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
The total light ship weight shall be divided in the following weight groups:
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
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.
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.
