Concept Design of Mooring Buoy

8/18/2019 Concept Design of Mooring Buoy

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Places of Refuge Initiative Mooring Buoy Initiative

Concept Design of Mooring Buoy

Prepared forState of AlaskaDepartment of Environmental Conservation

Anchorage, Alaska

Under ADEC SPAR Term Contract #18-5048-10

Prepared by

oastwise orporat ionNaval Architects Marine Engineers

Anchorage, Alaska

and

File No. 11073.0130 June 2011Rev. -

1201 Western Avenue, Suite 200, Seattle, Washington 98101-2921 TEL 206.624.7850 FAX 206.682.9117 www. glosten.com

Consulting Engineers Serving the Marine Community


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1201 Western Avenue, Suite 200, Seattle, Washington 98101-2921 TEL 206.624.7850 FAX 206.682.9117 www. glosten.com

Consulting Engineers Serving the Marine Community

Places of Refuge Initiative Mooring Buoy Initiative

Concept Design of Mooring Buoy

Prepared forState of AlaskaDepartment of Environmental Conservation

Anchorage, Alaska

Under ADEC SPAR Term Contract #18-5048-10

Prepared byCoastwise Corporation

Anchorage, Alaska

and

The Glosten Associates, Inc.Seattle, Washington

File No. 11073.0130 June 2011Rev. -

PREPARED:

Katherine V. Sultani-Wright , PEProject Manager

CHECKED:

David L . Gray, PESenior Principal

APPROVED:

Jus tin M. Morgan, PEPrincipal-In-Charge

Digitally Signed12-Jul-2011

Digitally signed byKatherine V. Sultani-Wright


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Coastwise Corporation i The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Contents Executive Summary .................................................................................................... i

Section 1 Background and Location ..................................................................... 1

Section 2 Climatology ............................................................................................ 3

2.1 Technical Approach ...................................................................................................... 3

2.2 Local Winds .................................................................................................................. 3

2.2.1 Design Wind: 100-Year Return Period Local Wind ............................................. 4

2.3 Local Current ................................................................................................................ 6

2.3.1 Design Current ...................................................................................................... 6

2.4 Local Waves ................................................................................................................. 6

2.4.1 Description of SWAN ........................................................................................... 7

2.4.2

Bering Sea Waves ................................................................................................. 8

2.4.3 Design Waves: 100-Year Return Period Local Waves ....................................... 12

2.5 Metocean Design Climatology ................................................................................... 16

Section 3 Design Vessel ....................................................................................... 17

3.1 Environmental Loads .................................................................................................. 18

3.1.1 Wind Forces ........................................................................................................ 19

3.1.2 Current Forces ..................................................................................................... 19

3.1.3 Wave Forces ........................................................................................................ 20

Section 4 Governing Regulations ........................................................................ 21 Section 5 Concept Moor ing Buoy Design ........................................................... 23

5.1 Overview .................................................................................................................... 23

5.2 Mooring Components ................................................................................................. 23

5.2.1 Buoy .................................................................................................................... 24

5.2.2 Mooring Lines ..................................................................................................... 25

5.2.3 Anchors ............................................................................................................... 25

5.2.4 Vessel Connection ............................................................................................... 27

5.3 Mooring Analysis ....................................................................................................... 28

Section 6 Cost Estimate ....................................................................................... 32

Section 7 Recommendations ............................................................................... 33

Appendix A Annual Ex tr eme Winds

Appendix B SWAN Analy si s Resu lt s


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Coastwise Corporation ii The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Revisio n HistorySection Rev Descriptio n Date Approved

All P0 Initial release. 6/22/11 ---

All - Final release. No changes from P0. 6/30/11 JMM

TermsAIRA Aleutian Islands Risk Assessment

ABS American Bureau of Shipping

ADEC Alaska Department of Environmental Conservatism

AIS Automatic Identification System

API American Petroleum Institute

CFR Code of Federal Regulations

FPI Floating Production Installation

IACS International Association of Classification Societies

GROW Global Reanalysis of Ocean Waves

Gumbel extreme value probabilitydistribution

A statistical probability distribution used to forecast further extremes based on observed extremes

JONSWAP Joint North Sea Wave Project

Metocean An abbreviation of the two words "Meteorology" and "Oceanography."The term is often used in the offshore industry to describe the physicalenvironment.

MODU Mobile Offshore Drilling Units

NDBC National Data Buoy Center

NPD Norwegian Petroleum Directorate

OCIMF Oil Companies International Marine Forum

OrcaFlex A time-domain dynamic analysis code that includes the effects ofunsteady wind, first-order wave excitation, second-order wave drift,current, and nonlinear mooring forces on floating bodies

ORQ Oil Rig Quality

PPOR Potential Place of Refuge

QTF Quadratic Transfer Functions

ROV Remotely Operated Vehicle

SPM Single Point Mooring

SWAN Simulating Waves Nearshore; a wave generation and propagation modelthat can be used to derive the wave conditions in a nearshore area

USCG United States Coast Guard

WAMIT Wave Analysis MIT; a 3D frequency-domain radiation-diffraction panel program for analyzing the interaction of surface waves with offshorestructures

WBAN Weather-Bureau-Army-Navy


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Coastwise Corporation iii The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

References

1. Buttolph, A, Technical Memorandum: Sediment Modeling Report for Unalaska Airport, Appendix E2, CH2M Hill, http://www.unalaskaairportproject.com/download.html , 29October 2010.

2. SWAN (Simulating WAves Nearshore); Software Version 40.72, Delft University ofTechnology, Netherlands, May 2008.

3. Ang, Alfredo H-S and Wilson H. Tang, Probability Concepts in Engineering Planning and Design, Volume II: Decision, Risk, and Reliability , John Wiley and Sons, 1984.

4. Integrated Surface Hourly Data, Dutch Harbor, NOAA, National Climatic Data Center,Asheville, NC, June 2011.

5. Standard Meteorological Data, Station 46035 (LLNR 1198) – Bering Sea 310 nm North ofAdak, AK, National Data Buoy Center, June 2011.

6. Standard Meteorological Data, Station 46073 (LLNR 1199) – Southeast Bering Sea, National Data Buoy Center, June 2011.

7. Design and Analysis of Stationkeeping Systems for Floating Structures , AmericanPetroleum Institute, API RP 2SK, October 2005.

8. Rules for Building and Classing Mobile Offshore Drilling Units, Part 3: Hull Constructionand Equipment , American Bureau of Shipping, 2008.

9. Owens R and P Palo, Wind Induced Steady Loads on Ships , Technical Note N-1628, NavalCivil Engineering Laboratory, April 1982.

10. Prediction of Wind and Current Loads on VLCCs , OCIMF, Second Edition, 1994.

11. “Design: Moorings,” Unified Facilities Criteria , Department of Defense, UFC-4-159-03,October 2005.

