cannot be simply offset by temporary IWP. This accident is actually the result of multiple factors.

Figure 13 shows the raising deformation of the upper flange. Compared with that of the upper stay ring, there exist several abnormal values in the in-situ measured data. This may be attributed to the structural deformation of the flange itself, which the FEA fails to simulate. Even though, the distribution of the raising deformation of the FEA results is still similar to that of the stay ring, and the peak value of the FEA results is 25.69 mm. However, it should be noted that negative value is observed around section 8, while all the FEA results are positive.

The peak value of the FEA results in pressurized case is 8.49 mm, which is only 33% of that in unwatered case. This result tends to be consistent with that of the upper stay ring, and temporary IWP helps a lot in controlling the raising deformation of the upper flange.

Apart from the deformation of the stay ring and the flange, the structural displacement of the SSC is also worth paying attention to. Figure 14 shows the vertical displacement of the SSC in the unwatered case. The maximum vertical displacement of the SSC appears at the external load area. The peak value of the vertical displacement of the SSC is 41.14 mm, and negative vertical displacement appears at the straight tube area. The result indicates that the accident causes severe localized uplift of the SSC, which needs to be taken care of before reconstruction.

Figure 15 shows the vertical displacement of the SSC in pressurized case. The distribution of vertical displacement of the SSC is similar to that of the unwatered case. However, the peak value drops to 22.45 mm, which is around half of that in unwatered case. Moreover, the maximum negative displacement is 18.96 mm, which is slightly larger than that in the unwatered case. This result verifies that temporary IWP also helps a lot in controlling the uplift of the SSC.

After the raising deformation accident, it is of great importance to acquire the post-accident stresses of steel structures, which determines whether it is necessary to replace them. Moreover, it also has a lot to do with the post-accident assessment of the structures and the rearrangement of the fixing devices.

Figure 16 shows the Mises stress of the SSC in the unwatered case. The stress value is below 257.7 MPa in most area, and the peak value is 385.6 MPa. Meanwhile, it is quite obvious that the stress value at the external load area is reasonably larger. Actually, the SSC has been designed to withstand the bursting IWP of maximum headwater plus water hammer with a conservative allowable stress intensity considered by reference to some design manuals or specifications for steel penstocks [27,28,29]. In this case, the yield strength of the SSC is designed to be 490 MPa. Therefore, the structural strength of the SSC should not be a major concern, but its severe deformation is still in need of repair and assessment.

In fact, however, it is worth mentioning that the reconstruction of this SCS was still carried out in an unwatered condition. With overall rearrangement and improved quality control of the concrete construction, the reconstruction went on well and it has been in operation since then.

4.2. Effect of the Shape of SSC

Another major difference in No.9 SCS of Xiluodu HPP is the shape of SSC. In this subsection, the effect of the shape of SSC under temporary IWP is investigated in multiple aspects. A hypothetical case is considered in which the cross sections between section 2 and section 5 are replaced by circular sections. The temporary IWP of both cases is chosen as 1.60 MPa, according to Chinese design code [26]. Hereafter, the terms non-circle case and circle case will be used to refer to the two cases for brevity.

Figure 17 shows the displacement of the SSC in the longitudinal axis of the powerhouse. The peak value in the non-circle case is 22.46 mm, while it is 6.98 mm in the circle case, which is only 30% of that in the non-circle case. Besides, the peak value appears at different locations in these two cases. It appears at the straight tube area in the non-circle case, while it appears around section 5 in the circle case. Actually, the displacement value around section 5 in the non-circle case is close to that of the circle case. That is to say, the displacement values of most areas are similar in these two cases, and non-circle sections may attribute to the larger displacement of the SSC in the longitudinal axis of the powerhouse.

A marked observation to emerge from Figure 17 is the large displacement at the inlet of the SSC in the non-circle case. In the operating period, the inlet of the SSC will be connected to an expansion joint or a penstock, which requires the inlet of the SSC to maintain its designed position. With such large displacement in non-circle case, it is actually impossible to connect the conduits smoothly. Therefore, an enhanced displacement control at the inlet of the SSC is highly suggested.

Figure 18 shows the displacement of the SSC in flow direction. The peak value in the non-circle case is 6.62 mm, and it spears around section 7. In the circle case, the maximum displacement is 1.73 mm, which is only 26% of that in the non-circle case. However, the distributions of the displacement in the two cases are similar.

