Development of a Port Facility Diagnostic System that Utilizes Data Measured by Strong Motion Seismographs

Abstract

The Authors developed a damage assessment system for mooring facilities. This system uses information on seismic motion observed by strong motion seismographs when an earthquake occurs in order to determine whether or not mooring facilities can be used based on the degree of damage suffered. The system creates maps estimating damage based on the results. Two methods are available for determining usability—immediate diagnoses and detailed diagnoses—and all calculations are performed automatically, from the acquisition of the seismic motion information to the determination of usability. The resulting maps estimating damage can be used to select surveyed facilities for on-site damage assessments, making this system an effective tool for supporting initial response systems after a disaster strikes.

1 Introduction

In Japan, there is concern that large-scale earthquakes will occur in the near future. When large-scale earthquakes occur, road and railway networks can become disrupted in many sections. Waterborne transport, including IWT, is therefore expected to play a crucial role in transporting emergency supplies. Prompt resumption of trunk line waterborne transport is also important for the economic recovery in disaster-affected areas. In order for waterborne transport to function properly in the event of a disaster, it is necessary to know which facilities at what ports can be utilized. Conventionally, this information has been obtained solely through field surveys after an earthquake. The Authors developed a diagnostic system for assessing the degree of damage to port facilities with the aim of streamlining on-site work at quays and facilitating the early resumption of waterborne transport.

This paper presents an example of a study on ports in Japan. The examples presented are expected to prove beneficial in the field of waterborne transport, which mainly involves large-scale canals. It is the Authors’ hope that this report will be helpful in the development of future disaster response measures in the field of waterborne transport.

2 Three Challenges with Field Surveys Following Large-Scale Earthquakes

Generally, the decision on whether or not mooring facilities can be used following a large-scale earthquake are based on the results of field surveys. However, there are major challenges to conducting these field surveys.

• (1) For piers and sheet pile quays, the degree of damage to underground and underwater areas cannot be ascertained with field surveys (simple visual surveys) immediately after a disaster occurs. For example, steel pipe piles at a facility that appeared sound had buckled, creating extremely dangerous conditions; members had no residual bearing capacity (Fig. 1).

• (2) The survey of damage to mooring facilities at larger ports requires considerable time and person-hours, demanding greater efficiency.

• (3) When a tsunami warning or advisory is issued, field surveys cannot be conducted immediately because people cannot approach coastal areas until the warning or advisory is lifted.

Fig. 1.

Example of damage to a pier following the 1995 Hyogo-ken Nanbu earthquake

3 Development of a Port Facility Diagnostic System

3.1 Outline of Port Facility Diagnastic System

Strong motion seismographs are installed at critical ports across Japan (Fig. 2), allowing seismic tremors to be observed. The overall process behind a Port Facility Diagnastic System (PFDS) is shown in Fig. 3. When an earthquake occurs, the data observed by the strong motion seismographs is stored in a data server at the National Institute of Maritime, Port and Aviation Technology, Port and Airport Research Institute (PARI). An e-mail notification is sent out indicating that an earthquake has occurred ((1), (2) in Fig. 3). After this email is distributed, PFDS accesses the PARI data server to obtain the observed seismic data ((3), (4) in Fig. 3). The obtained seismic waveforms are those observed on the surface or underground. The seismic motion used in the field of seismic design of port facilities in Japan is generally assumed to be the waveform in the engineering bedrock. In seismic engineering, the engineering bedrock refers to the boundary layer between the surface (soft ground), which has a significant influence on vibration characteristics, and the deeper area (hard ground), which has less influence on vibration characteristics. It is established for convenience. For the evaluation of the mooring facilities, a one-dimensional seismic response analysis is conducted using the equivalent linearization method (Yoshida 1996), which can consider the frequency dependence using the ground model for the observation point. The observed waveform data is used after being converted into an earthquake waveform in the engineering bedrock. The facility diagnosis registered in PFDS is performed using the obtained seismic data on the engineering bedrock, ((5), (6) in Fig. 3). There are two types of facility diagnostic methods. The first is an immediate diagnosis, where the diagnosis is performed immediately after an earthquake occurs. The second is a detailed diagnosis, where a detailed diagnosis is performed by conducting a two-dimensional seismic evaluation of the facility in question. Details are described below. After facility diagnoses are performed, a map estimating the damage is created for each registered facility at an individual port, and the map is distributed to the emergency operations center ((7), (8) in Fig. 3). PFDS performs these steps automatically.

Note that in the Japanese port sector, the ground vibrations are studied and divided into zones based on similar vibration characteristics. In the example of Shimizu Port in Japan shown in Fig. 4, the applicable area of strong motion seismograph observation data ranges from about 3 to 10 km.

Fig. 2.

Strong motion earthquake observation points in port areas and an example of strong-motion seismograph installations

Fig. 3.

The overall process behind the Port Facility Diagnostic System

Fig. 4.

Zones with similar seismic characteristics (Example: Shimizu Port)

3.2 Development of Diagnostic Methods

In the Japanese port sector, FLIP, a two-dimensional seismic evaluation code (Iai et al. 1990), is used as a seismic inspection and design tool for mooring facilities assuming large-scale earthquakes. FLIP is a seismic evaluation program developed to predict structural damage due to liquefaction. This analysis code is an analysis program based on the finite element method. It enables deformation analysis considering the dynamic interactions between the ground and the structure by modeling the port structures and the ground in an integrated manner. In addition, the analysis can reproduce the damage to port facilities caused by large-scale earthquakes in the past with a high degree of accuracy. The analysis code is extremely effective in performing detailed seismic inspections of facilities. A diagnostic method for facilities that utilizes FLIP was therefore developed in PFDS.

