Ocean response to a hurricane, part II : data tabulations and numerical modeling

Field observations of the ocean's forced stage response to three hurricanes, Norbert (1984), Josephine (1984) and Gloria (1985), are analyzed and presented in a storm-centered coordinate system. All three hurricanes had a non-dimensional speed of O(1) and produced a strongly rightward biased response of the ocean surface mixed layer (SML) transport and current. The maximum layer-averaged SML currents varried from 0.8 m S-1 in response to Josephine, which was a fairly weak hurricane, to 1.7 m S.l in response to Gloria, which was much stronger. In these two cases the current amplitude is set primarly by the strength of the wind stress and its efficiency of coupling with the SML current, and the depth of vertical mixing of the SML. The Norbert case (SML Burger number ≈ 1/2) was also affected by significant pressure-coupling with the thermocline that caused appreciable upwellng by inertial pumping and strong thermocline-depth currents, up to 0.3 m S-l, under the trailing edge of Norbert. The observed SML current has a vertical shear in the direction of the local wind of up to 0.01 S-l. This vertical shear causes the surface current to be larger than the layer-averaged SML current described above by typically 0.2 m S.l.Funding was provided by the Office of Naval Research under grant No. N00014-89-J-I053

An Exercise in Forecasting Loop Current and Eddy Frontal Positions in the Gulf of Mexico

As part of a model-evaluation exercise to forecast Loop Current and Loop Current eddy frontal positions in the Gulf of Mexico, the Princeton Regional Ocean Forecast System (PROFS) is tested to forecast 14 4-week periods Aug/25/99- Sep/20/00, during which a powerful eddy, Eddy Juggernaut (Eddy-J) separated from the Loop Current and propagated southwestward. To initialize each forecast, PROFS assimilates satellite sea surface height (SSH) anomaly and temperature (SST) by projecting them into subsurface density using a surface/subsurface correlation that is a function of the satellite SSH anomaly. The closest distances of the forecast fronts from seven fixed stations in the northern Gulf over a 4-week forecast horizon are then compared against frontal observations derived primarily from drifters. Model forecasts beat persistence and the major source of error is found to be due to the initial hindcast fields

ENVIRONMENTAL FORCES ON OFFSHORE LNG TERMINALS: THE COMPLICATIONS OF SHALLOW WATER

ABSTRACT Construction of large and expensive facilities in relatively shallow water demands that additional effort be paid to the extreme environmental conditions expected there. A review of the literature on waves in shallow water shows that many processes must be considered there which are not important in deep water. Bottom friction under waves depends on the detailed bottom conditions and parameterizing it properly may require calibration to local measurements. The limits on wave heights over the nearly flat bottoms that are common in water depths of 10-30 m are poorly known. Additional laboratory and field measurements appear to be necessary before depth limited waves can be confidently specified. The structures often respond differently to wave from different directions, so directional criteria could be useful. Commonly used methods of specifying directional criteria are un-conservative, but it is possible to adjust them so that the overall reliability of the structure is preserved. PURPOSE AND SCOPE Increasing demand and lessening supply of natural gas in North America and Europe is leading toward the need for significant imports of liquefied natural gas (LNG). Regulatory and environmental concerns make it desirable to locate LNG terminal facilities offshore. In general, however, those facilities will be as close to shore as possible in order to shorten the necessary pipelines and reduce the cost of the facilities. Thus they will be in water depths just sufficient for the navigation of the LNG tankers. Over at least the last decade, the thrust of the offshore oil industry has been toward exploration and production in ever deeper waters. Consequently, most of the metocean work that has gone on in support of the industry has been to understand deep water phenomena, in particular the complexities of current profiles. This emphasis has tended to make us forget that metocean processes in relatively shallow water are often more complicated and less understood than those in deep water. Figure 1. GBS as LNG terminal Most of the proposed terminals will be in water depths of 15-30 m. In these depths, bottom friction, refraction and depth limited wave breaking are added to the processes which affect waves in deep water. If conditions are severe enough, the limits on wave height given by bottom depth and wave steepness may completely govern the design criteria. Sinc

Asymmetries in cylindrical waveguides

Cylindrical geometries with axial symmetry are often used to model physical systems. Although such a model materially simplifies calculations it may ignore significant effects arising from small asymmetries in the system. In order to demonstrate this and to examine the nature of such effects a model was analyzed consisting of a cylindrical waveguide with perfectly reflecting walls and an isolated point source displaced a small distance from the axis of the hole. The subsequent analysis shows two distinct types of arrivals associated with the geometry. Both of these produce strikingly large asymmetries in the motion of the system. These effects are clearly displayed in the closed form solutions obtained for the problem and the light which they shed on the nature of the reflection process at a cylindrical interface may well be of significant value for ray theory methods in diffraction

A wave acquisition stereo system (WASS) for the prediction of the expected highest waves and directional spectra at the oceanographic tower 'acqua alta', Venice, Italy

Issued as final reportChevron U.S.A. Inc.Forristall Ocean Engineering, Inc

Worldwide Approximations of CUrrent Profiles - JIP WACUP

The design of catenary risers, riser towers, and export lines is key to oil and gas production in the deep ocean.  Winds, waves, surface currents and current profiles are essential inputs to the design of these riser systems.  With regards to current loading, the variety and abundance of current profiles can make it difficult to determine which profiles are most appropriate for determining extreme and fatigue loading on risers.  The choices become increasingly difficult on the frontier of deepwater oil and gas developments as risers are increasingly being required to successfully operate in harsher environments and deeper water.  However, longer and more detailed current measurements from in-situ deployments continue to become available.  Two or three year current profile records are now common. In order to progress in the specification of current profiles and in the methodologies of using these profiles in the riser design, four oil & gas companies, BP Exploration Operating Company Limited, PETRÓLEO BRASILEIRO S.A. - PETROBRAS, Statoil ASA and TOTAL E&P Recherche Développement SAS, have funded the WACUP project (Worldwide Approximations of Current Profiles). The first objective of the WACUP project is to establish best practices for reducing large current profile databases into a smaller, representative set of profiles that can be used for designing risers.   Three techniques have been compared in their ability in reducing measured in-situ databases:  Empirical Orthogonal Function (EOF), Self Organizing Maps (SOM) and classical Current Profile Characterisation (CPC).  For Vortex Induced Vibration (VIV) fatigue analysis we evaluate the skill of these three techniques in estimating VIV damage to a Steel Catenary Riser (SCR).  Our measure of skill is the comparison of the results from the reduced data sets with results from the complete database.  The standard techniques are modified, improving the accuracy of the database reduction.  We recommend that any of these techniques can be made suitable for concept selection or preliminary engineering, but it may be desirable to run the full set of Gold Standard profiles for full detailed structural design. The second objective is to better understand the means to deal with extreme current profiles, in one part for fatigue damage and in another part for static loads analyses, to compare to traditional methods and to propose, if necessary, better practices.  For fatigue, due to the complexity of the relation between current profile shape and damage, extrapolation on Gold Standard damage is the only way to produce quality n-year damage return values.  Concerning extreme static response, the first analyses show that CCA profiles, used in exhaustive directions, produce realistic n-year return values. The ability of the available numerical current models to complement the in-situ measurements is also assessed.  Models are superior to measurements for providing a large scale regional interpretation of key oceanographic processes.  They can also be used to cost-effectively quantify spatial variability in the current regime.  However they must be properly validated and calibrated before use in any engineering application.  The primary source of data for characterisation of current profiles in riser design remains full water column, site specific, in-situ measurement