Re: Bls: [Oil&Gas] Fatigue live meningkat?

 

Pak Jerry,

 

Trm ksh atas infonya. Sy coba mlihat lbh detil.

 

trm ksh

 

2011/9/26 Jerry Mintaredja <jerry.mintaredja@yahoo.co.id>

 

Pak Iqbal,
Analisa fatigue untuk natural period > 3 dan terdapatnya ketersediaan data gelombang yang mendukung dan dimana faktor dynamic menjadi hal yang mandatory sudah tepat metode yang digunakan adalah spectral analysis. Secara garis besarnya seperti penjelasan di bawah ini: 1. Calculated "center of damage" seastate for use in derivation of linearized foundation springs and wave steepnees.
2. From the structural model and linearized foundation springs, perform dynamic analysis to obtain structure natural periods and modal shape.
3. Select wave natural period based on the structure natural period and appropriate wave steepness for development of Transfer Function.
4. Perform dynamic wave response analysis for all the eight/twelve directions.
5. Compute hotspot stress transfer function for each by multiplying the nominal member stress transfer function with its appropriate stress concentration factor (SCF). The wave spectrum of each sea state in the wave scatter diagram is calculated using JONSWAP (JS) spectrum. Using the square of the hot spot stress transfer function and the seastate spectra, short term stress response statistic are derived for each hot spot, for every seastate and each wave approach angle. The stress statistic are then combined with the probability of occurrence of the seastate and the probability of occurrence of each wave approach angle, to determine the total number of stress cycles experienced in a particular seastate for a selected wave approach angle.
6. Calculate the short term damage per seastate per direction. The cumulative annual fatigue damage is calculated as the sum of the damage from every individual seastate and direction. Cumulative fatigue damage effect calculations based on Miner's rule linear accumulation with the assumed S-N curve. The fatigue damage calculation performed takes into account the probability distribution of hot spot stress range, the probability of occurrence of 8 or 12 sectors of wave directions, and the severity of individual seastate of the wave scatter diagram.
Semuanya yang terdapat pada list di atas adalah mandatory dan diperhatikan tiap step nya dilakukan dengan benar atau tidak. Karena untuk case dibawah, kok saya agak ragu dengan hasilnya. Coba diperhatikan transfer function yang digenerated, apakah "peak"conditionnya sudah meng-capture period-period dominan seperti period dari center of damage wave dan 1st to 5th (at least) natural period of platform.

Thanks.
JJM

Dari: Iqbal M <iqbaloffon@gmail.com>

Kepada: Migas_Indonesia@yahoogroups.com

Dikirim: Kamis, 22 September 2011 9:09
Judul: Re: Bls: [Oil&Gas] Fatigue live meningkat?

Rekan2 sekalian, trm ksh byk atas segala inputnya.

Pak Jerry, platform ini tmasuk manned platform dan blm lama terinstall di
offshore. Dari hasil analisa inplace, sejauh ini tdk ada masalah di pile.

Sy mcoba mbanding hasil bbrp studi fatigue yg dilakukn di platform tsb dan
mngambil case 1 joint.
1.  pada saat natural periode Tp = 3.76 sec, fatigue life FL= 55 tahun
2.  Tp = 3.47 sec, FL = 22 tahun
3.  Tp = 3.66 sec, FL = 51 tahun

Sy mcoba mbuat kesimpulan. CMIIW.
- Periode beban gelombang mdekati 3.4 sec. dekat dg periode platform, shg
DAF akan besar dan mningkatkan stress (dg suatu siklus ttentu) di member
jacket. Hal ini mnyebabkan turunnya fatigue life.

- dg asumsi periode gelombang yg digunakan di semua studi ini sama, maka
semakin jauh dari periode natural maka DAF akibat beban gelombang akan
bkurang. Hal ini bisa tlihat dari studi 1 dan 3. dan bdampak pd turunnya
stress dan umur fatigue jadi lebah besar.

Sbgmn rekan2 ketahui, analisa fatigue ini mgunakn analisa spektral. Hanya
saja, saya blm mngetahui bgmn mnemukan periode gelombang yg digunakn dlm
analisa tsb. Sy hanya berasumsi sptnya dekat dg 3.4 sec. Mohon bantuan
rekan2 sekalian. Sy msh awam dlm analisa spektral ini.

Trm ksh sblmnya.

Iqb

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