Lars Johanning

Offshore wind farm electrical cable layout optimization

This article explores an automated approach for the efficient placement of substations and the design of an inter-array electrical collection network for an offshore wind farm through the minimization of the cost. To accomplish this, the problem is represented as a number of sub-problems that are solved in series using a combination of heuristic algorithms. The overall problem is first solved by clustering the turbines to generate valid substation positions. From this, a navigational mesh pathfinding algorithm based on Delaunay triangulation is applied to identify valid cable paths, which are then used in a mixed-integer linear programming problem to solve for a constrained capacitated minimum spanning tree considering all realistic constraints. The final tree that is produced represents the solution to the inter-array cable problem. This method is applied to a planned wind farm to illustrate the suitability of the approach and the resulting layout that is generated.

Assessing mechanical loading regimes and fatigue life of marine power cables in marine energy applications

Reliable marine power cables are imperative for the cost-effective operation of marine energy conversion systems. There is considerable experience with marine power cables under static and dynamic load conditions but the loading regimes for floating marine energy converters are not well understood, due to a lack of field experience. This paper aims to assess mechanical load conditions and failure modes for a dynamic power cable that is connected to a floating wave energy converter. The applied approach combines experimental tank test data with numerical modelling and site-specific wave characteristics to identify maximum load points and to quantify the fatigue life. The effect of varying wave parameters on maximum loads and fatigue cycles is investigated and results are presented for two common umbilical configurations: catenary and lazy wave. In situations with limited field experience, the presented approach provides a tool to determine if critical components are fit for purpose and to assess the expected level of reliability prior to deployment. The cable conductor’s fatigue life is estimated for the lazy wave configuration and highlights component fatigue failure as a major concern that must be addressed in floating marine energy applications.

Measurement of the Effect of Power Absorption in the Lee of a Wave Energy Converter

Monitoring the effect of floating wave energy converter (WEC) devices on the surrounding wave field will be an important tool for monitoring impacts on the local wave climate and coastlines. Measurement will be hampered by the natural variability of ocean waves and the complex response of WEC devices, causing temporal and spatial variability in the effects. Measurements taken during wave tank tests at MARINTEK are used to analyse the effectiveness of point wave measurements at resolving the influence of an array of WEC on the local wave conditions. The variability of waves is measured in front and in the lee of a device, using spectral analysis to identify changes to the incident wave field due to the operating WEC. The power capture and radiation damping are analysed in order to predict the measured changes. Differences in the wave field across the device are clearly observable in the frequency domain. However, they do not unanimously show a reduction in wave energy in the lee of a device and are not well predicted by measured power capture.Copyright

Interaction Between Mooring Line Damping and Response Frequency as a Result of Stiffness Alteration in Surge

The design and operation of a chain mooring for a wave energy converter (WEC) is considered. Experimental measurements of a mooring line were conducted in the Heriot-Watt University wave basin at a scale of 1:10. The laboratory procedures were designed to resemble tests undertaken earlier in the year at ‘full’ scale in 24 m water depth. This paper describes and compares these measurements and relates the results to earlier work on mooring lines by Webster [1]. Measurements of both the damping and response frequencies of the mooring are described. Although the present results support partly the conclusions of the earlier work, care must be taken in how these are applied when one is considering mooring line design for WECs. It is concluded that there are significant differences for a WEC for both operational and limit state design in comparison with a more conventional offshore system such as an FPSO or CALM. Although the primary requirement is still one of station-keeping two further considerations may be of great importance. Firstly if a ‘farm’ of devices is to be considered then limitations in sea space may necessitate that the devices be relatively densely packed. This will mean that the ‘footprint’ of the mooring should be constrained, to ensure that the moorings from each device do not interfere with one another and this will have great significance for the loading experienced by the line. This can be exacerbated by variations in tidal range which will have a larger effect in comparison with a conventional deepwater mooring. A second factor may arise if the mooring system response is critical to the WEC energy extraction process. If the mooring becomes part of the ‘tuned’ system then changes in the mooring properties of damping and natural frequency could seriously affect energy conversion efficiencies.Copyright

Offshore Reliability Approach for Floating Renewable Energy Devices

This paper describes the test facilities developed within the Peninsular Research institution for Marine Renewable Energy (PRIMaRE) group and discusses the approach of the group to mitigate risk for marine renewable energy installations. The main consideration is given to the reliability assessment of components within mooring configurations and towards power umbilical for typical renewable energy sites. Load and response data from sea trial will be used to highlight the importance of these research activities, and a Dynamic Marine Component Test rig (DMaC) is introduced that allows four degree of freedom fatigue or destructive tests. Furthermore it is discussed how this facilities could also aid in the reliability assessment of wider offshore applications.Copyright

