· · · Institutional Repository

In dit rapport wordt een methode beschreven voor het maken van een economisch verantwoorde planning van de tijdstippen van herstel aan een waterbouwkundige construetie. Bij deze methode staat de contante waarde van de kosten centraal. Er wordt gesteld dat de planning optimaal is in het geval de contante waarde van de totale kosten minimaal is. ander totale kosten wordt hier verstaan de som van de kosten van het herstel, de inspecties en het risico. Om het onderhoud en risico voor elk willekeurig tijdstip te kunnen bepalen moet er een verouderingsmodel beschikbaar zijn. Dit model definieert op elk tijdstip de sterkte van de constructie. Verouderingsmodellen bestaan in principe uit drie essentiele componenten, te weten: - Belastingen die veroudering veroorzaken - Sterkte of weerstand van de constructie tegen veroudering - Een relatie tussen de weerstand, de belasting en de veroudering. Er zijn gevallen waarbij we deze drie componenten (nog) niet voldoende kennen. In dergelijke gevallen wordt veelal direct de afname van de sterkte per tijdseenheid gemeten aan prototypes. Deze methode kan een oplossing bieden maar verschaft ons geen inzicht in het verouderingsproces. De conditie van een constructie op een bepaald tijdstip is athankelijk van het verouderingsproces en de tijdstippen van herstel. Als het verouderingsproces gedefinieerd is middels een verouderingsmodel, dan is de conditie van een constructie alleen nog een functie van de tijdstippen van herstel, zo ook de contante waarde van de totale kosten. In hoofdstuk 3 is een classificatie gegeven voor verouderingsprocessen. De belangrijkste eigenschap van een verouderingsproces is het al dan niet periodiek zijn van het verouderingsgedrag voor de verschillende herstel-intervallen. Hier is het herstel-interval gedefinieerd als de tijdsduur tussen twee opeenvolgende malen te plegen herstel. Bij een periodiek verouderingsproces is het herstel-interval constant. De contante waarde van de totale kosten is nog slechts een functie van de grootte van het herstel-interval. Minimalisatie van de contante waarde van de totale kosten is dan een betrekkelijk eenvoudige zaak. In het geval we te maken hebben met een niet-periodiek verouderingsproces, is de grootte van het herstel-interval niet constant. De contante waarde van de totale kosten is dan een functie van alle tijdstippen van herstel. Het aantal tijdstippen is athankelijk van de gewenste levensduur van de construetie. Minimalisatie van de contante waarde van de totale kosten is in deze gevallen vaak een tijdrovende aangelegenheid. In hoofdstuk 7 is het nut van inspectie gedefinieerd als het verschil in de contante waarde van de totale kosten volgens de oorspronkelijke planning, met als beginvooraarde de gevonden sterkte bij inspectie, en de contante waarde van de totale kosten volgens een op grand van de inspectie geoptimaliseerde planning. Met behulp van de sterkte, die gedefinieerd is door het verouderingsmodel, kan men een verwachtingswaarde van het nut van inspectie bepalen, zodat er alvorens te gaan inspecteren een schatting kan worden gemaakt van het nut ervan. Door voor elk jaar de verwachtingswaarde van het nut van inspectie te berekenen kan de eerstvolgende inspectie worden gepland in het jaar waarbij het nut positief is.

