Geir Moe

HYDRODYNAMIC COEFFICIENTS FOR CALCULATION OF HYDRODYNAMIC LOADS ON OFFSHORE TRUSS STRUCTURES

Abstract The current American Petroleum Institutes recipe [API RP 2A WSD, Recommended practice for planning, designing and constructing fixed offshore platforms, working stress design. API, USA, 1993.] for calculation of hydrodynamic loads on offshore truss structures is compared with the corresponding North Sea Design Practice, as given by the rules of Det Norske Veritas. Most emphasis is put on the hydrodynamic coefficients and the estimation of design current as these issues are identified to be particularly critical. Use of the updated API (1993) recommendations in which the drag coefficient for roughened cylinders is increased from a minimum of 0·6 (API 1991) to 1·05 (API 1993) and where current is included, could lead to a general increase in the estimated load level on slender offshore structures [Petrauskas, C., Heideman, J.C. & Berek, E.P., Extreme wave force calculation procedure for the 20th edition of API RP 2A. OTC paper 7153, In Proc. OTC 1993, Houston, Texas, 1993, pp. 201–211]. The main emphasis with regard to the impact of the new API recommendations, however, is that a consistent approach is provided to the calculation of 100-yr directional loads. This includes taking into account the effect of marine growth on force coefficients, modifying the wave kinematics for directional spreading, and considering current blockage effects, conductor shielding effects, and joint occurrence of wave height and current (i.e., using the associated current as being representative of the current that would lead to the 100-yr load). It is concluded that a consistent approach, such as that underlying the new API RP 2A (1993) recipe, is preferable to the current North Sea Design Practice [Det Norske Veritas, Environmental conditions and environmental loads. DNV classification notes 30.5, 1001.] in this field, and thus that the North Sea Design Practice should be updated. This relates in particular to selection of hydrodynamic coefficients. Measurement programmes to obtain full scale global force data simultatneously with wave and current data are furthermore recommended.

Lattice Towers for Bottom-Fixed Offshore Wind Turbines in the Ultimate Limit State: Variation of Some Geometric Parameters

Optimal solutions for offshore wind turbines (OWTs) are expected to vary from those of their onshore counterparts because of the harsh offshore climate, and differences in loadings, transportation, access, etc. This definitely includes the support structures required for service in the sea. Lattice towers might be a competitive solution for OWTs due to less physical impact from waves and less concern for visual impact, This paper addresses the design methodology of lattice towers for OWTs in the ultimate limit state and presents a FEM code that has been developed to implement this methodology. The structural topologies are specified in terms of tower cross-section geometry, the inclination of bracings, and the number of segments along the tower height. For each topology a series of towers is designed in which the bottom distance between the legs has been varied; the resulting tower mass is evaluated as a major parameter for the cost assessment. The study was conducted using the NREL 5-MW baseline wind turbine for an offshore site at a water depth of 35 m. The optimal design is searched for according to tower mass and fabrication complexity. The most economical tower geometry appears to have a constant inclination of bracing owing to its simplicity of fabrication and strong antitorsion capacity. Three-legged and four-legged alternatives have different advantages, the former haring simpler geometry and the latter offering better torsion resistance. As a design driver for offshore steel structures, the fatigue life of the towers designed in the ultimate limit state should be assessed and the structures are consequently modified, if necessary. However, fatigue assessment is out of the scope of this paper and will be done in a later work.

Predictions and Model Tests of an SCR Undergoing VIV in Flow at Oblique Angles

The paper describes how an existing computer code, VICoMo, see Moe & Arntsen (2002), has been extended to analyse Vortex Induced Vibrations (VIV) of SCR’s for situations in which the flow is neither in the plane of the riser, nor perpendicular to it. Various strategies were attempted in order to make the solution deal with flows at oblique angles. It was found that the best approach was to assume that the riser motions occurred in plane perpendicular to the flow, while motions in the flow direction were disregarded. One source of uncertainty is associated with the empirical database in VICoMo which is based on tests in which the flow was perpendicular to the riser elements. The timeseries from the experiment showed large temporal variations. Actually the average motions amplitudes at given points may be as low half of the maximum value. Also the largest amplitudes at different points on the experimental envelope on the riser will usually have been reached at quite different times. These features were not included in the theoretical model, and made direct comparisons between predictions and measurements difficult. We ended up comparing the envelopes of motion response. In several of the cases the predictions matched the experiments closely, for frequency, modeshape as well as amplitudes. In other cases the match was somewhat less satisfactory, however.Copyright

