RAPID MOC

 

Home

 

Overview

 

Instrumentation

Intro | Florida Current | Western boundary | US buoy | Eastern boundary | Archive

Telemetry system

Intro | United Kingdom | United States

Data

Intro | Florida Current | Western boundary | US buoy | Eastern boundary | Archive | Time series

Restricted access — project participants only

Intro | Western boundary | US buoy | Eastern boundary | Archive | Data download

     Real time data of the RAPID MOC monitoring array at 26.5°N

Telemetry system - United Kingdom

Information about the design of the telemetry system and planned future adaptions can be found here.

Go to the top of this page

The Mark 1 design

The Mark 1 telemetry system design was completed in time for deployment on the RAPID MOC mooring array in Spring 2004. The complete system comprised self logging inductive instruments on an insulated mooring wire, with a surface telemetry buoy relaying the data from these instruments to the shore station. There were also inductive swivels to allow the mooring to rotate whilst still maintaining the electrical link, along with the mooring tether which continued the link to the surface buoy from the main subsurface buoyancy.

Mark 1 buoy ready for deployment.
© Mark 1 buoy ready for deployment.

The telemetry buoy in the Mark 1 design consisted of a 0.8 m diameter sphere with a stainless steel lower section topped by a transparent Perspex dome. The hull had two penetrations; one for the inductive link to the mooring, and the other for an RS-232 data port to allow communications on deck. The buoy’s electronic brain consisted of a Persistor CF2 microcomputer, which controlled communications with the inductive moored instruments, data telemetry via an Iridium transceiver, and a GPS receiver. There were also interfaces with the battery monitor and internal temperature sensors.

The Iridium system was used to telemeter the scientific data. The system places a phone call to a dedicated number at the National Oceanography Centre, Southampton (NOCS) to transfer large amounts of data cost effectively. Smaller amounts of engineering data were to be sent on a more regular basis through emails using the Iridium Short Burst Data message system. Power was provided by two 80 Ah lead acid batteries charged through four 10 W solar panels mounted at the top of the buoy chassis.

Mark 1 tether laid out on deck ready for deployment.
© Mark 1 tether laid out on deck ready for deployment.

A load bearing nylon rope tether with support floats connected the subsurface buoyancy and telemetry buoy, and supported a two-core conducting cable along which the data are passed to the telemetry buoy. The conducting cable was longer than the load bearing rope to allow slack when the rope was under tension. Both ends of the tether were terminated and potted to provide an insulated electrical connection to the telemetry buoy and the uppermost inductive swivel.

To allow free rotation of the mooring under varying current conditions, and still maintain an electrical conduction, it is necessary to use inductive swivels. The original swivels were designed by Elkins Engineering Services Limited and comprised an inductive link slip ring assembly in a titanium pressure balanced casing. Impulse bulkhead connectors were fitted, and the connecting leads fitted at NOCS.

Mark 1 swivel assembly and mooring wire termination.
© Mark 1 swivel assembly and mooring wire termination.

The main buoyancy of the mooring was provided by a 41 inch diameter steel sphere, which had the inductive swivels fitted above and below with a tube running through the buoy to hold the inductive linking wire. Below this sphere there was a continuous length of 3/16 inch jacketed mooring wire. The top end was terminated and potted to create an insulated join to the swivel, whilst the lower end was un-potted to allow a sea-earth loop back to the hull of the telemetry buoy.

Inductive instruments were clamped to this mooring wire at predetermined depths. Sea-Bird offers the only commercially available inductive telemetry system with a proven track record. The surface inductive modem board was fitted in the telemetry buoy allowing communication with Sea-Bird SBE37 IMP MicroCAT instruments on the mooring wire. Sontek/YSI negotiated with Sea-Bird to include the inductive modem capability in their Argonaut MD current meter, and these were also deployed on the mooring. See the Instrumentation Section for more information on the inductive instruments used.

Go to the top of this page

Summary of Mark 1 deployments

Imploded Mark 1 telemetry buoy.
© Imploded Mark 1 telemetry buoy.

