wave tranquillity performance criteria · pianc report wg no. 24, belgium. pianc (2013). design and...

America's Cup 36 Coastal Processes and Dredging Technical Report

Beca // 12 January 2018

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Appendix E

Wave Tranquillity Performance Criteria

Memo

To:

Tom Warren, Stephen Priestley, Philip Wardale, Doug Treloar, Connon Andrews Job No:

1005128

From: Richard Reinen-Hamill Date: 20 November 2017

Subject: Moored Boat wave criteria and response

1 Purpose

This memo sets out vessel tranquillity criteria for marinas based on published guidelines and recommendations focussing on weekly wave conditions, rather than more extreme wave events. It also discussed passenger discomfort criteria, particularly with regard to roll and transverse acceleration as these are important sources of discomfort (Dallinga et al., 2003).

2 General wave height criteria

Figure 1 shows the generally accepted international harbour tranquillity guidelines for small craft harbours for both severe wind events and weekly conditions and Table 1 extracts the criteria from PIANC (2016) for beam seas for annual and weekly conditions. Weekly recognises the more frequent but low wave height events, such as those caused by boat wake, are as important to consider as the less frequent higher energy events.

Figure 1 Tolerance for harbour tranquillity criteria for a range of return periods of head and beam seas (ASCE, 2012)

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Tonkin & Taylor Ltd Moored Boat wave criteria and response

20 November 2017 Job No: 1005128

Table 1 Recommended criteria for wave climates in small craft harbours for Beam Seas (PIANC, 2016)

Direction and peak period of design harbour wave

Wave event exceeded once each week (m)

Wave event exceeded once a year (m)

Less than 2 seconds Moderate 0.38 Moderate 0.38

Good 0.30 Good 0.30

Excellent 0.23 Excellent 0.23

Between 2 and 6 seconds Moderate 0.10 Moderate 0.19

Good 0.08 Good 0.15


Greater than 6 seconds1 Moderate 0.10 Moderate 0.19

Good 0.08 Good 0.15


1. For these longer periods a horizontal motion of less than 0.23 m each week and 0.3 m annually as an optional criteria in lieu of the wave height

The range of wave heights for weekly beam seas is between 0.1 m to 0.06 m for moderate to excellent conditions with between 0.11 m and 0.19 m for annual events. For weekly head seas can range from 0.12 m to 2.2 m.

These results do not recognise the effect on the moored vessels is also a function of vessel size. Table 2 shows recommended criteria for more conventional small craft from 4 to 20 m in length for annual exceedance events. The results in this table shows a reasonable increase in acceptable wave heights for short period waves (< 4 s) as the vessel size increases but still relatively low acceptable wave heights for beam/quartering seas, even with the larger vessels.

Table 2 Recommended wave criteria for small craft and pleasure boats. The acceptable frequency of occurrence is one to a few times per year (PIANC, 1995)

Ship length (m) Beam/Quartering seas Head seas

Period (s) Height, Hs (m) Period (s) Height, Hs (m)

4 – 10 < 2.0 0.20 <2.5 0.20

2.0 – 4.0 0.10 2.5 – 4.0 0.15

> 4.0 0.15 > 4.0 0.20

10 – 16 < 3.0 0.25 < 3.5 0.30

3.0 – 5.0 0.15 3.5 – 5.5 0.20

> 5.0 0.20 > 5.5 0.30

20 < 4.0 0.30 < 4.5 0.30

4.0 – 6.0 0.15 4.5 - 7.0 0.25

> 6.0 0.25 > 7.0 0.30

3 Passenger mobility and comfort criteria

While the values above provide wave height criteria, there is also an effect on passenger discomfort. Vertical accelerations have been the primary focus on establishing passenger discomfort for motor

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launches (Dallinga et al., 2003). However, rolling and transverse acceleration have been established as the most significant sources of discomfort. The following threshold values have been used (Dallinga et al., 2003):

10% Motion Sickness Indicator (MSI - probability that no one is seasick in a group of 5)

Transverse accelerations: 0.5 m/s2 rms

Roll: 10 degrees SDA roll (2.5 degrees rms)

Figure 2 shows the wave heights and period relationships required to exceed these criteria with and without roll stabilization options. This figure shows periods of between 4 to 11 seconds are the most critical for beam waves on super yacht comfort.

Figure 2 Effect of roll for a range of wave heights an periods with alternative stabilization measures on limiting wave height (Dallinga et al, 2003)

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The likelihood of roll is linked to the incident wave period coinciding with the natural roll period of the vessel. The following formula calculates the natural roll period of the vessel measured in seconds or T based on the beam in feet and the Metacentric Height (GM). A natural roll period is the time between two successive peak roll angles on the same side of a vessel.

