concept design of mooring buoy - aleutian islands risk

Places of Refuge Initiative Mooring Buoy Initiative

Concept Design of Mooring Buoy

Prepared for

State of Alaska Department of Environmental Conservation Anchorage, Alaska

Under ADEC SPAR Term Contract #18-5048-10

Prepared by

C oastwise Co rpo ration

Naval Architects Marine Engineers Anchorage, Alaska

and

File No. 11073.01 30 June 2011 Rev. -

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

Digitally Signed 12-Jul-2011

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 for

State of Alaska Department of Environmental Conservation Anchorage, Alaska

Under ADEC SPAR Term Contract #18-5048-10

Prepared by Coastwise Corporation Anchorage, Alaska and The Glosten Associates, Inc. Seattle, Washington File No. 11073.01 30 June 2011 Rev. -

PREPARED:

Katherine V. Sultani-Wright, PE Project Manager

CHECKED:

David L. Gray, PE Senior Principal

APPROVED:

Justin M. Morgan, PE Principal-In-Charge

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 Mooring 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 Extreme Winds

Appendix B SWAN Analysis Results

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

Revision History Section Rev Description Date Approved

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

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

Terms AIRA 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 probability distribution

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 physical environment.

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 of unsteady 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 model that 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 offshore structures

WBAN Weather-Bureau-Army-Navy

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, 29 October 2010.

2. SWAN (Simulating WAves Nearshore); Software Version 40.72, Delft University of Technology, 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 of Adak, 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, American Petroleum Institute, API RP 2SK, October 2005.

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

9. Owens R and P Palo, Wind Induced Steady Loads on Ships, Technical Note N-1628, Naval Civil 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 Tankers at 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/060922AleutiansVesselReportSCREEN.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.

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 is strategically located near Unimak Pass, which is frequently transited by large vessels following 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 is located approximately at 53º-55.35′ N and 166º-37.00′ W.

Climatology

This report presents a climatological study to select design environmental conditions for the mooring within Unalaska Bay. The design climatology represents a severe storm that is expected once every 100 years. The resulting local wave heights are 4-5 m, depending on the wind 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 vessel casualties near Dutch Harbor; it is not based on a comprehensive vessel traffic study, which was not in the scope of the concept study. The design vessel used to size the mooring components consists of an underwater hull form similar to a tanker or bulk carrier, and an above water hull form similar to a car carrier. It is 206 m in length and 32.4 m in breadth, with a displacement of 64,387 tonnes.

Mooring Design

A three (3) leg, twin line mooring configuration was selected for the concept design based on common Navy fleet moorings. A sketch of the concept design is shown in Figure 1.

Anchor Selection

An 18 tonne Bruce TS anchor is needed to develop the required holding capacity of 700 kips. The four (4) existing 15 tonne Bruce TS anchors do not provide enough holding capacity on an individual basis for the configuration shown in Figure 1, assuming soft bottom conditions. Variations from the assumed bottom conditions or other design assumptions 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 disabled vessels in Broad Bay, Alaska

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 introduced as a means of risk reduction in the Ports and Waterways Safety Assessment for the Aleutian Islands. Anticipated increases in vessel traffic in the Aleutians have focused the need to further improve the infrastructure to provide PPOR for vessels in distress. The Aleutian Islands Risk Assessment (AIRA) project identified the need for acquisition of a mooring buoy in Dutch Harbor to accommodate large disabled vessels.1 Dutch Harbor is strategically located near Unimak Pass, which is frequently transited by large vessels following the Great Circle route. 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 vessels anticipated to use the mooring buoy. Broad Bay was suggested by the Alaska Marine Pilots as a suitable location approximately four nautical miles northwest of Dutch Harbor within Unalaska 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 the northeast sector have an unobstructed path into Broad Bay; however, it is protected from ocean swell in other directions. Figure 2 illustrates the proposed mooring buoy site, located approximately at 53º-55.35′ N and 166º-37.00′ W.

1 http://www.aleutiansriskassessment.com/


Figure 3 Proposed Mooring Buoy Site

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

Proposed Mooring Buoy

Site


Section 2 Climatology

2.1 Technical Approach

A common methodology for offshore structures design, including moorings, is to use Metocean criteria associated with the 100-year return period condition. As consistent quality environmental data records are almost never available for periods exceeding 100 years, it is necessary to employ extrapolation methods to estimate the wind speeds or wave heights associated with a 100-year return period. Different methods have been developed for conditioning and extrapolating the available data in order to develop an estimate of the extreme (i.e., 100-year return) wind or wave. This extrapolation of wind speed and wave height 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 record to annual extremes provides a sufficient number of data points to lend confidence to this method of extrapolation.

