Dutch Continental Shelf Research Articles

On the parameterization of biological influences on offshore sand wave dynamics

The bed of the North Sea is covered by sand waves and houses a great number of macrobenthic animals. These bio-engineers are known to have a significant influence on the stability of the bed and thereby on the geomorphology of the seabed. This paper proposes a parameterization of these bio-geomorphological interactions. Given the abundance of three dominant bio-engineers on the Dutch Continental Shelf, the predicted occurrence of sand waves, in which the parameterization is included, shows significantly better results, compared to the prediction for the default case without biology. Therefore, the inclusion of biological activity could be important to predict the occurrence of sand waves.

Inverse Shallow-Water Flow Modeling Using Model Reduction

The idea presented in this paper is variational data assimilation based on model reduction using proper orthogonal decomposition. An ensemble of forward model simulations is used to determine the approximation of the covariance matrix of the model variability, and only the dominant eigenvectors of this matrix are used to define a model subspace. An approximate linear reduced model is obtained by projecting the original model onto this reduced subspace. Compared to the classical variational method, the adjoint of the tangent linear model is replaced by the adjoint of a linear reduced forward model. Thus, it does not require the implementation of the adjoint of the tangent linear model. The minimization process is carried out in reduced subspace and hence reduces the computational cost. Twin experiments using an operational storm surge prediction model in the Netherlands, the Dutch Continental Shelf Model are performed to estimate the water depth, with the findings that the approach with relatively little computational cost and without the burden of implementation of the adjoint model can be used in variational data assimilation.

Description of the long-term (1991–2005) temporal and spatial distribution of phytoplankton carbon biomass in the Dutch North Sea

Since the beginning of the 1990s phytoplankton species composition and abundance have been monitored at a high frequency (bi-weekly in the growing season and monthly in winter) at a number of fixed stations on the Dutch Continental Shelf, of which 18 are used in this study. Phytoplankton carbon biomass has been calculated from species-specific biovolume/cell data and summed over all species per functional group enumerated in the samples. The species are divided into four functional groups i. e. diatoms, flagellates, autotrophic and mixotrophic dinoflagellates and Phaeocystis spp. The total number of phytoplankton samples analysed up to and including 2005 is almost 4000. The annual mean phytoplankton biomass over all stations remained stable at around 145 mg C m − 3 . However, the phytoplankton composition has changed significantly, with increases in diatoms and dinoflagellates and compensating decreases in flagellates and Phaeocystis. With increasing distance from the shore, coinciding with a decrease in nutrient availability and increasing water depth, total phytoplankton biomass as well as the biomass of diatoms, flagellates and Phaeocystis spp. decreased. This pattern was not true for the dinoflagellates, which occurred at more or less the same biomass throughout the region. Stations near river mouths and in the Wadden Sea outlets had much higher phytoplankton biomass than stations further from freshwater discharges. The data, split in two periods (1991–1998) and (1999–2005) and averaged over the whole Dutch Continental Shelf, had been aggregated into seasonal biomass distributions. The seasonal phytoplankton biomass distribution was unimodal in both periods, with similar spring maxima of around 300 mg C m − 3 . The spring maximum occurred one month earlier, in April, in the second period. Phaeocystis over the whole study period remained the dominant near-shore species as it has been since the first phytoplankton observations at the end of the 19th century.

Sandbank occurrence on the Dutch continental shelf in the North Sea

Sandbanks, the largest of bed patterns in shallow sandy seas, pose a potential risk to shipping. They are also valuable elements of natural coastal protection, dissipating the energy of waves. In the Southern Bight of the North Sea, several sandbank areas have been reported in the literature. However, based on an objective crest–trough analysis of the bathymetry of the Dutch continental shelf, the present study shows that sandbanks are more widespread than commonly considered. These banks are relatively low, presumably explaining why they have not been documented before. This widespread occurrence of sandbanks in the North Sea is in agreement with theoretical predictions based on stability analysis techniques. The possible interference between large-scale human activity and low-amplitude open-shelf ridges implies that one should be careful not to overlook these patterns if none should appear in a preliminary (visual) assessment. The only part of the Southern Bight in which no ridges can be seen is a circular area with a diameter of about 50 km near the mouth of the river Rhine. Here, freshwater outflow affects the direction of tidal ellipses and residual flow, and suppresses the formation of open ridges.