𝑇 = 0.44 × 𝐵𝑒𝑎𝑚

√𝐺𝑀

The metacentric height (GM) is a measurement of the initial static stability of a floating body. It is calculated as the distance between the centre of gravity of a ship and its metacentre (refer Figure 3).

Figure 3 Ship stability diagram showing centre of gravity (G), centre of buoyancy (B), and metacentre (M) with ship upright and heeled over to one side. (By Life of Riley - Own work, CC BY-SA 2.5, https://commons.wikimedia.org/w/index.php?curid=16339428)

There is no standard value of GM as it is vessel specific, but a larger metacentric height implies greater initial stability against overturning. However, a larger metacentric height can cause a vessel to be too "stiff"; excessive stability is uncomfortable for passengers and crew. Published guidance is for the GM to be more than 4 ft (1.2 m). Table 3 shows calculated vessel roll periods based on standard super yacht dimensions from PIANC (2012) and two different metacentric heights based on the lower bound value of 1.2m and a notional value of 2.2 m.

Table 3 Calculated roll period for a range of vessel sizes and two assumed GM values

Vessel length (m) Beam (m) GM (m) Vessel roll period (s)

Lower Upper Lower Upper

25 – 35 6.8 1.2 2.2 4.9 3.7

35 – 45 8.3 1.2 2.2 6.0 4.5

45 – 55 9.6 1.2 2.2 7.0 5.2

55 – 65 10.2 1.2 2.2 7.4 5.5

65 – 75 10.6 1.2 2.2 7.7 5.7

These values suggest for beam seas the vessel roll period for the larger super yacht classes is between 4 and 8 seconds. It is noted that active fins and anti-roll tanks make a significant different to reducing roll effects (refer Figure 4) compared to passive fins. This could be a useful consideration for vessels moored outside the marina basin.

https://en.wikipedia.org/wiki/Centre_of_gravity

https://en.wikipedia.org/wiki/Metacentric_height#Metacentre

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Figure 4 Comparison of fins and ant-roll tank at anchor (Hooijmans and Gaillarde, 2006)

4 Summary

The wave height criteria based on the latest guidance show the need to maintain low wave height levels within the marina even on a weekly basis. Also that wave heights for beam seas of less than 0.1 m are required to provide a good climate and manage roll effects when wave periods are between 4 and 8 seconds. Achieving these heights should also be sufficient for comfort levels on board, but checks on transverse acceleration would be required for the sudden effect of ferry wakes on the moored vessel if modelling shows longer wave events from ferry wakes still entering the marina. Considering the specific properties of vessels with regard to active fins and anti-roll tanks may be a useful consideration regarding accommodating the vessel.

8-Jan-18 p:\1005128\workingmaterial\vessel motion characteristics\20171115.rrh.technical memo - wave motions.docx

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References

ASCE (2012). Planning and design guidelines for small craft harbours. American Society of Civil Engineers Manual 50, USA.

Dallinga, R.P., M.L. Levadou, J.E. Bos, F. Gumbs (2003). Seakeeping of motor yachts and passenger discomfort. Projects 03, USA.

Hooijmans, P. and G. Gaillarde (2006). Hydrodynamics of large motor yachts: past and future developments. II International Symposium on yacht design and production.

PIANC (1995). Criteria for movements of moored ships in harbours, a practical guide. PIANC report WG No. 24, Belgium.

PIANC (2013). Design and operational guidelines for superyacht facilities. PIANC report WG No. 134, Belgium.

PIANC (2016). Guideline for marina design. PIANC report WG No. @@@, Belgium.



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Appendix F

Wave Reflection and Transmission Coefficients



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Appendix F – Wave Reflection and Transmission Coefficients

F.1 Introduction

Incorporating wave protection works into an overall harbour layout is important to meet moored vessel

tranquillity criteria.

Wave protection works are generally either a rock mound breakwater or a structure incorporating (near)

vertical panels to give the desired wave transmission and reflection characteristics. These characteristics are

usually described in the following form:

• Kr = hr/hi – reflection coefficient

• Kt = ht/hi – transmission coefficient

• Where hi – incident wave

hr – (1 + Kr) hi is the reflected wave

ht – transmitted wave.

Both local wind waves and boat wakes need to be considered. These cover a range of wave periods

between 3 to 6s. Wave periods and water depth affects the local wave length which in turn affects the

reflection and transmission coefficients. This relationship is given in Table F.1.

