Guidelines for the design of offshore wind farms: a new approach in offshore installations
R. G. Nicolosi, A. Lo Nigro, F. Petrillo, S. Copello, L. Brunori
RINA Industry, via Ilva 2, 16128 Genova
Tel: +39 010 5385868 - Fax: +39 010 561499
Raffaele.Nicolosi@rina.org
Abstract
This paper presents an overview of the newly issued RINA guidelines for the design of offshore wind
farms, which address the different aspects and the new challenges of this type of offshore installation.
By examining different support structures for wind turbines, as opposed to oil & gas installations in the
Mediterranean, weakness and strength points in the application of existing RINA rules for the design of
fixed offshore structures to these new plants are analysed. As a result of the comparison between
traditional offshore structures and wind turbines installations, the importance of the new RINA guidelines
for the design of wind farms is presented. Finally, a case study is shown: the first Italian offshore wind
turbine design appraisal and installation.
Keywords: Wind Energy; Certification; Guidelines; Wind Farm; Offshore; Wind Turbine; Support Structure.
Introduction
Since the late 50s a great number of offshore structures have been installed and are still being installed
all around the world in order to exploit oil and gas reservoirs. Fifty years of research, engineering effort
and experience has led all the companies and parties involved in the different aspects and phases of
the life cycle of these structures to a consolidated know-how and a widely recognized modus
operandi.
Depending on the specific features of the site and the expected production capacity of the plant,
different types of structures are installed. Together with economic and productions needs, the
environmental loadings and installation capabilities have been the key drivers for the selection of the
most appropriate design.
Over the years offshore installations have become increasingly complex and challenging as the search
for new reservoirs has led companies to drill in deeper waters and harsher environments.
In this respect the North Sea is an excellent
example: gravity based structures (GBS),
traditional piled jackets, tension leg platforms
(TLP), spars, CALM (catenary anchor leg
mooring) and SALM (single anchor leg
mooring) buoys, FPSOs (floating production,
storage and offloading) systems etc. show
the offshore design evolution over the years.
The
demand
for
renewable
energy
generation is now rapidly growing and
offshore wind turbine installations are clearly
becoming one of the most attractive and
efficient answers. The lessons learnt in the oil
and gas field will be crucial for the rapid and
effective development of these new
structures. On the other hand, even though
some of the technical issues remain the same for wind and traditional oil and gas support structures,
there are significant differences related to topside facilities (turbine versus production plant) and
hence cost-benefit analyses which will lead to very different design choices.
The Mediterranean will therefore see installed a wide variety of offshore structure typologies that in the
oil and gas field can only be seen in environmentally more unfriendly areas such as the North Sea or
the Gulf of Mexico.
The choice of the support structure
The main factors governing the choice of the support structure are:
 Topside weight, size and induced loads (the “payload”);
 Site environmental conditions and water depth;
 Cost of the structure (materials and construction / technological complexity);
 Installation equipment and procedures.
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The purpose of an offshore installation is confined to what the topside facilities are intended to
accommodate (deck, wind turbine, etc.). The economic benefit of an investment lies in how many
MW or how much oil or gas the plant can produce. Therefore weight and size of the topside are the
starting point of the whole project and can rarely be changed.
Apart from the support supplied to the piping elements running from and to the seabed (conductors,
risers, cables, etc.), the main purpose of an offshore support structure is to hold the topside while
keeping the environmental loading acting on itself to a minimum in order to reduce the cost of the
structure itself (less loading means less structural steel).
The reduction of wave, wind and current loadings on a structure has been investigated for years and
the actual offshore installations are the product of this research: tubular “semi-transparent” structures
(such as jackets and monopods), catenary and tension leg moored topsides (TLP) are some clear
examples.
What kind of support best fits the purpose of a specific installation strongly depends on the cost of the
structure itself in terms of materials involved and construction yard capability and availability,
compared with the economic benefit of the production facilities to be operated.
