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ozziz (Structural) (OP)
31 Oct 08 20:51
Just a question on the recommended berthing direction for jetty. I see most of the berthing jetties are lined parallel to the shoreline. Suppose, a berthing jetty is to be built on the east side of a small island and the wind is blow from north to south for 6 months and then reverse for the second half, what will be the most appropriate orientation of this berthing jetty?

Also if the wind is on north-south direction, how does it create waves on the east or west side of the island. Would it not just have longshore movement rather than waves?

Lastly what will be the most appropriate alignment of ship vessels to the waves or current? Perpendicular to assist berthing or parallel to avoid having the vessels banging against the jetty?

 
swearingen (Civil/Environmental)
3 Nov 08 7:25
What size vessels are we talking about here?  Is this open water?  Any protection from quays or the like?  Is this for bulk material transfer, ferry or a cruise application?


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ozziz (Structural) (OP)
4 Nov 08 2:49
Swearingen

Appreciate if you can answer some of the questions. Yes suppose it is a remote unprotected island surround by open seas on all sides. A jetty is required to berth 7500 DWT barges or ferries. Based on initial environmental report, the wind is blowing from north for the first 6 months and then for another 6 months from south. There is a slight change in direction during the transition of wind direction.
Since the island is like a outcrop, the seabed dips quite steeply. My first question is since waves are created by wind, how would waves and surf are created on the east and west coast of the island as there are no wave runup to the shore if the winds are in north-south direction?
Second question is direction of the berth or wharf - to be arranged parallel to the waves or right angle to the waves?

Thanks  
GregLocock (Automotive)
6 Nov 08 5:54
I'd run an east west pier, giving the captain the option of berthing in the lee of the pier for both significant wind directions.

The disadvantage is that it exposes the hull to the prevailing wave direction.

However that is based solely on observation, and sailing yachts into harbours, not any professional expertise.
 

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

swearingen (Civil/Environmental)
6 Nov 08 8:54
Being on the lee side isn't as much of an advantage for larger vessels.  Berthing while heading into the wind is easier to do and there will be less roll from the waves.  I recommend running the jetty north-south.


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GregLocock (Automotive)
6 Nov 08 20:55
Yes that makes much more sense.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

pauljohn (Marine/Ocean)
6 Nov 08 22:55
I haven't had any formal training on this so my observations are based entirely on experience.

First I question your assumption that the wind will blow with so much reliability as you predict. It is very rare even with predictable trade winds this will happen and when highs, lows, (tropical?) storms come through there will always be wind changes from around the compass and often the worst winds will come from the unexpected direction.

Second if your dock is completely exposed you will get main swell from the direction of wind and will gradually diminish as you get about 90 degrees off the wind, i.e. the swell will wrap around the obstruction slightly. But this could change dramatically depending on underwater conditions, currents and surrounding terrain.

Third - facing a swell when tied to a dock will cause the bow to rise and stern to drop, called surge, and will be hard on mooring lines and fenders and cause problems with gangplanks unless you can haul the boat away from the dock using out-anchors. Personally I would rather have problems docking the boat with the wind pushing me off than have to deal with substantial movement when tied for long periods to the dock, which is why I always go for a dock where I can 'hang' of it. So my preference would be east-west orientation.

So my advice would be talk to someone who has good local knowledge or gain some yourself, before you invest time and money.
ozziz (Structural) (OP)
7 Nov 08 4:16
The wind directions are just a simplication to make things easy. I could not find any authority references as to which orientation of the jetty or berth. On maritime structural engineer in his course notes recommend vessels to berth at right angle to the wind/waves.

Appreciate if there are more vies on this subject  
pauljohn (Marine/Ocean)
7 Nov 08 10:20
Do you have any topography maps of the area? How deep will the dock be? Is there shallow water (less than 10 feet) for a long distance out that gradually tapers off or does the bottom drop off fast within 25 metres of the planned dock? What is the average wave height? Is there a channel being dredged for entry? Can you offload cargo then anchor out?

So many variables that will affect your decision. Local knowledge is invaluable.

Where is this anyway? Most places I have been prevailing winds are east-west. Just curious.
ozziz (Structural) (OP)
7 Nov 08 19:28

Paul
It is just a preliminary planning with some  environmental studies carried out by others at earlier date. The island is in the tropics and the owner wanted to develop part of the island. Foremost, barges have to bring in material and machineries as the island is quite hilly and undulating. The wave height is about 2m (6.5 ft) and LAT is assumed to be RL 0 m. The seabed at docking is at least 5m (16ft) and the topography of the seabed is quite steep. Channel dredging is unnecessay.
Agree - more local knowledge is required and further waves and current data are necessary. But as a preliminary design and trying to tell the client as to the cost, we have to come up with a structural arrangement.

Other question is since the wind is in North-South direction, I am curious that waves can turn east-west hitting the shore at almost right angle to the East and west coast shore?
pauljohn (Marine/Ocean)
7 Nov 08 22:16
ozziz

Go to google maps and type in 'cedar island, michigan 49726' and hit the search button. Switch to satellite mode and move the map north of the area until you pass over the US/Canadian border. You will see a very uniform wave pattern interacting with some islands. You can see how the waves wrap aroung the islands and other shallow areas. Once the waves start turning around the obstacle they will start to diminish untill after about 90 degrees they will be gone. The reason it does this is, I believe, the underwater friction with the shallow land that causes the wave to slow down on the inside and pull it around the obstruction. Also at sea the waves will will build not only from the direction of the wind but also 5 or 10 degrees off the wind either side of it, which helps to steer the wave around the obstruction.

However if your dock is pretty exposed you might not get any shelter even on the lee side and the swell will cause surge at the dock at all angles especially with a deep dock and a 6 foot wave height. (This is quite high are you sure that is correct?). Most docks I have been on are built parallel to the shore simply from a cost point of view with no regard for the boat. It is left to the crew to cope with whatever conditions they are dealt. Many docks in the Bahamas and Caribbean are built like this.
ozziz (Structural) (OP)
11 Nov 08 3:12
Pauljohn

Thanks for the explanation on waves wrapping around. Could the direction of the wharf depends on the type of vessels and also loading and unloading conditions?

Tow barge - if parallel to the wave, would it be easy for it to dock and undock as the waves would just push the vessel out?
Ferries or passenger boats - perpendicular to waves so that the forward and stern will not roll up and down.
Bulk carrier - as ferries otherwise difficulties in load and unload into the hatch

Are the above answers reasonable?

Cheers
gerhardl (Mechanical)
16 Nov 08 14:16


When you google the Earth, have a look at how relatively unprotected (by nature) harbours, for instance for fishing vessels and ferries, are being protected in Northern Europe, for instance in flat coastlandscapes in Denmark.

The answer is a complete half-moon construction with opening somwhat in the middle.

Advantages on this construction:
Protection from all directions, and all wave and wind directions.
Inlet will work as a 'turned around lince' spreading and diminishing waves.
Sudden squalls and shifting directions influence diminished.

Disadvantage: size, manouvre room and hence cost.

 

ozziz (Structural) (OP)
17 Nov 08 3:43
gerhardl

Building breakwater basin can be very costly and pose environmental problems in the basin if there is no flushing and stop longshore sediment transport.

The concept plan is try to maintain some privacy and not to build a marinas or harbour.

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