Ocean current Variability in Relation to Offshore Engineering

This work adresses ocean current variability in relation to offshore engineering.The offshore oil and gas activity has up until recently taken place mainly on the continental shelves around the world. During the last few years, however, the industry has moved past the continental shelf edge and down the continental slope towards increasingly deeper waters. In deep water locations, marine structures may span large spaces, marine operations may become more complicated and require longer time for completion and the effect of the surface waves is diminished. Therefore, the spatial and temporal variability of the current is expected to become more important in design and planning than before.The flow of water in the oceans of the world takes place on a wide variety of spatial scales, from the main forms of the global ocean circulation (~km), to the microstructure (~mm) of boundary layer turbulence. Similarly, the temporal variability is also large. In one end of the scale we find variations that take place over several decades, and in the other end we find small-scale turbulence (~seconds). Different features of the flow are driven by different mechanisms. Several processes and properties (stratification1, sloping boundary, Coriolis effect, friction, internal waves, etc.) interact on the continental slope to create a highly variable flow environment. Analysis of a set of observed data that were recorded close to the seabed on the continental slope west of Norway are presented. The data suggest that some strong and abrupt current events (changes in flow speed of ~0.4 m/s in just a couple of hours) were caused by motions of the deep pycnocline2, driven by variations in the surface wind field. This conjecture is partly supported by numerical simulations of an idealised continental slope and a two-layer ocean. The data also contains an event during which the flow direction at the sea bed changed very rapidly (within a few minutes) from down-slope to up-slope flow. The change in speed during this event was as high as 0.5 m/s.Another data set has been analyzed in order to illustrate the spatial variation in the current that can sometimes be found. It is shown that the flow in the upper layer is virtually decoupled from the flow in the lower layer at a location west of Norway. This is either caused by bottom topography, stratification or both.High variability of the current presents new requirements to the way that the current should be modelled by the offshore engineer. For instance, it is necessary to consider which type of operation/structure that is to be carried out or installed before selecting design current conditions. Reliable methods for obtaining design current conditions for a given deep water location have yet to be developed, only a brief discussion of this topic is given herein.It is shown, through calculations of VIV-response and simulations of typical marine operations, that the variability of the current will sometimes have a significant effect on the response/operation.1Vertical distribution of density. In a stratified ocean or flow, the density of the water varies in the vertical direction.2pycnocline=density surface between water masses. The pycnocline between two water masses of different density is defined by the maximum of the density gradient.

[1]  Ivar Holand.  Safety of structures under dynamic loading , 1978 .

[2]  H. Berntsen,et al.  Efficient numerical simulation of ocean hydrodynamics by a splitting procedure , 1981 .

[3]  W. Emery,et al.  Global water masses: summary and review , 1986 .

[4]  Olusegun Tunde Babalola.  Fatigue Strength of Titanium Risers - Defect Sensitivity , 2001 .

[5]  Sonya Legg,et al.  Internal Wave Breaking at Concave and Convex Continental Slopes , 2003 .

[6]  B. Ådlandsvik,et al.  Production of monthly mean climatological archives for the Nordic Seas , 1998 .

[7]  Jørgen Fredsøe,et al.  Hydrodynamics Around Cylindrical Structures , 2006 .

[8]  C. Garrett.  Mixing in the ocean interior , 1979 .

[9]  S. Thorpe.  On the Interactions of Internal Waves Reflecting from Slopes , 1997 .

[10]  C. Eriksen.  Implications of Ocean Bottom Reflection for Internal Wave Spectra and Mixing , 1985 .

[11]  J. C. Evans,et al.  Separation of Warm-Core Rings in the Gulf of Mexico , 1993 .

[12]  P. Hamilton,et al.  Observations of high speed deep currents in the northern Gulf of Mexico , 2001 .

[13]  Gunnar K. Furnes,et al.  On the response of a free span pipeline subjected to ocean currents , 2003 .

[14]  M. Zweng,et al.  Anticyclonic rings in the Gulf of Mexico , 2007 .

[15]  S. Østerhus,et al.  North Atlantic–Nordic Seas exchanges , 2000 .

[16]  T. N. Stevenson,et al.  Fluid Mechanics , 2021, Nature.

[17]  J. Lumley,et al.  A First Course in Turbulence , 1972 .

[18]  Kjetil Bjørn Skaugset.  On the suppression of vortex induced vibrations of circular cylinders by radial water jets , 2003 .

[19]  T. Sarpkaya,et al.  Mechanics of wave forces on offshore structures , 1981 .