De Waal is de meest bevaren rivier van Europa, jaarlijks wordt er zo'n 150 miljoen ton vracht over getransporteerd. Dit zal de komende 10 jaar met 40% toe nemen. Rijkswaterstaat heeft om de bevaarbaarheid en veiligheid te kunnen blijven waarborgen het Waalproject opgezet. Hoofddoelstelling van dit project is het vergroten van de vaargeul bij OLR (Overeengekomen Lage Rijnafvoer). Voor de Waal is OLR gelijk aan 777 m3/s en zijn de afmetingen van de vaargeul 2,50 m diep en 150 m breed. Voor de nieuwe eisen geldt een diepte van 2,80 m en een breedte van 170 m. Om aan deze eisen te voldoen is voor de Midden-Waal (tussen Nijmegen en Tiel) besloten om alle knelpunten weg te baggeren. Hierbij wordt het gebaggerde zand in diepere delen van de rivier teruggestort. Uit vooronderzoek is gebleken dat deze baggerwerkzaamheden kunnen worden geoptimaliseerd. Dit door gebruik te maken van de natuurlijke morfologische reactie van de rivier op ingrepen in het dwarsprofiel. Met name het nivelleren van bochten, waarbij zand vanuit de ondiepe binnenbocht in de diepe buitenbocht wordt gestort, en opeenvolgende zandvangen zijn veelbelovend. Het doel van dit onderzoek is een analyse van de mogelijkheden om de baggerhoeveelheden te minimaliseren via dergelijke ingrepen in het dwarsprofiel. De morfologische reacties op ingrepen zijn met Sobek-Sedredge berekend. Dit rekenpakket simuleert in een quasi twee-dimensionale omgeving de sedimentbeweging en morfologie in rivieren. Doordat niet eerder met dit pakket was gewerkt is veel tijd besteed aan het opzetten van het model. De optimalisatie van het nivelleren is gedaan door te varieren met de mate van nivellering, de lengte waarover de ingreep plaatsvindt en het tijdsinterval tussen onderhoudsbaggerwerkzaamheden. De resultaten van deze berekeningen zijn vergeleken met de door Rijkswaterstaat geplande werkzaamheden. Bij deze werkzaamheden wordt elk knelpunt weggebaggerd en wordt het vrijgekomen zand in diepe gedeeltes van de rivier teruggestort. Uit deze vergelijking bleek dat de jaarlijkse onderhoudsbaggerwerkzaamheden bij bochtnivellering groter zijn dan bij de huidige aanpak van knelpunten. Eveneens bleek dat door het snelle uitdempen van de morfologische reactie de breedte en diepte winst te gering te zijn om veel knelpunten op te lossen. Bij de berekeningen met zandvangen kon door beperkingen van het rekenmodel alleen worden gekeken naar de gevolgen van een enkele zandvang in plaats van opeenvolgende zandvangen. Hierbij bleek dat de morfologische reactie geYnduceerd door een zandvang snel uitdempt en dat onderhoudsbaggerwerkzaamheden groter zijn dan de geplande werkzaamheden. De hoofdconclusie van dit onderzoek is dat het onderzochte gebruik van de morfologische reactie op ingrepen in het dwarsprofiel om de baggervolumina te minimaliseren geen voordelen oplevert.

Het doel van dit afstuderen was het opstellen van een methode voor het bepalen van een optimale combinatie van aanleg en onderhoud. De optimale combinatie van aanleg en onderhoud is die combinatie waarbij sprake is van minimale kosten gedurende de levensduur. Hiertoe is een 'kasstroommodel' voor de aanleg en het onderhoud van een snelweg opgesteld. Met behulp van een onderliggend 'onderhoudsrisicomodel' kunnen de risico's in de tijd in het kasstroommodel worden meegenomen. In het 'onderhoudsrisicomodel' zijn drie mechanismen opgenomen. Er is een mechanisme voor zettingen, een mechanisme voor scheurvorming en een mechanisme voor rafeling. De ontwikkelde methode is toegepast op Rijksweg 11, die momenteel wordt aangelegd. Het contract van Rijksweg 11 is een DBM-contract (Design, Build en Maintain). Dit houdt in dat het consortium N11 naast het ontwerp en de aanleg, ook verantwoordelijk is voor onderhoud voor een periode van 10 jaar. Het voorlopige ontwerp, opgesteld door het Consortium N11, blijkt, na doorrekening met de opgestelde methode, relatief conservatief te zijn. De methode toont aan dat binnen de gestelde aannamen het consortium de mogelijkheid heeft om binnen de randvoorwaarden van het contract te optimaliseren. Dit kan het consortium N11 per kilometer snelweg ongeveer € 180.000,- besparen op de contante kosten. Rijkswaterstaat had voor het bedrag dat zij voor onderhoud betaalt aan het Consortium een langere onderhoudsperiode kunnen afspreken. Een andere mogelijkheid voor Rijkswaterstaat was om minder vergoeding te betalen voor het onderhoud. Het komt er op neer dat Rijkswaterstaat betaalt voor onderhoud dat met grote waarschijnlijkheid niet hoeft te worden uitgevoerd.

