NOAA Technical Memorandum OAR PMEL-130

KEO Mooring Engineering Analysis

Noah Lawrence-Slavas, Christian Meinig, and Hugh Milburn

Pacific Marine Environmental Laboratory

7600 Sand Point Way NE

Seattle, WA 98115-6349

June 2006

Contribution 2930 from NOAA/Pacific Marine Environmental Laboratory
NOTICE

Mention of a commercial company or product does not constitute an endorsement by

NOAA/OAR. Use of information from this publication concerning proprietary products

or the tests of such products for publicity or advertising purposes is not authorized.

Contribution No. 2930 from NOAA/Pacific Marine Environmental Laboratory

For sale by the National Technical Information Service, 5285 Port Royal Road

Springfield, VA 22161

ii
Contents iii

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.1 Mooring History . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Mooring Design . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2.1 2004 (Initial) Deployment . . . . . . . . . . . . . . . 5

1.2.2 2005 Buoy Turn-Around . . . . . . . . . . . . . . . 6

2. Environmental Conditions and Load Cell Data Leading to

Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.1 Environmental Conditions . . . . . . . . . . . . . . . . . . . . 7

2.2 2005 Load Cell Post calibration . . . . . . . . . . . . . . . . . 8

3. Mooring Failure Analysis . . . . . . . . . . . . . . . . . . . . 8

3.1 Line Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3.2 Mooring Wear Analysis . . . . . . . . . . . . . . . . . . . . . 11

3.3 Shock Loading Analysis . . . . . . . . . . . . . . . . . . . . . 11

4. WHOI CABLE Analysis . . . . . . . . . . . . . . . . . . . . . 17

4.1 Initial Mooring Analysis . . . . . . . . . . . . . . . . . . . . . 17

4.2 Post Failure Mooring Analysis . . . . . . . . . . . . . . . . . 18

5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.1 Line Failure . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

5.2 2006 KEO Mooring Design . . . . . . . . . . . . . . . . . . . 27

6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 29

List of Figures

1 Kess observing array. . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

2 Buoy positions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3 2004 mooring configuration. . . . . . . . . . . . . . . . . . . . . . . . 3

4 2005 mooring configuration. . . . . . . . . . . . . . . . . . . . . . . . 4

5 NOAA PMEL 3/8

jacketed wire rope fairing. . . . . . . . . . . . . . 5

6 2005 KEO buoy. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

7 KEO site current profiles. . . . . . . . . . . . . . . . . . . . . . . . . 8

8 Physical conditions for the 6 months leading to failure. . . . . . . . . 9

9 Physical conditions for the week prior to failure. . . . . . . . . . . . 10

10 Pre and post-2005 deployment load cell calibration curve. . . . . . . 11

11 3/4

8-strand plaited nylon: section A change in pik length. . . . . . 12

12 3/4

8-strand plaited nylon line structure in vicinity of break. . . . . 13

13 3/4

8-strand plaited nylon: section C pik detail. . . . . . . . . . . . . 14

14 3/4

8-strand plaited nylon: section C kink. . . . . . . . . . . . . . . 14

15 3/4

8-strand plaited nylon: section F (top view). . . . . . . . . . . . 15

16 3/4

8-strand plaited nylon: section F (bottom view). . . . . . . . . . 15

17 3/4

8-strand plaited nylon: section F–G parted end. . . . . . . . . . 16

18 3/4

8-strand plaited nylon: section G parted end. . . . . . . . . . . . 16

19 3/4

8-strand plaited nylon: KEO recovered nylon cut fiber patch. . . 17

20 Clevis wear between the 2004 deployment and the 2005 deployment. 17

21 Archived WW3 satellite wave data for wave heights. . . . . . . . . . 18

22 CABLE static line tension vs. water depth, 2004 deployment. . . . . 19

23a Physical conditions for 6 days bracketing 2005156190000. . . . . . . 21

23b CABLE dynamic line tension vs. time for 2005156190000. . . . . . . 21

24a Physical conditions for 6 days bracketing 2005187190000. . . . . . . 22
iv Contents

24b CABLE dynamic line tension vs. time for 2005187190000. . . . . . . 22

25a Physical conditions for 6 days bracketing 2005197130000. . . . . . . 23

25b CABLE dynamic line tension vs. time for 2005197130000. . . . . . . 23

26a Physical conditions for 6 days bracketing 2005285070000. . . . . . . 24

26b CABLE dynamic line tension vs. time for 2005285070000. . . . . . . 24

27 CABLE dynamic line tension vs. time for the week preceding failure. 25

28 Damage to 3/8

jacketed wire rope at a depth of ∼547 m. . . . . . . 26

29 Damage to 3/8

jacketed wire rope at a depth of ∼547 m. . . . . . . 26

30 TAO 3/4

8-strand plaited nylon: pic length change (E927). . . . . . 26

31 WHOI CABLE dynamic line tension vs. time, 2006 deployment vs.

2005 deployment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

32 WHOI cable dynamic line tension vs. time, 2006 deployment, for

mean currents, max currents, and max currents +30% . . . . . . . . 28

33 KEO 2006 deployment mooring profile during max currents. . . . . . 28

34 KEO 2006, mooring configuration. . . . . . . . . . . . . . . . . . . . 30
KEO Mooring Engineering Analysis

Noah Lawrence-Slavas, Christian Meinig, and Hugh Milburn

Abstract. As a contribution to the global network of OceanSITES time series reference sites a

Kuroshio Extension Observatory (KEO) mooring was deployed by NOAA/PMEL in the recirculation gyre in June 2004. The low-cost KEO mooring includes a suite of meteorological, chemical,

subsurface, and engineering instrumentation reporting in real time. The mooring was designed to

withstand the strong and deep western ocean boundary currents, and the harsh surface conditions

found in the Kuroshio Extension region. The observatory returned near 100% data return for 18

months, providing an important data set in an area that is sparsely sampled and has the largest

air-sea fluxes in the Pacific. On 6 November 2005, in relatively benign conditions, the KEO buoy

broke loose from its mooring and fortuitously was recovered 2 days later. Post failure inspection

of the mooring line revealed the mooring had parted in the middle of a continuous section of 3/4



nylon, 300 m below the lower terminus of the jacketed wire rope, ∼1000 m below the surface. Additionally, analysis of the load cell data exposed concerning patterns of shock loading throughout

the deployment period. This report focuses on the KEO mooring design and failure analysis. The

wealth of environmental and engineering data available for this mooring gave PMEL engineers a

rare opportunity to compare model results with actual observations.