12. Rules for Building and Classing Single Point Moorings , American Bureau of Shipping,1996.

13. Recommendations for Equipment Employed in the Bow Mooring of Conventional Tankersat Single Point Moorings , OCIMF, Fourth Edition, May 2007.

14. Vessel Traffic in the Aleutians Subarea , Nuka Research and Planning Group,http://www.aleutiansriskassessment.com/documents/060922AleutiansVesselReportSCREE

N.pdf , September 20, 2006.

15. Emergency Towing System for Aleutians, Alaska,http://www.dec.state.ak.us/spar/perp/aiets/home.htm .

16. WAMIT, WAMIT Incorporated, Software Version 6.41, December 2008.

17. OrcaFlex, Orcina Ltd, Software Version 9.4f, August 2010.


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Coastwise Corporation i The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Executive Summary

Background

The Aleutian Islands Risk Assessment (AIRA) project identified the need for acquisition of a

mooring buoy in Dutch Harbor to accommodate large disabled vessels. Dutch Harbor isstrategically located near Unimak Pass, which is frequently transited by large vesselsfollowing the Great Circle route. This report documents the conceptual design of a mooring

buoy in Broad Bay, to the northwest of Dutch Harbor. The proposed mooring buoy site islocated approximately at 53º-55.35 ′ N and 166º-37.00 ′ W.

Climatology

This report presents a climatological study to select design environmental conditions for themooring within Unalaska Bay. The design climatology represents a severe storm that isexpected once every 100 years. The resulting local wave heights are 4-5 m, depending on thewind direction. Wind speed is approximately 60 knots. Current is about 1.5 knots and is

always aligned with the wind, as tidal currents are negligible in the area.

Vessel Selection

The selection of the design vessel is based on limited information about recent vesselcasualties near Dutch Harbor; it is not based on a comprehensive vessel traffic study, whichwas not in the scope of the concept study. The design vessel used to size the mooringcomponents consists of an underwater hull form similar to a tanker or bulk carrier, and anabove water hull form similar to a car carrier. It is 206 m in length and 32.4 m in breadth, witha displacement of 64,387 tonnes.

Mooring Design

A three (3) leg, twin line mooringconfiguration was selected for theconcept design based on common

Navy fleet moorings. A sketch ofthe concept design is shown inFigure 1.

Anc ho r Se lect ion

An 18 tonne Bruce TS anchor isneeded to develop the required holdingcapacity of 700 kips. The four (4) existing 15 tonne Bruce TS anchors do not provide enoughholding capacity on an individual basis for the configuration shown in Figure 1, assuming soft

bottom conditions. Variations from the assumed bottom conditions or other designassumptions could have a significant impact on cost and design of the mooring system.

Cost Estimate

The total cost of installing this system with an ABS classification is estimated at $10.5 million,with an annual inspection cost of $250,000 excluding hardware replacement.

Figure 1 Concept design of mooring buoy for disabledvessels in Broad Bay, Alaska


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Coastwise Corporation 1 The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Section 1 Background and Location

Establishing Potential Places of Refuge (PPOR) in the Aleutian Island region was introducedas a means of risk reduction in the Ports and Waterways Safety Assessment for the AleutianIslands. Anticipated increases in vessel traffic in the Aleutians have focused the need tofurther improve the infrastructure to provide PPOR for vessels in distress. The AleutianIslands Risk Assessment (AIRA) project identified the need for acquisition of a mooring buoyin Dutch Harbor to accommodate large disabled vessels. 1 Dutch Harbor is strategically locatednear Unimak Pass, which is frequently transited by large vessels following the Great Circleroute. Figure 1 from Reference 13 illustrates the primary traffic routes.

Figure 2 Vessel Traffic on Great Circle Route (Reference 13)

The entrance to Dutch Harbor itself provides insufficient water depth for the largest vesselsanticipated to use the mooring buoy. Broad Bay was suggested by the Alaska Marine Pilots asa suitable location approximately four nautical miles northwest of Dutch Harbor withinUnalaska Bay. The area offers relatively open waters for maneuvering in water depths of 30-50 fathoms. Due to the geography of Unalaska Bay, Bering Sea waves approaching from thenortheast sector have an unobstructed path into Broad Bay; however, it is protected from oceanswell in other directions. Figure 2 illustrates the proposed mooring buoy site, locatedapproximately at 53º-55.35 ′ N and 166º-37.00 ′ W.

1 http://www.aleutiansriskassessment.com/


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Figure 3 Proposed Mooring Buoy Site

The Alaska Department of Environmental Conservatism (ADEC) commissioned this work toinvestigate further development of Unalaska Bay as a PPOR. This report documents theconceptual design of a mooring buoy to serve as a PPOR in Broad Bay. Subsequent sectionsaddress the climatology in the bay, the design vessel selection, governing regulations, themooring design concept, and the costs of installing and maintaining the system.

ProposedMooring Buoy

Site


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Section 2 Climatology

2.1 Technical ApproachA common methodology for offshore structures design, including moorings, is to use

Metocean criteria associated with the 100-year return period condition. As consistent qualityenvironmental data records are almost never available for periods exceeding 100 years, it isnecessary to employ extrapolation methods to estimate the wind speeds or wave heightsassociated with a 100-year return period. Different methods have been developed forconditioning and extrapolating the available data in order to develop an estimate of theextreme (i.e., 100-year return) wind or wave. This extrapolation of wind speed and waveheight was executed using a data set comprised of annual extremes, sorted by direction,

because the data record length was adequate. Reduction of the twenty-three year wind recordto annual extremes provides a sufficient number of data points to lend confidence to thismethod of extrapolation.

2.2 Local WindsDesign wind conditions were established using a twenty-three year wind record at DutchHarbor Airport for the years 1988-2010 (Reference 4). Table 1 contains details about theDutch Harbor Airport weather station.

Table 1 Weather Station Specifics

Dutch Harbor Airpo rt Alaska, Unit ed States

WBAN Identification Number 704890

Elevation 4 m above local ground

Latitude / Longitude 53º 54 ′ N / 166º 33 ′ W

The joint probability distribution of wind speed and direction at the Dutch Harbor Airport isshown in Figure 4.


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Figure 4 Joint probability distribution of wind speed and direction at Dutch Harbor Airport

2.2.1 Design Wind: 100-Year Return Period Local Wind

The 100-year return wind speed was determined by the expected value based on annualextremes by direction. The following describes the process by which that value is determined.

The annual maximum wind speeds were extracted from the 23-year Dutch Harbor data set bydirection, for eight direction sectors. Wind direction is defined as the direction from which thewind is blowing, in degrees from true north. The eight direction sectors were bounded asshown in Table 2. The directional data was reported to the nearest degree, so the ranges weredefined to the half-degree just above the upper bound and to the half-degree just below thelower bound. This resulted in nine sets, one for each direction and one across all directions, ofannual extremes, each containing 23 data points, one for each year.