BLADE RESPONSE ON OFFSHORE BOTTOM FIXED WIND TURBINES WITH DOWN-WIND ROTORS

Offshore wind turbines are becoming more common due to the scarcity of suitable land sites. By going offshore, maintenance costs become one of the driving expenses. Hence more reliable components should be implemented on offshore wind turbines. The down-wind rotor configuration does not require as powerful yaw drive as the upwind rotor configuration to align with the wind direction. Thus the yaw system can be simpler with fewer components that can fail and require maintenance. This paper presents numeric simulation studies of how the tower shadow impacts the blades when they pass through the wake behind the tower. The work concentrates on bottom-fixed offshore wind turbines designed for the specifications of the NREL offshore 5-MW baseline wind turbine. The blade response has been compared for a full truss tower and a conventional tubular tower to show how the different tower shadows influence the blades. The blades on the more transparent truss tower experience less root flapwise moment fluctuations due to the weaker tower shadow. The simulations were performed by means of GH Bladed, version 3.82.Copyright

What Is the Optimum Size for a Wind Turbine

Wind turbines have grown tremendously during the last 20 years and it is of interest to investigate how large they can be expected to be. In the present paper several structural aspects have been studied in order to see how they develop as a function of size. It was found that as long as thrust and centrifugal forces dominate, then most aspects of the turbine are independent of scale, provided geometric similarity scaling is performed. The most important deviation from this rule is that selfweight of the blades results in stresses that grow linearly under geometric scaling. The effects of selfweight appear to become important for the largest machines currently available. (5MW)Copyright

Individual pitch control of horizontal-axis wind turbines

An individual pitch controller (IPC) based on the multivariable Linear Quadratic Gaussian (LQG) concept is presented to reduce loads in megawatt-size wind turbines. Most turbines currently installed use collective pitch control to pitch the blades in order to limit the excess of wind power and to regulate the rotor speed above rated conditions. However, research has shown that IPC control is much more effective to reduce blade loads. Both collective and individual pitch control are implemented for the NREL 5 MW reference turbine. Simulation results are used to illustrate the advantage of the IPC approach, and its ability to reduce much of the flap-wise blade motion is demonstrated.

Design philosophy of floating bridges with emphasis on ways to ensure long life

The paper briefly outlines current design philosophies for floating bridges, with special reference to aspects that are deemed to be of interest in the context of very large floating structures. Since the design of submerged floating tunnels is done by people in the same milieu and deals with many of the same problems as in the design of floating bridges, while being in some respects more critical, some of the design philosophy of that subject is also included. The design philosophy and methodology for floating and submerged tunnel bridges draws heavily on Norwegian experiences in two large fields: offshore structures and conventional bridges.

A Review of Hydrodynamic Effects on Bottom-Fixed Offshore Wind Turbines

Recent and historical literature regarding hydrodynamics has been reviewed, with offshore wind turbine support structures in mind. Under conditions of separated flow, several relevant phenomena have been noted which are not covered by the commonly-used Morison equation: 1. damping of structural vibration or slow-drift motion; 2. the interaction of structural vibration and vortex shedding; 3. loads near the free-surface; and, 4. burst motions, caused by impulse-like loading from steep waves. References have been given to books and articles that describe the phenomena in more detail. A form of the Morison equation is proposed which has separate empirical coefficients for each of the velocity and acceleration terms. The coefficients can be determined from existing test data with use of least-squares error minimization. A simplified form of the equation provides a means to obtain conservative bias on both the applied load (bias towards a high drag coefficient) and damping (bias towards a low drag coefficient). Further investigation into free-surface and burst motion (ringing) phenomena is recommended, considering a slender wind turbine monotower in 20 to 50 m water depth.Copyright

Experimental Investigations of the Efficiency of Round-Sectioned Helical Strakes in Suppressing Vortex Induced Vibrations

