Some further comments wrt MOB-barges paper

[Eelco Hoogendoorn]

 

Miguel,

 

Some further points that occured to me wrt mooring:

 

You mention the average slope of a mooring line is 1:7. If we consider the arrangements you drew to be true-scale, then their waterdepth is either very limited, or mooring lines would need to cross under other barges. Is there a standard solution to this? I can imagine setting all anchors first, then placing both rows of barges, but if one line of barges is already in place (and that is the general scenarios for a gradually growing community), it would be very difficult to moor the ones next to it, requiring submarines or something, no? Is that also routinely done, and would it be affordable? Or is modularity along a single dimension simply much easier and thus preferrable to close-proximity modularity in arbitrary directions?

 

Most production vessels / storage barges are moored with a weathervaning turret. With a fixed mooring, waves coming from the side would induce large wave-drift forces and motions. If we use a fixed mooring, would it be best to limit locations to places that have a very dominant wave-direction? (gulf of finland being a good example; there are probably more)

 

 

On another note: having multiple hulls of 100-200m connected at their ends is very nice form a strength/comfort perspective; judging from that paper I sent (or maybe it was another one) you get significant pitch attenuation by connecting multiple modules together, but you dont have the ridiculous hogging/sagging problems of a single 400m+ hull.

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[Miguel Lamas Pardo]

Thank you Eelco for the comments. I do not know why, but I am not able of posting in the google groups.

 

My explanations:

* Arrangement: proposal arrangement is very schematic and not at scale of course. In the book "Barge Mooring" a lot of possibilities and complicated configurations are shown. These operations are done with AHTS (Anchor Handling Tug Supply) vessels, some of them equipped with submarine ROVs (Remote Operate Vehicles). For me it is very surprising that 1700 m could be reached with anchoring.

* A survival mooring line is installed in case of big storms, I assume doing the same work as a weathervaning turret.  According  to the report  “The current forces are relatively constant in direction in the offshore zones; in closer-in areas and opposite the mouths of great estuaries they may vary with the tidal cycle. The wave forces can be considered as comprising an oscillatory motion plus a steady, slow drift force. Both the mean forces of a quasi-static nature and dynamic forces must be resisted”.  Some examples are shown on the book with calculations of up to 15 m waves, so for a seastead in benign water that should not be a problem.

*The idea is to install some constant tension winches that connect the barges between them but with some free movement and to avoid the contact, to install a fender. Of course that will act obtaining some degree of pitch attenuation.

In any case, it would be good to have a feedback from an expert on offshore anchoring/mooring. Perhaps my ideas are not possible.

2010/5/5 Eelco Hoogendoorn <e.hoog...@seasteading.org>

--
Miguel Lamas, Engineering Research
The Seasteading Institute
www.seasteading.org

[Eelco Hoogendoorn]

* Arrangement: proposal arrangement is very schematic and not at scale of course. In the book "Barge Mooring" a lot of possibilities and complicated configurations are shown. These operations are done with AHTS (Anchor Handling Tug Supply) vessels, some of them equipped with submarine ROVs (Remote Operate Vehicles). For me it is very surprising that 1700 m could be reached with anchoring.

 

Ok, good to know that submarine ROVs have been used before as well. So it must be possible, but it does make me worry about the cost a bit. I have not found a lot of cost data regarding mooring, but some examples ive seen suggest that in deep waters, even a simple system can be of the same order of maggnitude in cost as the hull, so it is something to keep an eye on.

 

* A survival mooring line is installed in case of big storms, I assume doing the same work as a weathervaning turret.  According  to the report  “The current forces are relatively constant in direction in the offshore zones; in closer-in areas and opposite the mouths of great estuaries they may vary with the tidal cycle. The wave forces can be considered as comprising an oscillatory motion plus a steady, slow drift force. Both the mean forces of a quasi-static nature and dynamic forces must be resisted”.  Some examples are shown on the book with calculations of up to 15 m waves, so for a seastead in benign water that should not be a problem.

15 meters is indeed more than we ever hope to encounter, but perhaps there are restrictions to that particular scenario? Anything is possible if you throw enough money at it, but mooring a ship to hold against 15m waves (potentially) coming in right to the side seems like it would require a very strong mooring system. That mooring system might be many times more expensive than one that only needs to stop waves coming in at the bow.

 

*The idea is to install some constant tension winches that connect the barges between them but with some free movement and to avoid the contact, to install a fender. Of course that will act obtaining some degree of pitch attenuation.