The wind speed averaging period is not reported in Reference 4; however, for this analysis it

was assumed to be a one-minute average. Airport wind data is typically recorded as a one-minute average in our experience, and this assumption has been accepted by ABS in previouswork.


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Table 2 Definition of wind direction sectors

Direction Heading,deg true

Lower Heading,deg true

Upper Heading,deg true

N 0 337.5 22.5

NE 45 22.5 67.5

E 90 67.5 112.5

SE 135 112.5 157.5

S 180 157.5 202.5

SW 225 202.5 247.5

W 270 247.5 292.5

NW 315 292.5 337.5

Gumbel extreme value probability distributions were fit to all nine sets of data. Figuresshowing extrapolation are shown in Appendix A. A summary of the 100-year returnextrapolation for all eight (8) directional sectors is presented in Table 3. The wind speeds in

the table are the expected value one-minute average wind speed at 4 m.Table 3 Summary of 100-year return one-minute average wind speeds at 4 m based on Dutch Harbor

Airport data, knots

U(4.0 m, 60 sec), knots N NE E SE S SW W NW ALL

Expected Value 54 46 61 60 69 60 57 61 66

The one-hour average wind speeds are used in the SWAN wave hindcast and the OrcaFlexmooring analysis. Therefore, it was necessary to convert from one-minute averages to one-hour averages. According to the recommendations in the API RP2SK-Appendix B, the

Norwegian Petroleum Directorate (NPD) wind spectrum was assumed (Reference 7).

The method in Reference 7 was used to obtain U 0, the one-hour average wind speed at anelevation of 10 meters, as a function of direction based on the expected value of the one-minute average wind speeds at 4 m elevation. If the expected value for a directional sector fell

below the expected value for all directions, the expected value for all directions was used;otherwise the expected value for the directional sector was used. This procedure was used asan attempt to reduce the effects of local topography on the wind speed data.

The results of the transformation of one-minute average wind speeds at 4 m elevation to one-hour average wind speeds at 10 m are shown in Table 4.

Table 4 Transformation from one-minute average to one-hour average 100-year return period windspeeds using NPD Spectrum wind gust formulation

Wind Heading, deg true

One-minute average wind speedat 4 m, knots

U(4 m, 60 sec)

One-hour average wind speed at10 m, knots

U(10 m, 3600 sec)

0/360 66 58

45 66 58

90 66 58

135 66 58


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Wind Heading, deg true

One-minute average wind speedat 4 m, knots

U(4 m, 60 sec)

One-hour average wind speed at10 m, knots

U(10 m, 3600 sec)

180 69 61

225 66 58

270 66 58

315 66 58

2.3 Local CurrentA study on currents in Unalaska Bay was conducted and reported on in Reference 1. No otherdata on local currents was sought out, as the report indicated that currents were generally veryweak, less than 0.25 m/sec (0.5 knots). Modeling efforts showed that currents in Broad Baywere less than 0.02 m/sec (0.04 knots) during peak ebb and flow tidal events, which wasreported to correlate well with current measurements.

Due to the low tidal current speeds in the area of interest, tidal current forces were not includedin the mooring analysis. However, wind stress current may still be present and is accountedfor in the analysis.

2.3.1 Design Current

Wind stress current at the surface is estimated by many references to be 2.5% of the steadywind speed, which will be interpreted to be U (10 m, 3600 sec), the one-hour average windspeed at an elevation of 10 meters. There are diverse models for the vertical profile of thewind stress current in the literature, but it may be conservatively regarded as classical planeCouette flow with a profile that varies linearly from maximum at the surface to zero at the

bottom. ABS Mobile Offshore Drilling Unit (MODU) Rules (Reference 8) provide guidance

that, in agreement with several other sources, suggests that the wind stress current is confinedto a near surface region. Applying the guidance of the ABS MODU Rules the vertical profileof the wind stress current would vary linearly over the top five meters of depth, from amaximum at the surface to the combined tidal and storm surge current (in this case, effectivelyzero) at 5 m depth. However, due to the lack of measured data or computational modeling ofthe current flow in Unalaska Bay, the more conservative linear vertical profile was chosen.

Accordingly, for each of the eight (8) cases, the one-hour average wind speed at 10 meters wasused as the steady wind speed and the current velocity was taken as 2.5% of that value. Alinear vertical profile was assumed, with the current speed at the sea surface equal to 2.5% ofthe steady wind speed and the current speed at the sea floor equal to zero.

2.4 Local WavesThe wave environment near the proposed mooring buoy site in Broad Bay can be characterized

by waves generated by the local winds and by waves entering Broad Bay from the Bering Sea.There is no source of data available for the waves at proposed mooring buoy site, so waves aregenerated and propagated computationally over a domain including the proposed mooring

buoy site. The modeling software used to analyze the wave environment is SWAN, a third


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generation wave model that computes random, short-crested, and wind-generated waves incoastal regions and inland waters (Reference 2).

2.4.1 Descript ion of SWAN

SWAN is a wave generation and propagation model that can be used to derive the wave

conditions in a nearshore area. SWAN is also suitable for use as a wave hindcast model inwater of intermediate and shallow depth for situations where the wind field may be considereduniform. Typical areas for the application of SWAN range between 10 x 5 km 2 and30 x 100 km 2.

SWAN is a two dimensional full spectral wave model for wave propagation in shallow waterincluding refraction and shoaling, growth due to wind action, non-linear wave interactions(triad and quadruplet) and dissipation by bottom friction and breaking. SWAN is appropriatefor and typically used for the simulation of wave generation, propagation, and dissipation incoastal areas.

The processes modeled by SWAN are:

Wave generation by a spatially varying wind

Refraction over a bottom of variable depth

Refraction over a spatially varying ambient current

Dissipation by wave breaking

Dissipation by bottom friction

Wave blocking by current

Non-linear wave interactions

SWAN explicitly includes the effects of non-linear four wave interactions (quadruplets) andthree wave interactions (triads). The discrete representation of the frequency spectrum meansthat SWAN is more suitable than previous models for application in areas where strong growthdue to wind action may occur and where the remains of old sea states or swell is also present(e.g., behind island barriers or bank systems).

SWAN calculates the wave field on a two dimensional horizontal rectangular grid covering thecomputational area. At each grid point, SWAN represents the complete 2D-action densityspectrum discretely as a function of frequency and direction. SWAN calculates wave

propagation in all directions. The solution technique marches forward row by row over thegrid beginning at the incident wave boundary, where the incident wave characteristics aredefined. The results in each direction sector at each grid point are computed from the resultsfor the grid points in the previous row. The propagation of energy is modeled using an energy

balance equation adapted to include terms for wave growth by wind action or dissipation dueto bottom friction or wave breaking.

SWAN has been verified using results both from field measurements and from physical modeltests. The SWAN program can be obtained from the internet site of Delft University ofTechnology, see http://fluidmechanics.tudelft.nl/ (Reference 2).