Impact of the oil price and fiscal facilities on offshore mining at the Dutch Continental Shelf

The surge in the oil price has raised questions about the magnitude of global reserves of oil. According to some analysts, the current high oil prices indicate a looming decline in the global production of oil. Others believe, however, that the increased level of the oil price encourages exploration and production activities, bringing the oil price to a lower equilibrium level in the near future. In this paper, we assess the impact of the unit profit (depending on the oil price) as well as fiscal facilities on the level of exploration and development drillings in the Dutch Continental Shelf. We conducted an econometric analysis of exploration and development drillings in the Dutch Continental Shelf over the period 1981–2003. Except a few fiscal changes, the regulatory framework for offshore activities in the Netherlands, the so-called “small fields policy” was unchanged in this period. We find that the expected unit profit based on a moving average of the oil price significantly explains the level of both exploration and development drillings. In addition, the analysis suggests that fiscal facilities have only a temporary effect on exploration activities but are more important for development activities. We conclude that the oil price is a major economic incentive for activities of the mining industry.

The distribution of macrozoobenthos in the southern North Sea in relation to meso-scale bedforms

This study investigates the distribution of macrozoobenthos in relation to meso-scale bedforms in the southern North Sea. Three sites on the Dutch Continental Shelf were sampled that are representative of large areas of the North Sea and show diverse morphological settings. These sites are (i) part of a shoreface-connected ridge, (ii) the lower part of a concave shoreface and (iii) a sandwave area. Within these sites, two or three different morphological units were distinguished. Sampling was undertaken in two seasons for two consecutive years. The species composition was analysed for differences in benthic assemblage within sites, based on the meso-scale morphology, and between sites and seasons. In addition, the benthic assemblage was correlated to water depth, median grain size (D50) and sorting (D60/D10). Results show that significant differences in the benthic assemblage can be found related to meso-scale bedforms, but macro-scale morphological setting and seasonal effects are more important.

Operational storm surge forecasting in the Netherlands: developments in the last decade

The accurate forecasting of storm surges is an important issue in the Netherlands. With the emergence of the first numerical hydrodynamic models for surge forecasting at the beginning of the 1980s, new demands and possibilities were raised. This article describes the main phases of the development and the present operational set-up of the Dutch continental shelf model, which is the main hydrodynamic model for storm surges in the Netherlands. It includes a brief discussion of applied data-assimilation techniques, such as Kalman filtering, the model calibration process and some thoughts on quality assurance in an operational environment. After further describing some select recent investigations, the paper concludes with some remarks on future developments in a European context.

Mathematical modelling of sand wave migration and the interaction with pipelines

A new method is presented for identifying potential pipeline problems, such as hazardous exposures. This method comprises a newly developed sand wave amplitude and migration model, and an existing pipeline–seabed interaction model. The sand wave migration model is based on physical principles and tuned with field data through data assimilation techniques. Due to its physical basis, this method is trusted to be more reliable than other, mostly engineering-based methods. The model describes and predicts the dynamics of sand waves and provides the necessary bed level input for the pipeline–seabed interaction model. The method was tested by performing a hindcast on the basis of survey data for a specific submarine gas pipeline, diameter 0.4 m, on the Dutch continental shelf. Good agreement was found with the observed seabed–pipeline levels. The applicability of the method was investigated further through a number of test cases. The self-lowering of the pipeline, in response to exposures due to sand wave migration, can be predicted, both effectively and efficiently. This allows the use of the method as a tool for pipeline operation, maintenance and abandonment.

