Peristiwa ini terjadi sekitar di tahun 2010, tahun terakhir saya menjadi Kepala Lapangan atau OIM. Setiap hari Jum’at, offshore melakukan weekly meeting untuk meng-up-date kegiatan production dan maintenance seminggu ke depan. Ada yang menarik di meeting kali ini ketika Field Maintenance Supervisor (FMS) bermaksud akan memeriksa vibrasi di crude oil transfer pump unit A, padahal baru saja kemarin ada vibrasi di unit B, dan kalau tidak salah unit C juga mengalami hal yang serupa . Ada apa gerangan dengan pompa-pompa tersebut?
Sambil menunggu FMS memberikan sejarah record maintenance di semua crude oil pump, maka saya keliling platform.
Saya melihat kira-kira beginilah process flow diagramnya:
Pompa pentransfer minyak (crude oil transfer pump) digerakkan oleh motor listrik dengan umpannya dari surge vessel. Pompa mengalirkan cairan campuran minyak mentah+condensate+produced water melalui pipa 8” menuju tangki minyak di FPSO. Di keluaran pompa dipasang meter jenis orifice yang berguna untuk mengukur laju alir minimum pompa sebagai proteksinya (supaya tidak vibrasi). Jika laju alir campuran tersebut kurang dari setting point, maka flow control valve (FCV) di pipa yang mengarah balik ke vessel akan membuka. Dan jika laju alirnya di atas settingnya, maka FCV tersebut akan menutup. Kemudian di downstreamnya terdapat LCV yang berguna untuk memelihara aras cairan di surge vessel. Keluar dari LCV, campuran minyak mentah+condensate+produced water kemudian dipompakan menuju tangki minyak di FPSO.
Saya mengamati bahwa pompa yang beroperasi ternyata dua buah pompa …..dan FCV yang mengontrol minimum recirculation pumpnya tidak stabil, buka dan nutup silih berganti. Maka saya bertanya kepada Field Production Supervisor (FPS) tentang laju alir liquid yang dipompakan selama ini.
Laju alir liquid yang dipompakan ternyata sekitar 7,000 bpd!! Sedangkan kapasitas pompa masing-masing sekitar 16,500 bpd. Kalau dua pompa berarti (apakah 2 x 16,500 bpd? Kalau ini jawaban anda, maka anda salah karena laju alir pompa tunggal atau yang diparalel atau yang diseri itu ditentukan oleh titik perpotongan antara kurva pompa dengan system headnya dan bukan dengan perhitungan matematika sederhana !!).
Anyway, kita tidak fokus ke sana, namun fokus ke masalah ini. Lalu saya memerintahkan FPS untuk mematikan satu pompanya. Banyak macam alasannya untuk tidak jadi menutupnya, salah satunya adalah takut shutdown….Kalau shutdown satu saja, maka level di surge vessel akan cepat naik dan intervensi operator mungkin telat. Saya menduga, operasi seperti ini memang sudah sedemikian lama sehingga agak susah untuk merubah mind set. Anyway, this is the beauty to be the Boss, so mereka harus tetap patuh. Singkatnya, satu pompa pun dimatikan…..
Malam harinya, karena sedikit curiga, saya turun lagi ke process area, dan mendapatkan pompa tetap satu yang beroperasi…namun kenapa FCVnya nol? Ternyata setting minimum flownya diubah ke 0 bpd. Waduh….
Saya panggil lagi FPS dan minta supaya harga setting FCV-nya tidak dikutak-katik karena ada alasan engineering kenapa di set dengan angka demikian. Shift malam beralasan mereka melihat FCV-nya tidak stabil dan ketika dipaksa nutup, semua system jadi terlihat stabil….
Saya pun kemudian menulis surat kepada Jakarta:
To: Technical Integrity Manager
cc: Process Engineer
cc: Rotating Engineer
Paks,
The three crude oil transfer pumps P-8400 A/B/C were installed in parallel with current practice: two pumps are in operation and one pump is in standby. Each pump capacity was 480 gpm (16,457 bpd) at 82 psig differential pressure. That number was located on best efficiency points of the pump (see pump’s curve attachment). Each pump is vertical centrifugal type driven by an electrical motor with capacity about 40 bhp.
To protect pump damage from low flow, a by-pass line from pump’s discharge to suction line had been installed with capacity of 8,000 – 9,000 bpd. This capacity is limited by Flow control valve (FCV) on the line per data sheet (on attachment). The 8,000 – 9,000 bpd corresponding to 54% of best efficiency point’s (BEP) of the pump and refer to several centrifugal pump’s article (by Igor Karassik on Chemical Processing Magazine), that is sufficient (see attachment), and also considered appropriate for high suction specific speed (Nss) type pump. P-8400 A/B/C pumps have Nss of 11,420 and categorized as high Nss pump. Refer to the article again, the high Nss pump more vulnerable has internal recirculation phenomena which is at the end may cause cavitation or vibration or noise or pump damage. To minimise that impact, pump by-pass line to re-circulate some portion of the flow will help - as we have on the existing installation.
According to Anoa operating manual, the by-pass line is designed for one pump duty only. This confirmed via FCV data sheet and recalculation on FCV capacity. It means, eventhough two pumps are in operation, in case pump’s flow is less than setting of minimum flow (for some reason), the flow through the by-pass line is still on 8,000 – 9,000 bpd and not doubles it. During first day Anoa operation which is the peak crude oil production was around 32,000 bpd, this may not a major concern, however, for the current liquid production (only about 7,000 bpd), keep operating 2 pumps in parallel is not proper due to several problem that may arise as explained before. Also, from energy efficiency point of view, operating 2 pumps in parallel in current condition is very waste energy. According to liquid production profile, since year 2008, total liquid production decreased to around 14,000 bpd and this number is enough to be handled by single pump operation. Operating 2 pumps in parallel for long period may contribute significantly to the maintenance history for the pumps (attached). Further close monitoring for the pump performance and tracking on the next maintenance history onward explain better.
