By: Dr. Eng. Rudi W. Prastianto
Various loads acting on a fixed offshore platform
Various loads acting on a fixed offshore platform
Environmental Criteria • Determining forces imposed on the platform by waves, waves, current , & wind assessing: – Water depth
– Storm wind velocity
– Tide conditions
– Earthquake
– Storm wave height
– Ice conditions
• All of the environment environment factors which impose loads on the platform must be investigated
Loads • Classification • Functional Loads • Wind Loads • Wave & Current Loads • Ice Loads
• Seismic Loads
Loads Classification • Beban fungsional (Functional loads) • Beban lingkungan (Environmental loads) • Beban kombinasi (Combination loads) • Beban konstruksi & instalasi (Construction & installation loads)
• Beban akibat kecelakaan (Accidental loads)
Functional loads • Dead load the weight in air of the overall platform structure, it will not change with operation modes: – Piling, superstructure, jacket, stiffeners, piping & conductors, risers, corrosion anodes, fixed ballasts, decking, railing, grout, paint, boat landing, barge buffers, & other appurtenances.
• Imposed loads (beban variasi) excluded from the dead loads.
Imposed loads • Drilling equip. weight placed on the platform (incl. derrick, draw-works, mud pump, mud tanks). • Prod. Or treatment equipt. weight separators, compressors, piping manifolds, storage tanks. • Weight of drilling supplies the variable loads employed during production (incl. drilling mud, water, diesel fuel, casing). • Weight of treatment supplies the variable supplies employed during production (incl. fluid in the separators, storage in the tanks). • Drilling load any appropriate combination of derrick load, pipe storage (set-back), or rotary table load.
Environmental loads • • • • • • • • •
Beban angin (Wind load) Beban gelombang laut (Wave load) Beban arus laut (Current load) Beban impulse / slamming akibat hempasan gelombang (Impulse or slamming loads) Beban akibat vortex shedding (Loads due to vortex shedding) Beban berat dari salju atau bongkahan es Beban akibat gempa Beban akibat geseran tanah (land slide) Beban hidrostatik pada bagian BLP yang terendam air
Combination loads • • • •
Kenyataannya, beban-beban yang bekerja pada BLP bekerja secara simultan Contoh 1: Kondisi badai beban kombinasi: gelombang ekstrem + arus storm + angin badai ekstrem Contoh 2: Beban gelombang + arus + angin bekerja dalam arah yang sama 3 kategori beban kombinasi: 1. 2. 3.
Beban kondisi 1 : beban lingkungan ekstrem + beban fungsional maks. efek kritis: max. stress & kemampuan ikat pondasi Beban kondisi 2 : beban lingkungan ekstrem + beban mati + 50% beban variasi efek kritis: stabilitas & pergeseran posisi kaki pada pondasi (terutama untuk jenis Jack-up). Beban kondisi 3 : beban kecelakaan + beban lingkungan + 50% beban variasi.
Accidental loads •
Beban-beban yang timbul pada BLP akibat adanya kecelakaan yang tidak diduga sebelumnya – Tabrakan dengan kapal pemandu operasi atau tunda (supply or tug boats) – Putusnya tali katrol (crane) saat operasi – Putusnya tali tambat (rantai jangkar, tali baja pengikat struktur) – Blowouts – dll.
Deck Floor loads •
Beban-beban terdistribusi yang diperhitungkan untuk menentukan ukuran pelat, penegar, dll dari deck. – – – – – –
Beban pada lantai ruang kabin : 3.25 kN/m 2 Beban pada lantai ruang crew, gang, jalan-jalan masuk, dll : 4.5 kN/m 2 Beban pada lantai di daerah tempat kerja : 9 kN/m 2 Beban pada lantai ruang penyimpanan : 13 kN/m 2 Loads in the drilling rig & derrick : 11.975 kN/m 2 Loads under the mud, water, & fuel tanks : 71.851 kN/m2
Wind Loads • The Gulf of Mexico (GoM) the importance of correct wind & wave force prediction, after Hurricanes Hilda & Betsy (mid-1960s). • The force of the wind on a structure is a function of: – Wind velocity – Orientation of the structure – Aerodynamic characteristics of the structure & its members
Wind & Wind Spectrum • The wind effect on an offshore structure becomes important when the superstructure (portion above the MWL) is significant. • The wind generally has two effects : – from the mean speed – and the other from the fluctuation about this mean value.