Spatial patterns of the hyperbenthos of subtidal sandbanks in the southern North Sea

The hyperbenthos of the Belgian continental shelf, the Dutch continental shelf off Zeeland and the Westerschelde estuary was sampled. Seven biotic communities were identified and the distribution was correlated with the presence of isolated sandbanks. A community in the Westerschelde estuary was different from the shelf hyperbenthos and was characterised by the highest density and biomass and lowest diversity. Two gradients in community structure were detected on the continental shelf: the principal onshore-offshore gradient perpendicular to the coastline and a less pronounced east-west gradient parallel to the coastline. The first gradient mainly indicated differences in density and biomass, while the second reflected species richness and diversity. The holohyperbenthos reached highest densities onshore and consisted nearly exclusively of mysids; Schistomysis spiritus and S. kervillei were the most common species. The high levels of suspended matter and mud in this area probably advantage this motile fauna in terms of food. Community composition altered with distance from the shore and planktonic species increased in abundance. Estuarine influence may have caused a decline of species richness eastward. The merohyperbenthos was most abundant at a certain distance from the coast and was mainly composed of a variety of larval decapods. Biomass peaked onshore with a dominance of postlarval gobies. A strong diversity gradient was found for the merohyperbenthos with a richer fauna off the Belgian coast than in the waters off Zeeland. Current direction and habitat heterogeneity are believed to be the most important structuring factors.

Mortality in megafaunal benthic populations caused by trawl fisheries on the Dutch continental shelf in the North Sea in 1994

We estimated the direct mortality of benthic fauna caused by one single passage of commercial beam and otter trawls in field experiments. The benthos dredge Triple-D was used to sample megafauna (>1 cm), while macrofauna (>1 mm) were sampled by means of a Reineck boxcorer and, in some cases, a van Veen grab. Direct mortalities ranging from about 5 up to 40% of the initial densities were observed for a number of gastropods, starfishes, small and medium-sized crustaceans, and annelid worms. For bivalve species, direct mortalities were found from about 20 up to 65%. Mortality per m 2 trawled area due to fishing with a 12-m beam trawl was not higher than that due to a 4-m beam trawl. For all species considered, the direct mortality was largely attributed to animals that died in the trawl track, either as a direct result of physical damage inflicted by the passage of the trawl or indirectly owing to disturbance, exposure, and subsequent predation. In 1994, the 12 m beam trawl with tickler chains was the dominant gear type in the Dutch sector, resulting in a mean annual trawling frequency of 1.23. The mean annual trawling frequencies with the 4 m beam trawl using tickler chains, the 4 m beam trawl with a chain mat, and the otter trawl were 0.13, 0.01, and 0.06, respectively. The annual fishing mortality in invertebrate megafaunal populations in the Dutch sector ranged from 5 up to 39%, with half of the species showing values of more than 20%. For all species studied, the 12 m beam-trawl fisheries caused higher annual fishing mortalities than the concerted action of the other fisheries. Only with respect to species restricted to sandy coastal areas did the 4 m beam-trawl fleet contribute substantially to the annual mortality. Implications of the impact of trawling on the composition of benthic communities are discussed. 2000 International Council for the Exploration of the Sea

A modelling study of tidally induced equilibrium sand balances in the North Sea during the Holocene

The present paper describes a depth-integrated modelling approach for tide-induced sediment transport, and its application to the North Sea for various cases during the Holocene sea level rise. The flow model is based on the shallow water equations, while the transport model is based on Galappatti’s asymptotic approach for rivers, now applied to tidal flow situations in two horizontal dimensions ( Galappatti, 1983, Galappatti and Vreugdenhil, 1986). The empirical Engelund–Hansen formula for sedimentation / erosion is used as a closure for the sediment exchange term in the transport equation, while variation of the bed height is neglected. The various assumptions in the derivation of the sediment model, and their consequences are addressed step by step. The North Sea area considered in the application is the whole North west European continental shelf, as mapped onto the Dutch Continental Shelf Model. In a series of cyclic period simulations of the present day situation, the sensitivity to parameter variations such as diffusion, equilibrium concentration and adaptation time is analysed, as well as the effect of bathymetry smoothing. Using a suitable and fixed parameter setting, the North Sea tidally induced equilibrium sand balances are simulated for several epochs during the Holocene sea level rise. Both uniform sea level reductions (0, −5, −15, and −35 m. and non-uniform sea level reductions (8500 BP, 8100 BP are considered, based on Jelgersma’s geological uniform sea level curve, and glacio-hydro-isostatic rebound modelling results, respectively. It is concluded that the derived Galappatti model for 2D tidal flow is a feasible approach to get a first assessment of equilibrium sedimentation balance on North Sea scale, and of its possible changes due to the Holocene sea level rise under various simplified assumptions regarding the evolution of coastline and bathymetry, both uniform and non-uniform.