1. Introduction

1.1 Mooring History

The KEO (Kuroshio Extension recirculation gyre Observatory, www.pmel.

noaa.gov/keo) buoy was deployed by PMEL in the North Pacific Ocean,

southeast of Yokohama, Japan in 5685 m of water on 16 June 2004 at

32◦21.0

N, 144◦ 38.2E (see Fig. 1). The buoy and upper 700 m of mooring were replaced in May 2005 (see Figs. 3 and 4). The KEO mooring is a

modified TAO mooring, designed for the harsh conditions of the Kuroshio

Extension region (see Fig. 3 (2004) and Fig. 4 (2005)). Modifications include:

• Slack line reverse catenary mooring design, with scope (mooring line/

water depth) of 1.4

• Sonic anemometer

• Modifications to the buoy platform and mooring line for added buoyancy and reduced drag

• 3100 kg anchor

The KEO mooring carries a suite of meteorological sensors to measure pCO2,

winds, air temperature, relative humidity, rainfall, and solar and longwave

radiation. Additionally, the KEO mooring carries surface and subsurface

instrumentation for measuring temperature and salinity and currents from

1 m to 500 m, and a load cell located at the upper terminus of the mooring,

between the buoy bridle and inductive cable. Sensor information is transmitted back to PMEL in “near real time” using satellite communications.

The KEO buoy, sensors, and the upper 700 m (inductive cable) of the

mooring were replaced on 28 May 2005 as part of the regularly scheduled
2 N. Lawrence-Slavas et al.

maintenance cycle. PMEL’s pCO2 (air-sea CO2 flux) measurement system

was added to the instrumentation suite during this platform swap, three Sontek current sensors were added to the inductive cable, 17 additional temperature/salinity modules, and a 2nd-generation load cell (IPS V918) replaced

the 1st-generation load cell on the original mooring. The 1st-generation load

cell (IPS1/V910) recovered during the May swap had failed after ∼6 months,

when the data cable connecting the load cell to the CPU package failed due

to bending fatigue acting on a small section of unsupported cable. Additionally, post deployment calibration showed that the 1st-generation load

Figure 1: Kess observing array.

Figure 2: Buoy positions.
KEO Mooring Engineering Analysis 3

2,279 METERS BOUYANT 

POLYOLEFIN MOORING LINE

4,900 METERS

NYLON LINE,

CONTINUOUS

700 METERS

KEO

Rick Miller 1 MAY 2004

7600 Sandpoint Way NE

Seattle, Wa. 98115

(206) 526-6149

DRAWN BY:

APPROVED BY:

DATE:

DATE:

MOORING:

SCOPE:

NOAA-PMEL-KE

1.4:1

5 METERS TC

700 METERS WIRE ROPE 

FAIRING TO 500 METERS

10 METERS TC

50 METERS

25 METERS TC

TC

75 METERS TC

100 METERS TP

150 METERS TC

200 METERS TC

300 METERS TP

400 METERS TC

500 METERS TP

5/8" SAS

5/8" SAS WITH BUSHING

.5 METER CHAIN

5/8" SAS

5 TON SWIVEL

IRIDIUM/POSITIONING BEACON GPS

SHORT AND

LONG WAVE

RADIATION

AIR TEMPERATURE,

RELATIVE HUMIDITY

RAIN GAGE

HANDAR 425 SONIC ANEMOMETER

 KEO

RADAR REFLECTOR

DEPTH: 5650 METERS

10 METERS 

NYLON

STIFFENER

3/4" SAS

.5 M 1/2" CHAIN

5/8" SAS

4 M 1/2" CHAIN

5/8" SAS

ANCHOR 

WEIGHT

6,820#

1 M 1/2" CHAIN

5/8" SAS

EYE INTO EYE

SPLICE

17" GLASS BALL FLOATS

5/8" SAS

5/8" SAS

3 TON SWIVEL

ACOUSTIC RELEASE

20 METERS 1 1/8"

NYLON

5/8" SAS

5/8" SAS

3/4" SAS

.5 METER CHAIN

5/8" SAS

2 METER CHAIN

.5 METER CHAIN

3/4" SAS WITH BUSHING

INDUCTIVE CABLE BREAKOUT

1 METER

2 LINKS 1" CHAIN

1" SAS

1 1/8" SAS

LOAD CELL

SSC

1" SAS

CABLE BRACKET

Figure 3: 2004 mooring configuration.
4 N. Lawrence-Slavas et al.

2,279 METERS BOUYANT 

POLYOLEFIN MOORING LINE

10 METERS 

NYLON

STIFFENER

3/4" SAS

.5 M 1/2" CHAIN

5/8" SAS

4 M 1/2" CHAIN

5/8" SAS

ANCHOR 

WEIGHT

6,820#

(3,100 KG)