• The mean speed as a s teady load on the offshore structure. For a fixed structure it is only the mean s peed that is taken into account (The effect of the fluctuation of wind about the mean value has a little effect on the fixed structure). • This is not the case for a floating s tructure the dynamic wind effect may be significant and may not be ignored.
Wind Speed • The accepted s teady wind s peeds in a design of an offshore structure are generally : – The averag e s peed occurring for a period of 1-h
duration. – The mean speed measured at a reference height, typically 30 ft (10 m) above the mean still water level. – A 100-year return period mean wind speed should be used in the design. – The directionality of the wind may be important in some applications.
• Wind load on the structure: – Should be treated as a steady component based on the above mean speed.
Wind Spectrum – Additionally, a load with a time-varying wind component known as the gust should be calculated generates low-frequency motion . – The time varying wind is described by a wind g us t s pectrum. • Wind blowing over the deck structure is also random having a mean speed superimposed on it. This wind spectrum may be important to consider for certain types of offshore structures.
Wind Loads (cont. …) • Act on the portion of a platform above the water level • Cable structure susceptible to violent motion from vortex shedding at hig h wind s peed . • It is expressed as: F D = C D ½ p V z2 A (force parallel to wind) F L = C L ½ p V z2 A (force perpendicular to wind) Where: C D = drag coeff. (range: 2.1 to 0.7 depending on the shape of the structure) C L = lift coeff. p = density of the air V z = wind velocity at height z A = area perpendicular to wind velocity
Wind Loads (cont. …) • The wind velocity is not constant zero at the surface & increases exponentially to a limiting max. speed (the gradient wind). • The wind speed at any elevation above a water surface is represented as ( one-s eventh power law ):
V z = V 10 [z / 10]1/7 Where: V 10 = wind speed at a height of 10 m (the customary elevation for such measurements) V z = the desired elevation (m) 10 = the reference height (m)
Wind Loads (cont. …) • Only s us tained wind s peed ? No the g us t is also needed.
• The gust factor ( F g ) multiplier that must be used on the sustained wind speed to obtain the gust speed. • The average F g : 1.35 – 1.45 (the variation of the factor with height is negligible)
Wind Loads (cont. …) • Structural comp. primarily loaded by wind should be design for: – The “fastest -mile-velocity” , period 100 years. – Term “fastest -mile-velocity” sustained wind speed a gust factor
• “fastest -mile-velocity” should be used for designing: – Tie-down design
Wind loads in combination with wave loads • DNV rule recommends the most unfavorable of the following two loadings: • 1-minute sustained wind speeds combined with extreme waves. • 3-second gusts. • API-RP2A distinguishes between g lobal and local wind load effects. – Firs t cas e: it gives guideline values of mean 1hour average wind speeds to be combined with extreme waves and current. – S econd cas e: it gives values of extreme wind s peeds to be used without regard to waves.
Wind + wave loads (cont. …) • Wind loads are generally taken as static. When the ratio of height to the least horizontal dimension of the wind exposed object (or structure) > 5, this object (or structure) could be wind sensitive. • API-RP2A requires: – The dynamic effects of the wind to be taken into account in this case – The flow induced cyclic wind loads due to vortex s hedding must be investigated.
Design wind pressures on structural components for a 100-year sustained wind velocity of 125 mph
Wind shielding factors for structural components in series • Allowance for shielding effect for some components calculated force for fully exposed the shielding coeff.
DESIGN LOADS – WAVE & CURRENT LOADS Two different analysis concepts are used: •
Design/ regular wave concept: – a regular wave of given height and period is defined and the forces due to this wave are calculated using a high-order wave theory. – Usually the 100-year wave is chosen. – No dynamic behavior of the structure is considered. This static analysis is appropriate when the dominant wave periods are well above the period of the structure. – This is the case of extreme storm waves acting on shallow water structures.
