Assessment of Offshore Wind Farm Decommissioning Requirements
The decommissioning study gathered the best available science related to how wind turbines and other equipment should be managed at the end of their life and identify what type of financial assurance should be in place.
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The Ontario Ministry of the Environment and Climate Change (MOECC) retained GL Garrad Hassan Canada Inc. (DNV GL) to provide a study on decommissioning requirements for potential offshore wind projects in the Great Lakes. More specifically, the objectives of the mandate were:
- to collect and present an international overview of offshore structures decommissioning requirements, regulations and costs, focusing on the type of structures used in offshore wind
- to design likely scenarios for offshore wind decommissioning in Ontario’s fresh water lakes
- to develop cost estimates to inform requirements for decommissioning and financial assurance requirements
By the end of 2015, over 10 gigawatts (GW) of global offshore wind capacity were fully operational, the great majority off the coasts of northern Europe. There are no operational commercial-scale projects in North American waters, though there is significant potential including the Ontario waters of the Great Lakes.
To achieve the above goals in the Study, DNV GL has provided descriptions of current offshore wind technologies and other offshore structures with an emphasis on their relevance to the Great Lakes. In addition, a review has been made of international jurisdictional decommissioning requirements. These overviews are complemented by the definition of potential offshore wind development scenarios in the Great Lakes, the development of a decommissioning cost estimation tool relevant to these scenarios, and the estimation of decommissioning cost ranges that would be anticipated. The costing tool is delivered to MOECC separately for their internal future use.
Offshore wind farms – an overview
An offshore wind farm may comprise up to or over 100 wind turbines supported by foundation structures and electrically linked by array cables buried in the sea-bed. Most offshore wind farms also include an offshore substation to collect the power for transmission to shore via the export cable. Other major elements are the onshore cabling, substation and operations base. Offshore elements may include a meteorological mast.
In over 20 years of offshore wind development, the design of offshore wind turbines has moved from marine versions of onshore turbines to specifically-designed offshore machines, and offshore wind farms have moved from a handful of turbines with a few megawatts (MW) capacity to projects of over 100 machines totalling several hundred MW capacity. In the absence of transportation restrictions offshore, their sizes have grown to over 6 MW with 10 MW machines foreseeable in the near future. Turbine foundation structures may be steel monopiles, lattice or jacket structures, gravity-based structures, tri-piles or tripods, or suction buckets with the selection largely depending on the water depth, turbine size, and the nature of the sea-bed. In deep waters, floating platforms are more recently being developed for support of turbines. Depending on the site conditions, protection against sea-bed scour (erosion of the seabed around the foundation) and against the forces of ice may be needed. Port and shore facilities are required to support offshore construction, maintenance operations and ultimately the decommissioning activities.
More detail of offshore wind energy technologies are given as part of the consideration of decommissioning methods.
Other offshore infrastructure
Offshore Oil & Gas (O&G) and other marine structures, in particular bridges and harbour constructions, provide structural design and marine decommissioning experience that can be transferred, to some extent, into the offshore wind sector.
There are similarities in the range of vessels used in the O&G industry from survey vessels, jack-ups and crane vessels to support vessels; and in techniques ranging from design against corrosion protection and ice forces to the methods for working underwater. Ice cones, concrete collars and ice deflection skirts are some of the designs being employed by the O&G industry where there is floating or fixed ice. The main differences of offshore wind compared with O&G structures arise primarily from the scale and numbers of installations (multiple similar installations compared with mainly one-off designs), generally lower weights and dimensions, the nature of the structural loading with turbines experiencing high overturning moments, and the much lower potential for environmental pollution or risks of accident. Care should therefore be exercised when transferring O&G experience to the offshore wind industry. In the field of decommissioning, the main experiences gained from the O&G industry are the need to take decommissioning into account at the design stage and throughout the operational life, and the developments in underwater cutting and other subsea techniques.
Experience from bridges and ports in the Great Lakes can help inform offshore wind farm design, particularly for withstanding floating and solid ice pressure on support structures. For example, methods of ice protection are well established such as the use of ice-cones around bridge piers. In addition construction techniques are well established though typically in relatively shallow waters and in relatively benign coastal conditions.
Jurisdictional and literature review
The Study reviews existing guidelines, regulations, codes of practice and best practices for decommissioning offshore energy projects, including ensuring adequate financial assurance. The main comparators were the United Kingdom, Germany and Denmark which have the most mature offshore wind industries, with a secondary review of the United States, Canada, the Netherlands, Belgium, Sweden, Norway, Finland, Ireland, Japan, China, Taiwan and South Korea.
The rights and responsibilities of decommissioning offshore energy installations are determined by international, national and regional laws and regulations. The primary international regulation is the United Nations Convention on the Law of the Sea (UNCLoS) which is transposed through the International Maritime Organisation (IMO). In the North East Atlantic, the Oslo and Paris (OSPAR) treaty is also binding.