Civil structures, especially hydraulic structures, are subject to influences from the environment around the structure. As a result of these influences the structure degrades, which lowers the strength of the structure. Several actions are available to lower the speed of degradation and/or restore the structure to a certain condition. Within the operations and maintenance of a structure, the question is whether regularly small reparations or once a large replacement needs to be performed. In order to make a founded decision a quantification is needed. One of these is using risks as part of the decision making process. Risk is defined as the probability of occurrence times the consequences. Since this can be expressed as costs, it can be compared with the costs of the reparations or replacement to find the value of this action. In this thesis, a framework is presented to aid in the development of a one replacement-multiple reparations-strategy. The method describes the steps needed to find all information to make a stochastic based deterioration model. Also the steps to calculate the costs for each strategy, where the moment of replacement and reparation interval are variable, are shown. The framework ends with an optimisation, resulting in the most optimal strategy within the set boundaries and assumptions. Testing of the framework has been performed with a case study of a fictional lock. After collecting all needed information about the degradation models, distribution and parameters, several calculations have been performed. With both a level II as a level III method these calculations have been made. The framework proved to be a good guideline for the determination of sought-after strategy. Since degradation models can have a more complex (than adding and multiplying parameters) shape, the level II method became significantly more complex than what it is used for (an indicative calculation). Even though level III method (the Monte Carlo method was used) requires more processing time (on a commercial laptop) than the level II method, the time it took to program was equal. A level II method is therefor only of use when a comparison with an analytical method is wanted.

In this research a decision model designed for the optimal planning of maintenance for road constructions to a particular section of a highway for a longer period. Optimal refers to the minimization of construction maintaining the required quality of the product and performance conditions. The optimization problem consists of assigning maintenance actions (A) to segments (S) over a planning horizon (T) equal to the contract for the multi-year road maintenance.

Project complexities might deliver a contribution to failure of projects in reaching the project goals. Research towards project complexities has been done in engineering projects, but not in maintenance projects. There is a difference in maintenance projects and engineering projects. For that reason one cannot assume that the comparable results coming forward from the research towards project complexities in engineering projects account for maintenance projects as well. The differences between maintenance projects and engineering project can be found in e.g. the schedule and cost estimations. In many maintenance projects the full scope of work is only known when the project is already in the execution phase. This is the result of discoveries that are done e.g. when equipment is opened up. Consequently the schedule and cost estimations cannot be fixed before the project has started. Other differences are the resource levelling, which is more dynamic in maintenance projects or the short execution phase relative to the planning and preparation phase. Maintenance projects are executed during the operation phase of a plant or construction. A construction project earns back the upfront investment during the operation phase. Maintenance project have therefore an influence on the Return on Investment (ROI) of construction projects. There are many organizations that are trying to save on their maintenance expenditures. This, in combination with the influence maintenance projects have on the ROI of a construction make them an important and interesting topic to research. For this research the following main research question is formulated “How can project complexities in maintenance projects be assessed and managed?” This research has an exploratory character. Exploratory research has got a qualitative nature which has as a result knowledge on attitudes, behaviours and perspectives of involved key-persons in the projects. Hence this is suitable for this research. The a qualitative research using case studies as research instrument is conducted at Hertel Singapore Pte. Ltd. Hertel executes maintenance projects in the petrochemical industry. The research is limited to planned preventive and planned corrective maintenance projects. Two of the maintenance projects are executed at an oil refinery, the others at a tank farm and chemical plant. The projects at the oil refinery were the mitigation of Corrosion Under Insulation (CUI) and the shutdown of a Crude Distillator (CD-5 Unit). At the tank farm and chemical plant it was a tank bottom leak and shutdown respectively. The data for the research was collected in two ways. First a desk research was done towards maintenance projects, project complexities and the background history of the projects used for the case study. Next interviews were held with key-persons involved in the projects. Semi-structured interviews were held guided by a questionnaire. During the interviewees the interviewees were asked to assist in filling in the TOE framework. The TOE framework is a framework developed in engineering projects to assess the project complexities. Part of this research was to investigate whether or not this framework can be of value in maintenance projects. For every case study at least three interviewees assisted in filling in the TOE framework. The results of the assessment of the project complexities with the framework are briefly analysed. During the interviews the interviewees were also asked to point out three to five severe elements from the TOE framework and to mentioned complexities that are missing from the TOE framework, if possible. The results of the different cases where further processed in the cross case analysis. In the cross case analysis the high scoring elements of the combined cases have been compared with the results of the assessment of the individual cases. The elements that were present in at least three cases have been combined with the elements that were pointed out in the TOE framework as severe by the interviewees and the missing elements according to the interviewees. After eliminating elements that were not typical project complexities the following elements remained present. • Is there a strong project drive (cost, quality, schedule)? • What is the number and nature of dependencies between the tasks? • What is the required effort for communications about the project (internal)? • Are the resources (materials, personnel) and skills required in the project, available? • Are involved parties aware of HSSE (health, safety, security, environment) importance? • Are there interfaces between different disciplines involved in the project (mechanical, electrical, chemical, civil, finance, legal, communication, accounting, etc) that could lead to interface problems? • Is the project organization set up from the beginning of the project? • How congested is the working area/site location? • Are there many unusual workers involved in the project? A number of these severe complexity elements evolve over the project and managed. A discussion of possible improvement to manage the most severe complexity elements has been provided. This can be mostly related to more cooperation between owner and contractor and earlier involvement of the contractor. Between them a strong project organization should be set up. From the research it appeared that not only is the management of the individual elements important, but there is also a different outcome in the perspectives of the owner and contractor. Awareness of these perspectives should be created between the both. Additionally there are less complexity elements that have been given a high grade in existing relationships. It is therefore recommended to work towards a relationship between owner and contractor to reduce the project complexities. There is more (quantitative) research towards maintenance projects and its project complexities needed. In this future research a adapted TOE framework should be created that is based on the typical project complexities that resulted from this research. The TOE framework has been of value for this research as it helped the researcher to obtain the research. However to assess the project complexities an adapted framework has more value. It is recommend to Hertel to make use of this adapted framework to create an awareness of the project complexities with the owner and to build a better relationship in addition to the set up of a strong project organization in the early start of a project.

