Modelling the uncertainties in predicting produced water concentrations in the North Sea

A random walk model has been used to compute concentration distributions of dispersed oil in the North Sea resulting from produced water discharges. This formed part of a joint study commissioned by Statoil, OLF and BP International with modelling being undertaken by SINTEF and the Brixham Environmental Laboratory. The model has been set up using predicted tidal currents from the Norwegian Meteorological Office 20-km grid three-dimensional Continental Shelf model for the year 1990. Climatology data from the North Sea have been used to define the variation of the thermocline depth at monthly intervals over the year, and wind data from the East Shetland Basin have been used to compute the vertical mixing rates. Peak concentrations of dispersed oil were predicted to be approximately 3 μg l⁻¹ in the East Shetland Basin, assuming no biodegradation; this value is consistent with the measurements of Stagg et al. (In: Reed, M., Johnsen, S. (Eds), Produced Water 2, Environmental Issues and Mitigation Technologies, Plenum Press, 1996), where the data were collected in the immediate vicinity of the discharges and the effects of degradation would be expected to be negligible.A matrix of model runs was undertaken with different parameter values for the degradation of dispersed oil, horizontal and vertical mixing, and the mixing across the thermocline. These results were used to estimate the sensitivity of the model and the uncertainty in the predicted concentration fields. The results indicate that for the areas of high concentration, a band approximately 150-km wide stretching up the centre of the North Sea, parameterisation of the vertical mixing is most important for predicting the concentration levels. For areas more remote from the discharges and closer to the land masses, the degradation rate of material has most effect on predicted concentrations.     

Oil production optimization solved by piecewise linearization in a Branch & Price framework

This paper presents a method for optimizing oil production on large scale production networks such as the Troll west field in the North Sea. The method is based on piecewise linearization of all nonlinearities, and on decomposition of the full scale problem into smaller subproblems. Column generation in a Branch & Price framework is used to solve the decomposed problem. The method differs from most Branch & Price methods by branching only on continuous quantities and by solving the subproblems using commercial MILP software.     

Comparison of the performance of five different water quality models of the North Sea

During an international workshop, held within the auspices of the ASMO (Assessment and Monitoring) group of OSPARCOM (Oslo and Paris Commissions), the performance of five different water quality models of the North Sea was compared. Selected substances were cadmium, PCB-153, two PAHs (fluoranthene and benzo[a]pyrene) and atrazine. However during the workshop it appeared that it was only possible to compare the cadmium results of five models. The modelled concentrations were validated with a common field data set in different areas of the North Sea. Also the results of the responsiveness to a 50% reduction in the input were compared. In general it appeared that there is a lack of quality controlled field data to calibrate and validate the models. It was remarkable that the calculated concentrations of cadmium were low compared to the measured data. The performance of the models in the 50% reduction scenario appeared to be sensitive for the boundary conditions.     

Identification of gas flares in the North Sea using satellite data

This paper describes the identification of oil production platforms responsible for flaring off gas in the North Sea, using data received from the Advanced Very High Resolution Radiometer (AVHRR) carried on the polar-orbiting weather satellites TIROS-N, NOAA-6 and NOAA-7. A selected area of each of several AVHRR scenes of the North Sea and surrounding coastline has been rectified and the geographical co-ordinates of the gas flares ascertained. These have been compared with a map of platform locations. In most cases the identification of the platform responsible for producing the flare is unambiguous.     

Flow analysis of a ribbed helix lip seal with consideration of fluid–structure interaction

Ribbed helix lip seals for rotating shafts have been widely used to retain oil and exclude contaminants in many applications throughout the industry. The objective of this study is to better understand the basic flow behavior associated with the pumping process of a ribbed helix lip seal. The theoretical model consists of a flow analysis of the lubricating film of the hydraulic fluid in conjunction with a stress analysis of the lip seal distortion. The complicated mechanical interaction between the oil flow and rubber deformation was simulated using a coupled fluid–structure approach implemented in a commercial computational fluid dynamics (CFD) code ESI-CFD, ACE+®. The flow characteristics and rubber deformation around a ribbed helix lip seal were fully resolved in a pumping-rate test environment, where both air and oil sides were filled with oil initially. The three-dimensional pressure field solved by the model via the coupled flow-stress analysis was compared with the predictions obtained from the model via the nondeformable rubber assumption to elucidate the significant effect of the fluid–structure interaction on accurate simulation of the oil pumping behavior. In the rotating speed ranging from 1000 to 6000 rpm, both measured and calculated pumping rates increase with the shaft speed for a ribbed helix lip seal. As compared to the baseline case, calculations with considering the fluid–structure interaction at higher rotary speeds can result in thicker oil films, and in turn produce greater pumping rates.     

