1
ABSTRACT The Murchison structure is barge-launched steel drilling and production platform which is to be floated out in Spring 1979 and installed in the Northern North Sea in 156m of water. In the past, offshore steel structures have been designed on the basis of an extreme static design load. Subsequently, the completed design has been checked for dynamic amplification and fatigue life. As structures are designed for deeper waters, from 150m depth their natural periods become longer and dynamic excitation can have a profound effect on the stress levels achieved in all wave- conditions. In order to meet the design requirements for dynamic and fatigue analysis a new suite of computer programs called FATJACK has been developed which embodies several innovations in the analysis of steel structures. The whole system is centered on the Atkins Stress Analysis System (ASAS). It uses the subspace iteration technique to handle very large eigen-vector extractions which are necessary if the structure is to be modeled with sufficient accuracy to find the dynamic stress levels in the smaller diameter members. This is followed by a deterministic analysis using a steady-state response to relate the climatic conditions to the dynamic response and hence fatigue life duration. The application of this program to the Murchison structure is described in detail. For example, the extraction of 25 eigen-vectors from an uncondensed structural idealisation shows significant low frequencies in the conductor tube bracing which have previously been undetected and this could explain the relatively high failure rates of this type of structural component found in the platforms of the Southern North Sea. In addition, a transient analysis using a Newmark-ß integration scheme has been undertaken to establish the dynamic amplification factors by comparison with the base reactions and deck level deflections for corresponding static load cases. INTRODUCTION The escalating demand for hydrocarbon fuels has necessitated the exploration and development of offshore fields in ever increasing water depths. The enormous size of the steel structures required for these depths has lead to a new breed of problem and, in particular, the fatigue and dynamic response of such structures has become increasingly important. Experience in the North Sea has shown that some structures are more prone to fatigue damage from small, low period waves rather than overload failures due to the long period storm waves. In previous static analyses of offshore platforms the environmental conditions specified for extreme loading and fatigue life calculations have been related but different. In the case of the extreme '100 year' storm condition a theoretical design-life wave, with a specified height and period, is applied to the structure at a point in time usually chosen by reference to the maximum mud-line horizontal shear force or overturning moment. It is arbitrarily assumed that this criterion will define the worst load case for all members and joints. In addition the design-life current is applied to the structure simultaneously to give a combined maximum load condition.
44Tunnel valleys of the central and northern North Sea (56°N to 62°N): Distribution and characteristics
Tunnel valleys of the central and northern North Sea (56°N to 62°N): Distribution and characteristics
1Gravity Platform Tower Vibration
The paper presents results of vibration tests on two concrete gravity structures, comprising free tower vibration in air and in submerged position and from a platform installed in the North Sea. Dynamic properties of tested structures are compared with design assumptions, and their influence on extreme loads and fatigue life is discussed. The observations indicate a conservative design. INTRODUCTION The design of offshore gravity platforms necessitates a dynamic response analysis to account for dynamic load effects on the structure. Several gravity type structures are now in service, both in the North Sea and elsewhere. The purpose of this paper is to describe tests which have been carried out to determine platform dynamic characteristics and to discuss how these compare with some of the design analysis assumptions. A typical platform, designed for a water depth of 145 m, is the Mobil Statfjord A platform. It is fitted with a large caisson which provides buoyancy for deck and process equipment during tow-out, later on the caisson serves as oil storage tank. Drilling takes place through two of three towers which support the deck. The third tower serves as utility providing space for pumps, generators etc. concerned with ballasting and loading operations. The Substructure, comprising caisson and towers, is slip-formed in concrete, whilst the deck is made out of steel in order to reduce the top load during tow-out. Fig. 1 shows the complete platform with a height of more than 200 m. The caisson base is fitted with steel and concrete skirts to transfer horizontal loads at mudline. Grouting will take place to ensure that vertical loads are evenly distributed underneath the platform. The environmental loads acting on the platform are dynamic, and this affects the design analysis w.r.t:Extreme loads (waves, wind, earthquakes).A fatigue analysis for the anticipated platform life.Local vibration levels (i.e. at top deck) to satisfy facilities and equipment design. The behaviour of offshore platforms under dynamic loading conditions has received much attention in recent years, and the state of the art has been significantly advanced by contributions from Penzien and others (1), (2), (3). These are primarily concerned with spectral techniques, whereas an actual design load may be specified in deterministic terms (i.e. a design wave) as well as stochastic (4). The analysis of dynamic structural response requires that some model is formulated, both to represent the soil/ water/structure interaction system and to represent the forcing functions. Also, the complexity of the interaction problems makes it necessary to effect a compromise between the mathematical model accuracy and its size. Most investigations assume a linear system so that solutions can be based upon normal mode superposition. The principal advantage of this method is that fairly complex systems can be analysed over a wide frequency range without running up large computer costs, and parametric response studies are thus permitted. Such studies are valuable to single out response sensitivities and allow more efficient use of detailed analysis.
