Oleh:Oleh:Ir. Murdjito, MSc.EngIr. Murdjito, MSc.Eng

Dosen Jurusan Teknik Kelautan Fakultas Teknologi Kelautan

Institut Teknologi Sepuluh Nopember (ITS) Surabaya

DESIGN CRITERIA DESIGN CRITERIA

DESIGN LOADS & CONDITIONS-References

API-RP2A, "Recommended Practice for Planning, Designing and Constructing Fixed Offshore Platforms-Working Stress Design", American Petroleum Institute, Washington, D.C., 21st ed., 2000.DET NORSKE VERITAS, Offshore standard: structural design of offshore units (wsd method), APRIL 2002, DNV-OS-C201 BS6235, "Code of Practice for Fixed Offshore Structures", British Standards Institution, London, 1982.DOE-OG, "Offshore Installation: Guidance on Design and Construction", U.K., Dept. of Energy, London 1985.Clauss, G. T. et al: "Offshore Structures, Vol 1 - Conceptual Design and Hydromechanics", Springer, London 1992.Hsu, H.T., "Applied Offshore Structural Engineering", Gulf Publishing Co., Houston, 1981.Graff, W.J., "Introduction to Offshore Structures", Gulf Publishing Co., Houston, 1981.

DESIGN LOADS & CONDITIONS

Dead Loads:Weight of the platform structure in air incl:weight of piles, grout, & ballastWeight of appurtenant structures permanently mounted on the platformHydrostatic forces acting on the structure below the water line incl: external pressure & bouyancy

Functional loads:Operating Loads: Fluid, contents in piping and equipmentLive Loads: the loads imposed during its use and may change during a mode of operation: static or dynamic functional loads arising form personnel, helicopter, maintenance loads, etc.

Environmental Loads: arise from the action of wave, currents and winds on the structureSeismic loads: arise as result of the ground motionAccidental Loads: arise as result of accident or abuse or exceptional conditions: boat impact, dropped objects, etcConsctructions Loads: resulting from fabrication, load out, transportation & installationDynamic Loads: loads imposed due to response to an excitation of a cyclic nature as wave, wind, earthquake, etc.

Design loads

Loads criteria Permanent (dead) loads.Operating (live) loads.Environmental loads including earthquakes.Construction - installation loads.Accidental loads.

Environment criteria :US and Norwegian regulations:100 yearsBritish rules : 50 years or greater

LOADING CONDITIONS

The environmental conditions combined with appropriate dead and live loadsOperational (Normal) Condition:

1-year return period environmental loadsAllowable stresses max 1.0

Storm Condition100-year return period environmental loadsAllowable stresses: increased by 1/3

Seismic ConditionConsider the effects of all gravity loads in combinations with simulatanous and collinear of loads due to ground motionAllowable stresses: increased by 70%

Accidental LoadsConsider the effects of collision loads and due to dropped objectsAllowable stresses: increased by 1/3For local design of elements, a dynamic load factor of 2.0 shallbe used

DESIGN CODES

API RP 2A WSD OR LRFDRecommended Practice Planning, designing and Constructing of Fixed Offshore Platform

AISCManual of Steel Construction, Allowable Stress Design

AWS D1.1Structural Welding Code

API RP 2LRecommended Practice Planning, Designing and Construction Heliport for Fixed Offshore Platform

Wind Loads

act on the portion of a platform above the water levelThe wind velocity profile (API-RP2A )

Vh/VH = (h/H)1/n

1/n=1/13 to 1/7, depending on the sea state, the distance from land and the averaging time interval. approximately = 1/13 for gusts and 1/8 for sustained winds in the open ocean.

Wind loads

Fw = (1/2) ρ V2 Cs Aρ : the wind density (ρ ~ 1.225 Kg/m3)Cs : the shape coefficient

Cs = 1,5 for beams and sides of buildings, = 0,5 for cylindrical sections = 1,0 for total projected area of platform.

Shielding and solidity effects can be accounted for

Wind loads

combination with wave loads:DNV and DOE-OG rules recommend the most unfavorable of the following two loadings:

1-minute sustained wind speeds combined with extreme waves.3-second gusts.

API-RP2A distinguishes between global and local wind load effects. first case: it gives guideline values of mean 1-hour average wind speeds to be combined with extreme waves and current. second case: it gives values of extreme wind speeds to be used without regard to waves.