Investment Analysis of Offshore Concessions in the Netherlands

textabstractDescribes the valuation of a complex capital project, the staged development of an oil field concession in the Netherlands. Stages of offshore petroleum development on the Dutch continental shelf; Asset valuation based on replication in financial markets; Major insights and conclusions.

Impact of a wind climate change on the surge in the southern North Sea

The aim of this paper is to gain quantitative insights into the surge-impact of a possible change of the wind climate in the southern part of the North Sea. The approach taken is based upon an initial review of the results of General Circulation Model scenarios for climate change under greenhouse forcing and of their likely effects upon wind climate. Two scenarios of possible wind climate change are formulated. Scenario 1 assumes a northward shift of the entire wlnd climate system in the southern part of the North Sea; scenario 2 describes an increase of the intensity of storms in this area. To model these scenarios, the concept of 'parametric storms' is presented, which considers the character~stlcs of a storm to be conditional upon a limited number of parameters. Applying this concept to the well-known 1953 storm yields promising results. On the b a s ~ s of this parametric 1953 storm, a large set of storms which are assumed to be consistent with the present (surge generating) wind climate in the southern part of the North Sea is generated. Because these storms do not all have equal chances of occurrence, this set is supplemented by a probability distribution. The future wind climate, which might develop as a result of the 2 wind climate change scenarios, is then modelled by changing this probability distribution and/or some parameters of the storm set. The surge-impact of the 2 modelled scenarios of wind climate change is determined using the Dutch Continental Shelf Model, which describes the depth-averaged tidal flow and storm surges in the North Sea and surrounding areas. For several locations in the southern part of the North Sea, the model output is analyzed and presented in terms of a frequency distribution of storm surge levels. With respect to scenario 1, these simulation results show that a relatively large northward shift of the entire wind climate system in the southern part of the North Sea will only have a small impact on the storm surges in this area In the case of scenano 2, however, the results show that a relatively small increase of the intensity of depressions in the southern North Sea will result in a relatively large impact on the storm surges in this area.

On the short-term and long-term impact of drilling activities in the Dutch sector of the North Sea

A research programme on the effects of drill-cutting discharges on the benthic system around platforms on the Dutch continental shelf has been running since 1985, with emphasis on locations where oil-based drilling muds (OBM) have been used and discharged. A few locations where only water-based muds (WBM) have been used were also investigated. At OBM locations, elevated total hydrocarbon concentrations in the sediment occurred up to 750-1000 m from well sites during the first year after discharges had stopped. Concentrations tended to decrease to natural background levels during the following years at distances ≥500 m from the platform, but remained far higher than background levels even after 8 yr within a few hundred metres. One year after drilling, biological effects of OBM discharges were detectable up to ≥1000 m by reduced abundances of a few very sensitive species (particularly Echinocardium cordatum and Montacuta ferruginosa). Closer to well sites, increasing numbers of species experienced adverse effects. In the longer term, the macrofauna seemed to recover at distances ≥500 m, but within that range the macrofauna was still affected after 8 yr. Possible effects of WBM discharges were investigated at a few single-well sites during surveys carried out within 2 mo to 1 yr after discharges were terminated. Adverse effects on the benthic community were not observed, even within 25 m of a discharge site.

Well Construction Team Approach Creates a New Future for One Company's Operations on the Dutch Continental Shelf

