Au­to­matic gas pro­cess­ing in Taranaki

The Po­hokura field is New Zealand’s largest nat­u­ral gas re­source. Its pro­duc­tion sta­tion uses an in­te­grated au­to­ma­tion con­trol sys­tem, mean­ing staff can op­er­ate it re­motely.

DEMM Engineering & Manufacturing - - AUTOMATION -

The Po­hokura field, off­shore from Taranaki, is the coun­try’s largest nat­u­ral gas re­source, owned by a joint ven­ture be­tween Shell, Todd Po­hokura, and OMV New Zealand.

The first commercial gas flowed from three on­shore ‘ex­tended reach’ drilling wells in the South­ern part of the field in Septem­ber 2006. In March 2007, gas and con­den­sate be­gan to flow from the first of five off­shore wells via an un­der­sea pipe­line back to an on­shore pro­duc­tion sta­tion at Mo­tunui.

De­vel­op­ing an un­manned site – where op­er­a­tions are mon­i­tored from a con­trol room in New Ply­mouth – re­quired the com­bined ex­per­tise of en­gi­neers, con­sul­tants and sys­tems in­te­gra­tors.

The Po­hokura de­sign con­trac­tor, Trans­field Wor­ley, ap­pointed sys­tems in­te­gra­tor part­ner En­gi­neer­ing Con­trol (EC). One of the re­quire­ments for re­mote oper­a­tion was that the mo­tor con­trol cen­tres be in­te­grated into the main plant con­trol sys­tem on an in­tel­li­gent net­work so that the in­for­ma­tion from the gas sta­tion could be fed back to the con­trol room.

Trans­field Wor­ley based the new sys­tem on the De­viceNet net­work to­gether with the Allen-Bradley Con­trolLogix plat­form from Rock­well Au­to­ma­tion.

An­other re­quire­ment was that the sta­tus of power switchgear at the 400V MCCs and the plant 11kV main switch­board had to be avail­able in the con­trol room and this switchgear had to be op­er­a­ble from the con­trol room as well.


This was achieved by hard­wiring the switchgear to dis­crete I/O in the Con­trolLogix. Be­ing able to op­er­ate the switchgear by re­mote is also a wel­come safety fea­ture.

The op­er­a­tional phi­los­o­phy for the Po­hokura pro­duc­tion fa­cil­ity was to es­tab­lish an un­manned site with zero nor­mal op­er­at­ing pres­ence (ZNOP); plant oper­a­tion would be per­formed off-site from a re­mote con­trol room in New Ply­mouth us­ing a dis­trib­uted con­trol sys­tem (DCS).

Re­mote oper­a­tion is an ideal way to help keep per­son­nel safe and away from po­ten­tially haz­ardous equip­ment, but its op­er­a­tional suc­cess re­lies on ex­cel­lent con­trol, fault di­ag­nos­tics and net­work ca­pa­bil­i­ties.

“De­viceNet pro­vided a net­work so­lu­tion that could help pro­vide re­li­able com­mu­ni­ca­tion and also had the added fea­ture of au­to­matic de­vice re­place­ment (ADR), al­low­ing for re­duced down­time with au­to­matic down­load of de­vice pa­ram­e­ters,” says Prasad Nory, in­dus­try man­ager, Rock­well Au­to­ma­tion.

ADR con­sists of con­fig­u­ra­tion and auto-ad­dress re­cov­ery that ef­fec­tively low­ers main­te­nance re­quire­ments. Op­er­at­ing 24 hours a day, seven days a week, Po­hokura has a sched­uled proac­tive main­te­nance day ev­ery month to iden­tify and deal with po­ten­tial prob­lems.

The Trans­field Wor­ley elec­tri­cally en­gi­neered de­sign was the ba­sis for a com­pletely in­te­grated so­lu­tion that re­quired the ca­pa­bil­i­ties of leading board builders, Switch­build. The Po­hokura so­lu­tion evolved around the de­vel­op­ment of two low-volt­age mo­tor con­trol cen­tres (MCCs). Two 2.5MVA transformers feed into the first low-volt­age MCC via 4000A Air Cir­cuit Break­ers (ACBs). Power from the first MCC is then fed through to the sec­ond low volt­age MCC.


The in­tel­li­gent MCC de­sign used the De­viceNet net­work, com­mu­ni­cat­ing to the DOL start mo­tor starters for con­trol and mon­i­tor­ing. The E3 plus smart over­loads pro­vided mo­tor pro­tec­tion that closely matched the mo­tor op­er­at­ing char­ac­ter­is­tics with the en­hanced pro­tec­tion ca­pa­bil­i­ties such as earth fault, stall, ther­mis­tor and loss of load that are nor­mally pro­vided by much more ex­pen­sive mo­tor pro­tec­tive re­lays.

An Allen Bradley Con­trolLogix pro­gram­mable au­to­ma­tion con­troller is used to pro­vide the com­pre­hen­sive mon­i­tor­ing and con­trol of the MCCs and feeds back in­for­ma­tion to the dis­trib­uted con­trol sys­tem.

To avoid costly down­time at Po­hokura, EC de­vel­oped and in­cor­po­rated switch­ing con­trols in the PAC in the event that power is lost from one of the two 11kV in­com­ers.

Peter Huitema, an en­gi­neer at EC says: “Nor­mally when sup­plies switch over you lose power, even if it’s just a few mil­lisec­onds, the mo­tors will shut­down.

“To avoid this, we de­ter­mined how many sec­onds the mo­tor can be with no power with­out caus­ing any dam­age. We used in­er­tia to keep mo­tors run­ning for up to 1.5 sec­onds, to al­low time for the other sup­ply to switch over. By pro­gram­ming this in­for­ma­tion into the con­trol sys­tem we were able to avoid costly shut­downs.”

The Po­hokura field pro­duc­tion sta­tion is op­er­ated by Shell Ex­plo­ration NZ. State-of-the-art en­gi­neer­ing has en­abled the gas plant to be op­er­ated re­motely.

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