1 M 1/2" CHAIN

5/8" SAS

EYE INTO EYE

SPLICE

17" GLASS BALL FLOATS

5/8" SAS

5/8" SAS

3 TON SWIVEL

ACOUSTIC RELEASE

20 METERS 1 1/8"

NYLON

5/8" SAS

5/8" SAS

3/4" SAS

.5 METER CHAIN

5/8" SAS

2 METER CHAIN

.5 METER CHAIN

3/4" SAS WITH BUSHING

5 METERS TC INVERTED

697.4 METERS

 WIRE ROPE 

FAIRING TO 525 METERS

7 METERS SONTEK

10 METERS

8.3 METERS TV

TC INVERTED

37 METERS

75 METERS

100 METERS

TC INVERTED

125 METERS

TP

225 METERS

300 METERS

TC INVERTED

15 METERS

17 METERS SONTEK

25 METERS

TC INVERTED

TC INVERTED

35 METERS

SONTEK

18.3 METERS TV

38.3 METERS TV

50 METERS TC INVERTED

150 METERS

TC

TC

175 METERS

200 METERS

250 METERS

275 METERS

325 METERS

TP

TC

T

TP

TC

TP

TC

700 METERS

5/8" SAS

5/8" SAS WITH BUSHING

.5 METER CHAIN

5 TON SWIVEL

4,900 METERS

NYLON LINE,

CONTINUOUS

INDUCTIVE CABLE BREAKOUT

IRIDIUM/POSITIONING BEACON GPS

SHORT AND

LONG WAVE

RADIATION

AIR TEMPERATURE,

RELATIVE HUMIDITY

RAIN GAGE

GILL SONIC ANEMOMETER

 KEO

RADAR REFLECTOR

1 METER

2 LINKS 1" CHAIN

1" SAS

1 1/8" SAS

LOAD CELL

SSC

1" SAS

CABLE BRACKET TORQUE load cell

and isolator plate

bolts to 120 lb. ft.

INDUCTIVE CABLE BREAKOUT

SOLID CORE CO2 BUOY

375 METERS

400 METERS

425 METERS

450 METERS

475 METERS

500 METERS

525 METERS

TP

TC

TP

T

TC

TP

525 METERS TC

350 METERS T

KEO

Rick Miller 20 MAY, 2005

7600 Sandpoint Way NE

Seattle, Wa. 98115

(206) 526-6149

DRAWN BY:

APPROVED BY:

DATE:

DATE:

MOORING:

SCOPE:

NOAA-PMEL-KE

1.4:1

MOORING LINE FAILURE DEPTH: 5650 METERS

350m below upper nylon 

termination

DRAWING NOT TO SCALE

Nylon Segment Lower Splice

250m below failure point

Nylon Segment Upper Splice

300m above failure point

550m Fish Bite Damage 

Figure 4: 2005 mooring configuration.
KEO Mooring Engineering Analysis 5

cell had “drifted” during deployment, and that all of the recovered load cell

data was corrupt. The 2nd-generation load cell addresses the issues with

the 1st-generation load cell, including replacing the 1st generation’s voltage

output with a current output.

On 12 October 2005 the pCO2 system ceased transmitting (see pCO2

KEO Failure.doc), and at ∼1510Z 6 November 2005 position data indicated

that the KEO mooring had gone adrift (see Fig. 2). Before the pCO2 system

ceased transmitting, the lab was receiving GPS position data on the KEO

mooring every 3 hours. After the pCO2 system failed, GPS position data

was received every 3 days via the IPS-load cell instrument, and 11 times per

day via Argos.

The buoy, 700 m of inductive cable, 350 m of nylon mooring line, and

all instruments were recovered by the RV Kaiyo on 8 November 2005, only

2 days after the mooring went adrift. The recovered mooring was then

transported fully assembled to Japan, where it was carefully broken down

by a PMEL mooring technician, packaged, and shipped back to Seattle,

Washington.

1.2 Mooring Design

1.2.1 2004 (Initial) Deployment

The KEO mooring was designed as a slack line reverse catenary mooring.

The mooring was designed to have a scope of 1.4 to survive in harsh current

regimes, and the reverse catenary design keeps the upper segment of the

mooring fairly vertical. The mooring is composed of the following segments

(top to bottom):

1. The KEO buoy is a 2.3 m ATLAS toroid with the center “donut” hole

glassed over and filled with foam. The gross displacement of the buoy

is ∼5600 lbs, and the net positive displacement of the entire mooring

system is ∼5040 lbs.

2. Non-rotating 3/8

 (0.92 cm) diameter wire rope jacketed to 1/2 (1.27 cm)

is used in the upper 700 m to guard against damage from fish bite, and

to form the inductive loop. The upper 500 m of this cable are faired

with PMEL’s extruded plastic snap on fairings (see Fig. 5). The fairing

design is based on a standard airfoil shape with a t/c (width/length)

around 0.25.

Figure 5: NOAA PMEL 3/8

jacketed wire rope fairing.
6 N. Lawrence-Slavas et al.

3. Plaited 8-strand 3/4

 (11/16 nom., 1.9 cm) diameter nylon line is used

for the next 4900 m of the mooring (composed of ∼550 m line segments

“end to end braided short spliced” together). The upper 10 m is coated

with polyurethane nylon stiffener.

4. The nylon is then “end to end braided short spliced” to 2280 m of

buoyant 7/8

 polyolefin line.

5. The anchor is fabricated from scrap railroad wheels and weighs

∼3100 kg.

All hardware is standard equipment as used in other PMEL moorings and

deployments follow the traditional anchor last routine. Subsurface modules

are clamped around the jacketed cable, and are standard TAO instrumentation.

For the 2004 deployment, the mooring was rigged as shown in Fig. 3.

The mooring modeling program, WHOI CABLE, was used to evaluate the

mooring design prior to the initial deployment.

1.2.2 2005 Buoy Turn-Around

The KEO mooring survived for 18 months before failure. After 12 months

the mooring was serviced. Servicing was performed by lifting the buoy out of

the water with the ship’s crane, and then recovering the 700 m of wire rope

with a capstan. The procedure was designed to recover and replace only

the buoy and wire rope, not the nylon. Excellent ship handling, centering

the ship directly above the anchor using the signal from the acoustic release,

and the 1.4:1 scope of the mooring, allowed the recovery to be accomplished,

while keeping the load below the SWL of the wire and nylon. Redeployment

consisted of substituting a new buoy, a new piece of 700 m of wire and

attached instruments, a new swivel and 1/2 m piece of chain, above the nylon.

Additionally the following changes were made (see Fig. 4):

• Added 3 Sontek current sensors to the inductive cable (within the top

37 m of mooring, ∼ neutrally buoyant).

• Added 17 temperature/salinity modules to the inductive cable (+17

lbs. wet weight).