Read more61The Quaternary succession in the northern North Sea
The Quaternary succession in the northern North Sea
67Trends in chlorobiphenyl contents in livers of Atlantic cod (Gadus morhua) from the North Sea, 1979–1987
Trends in chlorobiphenyl contents in livers of Atlantic cod (Gadus morhua) from the North Sea, 1979–1987
61Population structure of harbour porpoises Phocoena phocoena in the seas around the UK and adjacent waters.
The population structure of harbour porpoises from British and adjacent waters was studied by examining variability in a 200 bp (base pair) section of the control region of mitochondrial DNA (mtDNA) extracted from 327 animals. This region contained 20 variable sites giving rise to 24 different haplotypes. Mean nucleotide diversity between all pairs of haplotypes was 0.81% (range 0-4%). The most common haplotype occurred in 63% of the samples and was recorded in all geographical areas; several other haplotypes were present in two or more of the sampling locations. This suggests considerable historical interconnections among populations, probably through gene flow. However, there were significant differences (p < 0.05) as determined by AMOVA (Analysis of Molecular Variance, Excoffier et al. 1992), between porpoises from the northern and southern North Sea, and between the northern North Sea and the Celtic/Irish Sea. The differences were predominantly due to variation among females. This sex-related difference in population genetic structure suggests that males disperse more than females. This has important consequences for evaluating the consequences of incidental catches of porpoises by fisheries in these seas since there may be a greater impact on local populations than is implied by simple calculations of mortality.
Read more156 - Standards and codes to control hydrogen-induced cracking in pressure vessels and pipes for hydrogen gas storage and transport
6 - Standards and codes to control hydrogen-induced cracking in pressure vessels and pipes for hydrogen gas storage and transport
Read more7Influence of Different Micro-Imprints to Fatigue Life of Components
To improve tribological properties of components, different kinds of textures are added to their surfaces. Although the added imprints or craters are only few tens of micrometers wide and a few micrometers deep, they form an initial crack from which a fatigue crack may propagate. In this article, a numerical approach for calculating the fatigue life of a body with present micro-imprints with standard finite-element software will be presented. The objective is to show how the imprint shape and its manufacturing process influences the fatigue life. The considered specimens are round bodies that were made from Boehler K890 microclean cold-working steel. Three shapes of the imprints are studied: the imprints produced by a Vickers pyramid and a Rockwell cone and a half-spherical crater void produced by a laser device. The flat-surface round body served as a reference. The imprint shape and its manufacturing process is also considered. For this reason, a simulation of impression of Vickers and Rockwell indenters is simulated first, which is followed by a numerical fatigue-life prediction. In this manner, the residual stresses from the imprinting process are considered during the fatigue-life prediction. For the laser-induced craters and the reference body, only the fatigue life is numerically predicted. Because the material inhomogeneities are much smaller than the geometrical ones, the calculated fatigue lives for different texturing represent the fatigue-life reduction caused by the applied textures. To validate the presented numerical approach, the calculated fatigue lives are compared to the experimentally determined.