[20]  D. E. Newland.  Random vibrations, Spectral & Wavelet Analysis , 1993 .

[21]  P. Hamilton,et al.  Deep Currents in the Gulf of Mexico , 1990 .

[22]  J. Horel.  Complex Principal Component Analysis: Theory and Examples , 1984 .

[23]  J. Imberger,et al.  Numerical study of internal wave reflection from sloping boundaries , 1999, Journal of Fluid Mechanics.

[24]  S. Thorpe.  Internal Wave Reflection and Scatter from Sloping Rough Topography , 2001 .

[25]  A. M. Goulois,et al.  Analysis of cable structures , 1979 .

[26]  C. Wunsch,et al.  Experimental study of internal waves over a slope , 1974, Journal of Fluid Mechanics.

[27]  A. Gordon,et al.  Geostrophic circulation of the Brazil-Falkland confluence , 1986 .

[28]  John R. Taylor.  Turbulence and mixing in the boundary layer generated by shoaling internal waves , 1993 .

[29]  A. Rosati,et al.  A Quasi-equilibrium Turbulent Energy Model for Geophysical Flows , 1988 .

[30]  N. Millard,et al.  Deep‐sea, high‐resolution, hydrography and current measurements using an autonomous underwater vehicle: The overflow from the Strait of Sicily , 2001 .

[31]  N Sodahl,et al.  METHODS FOR DESIGN AND ANALYSIS OF FLEXIBLE RISERS , 1991 .

[32]  A. Grossmann,et al.  Cycle-octave and related transforms in seismic signal analysis , 1984 .

[33]  B. Gjevik.  Simulations of shelf sea response due to travelling storms , 1991 .

[34]  S. Garzoli,et al.  Transports, frontal motions and eddies at the Brazil-Malvinas currents confluence , 1989 .

[35]  Roger Nokes,et al.  Vertical mixing due to the breaking of critical internal waves on sloping boundaries , 1989, Journal of Fluid Mechanics.

[36]  Alan F. Blumberg,et al.  Modeling vertical and horizontal diffusivities with the sigma coordinate system , 1985 .

[37]  Klaus Gersten,et al.  Boundary-Layer Theory - 8th Revised and Enlarged Edition , 2000 .

[38]  William J. Emery,et al.  Data Analysis Methods in Physical Oceanography , 1998 .

[39]  R. Blevins,et al.  Flow-Induced Vibration , 1977 .

[40]  Peter Bradshaw,et al.  An introduction to turbulence and its measurement , 1971 .

[41]  Henrik Schmidt,et al.  Real-time frontal mapping with AUVs in a coastal environment , 1996, OCEANS 96 MTS/IEEE Conference Proceedings. The Coastal Ocean - Prospects for the 21st Century.

[42]  P. K. Kundu,et al.  Some Three-Dimensional Characteristics of Low-Frequency Current Fluctuations near the Oregon Coast , 1976 .

[43]  S. Glenn,et al.  Drifting buoy observations of a loop current anticyclonic eddy , 1993 .

[44]  H. Stommel,et al.  The abyssal circulation , 1958 .

[45]  Carl M. Larsen,et al.  FREQUENCY AND TIME DOMAIN ANALYSIS OF VORTEX INDUCED VIBRATIONS FOR FREE SPAN PIPELINES , 2002 .

[46]  W. Zenk,et al.  The Vitória Eddy and Its Relation to the Brazil Current , 1995 .

[47]  Trond Stokka Meling,et al.  AN ASSESSMENT OF EOF CURRENT SCATTER DIAGRAMS WITH RESPECT TO RISER VIV FATIGUE DAMAGE , 2002 .

[48]  Ole David Økland.  Numerical and Experimental Investigation of whipping in Twin Hull Vessels exposed to serve wet Deck slamming , 2002 .

[49]  P. Rhines,et al.  Slippery Bottom Boundary Layers on a Slope , 1993 .

[50]  A. D. Heathershaw,et al.  The turbulent structure of the bottom boundary layer in a tidal current , 1979 .

[51]  P. Baines.  The reflexion of internal/inertial waves from bumpy surfaces , 1971, Journal of Fluid Mechanics.

[52]  A. Patera.  A spectral element method for fluid dynamics: Laminar flow in a channel expansion , 1984 .

[53]  J. Smagorinsky,et al.  GENERAL CIRCULATION EXPERIMENTS WITH THE PRIMITIVE EQUATIONS , 1963 .

