Dynamic Positioning System vs Mooring System

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One of the main disadvantages of dynamic positioning systems when compared to mooring systems, is the fuel costs incurred in operation. But it is necessary to quantify that difference in order to make an informed choice between the two possibilities: mooring or dynamic positioning.

In Ref [1], a comparison of fuel consumption between a passively moored FPSO and a Dynamic Positioned FPSO is made. The arrangement of the two versions are show in pictures on the right. The moored version of the FPSO has an internal turret with 3*3 chain mooring arrangement. The DP version of the FPSO has a thruster layout consisting on 7 thruster units.
 
The following simulations were carried out:
  • DP station keeping simulations.
  • Power use and fuel consumption simulations.
The analysis considers a typical annual scatter diagram of sea states for the Gulf of Mexico. The results are summarized in the table below:

Option
 
HFO consumption
t/year
HFO Cost
€m
DP FPSO
Annual Average consumption
6,930.7
1.7
Moored FPSO
(DP system out)
Annual Average consumption of ship systems = 2.5 MW
6,428.4
1.6
 
 
 
Diference = 0.1
 
 
 
 
 
 
MDO equiv.
t/year
 
Process Plant
Gas Turbines 2*25MW
30 MW load
24,737.2
 
 
Resume and analysis:
  • The results show that the DP FPSO concept requires about 7% extra fuel compared to the moored FPSO concept (total for the ship), while it is 1.5% if the power of the "process plant" for production is also considered.
  • If we consider the “process plant” consumption as equivalent to the “seastead consumer plant” (a very gross consideration for all lighting, HVAC,…) the expected difference between the two options is only that 1,5%, which is very small and put the balance on the side of DP system.
  • If the metocean conditions are worst than in Gulf of Mexico, the DP system would consume more, and the 1,5% extra fuel in difference would be higher. But it is not expected to establish a seastead in a rougher scenario than the GoM.
  • The net difference in the annual fuel consumption cost is only 0.1 €m between the DP and moored versions. Therefore, the annual consumption of the DP system is 100,000 € which implies:
DP fuel cost per year = 100,000 €
DP fuel cost per month = 8,333 €
 
If we do a gross estimation of the potential number of “seastead apartments” available on the FPSO, we obtain following results:
 
Length = 262.4 m
Beam = 46 m
Main Deck space = 10,863 m2
Number of decks = 5.00
Total deck space = 54,317 m2
Average space apartment = 100 m2
No. Apartments = 543
=> DP fuel cost per month and apartment = 15 €
 
That means that the cost per “seastead apartment” is quite low and shows that a DP system need not have high operating costs, for a big vessel such as this, with a somewhat hydrodynamic shape.
 
Notes and conclusions:
  • Bunker fuel is used for power generation, as opposed to diesel, which is used in the calculations of the ClubStead report. Diesel tends to be a lot more expensive. However, if for some reason diesel were to be considered more desirable, costs would still remain modest.
  • This analysis only considers OPEX and not to CAPEX; differences in installation costs between a mooring and DP system are not taken into account. For shallow waters, mooring would probably have a CAPEX advantage, but for deep waters, DP would probably be preferrable for CAPEX reasons as well.
  • We can conclude that for a large barge like seastead, a DP systems has only modest operating costs, as compared to the total operating costs of the seastead.
 
[1]   Aalbers, Albert B.; de Vries, Leo; van Vugt, Hans. Fuel consumption and emission predictions: application to a DP–FPSO concept. Houston: Dynamic Positioning Conference, October 2006.

 

16 comments

  1. vincecate 7:49 pm

    A solar powered SFS could do DP for no added cost.

    Seems 1.5 kw can produce 1780 lbs thrust.   Solar is down to $2/watt.

    http://wiki.seasteading.org/index.php/Thrusters

  2. Carl Pålsson 3:09 am

    A seastead probably needs a lot less accuracy than a FPSO (it can be allowed to drift around more). This should save energy.

    Is the FPSO a low-drag structure (like a ship), and is it thrusting only in the most efficient direction? If not I’m guessing a lot can be saved here.

    What is the depth for the mooring in the example? Deeper water will be more expensive.

