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This article describes  the procedure to calculate lightship weight  of any  drillship at the end of the basic design  stage. During the development  of the design the lightweight will need to be updated regularly as revised and vendor data becomes available.
 
INFORMATION REQUIRED
 
  • General Arrangement  Drawing
  • Steel Material Take-off:
  • Piping and appendages  list with MTO
  • Vendor Equipment details
 
The total light ship weight shall be divided in the following weight  groups: 

  • Steel
  • Marine systems
  • Power Generation,  DP and Electrical systems
  • Outfitting
  • Major Rig equipment
  • Mud systems
  • Shipboard Subsea equipment
  • Marine riser system
  • Pipe handling
  • Well Check / test equipment
  • Derrick & substructure  piping
 
In  general best  estimate   is  made  based  on available  typical  equipment  data  or  data  from  previous  comparable   projects.  Also "estimate"  may  well  include  calculations   or  MTO's  however  based  on  preliminary designs.
 
LIGHT  SHIP WEIGHT  MARGIN

Two margins are added to the weight estimations; design allowance  and contingency. The design  allowance  can be set Approx 6 to 8% when no material take off data or equipment  data is available. After receiving material take off  this  allowance can be reduced at  2.5 to 3%.  When  equipment  data  is  well  specified  the allowance is set to zero. This allowance is set for design changes  and unknown items at basic design stage. When design progresses this margin may be reduced further.

The contingency  can be set at 6% for steel MTO's to cope with welding,  structural and a general allowance. The contingency for the other items can be set at 4% to cope with minor items not accounted  for in the weight  breakdown,  such as small  connection  details, pipe supports  etc .. When  well  defined  equipment  data  is  used  the  contingency  is omitted in some places.
 
 
LIGHT  SHIP WEIGHT  DISTRIBUTION
 
Longitudinal members shall be distributed uniformly in their region. Transverse bulkheads and frames can be distributed in narrow band along frames. The individual weight items are spread over the number of frames where the  item  is located. Some items are distributed as point loads with very less area for support.Like derrick and many mud equipments with legs are distributed as point load.  This  lightweight  distribution  is  used to  calculate  the  still water bending moments and shear forces  in the loading conditions.

 STEEL
 
The steel weight data of the vessel hull shall be based on the material take off list at basic design stage. Which includes all basic structures drawings excluding secondary foundation and details. As the vessel design progresses, detailed weights from 3d hull simulation or weight control report will provide more accuracy. 
 
 
MARINE SYSTEMS
 

Material take off list of marine systems shall be prepared from basic drawings of following systems
  • Fresh water system
  • Sewage system
  • Fuel oil system
  • Brine system
  • Lube oil system
  • Fire fighting system

POWER GENERATION, DP AND ELECTRICAL SYSTEMS

 
This  weight  group  contains   all  power  generation,   electrical   and  instrumentation components   complete  with  cabling   and  cable  racks.  The  components   consist  of electrical motors, switchboards  etc. The data presented for this weight group shall be based on  estimations  and  typical   (manufacturer)  data  for  thrusters,   E-motors,  frequency converters and transformers.
 
OUTFITIING
 
This weight group contains the outfitting of the accommodation,  life saving appliances, thrusters ,anchoring   and  mooring  equipments,   painting   etc.  Most  of  the  data presented for this weight group shall be based on estimations.Later detail calculation shall be included.
 
MAJOR  RIG EQUIPMENT
 
This weight  group  contains  the  derrick,  drawworks  and  main  items  on or  near the rig floor as main components.  Drilling vendor provided weight info on the derrick, but this item is still marked as estimate as this appears more appropriate.
  
MUD SYSTEM
 
This weight  group contains  the mud pumps and mud preparation  / cleaning systems and  systems  associated  with  this.  The  weight  data shall be  based  on  vendor  data  for equipment and estimations form previous projects for the systems.
 
SHIPBOARD   SUBSEA  EQUIPMENT
 
This weight group contains the systems associated with the BOP incl. handling as well as the riser tensioners  and associated  equipment  and the ROV's.  The weight of the riser tensioners  shall be based on drilling equipment vendor data for required pull and number of tensioners.
 
MARINE  RISER SYSTEM
 
This weight group  shall be associated  with mainly the riser handling  systems.  At basic design stage these weights are estimated and later on based on vendor equipment data can be revised.
 
PIPE HANDLING
 
This  weight  group deals  with  the  drillpipe  casing  and tubular   handling  and racking systems. The weights are estimates.
 
WELLCHECK   / TEST EQUIPMENT
 
This weight group is a minor group  and contains the fixed  items  associated with well checking and logging.
  
DERRICK  & SUBSTRUCTURE   PIPING

 
This weight group contains the piping running from the vessel to rig floor and derrick. Weights for this group are estimated  based on previous projects. 

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  • Ship Design
    • Tug Designs
    • Boat Design
    • Ship Design Process >
      • Barge Design
      • Modular Spud Barge Design
    • Tanker Design
    • FPSO Engineering
    • Jetty Design
    • Offshore Support Vessel design
    • LNG Vessel Design
  • Consultancy
    • Naval Architecture
    • Structure FEM Analysis
    • Hydrodynamics
    • Stability Analysis
    • Production Design
    • Ship Manuals
    • Machinery & Piping
  • Offshore Engineering
    • Mooring Analysis
    • Transportation/Installation
    • Offshore Heavy Lifting
    • Offshore Pipeline Engineering
  • ONLINE RESOURCE
  • CONTACT