Regarding Initial Design of AHTS

Discussion in 'Class Societies' started by shettythunderz, Nov 19, 2010.

  1. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    I am Abhishek in my final year of Naval Architecture and Shipbuilding course and i have been given AHTS as my final year project.
    So can anyone please help me out with the Owner's Requirement Criteria to be selected for AHTS designing. I also need to know on what criteria should i select my Parent Ships, should it be Bollard pull or Deadweight????
     
  2. JRMacGregor
    Joined: Oct 2005
    Posts: 83
    Likes: 17, Points: 8, Legacy Rep: 255
    Location: Scotland, UK

    JRMacGregor Junior Member

    The key owners requirements for an AHTS are;
    - bollard pull
    - winch pull
    - speed
    - DP rating

    Then, the number of winches and wire storage are important. Also capacity of chain lockers. Then the decision is - should the vessel also carry rig cargo (drillwater, bulk powder tanks etc) so it can make some money when no AH work.

    Some owners of large AHTS are also adding more accommodation, an ROV capability and a reasonable sized crane to their AHTS. This allows the vessel to do some non AHTS work.

    For an AHTS you should select your parent ships on bollard pull. Deadweight is more or less irrelevant for an AHTS.

    A big part of the design for large high power AHTS is selecting the power plant;
    - 2 x big direct diesels and CP for the main props, plus smaller DGs for the other thrusters and winches
    - fully diesel electric
    - some diesel electric hybrid with electric motors on main shafts for good economy when steaming, but main shafts also driven direct by big diesels for max bollard pull

    Read up on the BOURBON DOLPHIN accident report to get an idea of what these ships have to do and what is important in their stability.
     
    1 person likes this.
  3. powerabout
    Joined: Nov 2007
    Posts: 2,925
    Likes: 66, Points: 48, Legacy Rep: 719
    Location: Melbourne/Singapore/Italy

    powerabout Senior Member

    fuel consumption is also important when not doing towing so think about the fact that you have 2 huge engines that only work at full power possibly less than 30% of the time the vessel is at sea.
    4 engined vessels get over this with 2 small and 2 large with a large and a small connected via a common gearbox
     
    1 person likes this.
  4. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Thank you JR MacGregor and Powerboat for your kind help. I got a lot help from you.
     
  5. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Can anyone please help me out with the selection criteria of Parent Ships for AHTS.Should it be deadweight or Bollard pull.If it is deadweight then i do not have any problem but if it is bollard pull then i need to know how to proceed and where to search for the procedures of main dimension fixing for AHTS.
     
  6. JRMacGregor
    Joined: Oct 2005
    Posts: 83
    Likes: 17, Points: 8, Legacy Rep: 255
    Location: Scotland, UK

    JRMacGregor Junior Member

    If you do not have access to DISPLACEMENT data, I recommend you try to make a new criteria for your family of potential parent ships. This should be LBD in cubic metres. Obtained from multiplying LOA (or LBP) x B x D.

    Then you can look at the bollard pull which is typically associated with each value of LxBxD. You can make a graph. Although there will be a scatter I think you will find quite a close trend relationship between the two parameters.

    This will give you an approx. required LBD for your desired bollard pull. Then you can start playing with the proportions of L/B, and B/D to give the desired LBD. You may need more L/B if you are also aiming for a fast AHT. Of course you should make sure that the L/B etc do not go far away from the ratios already proven in the family of parent ships.

    In my view, LxBxD is a more useful definition of AHT size/capability than is deadweight.

    If you have displacement data for your potential parent ships, that is better than LBD or deadweight.
     
  7. StianM
    Joined: May 2006
    Posts: 593
    Likes: 23, Points: 18, Legacy Rep: 114
    Location: Norway

    StianM Senior Member

    I worked as a motorman on a AHTS when I was 20, but after finishing off my as a engineer i som how ended up in automation and are now a service engineer in power and automation.
    I had the responsebilety for 4AHTS vessels with switchboards and 1 with the whole automation package.

    Our latest ATHS have the following layout.
    2x main engines + 4x aux engines.
    2x Gears with PTI and PTO in front of the main engines.

    The main engines can then run both electrical (trough PTO) and propellers during steaming or light towing.
    It can run diesel electrical during ROV or other DP operation trough the PTI on the gear.
    For heavy rowing it can run a boost mode where it are using both main engines and all 4 aux's are delivering electrical power to the PTI.

    Also reactive power has to be consider and the reactive power will be less if running 50Hz system, but will result in more heavy installation since you would need bigger engines and motors to get the same power.

    DP1 require only DP equipment while DP2 require a redundant system that will allow you to stay on DP even if a failure appear. If you want to have a DP2 system with closed bustie on the switchboard you will need a system that trips generators if they under or over excitate.
    DP3 might be required if you want to be involved in diving operations, but you are not allowed to go into DP3 with closed bus tie and you need two separate control and switchboard rooms. Also the bridge need a separate emergency bridge and you need two engine rooms.

    With the current changes towards operation in arctic environment de-ice systems and ice class might be a good idea.

