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Construction and installation engineering for
floating wind turbines
A.P. Crowle*, and PR Thies
University of Exeter, College of Engineering, Mathematics and Physical Sciences
Renewable Energy Group, Penryn Campus, Treliever Road, TR10 9FE, UK
Abstract. The construction and installation engineering of floating
offshore wind turbines is important to minimize schedules and costs.
Floating offshore wind turbine substructures are an expanding sector
within renewable power generation, offering an opportunity to
deliver green energy, in new areas offshore. The floating nature
of the substructures permits wind turbine placement in deep water
locations. This paper investigates the construction and installation
challenges for the various floating offshore wind types. It is concluded that
priority areas for project management and design engineers minimising
steel used in semi submersible construction, reducing the floating draft of
Spars and for Tension Leg Platforms developing equipment for a safe
installation.
Specifically tailored design for construction and installation includes
expanding the weather window in which these floating substructures can
be fabricated, transported to and from offshore site and making mooring
and electrical connection operations simpler. The simplification of
construction methodology will reduce time spent offshore and minimise
risks to installation equipment and personnel.
The paper will include the best practice for ease of towing for offshore
installation and the possible return to port for maintenance. The
construction and installation process for a floating offshore wind turbine
varies with substructure type and this will be developed in more detail in
the paper. Floating offshore wind structures require an international
collaboration of shipyards, ports and construction vessels, though to good
project management. It is concluded that return to port for maintenance is
possible for semi submersibles and barges whereas for Spars and TLP
updated equipment is required to carry out maintenance offshore.
In order to facilitate the construction and to minimize costs, the main
aspects have to be considered i.e., the required construction vessel types,
the distance from fit-out port to site and the weather restrictions.
1 Introduction
Floating offshore wind turbines are being developed to produce renewable energy in
water depths beyond about 65 metres. Options for floating offshore wind turbines are
* Corresponding author: ac1080@exeter.ac.uk
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© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative
Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).
complex structures and are described in section 2. The requirements and facilities for
construction are described in section 3. Typical construction techniques are presented in
section 4 and installation sequence is provided in section 5.
2 Structures
2.1 Size and complexity
Due to the size and complexity of floating offshore wind structures, as well as the weather
conditions in which they are deployed and installed, the actual construction translating the
design into a physical reality requires very sophisticated planning, engineering,
management, and verification. These construction activities are embodied under the overall
term constructability. The four main types of floating offshore wind turbine are) are shown
in figure 1 and are the barge, semi submersible, the Spar and the Tension Leg Platform
(TLP) and are classed per ref [1], [9], [10] and [11].
Fig. 1. Floating offshore wind turbine types (Ref [3]).
Floating offshore wind turbine (FOWT) structures and their associated subsea cables are
very capital-intensive. They constitute a high early expenditure which increases the need to
control construction costs. FOWT construction comprise thorough planning and competent
project management.
The type of Floating offshore wind turbine determines the selection of construction
methods, facilities and stages, the procurement and assembly of materials and fabricated
components, the organization and supervision of the work, and the training of workers.
During construction of the floating offshore wind turbine weight control is very important.
Table 1 gives details of the constraints per type are presented.
Table 1. Construction restraints for FOWT.
TLP SPAR BARGE
SEMI
SUBMERSIBLE
Very Low intact stability,
ref [4] and [5]
Needs solid ballast after
upending, ref [6] and [7)
Long fit-out
quay length
Long fit-out quay
length, ref [8]
Temporary buoyancy
needed offshore
Deep water required for
inshore construction Temporary buoyancy
in drydock
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Requires specialised
offshore crane vessel
Deep water required for tow
to offshore site
Base for tendons needs
offshore crane vessel
Requires inshore crane
vessel to install turbine
2.2 Floating wind turbine components
The main components of a floating offshore wind turbine are given in figure 2.
Constructability employs work simplifications and standardization techniques in order to
overcome the difficulties inherent in complex and sophisticated construction in an offshore
environment. Its scope includes construction, deployment and installation. Subsequent
removal, towing ashore for maintenance, relocation, or salvage can also be considered as
part of the constructability.
In constructability planning, it is essential to formally set these stages forth by title,
description, and schematic drawing. Each of these major stages can then be subdivided into
the detailed stages required. The stages should be further portrayed by a series of drawings
or sketches. Isometric drawings to the structural capacity under differential heads, the intact
stability performance afloat, and the instrumentation with its real-time readout are
developed during the design and planning stage.
Fig. 2. Floating offshore wind turbine components (Ref [1]).
Engineering challenges are shown in Table 2 which compares construction and
installation of floating offshore wind turbines (FOWT).
Table 2. Qualitative comparison of construction and installation FOWT options.
Item TLP SPAR BARGE SEMI
SUBMERSIBLE
Construction Land Area Medium Medium Medium Large
Ease of onshore construction Medium Medium Medium Medium
Seabed area Low Large Large Large
Intact stability in tow Low Large Medium Large
Attachment of moorings Complicated Standard Standard Standard
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3 Construction stages for floating offshore wind turbines
A floating offshore wind turbine (FOWT) structure goes through a series of very distinct
stages as it moves from construction to loadout (or float-out), to completion afloat, to
transport, to installation, and to mooring connection and subsea hook-up.