• Added a buoy mounted pCO2 system (+150 lbs. embedded in buoy

∼12 above waterline) (see Fig. 6).

• Replaced 1st-generation load cell with 2nd-generation load cell (physical dimensions nearly identical).
KEO Mooring Engineering Analysis 7

2. Environmental Conditions and Load Cell Data

Leading to Failure

2.1 Environmental Conditions

The following figures (Figs. 7–8) show the environmental conditions observed

at the KEO mooring site for the 6-month period ending in the mooring

failure. The load cell data (upper terminus of the mooring, between the

buoy bridle and inductive cable) collected during this time is of particular

interest.

The KEO mooring was subjected to disturbingly high levels of “shock

loading” during the last 6 months of its life. Since the 2004 deployment

load cell failed during deployment, it is possible the mooring may have been

experiencing shock loading over its entire 18-month life. Shock loading occurs when the tension within a segment of the mooring line drops to zero,

causing the line to momentarily go slack. When the tension is re-applied

to the line, the slack is rapidly taken in, causing an abrupt increase in line

tension. The instances of shock loading can be directly related to periods of

high wind at the mooring site (see Fig. 8). A comparison of wave “slopes”

(Wave Height/1/2 * Wave Period, obtained from WW3 satellite wave maps)

at the mooring site during these periods of high shock loading is shown in

Figs. 8 and 9. Additionally, Fig. 9 shows the loads and conditions seen by the

05/06

XX

D

C

B

A

B

C

D

8 7 6

8 7 6 5 4 3 2 1

5 4 3 2 1

A

APPROVALS

1

CAD GENERATED DRAWING, NOAA-PMEL DO NOT MANUALLY UPDATE

SCALE: 

CAD FILE:

SHEET:

OF

DATE

DRAWN CHECKED

UNLESS OTHERWISE SPECIFIED

DIMENSIONS ARE IN INCHES

B

TOLERANCES ARE:

FRACTIONS ANGLES

NAME:

DECIMALS SIZE:

1 2005 KEO Buoy

PROJECT:

Engineering Division

Sheet1

 Seattle, WA 98115

0.002 DO NOT SCALE DRAWING 1

1/16

.XXX

 206-526-6175

NTS

7600 Sand Point Wy NE

 .X 0.05

.XX 0.02 SHEET

NLS

2005 KEO BUOY COMPONENTS

TAO - KEO

pCO2 System Load Cell

Electronics Tube

Upper Terminus

of Mooring

Figure 6: 2005 KEO buoy.
8 N. Lawrence-Slavas et al.

mooring during the week prior to failure. The red dots labeled “interpolated

wind with load ≤200 lbs” correspond to the times at which the minimum

mooring line tension fell below 200 lbs.

2.2 2005 Load Cell Post calibration

Upon recovery, a post calibration routine was run on the load cell. Comparing this post-calibration to the pre-deployment calibration showed that

sometime during the KEO deployment the load cell drifted ∼75 lbs. in the

positive (heavy) direction (see Fig. 10). After consideration, it was determined that this drift could be attributed to errors in the calibration process,

and that a 75-lb. error would be lost in the “noise” during deployment and

considered insignificant to the overall load cell results.

3. Mooring Failure Analysis

3.1 Line Failure

The KEO mooring line parted at a point 350 m below the junction between

the inductive cable and the plaited 8-strand 3/4

 (11/16 nom.) nylon line,

approximately 1050 m below the surface (see Fig. 4). The mooring line

failed approximately in the middle of a 550 m piece of nylon (∼300 m below

0

1000

2000

3000

4000

5000

6000

7000

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

Current Velocity (m/s)

Water Depth (m)

Design Current

0

200

400

600

800

1000

1200

1400

1600

1800

2000

0 0.5 1 1.5 2 2.5

Current Velocity (m/s)

Water Depth (m)

Max +50% Current

Max +30% Current

Max Current

Mean Current

(a) (b)

Figure 7: (a) Initial “best guess” current profile, used to design the 2004–2005 KEO mooring. (b) Observed

2005 current profile at KEO site (please note that the current profile is only shown to 2000 m; beyond 2000 m

the current remains constant all the way to the bottom at ∼5650 m).
KEO Mooring Engineering Analysis

9

0

500

4000350030002500200015001000

Load (lbs.) 12

34

210

Slope (m/s)

50

25201510

Speed (m/s)

153 163 173 183 193 203 213 223 233 243 253 263 273 283 293 303 313

1

1.5

2

2.5

3

3.5

4

4.5

5

Distance (miles)

Days of Year

2005 143 2005 318

load mean

load minload max

wave slope

wind speed interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Figure 8: Loads, wave slope, winds, and KEO buoy distance from anchor for the 6 months leading tofailure.
10 N. Lawrence-Slavas et al.

0

500

1000

1500

2000

2500

3000

3500

4000

Load (lbs.)

4

3

2

1

0

1

2

Slope (m/s)

0

5

10

15

20

25

Speed (m/s)

304 305 306 307 308 309 310 311 2.7

2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

Distance (miles)

Days of Year

2005 303 2005 311

load mean

load min

load max

wave slope

wind speed

interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Adrift! Adrift!

Figure 9: Loads, wave slope, winds, and KEO buoy distance from anchor for the week prior to failure.

the upper line segment splice, and ∼250 m above the lower splice). The

recovered inductive cable did show apparent fish bite damage at ∼547 m of

water depth (Figs. 28 and 29).

The cause of the line failure is not readily apparent; the unusual mooring

line structure in the vicinity of the break is summarized below, and shown

in the following figure and photos (Fig. 12).

1. Variable pik lengths of the nylon weave in the segment of line within

∼3.0 m of the break (see Figs. 11, 13, 14, and 17). Visual inspection

of the rest of the 350 m of nylon line recovered did not show this

structural abnormality in any other location.

2. Strange loops in the line just before the break point that seem to be

caused by two strands of the 8-strand line getting hung on each other

and missing the weave for 2–3 pik lengths (see Figs. 15 and 16).