Read more49Elementary composition of suspended particulate matter in the North Sea
Elementary composition of suspended particulate matter in the North Sea
North Sea Exploration: ABSTRACT
The North Sea covers parts of the intracratonic Northwest European basin. Exploration efforts in onshore parts of this basin peaked in the 1950s and 1960s and resulted in the proving of ultimate recoverable reserves of 2.4 × 109 bbl of oil and 110 Tcf of gas. Exploration in the North Sea began in the early 1960s. Ultimate technically recoverable reserves in established offshore accumulations are estimated to amount to 24 × 109 bbl of oil and 90 Tcf of gas. The gas province of the southern North Sea forms a direct extension of the onshore Permian gas play. Onshore oil plays do not extend into the offshore. The prolific oil and gas province of the central and northern North Sea is closely tied to the Mesozoic North Sea rift. Reservoirs range in age from Devonian to early Tertiary. Upper Jurassic kerogenous shales are the principal source rocks; in large parts of the Viking and Central grabens the source shales reached maturity during the Tertiary by which time the North Sea rift had become inactive and was replaced by regional subsidence which led to the development of the Cenozoic North Sea basin. Most of the hydrocarbon reserves in the Viking and Central grabens is contained in structural traps. High reserve concentrations in relatively small areas are related to the availability of abundant, mature source rocks, the blanket development of reservoirs, and the close spacing of trap-providing structures. Many of the known oil accumulations are contained in overpressured reservoirs. Long-range migration is apparently of little significance in the oil habitat of the North Sea. End_of_Article - Last_Page 556------------
Read moreOperational Experiences With The Offshore Loading Units Auk-ELSBM And Brent-SPAR
Buyzen, J.P.M., Shell Expro UK Ltd. Scotland Introduction Shell Expro operates in the British sector of the North Sea on behalf of Shell and Exxon. The company has three operating locations: London, (Head Office), Lowesoft (Southern Gas fields) and Aberdeen (Middle and Northern North Sea). This paper deals with the operation of the Auk-ELSBM (Middle North Sea) and Brent-SPAR (Northern North Sea), activities which are controlled from Aberdeen (Fig. 1). DESCRIPTION OF THE INSTALLATIONS General The hostile environment of the North Sea demands a major effort with respect to the design and effective operation of the offshore loading facilities. Although designs show significant differences, each of these installations is a variation of the Single Point Mooring theme, in that they all incorporate some form of mooring turntable and fluid swivel, which permits the tanker, while remaining moored, to weathervane around the SPM in response to the prevailing weather and sea conditions. THE AUK-ELSBM (Exposed Location Single Buoy Mooring) Location The ELSBM is located 2,000 m from the Auk platform and moored to the seabed by 8 anchors, each weighing 15 tons, which are set around the buoy in a circle with a 460 m radius. Each anchor is attached to the ELSBM by a 3 in. diameter stud link chain and is pretensioned (Fig. 2). The water depth in the Auk field is 84 m.
Read more36Egg weight and fecundity in the north sea herring (clupea harengus)
Egg weight and fecundity in the north sea herring (clupea harengus)
Storm Xaver over Europe in December 2013 and its energy meteorological impacts
&lt;p&gt;Storm Xaver impacted the northern Europe on 5-6 December 2013. &amp;#160;It developed southeast of Greenland and passed north of Scotland and across southern Norway on a trajectory that led to a cold air outbreak across the North Sea and intense convection activity in northern Europe.&amp;#160; Strong sustained north winds led to a high storm surge that impacted all countries bordering the North Sea. &amp;#160;Storm Xaver was a century scale event with certain locations around the North Sea reporting their highest ever water levels since the start of modern records.&amp;#160; Media reports from the time of the storm chronicle the scale of the disruptions, including many cancelled flights, interrupted rail networks, closed bridges and roads, coastal building collapses, and power blackouts across northern Europe. &amp;#160;Much of this was due to the strong winds, but coastal storm surge flooding was important in the UK, and it led to interrupted port operations around the North Sea.&lt;/p&gt;&lt;p&gt;The storm was important for energy infrastructure and particularly for wind energy infrastructure.&amp;#160; In the northern North Sea, petroleum platforms were evacuated and operations closed ahead of the storm as a precautionary measure.&amp;#160; A number of onshore wind turbines were badly damaged by high winds and lightning strikes in the UK and Germany.&amp;#160; Over the North Sea, wind speeds exceeded the turbine shutdown threshold of 25 m/s for an extended period of time, with economic impacts from the loss of power generation.&amp;#160;&amp;#160; In the German Bight, the FINO1 offshore wind energy research platform was damaged at the 15 m level by large waves. &amp;#160;This was the third report of storm damage to this platform after Storm Britta in 2006 and Storm Tilo in 2007. &amp;#160;Researchers have highlighted the need to reassess&amp;#160; the design criteria for offshore wind turbines based on these kinds of extreme meteorological events. &amp;#160;For the offshore wind industry, an important element of energy meteorology is to characterize both the evolving wind and wave fields during severe storms as both elements contribute to turbine loads and potential damage.