Key considerations which have been inadequately addressed in the planning of previous
structures include:
x Draft, with relation to available water depth during initial stages of construction afloat
x Intact stability during all stages of installation; including the effects of free surfaces
x Tie-down seafastenings of structures on heavy transport vessels
x Hydrodynamic response of structure during tow, especially acceleration forces
x Effect of pressure and temperature changes on function of instrumentation, valves
x Wave and current forces during construction and installation period.
x Effect of shallow water and minimal under-keel clearance
x Human error in ballasting control
x Inadequate weight and tolerance control during construction, leading to mishaps
x Vortex shedding, vibration and fatigue
x Welding temporary attachments and closures without following prescribed procedures
The division of the project into stages and the subdivision of each stage into actual steps
is a procedure by which the most efficient method can be selected for each step. Sound
judgment and experience will tend to integrate closely related steps within each stage. In
floating offshore wind turbine construction, however, with its revolutionary developments
in equipment, tools, and instrumentation, with its new structures and systems and
environments, specific experience may not exist.
4 Facilities and methods for construction
4.1 Design and procurement
Design and procurement work is as follows:
x Seabed geotechnical surveys
x Design and procure materials
x Model Tests
x Set up substructure shipyard, ports for turbine fit-out, mooring laydown and cable
laydown
4.2 Early stages on shore
For floating offshore wind turbine structures, the early stages of construction are carried out
at a shore base. This base may be purpose-built for this one project or may be a relatively
permanent facility. The area for such a facility must be adequate to accommodate not only
the structure and/or components themselves but also storage of materials, access roads,
support buildings, and infrastructure facilities.
Adequate roads must be constructed, e.g., of gravel, around the FOWT, and adequate
drainage installed. The construction yard must be stable and firm enough to support the
new FOWT substructure and the construction equipment. Since yards are located near the
water, the original soils may require stabilization and fill such as compacted shell or
crushed rock on which to operate. In weak sediments, filter fabric or pile supports may be
required, over which rock may be placed, or a reinforced concrete slab. Particularly critical
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loadings occur with large crawler cranes, since when they pick their maximum loads,
almost the full load of the crane itself plus the lifted load are concentrated on one crawler.
A number of methods have been developed to facilitate this movement from onshore to
offshore. Some of these are briefly described below.
x Build on dry land and move onto a Heavy Transport Vessel, then travel to the fit-out
port for FOWT float off, figure 3.
x Build in a dry dock and float the FOWT to the fit-out port, figure 4
Fig. 3. Side loadout onto, tow to fit-out port, ocean tow complete FOWT(Ref [3]).
Fig. 4. Steel barge in dry dock and substructure tow and ocean towout (Ref [2]).
5 Installation sequence
5.1 Early offshore activities
Before towout of the FOWT the offshore moorings and inter array power cables are
installed
5.2 Offshore activities
Offshore activities in sequence are:
x Tow complete Floating Offshore Wind Turbine to the offshore site
x Connect offshore moorings
x Connect offshore power cables
x Final commissioning
6 Discussion and conclusions
6.1 Discussion
The Spar has the advantage of low motions during the tow out, but has a deep draft of over
70m. The Semi submersible only requires shallow water (less than 10m) at the fit-out port,
however it has a large substructure weight. The steel barge is simple to construct but
requires many mooring lines. The TLP requires temporary buoyancy and a floating crane
vessel for offshore installation.
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Engineering work for construction and installation includes:
Naval Architecture = ballast, intact stability, ocean tow motions
Structure engineers = strength for construction and installation
Marine engineers = ballast piping
Electrical engineers = subsea cable design
Safety engineers = transfer systems for personnel
6.2 Conclusions
There are several separate ports required for the construction and installation of floating
offshore wind turbines:
x Shipyard for floating offshore wind turbine substructure construction (this is either on
land or in a drydock)
x Laydown area for anchors (or mooring piles) and mooring lines
x Laydown area for the offshore power cables
x Fit-out port with laydown area for wind turbine components and space for a large crane.
Alan Crowle thanks his colleagues at the University of Exeter for their assistance in preparing this
paper. Professor PR Thies would like to acknowledge the support through the EPSRC Supergen ORE
Hub [EP/S000747/1].
References
1. DnV ules For Classification Floating Offshore Wind Turbine Installations, 2020
2. ‘www.bw-ideol.com photos, 2020
3. ‘www.principlepowerinc.com photos, 2020
4. ‘www.bw.com photos, 2020
5. ‘www.sbm.com photos, 2020
6. ‘www.equinor.com photos, 2020
7. ‘www.stiesdal.com photos, 2020
8. ‘www.gustomsc.com photos, 2020
9. ABS Guide For Building And Classing Floating Offshore Wind Turbines, 2020
10. Bureau Veritas Classification and Certification of Floating Offshore Wind Turbines
2019
11. Class NK Guidelines for offshore floating wind turbine structures, 2012
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