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Maritime Safety Aspects Regarding Installation and Maintenance of Offshore Wind Turbines

This research project is concerned with ship and offshore plant safety, and focuses on problems that can arise in the context of offshore wind turbines (OWT) in the Baltic and North Seas. Safety problems in connection with ships can occur during the installation, operation, maintenance and decomposition phases of offshore plants. While these safety problems may involve various scenarios, the physical reasons will have a high level of similarity. A deep understanding of the physical phenomena and the interdisciplinary development of the simulation models in the numerical methods is crucial in generating efficient technical solutions for safety issues without disproportionally increasing project costs or time.

The aim of this research project is to support the wind energy industry as well as national and international organisations by providing numerical methods in an effort to increase ship and offshore plant safety, to reduce the risks for humans, and to decrease the danger for environmental damage.

The research project focuses on computational strategies devoted to the prognosis of accident event sequences and possible consequences from resulting damages. A database for capsizing and sinking accidents with ships will be developed in order to provide test cases for other research projects and to validate numerical methods currently under development. These methods can be applied to analyse accident scenarios and to improve the safety performance of new ships and offshore plant designs. The evaluation of numerical results can also be used to define various categories of preventive and operational measures that can decrease the likelihood of accidents as well as damages and their consequences. Preventive measures would change the design characteristics in such a way that a particular problem would not arise and/or would have less serious consequences. Operational measures would reduce the consequences of an accident. These measures can be implemented during an accident in order to decrease its magnitude and to minimize the consequences; for example, the development of evacuation plans depending on the extent of the damage incurred.

A special feature of the planned numerical investigations is the simultaneous consideration of complex interaction between different phenomena. Examples included refer to the change in hydrostatic behaviour of offshore plants during installation, ship collision, global failure of ship structure due to local failure, and flooding simulation in rough seas under the influence of limited manoeuvring capabilities caused by damages of control devices in heavy weather conditions.

Six work packages (WP) on numerical simulation of dynamic ship and OWT behaviour in critical safety situations are included in the research program. A short summary of the six work packages is given below.

The sinking and capsizing of ships result in the largest number of sea-related fatalities. The offshore segment of OWT installation, maintenance and decomposition is especially high-risk in this regard, because larger structures are handled either by crane operations or by floating procedures, which can potentially lead to capsizing or sinking events. These kinds of events are extremely complex, as the event chain depends on many influences. The computation of such event chains requires a set of highly specialised numerical simulation methods, which, of course, must be adequately validated. This validation, however, can only be carried out on full-scale events, as model tests neglect too many important influences. It is, therefore, the aim of the project to establish a database of selected full-scale capsizing or sinking events which can then be used for the validation of numerical methods. The database will consist of ship and compartment models, major environmental data and a computed event chain that re-simulates the accident. Several steps for each individual accident can be derived from the event chain that can then serve as a validation basis for the other work packages.

The landing operation of a service ship with an offshore wind turbine structure is a complex manoeuvre, especially at high sea states. For an accurate simulation of such a manoeuvre, numerical methods will be developed. The results of the simulation allow defining the limits of the main parameters influencing the safety of the landing manoeuvre with respect to the transfer between the ship and the OWT. To ensure a safe passage between the ship and the offshore structure, only a limited amount of movement between them is allowed. Thus, the ship is equipped with a fender at its bow which is pressed against the offshore structure in order to create a friction force. The material combination and the normal force in the contact area determine the maximum permissible friction force at which limited movement occurs. Therefore, the main tasks of this study are to determine the wave loads on the ship and to model the contact problem between the ship and the offshore structure. By performing fluid-structure interaction simulations, the deformability of the structures, especially the fender, shall be taken into account. The developed methods will allow planning a safe landing manoeuvre, see Figure 1.

Figure 1: Simulation of landing manoeuvre to a monopile structure, Copyright TUHH

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Figure 1: Simulation of landing manoeuvre to a monopile structure, Copyright TUHH

Gravity foundations are being developed in Germany as the preferred OWT foundation and will be used in the Baltic and North Seas in the near future. One of the necessary requirements for permission to operate a German OWT is an analysis of ship collisions with wind turbines in order to estimate both the risk of ship and wind turbine damage, see Figure 2.

The numerical modelling of the collision of a ship with an OWT is highly non-linear. FEM models will be developed considering inelastic modelling of ship structures, inelastic modelling of wind turbine structure including gravity foundation, inelastic and two-phase modelling of soil, modelling of ship movement due to surface waves, surface-based contact between ship and gravity foundation as well as gravity foundation and soil including the possibility of relative normal and tangential movements.