PHYSICAL COMPONENT TESTING TO SIMULATE DYNAMIC MARINE LOAD CONDITIONS

The reliability and integrity of components used in the marine offshore environment is paramount for the safety and viability of offshore installations. The engineering challenge is to design components that are robust enough to meet reliability targets whilst lean enough to minimise cost. This is particularly the case for offshore marine renewable installations which operate in the same, possibly harsher, environment as offshore oil and gas installations, and are subjected to highly cyclic and dynamic wave, wind and operational load conditions. The cost of electricity produced has to compete with other means of electricity generation and does thus not offer the same profit margins available as oil and gas commodities. As a result, components for marine renewable installations have to meet the target reliability, without the application of costly safety factors to account for load and environmental uncertainties. Industries with similar design tasks such as the aviation or automotive industry have successfully used a service simulation test approach to develop robust yet lean designs.This paper builds on an approach to establish and validate the reliability of floating renewable energy devices in which dedicated component testing using the purpose built Dynamic Marine Component test rig (DMaC) plays a pivotal role to assess, validate and predict the reliability of components in the marine environment. This paper presents a test rig for both static and fatigue tests of marine components such as mooring lines and mooring shackles under simulated or measured load conditions and provides two case studies from recently conducted mooring component tests. This includes an investigation into the load behaviour of synthetic mooring ropes and the ageing of mooring shackles.© 2013 ASME

Long-term underwater sound measurements in the shipping noise indicator bands 63 Hz and 125 Hz from the port of Falmouth Bay, UK

Chronic low-frequency anthropogenic sound, such as shipping noise, may be negatively affecting marine life. The EUs Marine Strategy Framework Directive (MSFD) includes a specific indicator focused on this noise. This indicator is the yearly average sound level in third-octave bands with centre frequencies at 63Hz and 125Hz. These levels are described for Falmouth Bay, UK, an active port at the entrance to the English Channel. Underwater sound was recorded for 30min h(-1) over the period June 2012 to November 2013 for a total of 435days. Mean third-octave levels were louder in the 125-Hz band (annual mean level of 96.0dB re 1μPa) than in the 63-Hz band (92.6dB re 1 μPa). These levels and variations are assessed as a function of seasons, shipping activity and wave height, providing comparison points for future monitoring activities, including the MSFD and emerging international regulation.

Component reliability test approaches for marine renewable energy

An increasing number of marine renewable energy (MRE) systems are reaching the stage where a working prototype must be demonstrated in operation in order to progress to the next stage of commercial projects. This stage is often referred to as ‘valley of death’ where device developers face the challenge of raising capital needed to demonstrate the prototype. The dilemma is that investors understandably demand a proven track record and demonstrated reliability in order to provide capital. One way to resolve this dilemma is specific component reliability testing that not only satisfies investor expectations but holds the potential to improve and de-risk components for MRE. This paper gives an overview to different component reliability test approaches in established industries and for MRE, covering both wave and tidal energy technologies. There has been notable activity in the research community to develop and implement dedicated component reliability test rigs that allow the investigation and demonstration of component reliability under controlled, yet representative conditions. Two case studies of physical test rigs will illustrate the possible test approaches. The Nautilus Powertrain test rig, a facility at the Offshore Renewable Energy (ORE) Catapult, focuses on the demonstration and testing of drive train components including gearboxes, generators, mechanical couplings and bearings. The Dynamic Marine Component test rig (DMaC) at the University of Exeter aims to replicate the forces and motions for floating offshore applications and their subsystems, including mooring lines and power cables. This paper highlights the relevance of component testing and qualification prior to large-scale commercial deployments and gives an insight to some of the test capabilities available in the sector. Several case studies illustrate the component test approach for tidal energy (Nautilus) and wave energy (DMaC) applications.

Lifecycle Fatigue Load Spectrum Estimation for Mooring Lines of a Floating Marine Energy Converter

One of the key engineering challenges for the installation of floating marine energy converters is the fatigue of the loadbearing components. In particular the moorings which warrant the station-keeping of such devices are subject to highly cyclic, non-linear load conditions, mainly induced by the incident waves. To ensure the integrity of the mooring system the lifecycle fatigue spectrum must be predicted in order to compare the expected fatigue damage against the design limits. The fatigue design of components is commonly assessed through numerical modelling of representative load cases. However, for new applications such as floating marine energy converters numerical models are often scantily validated. This paper describes an experimental approach, where load measurements from tank tests are used to estimate the lifecycle fatigue load spectrum for a potential deployment site. The described procedure employs the commonly used Rainflow cycle analysis in conjunctionwith the Palmgren-Minerrule to estimate the accumulated damage for individual sea states, typical operational years and different design lives. This allows the fatigue assessment of mooring lines at a relatively early design stage, where both information from initial tank tests and the wave climate of potential field sites are available and can be used to optimise the mooring design regarding its lifecycle fatigue conditions.

Dynamic Response Characteristics of a Floating Wind Turbine Tower at Low Response Frequency

This paper presents experimental and numerical hydrodynamic load and damping results on a surface piercing pivoted circular cylinder, representing a monopile of a floating Marine Renewable installation (Spar-Buoy concept). The resulting load and damping characteristics are applied to predict the response characteristic of the pile and compared with experimental results. The main consideration is given to the characteristic of the cylinder due to steep wave action in the pitching mode. The experiments were conducted in the wave tank at the University of Glasgow using a cylinder model with a diameter of D = 0.21m that was constructed at the Imperial College London, to provide a structural to wave frequency ratio of 2 and 3. The outcome from the investigation supports the importance of the dynamic response to the successful operation of a deep water wind turbine installation.Copyright