Gunnar K. Furnes

Retroflection of Atlantic water in the Norwegian trench

Abstract Recent investigations off western Norway draw into question the classical concept of an inflow of Atlantic water as a continuous filament along the western slope of the Norwegian Trench from north of Shetland to the Skagerrak. Based on biological, hydrographical and current observations, as well as on the theoretical considerations, we conclude that a significant part of the inflowing Atlantic water to the Norwegian Trench retroflects within the northwestern part of the trench. Moreover, there is an apparent discontinuity in the inflow of Atlantic water to the Skagerrak off southwestern Norway at about 58° to 58°30′N.

A three-dimensional numerical sea model with eddy viscosity varying piecewise linearly in the vertical

Abstract The solution of the three-dimensional linear hydrodynamic equations which describe wind-driven flow in a homogeneous sea are solved using the eigenfunction method. The eddy viscosity is taken to vary piecewise linearly in the vertical over an arbitrary number of layers. Using this formulation the eigenfunctions are given in terms of Bessel functions. The coefficients of integration as well as the eigenvalues are determined accurately such that the boundary conditions are satisfied. Values of the eigenfunctions at any depth can then be determined very fast and to a high degree of accuracy. Current profiles at any position can hence be computed accurately. The expansion of the horizontal component of current converges very fast at all depths.

Wave height variations in the North Sea and on the Norwegian Continental Shelf, 1881–1999

Analyses of overlapping Norwegian Meteorological Institute (DNMI) hindcast squared monthly mean wind speeds and monthly mean significant wave heights show a linear relation. Based on available time series of wind speed, computed from sea level pressure (SLP), this enables us to establish long and consistent time series of monthly mean significant wave heights. Data sets of monthly mean 10 m wind speeds from nine locations in the North Sea and on the Norwegian Continental Shelf have been investigated. The data sets include World Meteorological Organization (WMO) data, 1881–1982, DNMI hindcast data, 1955–1999, and National Centers for Environmental Prediction/ National Center for Atmospheric Research (NCEP/NCAR) data, 1948–1999. From these time series it is evident that mean wave heights are subject to significant variations within a variety of time scales. There is a positive trend starting in the 1960s, mainly in northern parts of the North Sea, with significant regional variation. The increase is, however, not more dramatic than the decrease which occurred from 1881 and towards the beginning of the 20th century. Analyses of annual maximum significant wave heights based on 6 h values of DNMI data, 1955–1999, strongly indicate increasing wave heights and rougher wave climate at the stations off the coast of mid-Norway. At the other stations the trends are only weakly positive or not apparent at all. r 2003 Elsevier Science Ltd. All rights reserved.

On the response of a free span pipeline subjected to ocean currents

Abstract A mechanistic study is performed to examine the coupling between the in-line and the cross-flow motion of a cylindrical structure subjected to current forces. The structure represents a free span pipeline but concerns marine risers as well. A time domain model is formulated in which the in-line and cross-flow deflections are coupled through the axial tension which in turn is computed from the pipeline prolongation at any time. This formulation introduces time dependent tensions and non-linearity into the problem. Preliminary validation of the model simulations vs. physical test data are carried out for one specific case to ensure that the sag and the in-line deflection are correctly resolved by the model. Using this as the initial condition a series of calculations are carried out to examine cross-flow induced deflections induced by an in-line prescribed deflection and vice versa. Finally, an idealistic simulation of flow induced vibration is presented. The model simulations demonstrate that the coupling varies with the mode shape and with which component it is initially introduced into. However, it is evident that the coupling effects may be significant and not negligible.

Climatic variations of oceanographic processes in the north European seas: a review of the 1970s and 1980s

Abstract Winds and hydrographic conditions are examined in the light of a suggested long-term change in wave climate in the northeast Atlantic. Time series of wind stress direction and wind power are computed for some selected positions between the southern North Sea and the central Barents Sea. These data are subsequently examined with respect to trends, variability and changes of the extreme values with time. Observed data of salinity and temperature are considered to investigate a possible signature of changes in the ocean climate. The variability found in the records, with time scales above 1 year, is suggested to be caused by changes in the volume fluxes to the considered sea areas. It is demonstrated that the fluxes and hence the hydrographic parameters are strongly coupled to the time variations of the mid-1970s is related to a counterclockwise change of the prevailing wind directions, leading to more southerly distributions over the North Sea. Some features of the wind-induced fluxes in the North Sea in recent years are pointed out, and connected to the exceptional algal blooms. Finally, the ice extension in the Barents Sea in relation to wind stress direction is also touched upon.

Formulation of a continuously stratified sea model with three-dimensional representation of the upper layer

Abstract A two-layered model is considered in which the upper layer is continuously stratified and the lower layer is homogeneous. The system is driven by atmospheric forces. Bottom stress and topography are included in the model. The linear three-dimensional hydrodynamical equations are used to describe the system. Taking the eddy viscosity in the upper layer as inversely proportional to the static stability, the dependent variables are expanded in terms of continuous functions in the vertical (eigenfunctions). Using this method it is possible to compute currents and internal displacements at any depth in the upper layer. The three-dimensional structure of the lower layer is not considered in this model. The equations describing the lower layer are integrated over depth to give depth mean currents. Using a staggered finite-difference grid in the horizontal and a forward time-stepping procedure, numerical test experiments are carried out for a cross section and for a closed rectangular basin.

Fatigue From Vortex-Induced Vibrations of Free Span Pipelines Using Statistics of Current Speed and Direction

A section of a sub sea pipeline that is suspended between two points on an uneven seafloor is often referred to as ‘a free span pipeline’. Pipelines, installed on a seabed with a highly irregular topography, may have to be designed with several free spans. If a free span is exposed to a current flow, vortex-induced vibrations (VIV) of the suspended part of the pipeline may occur. These vibrations may cause unacceptable fatigue damage in the structure. Statistical distributions of current speed and direction close to a small mountain on the seabed (approximately 20 m high and 40 m wide) are established based on full-scale measurements of the current velocity in the area. Some results from recent model tests of VIV in free span pipelines, including some tests in which the flow direction was not perpendicular to the longitudinal axis of the pipe, are shown. These results indicate that it is sufficient to use the component of the current velocity vector that is normal to the pipe when using empirical models for estimating the response due to vortex shedding. An existing empirical model for analysis of VIV [1] is extended such as to include oscillations in the same plane as the current flow (in-line VIV). The effect of including the directional variability of the current when estimating the VIV fatigue damage, using the extended VIV model on a typical free span pipeline, is demonstrated, and found to be of great importance. A parameter study, in which the length of the free span is varied, is also carried out. The conclusion from this study is that a reduction of free span length affects the parameters that govern the accumulation of fatigue damage differently. Stresses are increased, but the number of current conditions capable of inducing VIV is reduced when the length of the span is reduced. It is therefore difficult to predict whether the accumulated damage will increase or decrease when the span length is reduced, and detailed analyses are required for each particular free span and current distribution. The damage from in-line VIV is generally lower than the damage from the cross flow VIV for all but the shortest span lengths.Copyright