Summary
1. Pumps had been running for long period in 2 x 100% mode which is not favour in term of pump reliability and energy efficiency.
2. Eventhough now only single pump in operation and 2 pumps are standby, it is still not energy efficient due to pump capacity is much higher than current liquid production.
Recommendation
1. Field Production Supervisor (FPS) to maintain crude oil transfer pump operation mode in single pump running and two pumps standby. Maintain minimum flow recirculation setting and do not change the setting without Offshore Installation Manager (OIM) approval.
2. Subject to cost benefit analysis, Jakarta engineer to strongly consider reviewing current pump design vs field liquid production and recommending lower capacity pump for energy efficiency and for reliability of the pump itself.
3. During study, OIM noticed that several production technician sounds did not understand the function of minimum circulation protection for centrifugal-type pump and this raise a worry since it was found at one day by OIM, the minimum circulation pump setting was changed to zero bpd. Request to Jakarta engineer to give a short course at offshore for production technicians about centrifugal pump principle operation and its control.
Dari maintenance data history, maka saya mulai mengerti akan seringnya pompa-pompa tersebut terkena vibrasi. Dan juga mengerti kenapa terkadang mereka overload (karena check valve-nya passing sehingga terjadi close recirculation thru other pump). Sekarang, setelah dua tahun berlalu dan saya pindah ke Jakarta, saya sudah jarang mendengar pompa-pompa tersebut harus dioverhaul karena vibrasi, etc. I think, change philosophy to be only one pump running - is working - meski harus dievaluasi lagi karena laju produksi minyak yang terus menurun….
Crude Oil Pump A Maintenance History Since 2006
NGX10-16xx 29 Jun 2010 REPLACE CRUDE OIL TRANSFER PUMP A DUE TO LEAK ON MECHANICAL SEAL NOPLAN
NGX09-33xx 08 Mar 2010 REPAIR CHECK VALVE OF CRUDE OIL TRANSF PUMP "A" DUE TO NOT WORKING PROP
NGX10-04xx 05 Feb 2010 TROUBLESHOOT CRUDE OIL TRANSF PUMP-A, DUE TO CHECK VALVE PASSING
NGX09-10xx 01 May 2009 CHECK AND REALIGNMENT CRUDE OIL TRANSFER PUMP "A" DUE TO VIBRATION
NGX08-41xx 28 Jan 2009 REPLACE 6" CHECK VALVE ON CRUDE OIL TRANSFER PUMP'A.
NGX08-18xx 27 Aug 2008 INSTALL PUMP OF CRUDE OIL TRANSFER PUMP "A"
NGX08-17xx 27 May 2008 FIX LEAK CRUDE OIL TRANSFER PUMP 'A
NGX08-05xx 18 Feb 2008 CHECK CRUDE OIL PUMP-A DUE TO BREAKER TRIP
NGX07-16xx 19 Jun 2007 INSPECT 8" CHECK VALVE CRUDE OIL PUMP A & C
NGX07-07xx 21 Mar 2007 CHECK AND DISMANTLE CRUDE OIL PUMP "A"
NGX06-26xx 25 Jul 2006 REINSTALL CRUDE OIL TRANSFER PUMP'A
NGX06-13xx 03 May 2006 OVERHAUL CRUDE OIL TRANSFER PUMP UNIT A
Crude Oil Pump B Maintenance History Since 2006
NGX10-xx27 18 Sep 2010 CHECK CRUDE OIL TRANSFER PUMP "B" DUE TO HARD NOISY
NGX09-xx20 01 Oct 2009 TROUBLESHOOT CRUDE OIL TRANSFER PUMP B DUE TO TRIP
NGX09-xx18 13 Jun 2009 REALIGNMENT CRUDE OIL TRANSFER PUMP "B"
NGX09-xx81 26 May 2009 REPLACE CRUDE OIL TRANSFER PUMP "B"
NGX09-xx82 25 May 2009 CHECK/INSPECT CRUDE OIL TRANSFER PUMP "B", DUE TO HIGH VIBRATION
NGX08-xx94 28 Jan 2009 REPLACE 6" CHECK VALVE ON CRUDE OIL TRANSFER PUMP "B"
NGX08-xx33 31 May 2008 TROUBLESHOOT ON CRUDE OIL TRANSFER PUMP "B" DUE TO HIGH AMPS.
NGX08-xx39 06 May 2008 CHECK CRUDE OIL TRANSFER PUMP MOTOR DUE TO HIGH VIB
NGX08-xx00 26 Apr 2008 CHECK AND REPAIR CRUDE OIL TRANSFER PUMP B DUE TO TRIP
NGX08-xx27 22 Feb 2008 REPLACE CRUDE OIL TRANSFER PUMP B.
Crude Oil Pump C Maintenance History Since 2006
NGX10-14xx 17 Jun 2010 CHECK CRUDE OIL TRANSFER PUMP UNIT 'C DUE TO VIBRATION
NGX08-27xx 29 Sep 2008 CHECK IMPELER OF CRUDE OIL TRANSFER PUMP'C
NGX08-29xx 27 Aug 2008 REPLACE CRUDE OIL TRANSFER PUMP'C.
NGX07-16xx 19 Jun 2007 INSPECT 8" CHECK VALVE CRUDE OIL PUMP A & C
Selesai
Cahyo Hardo
Vivat Process Engineering
Referensi:
Anoa Operating Manual – Premier Oil Natuna Sea B.V.
Centrifugal pump operation at off-design conditions – 3 series articles by Igor J. Karassik, Chemical Processing.