2.2 Local Winds

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

Table 1 Weather Station Specifics

Dutch Harbor Airport Alaska, United 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 is shown in Figure 4.


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 annual extremes 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 by direction, for eight direction sectors. Wind direction is defined as the direction from which the wind is blowing, in degrees from true north. The eight direction sectors were bounded as shown in Table 2. The directional data was reported to the nearest degree, so the ranges were defined to the half-degree just above the upper bound and to the half-degree just below the lower bound. This resulted in nine sets, one for each direction and one across all directions, of annual 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 previous work.


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. Figures showing extrapolation are shown in Appendix A. A summary of the 100-year return extrapolation 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 OrcaFlex mooring 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 U0, the one-hour average wind speed at an elevation 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 as an 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 wind speeds using NPD Spectrum wind gust formulation

Wind Heading, deg true

One-minute average wind speed at 4 m, knots

U(4 m, 60 sec)

One-hour average wind speed at 10 m, knots

U(10 m, 3600 sec)

0/360 66 58

45 66 58

90 66 58

135 66 58



One-minute average wind speed at 4 m, knots

U(4 m, 60 sec)

One-hour average wind speed at 10 m, knots

U(10 m, 3600 sec)

180 69 61

225 66 58

270 66 58

315 66 58

2.3 Local Current

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

Due to the low tidal current speeds in the area of interest, tidal current forces were not included in the mooring analysis. However, wind stress current may still be present and is accounted for 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 steady wind speed, which will be interpreted to be U (10 m, 3600 sec), the one-hour average wind speed at an elevation of 10 meters. There are diverse models for the vertical profile of the wind stress current in the literature, but it may be conservatively regarded as classical plane Couette 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 confined to a near surface region. Applying the guidance of the ABS MODU Rules the vertical profile of the wind stress current would vary linearly over the top five meters of depth, from a maximum at the surface to the combined tidal and storm surge current (in this case, effectively zero) at 5 m depth. However, due to the lack of measured data or computational modeling of the 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 was used as the steady wind speed and the current velocity was taken as 2.5% of that value. A linear vertical profile was assumed, with the current speed at the sea surface equal to 2.5% of the steady wind speed and the current speed at the sea floor equal to zero.

2.4 Local Waves The 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 are generated 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


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

2.4.1 Description 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 in water of intermediate and shallow depth for situations where the wind field may be considered uniform. Typical areas for the application of SWAN range between 10 x 5 km2 and 30 x 100 km2.

SWAN is a two dimensional full spectral wave model for wave propagation in shallow water including refraction and shoaling, growth due to wind action, non-linear wave interactions (triad and quadruplet) and dissipation by bottom friction and breaking. SWAN is appropriate for and typically used for the simulation of wave generation, propagation, and dissipation in coastal 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) and three wave interactions (triads). The discrete representation of the frequency spectrum means that SWAN is more suitable than previous models for application in areas where strong growth due 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 the computational area. At each grid point, SWAN represents the complete 2D-action density spectrum discretely as a function of frequency and direction. SWAN calculates wave propagation in all directions. The solution technique marches forward row by row over the grid beginning at the incident wave boundary, where the incident wave characteristics are defined. The results in each direction sector at each grid point are computed from the results for 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 due to bottom friction or wave breaking.

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


2.4.2 Bering Sea Waves

There are two sources of Bering Sea wave data: NDBC buoys 46035 and 43073, shown in Figure 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 Sea

Buoy 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 to characterize 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 those from 46035. Due to the geography of Unalaska Bay, Bering Sea waves approaching from the northeast sector have an unobstructed path into Broad Bay. Therefore, only Bering Sea waves from 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.


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 absence of any other information, the Bering Sea waves were assumed to be aligned with the wind, and so the wind direction was used as a proxy for the wave direction. The annual extreme events from 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 Buoy Significant Wave

Height, m Dominant Wave

Period, sec Wind Direction,

deg true

29-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 27

24-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


Date Buoy Significant Wave

Height, m Dominant Wave

Period, sec Wind 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 Gumbel extrapolation 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 data set was chosen to maximize the goodness-of-fit of the extrapolation to the upper end of the data. The resulting estimate of the expected 100-year return Bering Sea significant wave height from the northeast sector was 14.5 meters.


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

the 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 wave steepness 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 height was selected based on the line of constant wave steepness, and was determined to be 16.5 seconds.

