Structured Packing for FPSO and Floating Offshore Columns: Motion, Tilt and Liquid Maldistribution

Packed columns installed on FPSOs, FLNG units, floating production systems and offshore vessels operate under conditions that do not exist in a fixed onshore plant.

Roll, pitch, heave, permanent list and structural movement can change the apparent direction of gravity, liquid level and flow distribution inside the column. A liquid distributor that performs uniformly when perfectly level may feed one side of the packing much more heavily when the vessel tilts.

Structured packing can reduce column weight, pressure drop and required height—important advantages offshore. However, its performance depends strongly on liquid distribution, packing retention and mechanical installation.

For floating service, conventional steady, vertical tower design is not enough.

Why Use Structured Packing Offshore?

Offshore process equipment must operate within strict limits on:

• Weight
• Footprint
• Column height
• Utility consumption
• Maintenance access
• Gas compression power
• Structural loading

Structured packing may provide:

• High mass-transfer efficiency
• Low pressure drop
• Low liquid holdup
• High hydraulic capacity
• Lower installed weight
• Reduced column height
• Smaller process inventory
• Lower compressor or reboiler duty

These advantages can be valuable in:

• Gas sweetening
• Glycol dehydration
• Condensate stabilization
• Produced-water stripping
• Solvent recovery
• Hydrocarbon fractionation
• Acid-gas treatment
• Offshore chemical processing

However, packing selected from fixed-platform data may not maintain the same performance on a moving vessel.

How Does Vessel Motion Affect the Column?

Floating equipment may experience:

• Roll
• Pitch
• Heave
• Surge
• Sway
• Yaw
• Permanent list
• Trim changes

These movements alter liquid behavior inside:

• Distributors
• Collectors
• Reflux drums
• Column sumps
• Packing surfaces
• Downcomers
• Side-draw boxes

The effect depends on:

• Motion amplitude
• Motion period
• Column location on the vessel
• Orientation relative to vessel axes
• Liquid depth
• Distributor geometry
• Column diameter
• Operating load

A motion condition that is acceptable for one vessel or column location may be unacceptable for another.

Why Is Liquid Distribution the Main Concern?

Structured packing requires uniform liquid irrigation.

When the column tilts, liquid level inside a distributor changes across its width. Openings on the low side may receive more liquid, while openings on the high side may receive less or stop flowing.

This may create:

• Over-irrigated packing
• Dry packing regions
• Vapor channeling
• Local flooding
• Reduced mass-transfer efficiency
• Higher solvent loss
• Unstable product purity
• Increased entrainment

The average liquid flow may remain correct even while the distribution across the column becomes unacceptable.

Static Tilt and Dynamic Motion

Static tilt and dynamic motion should be evaluated separately.

Static Tilt

A permanent list or trim creates a sustained difference in liquid level.

This may cause continuous:

• Distributor flow imbalance
• Wall flow
• Unequal packing wetting
• Side-draw error
• Sump-level bias

Dynamic Motion

Roll and pitch continuously move the liquid from side to side.

This may cause:

• Oscillating distributor flow
• Intermittent dry areas
• Liquid sloshing
• Repeated local overload
• Entrainment
• Unstable instrument readings

A distributor that tolerates a small permanent tilt may still perform poorly under repeated dynamic motion.

Distributor Type and Motion Sensitivity

Different liquid distributors respond differently to tilt and changing acceleration.

The design should evaluate:

• Liquid-head requirement
• Channel depth
• Orifice position
• Overflow behavior
• Compartmentalization
• Liquid residence time
• Redistribution rate
• Sensitivity to level difference
• Drainage during shutdown

A shallow, wide distributor may experience a significant flow imbalance under tilt.

Compartmented designs may reduce long-distance liquid movement, but each compartment must receive the correct feed.

The final design should be verified against the vessel’s motion basis rather than selected from a standard onshore drawing.

Column Orientation

The orientation of internal channels relative to the vessel’s roll and pitch axes may influence liquid movement.

Engineering review may consider:

• Dividing channels parallel or perpendicular to the dominant motion
• Feed entry orientation
• Collector layout
• Support-beam direction
• Side-draw location
• Instrument position
• Manway access

There is no universal orientation that is correct for every vessel.

The motion envelope, column position and internal geometry must be considered together.

Packing Surface Area and Motion

Higher specific surface area can improve mass-transfer efficiency.

It may also create:

• Narrower channels
• Greater sensitivity to local over-irrigation
• Higher pressure drop
• Greater sensitivity to fouling
• More difficult drainage
• Higher distributor requirements

A more open packing may offer greater hydraulic tolerance during motion-induced local load changes.

The selection should balance:

• Required separation efficiency
• Maximum local vapor load
• Maximum local liquid load
• Motion envelope
• Pressure-drop allowance
• Fouling tendency
• Available column height

The normal average load is not sufficient. Local transient loading should also be considered.

Local Flooding Under Motion

Even when the overall column operates below its flooding limit, motion may send excess liquid to one side.