3. Kinks in the individual line strands in the areas of the line that have

an especially long (∼4) pik length (see Fig. 14)
KEO Mooring Engineering Analysis 11

-1000

0

1000

2000

3000

4000

0 200 400 600 800 1000

0 200 400 600 800 1000

IPS V918 &LCA0502075 postcal 1_20_2006

1/06 postcal

4/05

y = 65.361 + 4.6757x R= 1 

y = -9.9079 + 4.6803x R= 0.99999 

1/06 postcal

B

E

Figure 10: Pre and post-2005 deployment load cell calibration curve.

In addition to the unusual line structure in the vicinity of the break,

the rest of the 350 m of recovered line exhibited regularly spaced patches

of cut fibers along the outside of the woven strands (see Fig. 19). Studying

pictures and notes of the deployment layout failed to yield an explanation for

these cut fibers, although the line could have been damaged during recovery

operations.

3.2 Mooring Wear Analysis

The clevis is the uppermost terminus of the KEO mooring line, and is hard

bolted to the buoy bridle just below the load cell (see Fig. 4). Previous

observations indicate that of all the components within the mooring system,

the clevis exhibits the highest amount of wear. A comparison of the wear

patterns between the initial (12 month) KEO deployment and the second (6

month) deployment showed that the respective clevises exhibited approximately the same amount (∼1/32) of wear for both deployments. Since the

2nd deployment’s duration was half that of the first deployment, it is assumed that the 2nd deployment’s mooring system experienced significantly

higher levels of “working” than the initial KEO deployment’s mooring (see

Fig. 20).

3.3 Shock Loading Analysis

A careful comparison of the KEO load cell data to the wind data, and the

WW3 satellite (NOGAPS) wave data (see Fig. 21, and https://navy.ncdc.

noaa.gov/products/products.html) shows that the KEO mooring begins

to experience shock type loading when the wave slope becomes steeper than
12 N. Lawrence-Slavas et al.

∼0.70 m/s, and the worst shock loading occurs in wave slopes greater than

∼1 m/s (see Fig. 8).

A rough numerical calculation of the terminal vertical velocity of the

upper 700 m of the mooring was determined using the following equation:

Fdt = 1/2 *(Cdtflat plate)*(ρ)*(A)*(μ2) + 1/2 *(Cdtcable)*(ρ)*(A)*(μ2) +

1/2 *(Cdtline)*(ρ)*(A)*(μ2)

The drag force was assumed to be the weight of the mooring in water, the

end effects of the instruments were calculated by assuming the instruments

approximated a single flat plate (Cdtflat plate = 1.17), the tangential drag of

the faired cable was calculated using a drag coefficient (Cdtcable) of 0.03, and

the tangential drag coefficient (Cdtline) of the mooring line was assumed to be

0.003. The Cdtcable value of 0.03 was a best guess, “dart throw,” 0.03 being a

factor of 10 greater then the Cdt of bare cable. The results of this calculation

suggested that the instruments and the mooring line contribute relatively

little tangential (vertical) drag to the mooring system when compared to

the tangential drag of the faired cable, and that the calculated terminal

velocity of the upper 700 m of mooring is ∼1.10 m/s.

To calculate the mooring’s terminal velocity with the wave slope data,

it was assumed that the wave slope approximates the vertical rise and fall

velocity of the KEO buoy in a given sea state. Therefore it appears that the

terminal velocity of the KEO mooring is approximately 0.7 m/s (wave slope

at which shock loading begins to occur), not the 1.10 m/s calculated above.

This error can probably be attributed to an error in the initial guess for the

Cdt of the faired cable (0.03 should be higher), and the fact that in real life

the nylon and polyolefin segments of the mooring line are not truly vertical,

thus the Cdtline of 0.003 that was used for the line drag calculation should

be higher. Shock loading occurs when the KEO buoy, traveling down the

backside of a wave, falls faster than the mooring line, causing the mooring

Figure 11: 3/4

8-strand plaited nylon: section A change in pik length.
KEO Mooring Engineering Analysis 13

Figure 12: 3/4

8-strand plaited nylon line structure in vicinity of break.
14 N. Lawrence-Slavas et al.

Figure 13: 3/4

8-strand plaited nylon: section C pik detail.

Figure 14: 3/4

8-strand plaited nylon: section C kink.
KEO Mooring Engineering Analysis 15

Figure 15: 3/4

8-strand plaited nylon: section F (top view).

Figure 16: 3/4

8-strand plaited nylon: section F (bottom view).
16 N. Lawrence-Slavas et al.

Figure 17: 3/4

8-strand plaited nylon: section F–G parted end.

Figure 18: 3/4

8-strand plaited nylon: section G parted end.
KEO Mooring Engineering Analysis 17

Figure 19: 3/4

8-strand plaited nylon: KEO recovered nylon cut fiber patch.

Figure 20: Clevis wear between the 2004 (12-month) deployment and the 2005 (6-month) deployment.

to go momentarily slack. When the buoy begins to climb the face of the

next wave this slack is abruptly taken in, causing the shock loading seen in

the load cell data. After running many iterations of the KEO mooring in a

time domain numerical model, it was determined that the Cdt of the faired

cable is ∼0.0501 (see: 4.2, “Post Failure Mooring Analysis”).

4. WHOI CABLE Analysis

4.1 Initial Mooring Analysis

Before the initial deployment, the mooring was statically analyzed in WHOI

CABLE using a “best guess” current profile (see Fig. 7a), and without the

addition of the subsurface instrumentation (see Fig. 22). WHOI CABLE is a

time domain numerical simulation of moored and towed oceanographic sys-
18 N. Lawrence-Slavas et al.

Figure 21: Archived WW3 satellite wave data for wave heights.

tems developed at WHOI. Unfortunately, the load cell deployed during the

initial deployment failed, so we were unable to compare the initial CABLE

model with measured load data.

4.2 Post Failure Mooring Analysis

After the KEO mooring failed, a concerted effort was made to build an accurate KEO mooring model in WHOI CABLE. During this modeling process

all mooring material properties were double checked for accuracy, and the

subsurface instruments were modeled as materials and added to the mooring

model. Special attention was paid to the faired inductive cable, which was

assumed to be the most important unknown variable in the mooring system.

