the design process, evolution and deployment of the rapid-moc moorings in the atlantic at 26.5ºn...
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The Design Process, Evolution and Deployment of the Rapid-MOC
Moorings in the Atlantic at 26.5ºN
Darren Rayner and Rob McLachlan
National Oceanography Centre, Southampton
Introduction to the Rapid-MOC project
Combines array of moorings across the Atlantic at 26 N, with winds from satellite scatterometry and the Florida Straits flow from a disused telephone cable between Florida and the Bahamas
Collaborative project between NOCS, RSMAS (University of Miami) and AOML (NOAA, Miami)
Mooring array first deployed in 2004 and serviced annually
Array will continue to 2014 through funding of Rapid-WATCH project
Evolving array design
Project aim: To provide a 10 year time series of the strength and structure of the Atlantic Meridional Overturning Circulation
UK contribution currently consists of:• 21 moorings (9 of which “tall”) • 12 BPR landers• 2 Inverted Echosounders
US contribution currently consists of :-• 3 moorings (in WB sub-array)• 4 BPR landers (in WB sub-array)• Florida Straits Cable
3 sub-arrays - Eastern Boundary (EB), Western Boundary (WB) and the Mid-Atlantic Ridge (MAR)
Introduction to the Rapid-MOC project
Bottom Pressure Recorders (BPRs)
1st year “tall” moorings deployed using a drop off mechanism
Non-uniform drift in pressure sensor - looks exponential but detrending may remove signal we’re trying to detect
2nd year of drift more closely approximated to linear
Difficult to join subsequent time-series due to pressure drift so now use overlapping time-series
Now use a lander tripod separate from the rest of the mooring and deployed for two years
Example of 1st year mooring design
• Main hold-up from single large steel sphere
• Very little support/backup buoyancy used
• Light anchor (900kg)
• 3/16” diameter wire used throughout
• 9 Microcat CTDs for 4700m of water column
• BPR on drop-off mechanism
Example of current mooring design
• Staged top design with 2 steel spheres and buoyancy above this too
• Much more support/backup buoyancy
• Heavier anchor (twice as heavy)
• Mixture of 4 different wire diameters (4mm, 5mm, 3/16” and 1/4”)
• 16 Microcat CTDs
• No BPR
Multi-stage top design
• Top floats can be cut off and 24” SS will support the upper section
• Weakest link is above main buoyancy
• Assumes actual breaking load close to manufacturer’s stated minimum breaking load
3 x Trimsyns
50m depth
24” Steel Sphere
90m depth
37” Steel Sphere
150m depth
8 x 17” glass
780m depth
40m of 4mm wire960kg MBL
60m of 4mm wire960kg MBL
630m of 5mm wire1500kg MBL
3/16” wire1814kg MBL
25kg
66kg
371kg
543kg (at 2000m)
In-situ tension
935kg
894kg
1129kg
1271kg
Reserve
Weakest Link
Design Process - Flowchart
Currentprofile
Siteinfo
Sciencerequirements
Rough idea/sketchof mooring
.csv text file ofmooring design
Outputs:• Backup buoyancy
• Launch tension• Knockdown (max depths)
• Stretch (min depths)• Required anchor weight
• In-situ tension
Databaseof materials
DesignOK?
Workingdesign
Mooring package
No
Yes
Adjust design
Previousexperience
Backup buoyancy
MAR1 - 2007 MAR1 - 2004
Launch tension
• Peak launch tension calculated from modelling drag as the mooring falls to the seabed following an anchor last deployment.
• Design package gives warning if launch tension is over 50% of breaking load
• Heavier anchor = higher launch tension
• WHOI safe anchor weight determined from buoyancy, current profile and drag using:
Wet Anchor Weight = 1.5 x (VA + HA/0.6)
VA= vertical anchor loadHA = horizontal anchor load
• Need to convert to dry anchor weight for material being used
Knockdown
• Knockdown (Subduction) calculated from mooring drag and current profile
• Design package produces simple reference plot
• Aim to modify routines further to warn if exceeds maximum operating depth
• Instrument tilt can be calculated (routines been modified to automatically do this)
Summary
• Brief intro into the Rapid-MOC project.
• Discussed changes in the mooring designs – BPR landers– “tall” moorings
• Example of MAR1 design from 1st year and present
• Development of the multi-stage top design
• Ran through the design process and gave example outputs from the design package I use.
THE END