That makes sense. It just occured to me that usinging winches / hydraulic jacks, the orientation of a fixed moored vessel can also be adjusted substantially so that it has some freedom in how it chooses to face the waves.

 

[Jeff Chan]

On Wednesday, May 5, 2010, 8:24:30 AM, Miguel Pardo wrote:

> *The idea is to install some constant tension winches that connect the
> barges between them but with some free movement and to avoid the contact, to
> install a fender. Of course that will act obtaining some degree of pitch
> attenuation.

FWIW at the first Ephemerisle last tear we lashed the houseboats
together directly at the cleats and this may have put too much
torque on the cleats even in the very mild waves of the delta.
Using fenders between the houseboats would probably be better
since it allows some movement between them.

Cheers,

Jeff C.
--
Jeff Chan
mailto:nation-...@jeffchan.com
http://www.jeffchan.com/

[Jeff Chan]

On Wednesday, May 5, 2010, 10:16:17 AM, Jeff Chan wrote:
> On Wednesday, May 5, 2010, 8:24:30 AM, Miguel Pardo wrote:

>> *The idea is to install some constant tension winches that connect the
>> barges between them but with some free movement and to avoid the contact, to
>> install a fender. Of course that will act obtaining some degree of pitch
>> attenuation.

> FWIW at the first Ephemerisle last tear

er, last year. Ephemerisle wasn't that sad. Really. ;)

[Miguel Lamas Pardo]

Yeah Eelco!!  As you say, everything is possible with money !!  OffshoreMooring could be good if you do the operation from time to time, not very frequently. And I assume the deeper you go, more money you will need.

 

If you are thinking in changing the position very frequently, DP system could be better

 

2010/5/5 Jeff Chan <nation-...@jeffchan.com>

--
Miguel Lamas, Engineering Research
The Seasteading Institute
www.seasteading.org

[Eelco Hoogendoorn]

 

Yeah, the problem is, location must satisfy a lot of constraints; good waves, good politics, good accessibility. It would be great if we could also have shallow waters, but all these criteria are fairly restrictive by themselves, and we dont want to end up with the empty set after intersecting all these criteria . Id say that shallow-international waters should be the first criteria we should let go of if we cannot find one location that satisfies all four conditions; we can always bootstrap from scenario 3 to 1, since at least the operating costs of DP shouldnt be very significant when dealing with a ship/barge. So even if DP is somewhat more expensive, as long as it is not the dominant cost, I think we are better off paying for the DP than (for example) dealing with 3m higher waves than we otherwise would have to.

[Francisco Fernando Quintanilha]

Some Oceanography Data at Brazil’s Coast

 

As you know my knowledge is concrete and steel.

 

A lot of gaps:

 

The site is: http://www.scielo.br/scielo.php?pid=S0102-261X2009000300010&script=sci_arttext

 

 Is written in Portuguese, but Miguel, if he thinks is worthwhile, can read it.

 

No periods of recurrence, the data were got quite recently.

 

No information about sea depth and currents.

 

But as I told you, close to equatorial zone waves are so small, around 2.5 meters maximum.

 

That’s why I wrote previously a location around latitudes, -4º North; 17º South have calm waters.

 

Around the latitudes 23º and 26º south, perhaps we are going to get problems with Brazil’s government, cause is an exploitation oil area and, probably, the government considers it as an Area of National Security.  Unless, if moved from 40º Longitude towards African Coast

 

Getting back to the location, I commented, it’s just a suggestion for the surveying team.

 

Related to comfort. If the “crew” comes from USA or Europe perhaps, living in this zone, some of them would find monotonous. Just one season every year, just variations at rainfall.

 

Attached some figures of the link.

 

Note that:

 

 Altura de Onda = Wave´s height

 

Período de pico= Peak period.

 

Regards

 

Fernando

 

 

[Eelco Hoogendoorn]

 

I tried reading it using google translate, which is quite bad, but here is what I read:

 

In the year 2000, there was one day with Hs_max=5.5, and one with Hs_max=5.6

 

Hs_max = 6 days occur with 0.7% probability in the Clubstead scenario.

 

That is an improvement, but judging from this crude comparison, not a very big one.

[Gary Root]

Hi guys, yes I have been looking at that area as well and have the same concerns and optimism.  I think we would eventually run into issues of territoriality, but won't we eventually have them anywhere we go?  Look, for a long time we will be tiny, they won't even notice.  And after they notice, come on they are Brazilian authorities we are still small stuff, we can bribe them to keep going without interference as long as we don't get in the way of any oil ventures.  I really like this area, it matches shallow ocean depths AND gentle currents and wind levels. I really recommend this location.