The effects of changing different WHOI CABLE input variables were

carefully studied during the model development process. The results of

modeling the subsurface instruments as “connectors” vs. “materials” was

looked at and found to increase the mooring line tension results in the line

above the instruments by ∼6% when using “connectors.” The effects of

wind forcing upon the model was found to be negligible (>3%) to overall

results. Varying the tangential drag coefficient of the faired cable was found

to greatly impact the tension “spread” (max-min) of the mooring line, while

having a minimal impact on mean mooring tension. Manipulating the wave

forcing inputs was also found to impact the mooring tension “spread,” although to a lesser extent than the tangential drag coefficient. Varying the

normal drag coefficient of the faired cable by a factor of 10, using a current

profile of ∼(0, 0.10) (280, 0.83) (500, 0.75) (2000, 0.38) (6000, 0), was found

to have only a minimal impact on the mean mooring tension (∼100 lbs.).

The current forcing input was found to have significant impact on the mean
KEO Mooring Engineering Analysis 19

1900

2000

2100

2200

2300

2400

2500

2600

2700

0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000

Water Depth (meters)

Tension (Lbs)

Figure 22: CABLE static line tension vs. water depth, 2004 deployment no instruments.

tension of the mooring, and it is one of the most important forcing inputs

in the CABLE model.

In order to attempt to obtain more accurate tangential and normal drag

coefficients for the faired cable, a test procedure was developed in which the

terminal velocity of the faired inductive cable was obtained experimentally

through repetitive vertical and horizontal faired cable drops in the NOAA

dive tower. Unfortunately, while this method yielded accurate numbers for

the drag on an un-faired cable, the fairings reduced the drag force to such

an extent that noise in the data caused by the experimental techniques overwhelmed the drag caused by the faired cable, yielding erroneous results.

Equating the observed terminal fall velocity of the mooring to the calculated velocity (see 3.3, “Shock Loading Analysis”) yielded a tangential drag

coefficient for the faired inductive cable of ∼0.108.

Once the KEO mooring was built in CABLE (see: {KEO}{KEO2}{xxxx}

{0}.cab), the model was verified for accuracy. This was accomplished by

comparing the static mooring tensions, the tension “spread,” instrument

depths, and the horizontal distances of the buoy from the anchor obtained

from the CABLE model during four, distinct, real-life environmental conditions, to the actual load cell data, instrument depth recordings, and buoy

positions seen during these environmental conditions. These deployment

“snapshots” were picked to cover the range of environmental conditions observed at the deployment site, and special care was taken to eliminate the

effects of changing conditions by taking these “snapshots” in the middle

of times of relatively stable environmental conditions. The KEO mooring
20 N. Lawrence-Slavas et al.

presented PMEL with a unique post analysis opportunity, due to the large

amount of data available on the current profile (primary forcing function) at

the buoy site. The complete current profile at the KEO site was collected

from WHOI’s ADCP profiling buoy array (see Fig. 1). Inputting the mean

current profile at the KEO site from 2005 as the primary CABLE forcing

function (see Fig. 7b), yielded mean tension results within 60 lbs. for all snapshots except 20051871900. For that snapshot, the mean current profile was

adjusted by increasing the current profile by 0.1 m/s to yield accurate tension results. A study of the Navy’s current forecast model, NLOM (http://

www7320.nrlssc.navy.mil/global_nlom32/kur.html), did not yield conclusive evidence as to why snapshot 20051871900 had higher mean tensions

than the other periods sampled. However, NLOM did indicate that the

mooring site may have encountered a “spin off” current during that time.

The faired inductive cable’s normal and tangential drag coefficients were

then adjusted (starting at the guess of 0.108, from above) so the CABLE

results were in line with the measured results. Since the normal and tangential drag coefficients of circular line are well known, these numbers were not

adjusted. The final normal drag coefficient for the faired cable was set at 0.3,

and the tangential drag coefficient was set at 0.0501. The “instruments as

materials” model was used to mimic the KEO mooring because “materials”

more accurately mimic actual bodies in WHOI CABLE dynamic solutions

(materials treat drag forces based on shape, not the “approximate sphere”

used for connectors). CABLE model results for instrument depth compared

to within 2.5% of observed instrument depth (CABLE was always on the

deep side), and CABLE results for the horizontal distance from the anchor

were within 10% (CABLE always showed the buoy closer to the anchor then

observed results). It should be noted that the CABLE results for distance

from anchor were almost always ∼10% under, and might be due to a difference in actual deployment depth compared to designed deployment depth.

Tension “spread” results were within 8% (∼70 lbs.) of overall tension, as

were the mean tension results (∼60 lbs.).

The dynamic results of these mooring models, and the actual conditions

observed, are shown below (See Figs. 23–27). The dynamic output of this

CABLE file for the time frame in the vicinity of the break is shown in Fig. 27

(see Fig. 9 for observed forces).
KEO Mooring Engineering Analysis 21

0

500

1000

1500

2000

2500

3000

3500

4000

Load (lbs.)

4

3

2

1

0

1

2

Slope (m/s)

0

2

4

6

8

10

12

14

16

Speed (m/s)

153 154 155 156 157 158 159 160 3.2

3.4

3.6

3.8

4

4.2

4.4

4.6

Distance (miles)

Days of Year

2005 152 2005 160

load mean

load min

load max

wave slope

wind speed

interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Figure 23a: Loads, wave slope, winds, and KEO buoy distance from anchor for 6 days bracketing

2005156190000.

Snapshot 20051561900

0

500

1000

1500

2000

2500

3000

0 50 100 150

Time

Tension (lbs)

Tension at Buoy

Tension at Nylon Break

Figure 23b: CABLE dynamic line tension vs. time, 2005 deployment instruments modeled as materials for

2005156190000.
22 N. Lawrence-Slavas et al.

0

500

1000

1500

2000

2500

3000

3500

4000

Load (lbs.)