In addition to the above, the installation issues play a fundamental role: the structure can be upended
using a crane vessel, launched from a barge, towed in free floating and lowered to the sea bottom
using buoyancy devices, etc. All the different options embed two critical and fundamental aspects:
 Size of the structure to be installed;
 Cost of the operation.
For example a crane vessel lifting capacity limits the weight of the structure to be installed the cost of
an alternate procedure, such as a floatover operation, can be greater than the cost of the structure
itself.
Oil and gas structures in the Mediterranean Sea
The Mediterranean is a relatively calm and shallow sea in most of the areas rich of hydrocarbons. For
example, most of the Italian gas platforms are located in the Adriatic Sea with water depths going
from 20 to 80 meters.
These two factors, together with cost-benefit analyses, have brought to the selection of fixed tubular
beam structures for nearly all the Mediterranean installations. These structures go from small monopods
to eight leg jackets piled to the seabed.
The jacket installation procedure does not change a lot from case to case: the jacket is transported
from the fabrication yard to the offshore site on a towed barge, then it is lifted (and upended, if
necessary) using a Heavy Lift Vessel and with the aid of buoyancy tanks often part of the jacket legs.
Finally the jacket is piled to the seabed and all the appurtenances and the deck are installed onto it.
The installation capability in the
Mediterranean has therefore grown
to meet the market needs and the
crane vessels actually working in the
oil and gas field go up to thousands
of tons of lifting capacity.
Nowadays the estimation of money
and time needed to install a new
platform at sea is a known variable
and the economic evaluation of the
whole project is a relatively standard
procedure.
This lack of diversity has led to a high
specialization and focus on this type
of fixed offshore structure while generating an obvious loss of interest (and consequent engineering
know-how) in other typologies.
The previously mentioned RINA rules are an interesting and natural outcome of this process, as they are
aimed at giving a guideline for the construction of fixed offshore structures, while the other RINA
offshore codes (e.g. the rules for Mobile Units and FPSOs) apply to a relatively smaller number of
projects.
Offshore wind turbines: a new approach
A great part of the offshore oil and gas world technical knowledge is used tout court in the wind
energy field. This invaluable database of information and engineering excellence gives the offshore
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wind farm technology the opportunity to grow very rapidly simplifying and accelerating the
experimental phase.
The knowledge of phenomena such as moored structures fatigue, soil properties, floating structures
motion or flexible behaviours, installation problems and costs and many other give a great advantage.
On the other hand there is a great difference between the oil and gas and the wind energy
installations: the “payload”. In fact a traditional platform carries a deck which in essence is a very
complex plant. Risers, conductors, compressors, generators, piping systems with valves and pressure
vessels, flares, helidecks, living quarters etc, are all necessary ingredients that give birth to heavy, large
topsides. Moreover, the load that a deck transfers to a jacket is essentially gravitational (vertical) while
a wind turbine also induces a strong overturning moment due to the horizontal force generated by the
thrust of the propeller at the top of the tower.
In essence a wind turbine is much smaller and lighter, induces significantly different loads on the
structures below and is much more delicate to install. All these differences lead directly to a question:
will the support structures used in the oil and gas field be also suitable for offshore wind farms? The
answer is obviously no, but to what extent?
Another aspect to look into is the cost of the structure compared to the MW to be installed. The
construction cost of a traditional jacket can go into several millions of euro, depending on the amount
of steel and therefore the weight of the topside and the height of the structure (i.e. the water depth).
The majority of wind turbine towers installed offshore up to now are simple single piles, relatively cheap
to produce and to design. For this kind of structure the cost/benefit ratio can be kept down to
reasonable values for shallow water installations and on this basis wind farms offshore the United
Kingdom, Denmark, Holland, Germany and other European countries have been installed near the
coast, generally in water depths up to 20-30m.