    DP is superior to mooring because depth does not matter. You have the whole ocean at your disposal (within reason).

    Like Vince points out, if you optimize propulsion for slow speeds, you get lots of force from modest power. Drag rises with the square of the speed.

    Another way is to simply put sails on your seastead. If we assume it is something along the lines of a cruise ship or a container ship, a couple of small (in relation to the ship anyway) sails controlled by servo motors and a computer program might be enough (for rough station-keeping, not for speedy travel).

    A seastead is in the water. Use the water. Don’t fight it.

  3. Eelco 11:17 am

     A seastead probably needs a lot less accuracy than a FPSO (it can be allowed to drift around more). This should save energy.

    Perhaps; but if its close to many other structures, this advantage would diminish.

     

    Is the FPSO a low-drag structure (like a ship), and is it thrusting only in the most efficient direction? If not I’m guessing a lot can be saved here.

    It has a barge-like shape; not as hydrodynamic as a ship, but in terms of drag per ton, still way more efficient than the next closest competitor in this regard. Orientation would to some degree be constrained by waveheading; however, waveheading strongly correlates with wind and current speed. Either way, the same constraints would apply to a seastead. Perhaps even moreso, since were trying to be more squeamish about comfort than what oil companies offer to their roughnecks.

    Like Vince points out, if you optimize propulsion for slow speeds, you get lots of force from modest power. Drag rises with the square of the speed.

    The system has been designed for station keeping; I wouldnt expect to be able to improve on that to any significant degree.

    Another way is to simply put sails on your seastead. If we assume it is something along the lines of a cruise ship or a container ship, a couple of small (in relation to the ship anyway) sails controlled by servo motors and a computer program might be enough (for rough station-keeping, not for speedy travel).

    TSI doesnt really want to go there. We want hunderds if not millions of these things in close proximity eventually. Anything other than a DP system wont give you enough flexibility to pull that off.

  4. Eelco 11:31 am

     A solar powered SFS could do DP for no added cost.

    Because everyone knows, captial costs arnt real costs?

     

    Seems 1.5 kw can produce 1780 lbs thrust.   Solar is down to $2/watt.

    For a seastead of this size, solar might well suffice for station keeping needs; though note that in terms of drag/displacement, we are a few orders of magnitude away from say, a small spar. Its not a simple calculation though; its hard to include all costs, such as the market price for deck space. But these DP systems all something-electric anyway; plugging in a chunk of solar could always be done easily, even after commissioning.

    That said, 15 a month or a spacious deck – I know what id choose.

  5. Carl Pålsson 3:26 pm

    Perhaps; but if its close to many other structures, this advantage would diminish.

    Yes, but that is not a problem seasteading has at this point. And it might never happen. Close proximity is dangerous, and the ocean is big.

    Perhaps even moreso, since were trying to be more squeamish about comfort than what oil companies offer to their roughnecks.

    Eventually perhaps. Not today, I would hope. Today TSI should target the most likely people: Those who are willing to endure the stresses of a frontier lifestyle, including some uncomfortable waves. It will be cheaper to build for them.

    The system has been designed for station keeping; I wouldnt expect to be able to improve on that to any significant degree.

    You are probably right about this, assuming this is exactly the kind of station keeping seasteading requires at this point.

    TSI doesnt really want to go there. We want hunderds if not millions of these things in close proximity eventually. Anything other than a DP system wont give you enough flexibility to pull that off.

    I’m not sure the optimal way to do things when hundreds or millions of seasteads roam the seas is the right thing to be focusing on today, when the number of sesteads is zero.

  6. livefreeortry 4:09 pm

    It might be worthwhile seeking out locations initially which have relatively subdued currents/winds or small closed circular current paths. I’d suggested trying the Pacific Garbage Patch in this context. The patch persists and does not diffuse away because the prevailing winds and currents are mild and tend to concentrate floating debris. The plastic isn’t too bad either, as I understand it. More  such locations must exist in other gyres.

    http://seasteading.org/interact/forums/research/tsi-research/pacific-garbage-patch-favorable-location

    Another rather exotic possibility that occurs to me is to use some sort of underwater parachute. Wind driven currents are rather shallow, ( http://www.physicalgeography.net/fundamentals/8q.html, http://www.physicalgeography.net/fundamentals/8q_1.html; see maps) extending about 100-200 meters from the surface. Below this, currents, if they exist, are far slower and the direction seems uncorrelated with surface currents. A collapsible underwater parachute could serve like an anchor to slow down the effect of currents and winds.