–4

–3

–2

–1

0

1

2

Slope (m/s)

0

2

4

6

8

10

12

14

Speed (m/s)

184 185 186 187 188 189 190 191 3.4

3.45

3.5

3.55

3.6

3.65

Distance (miles)

Days of Year

2005 183 2005 191

load mean

load min

load max

wave slope

wind speed

interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Figure 24a: Wave slope, winds, and KEO buoy distance from anchor for 6 days bracketing 2005187190000.

Snapshot 20051871900

0

500

1000

1500

2000

2500

3000

150 200 250 300

Time

Tension (lbs)

Tension at Buoy

Tension at Nylon Break

Figure 24b: CABLE dynamic line tension vs. time, 2005 deployment instruments modeled as materials for

2005187190000.
KEO Mooring Engineering Analysis 23

0

500

1000

1500

2000

2500

3000

3500

4000

Load (lbs.)

–4

–3

–2

–1

0

1

2

Slope (m/s)

0

2

4

6

8

10

12

14

Speed (m/s)

194 195 196 197 198 199 200 201 3.2

3.25

3.3

3.35

3.4

3.45

3.5

3.55

3.6

Distance (miles)

Days of Year

2005 193 2005 201

load mean

load min

load max

wave slope

wind speed

interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Figure 25a: Wave slope, winds, and KEO buoy distance from anchor for 6 days bracketing 2005197130000.

Snapshot 20051971300

0

500

1000

1500

2000

2500

3000

350 400 450 500

Time

Tension (lbs)

Tension at Buoy

Tension at Nylon Break

Figure 25b: CABLE dynamic line tension vs. time, 2005 deployment instruments modeled as materials for

2005197130000.
24 N. Lawrence-Slavas et al.

0

500

1000

1500

2000

2500

3000

3500

4000

Load (lbs.)

–4

–3

–2

–1

0

1

2

Slope (m/s)

0

2

4

6

8

10

12

14

Speed (m/s)

282 283 284 285 286 287 288 289 3.45

3.46

3.47

3.48

3.49

3.5

3.51

3.52

Distance (miles)

Days of Year

2005 281 2005 289

load mean

load min

load max

wave slope

wind speed

interpolated wind w/load <= 200

distance from anchor

interpolated distance w/load <= 200

Figure 26a: Wave slope, winds, and KEO buoy distance from anchor for 6 days bracketing 2005285070000.

Snapshot 20052850700

-200

300

800

1300

1800

2300

2800

125 175 225 275 Time

Tension (lbs)

Tension at Buoy

Tension at Nylon Break

Figure 26b: CABLE dynamic line tension vs. time, 2005 deployment instruments modeled as materials for

2005285070000.
KEO Mooring Engineering Analysis 25

Week to Failure

0

500

1000

1500

2000

2500

3000

350 400 450 500

Time

Tension (lbs)

Tension at Buoy

Tension at Nylon Break

Figure 27: CABLE dynamic line tension vs. time, 2005 deployment instruments modeled as materials for

the week preceding failure. Note the many instances of zero tension.

5. Conclusions

5.1 Line Failure

The cause of the line failure is still unknown. Discussions with the rope

manufacturer on the baffling line structure in the vicinity of the break have

yielded inconclusive results. It is believed that at some point during the

deployment some (∼1/5) of the outer strand fibers line were cut and this,

combined with the strange line structure, and shock loading eventually resulted in line failure. The strange cuts to the outside of the rope fibers

(see Fig. 19) could have been a result of deployment/recovery operations, or

damage due to fishing gear. Fish bite at the depth of the break location,

∼1000 m, seems unlikely, however we did see heavy fish bite damage to the

inductive cable at ∼547 m (see Figs. 28 and 29). It is still a mystery as to

why the line pik lengths varied so substantially (300%) in the vicinity of the

break, why there were the strange braiding problems at the point of failure,

and why even with these cut fibers the line failed under such low loads. It is

important to note that this change in pik length was also recently observed

(to a much less extreme extent) in a small (0.25 m) segment of nylon line

recovered off of a taught line TAO buoy moored at 0◦, 125◦W (see Fig. 30).

The TAO line was also 3/4

 8-strand plaited nylon. However, the TAO line

was in no way related to the KEO line, having been made by a different

manufacturer.

Segments from both of these recovered lines were tested to determine if

the pik length could have been caused during the line stretching and measurement operations. Tests proved that stretching the line to 3500 lbs. and
26 N. Lawrence-Slavas et al.

Figure 28: Damage to 3/8

jacketed wire rope at a depth of ∼547 m.

Figure 29: Damage to 3/8

jacketed wire rope at a depth of ∼547 m.

Figure 30: TAO 3/4

8-strand plaited nylon: pic length change (E927).

wrapping it around blocks could not cause the change in pik length that was

observed. The segments of the nylon line were also pulled to failure, resulting in a 10,300 lb. breaking strength (vs. 15,000 lb. RBS), these numbers are

probably not valid, due to the test piece not being long enough (end effects

of splices).

The effects of shock loading on the nilspin cable were studied by testing

to failure the upper section of nilspin from both KEO deployments. This

resulted in an ultimate breaking strength of ∼15,400 lbs. (vs. 13,900 lb.

RBS), so there was no strength degradation due to shock loading during

deployment.

For future KEO deployments the primary design issue that needs to be
KEO Mooring Engineering Analysis 27

-100

400

900

1400

1900

160 165 170 175 180 185 190 195 200 205 210 215 220 225 230

Time

Tension (lbs)

2005 Tension at Buoy

2005 Tension at Break

2006 Tension at Buoy

2006 Tension at Location

of 2005 Break

Figure 31: WHOI CABLE dynamic line tension vs. time, 2006 deployment vs. 2005 deployment, instruments modeled as materials for worst case environmental conditions seen at site last year (snapshot

285070000), mean currents, harmonic wave.

addressed is the shock loading of the mooring system. Addressing this design

problem, as well as performing a careful physical inspection of the mooring

line structure prior to deployment, and tailoring the deployment deck layout

to eliminate possible line abrasion, should result in a survivable mooring.