This research is aimed at developing a cost-effective maintenance planning and packaging method that will lead to the reduction in direct maintenance costs, yet maintaining the reliability of the Boeing 737 NG aircraft.

The thesis reports on the Dutch railway infrastructure manager ProRail, on the literature study, on the determined Top 10 of maintenance activities that are determining the maintenance schedule, on the developing of the optimization model that finds such a maintenance schedule, and finally on the results and conclusions.

This report discusses the process that was used to redesign the service of routine infra maintenance. New governmental demands have led to new contract demands for infrastructure contracts. These new contracts require a new approach to infrastructure maintenance. Contractor bid's are no longer selected on a base of price only, but EMVI (Economically Most Advantageous Bid) criteria are used for the selection. Analysis of the new tendering requirements and the new demands for bids has led to a shortlist of new demands. Service theory and blueprinting is used to visualize the elements involved in the demanded new service. After visualising the demanded new service, a specific element was redesigned, to create a better fit with the new EMVI selection criteria. Several product concepts were designed to provide a different service. Using stakeholder assessment the feasibility of the developed new concept was estimated and improvement suggestions were made.

This book shows the essentials of the work of students on a theoretical commission to renovate 'het werkplaatsengebouw' for new functions. The students from the TU Delft, the PT Krakow and the FH Münster tried to find the best adaptations to give the building a new life. 'Het werkplaatsengebouw' is situated on a peninsula in the city of Rotterdam. This area is important because of the booming building activities and, because of these activities, it serves as a visiting card for the municipality The functions of buildings on th is peninsula are mainly related to the high quality offices, to several types of housing and to special activities required in such a high quality area, includes facilities for shopping and relaxation. The building designed by the architectural firm 'Van de Broek & Bakema' can be considered a young monument.

In the 1ge century what had been the traditional building techniques up to th at time were considerably changed owing to the use of cast iron and later wrought iron and steel. In fact, the industrial revolution, initially based in England, would not have been possible without these materiais. Not only the development of the product, in which the ditlerence bet ween iron and steellay purely !he fact that steel could be tempered, while iron could be hammered into shape, but also the manufacturing processes. These progressed via the puddling fumace, introduced by Henry Cort in 1784, which made possible the production of wrought iron, the Bessemer converter 1855, and the Siemens-Martin open hearth process and led to continuous improvements in building technology.