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, then 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 and the flow induced cyclic wind loads due to vortex shedding must be investigated.

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.

WAVE THEORY

Wave Pattern

Wave Statistics

Wave SpectrumS (f,σ ) = S(f).D (f,σ )

S(f): wave energy density spectrumD(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 equationFor deep water: requires a load analysis involving the dynamic action of the structureFor global structure: ignored lift forces, slam forces, and axial Froude-Krylov forces If D/L >0.2, use diffraction theoryTotal base shear and overturning moment are calculated for global structure forcesLocal 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 structureCD ≈0,6 to 1,2 and CM ≈ 1,3 to 2,0.

PROCEDURE FOR CALCULATION OF WAVE PLUS CURRENT FORCES

WAVE DIRECTION

1 2

A

B

1 2

A

B

WAVE PARAMETERWave Kinematic factor:

Consider wave directional spreading or irregularity in wave profile shapeTropical storm: 0.85 – 0.95Extra tropical storm: 0.95 – 1.0

Current Blockage Factor:Reducing current speed due to the presence of the structure

Marine Growth: Increased in cross sectional areaDrag and Inertia Coefficient, depend on:

Reynold Number : R = Um D/νK-C number : K = 2 Um T2/DRoughness : e = k/DCurrent/Wave velcity : r = V1/VmoMember Orientation

CD, CM vs Re

CD, CM vs KC

CURRENT BLOCKAGE FACTOR

# of legs Heading factor

3 All 0.90

4End-on 0.80Diagonal 0.85Broadside 0.80

6End-on 0.75Diagonal 0.85Broadside 0.80

8End-on 0.70Diagonal 0.85Broadside 0.80

CONDUCTOR SHIELDING FACTOR

Depending upon the configuation of the structure and the number of conductorTo be applied to the drag and inertia coefficient for conductor arrayAppropriate for:

Steady current with negligible wavesExtreme waves with Umo Tapp/S > 5π

DIAGRAM CONDUCTOR SHIELDING FACTOR

Wave lift and slamming Loads

In addition to the forces given by Morison's equation, the lift forces FD and the slamming forces FS, typically neglected in global response computations, can be important for local member design.

FL = (1/2) ρ CL Dv2

FS = (1/2) ρ Cs Dv2

CL ≈1,3 CD. Cs ≈ π For tubular members

Earthquakes

Two levels of earthquake intensity: strength level (SLE)ductility level (DLE).

SLE: reasonable likelihood of not being exceeded during the platform's life (mean recurrence interval ~ 200 - 500 years), the structure is designed to respond elastically. DLE: maximum credible earthquake at the site, the structure is designed for inelastic response and to have adequate reserve strength to avoid collapse.API-RP2A recommends: X, Y, 0.5 ZDNV rules: 0,7X, O,7 Y and 0,5 ZThe value of a max and often the spectral shapes are determined by site specific seismological studies.

Ground acceleration

Design Spectra

Marine GrowthMarine growth is accumulated on submerged members. Its main effect is to increase the wave forces on the members by increasing not only exposed areas and volumes, but also the drag coefficient due to higher surface roughness.It increases the unit mass of the member, resulting in higher gravity loads and in lower member frequencies. Depending upon geographic location, the thickness of marine growth can reach 0,3m or more. It is accounted for in design through appropriate increases in the diameters and masses of the submerged members.

Tides

Tides affect the wave and current loads indirectly, i.e. through the variation of the level of the sea surface. The tides are classified as: (a) astronomical tides - caused essentially from the gravitational pull of the moon and the sun and (b) storm surges -caused by the combined action of wind and barometric pressure differentials during a storm. The combined effect of the two types of tide is called the storm tide. The astronomical tide range depends on the geographic location and the phase of the moon.Storm surges depend upon the return period considered and their range is on the order of 1,0 to 3,0m. When designing a platform, extreme storm waves are superimposed on the still water level while for design considerations such as levels for boat landing places, barge fenders, upper limits of marine growth, etc., the dailyvariations of the astronomical tide are used.

Tides

Seafloor Movements

Scour:Removal of seafloor soil caused by currents and wavesCan result in removal of vertical and lateral support for foundationsDesign condition scour depth ~ 3 ft

SettlementsGround motion due to overstressing of foundation elements

SubsidenceGround motion due to failure of seafloor slope