envirtech abyssal remote controlled moored ctd-adcp chain to measure mindanao current

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Envirtech SpA Venice, 14th April 2010 Envirtech Code: 29111-OFF-001.8

Abyssal moored CTD/ADCP to measure Mindanao Current

ENVIRTECH Code 29111-OFF-001.8

Date 14/04/2010

Abyssal moored CTD/ADCP Chain to measure Mindanao Current

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1 TECHNICAL DESCRIPTION This paper relates a multipoint abyssal moored CTD/ADCP to drive an exhaustive study of the Mindanao Current collecting 3D parameters to model the seasonal current behavior. The Mindanao Current is an ocean current along the eastward side of the southern Philippines. It is a narrow, southward flowing current starting at 14°N with 13Sv increasing in strength to 33Sv at 5.5°N. The current splits before the coast of south-east Mindanao where one part flows cyclonically into the Celebes Sea later feeding the North Equatorial Counter Current (NECC) while another part feeds the NECC directly.

The basic problem is how to recover collected data on a periodic base to avoid data lost in case of moored equipments accidents. In order to measure

• CTD data • Current data (up and down)

in the water column close to the eastern side of Mindanao it is necessary to deploy an instrumented mooring line with a dead weight at 6000m of water depth.


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The configuration of the system is depicted in the following picture where the following main parts can be identified:

- a stopper buoy at 400mwd with a buoyancy of 1200-1500kg, (a cylinder with diameter of 70cm and length of 4.5m) integrating a frame in AISI316-L or in Aluminum hosting

o power/acquisition electronic unit integrating a § battery pack with an autonomy of 1 year § Main electronic Data Acquisition Unit (M-DAU)

o underwater winch with 300m of metal cable plastic impregnated o a magneto inductive link connecting the M-DAU with the Data Transmission Unit

(DTU). o The DTU is composed of an electronic housing integrated in a floating part that can

come on surface a time per month as programmed in the M-DAU. When the DTU is on surface it can transfer the data via satellite modem (Iridium modem) to a standard PSTN modem (or equivalent solution) of a dedicated land base station.

- The mooring line up to 1600mwd is composed of a plastic impregnated metal cable that allows the magneto inductive link between the M-DAU and the instrumentation along the mooring line. The remaining part from 1600mwd to 9800mwd is composed of a synthetic rope. The motion due to the currents and waves are accommodated by elastic stretching of syntetic rope, which is chosen for its low elastic spring constant.

- An acoustic release in tandem configuration at 6000mwd allows to recover on surface the instrumented part of the mooring, the buoyant, the electronic unit, the winch and the DTU.

- The instrumentation is composed of

o 6 CTD with Magneto Inductive interface installed at 400, 550, 700, 850, 1000, 5500mwd.

o 3 single point Doppler current meter installed at 700, 1200 and 5500mwd. o 2 CT of installed at 1200 and 1500mwd. o 2 long range ADCP 75kHz installed at 400mwd (one looking up and one looking

down)

All instruments are equipped with internal battery pack to get an autonomy of acquisition of 12 months. The data link between each instrument and the M-DAU is relied on magneto inductive modems through the upper part of the mooring line made of metal cable plastic impregnated.


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System Configuration


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The main functionalities provided by the system are the following:

- Hourly / 3 hours acquisition of sea current and CTD data; - Logging of all acquired data (1 year); - Monitoring of underwater diagnostic data (battery voltage and current, water alarm, internal

pressure and temperature); - Transfer of summary data (a reduced data set to be detailed in detail design phase) a time

a month via satellite modem to a dedicated land station. All logged data will be downloaded at the recovery of the system on surface. The whole system is designed to facilitate the localization in the recovery phase: a beacon with light and satellite link will be integrated in the upper part coming on surface.


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2 RESULTS OF ANALYSIS OF THE FORCES The effects of gravity and viscous drag as well as currents have been included in this analysis.

We considered the worst hypothesis with a uniform current speed of 104,0 −= msvc that works on the mooring line from 200m to 5000m depth and a current speed of 0,2m/s on the underwater buoyancy.

The net buoyancy at the offered buoy is in the range 12-14 KN whereas the total drag force is about 400-500N. Thus the tilt angle due to the sea current is about 2° respect to the vertical line.

