Envirtech Tsunami Warning System (eTWS)

(This paper has been extracted by presentation to United Nation – Unescap - High level Expert Group Meeting on technical options for disaster management systems - Bangkok June 22, 2005)

Introduction:

In this technical sheet an innovative and reliable system able to forecast the arrival of a tsunami is described. It is the result of several experiences of the Italian company Envirtech SpA in marine project combining the company competence in the scientific/environmental field, the newest offshore technologies, the more advanced electronics and data communication solutions and the knowledge of logistic/operational aspects related to the installation of sea surface and underwater systems.

Architecture:

The system is composed of the following main parts:

* An underwater monitoring module (UM) installed at the seabed;
* A surface buoy (SB) moored in the area of the UM;
* An “in water” communication segment connecting the UM with SB;
* An onshore centre (OC) hosting a standard PC server;
* A satellite communication segment connecting SB and OC.

Two solutions are available:

TWS-M with a magneto-inductive data link between UM and SB: in this case a galvanized metal cable with a plastic cladding is used to connect SB and UM. This cable is used to deploy/recover UM and as mooring line for the SB: in this solution the UM hosts some ballasts to work also as anchor for the SB.

TWS-A with an acoustic data link between UM and SB: in this case the SB requires a proper nylon mooring line and dead weight whereas the UM is a free fall installation and pop up recovery system with a proper ballast, buoyancy and acoustic release.

The solution TWS-M is suitable for installation sites with water depth up to 3000m. For deeper sites it is suggested the TWS-A solution as the size of the metallic cable increases significantly and so the total cost of the system and related installation infrastructure.

The main advantages of the TWS-M solution respect to the TWS-A are:

* The availability of a quicker and more reliable “in water” data link;
* The possibility to use of a single cable for a controlled installation/recovery of the UM and the SB mooring;

In the TWS-M configuration the frame of the UM is made of stainless steel and its total weight has to be increased with additional ballast in order to assure its stability at the sea bed in case of movements of the SB and mooring cable induced by the water waves and currents.

For the TWS-A configuration the weight of the UM is minimised using an aluminium frame. Also in this case additional ballast at the bottom of the frame is required (~400 kg): it is released (on acoustic command from the surface) and left at the seabed at the recovery phase when a dedicated deep-water buoyancy system pops up the UM on surface. The buoyancy system is composed of the same high resistant foams used during the installation of the sub sea production systems in the offshore industry.

Underwater Module:

The UM is composed of an aluminium or stainless steel frame with four legs for a stable placement at the seabed. A 3D view of the UM is reported in picture where the following main parts of the system are showed:

* A titanium vessel containing the UM electronic Data Acquisition and Communication System (UM-DACS);
* A titanium vessel containing an high capacity and reliability primary Lithium battery pack: the frame is configured to host two battery vessels (the green ones) to increase the autonomy up to 42-48 months;
* The instrumentation composed of:
1. A heading sensor and tilts sensors inside the electronic vessel.
2. Two Paroscientific pressure sensors (1 ppm of resolution);
3. An hydrophone (optional);
4. A broadband three components seismometer (optional);
5. An altimeter (echo sounder) useful during the deployment phase;
* The “in water” telemetry system composed of a bi-directional magneto-inductive data link through the buoy mooring line or by a bi-directional multi-modulation acoustic link.

Surface Buoy:

The SB is composed by a metallic pole and a foam body having a diameter of 1.45 m. The main parts installed on the buoy are:

The electronic box containing the SB Data Acquisition and Communication System (SB-DACS) relied on the same type of electronics of the UM;

An autonomous power supply system composed of 3 photovoltaic panels (12V- 50W each) and a gel battery pack (12V- 400Ah);

A magneto-inductive surface modem or the acoustic modem for the data link with the underwater unit;

A satellite modem Inmarsat C for reliable data connection with the Onshore Centre (OC);

A meteorological station (optional);

A multi-parametric probe to monitor the main physical/chemical properties of the surface water layer (optional).

System Functionalities:

The TWS provides the main functionalities listed below:

F001: continuous measurement of the sea bottom pressure with a rate of 15s, 30s, 1min, 2min, 5min selectable be the user in the OC. Optional monitoring of earthquakes occurence.

F002: on line processing of the pressure data with a digital Kalman filter to detect a frequency component typical of a tsunami; the thresholds for the detection of tsunami waves can be configured by the OC user.

F003: the beginning of a possible event is automatically triggered by the pressure sensors (able to detect earthquake waves) and also by the hydrophone/seismometer if installed in UM.

F004: the UM can start the tsunami detection algorithm also on user request from the OC in case of identification of seismic activity in the interested area.

F005: daily synchronisation of the SB and UM clock with the GPS.

F006: self-diagnostic and periodical notification to the OC.

F007: internal logging in UM and SB of all acquired data, all detected events, all diagnostic status and exchanged messages (black box).

F008: remote configuration of the UM (change of communication settings, filtering parameters, on/off of sensors and devices, software updating).

F009: reception of commands from OC and notification of its execution;

F010: reception of data request from OC and reply with the requested data.

The main scenario in case of detection of an anomaly in the pressure signal is the following:

1. The UM-DACS in its standard operating mode IDLE MODE detects an unexpected variation in the pressure signal;

2. A notification message is sent to the OC and the UM-MODULE changes in the new status ALARM MODE;
3. In ALARM MODE the UM sends periodically a message to the OC: on request the user in the OC can transfer all pressure data acquired in ALARM MODE.
4. In case of detection of a tsunami events (frequency component in the range 0.01.0.0005Hz) an TSUNAMI DETECTION message is sent to the OC.
5. The user in the OC can verify the pressure data acquired during the ALARM MODE to validate the alarm condition and to verify its amplitude.
6. After the decrease of the tsunami wave components under some minimal threshold (parameter remotely configurable by the OC user) and after a period of some hours (parameter remotely configurable by the OC user), the UM chages from ALARM MODE to IDLE MODE.

Comparison with NOAA DART system:

Respect to the NOAA DART system, the Envirtech TWS implements more reliable and robust technologies/solutions: the main differences are summarised here below.

1. In the Envirtech TWS a dedicated mechanical frame for the UM has been designed and developed: in the DART system the supporting frame for sensors and electronics is a sort of simple table not able to protect adequately the devices;

2. Envirtech TWS does not use glass spheres that can be dangerous during the installation and recovery phase: the implosion of a glass sphere can damage the other devices;

3. Robust titanium vessels are used in Envirtech TWS.

4. High reliability and energy capacity primary Lithium battery are used in Envirtech TWS respect to the standard Alkaline D-Cell powering the DART.

5. The electronics of Envirtech TWS system is relied of ARM microprocessor with RISC architecture whereas the DART uses a Motorola 68332 that is becoming obsolete.

6. In the Envirtech solution a well-integrated seismometer can be used to implement event driven solutions capable to increase alarm reliability and tsunami-genic events models tuning.

Probably the cost of the hardware of a DART system is more convenient respect to an Envirtech TWS but the reliability of the whole system during the installation/recovery phases and during the working period at the seabed is not comparable with Envirtech TWS solution.

Taking into account the type of application of these systems (they are installed to save human life) and the high logistic costs for their installation and maintenance in open sea/ocean, a significant increase in the system reliability deserves higher priority respect to an initial partial reduction of materials expenditures.