SANCOR Newsletter Issue #172:March 2003

 

 

REPORT ON THE PIRATA FR-11 CRUISE.

12/16/2002 to 01/03/2003. Dakar (Senegal) to Lomé (Togo).

Mathieu Rouault (rouault@physci.uct.ac.za).

Dept of Oceanography, University of Cape Town, South Africa

 

1. Introduction

In December 2002 I flew to Dakar Senegal to board the N/O SUROIT, a 60 m French research vessel from IFREMER, and participate in the PIRATA FR’s 11 cruise. Its main mission was to recover and replace five Atlas moorings in the Tropical Ocean Atlantic.

The food was outstanding and served with fine wine by a chief steward. We all received gifts on Xmas day and spent New Years Eve at the Equator. The crew was competent and friendly. My trip was sponsored by a POGO SCOR fellowship that was awarded in 2002. The POGO SCOR fellowship opportunity was widely advertised via the SANCOR distribution list. I applied and on being awarded the fellowship, I received a flight to Senegal and a stipend to cover my expenses.

 

South Africa’s participation is a key element of the feasibility study for the extension of PIRATA to the South East Atlantic Ocean, a project of the Benguela Current Large Marine Ecosystem program. My goals were to assess what it takes to recover and deploy an Atlas mooring, to understand what the technical requirements and constraints are and to report back to the various stakeholders from Angola, Namibia and South Africa. More information on PIRATA can be found on the PIRATA SE extension web site: http://www.egs.uct.ac.za/~rouault/piratase.html

 

2. Loading

Operations started when the first container arrived by truck near the N/O Suroit. The containers housed four Atlas moorings and a spare system. A number of sensors were on board the Suroit such as an anemometer, air temperature and humidity probe, pyranometer (short wave radiation), rain gauge, sea surface temperature and conductivity sensor, 3 temperature/conductivity sensors to be deployed at 20, 40, 120 meter depth, 5 temperature sensors to be deployed at 60, 80, 100, 140, 180 meter depth and 2 temperature/pressure sensors to be deployed at 300 and 500 meters. The N/O Suroit is well suited for PIRATA with a large platform at the rear near the A-frame that will be used in conjunction with a crane to deploy and recover moorings.

 

3. Assembling and testing

During the transit to the first mooring, Linda Stratton (NOAA, PMEL), Brian Lake, (NOAA, PMEL) and Fabrice Roubaud (IRD) had plenty of time to assemble the first mooring, install and test the sensors that are connected to a central cylindrical housing. The sensors sampling, averaging, storing and transmissions are done in the housing that can store up to 64 megabytes of data. High-resolution underwater temperature sensors are also stored internally. The mooring transmits the data signal to a satellite via an ARGOS system.

Linda and Brian are well-trained PIRATA operators. Fabrice is a relatively new player but after 2 PIRATA cruises and a two months training period at PMEL (Seattle), he already knows what it takes to install, deploy and recover a PIRATA mooring. The chief scientist Jacques Grelet (IRD) has also spent two stays at PMEL and participated in most of the former PIRATA cruises.

 

4. Recovering the first mooring

The Atlas mooring was sighted at 19 p.m., 30 min before darkness. The acoustic release was triggered - it worked. We did 3 passes nearby to check current and wind speed and direction. We also caught a dozen big Dorado and tuna, which we braaied the next day. An incredible ecosystem had developed within a year around the mooring. There was a 1-knot current and 10 to 15 knots wind with a 2 meters large period swell. We sent a rubber duck to get Brian onto the mooring to recover the meteorological sensors. When this was done the ships rear was brought near the mooring and the buoy was brought closer to the ship under the A-frame with a ropes. The bulky and heavy buoy (5 by 2 meters and 600 kg) was lifted on board using the A-frame. Bringing the mooring to the rear of the ship and securing it is a delicate operation. When secured, the mooring, (still bearing all underwater sensors, 1000 meter steel wire, 4000 meter nylon ropes, acoustic release, shackles, swivels and chains), was taken out of the buoy and connected to the ship before being connected to the capstan and rewound. Then we started removing the cable and taking out the various underwater probes. We were finished at 2 a.m. It takes between 4 and 6 hours to recover a mooring

 

5. Deploying the first mooring

We started the next day at 6 a.m. just after 4 hours of sleep. The mooring was put down on the side of the boat with the crane; the first section of cable already plugged in. We knew the exact bathymetry of the place from previous cruises. At 10 a.m. all sensors, cables and ropes where deployed and we dragged the mooring at the right place. We installed the acoustic release, the last chain and connected the mooring to the 2.2ton anchor. On reaching the bottom the anchors will move about one third of the total mooring length towards the buoy and the buoy will move forward two third of the way towards the anchor. When everything is stabilized the position of the mooring is checked.

 

6. Conclusion

Installing, testing, deploying and recovering an Atlas mooring involved many different operations. This provides an excellent opportunity for all round training and capacity building for technicians, students and crewmembers. Procedures have been well tested and improved over 15 years during TAO and PIRATA. Nonetheless great care must be taken when moving, installing, recovering or deploying the mooring. It would two or three PIRATA or TAO cruises and training at PMEL (Seattle) before a technician will be able to do the operations on his/her own. Invitations to participate in TAO or PIRATA cruises as well as in training period at Seattle (PMEL, NOAA) are proposed by NOAA and IRD.