Taking into account the high ratio Net Buoyancy / Drag Force the oscillation of the mooring line can be neglected. Anyway the instrumented platform over the buoyant located at 200mwd is equipped with tilt and heading sensor to verify these movements and to compensate the measurement of the current speed. The static analysis demonstrates that the max tilted angle can be 2 deg. In order to reduce this angle it is possible to increase the net buoyancy of the whole system.

Buoyant force

Drag force

Resultant force

6000m of mooring line

°< 2α


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3 FEATURES OF THE MAIN COMPONENTS In this paragraph the features of the main components are described

3.1 CTD with MI interface

The CTD selected has a Magneto Inductive interface and is equipped with a Conductivity, Temperature and pressure sensors having the following features.


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3.2 Profiler current meter (ADCP)

It is the model operating at 75kHz or 150kHz able to measure the sea current on a range of 900m or 600m with maximum cell size of 32m or 16m and cells number 170. The instrument integrates also tilt, heading and water temperature sensors. The accuracy is 1% with a maximum speed of 20knot. Two ADCP will be integrated in the underwater instrumented module: one looking up and the second looking down.

ADCP 75kHz and 150 kHz

3.3 Single Point Doppler Current Meter

The sensor utilizes the well-known Doppler Shift principle as the basis for its measurements. The sensor transmits acoustic pulses of 2MHz into the water in sequence. As the sound propagates, small particles or air bubbles in the water reflect a portion of the energy. The back-scattered energy from the area between 0.4 to 2,2 meters from the sensor is picked up by the transducers and analyzed to find any change in frequency. The current direction is found by taking the measurements along two orthogonal axes, x and y. These measurements are compensated for tilt by use of an electrolytic tilt sensor and referred to magnetic North by means of an internal Hall-effect compass. A microprocessor computes vector averaged current speed and direction over the last sampling interval. Four piezoceramic acoustic transducers are placed 90° apart around the circumference of the sensor that ismolded in a polyurethane material.


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RCM11 Current Meter


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3.4 Thermoaline sensor

This is a temperature and conductivity meter of high accuracy that has seven electrodes internal sensor for the conductivity sensor. This equipment has 2,000m water pressure resisting performance, and can be used for the observation of deep sea.


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3.5 Acoustic release

The Acoustic Release System provides a reliable and secure acoustic link permitting the relocation, and recovery of instrument packages or other item from underwater locations up to 8000 meters deep. The Release receives coded acoustic signals and processes them. If the received code matches any one of the commands performed by the release, the microprocessor performs that function. In order to increase the reliability of the recovery system, two releases in tandem configuration will be adopted.


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3.6 Frame, Electronic module, Underwater winch, Transmission Unit

A frame in Aluminium or Stainless steel AIS316L integrates the following parts:

- Electronic Unit M-DAU to acquire and to store all CTD, ADCP, 3D-Single Point Current Meter and diagnostic data of one year. Battery pack (lithium battery) integrated in the protective housing of the M-DAU.

- Underwater winch with the following main features:

o Construction in stainless steel AISI316L (drum, frame, motor, electronics enclosure); o Submersible Motor DC brushless in stainless steel housing filled with mixture of

water and glycol and pressure compensated. o Two stage worm-gear reducer oil filled and pressure compensated. o Breaking provided by worm-gear reducer thus no external breaking is required. o Light metal cable plastic impregnated to allow magneto inductive communication

between M-DAU and DTU. o Winch operations are controlled by M-DAU. Feedback for DTU positioning is

provided by pressure sensor and limit switch.

- DTU is the data transmission unit composed of o A protective housing hosting the electronic unit, the mass memory for summary data

logging, the magneto inductive modem for data link with M-DAU via winch cable, an Iridium satellite modem for data transfer to the land station.

o An external foam body providing a net buoyancy of 10kg to allows the DTU coming on surface.

o External satellite antenna and pressure switch powering on the Iridium modem only on surface.

A layout of the underwater winch is depicted hereinafter.

Underwater winch


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The transfer of data will be carried out via magneto inductive link from the M-DAU to the DTU and from DTU to land station via satellite link. The adoption of the Magneto Inductive link allow to avoid Electro Mechanical cables in the underwater winch that are critical to be used in the sea surface layer. Also underwater electrical slip ring in the winch can be avoided that are expensive and not suitable for long term applications. The DTU will be powered by a dedicated internal primary battery pack (Lithium battery). The Iridium modem will powered only when the DTU is on surface (it will be activated by a dedicated pressure switch).

End document

envirtech abyssal remote controlled moored ctd-adcp chain to measure mindanao current

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