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2.4.2 Bering Sea Waves

There are two sources of Bering Sea wave data: NDBC buoys 46035 and 43073, shown inFigure 5 (References 5 and 6). Table 5 summarizes the data provided at each buoy.

Table 5 Bering Sea wave buoys near Dutch Harbor

Wave Buoys in the Bering SeaBuoy Identifier 46035 46073

Description Bering Sea 310 nm North of Adak, AK Southeast Bering Sea

Date range available 1985 - 2010 2005- 2010

Latitude / Longitude 52.067º N / 177.75º W 57.011º N / 170.981º W

Figure 5 Offshore wave data buoys (46035 and 43073) in relation to Dutch Harbor

Despite the fact that buoy 46073 is closer to Dutch Harbor, buoy 46035 was selected tocharacterize the Bering Sea waves because it had a longer time record of wave data. However,for the years that data was available for 46073, those data points were used instead of thosefrom 46035. Due to the geography of Unalaska Bay, Bering Sea waves approaching from thenortheast sector have an unobstructed path into Broad Bay. Therefore, only Bering Sea wavesfrom a sector defined by 15 deg true and 75 deg true were considered in developing the 100-year return period design climatology. This is shown in Figure 6.


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Figure 6 Bering Sea waves sector from which annual extreme wave heights were determined

The buoy data did not contain wave direction, but it did contain wind direction. In the absenceof any other information, the Bering Sea waves were assumed to be aligned with the wind, andso the wind direction was used as a proxy for the wave direction. The annual extreme eventsfrom the northeast sector are shown in Table 6.

Table 6 Annual extreme Bering Sea wave heights from northeast sector (15 deg true – 75 deg true)

Date BuoySignificant Wave

Height, mDominant Wave

Period, secWind Direction,

deg true29-Oct-1985 46035 8.5 12.5 70

5-Mar-1986 46035 8.6 14.3 69

21-Mar-1987 46035 10.1 12.5 29

14-Dec-1988 46035 7.7 12.5 21

22-Dec-1989 46035 6.8 12.5 50

19-Jan-1990 46035 12.4 16.7 24

22-Dec-1991 46035 10.5 14.3 44

4-Feb-1992 46035 10.2 14.3 57

25-Dec-1993 46035 8.6 12.5 2724-Feb-1994 46035 7.7 12.5 40

20-Nov-1995 46035 8.5 12.5 36

4-Feb-1996 46035 7.7 12.5 47

8-Jan-1997 46035 10.96 14.29 42

23-Feb-1998 46035 8.65 11.11 59


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Date BuoySignificant Wave

Height, mDominant Wave

Period, secWind Direction,

deg true

7-Nov-1999 46035 9.07 12.5 55

2-Dec-2000 46035 9.97 12.5 40

11-Nov-2001 46035 8.12 11.11 64

1-Jan-2002 46035 8.92 12.5 36

7-Jan-2003 46035 9.02 12.12 68

9-Feb-2004 46035 8.29 12.12 30

8-Nov-2005 46073 9.28 12.9 24

28-Dec-2006 46073 4.79 11.43 22

16-Jan-2007 46073 7.22 12.9 65

22-Oct-2008 46073 8.26 12.12 39

6-Oct-2009 46073 6.82 10.81 47

8-Feb-2010 46073 7.78 10.81 47

These annual extremes over twenty-six years of record were used as a basis for a Gumbelextrapolation of the extreme significant wave height expected in one hundred years of record,as shown in Figure 7. Not all of the annual extremes were used in the extrapolation; the dataset was chosen to maximize the goodness-of-fit of the extrapolation to the upper end of thedata. The resulting estimate of the expected 100-year return Bering Sea significant waveheight from the northeast sector was 14.5 meters.


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Figure 7 Extrapolation of 100-year return Bering Sea significant wave height for waves arriving fromthe northeast sector

The peak period associated with the 100-year return Bering Sea significant wave height was

determined according to the method shown in Figure 8. A line of best-fit constant wavesteepness was calculated for the 26 extreme combined significant wave height – dominant period points. The dominant period for the 100-year return Bering Sea significant wave heightwas selected based on the line of constant wave steepness, and was determined to be 16.5seconds.

0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

5

6

7

8

9

10

12

14

16

18

20

22

S i g n i

f i c a n

t W a v e

H e i g h

t , m e t e r s

Cumulative Probability

Annual Maximum Wave Heights (1985-2010)Buoy 46035 and 46073NE Sector Only (15 deg true - 75 deg true)

Return Period, years

Upper 90% Prediction Bound andLower 90% Prediction Bound shownas dashed lines about Expected Value.


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Figure 8 Conditional peak period expected in association with 100-year return Bering Sea significantwave height for waves arriving from the northeast sector

2.4.3 Design Waves: 100-Year Return Period Local Waves

Both the local winds and Bering Sea waves were input to a SWAN model of the proposedmooring buoy site. The 100-year return one-hour average wind speeds at 10 m elevation andthe 100-year return Bering Sea wave data were used as inputs to a SWAN wave generation and

propagation model. The SWAN model is based on the bathymetry surrounding the proposedmooring buoy site in Unalaska Bay.

2.4.3.1 SWAN Model of Unalaska Bay

For the wave analysis at the proposed mooring buoy site, SWAN was used with temporal andspatial stationary wind and boundary waves, thus producing the fully-developed solution thatwould be obtained if the forcing conditions persisted forever. SWAN is capable of modelingresponse to temporally and spatially non-stationary (transient) forcing. However, the adoptedapproach of assuming uniform, homogenous, and stationary forcing conditions is conservative,i.e. predicting higher wave heights at the mooring site.

The SWAN model was set up according to the following: a JONSWAP spectrum representingthe Bering Sea waves was applied on three of the four boundaries; the southern boundarycontained almost exclusively land, so waves were not applicable. The JONSWAP spectrumwas derived as described above using two buoys. Figure 9 below shows the SWANcomputational domain. The computational domain encompasses an area of approximately400 km 2 with grid points every 60 m, which is within the normal operating limits of the

0

2

4

6

8

10

12

14

16

18

20

0 2 4 6 8 10 12 14 16 18

D o m

i n a n

t P e r

i o d ( T p

) , s e c

Wave Height (Hs), m

Annual Maximum Wave Height - 46035

Line of b est fit - 46035

Annual Maximum Wave Height - 46073

Line of b est fit - 46073

Hs/gTp 2 = 0.00539

Hs/gTp 2 = 0.00543

Tp = 16.5 sec

H s =

1 4

. 5 m


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SWAN software. The Bering Sea waves were applied on the northern boundary. Winds wereapplied over the entire computational domain according to the speed and direction calculatedand shown in Table 4. The selected combinations of Bering Sea waves and local windconditions results in eight (8) separate cases for analysis.

