APPLICATION OF A HIGH RESISTANCE GROUNDING (HRG) UNIT TO AN ESP WITHIN A HAZARDOUS AREA 2014 EUALF

When conventional attempts to eliminate electrical discharge activity in a zone 1 hazardous area associated with an earth faulted ESP failed, the system was isolated and the back-up system brought online. When the backup system failed an alternative approach to the problem was attempted.

This presentation will look at the following:

How the fault led to above surface electrical discharge activity

How the application of a High Resistance Grounding Unit eliminated the problem

INTRODUCTION

2

Well completed as a dual ESP operated via a Variable Speed Drive (VSD)

Primary ESP developed earth fault within first weeks of operation

Floating neutral VSD system allowed for continued operation of faulted ESP

Gauge of backup ESP connected to maintain downhole data

After 16 months operation electrical discharge noticed in wellbay area during routine checks

Well taken offline and investigations commenced – earlier earth faulted had further degraded

BACKGROUND

3

Electrical discharge from instrument pipework

Three leg electrical penetrator

Tree earthing checked and supplemented

Equipotential bonding applied to all extraneous metalwork

Backup gauge system isolated (remove alternative power sources)

Backup ESP system isolated (removed circulating current paths)

Voltage in offline ESP system checked

Electrical discharge potential still present – measurable as voltage difference between adjacent bonded metalwork

CONVENTIONAL SOLUTIONS

4

Primary ESP mothballed due to discharge

Backup ESP brought into operation

Well operated for further 22 months until backup ESP failure (suspect motor bearing failure)

Opportunity to to re-use primary ESP if electrical discharge potential could be eliminated

INTERIM OPERATION

5

Sharp increase in amps prior to eventual failure

Time Domain Reflectrometry employed to determine location of fault

Information indicates low impedance earth fault located high in completion (immediately below or within tree)

Repair attempted by replacing intermediate connectors but ineffective – required completion to be pulled

Multiple additional earth bonding jumpers attached to tree and pipework – no success

Fault location and severity causing capacitive charging of local earth plane

Capacitive charge unable to dissipate as supply system (VSD) unearthed

FURTHER INVESTIGATIONS

6

Fault current requires return path to prevent capacitive charging

VSD not suitable for operating with earthed star point

High Resistance Grounding Unit (HRGU) being evaluated to extend run life of earth faulted ESP wells

Decision to employ HRGU to provide return path for fault current

ALTERNATIVE SOLUTION

7

HV/LV transformer in Star/Broken Delta configuration

HV star point grounded to provide earth fault current return path

Open Delta secondary winding closed using high power rating resistor

Current flow in primary matched (at transformer ratio) with current flow in secondary

Secondary winding resistor limits primary (fault) current

HIGH RESISTANCE GROUNDING UNIT

8

HIGH RESISTANCE GROUNDING UNIT-BETA UNIT

9

HIGH RESISTANCE GROUNDING UNIT-METERS WITH GROUND FAULT

10

HIGH RESISTANCE GROUNDING UNIT PRODUCTION UNIT

11

HRGU connected in parallel with VSD output-at VSD

VSD operated at low output voltage to determine fault levels

HRGU instrumentation indicates earth fault impedance near zero

Secondary winding resistor value altered to limit fault current to ~2 Amps

VSD run up to operating voltage and frequency

Electrical discharge potential voltages still present in well bay area

INITIAL OPERATION

12

Analysis of installation indicates parallel earth paths to HRGU through platform metalwork and supply cable braid

Need to provide ‘preferential’ path for earth fault current

Low impedance path provided from close to fault site to HRGU (large diameter earth conductor following path of main conductors)

Makes the Tree the New Earth Fault Location – No Stray Currents thru Piping!

VSD run up to operating voltage and frequency

No electrical discharge potential voltages measurable in well bay area

REVISED OPERATION

13

Well brought back online at production rate of ~980 BOPD

Well operated for further 10 days

Suspect motor failed mechanically (i.e. not directly related to earth fault)

Cost of HRGU purchase and installation recouped within first 3 days of additional production

HRGU technology and application proven – electrical discharge potential from earth faulted ESP eliminated

EXTENDED RUN-LIFE

14

8.4Mbo additional production

ACKNOWLEDGEMENTS

15