Final report for WP4.3: Enhancement of design methods ... - Upwind

UPWIND WP4: Offshore Support Structures and Foundations
clusion from these studies was that line dynamic analysis should be conducted when the wave frequency response
of the vessel is large, when the water depth exceeds 150m, or when the mooring line includes large
drag elements such as chain moorings [79].
In order to make the design of floating offshore wind turbine platforms as efficient and commercially viable as
possible, it would be useful to derive equivalent results for FOWTs. In order to do this, the existing floating wind
turbine design tools must be extended to include accurate modelling techniques for mooring lines, including
dynamic effects. Further research into mooring systems specific to floating wind turbines could then be performed,
incorporating the following elements:
• Quantitative comparison between the different methods for calculating mooring line tension forces for
FOWTs
• Analysis of which aspects of mooring line behaviour are important specifically for FOWTs
• Analysis of which mooring system types and configurations have the most dynamic effect on FOWTs
and therefore need to be designed using a full dynamic analysis
• Investigation into the transition depth at which dynamic mooring line effects become non-negligible for
FOWTs.
There are two main approaches to modelling mooring lines for floating offshore wind turbines. The first option is
to couple a dedicated mooring line code with a wind turbine analysis code. The advantage of this approach is
that it enables mooring line dynamic effects such as line inertia, drag of the line through fluid and vortex shedding
to be fully accounted for. An attempt has been made by Jonkman et al. to couple the dynamic mooring line
system LINES of SML with the aero-elastic wind turbine codes FAST and ADAMS. However, this attempt was
abandoned after it was found that LINES encountered numerical instabilities when modelling the slack catenary
mooring lines of interest [82]. The most fruitful attempt to date is the coupling between offshore floating structures
code SIMO/RIFLEX and the multi-body wind turbine code HAWC2, described in [88]. However this approach
is still limited in that the floating wind turbine cannot be modelled as a single integrated dynamic structure,
since the two problems must be solved in separate programs and information exchanged between the
programs at a single interface point. This interface was also known to be quite numerically unstable.
The alternative approach is to extend the capabilities of existing wind turbine design tools to incorporate mooring
line modelling techniques. The force-displacement and quasi-static representations discussed in Section
6.1.5 fit into this category. This integrated approach is more common than the coupled approach because it is
simpler to implement and generally gives greater numerical stability. The remainder of this section therefore
deals with this second approach.
In order to enhance existing FOWT design tools it is useful to incorporate methodologies and techniques currently
available from other industries e.g. oil and gas. A number of software tools exist which can model the
behaviour of mooring configurations for floating platforms. Section 8.3.1 presents a review of the available
mooring line codes, with the aim of better understanding the commercial options available for incorporation into
FOWT design tools. Section 8.3.2 presents initial results from simulations performed with the multi-body system
approach. It is hoped in the future to be able to make comparisons with results from full dynamic mooring
line codes, in order to analyse the relative strengths and limitations of the different modelling methods.
8.3.1 Review of mooring codes and approaches
A list of candidate mooring codes is drawn up and the most suitable options for FOWTs are assessed, taking
into account the associated computational effort, flexibility and ease of adoption. As well as comparing tools,
the several types of modelling approaches are also compared. The aim of this exercise is to obtain a clear view
of the modelling approaches that are applied in commercial packages and their accuracy, in order to decide
which is the most appropriate package and modelling approach for incorporation into FOWT design tools.
Table 8.20 shows a list of various candidate tools/packages which currently exist, along with a brief account of
their properties. The tools shown in bold are those which have been investigated in detail. These four were
chosen for detailed investigation mostly because:
• ROMEO was developed within the GL group, so GL Garrad Hassan has access to the source code.
• MDD is open-source so can be altered and used freely.
• AQWA and OrcaFlex are both widely used within the offshore engineering community.
120