Figure 9 SWAN computational domain (highlighted area shown in Figure 10)


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Figure 10 SWAN computation domain in the immediate vicinity of the proposed mooring buoy site

Eight cases were modeled using SWAN. A summary of these eight (8) cases is shown inTable 7. A heading of 0 degrees corresponds to a wind or wave coming from true north, aheading of 180 degrees corresponds to a wind or wave coming from true south. Each casecorresponds to local wind from all directions at 45 degree increments and Bering Sea wavesfrom 45 degrees. A sensitivity study was conducted to find the angle of Bering Sea waves thatmaximized the waves at the proposed mooring buoy site. It was concluded that Bering Seawaves coming into the bay at a 45 degree heading produced the largest waves at the mooringsite.


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Table 7 Summary of SWAN cases

CaseNumber

Wind Speed,knots

U(10 m,3600 sec)

WindHeading,deg true

Bering SeaSignificant Wave

Height, meters

Bering SeaWave PeakPeriod, sec

Bering SeaWave Heading,

deg true

1 58 0 14.5 16.5 452 58 45 14.5 16.5 45

3 58 90 14.5 16.5 45

4 58 135 14.5 16.5 45

5 61 180 14.5 16.5 45

6 58 225 14.5 16.5 45

7 58 270 14.5 16.5 45

8 58 315 14.5 16.5 45

Table 8 shows a summary of the local waves at the proposed mooring buoy site as predicted by SWAN. The full set of SWAN results is presented in Appendix B.

Table 8 Summary of local waves based on SWAN analysis

Case Number

SignificantWave Height,

mPeak WavePeriod, sec

Peak WaveHeading , deg

true

1 4.37 9.0 57.5

2 5.04 8.9 57.5

3 4.95 8.4 57.5

4 4.31 8.0 57.55 4.18 8.0 57.5

6 4.06 8.5 57.5

7 4.03 8.6 57.5

8 4.05 8.5 57.5


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2.5 Metocean Design ClimatologyTable 9 summarizes the design climatology for the proposed mooring buoy site. All headingsare in degrees true.

Table 9 Eight climatology cases at proposed mooring buoy site

CaseNumber

WindSpeed,knots

U(10 m,3600 sec)


SignificantWave

Height, m

PeakWave

Period,sec

PeakWave

Heading,deg true

CurrentSpeed,knots

CurrentHeading,deg true

1 58 0 4.37 9.0 57.5 1.45 ---- S a m e a s Wi n

d H e a d i n g----

2 58 45 5.04 8.9 57.5 1.45

3 58 90 4.95 8.4 57.5 1.45

4 58 135 4.31 8.0 57.5 1.45

5 61 180 4.18 8.0 57.5 1.53

6 58 225 4.06 8.5 57.5 1.45

7 58 270 4.03 8.6 57.5 1.45

8 58 315 4.05 8.5 57.5 1.45


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Section 3 Design Vessel

A comprehensive vessel traffic study, in order to identify a possible range of vessels that mightuse the mooring buoy, was not conducted during this phase of the design.

There was a vessel traffic study that was conducted in the area, Reference 14; however itsummarizes traffic primarily based on oil capacity and gross tonnage. These parameters are oflimited use in selection of a design vessel for the mooring system. ADEC owns the sourceAIS data used to develop the traffic study in Reference 14 2, but could not provide it in time forthe present work. This data could be analyzed in the next design phase to link vessel size totraffic through Unimak Pass and validate the design vessel selection.

In lieu of that analysis, a design vessel was chosen based on recent vessel casualties in thearea. Two recent casualties involved Panamax vessels shown in Table 10.

Table 10 Reference vessels for design vessel selection

Vessel Vessel Type Nominal Capacity

Length

Overall, m Beam, m Draft, mCougar Ace Car Carrier 5500 cars 199 32.26 9.72

Selendang Ayu Bulk Carrier 75000 DWT 225 32.26 12.62

Figure 11 Serenity Ace , a similar vessel to the Cougar Ace , which has a large windage area

Figure 12 Selendang Ayu , which has a small windage area, but a deeper draft (see Table 10)

A composite vessel was created based on the underwater area of the bulk carrier and thesuperstructure of the car carrier. This composite vessel captures the effects of a deeper draftand a large windage area. For a concept level mooring buoy design, this composite vessel isconsidered appropriate. The particulars of the design vessel are shown in Table 11.

2 Reference 14 analyzed nine months of data (October 1, 2005 through June 30, 2006) from the automatedidentification system (AIS) installed at Scotch Cap, Unimak Pass. AIS data information for each vessel detected

provides the vessel name, vessel type, next and last port, and call-sign. Vessel identification information can becross-referenced with the Lloyd’s Register database to identify vessel size.


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Table 11 Particulars of design vessel

Length Overall (LOA) 206 mBeam 32.4 mDraft 12 mDisplacement 64,387 tonnesFreeboard 23.8 m

A vessel of this size (~55,000 DWT) matches the capability of the Emergency Towing Systemin Dutch Harbor. The City of Unalaska has purchased a system suitable for vessels up to50,000 DWT, and the Alaska Department of Environmental Conservation is purchasing asystem capable of towing vessels greater than 50,000 DWT according to Reference 15.

The design vessel selected does not represent the largest known Panamax vessel or account forfuture vessel traffic trends. “New Panamax” size vessels are expected to transit Unimak Passwhen the Panama Canal’s Third Set of Locks Project is complete and open for vessel traffic.Table 12 below provides a comparison between notional Panamax and “New Panamax” vessel

characteristics.Table 12 Panamax vs. “New Panamax” vessel size

VesselType Nominal Capacity LOA Beam Depth

MaximumDraft

Displacementat Maximum

Draft

Freeboardat

MaximumDraft

m m m m tonnes mPanamax:Container 4,000-5,000 TEU 294 32.2 22.6 13.3 80,000 9.3

Tanker 75,000 DWT 230 32.26 20.7 14.6 90,000 6.1 New Panamax:Container 12,000 TEU 366 49 27.9 15.2 160,000 12.7

Tanker 145,000 DWT 274 48 24.4 15.2 165,000 9.2

3.1 Environmental LoadsWind, current, and wave loads on the design vessel were calculated and included in theanalysis of the mooring buoy. The forces on the vessel are defined in the coordinate systemshown in Figure 13.

Figure 13 Environmental loads sign convention


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The following sections describe the environmental loads on the design vessel.

3.1.1 Wind Forces

The wind forces on the design vessel were estimated using Reference 9 based on the Cougar Ace above water profile. The wind force coefficients are shown in Figure 14.

Figure 14 Wind force coefficients for design vessel

3.1.2 Current Forces

The current forces on the design vessel were estimated using Reference 10 based on a bulkcarrier/tanker hull form. The current force coefficients are shown in Figure 15.


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Figure 15 Current force coefficients for design vessel

3.1.3 Wave Forces

First and second order wave forces were calculated for the design vessel using WAMIT(Reference 16).