The basic principles are that:
- ideally, all offshore installations or structures should be completely removed when no longer used. In practice, however, there is some flexibility provided. This is usually on the basis of extreme cost, extreme safety risk, or when removal will cause greater environmental damage than leaving in situ. In some instances, installation may be left in situ if they will serve a new use
- polluter pays, with regulators seeking to ensure that developers make adequate provision to meet decommissioning liabilities
Most nations make an adequate decommissioning concept or plan a requirement for granting either a lease or permission to construct the project. Denmark, however, does not require this plan until fairly late in the life-time of the project. All nations require that some form of financial assurance is made that ensures that decommissioning can be funded. In general, the value is calculated by the project owner and approved by the lead regulatory agency. In all countries an irrevocable bond or cash deposit is acceptable as a guarantee.
According to the IMO convention, it may be permitted in certain cases to allow structures to remain at least partially in place, for example if the removal processes are likely to cause more environmental damage than leaving them in situ. In practice for offshore wind projects, this condition is most likely to apply to piles, to buried cables, and to some scour protection.
Beyond these basic principles of the extent of decommissioning and the provision of financial assurance, main areas to be considered are:
- the strength of legislative backing and the liabilities on developers
- the flexibility in the provision of guarantees by developers, and the appreciation of by regulators of uncertainties in the estimation of decommissioning methods and costs
- the balance between ensuring environmental protection while minimising the burden on developers, for example taking into account the differences in pollution risk between offshore wind and oil and gas decommissioning
- the reflection of broader ideological and cultural approaches in the regulatory involvement. For example in Denmark the regulator has a very active role undertaking the initial decommissioning cost assessment on behalf of the developer; whereas in the UK the regulator issues guidance and works with the developers on a case by case basis to approve the plan
Technology selection and description
Offshore wind technologies are reviewed with a focus on the selection and description of the most likely technologies for Great Lakes offshore wind projects. The selection of turbine foundation is a critical decision based on the site-specific conditions and the turbine type. Generally an iterative approach is adopted as increasingly detailed data is collected during the development process.
In the Canadian Great Lakes the very wide range of water depths, geological characteristics, wind speeds and extent of fresh-water ice lead to the conclusion that none of the major foundation types for wind turbine generators can be ruled out at this stage. Wind speeds tend to be higher further away from the shore and are greater in Lake Superior and Lake Huron, and therefore more remote from populations. Water depths range from an average 19 m in Lake Erie to 149 m in Lake Superior. Lake bed geology plays an important part in determining the optimum foundation and ranges greatly from regions with very hard bedrock and little overlying sediment which would prevent the driving of piles, to regions with softer sedimentary rock. A particular feature of the Great Lakes is the winter ice cover which will influence the choice and design of foundations and also restricts the length of the shipping season. Ice-free waters occur where water depths are greatest. However, tides and water currents are negligible in the Great Lakes, and wave heights are less restrictive than in the open ocean. There are some limits to marine operations from fog.
The selection of specific locations for offshore wind farms is outside the scope of this Study. Major deciding factors will be the wind speeds, water depths and sea-bed geology, though the selection will also be influenced by other factors such as remoteness from ports, proximity of electrical connections and environmental permitting. For the purposes of the Study, a range of site-types is considered with different combinations of these factors. Similarly, the selection of specific turbine types is not included in this Study and for the more detailed technical descriptions, generic 4 MW and 8 MW turbine sizes are considered.
During the construction or decommissioning of offshore wind turbines, in general jack-up platforms or vessels (lift-boats) are needed. With this static rather than floating base, the crane can work to the precision needed. In Europe, turbines are generally installed using specialised jack-up vessels with integral cranes capable of over 1000 t lift and reaching to the turbine hub heights. Similarly foundations are installed using specialised heavy lift crane vessels capable of handling monopiles and jackets that may weigh well over 1000 t, or greater for gravity base foundations. Offshore substations are installed with similar heavy lift crane vessels, with the jacket and topside installed in separate operations.
Turbine decommissioning involves the reverse of installation techniques using multiple lifts to remove blades, nacelles and towers. Foundation decommissioning involves different techniques from installation, especially for piled structures: cutting monopiles or jacket piles at just below sea-bed level using high pressure water jet techniques; or removing the ballast from gravity base and releasing the foundation; or releasing suction buckets by water injection. In each case a heavy lift floating vessel is generally used to lift out the foundations. With floating support structures, they are released from their moorings and towed to shore; turbines would be removed at shore. Cables are de-buried using ploughs or water jets, similar to installation. Offshore substations are decommissioned by removing the topside using similar heavy lift methods to installation, and then removing the foundation.