    With current and future requirements on emissions LNG is a good idea, but since it could be hard to get some places in the world duel fuel might be the way to go.
    With LNG you need a ESD systems that will shutdown zones of the vessel if gas is detected. If gas is detected in the engine room a shutdown would mean a breach of DP requirements so if gas mode is to be used with DP mode a separated engine room and take DP3 as well might make sense.

    UREA systems are used, but personally I think they make no sense since you need more weight for tank and chemicals. You can no longer use a exhaust boiler. And the production and transportation of the chemicals + added cost of stainless exhaust and increased pollution from producing nickel don't justify the reduced emissions from the vessel.

    The vessel need to put on more generator load since there is no exhaust boiler.
    The vessel need to use more fuel in the main engines to push the vessel trough the sea because it carry chemicals for UREA.
    The chemicals need to be produced in a energy consuming chemical factory.
    The chemicals need to be transported by boat/truck to the vessel.

    Depending on future fuel prices it really is interesting if the oil companies will ask vessels to start operation on HFO or not, but this as far as I know not legal while being inside of 200m from a oil installation in the North Sea.
     
  8. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Thank you sir for your reply. I have statred my project with all your help. I have fixed my main dimensions and now i am into my checks. So if there is no sheer given to my ship do we have to consider "correction for sheer" under FREEBOARD CHECK?
     
  9. JRMacGregor
    Joined: Oct 2005
    Posts: 83
    Likes: 17, Points: 8, Legacy Rep: 255
    Location: Scotland, UK

    JRMacGregor Junior Member

    Yes - even if the ship has a flat deck (no sheer) you have to "correct for sheer" due to the existence of raised superstructures (forecastle etc).

    The load line rules make clear what you have to do.
     
  10. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    thank you sir for your kind reply. Where can i get the stability criteria for GZ curve? According to IMO the maximum value of GZ should be @ 30 deg or more but my max. GZ value is coming @ angle between 15 to 30 degree. So is there some special satbility criteria for AHTS or am i wrong somewhere?? And also may i know what is the range of block coefficients for AHTS or offshore supply vessels?
     
  11. JRMacGregor
    Joined: Oct 2005
    Posts: 83
    Likes: 17, Points: 8, Legacy Rep: 255
    Location: Scotland, UK

    JRMacGregor Junior Member

    Hi

    For stability regulations applicable to AHTS you should check out IMO resolutions A469 (OSVs) and A534 (Special Purpose Ship code). Concentrate on the intact stability issues as the application of probabilistic damage scenarios under SPS Code will take you a long time.

    For OSVs less than 100m there are relaxed requirements for the angle of max GZ. It should not occur before 15 degrees. This code was developed to recognise the special stability characteristics of ships with wide beam and low freeboard (OSVs).

    Block coefficient for an AHTS or OSV varies on the design and the draught. But a typical value at operational draught would be about 0.65.

    If you have not already done so you should read the report on the Bourbon Dolphin accident. Preliminary version here.............

    http://www.regjeringen.no/pages/2061386/PDFS/NOU200820080008000EN_PDFS.pdf

    This report points out deficiencies in the application of the stability codes to AHTS. Mainly failure to take proper account of the effect of the applied anchor/chain tension.
     
  12. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Hi
    Again i am thankful to you sir as i have done my initial stability check successfully. Now i will be doing my Volume Check. Sir may i get any free link for books to get more knowledge about AHTS. We have some specific books for RO-RO or LNG or Bulk Carrier giving all the detailed idea about the ship. Similarly if i could get any book on AHTS that would help me to know more about the ship and thus i would easily proceed with volume checks and type of machinery to be selected.
     
  13. JRMacGregor
    Joined: Oct 2005
    Posts: 83
    Likes: 17, Points: 8, Legacy Rep: 255
    Location: Scotland, UK

    JRMacGregor Junior Member

    Hi

    As far as I am aware there are no good technical books on AHTS design.

    There is an interesting and expensive book on the history of supply boats and AHTS - called THE HISTORY OF THE SUPPLY SHIP by VICTOR GIBSON but this will not help you much.

    There are a few SNAME papers on the design of offshore supply/AH vessels - mostly American designs.

    To get info on the layout and sizes of compartments you can find a lot on the vessel data sheets held of the websites of the main operators of AHTS.

    I dont know what size of AHTS you are designing, but you can try the websites of;
    Maersk, Bourbon, Farstad, Viking Supply, REM, Havila etc.

    The internal volume for tanks will come simply from the selected fuel volume, water ballast volume, FW volume etc. The machinery space size will be driven mainly by the number and sizes of main engines you need to drive your shafts. The main engine makers (MAK, MAN/B&W, Wartsila, Bergen) have catalogues which give engine (or generator set) dimensions for different power rating. For reliability you should aim for a medium speed engine of less than 1000 rpm and probably 26cm or 32cm cylinder bore.
     
  14. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Sir the main dimension of my ship is L=58m B=15.2 D=6.39 T=5.4 Cb=0.72 V=13knots
    B.P= 80t.
     

  15. shettythunderz
    Joined: Nov 2010
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: India

    shettythunderz Junior Member

    Wish you a very Happy New Year sir. Hope this new year bring all the happiness for you and your family.
    It has been my pleasure to get knowledge from you. Thank you for your kind help and hope to continue with this. Thank you very much sir.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.