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