Concrete as a conglomerate of sand, stone and a binder, is a very old material indeed. In the Roman period earth from Puozzoli, together with lime and water could bind the sand and the stones to form a conglomerate that has an affmity to our modem concrete. Later, in the more northem areas of Europe, the use of trass, plus water for a reaction, also produced cement. The Romans proved that they could build in a durable way with earth from Puozzoli, because even today we can still enjoy the aquaducts built all over Europe and the water reservoirs in Rome, which are still functioning. From building specifications for the Basilica of Constantine and the Pantheon, both in Rome, we know that cost could be reduced by using a mixture of stone fragments, Puzzuoli earth, lime and water for the inner parts of the walls.

What do we understand by the term 'foundations'? This general idea may embody the literal ' groundwork' that provides support for a building and may possibly include the entire structural works that serve to pre vent subsidence. Remarkably, in this definition the word 'groundwork' incorporates both the 'ground' itself and the structural 'works'. Ground itself is a general word for the usually layered formations containing elements such as sand, clay peat and silt deposited beside each other or are intermingled, but each having its own characteristics.

The port of Kandla, which is situated in the north west of India and is subject to a large tidal variation of about 5 metres, is preparing a major expansion plan. The capacity of the harbour will be increased in two ways. Firstly the construction of several berths and jetties will allow more ships to be handled on a yearly basis. Secondly deepening of the harbour and its approaches will allow ships with a larger draught to call upon the port. It is this deepening that causes concern. The Kandla Port Trust (KPT) and the Central Water and Power Research Station (CWPRS) have calculated the amounts of cubic metres that will have to be dredged in order to raise the permissible draught from 10.7 to 14 metres. Especially the maintenance dredging posts a problem. KPT and CWPRS have calculated that between 90 and 111 million m3 have to be dredged annually to maintain the required depths. This is an increase by a factor 20 from the present 5 million m3. The costs that accompany this huge increase of dredging exceeds the extra generated income by large. As no details are known about the calculation methods of both KPT and CWPRS and the expected dredging amounts seem unrealistically high, a goal of this study is to simulate the future situation and determine the dredging amounts that will arise. A second goal is to determine the effects of training walls on the sedimentation in the approach channel, where the largest increase in maintenance dredging is expected. A process-based software package, Delft3D, developed by WLI Delft Hydraulics will be used to simulate the reality. The set up and calibration of the model has led to mixed results. In Kandla Creek, where the port is situated, the flow velocities calculated by the model resemble the measured data quite good. In the area around the approach channel (Sogal Channel), the flow velocities are too low in comparison with reality. This has consequences for the morphodynamics in the region. The morphological development in Kandla Creek shows better results than the morphological activity in the shallower vicinity of the approach channel, where little to no morphological change has occurred. The simulation of the future situation has led to the calculation of a significantly lower increase in maintenance dredging than found by KPT and CWPRS. Even though assumptions have been made the increase amounted to a more realistic magnitude of a factor 3. The costs that accompany this increase are significantly smaller than the extra income generated by the expansion. Implementing training walls to divert or contract the flow in order to decrease sedimentation by means of natural flushing of Sogal Channel will not be necessary from an economical point of view. The simulation of a few training wall layouts however has been done to obtain insight in the morphological development of the approach channel. The first concept consists of a wall in line with the main flow direction. The idea is to guide the flow more into the main channel creating more natural flushing and stabilising the channel in its present position. The effect of this proposal however is negligible, as the main flow isn't deflected enough. The flow velocity in the bend of the channel is increased only marginally, especially in the creek outfall. The extra erosion in that area isn't beneficial as the depth is large enough already. In order for this concept to be more effective a larger portion of the flow (both ebb and flood) needs to be directed into the channel and therefore the training wall has to be less in line with the existing flow direction. The model has to be adjusted to create a different alignment of the wall. The other two concepts block a part of the flow and only allow water to flow through the channel. This is done by implementing training walls perpendicular to the flow direction. The first of these concepts consists of a staggered wall, which does not block the flow completely. Water is able to flow through the wall, albeit that the flow is severely hindered. The second layout blocks the flow completely and all of the water is directed through the channel. Both concepts have a favourable effect on the morphological development of the bend in the channel, which becomes subject to erosion. The problem however is shifted towards other parts of the approach channel, which become subject to sedimentation. The effect is roughly twice -as large in case of total blockage than in case of the staggered training wall. The impact of these proposals is very severe. The rate of morphological change is very high as the present hydrodynamic situation is gravely affected. It is not clear what the effect will be in the long run. The results of the simulations have to be interpreted cautiously. In the process of modelling assumptions have been made. A few of these assumptions had to be made due to a shortage of data, while others have been made in order to simplify the problem so that it could be dealt within the scope of this study. The quantitative results cannot serve as a basis on which conclusions are formulated. A qualitative analysis however is possible, which can be found in this report. The overall conclusion of this study is that the best way to enlarge and maintain the maximum permissible draught is to increase dredging. The extra costs of the increase in maintenance dredging weighs up to the extra-generated income. Furthermore the impact of the different training walls is too severe and the uncertainties that accompany this impact in the long run are too plentiful.