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

Sig

nif

ican

t Wav

e H

eig

ht,

met

ers

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.


Figure 8 Conditional peak period expected in association with 100-year return Bering Sea significant

wave 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 proposed mooring buoy site. The 100-year return one-hour average wind speeds at 10 m elevation and the 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 proposed mooring 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 and spatial stationary wind and boundary waves, thus producing the fully-developed solution that would be obtained if the forcing conditions persisted forever. SWAN is capable of modeling response to temporally and spatially non-stationary (transient) forcing. However, the adopted approach 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 representing the Bering Sea waves was applied on three of the four boundaries; the southern boundary contained almost exclusively land, so waves were not applicable. The JONSWAP spectrum was derived as described above using two buoys. Figure 9 below shows the SWAN computational domain. The computational domain encompasses an area of approximately 400 km2 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

Do

min

ant

Per

iod

(T

p),

sec

Wave Height (Hs), m

Annual Maximum Wave Height - 46035

Line of best f it - 46035

Annual Maximum Wave Height - 46073

Line of best f it - 46073

Hs/gTp2= 0.00539

Hs/gTp2= 0.00543

Tp = 16.5 sec

Hs

= 1

4.5

m


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

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


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 in Table 7. A heading of 0 degrees corresponds to a wind or wave coming from true north, a heading of 180 degrees corresponds to a wind or wave coming from true south. Each case corresponds to local wind from all directions at 45 degree increments and Bering Sea waves from 45 degrees. A sensitivity study was conducted to find the angle of Bering Sea waves that maximized the waves at the proposed mooring buoy site. It was concluded that Bering Sea waves coming into the bay at a 45 degree heading produced the largest waves at the mooring site.


Table 7 Summary of SWAN cases

Case Number

Wind Speed, knots

U(10 m, 3600 sec)


Bering Sea Significant Wave Height, meters

Bering Sea Wave Peak Period, sec

Bering Sea Wave Heading,

deg true

1 58 0 14.5 16.5 45

2 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

Significant Wave Height,

m Peak Wave Period, sec

Peak Wave Heading, 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 57.5

5 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


2.5 Metocean Design Climatology

Table 9 summarizes the design climatology for the proposed mooring buoy site. All headings are in degrees true.

Table 9 Eight climatology cases at proposed mooring buoy site

Case Number

Wind Speed, knots

U(10 m, 3600 sec)


Significant Wave

Height, m

Peak Wave

Period, sec

Peak Wave

Heading, deg true

Current Speed, knots

Current Heading, deg true

1 58 0 4.37 9.0 57.5 1.45 ----Sam

e as Wind H

eading----

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


Section 3 Design Vessel

A comprehensive vessel traffic study, in order to identify a possible range of vessels that might use 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 it summarizes traffic primarily based on oil capacity and gross tonnage. These parameters are of limited use in selection of a design vessel for the mooring system. ADEC owns the source AIS data used to develop the traffic study in Reference 142, but could not provide it in time for the present work. This data could be analyzed in the next design phase to link vessel size to traffic 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 the area. 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, m

Cougar 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 the superstructure of the car carrier. This composite vessel captures the effects of a deeper draft and a large windage area. For a concept level mooring buoy design, this composite vessel is considered 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 automated identification 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 be cross-referenced with the Lloyd’s Register database to identify vessel size.


Table 11 Particulars of design vessel

Length Overall (LOA) 206 m

Beam 32.4 m

Draft 12 m

Displacement 64,387 tonnes

Freeboard 23.8 m

A vessel of this size (~55,000 DWT) matches the capability of the Emergency Towing System in Dutch Harbor. The City of Unalaska has purchased a system suitable for vessels up to 50,000 DWT, and the Alaska Department of Environmental Conservation is purchasing a system 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 for future vessel traffic trends. “New Panamax” size vessels are expected to transit Unimak Pass when 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

Vessel Type Nominal Capacity LOA Beam Depth

Maximum Draft

Displacement at Maximum

Draft

Freeboard at

Maximum Draft

m m m m tonnes m Panamax: 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 Loads

Wind, current, and wave loads on the design vessel were calculated and included in the analysis of the mooring buoy. The forces on the vessel are defined in the coordinate system shown in Figure 13.

Figure 13 Environmental loads sign convention


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 bulk carrier/tanker hull form. The current force coefficients are shown in Figure 15.