The present work highlights some aspects related to the analyses of Arctic offshore floating structures. This thesis consists of five papers, which can be divided into two main categories. One category deals with the dynamics of slender structures with an emphasis on the prediction and suppression of vortex induced vibrations (VIV), and the other category examines the process of interaction between sloping structures and sea ice with focus on developing a numerical model to simulate this process in real time. Slender structures, such as mooring lines and marine risers, are very important for the offshore petroleum industry, which is currently approaching deeper waters. Increasingly, attention has been focused on predicting the susceptibility of these structures to VIV. In this thesis, two asymptotic techniques namely, the local analysis and the WKB methods, were used to derive closed-form solutions for the natural frequencies and mode shapes of slender line-like structures. Both the top-tensioned nearly-vertical configuration and the catenary configuration were considered. The accuracy of the solutions derived was established through comparison with other analytic solution techniques and with results of numerical finite element solutions. The effects of the bending stiffness and the effects of approximating the tension variation as a linear function were discussed. Experimental data on the multi-modal in-line and cross-flow response behaviour of a towed catenary model were analysed to examine the usefulness of the solutions for predicting the response frequencies and envelopes due to VIV. Helical strakes are often used as a mitigating measure to suppress the VIV of slender structures. This thesis presented an innovative method to fit ropes helically to a riser in the installation phase. Such a procedure will help to overcome the handling problem associated with the use of conventional sharp-edged strakes. Experimental investigations were then performed to verify the efficiency of these ropes (round-sectioned helical strakes) in suppressing VIV. Systematic experimental investigations including twenty-eight configurations of round-sectioned helical strakes were tested in an attempt to find the most suitable strake configuration. The effects of varying pitch, the surface roughness and the ratio between the cross-flow and in-line natural frequencies on the efficiency of the proposed configuration of round-sectioned helical strakes were also investigated. The process of interaction between sea ice and offshore sloping structures (e.g., conical structures and ship-shaped structures) is quite complex. Modelling this process is very demanding and often computationally expensive, which typically hinders the chances for realtime simulations. This kind of simulation can be very useful for training personnel for Arctic offshore operations and procedures, for analysing the efficiency of various ice management concepts and as a part of the onboard support systems for station keeping. The challenge of meeting the real-time criterion was overcome in the present work. This thesis developed a numerical model to simulate the process of interaction between sea ice and sloping structures in real time. In this model, only level- and broken-ice features were studied. New analytical closed-form solutions were established and used to represent the ice breaking process. PhysX was used for the first time to solve the equations of rigid body motions with six degrees of freedom for all ice floes in the calculation domain. The results of the simulator were validated against experimental data from model-scale and full-scale tests. Accurate predictions of ice actions are also vital to optimise the design of the structures in the Arctic regions. A good understanding of the role of seawater in the process of interaction between the sloping structures and level ice will help to establish reliable models to estimate the ice forces. This work formulated both the static and dynamic bending problems for a floating wedge-shaped ice beam interacting with an offshore sloping structure. For the dynamic interaction, the effects of the water foundation on the bending failure of the ice were studied by comparing the results of an elastohydrodynamic approach with a model of a Winkler foundation. The thesis also investigated the breaking lengths of the ice wedges (i.e., the frequency of the ice loads) as a function of the ice thickness, the compression in the ice and the acceleration of the interaction.

Decentralised Control Design for Load Mitigation in Horizontal Axis Wind Turbines (HAWTS)

In modern MW-size machines it has become a common practice to introduce controllers that provide active damping of turbine components to reduce blade, tower and drive-train loads, whilst optimising energy capture. However, as wind turbines become larger and more flexible, these controllers have to be designed with great care as the coupling between flexible modes increases and so does the potential to destabilise the turbine. The most direct method to address the above issues has been to exploit the pitch control capabilities. Individual Pitch Control (IPC) has been proposed many times over the last few years for load mitigation. Bearing this in mind, this paper investigates two different approaches to design a controller to pitch each blade individually in the wind turbine operating region III. The first one is a decentralised control algorithm and the second one is an H ∞ loop shaping design. A controllability analysis of the wind turbine is also included in the paper. The investigation is conducted based on the NREL 5MW benchmark wind turbine. Turbine modeling and control is conducted in FAST and Simulink.Copyright