The low side may experience:

• Higher local liquid load
• Increased wet pressure drop
• Reduced vapor-flow area
• Entrainment
• Local flooding

Vapor may then move toward the less-wetted high side, increasing gas maldistribution.

This interaction can amplify separation loss.

A suitable offshore design requires margin for local hydraulic imbalance, not only average column capacity.

Wall Flow

Tilt encourages liquid to migrate toward the lower shell wall.

Once liquid reaches the wall, it may bypass the packing.

Consequences include:

• Reduced effective mass-transfer area
• Uneven liquid distribution
• Local corrosion
• Higher liquid accumulation
• Reduced separation efficiency

Wall-flow control may require:

• Accurate packing fit
• Wall wipers
• Intermediate collection
• Redistribution
• Correct packing-edge design
• Controlled segment gaps

The wall-wiper design must accommodate shell tolerance, thermal expansion and vessel motion.

Packed-Bed Height and Redistribution

Liquid maldistribution grows as liquid travels through a packed bed.

Floating motion may accelerate this deterioration.

Dividing a tall bed into shorter sections with collection and redistribution may improve performance, but it also adds:

• Column height
• Weight
• Pressure drop
• Liquid holdup
• Fabrication complexity
• Maintenance requirements

The optimum bed height should balance redistribution performance against offshore weight and space constraints.

Standard onshore bed-height rules may require adjustment for the vessel motion basis.

Gas Distribution

Gas distribution may also be affected by motion and unequal liquid loading.

Vapor tends to move toward the lower-resistance region. If one side of the packing becomes heavily irrigated, more gas may pass through the drier side.

The gas inlet device should:

• Reduce inlet momentum
• Distribute flow across the column
• Avoid direct impact on packing
• Operate over the required turndown
• Maintain low pressure drop

Two-phase feed entry requires particular attention because liquid and vapor may respond differently to vessel acceleration.

Mechanical Retention of Packing

Packing blocks must remain in position during:

• Normal operation
• Vessel motion
• Gas surges
• Startup and shutdown
• Transport
• Emergency depressurization

The mechanical system may include:

• Packing support grid
• Hold-down grid
• Retaining clips
• Segment bands
• Wall restraints
• Anti-lifting devices

The design must prevent:

• Packing uplift
• Segment migration
• Corrugation damage
• Gaps opening between blocks
• Rubbing against the shell
• Loss of layer orientation

A standard lightweight hold-down ring may be insufficient without checking the offshore acceleration and dynamic-load basis.

Support-Grid Loads

The support grid must carry:

• Packing dry weight
• Operating liquid holdup
• Motion-induced load
• Vertical and lateral acceleration
• Differential pressure
• Upset liquid load
• Installation load
• Transport load

Offshore motion can introduce lateral forces that are small or absent in an onshore vertical column.

Support beams, grid panels and wall attachments should be evaluated by the qualified mechanical designer.

High open area remains important because a restrictive support may increase pressure drop and gas maldistribution.

Packing Segment Design

Large-diameter structured packing is normally installed in segments.

For offshore service, segment design should consider:

• Manway size
• Maximum handling weight
• Internal lifting space
• Segment locking
• Layer orientation
• Wall clearance
• Movement between adjacent blocks
• Identification and installation sequence

Blocks should fit closely enough to limit bypass but not so tightly that thermal expansion or shell deformation crushes the packing.

Segment joints should not align into continuous vertical bypass paths.

Transport and Module Integration

Offshore equipment may be fabricated, transported and lifted as a module before final operation.

Packing and internals may experience loads during:

• Road transport
• Marine transport
• Module lifting
• Vessel integration
• Tow-out
• Commissioning

These loads may differ from normal process loads.

The project should define whether packing will be:

• Installed before transport
• Installed after module placement
• Temporarily restrained
• Inspected after transport
• Releveled before commissioning

Packing installation planning should be integrated with the project construction sequence.

Small Footprint Does Not Eliminate Access Requirements

Offshore equipment is compact, but packed columns still require access for:

• Distributor inspection
• Packing installation
• Support-grid inspection
• Cleaning
• Segment removal
• Instrument maintenance

Manway size and location should be confirmed before packing segmentation.

A packing block that fits the column cross-section may not pass through the available access route.

Internal platforms, piping and the dividing structure of the module may also restrict movement.

Material Selection

Offshore columns may handle:

• Acid gas
• Amine solvents
• Glycol
• Hydrocarbons
• Produced water
• Chlorides
• Oxygen-scavenger chemicals
• Antifoam
• Cleaning solutions

Material selection must consider both process chemistry and the offshore environment.

Metal structured packing offers:

• High strength
• Thin sheets
• Large open area
• Accurate geometry
• Temperature resistance
• Stable segment construction

Plastic structured packing may offer corrosion resistance and lower weight in selected duties, but temperature, fire, static and mechanical creep require evaluation.

The complete material selection should include packing, supports, hold-downs, distributors and fasteners.

Foaming and Motion

Amine, glycol and produced-water systems may foam because of:

• Hydrocarbon contamination
• Compressor oil
• Degradation products
• Fine solids
• Excessive antifoam
• Upstream liquid carryover

Vessel motion may worsen liquid entrainment and apparent level instability.

