Numerical

Soil Investigation and Stability Analysis of Oil Rigs in Western Offshore Basin of IndiaPresented by ATUL KUMAR SINGH

Under the Guidance ofProf Dr. P.K.Maiti (Dept. of Civil Engg., IIT(BHU))Prof. Dr. M.S.Kulkarni (Dept. of Civil Engg., MIT, Pune)Prof G.S.Ingle (Dept. of Civil Engg., MIT, Pune)In Association with

IndexIntroductionSoil Investigation For the establishment of Oil RigsStability Analysis of oil RigsIntroduction

Anoil platform,offshore platform, or (colloquially)oil rigis a large structure with facilities to drill wells, to extract and processoiland natural gas, or to temporarily store product until it can be brought to shore for refining and marketing. In many cases, the platform contains facilities to house the workforce as well.

Depending on the circumstances, the platform may befixedto the ocean floor, may consist of anartificial island, or mayfloat. Remote subseawells may also be connected to a platform by flow lines and byumbilicalconnections. These subsea solutions may consist of one or more subsea wells, or of one or more manifold centres for multiple wells.Fig.1 Standard ONGC Oil RigSoil Investigation for Oil RigsGeotechnical Field investigation for the two types of Oil Rigs (Jacket and Jack-Up Rigs) is performed by ONGCs Geotechnical Vessel Samudra Sarvekshak.The purpose of Investigation was to determine the Soil Condition at the location for the assessment of Ultimate Axial Capacity, Load Deformation, p-y data for tubular piles, Jack-up rigs Penetration for Standard ONGC rig.CPTU was done on spot up to a depth of 107.50m below the sea bed after every 5m interval so as to determine various parameters, and after that the Soil Sample was sent to IEOT, ONGC for laboratory experiments so as to determine Shear Strength, Atterberg Limit etc. so as to compare the results of both the testing's.Axial pile Capacities and Axial Load-deformation characteristics (t-z and q-z data) for open ended steel piles of diameter 1.5 m.Mudmat Bearing Capacity was also determined for Jacket type Structure.

Fig. 3 Jack-up Laboratory Testing's (Offshore and Onshore)OFFSHOREVisual ClassificationWater ContentUndrained Shear Strength by Torvane, motorvane, and Pocket PenetrometerUU Triaxial Tests

ONSHOREGrain Size distribution (Using Hydrometer Analysis)Atterberg Limit Carbonate Content UU (Triaxial) (Cyclic as well as Non Cyclic Loading)ResultsComplete soil profile and soil parameters were determined with the help of Offshore and Onshore Lab experiments.Various Layer of Soil and Sand were present at particular depth below the sea bed.(No layer of Silt were found out)Axial Pile Capacity is determined with the help of properties of Soil.t-z and q-z data was determined (Axial Load Deformation Data).Bending moments and Deflections induced in laterally loaded piles are evaluated numerically and after that p-y data is generated.Penetration analysis for Standard ONGC jack up rig with the spudcan of diameter 14m and Maximum preload of 45MN is carried out, and the bearing capacity curves are plotted.

Stability Analysis (Numerical Modelling of Jack-Up Rigs and Jacket)There are more than 9000 xed offshore platforms around the world related to hydrocarbon production, the largest numbers of platforms are located in South East Asia, Gulf of Mexico and the North Sea followed by the coast of India, Nigeria, Venezuela and the Mediterranean Sea.

The Design of Marine Structure should be compatible with the extreme Offshore environmental conditions (external loads) like:-a) Wind Loadb) Hydrodynamic Loading c) Wave in deck loadingd) Earthquake Loading e) Ice loadingTypes of Oil Rigs (Platforms)

Jacket Structure and Jack-Up rigs

Typical Jacket StructureSkirt pile and Mudmat

Typical Jack-up

Analysis On the basis of Loadings

a) Hydrodynamic Loading (Wind Generated Waves)Hydrodynamic loading is the load that is applied by the Oscillatory flow of Oceanic waves. Influid dynamics,Airy wave theory(often referred to aslinear wave theory) gives alinearizeddescription of thepropagationofgravity waveson the surface of a homogeneous fluidlayer.This linear theory is often used to get a quick and rough estimate of wave characteristics and their effects. This approximation is accurate for small ratios of thewave heightto water depth (for waves inshallow water), and wave height to wavelength (for waves in deep water).

The surface elevation of an Airy wave of amplitude a, at any instance of time t and horizontal position x in the direction of travel of the wave, is denoted by (x,t) and is given by: (x,t) = a cos(x t)

where wave number = 2/ L in which L represents the wavelength and circular frequency = 2/T in which T represents the period of the wave. The celerity, or speed, of the wave C is given by L/T or /, and the crest to trough wave height, H, is given by 2a.

p-m wave spectrumOcean waves are predominantly generated by wind and although they appear to be irregular in character, tend to exhibit frequency-dependent characteristics that conform to an identifiable spectral description. Pierson and Moskowitz (1964), proposed a spectral description for a fully-developed sea state from data captured in the North Atlantic ocean:

was a good fit to the observed spectra, where = 2f, f is the wave frequency in Hertz, = 8.1 103, = 0.74 , 0 = g/U19.5 and U19.5 is the wind speed at a height of 19.5 m above the sea surface.

By integrating S() over all we get the variance of surface elevation:

The significant wave height is:

b) Morison EquationInertial ForceDrag ForceLoads

Self weight The generated self weight of all members sums up to 400000 N.

