FLOATING PRODUCTION STORAGE AND OFFLOADING (FPSO) VESSELSGENERAL DESIGN CONSIDERATIONS

George Gougoulidis

Introduction• A Floating Production

Storage and Offloading (FPSO) Vessel is a ship-shaped offshore, self-sustaining, seaworthy platform able to produce oil, continuously over the life of an oil field

FPSO Anatomy

Economics

• Very different form trading tankers• Typical charter rates of tankers may be

measured in terms of thousands of dollars per day

• FPSOs operate with a cash flow measured in millions of dollars per day

• A service interruption is more important than tankers

• Dry docking every ~20 years

Hull

• Either new buildings or tanker conversions• Barge-shape or similar to crude tankers• Exempt from MARPOL, thus they can be:▫ Single hull▫ Double hull▫ Double sided

• Built to stay on the field for very long periods

Hull Ratio TrendsNew FPSOs Tankers

L/B 5.1 5.6

B/D 1.9 2.5

L/D 9.2 13

Cb 0.95 0.9

Evolution of FPSOsTanker conversion New FPSO

Internal Arrangements• The major part of the hull consists of the oil

storage tanks• Collision bulkhead at the bow: 5% x LBP or 10 m

aft of FP whichever is less• Since tandem off-loading at the stern, also use

aft collision bulkhead• Cargo tanks sizing according to MARPOL

regulation 24• at least 2 slop tanks with 2% of storage capacity,

if segregated ballast

Mooring Systems

• Definition - weathervane means that the ship can rotate in the horizontal plane (yaw) into the direction where environmental loading due to wind, waves and currents is minimal

• Most common▫ Spread mooring▫ Turret mooring

Spread Mooring

• Groups of mooring lines are terminated at the corners of the FPSO, holding a stable heading for the vessel

• Vessel not able to weathervane, thus higher environmental loads

• Limited to areas of mild environment and relatively shallow waters (up to 330 meters)

• Simple system, with low cost and doesn’t require conversions

Spread Mooring @ 288 m

Turret Mooring

• a number of mooring legs are attached to a turret, which includes bearings to allow the vessel to rotate 360° around the anchor legs

• Minimizes environmental loads on the vessel by heading into the prevailing weather

• It can be:▫ External ▫ Internal

External Turret

• At the bow▫ Single cantilever (deck)▫ Double cantilever (deck and bulbous bow)

• (+) Simplicity, minimal requirement for integration into the hull, low conversion costs

• (-) Shallow waters, mild environments

External TurretSingle cantilever Double cantilever

Internal Turret

• Typically 5%-45% x L aft of FP• Usually 10%-30% x L aft of FP • (+) Suitable for deep waters and severe

environments• (-) Complex integration into the hull, loss of

cargo area

Internal Turret

Topside Facilities

• Oil and water separation• Gas compression• Water injection• Cargo handling and offloading• Utility and support• Safeguards

Processing Facilities

• Crude stabilization plant on the main deck• Minimum distance between main deck and

processing units 3 m▫ For safety and accessibility purposes

• Storage capacity usually 10 times the processing capacity

Processing Facilities

OffloadingAlongside Tandem

Accommodation

• Fire safe wall facing the process systems• Separation distance between process facilities

and accommodation at least 100 ft• The location of accommodation affects the

location of the turret and machinery space▫ Accommodation at the bow – turret towards

midship – decrease of natural weathervaning▫ Accommodation at the stern – turret closer to the

bow – able to weathervane

Accommodation ConfigurationsAt the bow At the stern

Structures

• Topside facilities can weigh 5.000-30.000 tn• Point loads on main deck• Allow 10%-15% additional strength to cover

increase in weight• The supports of the process plant should be

placed over transverse, longitudinal bulkheads, and web frames

• Box-shaped hull means more buoyancy near the ends resulting in high sagging moments

Stability

• Process facilities add weight high• Not a big problem since L/B is low• Critical conditions▫ Wind heel moments of the process plant in light

ship condition▫ Free surface effects in partially filled tanks

Seakeeping

• To avoid any special issues related to the process facilities, motions are commonly limited to:

• pitch within 10 degrees double amplitude (±5 degrees) or similar

• roll within 20 degrees double amplitude (±10 degrees) or similar.

Water Loads

Cylindrical Hull

• Effect of wind direction and current is minimized

• Excellent motion characteristics• No turret or swivel

SEVAN PIRANEMA

SEVAN PIRANEMALocation: Brazil

Overall length: 66 m

Diam. in waterline: 60 m

Displacement at 18m draft: 55,000 mT

Deck area capacity: 5,000 mT

Deck area: 2,800 m2

Accommodation: 63 persons

Mooring: 9 lines (anchor-chain-polyester-chain)

Riser slots: 25

Liquid production: 30,000 bbl/day

Crude storage capacity: 250,000 bbls/40,000 m3

Export to shuttle tanker: 1,800 m3/h

SEVAN VOYAGEUR

SEVAN VOYAGEURLocation: North Sea

Overall length: 66 m

Diam. in waterline: 60 m

Displacement at 18m draft: 55,000 mT

Deck area capacity: 5,000 mT

Deck area: 3,200 m2

Accommodation: 54 persons

Mooring: 12 lines (anchor-chain-polyester-chain)

Riser slots: 10

Liquid production: 30,000 bbl/day

Crude storage capacity: 270,000 bbls/43,000 m3

Export to shuttle tanker: 3,600 m3/h