In other areas, like many of the Mediterranean coastal sites where the wind speeds are sufficiently high
to justify a possible wind farm installation, the water depths increase rapidly offshore and the cost of a
column structure piled to the seabed can be unreasonably high.
Moreover, the rapid increase of the wind turbines efficiency and dimensions, leading to a massive
growth of potential MW installed per unit, will require growing design wind force and therefore the
need to look towards installations in outer seas and thus in deeper waters.
Installation costs play another
important role. Most of the wind
farms around the world have been
installed using crane vessels. This
method is the traditional one used
for typical offshore structures like
jackets
and
monopods.
For
example the single crane upending
procedure has been experienced
for years and used for nearly all of
the
oil
and
gas
platforms
installations in the Mediterranean.
On the other hand the daily rate of
suitable crane vessels and their
availability (also in terms of weather
window for the operation, i.e.
possible standby time) can prove to
be economically challenging for
the project.
All the issues above have urged engineers involved in this field to find possible alternative solutions to
the traditional single column structure both in the design of the tower and the installation procedure.
From a simple examination of the evolution of the oil and gas offshore structures from shallow to deep
waters it is clear that the classic jacket type structure cannot be the appropriate answer for the
previously described reasons related to cost of the structure, wave and current loadings and
installation capabilities.
Apart from some feasibility study, aimed to find and revamp existing jackets placed on reservoirs that
are no longer productive, in order to replace their decks with wind generators, the most popular and
convincing alternative that has been brought forward in these last years is the floating structure,
specifically tension leg platform type (TLP). This kind of platform can be often seen in very deep waters
in the North Sea and the Gulf of Mexico. The main advantages are:
 reduction of wave loading;
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 drop of construction and steel costs.
Nonetheless there are some drawbacks: the engineering effort and expertise necessarily involved has
to be massively increased. Aspects like motions and stability of the floater, tendon (or chain) fatigue
and hydrodynamics and many other, need to be investigated thoroughly.
It is therefore clear how the challenges before wind farms development are still open and in many
ways different to those already faced in the oil and gas filed.
Cables and pipelines
The electric power generated by wind turbines, if not stored at site, for example with the production of
hydrogen from the sea water, needs to be transferred through cables laid on the seabed. The
engineering knowledge built in the oil and gas field in this specific branch is huge. Pipes and cables
have been laid all around the world, in shallow
and ultra deep waters, in sandy and rocky
uneven sea beds.
FPSOs flexible risers and their underwater
behaviour now have had years of study and
assessment.
In order to reduce loadings on the umbilical and
to allow it to follow the motions of the floating
unit without suffering from wear or damages, a
series of subsea configurations now are
implemented through the use of mid water
arches, tethers, floaters etc.
All this know-how will have to be transposed
and some kind of migration of engineers specialized in pipeline and flexible laying and design towards
wind energy is absolutely necessary. Especially for floating platforms the issues related to the fatigue
and motions of the flexible cables induced by both the dynamics of the platform and the wave and
current loading, can be determining for the whole project and the profound knowledge of this
particular engineering art, which includes the understanding of complex phenomena like VIV (vortex
induced vibrations), is fundamental.
The role of RINA: a renewed effort and new guidelines
The presence of RINA’s Industry Division in the offshore field in the last fifty
years has been constant and essential for many oil and gas operators,
especially in the Mediterranean Sea.
Working together with the major oil and gas companies (ENI above all),
the Industry Division of RINA has become to be the leading offshore
certification societies operating in the Mediterranean. As a result of this
synergic effort and continuous cooperation between all stakeholders in
1973 was issued the first edition of “RINA Rules for the Classification of Steel
Fixed Offshore Platforms”. These guidelines cover all aspects of an
offshore fixed structure, going from design and construction to load-out,
transportation, and final installation stages.
The continuous support during all the above mentioned stages has lead
to the construction of safe and reliable structures.