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 program for analyzing the interaction of surface waves with offshore structures. Forward speed effects are not accounted. The program implements a set of highly efficient algorithms for the Green’s function computations which are at the core of the boundary element method. The version currently available in the office is the most up-to-date PC executable V6.41PC which solves 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, and steady drift forces. In addition to the six rigid-body modes of a floating body, WAMIT V6.41PC can handle multiple bodies, body near vertical walls, user-specified generalized modes, etc. Several tools are available for use in conjunction with WAMIT to model external springs, connections between bodies, etc., and to evaluate response statistics including motions at a point in specified sea states.


Section 4 Governing Regulations

The concept design presented in this report is designed to ABS Rules for Building and Classing Single Point Moorings (Reference 12). The design conditions and safety factors defined by these rules are summarized in Table 13. For the purposes of this analysis, the design 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 safety on anchor 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 capacity

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 be considered 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. The Single Point Mooring (SPM) rules were developed around CALM buoy type installations where product is loaded through a riser and floating hose. The safety factors are set so that a fatigue analysis is not required.

b. ABS Guide for Building and Classing Floating Production Installations, November 2010. This guide provides an alternative classification path for SPMs with lower safety 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 equipment identified in this guide. The guide contains the following description of its purpose:

“This Guide has been developed in response to industry requests for an optional 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.


2) International Association of Classification Societies:

a. IACS Recommendation No. 38 [(1995), Rev 1, October 2010] “Guidelines for the Survey 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 for the design and analysis of mooring systems.

b. API RP 2I: Recommended Practice for In-Service Inspection of Mooring Hardware for Floating 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 Edition 1995.

c. OCIMF, “Recommendations for Equipment Employed in the Bow Moorings of Conventional 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.


Section 5 Concept Mooring Buoy Design

5.1 Overview

The 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 the riser chain. The advantage of the riser-type mooring is that vessels can weather-vane about the mooring unrestricted by the ground legs. ABS recently suggested considering a higher, but as yet 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-shelf hardware for a more dependable cost estimate. Alternate means of connecting the anchor legs may be explored as the design is refined in future development.

5.2 Mooring Components

The mooring consists of a hawse pipe type mooring buoy, riser chain, ground ring, anchor legs, and drag embedment anchors. A three leg, twin anchor line configuration was selected for the concept design. Figure 16 illustrates the arrangement. The riser chain, equipped with a chain swivel, connects the ground ring to the buoy. Twin anchor legs laid out with a nominal 120 degree spread connect the anchors to the ground ring. A twin anchor leg arrangement is used to develop the required capacity. Triangular “spider” plates equalize the twin anchor leg loads 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 under tension and so that the connection hardware does not contact the seabed.

Riser Chain

Anchor Leg

Drag Anchor

Mooring Buoy

Ground Ring


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

Figure 17 Plan view of concept mooring buoy mooring arrangement

5.2.1 Buoy

A foam-filled mooring buoy fitted with a through-chain hawse pipe and capture plate provides a net buoyancy of 45 tonnes. This design assumes a Trelleborg MB-45000 mooring buoy with an overall diameter of 4.2 meters and a height of 4.1 meters (excluding hawse pipe and hardware). The buoy has a nylon filament reinforced polyurethane skin which has excellent resistance 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 to navigation requirements and a chafing guard will be required accessories. In the 100-year storm 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 be replaced with a hawse pipe.

100 m

N

W

S

E

45 deg


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 which are shown in Table 14. The chain types correspond to the parts labeled in Figure 16.

Table 14 Properties of mooring chain

Component Size Number x Length Breaking Strength Mass

Riser Chain 90 mm R4

stud-link chain 1 x 48 m 8,167 kN 177 kg/m

Anchor Leg 76 mm ORQ

stud-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 selection of 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 (343 kips). According to the guidance in Reference 12, the minimum factor of safety on anchor holding capacity is 2.00. Therefore, the anchors must have a minimum holding capacity of 3051 kN (686 kips). Assuming a minimum holding capacity of 700 kips, the anchor sizes shown in Table 15 are possibilities. The anchor types were selected based on Reference 7.


Table 15 Drag embedment anchor candidates for concept mooring design

Anchor Type Anchor Weight Anchor Shank

Length Anchor Fluke

Width

Holding Capacity (in mud)

Bruce FFTS MK 4

15,000 kg (approximately 30

kips)

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

Vryhof Stevpris MK 5


kips)

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

Bruce TS


kips)

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

Moorfast


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 in Alaska. These anchors are not quite large enough to develop the required holding power and safety factor for the mooring design. An 18 tonne Bruce TS anchor is required as shown in Table 15. If the anchors can be obtained at a substantial discount (i.e. less than half the cost for 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 anchor selection 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 be designed for the site-specific soil conditions. Since these are presently unknown, a plate anchor design was not pursued. Design features of both types of anchors are presented in Table 16 (Reference 11).