WAMIT (Wave Analysis MIT) is a 3D frequency-domain radiation-diffraction panel programfor analyzing the interaction of surface waves with offshore structures. Forward speed effectsare not accounted. The program implements a set of highly efficient algorithms for theGreen’s function computations which are at the core of the boundary element method. Theversion currently available in the office is the most up-to-date PC executable V6.41PC whichsolves the linear hydrodynamic problem. It can be used to evaluate the added masses,damping coefficients, wave exciting forces, motions, hydrodynamic pressure at specified

points, fluid velocity vector at specified points, free surface elevation at field points, andsteady drift forces. In addition to the six rigid-body modes of a floating body, WAMITV6.41PC can handle multiple bodies, body near vertical walls, user-specified generalizedmodes, etc. Several tools are available for use in conjunction with WAMIT to model externalsprings, connections between bodies, etc., and to evaluate response statistics including motionsat a point in specified sea states.


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Section 4 Governing Regulations

The concept design presented in this report is designed to ABS Rules for Building andClassing Single Point Moorings (Reference 12). The design conditions and safety factorsdefined by these rules are summarized in Table 13. For the purposes of this analysis, thedesign operating condition was assumed to be the 100-year storm. A damage analysis was not

performed for the concept design.

Table 13 Factors of safety used for concept design (Reference 12)

Required factors of s afety on ancho r legs

Design storm (100 year) without vessel, Intact 2.50 Design operating with vessel, Intact 3.00 (2.50 can be used if damaged case

(one line damaged) meets 2.00 FOS) Required factor of safety on mooring hardware Maximum of:

Design storm (100 year) without vessel, Intact 2.50 x Maximum anchor leg tension

Design operating with vessel, Intact 3.00 x Maximum anchor leg tension Required factor of safety on anchor c apacity

Design storm (100 year) without vessel, Intact 1.50 Design operating with vessel, Intact 2.00

The following regulations and industry standards related to single point moorings should beconsidered in subsequent design and development.

1) American Bureau of Shipping:

a. ABS Rules for Building and Classing Single Point Moorings , 1996.As noted above these rules form the design basis for the concept in this report. TheSingle Point Mooring (SPM) rules were developed around CALM buoy typeinstallations where product is loaded through a riser and floating hose. The safetyfactors are set so that a fatigue analysis is not required.

b. ABS Guide for Building and Classing Floating Production Installations , November2010. This guide provides an alternative classification path for SPMs with lowersafety factors, but more rigorous analysis requirements. A fatigue analysis is required.The guide generally follows and references API RP 2SK.

c. ABS Guide for the Mooring of Oil Carriers at Single Point Moorings , December 2010.The mooring design should accommodate vessels fitted for standard SPM equipmentidentified in this guide. The guide contains the following description of its purpose:

“This Guide has been developed in response to industry requests for anoptional ABS Class notation to address arrangements where an Oil Carrier is

fitted with equipment enabling it to be moored to single point moorings.”

d. ABS Guidance Notes on the Application of Synthetic Ropes for Offshore Mooring,March 1999. This guide applies to the hawser.

e. ABS Guide of the Certification of Offshore Mooring Chain, December 2009.


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2) International Association of Classification Societies:

a. IACS Recommendation No. 38 [(1995), Rev 1, October 2010] “Guidelines for theSurvey of Offshore Mooring Chain Cable in Use.”

3) American Petroleum Institute:

a. API RP 2SK: Design and Analysis of Stationkeeping Systems for Floating Structures,October 2005. This publication provides supplementary information and guidance forthe design and analysis of mooring systems.

b. API RP 2I: Recommended Practice for In-Service Inspection of Mooring Hardware forFloating Drilling Units, May 1987.

4) Oil Companies International Marine Forum:

a. OCIMF, “Prediction of Wind and Current Loads on VLCCs,” Second Edition, 1994.

b. OCIMF, “Single Point Mooring Maintenance and Operations Guide,” Second Edition1995.

c. OCIMF, “Recommendations for Equipment Employed in the Bow Moorings ofConventional Tankers at Single Point Moorings,” Fourth Edition, May 2007.

5) Department of Defense, Unified Facilities Criteria, “Design: Moorings,” UFC-4-159-03,October 2005.

6) Owens R and P Palo, “Wind Induced Steady Loads on Ships,” Technical Note N-1628, Naval Civil Engineering Laboratory, April 1982.

7) United States Aids to Navigation System , 33 CFR 62, USCG, July 2010.


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Section 5 Concept Mooring Buoy Design

5.1 OverviewThe proposed design is a riser-type mooring commonly used by the Navy for fleet moorings.

The main components of the mooring configuration are described below.It is recognized that the riser-type mooring does not provide redundancy for damage to theriser chain. The advantage of the riser-type mooring is that vessels can weather-vane about themooring unrestricted by the ground legs. ABS recently suggested considering a higher, but asyet undefined, safety factor for the riser chain and mooring components without redundancy.

The design philosophy at this early concept level was to select commercial off-the-shelfhardware for a more dependable cost estimate. Alternate means of connecting the anchor legsmay be explored as the design is refined in future development.

5.2 Mooring ComponentsThe mooring consists of a hawse pipe type mooring buoy, riser chain, ground ring, anchorlegs, and drag embedment anchors. A three leg, twin anchor line configuration was selectedfor the concept design. Figure 16 illustrates the arrangement. The riser chain, equipped with achain swivel, connects the ground ring to the buoy. Twin anchor legs laid out with a nominal120 degree spread connect the anchors to the ground ring. A twin anchor leg arrangement isused to develop the required capacity. Triangular “spider” plates equalize the twin anchor legloads before connecting to the ground ring.

The primary components of the proposed mooring buoy are shown in Figure 16.

Figure 16 Components of mooring system (not to scale)

The ground ring is suspended above the seafloor to ensure that the riser chain is always undertension and so that the connection hardware does not contact the seabed.

Riser Chain

Anchor Leg

Drag Anchor

Mooring Buoy

Ground Ring


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The alignment of the six-line mooring is shown in Figure 17. The alignment was chosen suchthat the mooring arrangement could best accommodate forces due to Bering Sea wavesentering Unalaska Bay from the northeast.

Figure 17 Plan view of concept mooring buoy mooring arrangement

5.2.1 BuoyA foam-filled mooring buoy fitted with a through-chain hawse pipe and capture plate providesa net buoyancy of 45 tonnes. This design assumes a Trelleborg MB-45000 mooring buoy withan overall diameter of 4.2 meters and a height of 4.1 meters (excluding hawse pipe andhardware). The buoy has a nylon filament reinforced polyurethane skin which has excellentresistance to water, oil, ice, strong sunlight, and abrasive surfaces. It remains flexible even at -40°C (-40°F) making it suitable for Arctic installations. Lighting to suit USCG aids tonavigation requirements and a chafing guard will be required accessories. In the 100-yearstorm event the buoy will fully submerge. Several vendors offer equivalent mooring buoys.Figure 18 illustrates the buoy construction; however, the internal core in the figure would bereplaced with a hawse pipe.

