BENEFITS OF VARIABLE SPEED DRIVES FOR SUBSEA PRODUCTION AND WELLHEAD HYDRAULIC POWER UNITS

METHODS OF SUBSEA HYDRAULIC SUPPLY PRESSURE CONTROL

Options for pressure control of subsea hydraulic systems:

1. Gap Control – On‐Off Control of Electric Motor Driven pumps

2. Pressure Regulation – pressure reducing valves

3. Gap control with variable speed electric motor driven pumps

TYPICAL SUBSEA LAYOUT

SUBSEA HYDRAULIC SYSTEM

SUBSEA HYD SYSTEM SCHEMATIC 

SUBSEA HYD SYSTEM – PHPU SCHEMATIC

SUBSEA HYD SYSTEM – SCM SCHEMATIC

SUBSEA HYD SYSTEM – PHPU, GAP CONTROL

SUBSEA HYD SYSTEM – PHPU, GAP CONTROL

• Simplest, lowest cost• Minimal internal leakage• Higher accumulation• More frequent pump starts

SUBSEA HYD SYSTEM – PRESSURE REGULATION

PRESSURE REGULATION

Sectioned Pressure Regulator‐ Control Spring‐ Sensor (piston)‐ Vent Seat‐ Main Valve Seat

Variable outlet pressureMinimum = Set pointMaximum = Internal relief settingOutlet pressure within the min/max is dependent on relative internal leakage of regulator and downstream components.

PRESSURE REGULATION

• Highest internal Leakage• Higher maintenance• Higher variation in pressure• Allows independent pressure control

SUBSEA HYD SYSTEM – GAP CONTROL WITH VSD

GAP CONTROL WITH VSD –SPEED/FLOW v PRESSURE

GAP CONTROL WITH VSD • Lowest pressure variation• Least pressure cycling system fatigue• Lowest maintenance• Lower accumulation

ACTUAL PRESSURE TRENDS DATA – 3 OPTIONS

BENEFITS OF VSD DRIVEN PUMPS FOR PHPU’S

Criteria Single Speed Electric Motor Driven Pump (Gap control with or without pressure regulation) Variable Speed Electric Motor Driven Pump

Pump starting Motor starts abruptly at maximum speed Soft-start capability.

Pressure spikes on start up High Low

Frequency of Pump Starts High Low

Flow rate Constant at the maximum flow irrespective of demand. Variable to match demand

Accumulator Capacity Required High Low

Pump Starting

BENEFITS OF VSD DRIVEN PUMPS FOR PHPU’S

Criteria Single Speed Electric Motor Driven Pump (Gap control with or without pressure regulation) Variable Speed Electric Motor Driven Pump

Output Pressure Fluctuation(without pressure regulation)

High.1.0 ~ 2.0 MPa

Low. 0.2 ~ 0.5MPa

Output Pressure Fluctuation(with pressure regulation)

High.1.0 ~ 2.0 MPa

Low. 0.2 ~ 0.5MPa

Pressure Variation

BENEFITS OF VSD DRIVEN PUMPS FOR PHPU’S

Criteria Single Speed Electric Motor Driven Pump (Gap control with or without pressure regulation) Variable Speed Electric Motor Driven Pump

Internal Leakage(without pressure regulation)

High Low

Internal Leakage(with pressure regulation)

Very High Low

Component Fatigue High Low

Maintenance High Low

Space envelope High Low

Life cycle cost High Low

Internal Leakage, Maintenance, Space envelope, Life Cycle Cost

VSD SYSTEM DESIGN CONSIDERATIONS

• Motor cooling at slow speed.

• Consider oversizing motor so there is adequate cooling for motor running continuously at slowest speed for worst case ambient conditions.

• Electric motor supplier can provide motor simulation data

• Adequate motor overload and thermal protection – thermistors

• Filters to remove harmonic distortion 

• Shielded power supply cables with 3 symmetrical grounding wires from VSD to electric motors to minimise Radio Frequency/Electro Magnetic Interference EMI/RFI

• Parameter set points on VSD will need adjustment for optimum performance and monitoring 

Thankyou for your interest.

Questions welcome.