3.2.1 Immediate Diagnostic Method

Performing a FLIP analysis—used in detailed diagnoses—can provide detailed estimates of damage to each member component comprising a facility. However, doing so requires several to more than ten hours of analysis time, whereas immediacy is demanded following an earthquake, Therefore, FLIP analyses were conducted for all facilities registered in PFDS beforehand, looking at seismic motions of various sizes with earthquakes of different magnitudes. The results of these analyses were compiled into evaluation curves that show the relationship between the magnitude of the earthquake and the degree of damage to the main member components of the facility structure (Fig. 5 and Fig. 6). The immediate diagnostic method uses these evaluation curves to perform diagnoses. The PSI of the velocity is used as an indicator of earthquake magnitude. The PSI of the velocity is defined by Eq. (1) (Nozu and Iai 2001) and is a seismic motion index that is highly correlated with the deformation of port structures and can be easily calculated from observed seismic data. A typical example of the extent of damage to main member components of pier or sheet pile quay structures is the maximum curvature ratio of steel pipe members, as defined by Eq. (2). For example, according to the technical standards and explanations of Japanese port facilities (Japan Port Association 2018), if the maximum curvature ratio of a sheet pile quay is less than 1.0, the mooring is structurally stable and can be judged to have residual bearing capacity. In other words, if the maximum curvature ratio exceeds 1.0, this indicates the steel pipe members are in an extremely dangerous condition, as they have lost their residual bearing capacity and have plasticized. The steel members are judged to be unusable. In addition to this structural judgment, as a utilization judgment, it is assumed that vessels can berth if the residual horizontal displacement at the top of the facility is 2.0 m or less. Judging using the evaluation curve shown in Fig. 6, a PSI of the velocity of 61 cm/s^0.5 or less is considered provisionally usable, while 61 cm/s^0.5 or greater is considered unusable.

Fig. 5.

Example of an evaluation curve used in the immediate diagnostic method (1) Relationship between PSI of the velocity and residual horizontal displacement

Fig. 6.

Example of an evaluation curve used in the immediate diagnostic method (2) Relationship between PSI of the velocity and maximum curvature ratio

$$ {\text{PSI}}\;{\text{of}}\;{\text{the}}\;{\text{velocity}} = \sqrt {\int_{0}^{\infty } {v^{2} (t)dt} } $$

(1)

where, v(t): velocity of earthquake motion at each time (cm/sec)

dt: integration time (sec)

$$ {\text{Maximum}}\;{\text{curvature}}\;{\text{ratio}} = \frac{{\varphi_{max} }}{{\varphi_{u} }} $$

(2)

where, φ_max: maximum curvature generated in steel pipe members based on FLIP analysis results (1/m), φ_u: limit curvature of steel pipe members (1/m).

3.2.2 Detailed Diagnostic Method

Detailed diagnostics is a method of analysis using FLIP based on pre-created analysis models. The analysis uses seismic waveforms converted from observed seismic waveform data to waveforms in the engineering bedrock. A residual deformation diagram is shown in Fig. 7 as an example of the analysis results. In addition to residual deformation, the degree of damage to each of the other main components can be estimated. For mooring facilities with container cranes, the container cranes are also modeled and analyzed using FLIP. This gives the maximum response acceleration at the center of gravity mass of a crane. This maximum response acceleration can be used to verify the crane’s lifting limit acceleration and design seismic intensity.

3.3 Creation of Maps Estimating Damage

An example of output results of immediate and detailed diagnoses as a damage estimation map is shown in Fig. 8. It is color-coded according to the usability classification. The map is intended to be used for reporting to the emergency operations center in the event of a disaster.

Fig. 7.

Example of FLIP analysis results (added to the residual deformation diagram)

Fig. 8.

Creation of Maps Estimating Damage (Example: Shimizu Port)

3.4 Addressing Challenges (Effectiveness of PFDS)

The challenges facing field surveys described in 2. were solved through the development of PFDS as follows.

• (1) Ascertaining damage underground, where judgements are not possible with visual surveys: PFDS, using FLIP analyses, has made it possible to ascertain the extent of damage to underground components.

• (2) Field surveys require considerable time and person-hours: the maps estimating damage created by PFDS makes it possible to improve the efficiency of field surveys, shorten the time required, and save labor.

• (3) When a tsunami warning or advisory is issued, field surveys cannot be conducted for a certain span of time: immediate and detailed diagnoses can be performed by PFDS during these spans of time. Therefore, the time lost due to tsunami warnings, etc. can be reduced.

4 Conclusions

PFDS brings together, and was developed based on, current technologies, including strong motion seismographs installed in port areas, records of damage to port facilities to date, a two-dimensional effective stress seismic evaluation program (FLIP) that can accurately reproduce the extent of damage to mooring facilities caused by earthquake motion, and the latest research results on the damage mechanisms seen in port facilities. Although a comprehensive decision on whether or not a facility can be used must be made after confirming the damage on site, PFDS can be used to narrow down the list of facilities that have a high possibility of being usable from among many facilities, enabling more efficient and effective field surveys.

Even in countries where there are insufficient seismographs installed, it may be possible to develop a system with the contents presented in this paper by collaborating with local weather stations and existing observatories. It is the Authors hope that our study will help the development of waterborne transportation in times of disaster.