Much of this know-how is or will be transferred with a natural movement of
experts from one field to the other, but it is also of primary importance
that all parties involved cooperate during this learning process as much
still needs to be done.
In this frame RINA wants cover a leading role. Being a major offshore
certification society in the Mediterranean, RINA can provide support
thanks to the many years of offshore activity and will also have a global
view of many projects which will bring forth a wide range of technical
issues and challenges.
In these first stages, in which the investments in this sector are still negligible compared to the oil and
gas market and the know-how flux from one field to the other is minimum, RINA will have to play a role
which goes far beyond the pure certification activity. A new generation of engineers will emerge and
will need to be guided in order to gain in the quickest possible way the basic and consolidated
knowledge of offshore structures and operations.
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RINA is working to confirm and expand its role as a classification and certification society not only in the
Mediterranean and the offshore wind development requires new rules and guidelines. Even if the
traditional rules for fixed platforms have proven to be strong and reliable, all the arguments mentioned
in this paper lead us to the firm understanding that a new set of guidelines is necessary in order to
include structures typologies and technical features that have not been yet addressed. The guidelines
include new sets of regulations and indications especially related to loadings due to the turbine trust
overturning moment (and its effect on the tower structure and foundations) and the dynamic
behaviour of floating structures. The latter will address in particular seakeeping calculations, stability
aspects, motion induced fatigue (for example on the tendons or chains) and installation procedures.
In order to properly address these new challenges, the Industry Division of RINA is increasing its
technical knowledge, also with the aid of state of the art tools for the motion and stability analysis of
floating structures and the investigation of complex flexible dynamics.
This great effort will result in providing a solid base on which designers, offshore operators and
essentially all offshore wind energy stakeholders will be safely and reliably able to build their future.
Sky Saver and the Tricase project
RINA Industry Division is fully involved in the
construction of the first Italian commercially operating
wind energy converter. The FCU (First Commercial
Unit) is the first tension leg platform wind turbine ever
built.
The construction is now underway in Brindisi (Italy)
and Varna (Romania),
and the wind tower will
be installed at the
Tricase site (Otranto
Channel). This first unit
will be followed shortly
by 25 more. This offshore
wind farm will have an
installed capacity of 92
MW, enough to cover the needs of more than 75,000 households.
Being a new design and experimental project the construction of the FCU
has been preceded by a prototype that has already been built and
installed offshore.
RINA has followed all design and construction aspects from design
appraisal to manufacturing survey of the complete structure, both of the
prototype and the FCU, providing Sky Saver with all round support.
The activities performed include:
 Complete environmental study for extreme loads and fatigue;
 Review of the dynamic analyses of the interactions between the loads on the wind turbine and
the loads on the platform, due to sea current, sea wave, tide and wind;
 Approval of the design of the platform;
 Supervision in the shipyard during the construction of the platform;
 Definition of quality control plans;
 Approval of the installation procedure;
 Supervision during the marine operations;
 Issuance of the certification.
During the whole project RINA has provided the project all the know-how and assistance going from
the basics (for example the choice of the proper materials for the different parts of the structure) up to
the most complex issues (like platform dynamics and environmental loadings).
The constant cooperation between RINA and Sky Saver has lead to two important goals: the
construction of the first Italian tension leg wind tower and the improved technical knowledge on both
sides.
Conclusions
Offshore wind farms are proving to be one of the most attractive and efficient answer to climate
change and a valuable source of green power generation.
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Through a simple analysis of similarities and differences with traditional oil and gas installations, it is clear
how if on one side the experience and skills gathered during the last 50 years in the offshore field are
the necessary basis, it is also essential to make a step forward.
The new challenges demand a research and experimental effort which has brought and will hopefully
go on bringing all parties to work together towards increasing know-how and efficiency.
The role of RINA is to guide all the stakeholders through this process and provide support throughout
the project. RINA is constantly working to achieve these goals and the issue of the guidelines for
offshore wind farms will give a fundamental support.
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