Foaming can cause:

• Rapid pressure-drop increase
• Solvent loss
• Reduced capacity
• Poor separation
• Unstable level control

Packing selection cannot solve contaminated solvent. Feed separation, filtration and solvent-condition control remain necessary.

Fouling and Offshore Maintenance

Offshore cleaning and packing replacement are expensive because of:

• Limited manpower
• Restricted access
• Safety requirements
• Production shutdown cost
• Waste-handling limitations
• Crane and lifting constraints

The highest-efficiency packing may not provide the lowest lifecycle cost if it fouls rapidly.

For contaminated service, a more open and cleanable geometry may provide better reliability.

Maintenance planning should include:

• Cleaning method
• Distributor flushing
• Packing-removal route
• Segment weight
• Waste containment
• Spare packing strategy

Pressure Drop Monitoring

Total column pressure drop may not reveal side-to-side maldistribution.

Useful monitoring may include:

• Differential pressure across each bed
• Temperature profile
• Product composition
• Solvent loss
• Vessel-motion data
• Distributor level
• Side-specific measurements where practical

Performance changes correlated with sea state may indicate motion-related distribution problems rather than permanent packing damage.

Operating data should distinguish:

• Calm-condition performance
• Normal motion
• Severe but allowable motion
• Startup and shutdown

Instrumentation Under Motion

Level and differential-pressure instruments may be affected by vessel acceleration.

Measurement systems should be located and configured to reduce false indications.

Potential issues include:

• Oscillating level
• Unequal impulse-line pressure
• Liquid accumulation in lines
• Apparent pressure-drop variation
• Control-valve hunting

A control problem may worsen liquid distribution even if the packing and distributor are mechanically correct.

Process-control design is therefore part of offshore packed-column performance.

Retrofit Projects

Replacing trays or old packing with structured packing may reduce:

• Pressure drop
• Column height requirement
• Weight
• Solvent inventory

But a retrofit should first determine whether existing problems arise from:

• Motion-sensitive distributors
• Insufficient support
• Wall flow
• Foaming
• Fouling
• Incorrect vapor split
• Damaged internals

Installing new packing below an unchanged motion-sensitive distributor may not improve performance.

Field measurements and inspection records should be included in the retrofit design.

What Information Should Be Included in the RFQ?

An offshore structured-packing inquiry should include:

• Process application
• Complete gas and liquid composition
• Operating pressure and temperature
• Vapor and liquid flow rates
• Minimum and maximum loads
• Column internal diameter
• Available packed height
• Required separation performance
• Maximum allowable pressure drop
• Vessel type
• Column location on the vessel
• Static list and trim
• Roll, pitch and heave design basis
• Acceleration design loads
• Column orientation
• Material requirements
• Distributor and collector scope
• Support and hold-down scope
• Manway dimensions
• Segment weight limits
• Transport and lifting conditions
• Cleaning and maintenance strategy

Without vessel-motion data, the supplier can provide only an onshore-style preliminary design.

Common Engineering Mistakes

Using an Onshore Distributor Without Motion Review

A level-sensitive distributor may feed the low side excessively during tilt.

Calculating Only Average Hydraulic Load

Local flow on one side may approach flooding even when total flow is acceptable.

Ignoring Lateral Mechanical Loads

Packing supports and restraints must consider vessel acceleration.

Selecting Packing Without Confirming Column Orientation

Roll and pitch direction may affect distributor and collector layout.

Installing New Packing Below an Old Distributor

The distributor may remain the real cause of poor performance.

Ignoring Transport Loads

Packing may be damaged before the offshore module begins operation.

Frequently Asked Questions

Can structured packing be used on an FPSO?

Yes, but the distributor, support, hold-down and packing segmentation must be designed for the vessel’s motion and acceleration basis.

Why does vessel tilt reduce packing efficiency?

Tilt changes distributor liquid level, sending more liquid to the low side and less to the high side. This creates maldistribution and possible local flooding.

Is a lower-surface-area packing better offshore?

Not automatically. More open packing may provide greater hydraulic tolerance, but the required separation efficiency and available column height must also be considered.

Does low pressure drop solve motion problems?

No. Low pressure drop is beneficial, but liquid-distribution and mechanical-retention problems require separate design measures.

What additional information is required compared with an onshore project?

Vessel motion, static tilt, acceleration loads, column orientation, transport conditions and offshore access limitations.

Conclusion

Structured packing can provide valuable weight, height, capacity and pressure-drop advantages in FPSO and floating offshore columns.

Its performance depends on whether liquid and vapor remain acceptably distributed while the vessel rolls, pitches and heaves. Distributor design, packing fit, wall-flow control and local flooding margin must therefore be based on the vessel’s motion envelope.

Support grids, hold-down devices and packing segments must also resist lateral and dynamic loads during operation and transport.

For floating service, structured packing cannot be selected from steady vertical-column data alone. The process hydraulics, vessel motion, mechanical restraints and installation sequence must be engineered as one integrated offshore internal system.