Charting the course for offshore access systems

There are no clear strategies at present in the global offshore industry on setting up and running cost-effective operations and maintenance (O&M) for far-from-shore or large projects in rough environments. But many options are now being analysed.

The Offshore Wind Accelerator (OWA) is the UK Carbon Trust's flagship collaborative research programme. Set up in 2008, the OWA is a joint industry project involving nine offshore wind developers with 72% (31GW) of the UK's licensed capacity. It aims to lower the cost of offshore wind by 10% in time for cost savings to be realised for the UK's Round 3 projects, currently in development. This will be achieved through innovation, not cutting corners.

As part of the wider programme, there is an OWA work stream focusing on O&M and, more specifically, access systems — that is, transportation of technicians and materials, including vessels, transfer systems, offshore support vessels, motherships and helicopters. This group's main objective is to support the development of cheaper, safer and more efficient access to projects in more onerous conditions, considering that an increase in accessibility will improve turbine availability, in turn boosting annual energy production and thus lowering the levelised cost of energy.

The OWA access expert group's first focus was a market screening suggesting that the technologies available in 2008 were unsuited for medium-to-large offshore projects, that is, more than 100 turbines, sited more than 80 kilometres from shore. It also considered the potential to significantly improve O&M procedures.

Next was modelling of different offshore scenarios, with varying locations, numbers of turbines and wave climates, taking into account the cost and capabilities of different access systems. The study concluded that crew transfer vessels (CTVs), also called wind farm support vessels, and motherships were the areas that needed more attention in terms of technology development.

The concentrated effort on R&D driven by the OWA has speeded up commercialisation of many innovations it has identified and supported. The Windserver from Fjellstrand and the WaveCraft from Umoe Mandal offer good examples. However, there is still a major question that needs to be solved, which not only affects new innovations, but also other types of access system: how good are they, or how can they be combined?

Performance evaluation

Limited understanding about CTV performance persists. While the sea-keeping and station-keeping capabilities of large vessels and the operation limitations of helicopters are well known, CTVs are largely considered in terms of one parameter: the maximum significant wave height (Hs), in which they can operate safely.

From a naval architecture or marine operations point of view, many other factors must be considered, such as the combination of meteorological and oceanic conditions - wave direction, Hs, period, current - plus speed, capacity, comfort, safety, fuel economy and charter cost. Some developers have also said actual vessel parameters are not always achieved in practice. As a result of these limitations, selection methods for vessels or transfer systems for specific sites are highly subjective, leading to inadequate O&M strategies.

Through OWA's experience, a new methodology to assess CTV performance has been developed, based on different R&D projects and full-scale sea trials. Three different approaches have been investigated. The first of these is numerical modelling, a relatively fast method to obtain several combinations of different met-ocean conditions with a very cost-effective ratio of time-cost per scenario run, but with low certainty on the results owing to unclear fender friction coefficients.

The second is tank testing. This is more certain than numerical modelling, but also more time-consuming and costly. Two different types of tank-testing facilities exist: testing channels and ocean basins. Full-scale sea trials are the third and most certain of all the approaches, but are costly and provide limited statistical confidence due to the lower number that can be performed. A fourth option is based on open-sea trials with remote-controlled scale models of the designs.

Best option

The preferred approach, considered to be the most cost-effective, is based on a combination of tank testing or scale model testing and numerical modelling. This is complemented by a brief validation in full-scale trials if possible, as it is not feasible to measure performance using sea trials in all conditions due to the vast number of permutations. The uncertainties around numerical modelling are currently being assessed in an OWA R&D project to correlate full-scale trials, tank testing and modelling while defining fender friction properties.

The outcome of the performance metric assessment will be a set of performance plots (P-Plots) enabling developers and operators to better select access systems according to each site's requirements, providing robust quantitative and objective values to improve O&M modelling tools. The P-Plots will also enable a vessel's expected performance to be assessed from design to full-scale operation, providing a tool that will give feedback to designers to improve concept development.

It should be noted that the OWA methodology does not aim to specify the best vessel for a specific site, but to help O&M decision-makers create a shortlist for a more detailed analysis considering the parameters that have not been included in the assessment. As part of the performance evaluation, three phases have been identified that split the trips of CTVs to and from offshore projects, always aiming to reduce the length of these to maximise the technician's time at the turbines.

A key lesson learned has been that transfer systems, because they are installed on a vessel to improve transfer operations, cannot be considered separately. Each combination of vessel and transfer system must be considered as a different vessel in performance terms, despite having the same hull forms.

The way forward

The OWA is validating the performance evaluation assessment with an extensive fulland model-scale testing of generic and representative vessel hull forms and lengths, and completing understanding of friction behaviour. This work is due to be completed later this year. Collaborative work is also being discussed with Offshore Maintenance JIP in the Netherlands.

There are recognised limitations to the proposed methodology, but it is understood that it is almost impossible to predict a vessel's performance entirely accurately. A given vessel could also see its performance vary depending on cargo, fuel loading and propulsion. However, the suggested methodology is a step forward in quantifying and understanding vessel performance.

CTVs will have a role to play in further-from-shore sites, whether they are large and working on a standalone basis or smaller and acting as daughter crafts, as large O&M vessels are still considered costly despite their increased capabilities and performance.

With the new methodology more robust strategies will allow CTVs to be combined with helicopters or medium-to-large support vessels, such as the Damen or Ulstein designs, with commercially ready walk-to-work systems such as the Amplemann or the Uptime, and improved dynamic positioning systems such as Siem Moxie's. More motherships with daughter craft like the Esvagt Froude vessel or designs such as the Launch and Recovery stems from Divex and purpose-built jack-up vessels like the DBB Wind Server, will have a role. Each wind project will need its own optimised strategy.

But the issue remains of accurately understanding met-ocean conditions in different areas of an offshore project, to provide tools allowing operators to modify O&M activities according to conditions. This topic is likely to be the focus of future OWA R&D activities.

Insight first published on Windpower Monthly.