•
Statistical analysis: – on the basis of a wave scatter diagram for the location of the structure. – Appropriate wave spectra are defined to perform the analysis in the frequency domain and to generate random waves, if dynamic analyses for extreme wave loadings are required for deepwater structures. – With statistical methods, the most probable maximum force during the lifetime of the structure is calculated using linear wave theory. – The statistical approach has to be chosen to analyze the fatigue strength and the dynamic behavior of the structure.
Wave Theories
Linear Airy theory,
Stokes fifth-order theory
Solitary wave theory,
Cnoidal theory,
Dean's stream function theory
Numerical theory by Chappelear.
f o y t i y r d i l o e a h V T f o e e v a m i W g e R
Wave Pattern
Wave Statistics
Wave Spectrum •
S (f,σ ) = S(f).D (f,σ ) – S(f): wave energy density spectrum – D(f,σ): directional spreading function – σ : the angle of the wave approach direction
DESIGN LOADS – WAVE & CURRENT LOADS • Represented by their static equivalent using Morisson’s equation • For deep water: requires a load analysis involving the dynamic action of the structure • For global structure: ignored lift forces, slam forces, and axial Froude-Krylov forces • If D/L > 0.2, use Diffraction theory • Total base shear and overturning moment are calculated for global structure forces • Local member stresses: due to local hydrodynamic forces (incl. slam, lift, Froude-Krylov, buoyancy) and loads transferred due to global fluid-dynamic force and dynamic response of the structure • CD ≈ 0,6 to 1,2 and C M ≈ 1,3 to 2,0.
PROCEDURE FOR CALCULATION OF WAVE PLUS CURRENT FORCES ON A FIXED PLATFORM
WAVE DIRECTION 1 A
B
2
Faktor Kinematika Gelombang
Dalam kinematika gelombang regular 2 dimensi dari teori gelombang seperti Stream Function atau Stokes orde 5, karakteristik nyata dari suatu gelombang seperti penyebaran arah gelombang (wave directional spreading ) atau ketidakteraturan bentuk profil gelombang (irregularity in wave profile shape) tidaklah masuk dalam perumusan.
Untuk mengakomodasi hal tersebut agar mendekati kondisi nyatanya, maka suatu cara pendekatan dalam analisa gelombang deterministik dapat digunakan yaitu dengan cara mengalikan kecepatan dan percepatan horisontal yang didapat dari solusi gelombang regular 2 dimensi dengan suatu faktor yang disebut Faktor Kinematika Gelombang (Wave Kinematics Factor ).
Nilai Faktor Kinematika Gelombang dari hasil pengukuran adalah:
Tropical storms = 0.85 – 0.95
Extra-Tropical storms = 0.95 – 1.00
Faktor Halangan Arus • Keberadaan struktur di tengah aliran fluida menjadi sebuah halangan (blockage) yang menyebabkan aliran-datangnya mengalami pemisahan. Sebagiannya cenderung bergerak mengitari struktur dan tidak meninggalkannya, sehingga kecepatan alirannya di sekitar struktur tersebut menjadi berkurang. • Untuk itu penentuan kacepatan arus lokal di sekitar struktur yang tepat menjadi sangat penting, mengingat besar beban global pada platform didapatkan dari jumlahan semua beban-beban lokal dari persamaan Morisonnya. • Suatu faktor untuk mengakomodasi hal tersebut disebut Faktor Halangan Arus (C urrent B lockag e Factor ).
Faktor Halangan Arus
Nilai-nilai perkiraan untuk struktur jenis jacket tipikal untuk perairan Teluk Meksiko – USA adalah sbb.:
Penempelan Organisme Laut
Organisme laut yang menempel dan tumbuh pada elemenelemen struktur, konduktor, riser dan appurtenances (Marine G rowth) akan menambah luas penampang melintangnya menambah beban pada struktur.
Koefisien Drag & Inersia • Koefisien Drag dan Inersia merupakan fungsi dari:
Koefisien Drag & Inersia
CD, CM vs Re
CD, CM vs KC
CONDUCTOR SHIELDING FACTOR • Depending upon the configuation of the structure and the number of conductor • To be applied to the drag and inertia coefficient for conductor array • Appropriate for: – Steady current with negligible waves – Extreme waves with Umo Tapp/S > 5 π
DIAGRAM CONDUCTOR SHIELDING FACTOR