Unit testing is generally accepted as an aid to produce high quality code, and can provide quick feedback to developers on the quality of the software. To have a high quality and well maintained test suite requires the production and test code to synchronously co-evolve, as added or changed production code should be tested as soon as possible. Traditionally the quality of a test suite is measured using code coverage, but this measurement does not provide insight in how tests are used by developers. In this thesis we explore a new approach to analyse how tests in a system are used based on association rules mined from the system’s change history. The approach is based on the reasoning that an association rule between two entities, possibly of a different type, is a measure for the co-use of the entities. Case studies show that analysing all the resulting rules allows us to uncover the distribution of programmer effort over pure coding, pure testing, or a more test-driven practice. Another application of our approach is that we can express the number of tests that are truly co-evolving with their associated production class.

Rijkswaterstaat is responsible for maintaining the fairway of the river Waal for navigation for more than a hundred years. In the past problem areas, which cause hindrance for navigation and block the fairway, were prevented with structural measures. Dredging was used as a last resort. In 2005 the desired fairway depth was increased from BRV -2,5m to BRV -2,8m which changed the role of dredging in the maintenance of the fairway. The dredging of the fairway is now executed through a performance-based contract. Today dredging is seen more as a permanent measure and is constantly used to maintain the fairway. In the last hundred years an increasing number of measurements have been executed on the river Waal and have been stored in databases. This data is currently hardly used for the day-to-day maintenance of the river system. This research aims at gaining more insight in the dredging efforts needed to maintain the fairway using the available data from Rijkswaterstaat. Former dredging contracts as well as the currently used performance-based contract were studied in order to get a better understanding of the efforts needed to maintain the fairway for navigation. It was tried to deduce a relation, between the discharge hydrograph and the efforts that are needed to maintain the fairway, in order to obtain an indication of the consequences of expected future changes to the river system. The expected future changes which were studied in this research are changing the currently used river groynes and climate change. The relation found could unfortunately not be used to indicate the possible consequences of changing the groynes on the necessary dredging efforts but it could be used to indicate the consequences of climate change on the necessary dredging efforts. Climate change will cause a major increase in the necessary dredging efforts.

Due to the closure of the Haringvliet in 1970 as part of the Delta works and the opening of the Beerdam in 1997, the course of the tidal flow in the Rhine-Meuse delta has changed. The tidal flow from and towards the Haringvliet is now completely taking place through the branches that connect the Haringvliet with the Nieuwe Maas and Nieuwe Waterweg. This causes these branches to erode by the increase of the tidal prism. The erosion problems occur in the central parts of the Rhine-Meuse delta, which, amongst other rivers, includes the rivers Spui, Oude Maas, Noord and Dordtsche Kil. The presence of sand, clay and peat layers in the branches causes the erosion mechanism to be rather complex. The alternation of sand and clay layers in the area causes the erosion to be non-uniformly distributed, resulting in deep local scour holes. The goal of this research is to determine an adequate maintenance strategy for the problem area. In order to do so the erosion processes are modelled in an existing Delft3D model. In this study, the morphodynamics in a depth averaged model, covering the Dordtsche Kil and a part of the Oude Maas, are calibrated in such a way that it quantitatively fits the existing long term erosion trend and that the qualitative processes of scour in the model simulations comply with the erosion occurring in reality. This is achieved by implementing multiple sediment layers simulating a bed topography where clay layers and local sand packages alternate. Subsequently modelling a variety of possible maintenance strategies has shown that several effective strategies are possible to deal with the erosion problems. Two strategies are suggested regarding the shortage of sediment in the system. The best way to deal with the local scour problems is to prevent further scouring of existing holes by fixation with riprap. Continuous nourishment in scour holes prevents the holes from scouring any further and moreover provides a solution for the long-term problem due to the fact that outflowing sediment diffuses and spreads out over the whole length of the river. A disadvantage of this strategy is the future obligation of continuous maintenance and monitoring for the whole Rhine-Meuse delta. The uncontrolled character of this strategy can also cause navigation nuisance. The other type of maintenance strategies considered is the reduction of erosive flow in the system. A situation where the Haringvliet gates are always opened, even during high tide, leads to a remarkable reduction in erosion. The scour holes still undergo erosion, but at a slower rate, and the erosion of the clay layer decreases to a negligible amount. A combination of fixing scour holes and opening the Haringvliet gates is also considered, which solves the problem for long-term erosion and the local scour, but comes at a high cost to implement. Besides in the Oude Maas, the erosion in the Spui branch is also alarmingly severe. In order to reduce erosion in this river branch, an option is to close it off. This causes the flow velocity in the Oude Maas to increase with approximately 10% and to reduce the flow velocity in the river Spui by half. It is strongly recommended to investigate the effects and possibilities of opening the Haringvliet sluices further. Further research should also be done with the extension of the Delft3D model to the Spui river, the downstream part of the Oude Maas and the Noord. Since the numerical model can become (even more) extensive with the implementations of 3 more domains, much more insight in the problems can be obtained with the interaction of the different eroding branches in a Delft3D model.