    After drifting some distance the parachute could be collapsed and the seastead moved back to the original spot.

  7. Eelco 4:55 pm

    Eventually perhaps. Not today, I would hope. Today TSI should target the most likely people: Those who are willing to endure the stresses of a frontier lifestyle, including some uncomfortable waves. It will be cheaper to build for them.

    It doesnt really matter for the cost, as a matter of fact. It matters for scale though; you can get away with a much smaller seatead if you compromise on comfort. Either way, the question depends a lot on what busines model we go with. You are right, we shouldnt exclude the possibility of building a seastead of similar comfort to some of the less comfortable oil rigs. But at least, we dont want to go much worse than that; perhaps building a society isnt impossible with 10% downtime, but there comes a point at which people are going to deem the ocean tax too high. Relating that to the original point in question; no, we probably wouldnt be able to do much better on fuel consumption because of that.

     

     Yes, but that is not a problem seasteading has at this point. And it might never happen. Close proximity is dangerous, and the ocean is big.

    I’m not sure the optimal way to do things when hundreds or millions of seasteads roam the seas is the right thing to be focusing on today, when the number of sesteads is zero.

    We want to provide a credible path from here to there. One of the biggest problems in making this work is overcoming isolation. Sure, one seastead doesnt need accurate positioning, but as soon as there are two, it does. Randomly drifting around on the ocean may be a necessary intermediate step, but it wont do for building a lasting society of any significance.

    Of cource ambition needs to be balanced by realism, and the simpler we start the better. But lets not forget the conclusion of this post; we can both have our cake and eat it as well!

  8. i_is_j_smith 5:34 pm

    This is very confusing.  What is the fuel consumption of a moored barge?  What is the depth?  What type of mooring system is it using?

    You expect to establish a seastead in an area as calm as the Gulf of Mexico?  Where would that be?

    Do both designs have the same level of redundancy?  How many mooring lines can fail vs. how many thrusters?

    Sorry, there is just too little information to understand this conclusion.

  9. Miguel Lamas 6:53 pm

    The complete report of Ref[1] with data and description of the two FPSO versions can be downloaded here:

    http://www.dynamic-positioning.com/dp2006/control_aalbers.pdf

    The idea of the blog post is to show that DP does not consume so much fuel as it would be expected, and therefore, it is a good alternative for keeping the position in deep waters. Mooring systems for deep waters, like the turret system shown, is also very expensive and in this case, a DP would be better.

    There are some points in the Gulf of Mexico outside of the EEZ that could be a good place for a seastead, but it should be studied more in detail.

  10. Eelco 9:36 pm

     This is very confusing.  What is the fuel consumption of a moored barge?  What is the depth?  What type of mooring system is it using?


    Good question; you are right, that is rather confusing. Its a floating oil production vessel; it uses shitloads of power, DP or no.
     
    The current scenario employed in the paper seems (at first glance) comparable to the one used in other calculations (that arrive at 100$ per person per month for clubstead diesle costs, for example), of a one knot continuous current. The gulf of mexico has rather strong currents, in most places, actually.
     

    Do both designs have the same level of redundancy?  How many mooring lines can fail vs. how many thrusters?

    The blog post is meant to convey a summary of the paper. Either way, do these details matter in the context of a cost of 15e per apartment per month?

  11. wohl1917 9:50 am

    is that your numbers don’t take into acount the fact that the energy consumption figures are what they are to support industry processes: a need that would not exist on your typical seastead. In the situation presented Dynamic Positioning Systems would be and are a small drain on the total consumption and needs. On a Seastead where the energy consumption/needs would be much less and where the economic activity would necessarlily be less productive than say PUMPING OIL, a DPS would be prohibitivly expensive to operate and maintain.