100 m

N

W

S

E

45 deg


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Figure 18 Mooring buoy construction

5.2.2 Mooring Lines

There are two types of chain used in the mooring buoy arrangement, the properties of whichare shown in Table 14. The chain types correspond to the parts labeled in Figure 16.

Table 14 Properties of mooring chainComponent Size Number x Length Breaking Strength Mass

Riser Chain 90 mm R4stud-link chain 1 x 48 m 8,167 kN 177 kg/m

Anchor Leg 76 mm ORQstud-link chain 6 x 350 m 4,621 kN 126 kg/m

5.2.3 Anchors

The composition of the seafloor at the mooring site is presently unknown, so proper selectionof anchors is not possible at this stage of design. For the concept design, drag-embedment

anchors were selected based on the assumption of a mud seafloor.Preliminary analysis showed that the highest tension in any of the anchor legs is 1526 kN (343kips). According to the guidance in Reference 12, the minimum factor of safety on anchorholding capacity is 2.00. Therefore, the anchors must have a minimum holding capacity of3051 kN (686 kips). Assuming a minimum holding capacity of 700 kips, the anchor sizesshown in Table 15 are possibilities. The anchor types were selected based on Reference 7.


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Table 15 Drag embedment anchor candidates for concept mooring design

Anchor Type Anchor Weight Anchor Shank

Length Anchor Fluke

Width

HoldingCapacity(in mud)

Bruce FFTS MK 4 15,000 kg

(approximately 30kips)

5.7 m 6.8 m 3,100 kN

(700 kips)

Vryhof Stevpris MK 5 15,000 kg(approximately 30

kips)

6.3 m 6.8 m 3,100 kN(700 kips)

Bruce TS 18,000 kg(approximately 40

kips)

6.9 m 5.3 m 3,100 kN(700 kips)

Moorfast 31,780 kg(approximately 65

kips)

6.2 m 6.9 m 3,100 kN(700 kips)

ADEC indicated that four Bruce TS anchors weighing 15 tonnes are available for purchase inAlaska. These anchors are not quite large enough to develop the required holding power andsafety factor for the mooring design. An 18 tonne Bruce TS anchor is required as shown inTable 15. If the anchors can be obtained at a substantial discount (i.e. less than half the costfor a new anchor of the required size), then they could be utilized in tandem on two of the six

legs. Additional anchors for the remaining four legs would be required from another source.We do not recommend purchasing anchors until a bottom survey is complete. The anchorselection will need to be revisited in the next design phase after bottom surveys are complete.

The concept mooring buoy design incorporates drag-embedment anchors. However, pile-driven plate anchors are another option that may be considered. Driven plate anchors must bedesigned for the site-specific soil conditions. Since these are presently unknown, a plateanchor design was not pursued. Design features of both types of anchors are presented inTable 16 (Reference 11).


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Table 16 Properties of drag embedment and driven plate anchors (Reference 11)

Drag Embedment Anchors Driven Plate Anchors

Works primarily in one horizontal direction(cannot tolerate any uplift)

Multi-directional and can resist anchor uplift

Requires large scope of chain to ensure nouplift at anchor

Can be used with a short scope of chain because anchor can resist uplift

Performance depends strongly on soil type;May not work with all seafloor typesincluding hard clay, gravel, coral, rock, orhighly layered seafloors; May not work wellfor sloping seafloors more than severaldegrees

Anchors must be designed for the site-specificsoil characteristics

Adequate seafloor sediment is required for proper setting

Adequate seafloor sediment is required for proper setting

Anchor may drag if overloaded at a slowenough rate Anchor is fixed and will not drag

Anchors can be recovered and reused Anchors cannot typically be recovered

Proof loading is recommended Proof loading is recommended; Mobilizationof installation equipment can be expensive

5.2.4 Vessel Connection

The connection hardware between the disabled vessel and the mooring buoy has not beendeveloped in detail as part of the concept design. However, a typical arrangement is shown in

Figure 19 for a single point mooring used exclusively by tanker vessels. The initial hardwaresizes selected for the hawser and chafing chain accommodate vessels less than 100,000 tonneDWT with OCIMF recommended SPM equipment. Not all vessels will have standard SPMfixtures; however, the chain termination of the hawser presents a generic connection interfaceto accommodate as many vessels as possible.

The chafing chain is 76 mm ORQ stud-link chain and the hawser is 144 mm nylon double braid. Complete specifications of the vessel connection hardware remain to be developed.


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Figure 19 Typical arrangement to facilitate connection of vessel to mooring buoy (adapted fromReference 13)

5.3 Mooring AnalysisA preliminary screening analysis of the concept mooring design was executed using OrcaFlex.

Figure 20 shows a three-dimensional rendering of the OrcaFlex model.OrcaFlex (Reference 17) is a time-domain dynamic analysis code that includes the effects ofunsteady wind, first-order wave excitation, second-order wave drift, current, and nonlinearmooring forces on floating bodies. At each time step, the vessel acceleration is computed fromthe instantaneous forces arising from these sources. This code makes use of first-order waveexcitation forces, radiation added mass and damping, and second-order drift force quadratictransfer functions (QTFs) that are computed by external three-dimensional hydrodynamicradiation-diffraction codes.

To Mooring BuoyTo Vessel


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Figure 20 Fish-eye view of concept mooring buoy mooring design as modeled in OrcaFlex

One-hour simulations of each design climatology case (see Table 9) were performed inOrcaFlex with the design vessel connected to the mooring buoy with all mooring lines intact.At the start of each simulation, the design vessel was oriented with its bow into the wind. The

bathymetry was directly taken from the bathymetry used in the SWAN analysis and reflectsthe water depth at mean high water.

Line tension results of the intact simulations are shown in Table 17 corresponding to the linenumbers shown in Figure 21. The maximum line tension is presented for each designenvironmental case.


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Figure 21 Line numbering in OrcaFlex model

Table 17 Anchor leg tension results

Case

Number

WaveHeading,

deg true

WindHeading,

deg true

Maximum Anchor

LegTension,

kN

Anchor

Leg

SecondHighest Anchor

LegTension,

kN

Anchor

Leg1 57.5 0 863 L0 681 L012 57.5 45 1039 L21 1027 L03 57.5 90 1158 L2 1103 L214 57.5 135 1526 L2 1277 L115 57.5 180 1504 L11 1261 L16 57.5 225 1115 L11 763 L17 57.5 270 861 L0 783 L018 57.5 315 1400 L1 1299 L0

Max.: 1526 Max.: 1299

FOS: 3.03 FOS: 3.56

The maximum tension in the riser chain was 2607 kN, which yields a safety factor of 3.13 on breaking strength.