Table F.1: Local Wave Lengths (m)

Wave Period(s)

Water Depth

6m

(low water)

7.5m

(mean sea level)

9m

(high water)

3 14 14 14

4.5 28 29 30

6.0 41 45 47

F.2 Full Depth Impermeable Panels

Theoretically these panels have a Kr = 1.0, but local effects limit it to Kr = 0.9 (CEM, 2011) and Kt = 0.0.

F.3 Vertical Panel with Gap at Base

Existing full depth panels have a 1.0m gap at the base. PIANC (2016) recommends use of the Kreibel

formulae:

• Kt = 2P / (1 – P)

• � � �

��

��

��

��

��

��

• Where d = water depth

D = depth of panel below water

L = wave length.



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• Kr =(1-Kt2)0.5 but <0.9

For the AC 36 project the coefficients listed in Table F.2 will apply:

Table F.2: Transmission and Reflection Coefficients for Panels with gap at base

Wave Period

(s)

Water Depth

6m 9m 7.5 m

Kt Kr Kt Kr Kt Kr

3.0 0.03 0.9 0.0 0.9 0.01 0.9

4.5 0.18 0.9 0.06 0.9 0.11 0.9

6.0 0.24 0.9 0.13 0.9 0.17 0.9

F.4 Panels with Porosity

Wave reflection can be reduced by having gaps or porosity (area of gaps/total area) in the vertical panels.

Table F.3 gives the coefficients for a range of porosities for mean sea level (PIANC 2016, Hydraulic

Research, 1989)

Table F.3 Transmission and Reflection Coefficients for panels with porosity

Porosity Wave Period (s) Kt Kr

0.04 3 0.2 0.9

4.5 0.15 0.85

6.0 0.2 0.8

0.05 3 0.2 0.8

4.5 0.2 0.8

6.0 0.3 0.65

0.06 3 0.2 0.8

4.5 0.25 0.7

6.0 0.35 0.55

0.08 3 0.25 0.7



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Porosity Wave Period (s) Kt Kr

4.5 0.30 0.6

6.0 0.5 0.5

0.1 3 0.3 0.6

4.5 0.5 0.5

6.0 0.6 0.4

0.2 3 0.4 0.5

4.5 0.6 0.35

6.0 0.8 0.2

0.3 3 0.6 0.35

4.5 0.7 0.25

6.0 0.85 0.10

0.4 3

4.5 0.9 0.10

6.0

0.5 3

0.9

0.10 4.5

6.0

F.5 Minimal Reflective Structures

For practical considerations, a piled structure to accommodate lateral wave loadings needs to be about 10m

wide. This structure would have a full panel on the leeward face and a perforation panel on the seaward

face. The optimum arrangement to minimise wave reflection is a width/wave length ratio of 0.2 (Hydraulic

Research, 1989). Based on a 10m width the performance of this type of structure is summarised in Table F.4

at mean sea level for a porosity of 15%.



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Table F.4: Transmission and Reflection Coefficients for a ‘non-reflective’ Structure

Wave Period Kt Kr

3 0.0 0.50

4.5 0.0 0.35

6 0.0 0.2

F.6 Rock Rubble Mounds

These mounds are common breakwaters which are generally considered to have lower reflection

characteristics. Based on Van Der Meer et al (2005), the coefficients are given in Table F.5 for an

impermeable core with a mound slope of 1:2.

Table F.5: Transmission and Reflection Coefficients for a Rock Rubble Mound

Wave Period Wave Type Kt Kr

3

4.5

6.0

Boat Wake (Hs=0.3m) 0.0 0.43

0.58

0.66

3

4.5

1 year ARI Wind Wave

(Hs=0.6m)

0.0 0.3

0.45

F.7 Approach

The moored craft tranquillity criteria (Appendix F) are more stringent for boat wakes than for wind waves. For

wave modelling purposes, the focus has been on boat wakes with transmission and reflection coefficients

representative of the worst case at mean sea level. Example coefficients are given on Figure G1 for the

proposed layouts.

F.8 References

Coastal Engineering Manual, 2011. Part 6, USACE.

Hydraulic Research, 1989, Wave Reflection in Harbours. Report OD 89.

PIANC, 2016, Guidelines for Marina Design, Report No.149, Part 1.

Van der Meer J, Brigante R, Zanuttigh B, Wang B, 2005. “Wave transmission and Reflection at low crested

structures”: Coastal Engineering.



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Appendix G

Sediment Plume and Fate Analysis



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Australia

Fiji

Indonesia

Myanmar

New Caledonia

New Zealand

Singapore

Thailand

wave tranquillity performance criteria · pianc report wg no. 24, belgium. pianc (2013). design and...

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