The trend of highly energy-efficient, low carbon economies leads to high targets for renewable energy production in 2020, with main contributions by onshore wind, offshore wind and biomass. Many offshore wind farms have to be built to meet these targets. These wind turbines will be built farther and farther away from the coast. When a failure occurs, the maintenance crew has to travel farther and therefore needs a longer good weather window. Since these longer windows occur less often than small good weather windows, waiting times until a good weather window appears will probably be larger for these wind turbines. As long as the maintenance crew can not reach the failed turbine, this turbine cannot produce power and will thus not provide revenue. To reduce the downtime caused by these weather waiting times, a service island can be used. This is a fixed point at sea from where maintenance can take place, where engineers can stay and spare parts can be kept. To determine an optimal location for such a service island, the problem can be written as the Weber problem. The problem is to find the point for which the weighted sum of distances to given points is minimized. Using the wind farms or wind turbines as given points, and the ratio of weighted power as weights, this problem can be solved with the Weiszfeld algorithm. This algorithm finds a solution by using a converging sequence, based on the first-order necessary conditions for a point to be optimal. Since the objective function is strictly convex, the Weiszfeld algorithm finds the global minimum. In this solution all the maintenance for all wind turbines is done from this location, even of the ones that are closer to a port than to the service island. Because transportation time is to be minimized this might not be the best location for the purpose of a service island, and for this reason a generalization of the problem is considered. By adding more service locations, a port can be incorporated in the solution and each wind farm or wind turbine is assigned to one service location. The problem is to find the points for which the sum of distances to the given points, with the corresponding allocation, is minimized and is known as the unconstrained continuous location-allocation problem. The objective function of this problem is neither convex nor concave, which can cause a large number of local minima. If the number of new locations is unknown, the problem is NP-hard, but if the number of new locations is known, the problem is polynomial solvable. The problem can then be solved with the MALA algorithm. This algorithm allocates each wind farm to the closest service location and then solves the problem for each service location individually with the Weiszfeld algorithm. This procedure is repeated until no further reduction in total cost can be made. With the solutions of the MALA algorithm, the maintenance during the lifetime of the wind turbines is simulated. Hereto failure rates en repair time distributions are used to simulate failures and repairs. Four maintenance categories are considered for failures of different components of the wind turbine. It is assumed that there is only one maintenance crew available and that each maintenance category has its own vessel with its own weather specifications. Wind and wave heights of the period 2001 until 2010 are used to simulate the wind and wave pattern. This simulation gives insight into the availability of wind turbines and the influence of service islands. The presence of a service island increases the availability of the wind turbines and increases the number of repairs. A cost comparison is made to determine when a service island is profitable, considering the cost of an island, the kilowatt hour price and the losses caused by the downtime of turbines. The maximum investment budget increases when the kilowatt hour prices increases, but also the total cost involved increase. Therefore the profitability of a service islands depends on the (expected) kilowatt hour price.