Hydrodynamic Loading

Wave Height (m)Wave Speed (U19.5)Time Period (sec)Wavelength(m)Drag Force (MN)16.674.8932.2.0511.58.176.0549.4.08029.447.0066.08.1162.510.558.3788.30.1413 11.568.5598.84.1643.512.49.176113.78.186413.39.84130.8.2114.514.1710.48148.50.236514.911.02169.3.244Table 1. Input Data Using NOAA and ONGC data for water Depth 75mResults from analysesIn the dynamic analyses the time steps used range from 0.001 s to 0.05 s. This corresponds to 0.0008Tn and 0.04Tn, respectively. The tiny time steps have been necessary to capture all nonlinear incidents.

Performance based on pushover analysis The static ultimate capacity for base shear show only minor variations, ranging from 83.8 MN to 86.3 MN. The largest capacity is found for a water depth of 78 m, corresponding to an Wave of 2.81 m. Whereas the ultimate capacity does not show any significant sensitivity to the load distribution (limited to those distributions analysed herein), the initial elastic stiffness clearly does.00.10.20.30.40.50.60.70.80.9020406080100Displacement [m]Structural resistance [MN]d 76 m=d = 77 md = 78 md 79 m=d 80 m=d = 81 mDepth 75 m / Wave 0.95 m(b) Depth 81 m / Wave 5.63 m

WaterDeckTotal waveFirst yieldSec. yieldMain yieldUlt. cap.depthinund.load BSaBsauBSauBSauBSau[m][m][MN][MN][m][MN][m][MN][m][MN][m]76.00.9341.1133.770.13648.480.19668.620.27883.830.34777.01.8747.6925.010.11444.710.20567.570.31085.910.40378.02.8158.7120.030.10041.600.21063.490.32186.310.45079.03.7566.1818.100.09540.060.21261.180.32585.650.46780.04.6972.8616.880.09239.070.21458.900.32485.050.48281.05.6379.6616.060.09038.340.21655.800.31684.520.494Performance based on time domain analysis The resulting displacement histories for different water depths (and corresponding wave height) are given in Figure Below. It has not been possible to produce time domain analyses of acceptable numerical quality for water depths from 79.5 m and beyond, due to numerical instability. The largest depth analysed is therefore 79 m, corresponding to an Wave of 3.75 m. The brace configuration of the model causes instability for responses resulting from loading above this level.

Time histories of accelerations are given for the four relevant analysis cases. For d = 76 m the response is close to purely elastic and the largest accelerations are approximately 1.5 m/s2. At d = 77 m the acceleration peaks are 3.2 - 3.3 m/s2, and for d = 78 m the peaks are rather close to 5.9 m/s2. The last case, d = 79 m, has very irregular accelerations due to many plastic incidents. The largest acceleration value is negative, and is close to 6.1 m/s2. This negative peak is followed by a positive peak of 4.8 m/s2. Thereafter the acceleration peaks remain at 2 - 4 m/s2, but are decreasing due to material damping.

ReferencesAmdahl, J., Skallerud, B. H., Eide, O. I., and Johansen, A. (1995). Recent developments in reassessment of jacket structures under extreme storm cyclic loading, part II: Cyclic capacity of tubular members. In Proceedings of the 14th International Conference on Offshore Mechanics and Arctic Engineering (OMAE) 1995, Copenhagen, Denmark.API LRFD (1993). Recommended practice for planning, designing and constructing fixed offshore platforms - Load and resistance factor design (API RP2A-LRFD). American Petroleum Institute, Washington, DC, USA, first edition.API LRFD (2003). Recommended practice for planning, designing and constructing fixed offshore platforms - Load and resistance factor design (API RP2A-LRFD). American Petroleum Institute, Washington, DC, USA, first edition. Reaffirmed May 2003.API WSD (2002). Recommended practice for planning, designing and constructing fixed offshore platforms - Working stress design (API RP2A-WSD). American Petroleum Institute, Washington, DC, USA, twenty-first (2000) edition. Including errata and supplement 1.Bea, R., Mortazavi, M., Stear, J., and Jin, Z. (2000). Development and verification of Template Offshore Capacity Analysis Tools (TOPCAT). In Proceedings of the 32nd Annual Offshore Technology Conference 2000, Houston, Texas, USA.Bea, R. G. (1993). Reliability based requalification criteria for offshore platforms. In Proceedings of the 12th International Conference on Offshore Mechanics and Arctic Engineering (OMAE) 1993, Glasgow, Scotland.Bea, R. G., Iversen, R., and Xu, T. (2001). Wave-in-deck forces on offshore platforms. Journal of Offshore Mechanics and Arctic Engineering, volume 123:pp. 10 21.Bea, R. G. and Lai, N. W. (1978). Hydrodynamic loadings on offshore platforms. In Proceedings of the 10th Annual Offshore Technology Conference 1978, volume 1, pages 155 168, Houston, Texas, USA. OTC 3064.Bea, R. G. and Mortazavi, M. M. (1996). ULSLEA: A limit equilibrium procedure to determine the ultimate limit state loading capacities of template-type platforms. Journal of Offshore Mechanics and Arctic Engineering, volume 118(no. 4):pp. 267 275.Bea, R. G., Xu, T., Stear, J., and Ramos, R. (1999). Wave forces on decks of offshore platforms. Journal of Waterway, Port, Coastal, and Ocean Engineering, volume 125(no. 3):pp. 136 144.

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