In addition to line tension, the suspended length of each anchor leg was examined to ensurethat the drag anchors did not experience any uplift forces. This was shown to be the case andthe full set of results is shown in Table 18.

100 m

N

W

S

E

L0

L01

L1

L11

L2

L21


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Table 18 Suspended length results

CaseNumber

WaveHeading,deg true


MaximumSuspendedLength, m

MinimumLeg Lengthon Seabed,

m Anchor

Leg1 57.5 0 261 90 L02 57.5 45 291 60 L03 57.5 90 271 80 L04 57.5 135 221 131 L05 57.5 180 330 20 L116 57.5 225 180 170 L117 57.5 270 261 90 L08 57.5 315 322 30 L01


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Section 6 Cost Estimate

The estimated costs associated with designing, installing, and maintaining an ABS-certifiedsystem are summarized in Table 19. The engineering and installation cost estimates are basedon two projects with similar sized hardware. The bulk of the installation costs were developedfrom our experience with an ABS classed FPI eight-leg catenary spread mooring installed inthe Aleutian Islands.

ABS was consulted regarding review fees for certification. The fees will vary slightlydepending on whether the mooring is classed per SPM or FPI rules.

Maintenance costs assume on-site ABS inspection with an ROV and survey support vessel forseven days on an annual inspection interval. Replacement hardware would be additional ifrequired.

Table 19 Cost Estimate

Description Cost

Design, Review, & Bid Support Analysis & Engineering 330,000$Fatigue analysis (optional) 120,000$Bottom survey 100,000$On-site wave data survey 167,000$

ABS Review Fees 40,000$Bid Package Prep 40,000$Bid Package Review 20,000$Subtotal 817,000$

InstallationPre-installation Engineering Support 40,000$On-site Installation Engineering Support 110,000$On-site ABS Survey 40,000$Mooring Component Acquisition 1,810,000$Connecting Hardware Acquisition 90,000$Pre-moorage Installation Work 450,000$Mobilization 1,305,000$Installation 3,383,000$Demobilization 705,000$

Added Cost for Weather 1,725,000$Subtotal Installation 9,658,000$

Total Design & Installation 10,475,000$

Annual Mai ntenance 250,000$


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Section 7 Recommendations

We have the following recommendations for future design and analysis work:

Purchase a Global Reanalysis of Ocean Waves (GROW) 3 point closer to the northeast

of Unalaska Bay to better characterize Bering Sea waves for the 100-year climatologyextrapolation.

Define maximum operating condition, if less than 100-year storm, in which a vessel isallowed to moor to the buoy.

Collect local wave data to validate wave model.

Perform a bottom survey to inform the anchor design.

Revisit anchor design and consider trade-offs between drag embedment and drive plateanchors.

Consider trade-offs between SPM and FPI certification paths.

Consider refining buoy location for optimal seafloor topography.

3 GROW couples Oceanweather's global wave model, tropical boundary layer model, and its experience indeveloping marine surface wind fields to produce a global wave hindcast. The result is a long term analysis ofthe global wave climate which can be applied to offshore structure design, tow-analysis, operability, and otherapplications where wind and wave data are required. The analysis can be sampled for project specific sites toestimate local design conditions


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Coastwise Corporation A-1 The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Appendix A Annual Extreme Winds

The following figures show the annual extreme wind speeds, sorted by directional sector, and plotted on log-extremal probability paper according to the Gumbel Type II extreme value probability distribution. The linear trendline and the upper and lower 90% prediction boundsare shown.

Procedure for Extrapolation

This analysis involves extrapolation of empirical and hindcast data to a 100-year return periodstatistical level. The accuracy of extrapolation is highly dependent on the statisticaldistribution of events of the process. The two processes examined here are maximum waveheights and maximum wind speeds.

A Gumbel Type II asymptotic distribution was chosen as the best fit to the wind speed andwave height data. To capture the long-term, or tail behavior, of the process most accuratelyonly the data points that had a cumulative probability of 10% and higher were used for the

extrapolations. The cumulative probability function, F(s), of a Gumbel Type II distribution isdefined as:

))sexp(exp()s(F where s is a standardized extremal variate. (1)

The natural logarithm (base e) of the extremes was regressed against the standardized extremalvariate. The relationship was highly linear, which is a strong indication that the GumbelType II distribution models the long-term behavior of the process well (Reference 3).

The linear relationships were used to calculate the expected value of the 100-year returnvalues. Since there is some scatter in the data, prediction bounds can be placed around thedata. One-sided 90% prediction bounds were chosen to bound the expected value of theannual maxima. With this approach, there is a 10% chance that a value of the process exceeds

the upper one-sided 90% prediction bound, or a 10% chance that a value of the process falls below the lower one-sided 90% prediction bound. 4

4 It should be noted that this differs from a 90% prediction interval. A 90% prediction interval is based on a two-sided probability distribution. So, there is a 5% chance that a value of the process exceeds the upper limit of the90% prediction interval, and there is a 5% chance that a value of the process falls below the lower limit of the90% prediction interval.


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0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m

i n u t e a v e r a g e w

i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: North (337.5 deg - 22.5 deg, across 0/360 deg)



0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m i n u

t e a v e r a g e w

i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: Northeast (22.5 deg - 67.5 deg)




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0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m


i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: East (67.5 deg - 112.5 deg)



0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m i n u

t e a v e r a g e w

i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: Southeast (112.5 deg - 157.5 deg)




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0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m


i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: South (157.5 deg - 202.5 deg)

Return Period , years


0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m i n

u t e a v e r a g e w

i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: Southwest (202.5 deg - 247.5 deg)




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0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m


i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: West (247.5 deg - 292.5 deg)

Return Period , years


0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m


i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: Northwest (292.5 deg - 337.5 deg)




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0.010.001 0.1 0.2 0.5 0.8 0.9 0.95 0.98 0.99 0.995 0.998 0.999

1.001 1.01 1.1111.25 2 5 10 20 50 100 200 500 1000

30

40

50

60

70

80

O n e - m


i n d s p e e

d , k n o t s


Annual Maximum One-minute Average Wind Speed, knotsDutch Harbor Airport (USAF ID# 704890), Elevation = 4 m above local groundSorted by Direction: All Directions




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Coastwise Corporation B-1 The Glosten Associates, Inc.Concept Design of Mooring Buoy, Rev. - File No.11073, 30 June 2011

Appendix B SWAN Analysis Results

The following figures show the results of the SWAN analysis. For each case, there is a plot ofthe marginal wave spectra: one as a function of heading, and one as a function of frequency.These are 2-dimensional visualizations of a 3-dimensional directional power wave spectrum.The figure underneath the figure showing the marginal spectra illustrates the significant waveheight distribution over the SWAN computational domain. The proposed mooring buoy site isindicated.


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Concept Design of Mooring Buoy

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