The system deployed in the Eastern Atlantic near the Canary Islands proved the telemetry buoy electronics worked, with data received from instruments as they were attached to the mooring. However, when the anchor was launched, communications between the moored instruments and the telemetry buoy ceased. This was thought to be caused by a failure of the inductive swivel connector. The buoy continued transmissions of its engineering data until the mooring was damaged through suspected trawling activity approximately 1 month after deployment. Unfortunately the telemetry buoy was not recovered in the Spring 2005 service cruise as the mooring wire had been severed.

The system deployed in the Western Atlantic near the Bahamas was dragged down by the strong currents after deployment. This submerged the buoy past its implosion depth and it was destroyed. To combat the problem of mooring knockdown in the strong currents experienced in the west the telemetry buoy was redesigned to withstand submergence to at least 500 m.

Go to the top of this page

The Mark 2 design

Cutaway design of the Mark 2 telemetry buoy.
© Cutaway design of the Mark 2 telemetry buoy.

The Mark 2 design is constructed from a 1 m diameter syntactic sphere assembled in two halves. Inside the buoy are two pressure cases; one for the single 100 Ah sealed lead acid gel battery, and the other for the electronics. The battery is charged through four 10 W solar panels that have been potted and mounted to the outside of the buoy.

The electronics casing is topped by a Perspex dome to allow the antennae to “see” the sky, and both pressure cases and the solar panels were pressure tested to an equivalent depth of 500 m prior to assembly of the buoy. The syntactic buoyancy has been painted with an anti-fouling paint that produces a smooth surface to prevent growth.

The buoy electronics are only slightly modified from the Mark 1 system as these had been proven to work well. The modifications included a pressure sensor to record the depth of the telemetry buoy if it became submerged, and a pitch, roll and compass sensor. As before, data telemetry to shore is a mixture of Iridium dial-up and email messages.

Mark 2 telemetry buoy during dock test.
© Mark 2 telemetry buoy during dock test.

The tether linking the subsurface buoyancy with the telemetry buoy is now 500 m long and is stronger, using a wire core encased in Kevlar. The greater length should allow the buoy to resist being pulled under the surface. The new material of the tether reduces drag as the inductive cable is now also load bearing and therefore there is not the need for a second line. The tether support buoyancy used in the Mark 2 design is also lower drag than in the Mark 1 design.

Termination of the tether is a simple potted termination to a spade fitting at the lower end, with the swivel being the fork part of the link. For the upper end that goes to the telemetry buoy, a friction system has been designed to take the load with the tether wrapping around the diameter of the battery case.

Mark 2 swivel assembly with fixed mounting to subsurface buoyancy and robust electrical connections.
© Mark 2 swivel assembly with fixed mounting to subsurface buoyancy and robust electrical connections.

The swivel connections have been made more substantial to prevent a reoccurrence of this failure mode on deployment. The swivels themselves have also been redesigned by Van De Weyden Engineering and are firmly mounted above and below the subsurface buoyancy to reduce the degree of free sideways movement whilst still allowing full rotation. The connecting electrical cable runs through a free-flooding channel in the syntactic, which also can withstand greater submergence than the previously used steel sphere. As with the telemetry buoy components, the swivels were pressure tested at NOCS prior to deployment.

For the mooring to resist knockdown better than the first deployment, the mooring design was changed to include a second deeper syntactic about halfway down the mooring. This stiffens the mooring and allows stronger wires to be used in the lower section.

Go to the top of this page

Summary of Mark 2 deployments

Mark 2 telemetry buoy prior to deployment showing tether with low drag support floats and syntactic buoys with swivel assemblies.
© Mark 2 telemetry buoy prior to deployment showing tether with low drag support floats and syntactic buoys with swivel assemblies.

So far there has only been one deployment of the Mark 2 design, in the strong currents of the western boundary. The system worked well with data transmitted for all the instruments on the mooring, and the buoy survived occasional submergence as required. Data were received at NOCS for 4 weeks after deployment, at which point the telemetry buoy broke loose from the mooring. Inspection of the tether revealed that fishing activity most likely caused the break. The buoy was recovered intact and in working order and will be redeployed in Spring 2006. The system for the eastern boundary will be deployed in November 2005.

Future adaptations

For the 2006 deployment it is planned to use a fairing on the WB2 mooring to reduce drag on the wire and therefore allow the whole mooring to stand up straighter in the strong currents.

Go to the top of this page

The fairing will not be needed at the more benign eastern site.

 
Visit the official RAPID web site
Visit the official RAPID MOC web site