&lt;/p&gt;&lt;p&gt;The present conference contribution presents a literature review of the major events during Storm Xaver and impacts on energy infrastructure.&amp;#160; Tide gauge records are reanalyzed to trace the progress of the storm surge wave around the North Sea.&amp;#160; A spectral analysis is used to separate the long period storm surge component, diurnal/semidiurnal tide, and short period components in the original water level record. &amp;#160;The short period component of the tide gauge record is important as it may be linked with infragravity waves that have been implicated in certain cases of offshore infrastructure damage in addition to coastal erosion. &amp;#160;Discussion is made of offshore wave records during the storm.&amp;#160; Storm Xaver is compared with two damaging offshore storms in 2006 and 2007.&lt;/p&gt;
Read more36The influence of high R ratio on unnotched fatigue behavior of 1045 steel with three different heat treatments
The influence of high R ratio on unnotched fatigue behavior of 1045 steel with three different heat treatments
17Vibrational Fatigue and Structural Dynamics for Harmonic and Random Loads
The presented study experimentally and theoretically researches vibrational fatigue of an aluminum-alloy specimen for harmonic and random loads. The main aim of this study is to determine the influence of modal parameter changes that occur during the experimental fatigue test, on the correctness of the numerical fatigue life prediction. Firstly, the material’s fatigue parameters were obtained with harmonic base excitation of the specimen near its natural frequency. During the harmonic fatigue testing the changes in the specimen’s natural frequency and damping loss factor were monitored as the fatigue damage was accumulated at the fatigue zone. Secondly, with a validated numerical model of the specimen the stress transmissibility was obtained for the case of the random-vibration base excitation. Finally, by respecting the stress response along with the experimentally obtained material fatigue parameters the vibration fatigue life was estimated for the case of the random vibration load. The numerically predicted fatigue life was compared to the experimental results, obtained with the electro-dynamic shaker. From this comparison the influence of the damping-loss-factor changes on the calculated fatigue life was clearly shown. For the case of the observed specimen, the damping loss factor included in the fatigue life estimation should be increased by more than 100 percent to give reliable prediction of the fatigue life. The presented research shows new possibilities and critical aspects in the area of accurate high-cycle vibration fatigue life-estimation of dynamic structures.
Read more11Metazoan plankton and the structure of the plankton community in the stratified North Sea
MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 175:191-200 (1998) - doi:10.3354/meps175191 Metazoan plankton and the structure of the plankton community in the stratified North Sea H. G. Fransz1,*, S. R. Gonzalez1, S. F. Steeneken2 1Netherlands Institute for Sea Research, PO Box 59, 1790 AB Den Burg, The Netherlands 2Department of Marine Biology of the University of Groningen, PO Box 14, 9750 AA Haren, The Netherlands *E-mail: fransz@nioz.nl ABSTRACT: The hypothesis of size-differential control of phytoplankton was tested by sampling stations on a transect from the Dogger Bank to the Shetland Isles in the North Sea on cruises in March/April and July/August 1994. On this transect light limitation was expected in spring, and an increase in nutrient limitation was expected from north to south in summer. The Shetland area, where nutrient levels and primary production rates were highest in summer, did not show the correspondingly higher relative abundances of large algae which would have been predicted by the hypothesis. The question addressed in this study is how the highly abundant zooplankton in the stratified North Sea in summer affect the structure of the plankton community of interacting algae, heterotrophic protists and metazoans. Variations in zooplankton species distributions revealed 4 latitudinal subregions. The Calanus finmarchicus population in the central North Sea reproduced in March. In August, reproduction rates were highest in the 2 northernmost regions. Carbon specific ingestion based on in vitro egg production rates led to an estimate of 10.4% for these stations in summer. It was concluded that the stratified central and northern North Sea can vary with latitude in zooplankton species composition, population structure and vertical distribution. In summer, adjacent subregions can differ widely in vertical distribution of C. finmarchicus biomass. High copepod abundance and grazing rates can promote the dominance of small algae by causing mortality of large algae and heterotrophic protists (grazers of small algae). During summer in the North Sea, egg production rates indicated an increased grazing activity of C. finmarchicus towards the north. This could have obscured the negative effect of decreasing nutrient limitation on the dominance of small algae predicted by the hypothesis of size-differential control of phytoplankton. KEY WORDS: Mesozooplankton · Calanus finmarchicus · Biomass · Egg production · Plankton community structure · Size-differential control · North Sea Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 175. Publication date: December 17, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.
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