Summary Rising costs and low energy prices were inhibiting the development of marginal fields on the Dutch Continental Shelf, resulting in a reduction in drilling activity and a loss of business opportunities. Independent benchmark surveys and critical analysis of total well costs showed the potential for major cost reductions that could turn many of the smaller offshore gas finds into economic development prospects. This triggered a change from traditional operating and contracting strategies to a partnering approach with the principal drilling and service contractors and the institution of a well-construction team (WCT). The WCT proved so successful, with an average well cost reduction of 21% in the first year, that a similar organization has been set up for all the land drilling and workover operations. This paper describes the strategies used and presents a comparison between the traditional performance and the results from the first year of operating as a WCT. The comparison includes the results of an independent benchmark study of 20 North Sea operating companies. Introduction The operator's Well Engineering Dept. delivered 22 offshore wells of various types during 1992 at a cost of $260 million. Subsequent remedial work with associated production deferment cost a further $60 million, giving a total cost to the operator of $320 million. Although this was considered to be representative of the general industry performance on the Dutch Continental Shelf, these cost levels made many of the smaller gas finds subeconomic development prospects. This resulted in a reduction in activity from five rigs in 1992 to two rigs in 1993. Further reductions in drilling activity were likely in the following months unless major cost savings could be realized. The prospect of minimal activity threatened both the contractors' survival and the continuation of the operator's offshore activities. These threats dictated a need for a radical change in strategy. The following options for achieving this change were considered by the operator:terminating the current contracts and retendering in order to obtain lower rates oraggressively pursuing improvements in all elements of the well-construction process while retaining the existing contracts. The latter option was selected as the only sure way of delivering the "best -value" wells to the operator while providing the contractors with the profit margins necessary to sustain and improve their business into the future. This decision was supported by an analysis of the 1992 well costs, which indicated that, in an ideal world, the total costs could be halved by a combination of optimization of well design and working practices, the development and application of new technology, and avoiding workovers through improved quality.

Protected areas in the North Sea: An absolute need for future marine research

There are many signals that different human activities affect the marine ecosystem on local and sometimes regional scales. There is evidence that in the Dutch sector of the North Sea at least 25 species have decreased tremendously in numbers or have totally disappeared. But what has caused their disappearance: fisheries, pollution, eutrophication, climatic changes, or a combination of causes? On the Dutch Continental Shelf, the fisheries are now so intensive that every square metre is trawled, on an average, once to twice a year. Furthermore, it has been shown that trawling causes direct damage to the marine ecosystem. This indicates that the “natural” North Sea ecosystem we are studying is already a heavily influenced system. And what is the value of data on the diversity and production of benthic animals, if the research area has been raked by beamtrawl gear an unknown amount of times before sampling? To be able to study the natural trends in the marine ecosystem, or to answer the question which human activity has most influenced the ecosystem, there is an absolute and immediate need for protected areas to be established. The size of the protected areas must be determined by the behaviour of that species characteristic for the area. In such areas, where fisheries and local pollution would be forbidden or very limited, scientific research into the species composition and age distribution of different populations should be carried out and trends should be established.

Elemental composition of aircraft-sampled aerosols above the southern bight of the North Sea

During the period from September 1988 to October 1989, 23 flights were carried out over the Dutch Continental Shelf of the North Sea and a total of 108 aerosol filter samples were collected. The samples were analyzed for Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, Br and Pb using both particle-induced X-ray emission (PIXE) and energy-dispersive X-ray fluorescence (EDXRF). The results from PIXE exhibited better precision and lower detection limits than those from EDXRF. Therefore the further discussion and interpretation was based on the PIXE data only. It was observed that Si, S, V, Mn, Ni, Cu and Pb were enriched with respect to the earth's crust. It was seen that winds from the sector Southeast-South contributed most significantly to the total aerosol concentration. The vertical profiles of several elements indicated that their concentration increases with proximity to the sea. Principal factor analysis on the data matrix containing elemental concentrations, height and wind direction parameters allowed us to identify 6 possible aerosol sources, namely, a composite of CaSO4 and metallurgical activities, refuse incineration, residual oil combustion, quartz, soil dust and sea-salt aerosol.

Influence of Salt Structures on Reservoir Rocks in Block L-2, Dutch Continental Shelf

Influence of Salt Structures on Reservoir Rocks in Block L-2, Dutch Continental Shelf

Macro-Pollutants on the Dutch Continental Shelf

According to national and foreign literature it was known that the great variety and magnitude of macro-pollutants creates a serious problem for the marine environment. Several effects of plastics on marine life are known e.g. mammals entangled in fishing nets and plastic ingestion by birds. Macro-pollutants are all the objects floating in and on the water surface, and which are present on the bottom of the North Sea. Their size varies from several centimetres to approximately one metre. About the origin of the macro-pollutants there is no absolute certainty but one can assume that a major source is shipping and the dumping of dredged material. Apart from a small research in 1972 there were no recent data available. Therefore in 1987 and 1988 a survey was carried out to determine the amount of macro-pollutants present on the bottom of the Dutch Continental Shelf. This survey forms part of the regular monitoring activities for biotic and abiotic parameters (e.g. fish diseases and water quality). The survey was carried out with a vessel of the Dutch Coast Guard with a bottom trawl dredge net. The areas that were examined are the same as those used for the examination of fish diseases.