     <http://ocr.wikia.com/wiki/Oceanic_Citizens_Republic_Wiki>

  12. Carl Pålsson 3:25 pm

    Wohl, the analysis uses the difference in fuel oil consumption between a DP platform and a moored one.

    So the oil pumping activites are excluded.

    Another thing that hasn’t been included that could make DP even more attractive is the comparison between DP and mooring equipment. I have a feeling the mooring gear will be more expensive.

  13. Miguel Lamas 11:28 pm

    Yes Carl, you are right. Oil pumping activities should be included in the "process plant" consumption. The difference of 7% of fuel oil consumption is related to common ship systems excluding the "process plant". That difference of 7% in terms of money is only 100,000 €. That means, as Eelco has suggested, that the current forces assumed should not be too big.

    I am also agree with you regarding the equipment, and it is something not compared in the paper: CAPEX of the two versions. A mooring system of turret type for deep waters seems to be more expensive than a dynamic positioninng system. And with the DP we have an important advantage for a seastead: the freedom for mobility.

  14. vincecate 2:24 am

     

    Vince: A solar powered SFS could do DP for no added cost.

    Eelco:  Because everyone knows, captial costs arnt real costs?

    I expect to use solar power and electric propulsion for a SFS.  For a SFS size it is easy to get plenty of solar area.   Solar costs are not that high compared to the rest  of the SFS and the freedom to stay in the ocean as long as you want is nice.  If you are not in a hurry you can get a lot of thrust from little power.   I really expect most SFSs to be solar/electric.

    With a solar/electric SFS the ability to do DP will not take any extra fuel or additional equipment.   As we get groups traveling together we probably will have some open source software that we use to control group movement.    I am also assuming the group already has networking.

      — Vince

  15. i_is_j_smith 7:02 pm

    "do these details matter in the context of a cost of 15e per apartment per month?"

    The details matter a lot, because the capital and maintenance costs are going to determine if you can get that 15e/month apartment at all.

    I still don’t understand the numbers from the paper.  It’s saying that the auxillary systems on the FPSO (lights, comms, A/C, etc) use a total of 2655kW and use 6400 tons of fuel per year, while the DP system uses between 1000kW and 31000kW and only uses 500 tons of fuel per year.  How does that make any sense?  Even if you use only the 1000kW number for the DP systems you are still half of the auxillary systems, so you should be using half the fuel that the auxillary systems do.  Something doesn’t make sense here.

    There are way too many variables to make a DP vs mooring decision.  In this example they are talking about a depth of 1500m which is going to be an enormous expense when mooring, so we just need to look for areas that aren’t as deep.  If that leans you towards DP you have to consider the fact that the highest Hs they considered in this sim was 7.5, and that only occurred less than 1% of the time.  A DP seastead outside of protected waters is going to see seastates like that a lot more.

    Capital costs are another variable.  Sure, mooring is expensive.  But DP isn’t cheap either.  You need the generators, the thrusters, the entire electrical subsystem…plus you need backups.  Some quick google-fu found used 3300kW Wartsila diesel generators for US$800k each.  Thrusters are almost US$250k each, and I don’t even know how much 5000kw AC motors cost.  I do know that high-voltage AC drive systems (for converting the DC from the generator to AC for the motor) can run more than a million US.  I don’t think a DP system will be as cheap as you think.

    As for solar-powered DP…you better have a good backup because the last thing you want is to start drifting away because you had a few days of cloudy weather…

  16. Miguel Lamas 11:32 pm

    Yes, I am agree: why to install so much DP power (31000 kW)  if then you are not using it? In the report they insist in the fact that in the point of GoM that they are studing, the meteocean conditions are not so hard, and as a consequence, the DP effort is very little. Perhaps the thruster plant of the FPSO was designed for a harder enviroment like North Sea where you could require more power to keep the vessel in position, and therefore, the DP fuel costs would be higher.

    Also agree with cost of a DP plant: it is not cheap at all, but compared to a mooring system for deep waters, it would not be so expensive, and it would be more flexible.

    In any case, a study similar to this one would be very useful when making a decision about the first place in the ocean for a seastead.

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