A Geosat-derived noise spectrum for detecting seamounts in the Arabian Sea

Abstract

Noise power spectral densities (PSDs) have been derived from the analysis often seamount-free Geosat tracks in the Indian seas. The average noise PSD has been fitted by a model function. The model noise PSD has been used in conjunction with a model seamount signature to construct a matched filter for detecting seamounts in the Arabian Sea. Performance of the technique has been verified by successfully detecting the Somali ridge and the Error Seamount which are located in the Arabian Sea. Several uncharted seamounts have also been predicted.     

Non-linear features related to the dual-frequency technique in maritime applications

Abstract

Field measurements of non-linear characteristics, related to the dual-frequency technique applied to the ocean surface, were carried out in the North Sea. The data observations were made with a CW multi-frequency C-band radar onboard the offshore research platform ‘Forschungsplattform Nordsee’. It is verified that the cross-product spectrum contains additional information on distinct non-linear features, when compared to existing theories previously published in the literature. In particular, a relatively sharp second order Bragg resonance line is found on the right-hand side of the pronounced first order peak. This Doppler line occurs, within error bars, in the spectrum at √2 times the first order resonance frequency

To substantiate this observational feature two alternative models are presented which separately describe non-linear hydrodynamical and scattering mechanisms. Both models are based on a locally homogeneous and stationary ripple wave field. The results from the field experiment have been examined and found to provide firm evidence supporting both proposed models. Furthermore, for longer matched wavelengths (typically > 17 m) energetic lines with very small Doppler shift (<0·1 Hz) were observed. This slowly propagating phenomenon is empirically deduced to be associated with wave group structures.     

All at sea: An ergonomic analysis of oil production platform control rooms

Control rooms on offshore production platforms are the focal point for their safe and efficient operation. Following the Piper Alpha disaster in 1988 a sizeable body of safety literature was generated covering the Ergonomic/Human Factors issues then in play. More than twenty years have passed since that time and significant changes have occurred to how control rooms are manned and the technology now in use. As the North Sea oil industry in the UK enters a new phase in its life cycle, and becomes subject to unprecedented production and cost pressures, it is time to revisit these issues. This paper reports on a high-level ergonomic survey covering approximately a third of all North Sea control rooms. The focus is on the adaptive capacity of the highly experienced control room operators and the current challenges to that capacity. Areas of concern include the support provided for dealing with non-routine events, the persistent issue of ‘alarm overload’, the flexibility and control of current SCADA systems, the use of control rooms for non-related tasks, and the possible role of non-technical skills training.     

Atmospheric correction algorithm for MERIS above case‐2 waters

The development and validation of an atmospheric correction algorithm designed for the Medium Resolution Imaging Spectrometer (MERIS) with special emphasis on case‐2 waters is described. The algorithm is based on inverse modelling of radiative transfer (RT) calculations using artificial neural network (ANN) techniques. The presented correction scheme is implemented as a direct inversion of spectral top‐of‐atmosphere (TOA) radiances into spectral remote sensing reflectances at the bottom‐of‐atmosphere (BOA), with additional output of the aerosol optical thickness (AOT) at four wavelengths for validation purposes. The inversion algorithm was applied to 13 MERIS Level1b data tracks of 2002–2003, covering the optically complex waters of the North and Baltic Sea region. A validation of the retrieved AOTs was performed with coincident in situ automatic sun–sky scanning radiometer measurements of the Aerosol Robotic Network (AERONET) from Helgoland Island located in the German Bight. The accuracy of the derived reflectances was validated with concurrent ship‐borne reflectance measurements of the SIMBADA hand‐held field radiometer. Compared to the MERIS Level2 standard reflectance product generated by the processor versions 3.55, 4.06 and 6.3, the results of the proposed algorithm show a significant improvement in accuracy, especially in the blue part of the spectrum, where the MERIS Level2 reflectances result in errors up to 122% compared to only 19% with the proposed algorithm. The overall mean errors within the spectral range of 412.5–708.75 nm are calculated to be 46.2% and 18.9% for the MERIS Level2 product and the presented algorithm, respectively.     