5.2 2006 KEO Mooring Design

In order to alleviate the shock loading on the KEO mooring system, the

vertical fall velocity of the inductive cable must be increased. As shown previously (see 3.3, “Shock Loading”), the primary factor impacting the vertical

velocity of the inductive cable is the tangential drag on the faired inductive

cable, not the vertical drag from the subsurface instruments. Because the

currents at the KEO deployment site are in reality much lower than the

mooring was initially designed for, fairings can be removed from the inductive cable, decreasing the tangential drag of the cable. After carefully

studying the current data for the deployment site (see Fig. 7b), 275 m of

fairings were removed from the inductive cable, and the remaining 250 m of

fairings were placed, based on the 2005 current profile, between 0–140 m and

240–350 m. WHOI CABLE predicts that the 2006 KEO mooring will survive current conditions up to the maximum currents seen at the site last year

+20% (over entire current profile, ∼4 knt surface current, 0.8 knt current

to the bottom). The dynamic output of the KEO CABLE mooring model,

for both random and harmonic wave states, after the fairings have been re-
28 N. Lawrence-Slavas et al.

-250

750

1750

2750

3750

4750

5750

6750

-100 0 100 200 300 400 500

Time

Tension (lbs)

Mean Current

Max Current

Max+30% Current

Buoy Displacement

Figure 32: WHOI cable dynamic line tension vs. time, 2006 deployment, instruments modeled as materials

for worst case environmental conditions seen at site in 2005, mean currents, max currents, and max currents

+30%, random wave.

0

1000

2000

3000

4000

5000

0 1000 2000 3000 4000 5000 6000

Horizontal Distance from Anchor

Water Depth (m)

KEO Mooring Catenary

Figure 33: KEO 2006 deployment mooring profile (catenary) during max currents.
KEO Mooring Engineering Analysis 29

moved from the cable, is shown in Figs. 31 and 32. Figure 32 shows that

the 2006 mooring might experience shock loading under extreme wave conditions. However, a direct comparison of harmonic wave CABLE output for

worst cast shock loading conditions from 2005 to the 2006 model illustrates

how removing 275 m of fairings have decreased the overall tension spread,

greatly reducing possible instances of shock loading, while maintaining the

mean tension from the 2005 deployment (see Fig. 31). Figure 33 shows the

2006 mooring’s catenary under maximum current conditions, and the new

and improved mooring diagram is shown in Fig. 34.

6. Acknowledgments

The authors would like to thank the following people, for their valuable

contributions to this paper: Dr. Meghan Cronin, for her scientific leadership, encouragement, and enthusiasm; Rick Miller, for his mooring expertise

and his thorough inspection and analysis of the KEO mooring components;

Curran Fey, for his invaluable help in filtering and graphing the various KEO

data sets; Ryan Layne Whitney for his help turning this report into a cohesive document; and everybody at PMEL EDD for their comments and

suggestions. Thank You!
30 N. Lawrence-Slavas et al.

2,279 METERS BOUYANT 

POLYOLEFIN MOORING LINE

10 METERS 

NYLON

STIFFENER

3/4" SAS

.5 M 1/2" CHAIN

5/8" SAS

4 M 1/2" CHAIN

5/8" SAS

ANCHOR 

WEIGHT

6,820#

(3,100 KG)

1 M 1/2" CHAIN

5/8" SAS

EYE INTO EYE

SPLICE

17" GLASS BALL FLOATS

5/8" SAS

5/8" SAS

3 TON SWIVEL

ACOUSTIC RELEASE

20 METERS 1 1/8"

NYLON

5/8" SAS

5/8" SAS

3/4" SAS

.5 METER CHAIN

5/8" SAS

2 METER CHAIN

.5 METER CHAIN

3/4" SAS WITH BUSHING

5 METERS TC INVERTED

7 METERS SONTEK

10 METERS

8.3 METERS TV

TC INVERTED

37 METERS

75 METERS

100 METERS

TC INVERTED

125 METERS

TP

225 METERS

300 METERS

TC INVERTED

15 METERS

17 METERS SONTEK

25 METERS

TC INVERTED

TC INVERTED

35 METERS

SONTEK

18.3 METERS TV

38.3 METERS TV

50 METERS TC INVERTED

150 METERS

TC

TC

175 METERS

200 METERS

250 METERS

275 METERS

325 METERS

TP

TC

T

TP

TC

TP

TC

700 METERS

5/8" SAS

5/8" SAS WITH BUSHING

.5 METER CHAIN

5 TON SWIVEL

4,900 METERS

NYLON LINE,

CONTINUOUS

INDUCTIVE CABLE BREAKOUT

IRIDIUM/POSITIONING BEACON GPS

SHORT AND

LONG WAVE

RADIATION

AIR TEMPERATURE,

RELATIVE HUMIDITY

RAIN GAGE

GILL SONIC ANEMOMETER

 KEO

RADAR REFLECTOR

1 METER

2 LINKS 1" CHAIN

1" SAS

1 1/8" SAS

LOAD CELL

SSC

1" SAS

CABLE BRACKET TORQUE load cell

and isolator plate

bolts to 120 lb. ft.

INDUCTIVE CABLE BREAKOUT

SOLID CORE CO2 BUOY

375 METERS

400 METERS

425 METERS

450 METERS

475 METERS

500 METERS

525 METERS

TP

TC

TP

T

TC

TP

525 METERS TC

350 METERS T

KEO

Rick Miller March, 2006

7600 Sandpoint Way NE

Seattle, Wa. 98115

(206) 526-6149

DRAWN BY:

APPROVED BY:

DATE:

DATE:

MOORING:

SCOPE:

NOAA-PMEL-KE

1.4:1

DEPTH: 5650 METERS

DRAWING NOT TO SCALE

FAIRINGS:

0 - 140 METERS

697.4 METERS

WIRE ROPE

FAIRINGS:

240 - 350 METERS

Add an Additional 

Pair (2) Glass Balls

Figure 34: KEO 2006, mooring configuration.