In the Great Lakes, the restricted capabilities of vessels are a major issue for construction and similarly for decommissioning operations. This is because vessels wider than 23.7 m (78 ft) cannot enter the St Lawrence Seaway locks into the Great Lakes system and also that the heavy lift vessels within the system are very limited in capability. Mobile cranes on modular jack-up platforms may need to be used for turbine installation as an alternative to the heavy lift jack-up vessels used in Europe. Turbine construction and decommissioning are likely to require mobile crawler cranes mounted on modular jack-up platforms or pontoons with added legs. If a specialised vessel is built within the Lakes it is unlikely to be as capable as those in Europe. Foundation construction and decommissioning is likely to be carried out with the foundations in more than one section to reduce the maximum lift needed. With the larger turbine and foundation sizes, techniques involving floating the pieces in a controlled manner may be adopted to reduce the maximum lifts required. For the topside of offshore substations, which may weigh 2000 t, they would be designed for construction and removal in several modules.
Cost estimates for decommissioning
Indicative cost estimates for the decommissioning of selected offshore wind technologies for different hypothetical scenarios in the Great Lakes are considered. The analysis includes the description and application of a custom-built cost modelling tool that is provided as part of the work.
In general, the main decommissioning cost categories are:
- pre-decommissioning work to review and fulfill regulations and Environmental Impact Assessment (EIA) requirements, and engineering surveys and planning
- marine operations to prepare and remove each of the components
- post-decommissioning survey work after completion of the marine operations
- overheads covering management costs, insurance, port fees etc.
- costs for materials disposal and potential revenues from recycling
The cost modelling focuses on notional 75-turbine offshore wind farms in the Great Lakes, each using a 4 MW generic turbine that represents today’s established technology. A future 8 MW turbine is expected to require very different methods of decommissioning and therefore its cost modelling is considered too speculative to be included.
The scope of the modelling encompasses all the major types of foundations considered for offshore wind turbines by means of the following main strategies based on the removal logistics:
- Cut, Lift, Carry (e.g. for monopiles and jackets)
- Lift, Float, Tow (e.g. gravity base or suction foundations)
- Detach and Tow (e.g. floating wind)
- Offshore sub-station (one or two by Cut, Lift, Carry method)
For removing 75 offshore wind turbines, the logistics play a large part in the cost, with the optimum generally utilising separate vessels for the turbine and foundation removal following behind one another, thus reducing the overall duration of the operations and allowing each vessel to make best use of its specialist capabilities. In contrast, for the offshore sub-station the same heavy lift vessel is employed for both the topside and foundation removal. The scope of the costing allows options to include or exclude the removal of array cables, export cables and offshore sub-stations, and to separate out the disposal costs and recycling revenue.
For this modelling, cost estimates for each scenario are derived in CAD $ and using current values which is standard practice. The results presented here are based on inputs at 2015 costs. In the future, the overall costs may change in line with changes due to inflation and market forces, and also changes in the underlying fundamentals of methodologies and equipment used.
In the Base Case, the values of the main inputs are chosen as the most likely “central” values, though the modelling includes a necessary allowance for weather delays and a 10% cost contingency. The other scenarios explore the sensitivity to the main variables and the range of potential foundation types with their different decommissioning methods.
The Base Case scenario of 4 MW turbines on monopile foundations in 25 m water depth and 20 km from the disposal port yields an overall estimated cost of the decommissioning phase of CAD $198 million, or CAD $187 million after recycling revenue is included, representing CAD $2.6 or $2.5 million per turbine. An overall duration of the marine operations of 14½ months is predicted, including estimated weather delays. Charter costs of the main vessels form over 85 percent of the cost, and as a consequence the overall cost is almost linearly proportional to the duration of the marine operations and to the day-rates for the vessels. The overall cost is only weakly dependent on the distance to the disposal port as the transit times form a small proportion of the overall time. An increase in water depth to 40 m results in an approximately 30 percent increase in overall estimated cost because more capable vessels are needed with longer legs for turbine removal and larger crane capacity for foundation removal. Decommissioning costs for a concrete gravity base and for steel suction bucket are slightly higher than the base case (17 percent and 10 percent higher respectively), the only difference in the modelling being the disposal cost analysis with similar durations being assigned to the removal operations. For floating structures the overall decommissioning cost is less than 50 percent of the Base Case.
The design of the cost model Tool provides sufficient detail to allow all the main elements of the decommissioning costs to be captured and the strong dependence on the marine logistics to be explored. The guidance notes for the Tool include indicative ranges of values for the main user inputs and the circumstances affecting the choices.
While the indicative values used in the modelling are all based on DNV GL’s experience in the current offshore wind industry, the parameters used are necessarily somewhat generic since the design and location of any Great Lakes offshore wind farm is as yet undecided. The restricted options for suitable decommissioning vessels in the Great Lakes also provides further sources of uncertainty in the cost estimates. In the future, when decommissioning cost estimates are required for a specific offshore wind farms, DNV GL recommends that more detailed and project-specific cost modelling be carried out.