New Applications

After more than two years of planning, design, and fabrication, acceptance tests are in progress on more than $1-million worth of equipment that will become the computer control/telemetry system for the huge Ekofisk oil fields in the North Sea.     

Application of the regularized shallow water equations for numerical simulation of seiche level oscillations in the Sea of Azov

A mathematical model for calculating the currents in the sea area scale was developed for the first time within an algorithm of regularized shallow water equations. The model and the numerical algorithm are described as applied to the topology and natural features of the Sea of Azov. The results of the calculations of hydrodynamic currents in the Sea of Azov in the presence of typical seiche waves caused by tidal or wind influences are presented.     

Island connected sea bed signatures observed by multi-frequency synthetic aperture radar

Multifrequency X-, C-, and L-band synthetic aperture radar (SAR) images of the northern sea area off the isle of Heligoland in the German Bight of the North Sea have been analysed. The data were collected during the SAR and X-band Ocean Nonlinearities Research Platform North Sea Experiment (SAXON-FPN) which was carried out in November, 1990. Different oceanographic phenomena are visible on simultaneously obtained SAR images. Wind streaks and a vortex street can be identified only on the X- and C-band SAR images. Elongated streaks of predominantly low radar return are related to nearshore reefs and are imaged on all available radar scenes. The imaging mechanism of these submarine ridges is investigated and discussed using with some modifications the simple Bragg relaxation model proposed by Alpers and Hennings. The improved model differs from the original version in the representation of the unperturbed wave-energy spectral density. Also the advection term and the phase modulation (velocity bunching) have been included in the model. Due to the improvements it is now possible to simulate the radar cross-section modulation with the same order of magnitude at 0.4 GHz (X-band) and 5.3GHz (C-band) as well as at 1.3 GHz (L-band). However, the simulated radar cross-section modulation is still underestimated compared with the SAR data, but the phase of the calculated radar cross-section modulation agrees quite well with the SAR measurements.     

Mapping bottom depth and albedo in coastal waters of the South China Sea islands and reefs using Landsat TM and ETM+ data

Optical models for the retrieval of shallow water bottom depth and albedo using multispectral data usually require in situ water depth data to tune the model parameters. In the South China Sea (SCS), however, such in situ data are often lacking or obsolete (perhaps from half a century ago) for most coastal waters around its islands and reefs. Here, we combine multispectral data collected by MODIS and Landsat to estimate bottom depth and albedo for four coral reef regions in the SCS, with results partially validated by some scarce in situ data. The waters in these remote regions are oligotrophic whose optical properties can be well derived from Moderate Resolution Imaging Spectroradiometer (MODIS) measurements when the waters are optically deep. The MODIS-derived optical properties are used to estimate the water column attenuation to the Landsat measurements over shallow waters, thus eliminating the requirement of model tuning using field measured water depths. The model is applied to four Landsat Thematic Mapper (TM) and Enhanced Thematic Mapper Plus (ETM+) images covering Pratas Atoll, Woody Island, Scarborough Shoal, and North Danger Reefs. The retrieved bathymetry around Pratas Atoll and North Danger Reefs are validated with some in situ data between 1 and 25 m. The relative difference and root mean square difference between the two measurements were 17% and 1.6 m, for Pratas Atoll and 11% and 1.1 m for North Danger Reefs, respectively. These results suggest that the approach developed here may be extended to other shallow, clear waters in the SCS.     

Clean seas: a North Sea test site

The Clean Seas project focused on the role that existing Earth observing satellites might play in monitoring marine pollution. Results are presented here from August 1997, for the North Sea test site, using sea surface temperature (SST), colour and synthetic aperture radar (SAR) images in conjunction with a hydrodynamic model. There was good correlation between data sources, e.g. between SST and ERS-2 SAR images. Both datasets showed the development of fine plume structures close to the Rhine outflow, apparently associated with the outflow, and possibly caused by tidal pulsing of the Rhine Plume.