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 no uplift 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 types including hard clay, gravel, coral, rock, or highly layered seafloors; May not work well for sloping seafloors more than several degrees

Anchors must be designed for the site-specific soil characteristics

Adequate seafloor sediment is required for proper setting

Adequate seafloor sediment is required for proper setting

Anchor may drag if overloaded at a slow enough 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; Mobilization of installation equipment can be expensive

5.2.4 Vessel Connection

The connection hardware between the disabled vessel and the mooring buoy has not been developed 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 hardware sizes selected for the hawser and chafing chain accommodate vessels less than 100,000 tonne DWT with OCIMF recommended SPM equipment. Not all vessels will have standard SPM fixtures; however, the chain termination of the hawser presents a generic connection interface to 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.


Figure 19 Typical arrangement to facilitate connection of vessel to mooring buoy (adapted from

Reference 13)

5.3 Mooring Analysis

A 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 of unsteady wind, first-order wave excitation, second-order wave drift, current, and nonlinear mooring forces on floating bodies. At each time step, the vessel acceleration is computed from the instantaneous forces arising from these sources. This code makes use of first-order wave excitation forces, radiation added mass and damping, and second-order drift force quadratic transfer functions (QTFs) that are computed by external three-dimensional hydrodynamic radiation-diffraction codes.

To Mooring Buoy

To Vessel


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 in OrcaFlex 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 reflects the water depth at mean high water.

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


Figure 21 Line numbering in OrcaFlex model

Table 17 Anchor leg tension results

Case Number

Wave Heading, deg true


Maximum Anchor

Leg Tension,

kN Anchor

Leg

Second Highest Anchor

Leg Tension,

kN Anchor

Leg

1 57.5 0 863 L0 681 L01 2 57.5 45 1039 L21 1027 L0 3 57.5 90 1158 L2 1103 L21 4 57.5 135 1526 L2 1277 L11 5 57.5 180 1504 L11 1261 L1 6 57.5 225 1115 L11 763 L1 7 57.5 270 861 L0 783 L01 8 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 ensure that the drag anchors did not experience any uplift forces. This was shown to be the case and the full set of results is shown in Table 18.

100 m

N

W

S

E

L0

L01

L1

L11

L2

L21


Table 18 Suspended length results

Case Number

Wave Heading, deg true


Maximum Suspended Length, m

Minimum Leg Length on Seabed,

m Anchor

Leg

1 57.5 0 261 90 L0 2 57.5 45 291 60 L0 3 57.5 90 271 80 L0 4 57.5 135 221 131 L0 5 57.5 180 330 20 L11 6 57.5 225 180 170 L11 7 57.5 270 261 90 L0 8 57.5 315 322 30 L01


Section 6 Cost Estimate

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

ABS was consulted regarding review fees for certification. The fees will vary slightly depending 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 for seven days on an annual inspection interval. Replacement hardware would be additional if required.

Table 19 Cost Estimate

Description CostDesign, Review, & Bid SupportAnalysis & 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 Maintenance 250,000$


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 climatology extrapolation.

Define maximum operating condition, if less than 100-year storm, in which a vessel is allowed 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 plate anchors.

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 in developing marine surface wind fields to produce a global wave hindcast. The result is a long term analysis of the global wave climate which can be applied to offshore structure design, tow-analysis, operability, and other applications where wind and wave data are required. The analysis can be sampled for project specific sites to estimate local design conditions

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 bounds are shown.

Procedure for Extrapolation

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

A Gumbel Type II asymptotic distribution was chosen as the best fit to the wind speed and wave height data. To capture the long-term, or tail behavior, of the process most accurately only 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 is defined 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 extremal variate. The relationship was highly linear, which is a strong indication that the Gumbel Type 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 return values. Since there is some scatter in the data, prediction bounds can be placed around the data. One-sided 90% prediction bounds were chosen to bound the expected value of the annual 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 the 90% prediction interval, and there is a 5% chance that a value of the process falls below the lower limit of the 90% prediction interval.


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

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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

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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)




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

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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

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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)




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

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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)



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

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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)




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

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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)



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

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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)




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

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



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 of the 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 wave height distribution over the SWAN computational domain. The proposed mooring buoy site is indicated.

concept design of mooring buoy - aleutian islands risk

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