Figure 17 and Figure 18 show the overall distribution of structural displacement. However, since the difference between these two cases only exists between section 2 and section 5, a more detailed comparison is necessary.

Figure 19 shows the radial deformation of section 4. The maximum radial deformation in the non-circle case is 15.62 mm, and it appears at the left waist of the section. As for the circle case, the peak value is 6.12 mm, which is 39% of that in the non-circle case. What is more, a marked observation to emerge from the data comparison in Figure 19 is the difference in the distribution of radial deformation. In the circle case, the radial deformation in the right half of the section is small, especially in the lower half. Overall, the section tends to maintain its original shape with slight lateral displacement under the effect of the temporary IWP. In contrast, relatively severe structural distortion happens in the non-circle case. Negative radial deformation is observed at both the upper and lower half of the section, and the peak value reaches 14.05 mm at the upper half. In general, the shape of the section in the non-circle case tends to get rounded under the effect of the temporary IWP.

In order to evaluate the stress condition of these two cases, Figure 20 shows the Mises stress of section 4. The value and distribution of the Mises stress are similar, with the stress value of the non-circle cases slightly larger. This happens because the loads of these two cases are actually the same, and the differences should be attributed to the shape of the sections. Besides, the stresses of both cases are at a relatively low level, indicating that the structural strength of both cases should not be a major concern.

Apart from the issues mentioned above, it should be noted that the stay ring plays an important role as the foundation of the turbine set and one of the main force-transferring structures. Accordingly, the deformation control of the stay ring is of critical importance in the safe and reliable operation of turbine set. Figure 21 shows the displacement of the inner line of the upper stay ring in a horizontal plane. In section 4, the radial displacement is negative in the non-circle case, while it is positive in the circle case. In general, there exists a tendency towards getting elliptical in both cases. However, the deformation in the non-circle case is more severe, and there also exists a slight difference where the displacement is towards the water intake direction in the circle case, while there is approximately an angle of 15° in the non-circle case.

The results reveal that the deformation of the stay ring needs more attention in the construction period, no matter the shape of the SSC. However, it should not be neglected that the deformation in the non-circle case is more severe than that in the circle case.

Figure 22 shows the Mises stress of the stay ring in two cases. Significantly, the stress values and distributions of these two cases are similar, with the peak value of the non-circle case slightly higher. The calculation results in Figure 22 suggest that the shape of the SSC has little effect on the stress level of the stay ring.

Overall, the SSC with non-circle sections requires higher standard for construction, and SSCs with both non-circle and circle sections are actually feasible with high-quality construction management.

5. Conclusions

This paper seeks to reveal the cause of a raising deformation accident of SCS in the construction period. The accident is numerically reproduced, and the results agree well with the in-situ measured data at multiple key aspects, such as the raising deformation of the upper stay ring and the upper flange. This numerical study has identified two important factors: (a) the construction condition (unwatered or pressurized), and (b) the shape of SSC.

The conventional design philosophy of temporary IWP is to create an initial gap between the SSC and the concrete. However, the findings of this study show that the temporary IWP also helps a lot in controlling the raising deformation of the structure. Even under such high external load of this accident, the temporary IWP managed to reduce the raising deformation to approximately one third of that in the unwatered condition. Moreover, it should be noted that the post-accident stresses of the SSC are still well below the yield point of steel. However, considering the need for safe and reliable operation of turbine set, such severe deformation is still unacceptable, and measures should be taken to restore the original state of steel structures. Accordingly, all the second-phase concrete should be removed, and a thorough check of steel structures is needed. Stronger restrains and better ventilation should be adopted for construction, and in-situ monitoring should be enhanced as well.

Meanwhile, the study has shown that the shape of SSC is also a major factor accounting for the severe deformation. Under temporary IWP, the SSC with circle sections tends to maintain its original shape with slight lateral displacement, while the SSC with non-circle sections tends to get rounded, leading to severe deformation. As for the upper stay ring, the displacement of the non-circle case is larger than that of the circle case, and the location of the maximum displacement is different. There is, therefore, a definite need to strengthen the restrains on the stay ring in the construction period, even for a conventional circle-section SSC.

The major limitation of this study is that this study merely focuses on the construction period. If such SCSs were to be used in a wider range, a follow-up study focusing on the operation period should be considered. Despite its exploratory nature, this work offers valuable insights into the construction of non-circle M-type SCSs, thus helping practitioners acquire precious lessons from this accident and prevent similar accidents in the future.