The model reproduced general temperature and sediment distributions well, but fine structures were not reproduced. Model sediment distribution patterns were verified using ‘chlorophyll concentration’ data from colour sensors, representative of sediment concentration in turbid water. In conjunction with the visible channels of the Advanced Very High Resolution Radiometer and Along-Track Scanning Radiometer, they give an uncalibrated measure of the sediment load. The model gives a more complete picture of the temporal dispersion of the Rhine Plume over time than is evident from the remotely sensed data alone.     

一种新的材料微结构电磁场效应数值模拟方法

为了定量揭示材料微结构与入射电磁波引起的材料内电磁场效应之间的内在关联性,借助可以表征能量流动方向和大小的Poynting矢量,通过程序设计结合有限元数值模拟的方法实现了材料微结构电磁场效应的计算。建立了材料微结构电磁场效应数值分析有限元模型。在数值计算中,将Poynting矢量正交分解为三个不同方向分量,计算了材料微结构的吸收、透射和波源方向反射效应;根据Poynting矢量在反射方向的分量,计算了任意反射方向的反射效应。研究方法对任意微结构均具有适用性。     

Analysis of the interaction between the module structures and the deck of an offshore oil production platform

A matrix method of analysis of the interaction between the module structures and the deck of an offshore oil production platform is presented. The interaction problem does not appear to have been given adequate attention before and as a result, design problems have occurred in some platforms currently under construction for the North Sea. The proposed method of analysis is very simple and is suitable for use in practical design office work.     

Air-Pollution Monitoring Network in the Netherlands

Athough the Netherlands is only a small country, it has the world's largest port in terms of annual tonnage. This is Rotterdam, situated in the area where the rivers Rijn and Maas loin, prior to reaching the North Sea. Their estuary forms a complex waterway; the main one being the "New Waterway," shown on the map Figure 1. South of this is a complex of harbours and industrial areas embracing six oil retineries, chemical plants, shipyards, metal works, etc. The residential areas within this complex suffer tfrom the air pollution associated with heavy industry.     

A two-look technique for studying atmospheric effects in optical scanner data for the ocean

Abstract

The traditional method for validating calculated values of chlorophyll concentration in the sea is based on comparisons with in situ point measurements made from ships. In this paper we present an alternative approach based on a two-look method involving data from successive orbits of the NIMBUS-7 satellite. We present the results of some calculations based on the use of Coastal Zone Colour Scanner (CZCS) data for the Irish Sea viewed from two successive orbits of the NIMBUS-7 satellite. The method used involves a first attempt at a self-consistent approach to calculating the aerosol path radiance. Results are presented for two pairs of scenes from 6 April 1980 and 4 May 1980.     

Assessing the effect of hydrocarbon oil type and thickness on a remote sensing signal: A sensitivity study based on the optical properties of two different oil types and the HYMAP and Quickbird sensors

We measured the light absorption properties of two naturally occurring Australian hydrocarbon oils, a Gippsland light crude oil and a North West Shelf light condensate. Using the results from these measurements in conjunction with estimated sensor environmental noise thresholds, the theoretical minimum limit of detectability of each oil type (as a function of oil thickness) was calculated for both the hyperspectral HYMAP and multispectral Quickbird sensors. The Gippsland crude oil is discernable at layer thickness of 20 µm or more in the Quickbird green channel. The HYMAP sensor was found to be theoretically capable of detecting a layer of Gippsland crude oil with a thickness of 10 µm in approximately six sensor channels. By contrast, the North West Shelf light condensate was not able to be detected by either sensor for any thickness up to 200 µm. Optical remote sensing is therefore not applicable for detecting diagnostic absorption features associated with this light condensate oil type, which is typical of the chemistry of many hydrocarbon oils found in the Australian Northwest Shelf area and condensates world wide. We conclude that oil type is critical to the applicability of optical remote sensing for natural oil slick detection and identification. We recommend that a sensor- and oil-specific sensitivity study should be conducted prior to applying optical remote sensors for oil exploration.The oil optical properties were obtained using two different laboratory methods, a reflectance-based approach and transmittance-based approach. The reflectance-based approach was relatively complex to implement, but was chosen in order to replicate as closely as possible real world remote sensing measurement conditions of an oil film on water. The transmittance-based approach, based upon standard laboratory spectrophotometric measurements was found to generate results in good agreement with the reflectance-based approach. Therefore, for future oil- and sensor-specific sensitivity studies, we recommend the relatively accessible transmittance-based approach, which is detailed in this paper.