The objectives of this work package are:

Study of designs of gravity foundations and alternative foundation concepts for offshore wind turbines, especially with respect to risks caused by ship collision
Analysis of risk of ship and foundation damage due to collision based on FEM analyses
Design optimization of existing gravity foundations to minimize the risk of ship and foundation damage based on FEM analyses

Figure 2: Simulation of collision of ships with gravity foundations of offshore wind turbines, Copyright TUHH

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Figure 2: Simulation of collision of ships with gravity foundations of offshore wind turbines, Copyright TUHH

Gravity foundations are favourable concepts for future installations of wind turbines located in German offshore fields. The foundation consists of cylindrical- and/or girder-shaped elements made of concrete or preferably steel. Prior to the final assembly of the power plant, the foundations are towed to their location and subsequently lowered to the ground. Filled with dense material, the foundation rests on the seabed. Their design is subjected to cost-related weight restrictions, but not to hydrodynamic aspects. This poses severe challenges to the towing and installation process where hydrodynamic issues are of relevance. The hydrodynamic performance of a foundation during towing and sinking is fairly complex and has not yet been given detailed attention. The complexity occurs when emphasis is given to the interactions of seaways, ground topography, various fluid-phases and free surfaces as well as arbitrary body motions and hydrodynamically interacting floating bodies. Such interactions can only be investigated using numerical methods for viscous multi-phase flows in combination with either meshless or overset-grid techniques.

The main objective of this research is the development of mathematical models for critical offshore operations using realistic wave and wind forces, which are modelled by stochastic diffusion processes. The dynamics of floating and fixed structures and supply vessels under extreme conditions during offshore operations have to be calculated. Furthermore, the influence of strong tidal currents will be analysed. The grounding of jack-up vessels is a very critical operation which involves impacts of jack-up legs into the seabed leading to an instantaneous change of ship stability behaviour, and operation limits of jack-up vessels and floating cranes need to be determined and compared.

In the project, the ultimate strength of windows in ships will be investigated and the strength of laminated glass will be analysed. Recently, several relatively large windows have been arranged in ship hulls and superstructures. In order to ensure sufficient damage stability, it is important that windows do not fail under the static and dynamic loads occurring during an accident. These problems are also relevant for offshore service vessels operating in harsh marine environments. Lower bound estimates for window panes and laminated windows are known; however, failure criteria for such windows have not been precisely defined. The surrounding structure and possible pre-damages can influence the ultimate strength and the failure behaviour considerably. In this project, a method for evaluating the failure probability of window structures will be developed and subjected to high hydrostatic and hydrodynamic loads, and will be verified by experiments, see Figures 3 and 4. The results of experiments currently (2011/12) being performed in an AiF-project will be the basis of the more fundamental research on this problem. Influences from the flexible framing (bonding, clamping) and steel structure as well as pre-damages will be taken into account.

Figure 3: Model test of ultimate strength of windows in ships, Copyright TUHH

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Figure 3: Model test of ultimate strength of windows in ships, Copyright TUHH

Figure 4: Numerical simulation of behaviour of the flexible framing (bonding, clamping) and steel structure, Copyright TUHH

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Figure 4: Numerical simulation of behaviour of the flexible framing (bonding, clamping) and steel structure, Copyright TUHH

The project is funded by

Forschungs- und Wissenschaftsstiftung Hamburg

Figure 3: Turning Circles on deep and shallow (marked with o) water

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Figure 3: Turning Circles on deep and shallow (marked with o) water

Work package Number	Title	Name	Institute
1	Development of a database of full-scale capsizing and sinking accidents for validation purposes	Prof. Dr.-Ing. S. Krüger	Ship Design and Ship Safety
2	Simulation of safety-relevant situations regarding the interaction of service ships with offshore wind turbine plants	Prof. Dr.-Ing. habil. A. Düster Prof. Dr.-Ing. M. Abdel-Maksoud	Ship Structural Design and Analysis Fluid Dynamics and Ship Theory
3	Collision of ships with gravity foundations of offshore wind turbines	Prof. Dr.-Ing. J. Grabe	Geotechnical Engineering and Construction Management
4	Hydrodynamic analysis of offshore turbine gravity foundations	Prof. Dr.-Ing. Th. Rung	Fluid Dynamics and Ship Theory
5	Offshore operations under random sea and wind conditions	Prof. Dr.-Ing. habil. Prof. E.h.E. Kreuzer	Mechanics and Ocean Engineering
6	Ultimate strength of window structures in areas relevant for damage stability and safety	Prof. Dr.-Ing. W. Fricke	Ship Structural Design and Analysis

Prof. Dr.-Ing. Moustafa Abdel-Maksoud
E-Mail: m.abdel-maksoud@tuhh.de

Maritime Safety Aspects Regarding Installation and Maintenance of Offshore Wind Turbines

TU Hamburg

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