[54]  John W. Miles,et al.  The Dynamics of the Upper Ocean, 2nd Edition. By 0. M. PHILLIPS. Cambridge University Press, 1977. 336 pp. $16.00 or $34.00. , 1978, Journal of Fluid Mechanics.

[55]  Chunhua Ge,et al.  Global Hydroelastic Response of Catamarans Due to Wetdeck Slamming , 2002 .

[56]  Yngve Heggelund,et al.  Hydrostatic and non-hydrostatic studies of gravitational adjustment over a slope , 2004 .

[57]  F. Vukovich.  Loop Current boundary variations , 1988 .

[58]  Roberto Bruschi,et al.  Response of very long and multi-span pipelines in turbulent flows -- Experimental results , 1995 .

[59]  A. Heathershaw.  “Bursting” phenomena in the sea , 1974, Nature.

[60]  Colin Grant,et al.  Improved Current Profile Criteria for Deepwater Riser Design , 2003 .

[61]  N. A. Phillips,et al.  A COORDINATE SYSTEM HAVING SOME SPECIAL ADVANTAGES FOR NUMERICAL FORECASTING , 1957 .

[62]  V. Buchwald,et al.  The generation of continental shelf waves , 1969, Journal of Fluid Mechanics.

[63]  A. M. Davies,et al.  Observed and Computed M2 Tidal Currents in the North Sea , 1980 .

[64]  Odd Halvdan Holt Kristensen,et al.  Ultimate capacity of aluminium plates under multiple loads, considering HAZ properties , 2001 .

[65]  S. Thorpe,et al.  On the reflection of a train of finite-amplitude internal waves from a uniform slope , 1987, Journal of Fluid Mechanics.

[66]  G. S. Fargion,et al.  Loop Current Eddy Paths in the Western Gulf of Mexico , 1999 .

[67]  H. Haren,et al.  Ekman-induced turbulence over the continental slope in the Faeroe–Shetland Channel as inferred from spikes in current meter observations , 2003 .

[68]  L. Stramma,et al.  Upper-level circulation in the South Atlantic Ocean , 1991 .

[69]  George Z. Forristall,et al.  Design Current Profiles Using Empirical Orthogonal Function (EOF) and Inverse FORM Methods , 1997 .

[70]  H. Sverdrup.  Wind-Driven Currents in a Baroclinic Ocean; with Application to the Equatorial Currents of the Eastern Pacific. , 1947, Proceedings of the National Academy of Sciences of the United States of America.

[71]  Eskild Storteig.  Dynamic characteristics and leakage performance of liquid annular seals in centrifugal pumps , 2000 .

[72]  G. Mellor,et al.  A Hierarchy of Turbulence Closure Models for Planetary Boundary Layers. , 1974 .

[73]  J. Kim Vandiver,et al.  Drag Coefficients of Long Flexible Cylinders , 1983 .

[74]  J. Hamilton,et al.  Mooring Vibration as a Source of Current Meter Error and Its Correction , 1997 .

[75]  Martin W. Johnson,et al.  The Oceans, Their Physics, Chemistry and General Biology , 1945 .

[76]  Dale B. Haidvogel,et al.  Numerical Ocean Circulation Modeling , 1999 .

[77]  Gudmund Kleiven,et al.  Identifying VIV Vibration Modes by Use of the Empirical Orthogonal Functions Technique , 2002 .

[78]  G. Mellor,et al.  Development of a turbulence closure model for geophysical fluid problems , 1982 .

[79]  S. Pond,et al.  Introductory Dynamical Oceanography , 1983 .

[80]  S. Thorpe.  Current and temperature variability on the continental slope , 1987, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[81]  B. Cushman-Roisin.  Introduction to Geophysical Fluid Dynamics , 1994 .

[82]  K. F. Bowden.  Measurements of turbulence near the sea bed in a tidal current , 1962 .

[83]  J. L. Thorogood,et al.  On The Choice of Current Profiles for Deep-Water Riser Design , 1998 .

[84]  Lars Petter Røed,et al.  Mean and eddy motion in the Skagerrak/northern North Sea: insight from a numerical model , 2004 .

[85]  D. Cacchione,et al.  Internal tides and the continental slope , 2004 .

[86]  A. D. Heathershaw,et al.  Sea-bed noises reveal role of turbulent bursting phenomenon in sediment transport by tidal currents , 1985, Nature.

[87]  N. Garfield.  The Brazil Current at subtropical latitudes , 1990 .

[88]  B. Butman,et al.  Near-bottom currents over the continental slope in the Mid-Atlantic Bight , 1988 .

[89]  Louise Levathes,et al.  When China ruled the seas , 1994 .

