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API RECOMMENDED PRACTICE 2A-WSD (RP 2A-WSD)
TWENTY-FIRST EDITION, DECEMBER 2000
ERRATA AND SUPPLEMENT 1, DECEMBER 2002
Recommended Practice for
Planning, Designing and Constructing
Fixed Offshore Platforms—Working
Stress Design
Natural Period
For structural natural periods above three seconds,
dynamic amplification is important, particularly for the lower
sea states which may contribute the most to long term fatigue
damage. Several authors have shown the desirability of
retaining the detailed information available from a full static
analysis and adding the inertial forces due to dynamic amplification
of the first few modes (mode acceleration or static
back-substitution method, Ref. 24). A pure modal analysis
using a limited number of modes misses the essentially static
response of some modes.
Since the natural period of a platform can vary considerably
depending upon design assumptions and operational
deck mass, a theoretical period should be viewed critically if
it falls in a valley in the platform base shear transfer function.
The period should be shifted by as much as 5 to 10% to a
more conservative location with respect to the transfer function.
This should be accomplished by adjusting mass or stiffness
within reasonable limits. The choice of which parameter
to modify is platform specific and depends upon deck mass,
soil conditions and structural configuration. It should be recognized
that adjusting the foundation stiffness will alter the
member loads in the base of the structure which can be
fatigue.
5 Fatigue
5.1 FATIGUE DESIGN
In the design of tubular connections, due consideration
should be given to fatigue problems as related to local cyclic
stresses.
A detailed fatigue analysis should be performed for template
type structures. It is recommended that a spectral analysis
technique be used. Other rational methods may be used
provided adequate representation of the forces and member
responses can be shown.
In lieu of detailed fatigue analysis, simplified fatigue analyses,
which have been calibrated for the design wave climate,
may be applied to tubular joints in template type platforms
that:
1. Are in less than 400 feet (122 m) of water.
2. Are constructed of ductile steels.
3. Have redundant structural framing.
4. Have natural periods less than 3 seconds.
5.2 FATIGUE ANALYSIS
A detailed analysis of cumulative fatigue damage, when
required, should be performed as follows:
5.2.1 The wave climate should be derived as the aggregate
of all sea states to be expected over the long term. This
may be condensed for purposes of structural analysis into
representative sea states characterized by wave energy spectra
and physical parameters together with a probability of
occurrence.
Figure 4.3.4-1—Definition of Effective Cord Length
5.2.2 A space frame analysis should be performed to
obtain the structural response in terms of nominal member
stress for given wave forces applied to the structure. In general,
wave force calculations should follow the procedures
described in Section 2.3.1. However, current may be
neglected and, therefore, considerations for apparent wave
period and current blockage are not required. In addition,
wave kinematics factor equal to 1.0 and conductor shielding
factor equal to 1.0 should be applied for fatigue waves. The
drag and inertia coefficients depend on the sea state level, as
parameterized by the Keulegan-Carpenter Number K (see
Commentary C2.3.1b7). For small waves (1.0 < K < 6.0 for
platform legs at mean water level), values of Cm = 2.0, Cd =
0.8 for rough members and Cd = 0.5 for smooth members
should be used. Guidelines for considering directionality,
spreading, tides and marine growth are provided in the commentary
for this section.
A spectral analysis technique should be used to determine
the stress response for each sea state. Dynamic effects should
be considered for sea states having significant energy near a
platform's natural period.
5.2.3 Local stresses that occur within tubular connections
should be considered in terms of hot spot stresses located
immediately adjacent to the joint intersection using suitable
stress concentration factors. The microscale effects occurring
at the toe of the weld are reflected in the appropriate choice of
the S-N curve.
5.2.4 For each location around each member intersection
of interest in the structure, the stress response for each sea
state should be computed, giving adequate consideration to
both global and local stress effects.
The stress responses should be combined into the long
term stress distribution, which should then be used to calculate
the cumulative fatigue damage ratio, D, where
D = S (n/N) (5.2.4-1)
and n = number of cycles applied at a given stress range,
N = number of cycles for which the given stress
range would be allowed by the appropriate S-N
curve.
Alternatively, the damage ratio may be computed for each
sea state and combined to obtain the cumulative damage
ratio.
5.2.5 In general the design fatigue life of each joint and
member should be at least twice the intended service life of
the structure (i.e., Safety Factor = 2.0). For the design fatigue
life, D should not exceed unity. For critical elements whose
sole failure could be catastrophic, use of a larger safety factor
should be considered.
When fatigue damage can occur due to other cyclic loadings,
such as transportation, the following equation should be
satisfied:
SFiDi < 1.0 (5.2.5-1)
Where Di is the fatigue damage ratio for each type of loading
and SFi is the associated safety factor. For transportation
where long term wave distributions are used to predict short
term damage a larger safety factor should be c
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Pak ZA,
Maaf, boleh tau dari standard mana syarat tersebut? Kebetulan saya sedang mengkaji masalah fatigue life dalam tugas saat ini.
Karena yang disyaratkan tersebut adalah natural frequency, maka bagaimana hubungan nya dengan frequency yang terjadi? Dalam pemikiran saya, natural freq. ini ada benda yang diam kemudian kerusakan akibat kelelahan material yang terjadi pada struktur benda tersebut akibat freq. yg terjadi menimpa benda tersebut. CMIIW.
Regards,
Yose M.
Sent from just a simple thought®
Ralat sedikit:
bila natural frekuensi platform < 0.33 Hz (atau perioda natural > 3 detik) maka standard mensyaratkan perhitungan fatigue life.