Reservoir Simulation of Horizontal Wells in the Helder Field

Summary. Results of a reservoir simulation study of horizontal wells in a field that is entirely underlain by water and has a highly unfavorable mobility ratio are compared with about 4 years of actual field data. Finely gridded single-well models were used to investigate the short-term performance of the horizontal wells and to generate pseudofunctions for performance of the horizontal wells and to generate pseudofunctions for use in the field model. Introduction The Helder oil field, on the Dutch Continental Shelf, was brought on production 1982 and developed with 13 conventional wells. To improve oil recovery and to minimize water coning, the field was redeveloped during 1987 and 1988 with the drilling of 10 horizontal wells by sidetracking from existing wells. The simulation study described here was initiated to answer the following questions posed as a result of observing the performance posed as a result of observing the performance of the horizontal wells.Is the water-cut performance adversely affected by producing the wells at high gross rates?What effects do various well and reservoir parameters have on short-and long-term well parameters have on short-and long-term well behavior? Parameters to be considered are horizontal-well length, height above the water/oil contact (WOC) reservoir heterogeneity, localized vertical permeability barriers, and drainage area shape.Is water production distributed evenly along the horizontal wellbore, or does it enter preferentially at one or more intervals?Can production performance be improved by water shutoff treatments on existing wells or by modified completion design in future wells?Can the ultimate recovery be improved with additional infill horizontal wells? Field Description. The Helder field is an Block Q/1 of the Dutch North Sea, about 62 miles northwest of Amsterdam in 85 ft of water. The structure is a slightly faulted anticline at a depth of 4,600 ft. Formation dip along the northwest/southwest axis of the field is generally less than 1 degree. The field is underlain by water over its entire 1,140 acres of closure and has a maximum oil column of 131 ft. The reservoir is supported by a strong aquifer. Two other fields, Helm and Hoorn, brought on production at the same time as Helder, share the same aquifer. The original oil in place us estimated to be 72 × 10(6) STB. Oil gravity is 22 degrees API, and initial viscosity at reservoir conditions is 30 cp. Production is from the Vlieland sand, which has permeabilities ranging from 500 to 6,000 md. Production History With Conventional Wells. Production History With Conventional Wells. Helder was put on production in Oct. 1982. During 1982–84, the field was developed with 12 conventional (vertical/deviated) wells from a centrally located platform. In 1986, a previously drilled appraisal well (Well B1) was tied back to the platform by use of satellite tripod tower. Early water breakthrough and rapidly increasing water cut were expected in the conventional wells because of the unfavorable mobility ratio (M=25), flat field structure, and high vertical permeability. The majority of the wells produced water within a few days of production startup. This performance was predicted by initial computer simulations of single-well models, which showed that the critical rate to prevent water production had to be exceeded if the field was to be produced economically. produced economically. Subsequent simulations and preliminary history matching predicted that water-cut performance was not rate-sensitive for practical performance was not rate-sensitive for practical purposes. This led to the conclusion that purposes. This led to the conclusion that maximizing gross fluid off-take was necessary to improve economics. Individual wells were produced at rates up to 12,000 BFPD with water cuts from 85 to 97%. Some of the implications of the high-gross-fluid/ high-water-cut production strategy were decline in reservoir pressure and increase in workover frequency with conversion to larger pumps. To overcome these extreme coning conditions, Unocal Netherlands investigated the feasibility of redevelopment with horizontal wells. Redevelopment With Horizontal Wells. The first horizontal well (Well A4RD) was put on production in Jan. 1987. After the success of this well, nine more wells were horizontally sidetracked. Figs 1 and 2 show the locations of the original wells and the horizontal sidetracks. Table 1 summarizes the horizontal-well parameters. The performance aspects of parameters. The performance aspects of the Helder horizontal wells were the subject of a previous paper. Model Rock and PVT Properties. Seven layers were used in the field model and 15 layers in the single-well models. JPT P. 906