[90]  M. Ludvigsen,et al.  Model supported AUV freshwater distribution surveys in the upper layers of deep fjords , 2003, Oceans 2003. Celebrating the Past ... Teaming Toward the Future (IEEE Cat. No.03CH37492).

[91]  S. Thorpe.  The generation of internal waves by flow over the rough topography of a continental slope , 1992, Proceedings of the Royal Society of London. Series A: Mathematical and Physical Sciences.

[92]  P. Rhines,et al.  Buoyant inhibition of Ekman transport on a slope and its effect on stratified spin-up , 1991, Journal of Fluid Mechanics.

[93]  John M. Huthnance,et al.  Circulation, exchange and water masses at the ocean margin: the role of physical processes at the shelf edge , 1995 .

[94]  K. A. Orvik,et al.  Major pathways of Atlantic water in the northern North Atlantic and Nordic Seas toward Arctic , 2002 .

[95]  P. E. An,et al.  An AUV survey in the littoral zone: small-scale subsurface variability accompanying synoptic observations of surface currents , 2001 .

[96]  S. Levitus.  Climatological Atlas of the World Ocean , 1982 .

[97]  R. Gopalkrishnan.  Vortex-induced forces on oscillating bluff cylinders , 1993 .

[98]  S. Thorpe.  Thermal Fronts Caused by Internal Gravity Waves Reflecting from a Slope , 1992 .

[99]  J. Riley,et al.  Turbulent mixing in the oceanic boundary layer caused by internal wave reflection from sloping terrain , 1996 .

[100]  M. P. M. Reddy,et al.  Descriptive physical oceanography , 2001 .

[101]  J. Riley,et al.  Turbulent Dynamics of a Critically Reflecting Internal Gravity Wave , 1998 .

[102]  L. Kantha,et al.  Numerical models of oceans and oceanic processes , 2000 .

[103]  R. L. Soulsby,et al.  Similarity Scaling of Turbulence Spectra in Marine and Atmospheric Boundary Layers , 1977 .

[104]  P. Leblond,et al.  The turbulent bottom boundary layer and its influence on local dynamics over the continental shelf , 1988 .

[105]  M. Tucker,et al.  Numerical simulation of a random sea: a common error and its effect upon wave group statistics , 1984 .

[106]  J. Berntsen,et al.  Numerical studies of the current response at Ormen Lange to a travelling storm , 2004 .

[107]  J. Gemmrich,et al.  Thermal Fronts Generated by Internal Waves Propagating Obliquely along the Continental Slope , 2001 .

[108]  S. Lentz,et al.  Asymmetric Behavior of an Oceanic Boundary Layer above a Sloping Bottom , 1991 .

[109]  G. Pawlak,et al.  Stratified Flow along a Corrugated Slope: Separation Drag and Wave Drag , 2001 .

[110]  Rune Yttervik,et al.  Current measurements on the continental slope west of Norway in an area with a pronounced two-layer density profile , 2005 .

[111]  P. K. Kundu,et al.  Ekman Veering Observed near the Ocean Bottom , 1976 .

[112]  Abuu Khalifa Mohammed.  Nonlinear Shell Finite Elements for Ultimate Strength and Collapse Analysis of Ship Structures , 2001 .

[113]  K. Hamilton.  Gravity Currents in the Environment and the Laboratory , 1998 .

[114]  C. Torrence,et al.  A Practical Guide to Wavelet Analysis. , 1998 .

[115]  R. C. Dyer,et al.  Development of a new metocean design basis for the NW shelf of Europe , 1995 .

[116]  Louis N. Howard,et al.  Note on a paper of John W. Miles , 1961, Journal of Fluid Mechanics.

[117]  P. Richardson,et al.  Seasonal cycle of velocity in the Atlantic North Equatorial Countercurrent as measured by surface drifters, current meters, and ship drifts , 1987 .

[118]  Steven J. Lentz,et al.  The Bottom Boundary Layer Over the Northern California Shelf , 1991 .

[119]  P. Müller,et al.  Scattering of Oceanic Internal Gravity Waves off Random Bottom Topography , 1992 .

[120]  A. E. Gill.  Atmosphere-Ocean Dynamics , 1982 .

[121]  H. Stommel,et al.  The westward intensification of wind‐driven ocean currents , 1948 .

[122]  William W. Hsieh,et al.  Forecasting the equatorial Pacific sea surface temperatures by neural network models , 1997 .

[123]  M. White.  Tidal and subtidal variability in the sloping benthic boundary layer , 1994 .