ZA
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Kalau dilihat dari perubahan perioda (3.66-3.47)/3.47x 100% = 5,5% atau perubahan natural frek (1/3.47-1/3.66)/(1/3.47) =5.2% dapat meningkatkan fatique life sampai puluhan atau ratusan kali lipat 'kelihatannya' aneh. Namun grafik magnification factor untuk struktur baja dengan rasio redaman (damping ratio) yang kecil (biasanya kira2 0.005) sangatlah terjal dan kurus (apalagi kalau redamannya dianggap nol). Pada kondisi spt ini maka penggeseran sedikit saja pada frek pribadi dapat menurunkan getaran secara signifikan.
Agar mudah dipahami bayangkan grafik tsb berupa gunung yang sangat terjal dan menjulang tinggi. Mula2 bayangkan kt berdiri di dekat puncak gunung tersebut dan diikat pada helikopter yang 'mengapung di atas kita'. Selanjutnya bayangkan gunung terjal tsb digeser menjauh sedikit saja tapi krn kt digantung di helikopter maka posisi horizontal kt tdk bs pindah tapi kedudukan vertikal kt akan turun jauh kalau kaki kita tetap harus menginjak tebing (jadi tali dari helikopter harus diulur banyak secara vertikal). Jadi dengan menggeser frekuensi pribadi sedikit saya, getaran platforn turunnya luar biasa shg fatique life naik secara mentakjubkan.
Spekrum frekuensi ombak, seingat saya seperti gunung yg tdk curam dan tersebar dari 0 sd 0.3 Hz jadi pergeseran natural frekuensi sebesar 5% tdk akan banyak menurunkan getaran dilihat dari spektrum ombak. Karena spektrum ombak tersebar landai (hampir merata) di bawah 0,33 Hz (perioda > 3 detik) maka bila natural frekuensi platform > 0.33 Hz (atau perioda natura > 3 Hz) maka standard mensyaratkan perhitungan fatigue life.
Jadi alasan utama kenapa pergeseran frek pribadi 5% dapat meningkatkan fatique life puluhan (ratusan) kali ya karena redaman struktur dianggap kecil sekali (untuk struktur yg tercelup dlm fluida apa valid? Perlu cfd rasanya krn tergantung luas dan panjang kaki). Jadi fatique life meningkat bukan karena pergeseran natural frekuensi menjauhi frekuensi eksitasi ombak krn spektrum ombak walau spt gunung tapi gunungnya tidak terjal.
Smoga dpt membantu.
Salam,
ZA
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Pak Igbal,
Dari Rumus Frekuensi natural dari SDOF sederhana spt dibawah (asumsi, damping tidak ada dan lokasi penambahan masa tidak diperhitungkan)
maka variabelnya hanya k (stiffness) dan m (mass).
Dimensi, ukuran dan tebal dari material kaki kaki jacket tidak berubah, sehingga dianggap stifness (k) tidak berubah. Karena ada penambahan berat di deck, yang artinya adanya penambahan massa (m), maka natural frekuensi menjadi lebih kecil (pendek) atau periode natural menjadi lebih besar.
Dalam analisa struktur dengan beban periodik (cyclic loads) yang salah satunya bisa dari beban gelombang, maka yang paling diperhatikan dalam analisa dinamik adalah kedekatan peride natural stuktur dengan periode bebabncyclic (dalam hal ini periode gelombang) itu sendiri. Karena jika berdekatan akan terjadi resonansi, sehingga faktor dynamic DAF (Dynamic Amplification Factor) juga akan meloncat tinggi juga.
Hal ini yang mendasari struktur fixed (spt jacket) didesain cukup rigid, agar periode naturalnya jauh dari periode gelombang utama, sehingga DAF nya juga tidak terlalu berakibat significant ke struktur integritas (CMIIW). Jadi mohon hati hati untuk menambah massa (berat) pada desain bangunan laut, baik fixed, maupun floating.
Matur suwun,
Budi
--- On Mon, 9/19/11, Galih Heru Prasetyo <galihheruprasetyo@gmail.com> wrote:
From: Galih Heru Prasetyo <galihheruprasetyo@gmail.com>
Subject: Re: [Oil&Gas] Fatigue live meningkat? Date: Monday, September 19, 2011, 6:17 PM
Pak Iqbal,Menurut saya, kalau yang berubah hanya massa (sehingga mempengaruhi Natural Period), bisa di check data gelombangnya (scatter diagram), mungkin pada Periode "yg mendekati 3.47 s" dan wave height besar memiliki occurance yg lebih tinggi . CMIIW..Salam,Galih Heru Prasetyo2011/9/19 Iqbal M <iqbaloffon@gmail.com>
Rekan-rekan sekalian,Mohon advice rekan2 sekalian terkait analisa fatigue di offshore platform. Case sbg berikut:1. Platform empat kaki2. kedalaman air sekitar 60mdengan adanya bbrp perubahan di topside maka dilakukan analisa ulang thd fatigue di Jacket. Jacket tetap sama, hanya topside yg berubahMuncul pertanyaan, mengapa hasil analisa ulang mnunjukkan fatigue life naik hampir 2 kali lipat di joint2 jacket. padahal member di jacket tsb tetap sama size dan thicknessnya?sbg info tambahan:1. Analisa awal. converted load for modal analysis = 3290 mton. periode natural = 3.47 secs2. Analisa ulang. converted load for modal analysis = 3565 mton. periode natural = 3.66 secsMohon advice rekan-rekan. mengapa hal ini bisa terjadi? Saya masih awam terkait fatigue.Trm ksh sebelumnyaIqb
--
Regards,Galih Heru PrasetyoOffshore Structural Engineer
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