Jacking-up windfarm maintenance efficiency

Mon, 29 September 2014

The UK's Crown Estate has commissioned a study exploring how optimisation of jack-up vessel deployment can improve windfarm performance as projects move into the operations and maintenance phase.

It is easy to forget that while offshore wind is still viewed as a maturing industry, increasing numbers of projects are now well-established. Vatenfall’s Kentish Flats, one of the earliest UK Round 1 projects has in fact been in production since 2005. The study looks at how changing profiles of the operation and maintenance (O&M) phase are presenting opportunities for optimising resources in what will become an increasingly busy and demanding market.

The relevant O&M activities referred to in the study involve maintenance and repair work such as replacing single heavy components at height, beyond the reach of small personnel transfer craft that tend to a windfarm’s day-to-day needs.

The study reports that over 500 jack-up vessel interventions have taken place in the UK to date with evidence of long periods of turbine downtime while repair campaigns are planned and delivered. Losses range from a few hundred thousand, to several million pounds per event. Vessel deployment and mobilisation costs are substantial, making single turbine repairs challenging to justify in isolation. Opportunities for cost reductions identified include: faster response times undertaking repairs, more efficient project planning and reduced charter costs through a more structured, proactive approach with greater optimisation of geographical campaigns.

Until recently, Original Equipment Manufacturers (OEMs) have been the main users of jack-ups for O&M operations, undertaken during the warranty period, generally between two and five years. As warranties expire, windfarm owners have a choice for O&M provision including extending existing arrangements, switching to independent service providers or providing maintenance with their own teams.

O&M costs make up around a quarter of a windfarm’s lifetime costs and it is estimated this sector of the O&M industry in the UK alone could become a £2bn a year industry by 2025. With over 1,200 turbines now operational and around 200 under construction in UK waters (a figure that could rise to over 3,000 by 2020 with current and planned developments) there is clearly scope for efficiency gains.

It is relevant to identify what is involved. Repair options can include: single turbine repairs, reactive and proactive batch repairs, and serial defect campaigns. Although the cost of repairing faults by breaking components down and replacing them using the turbine's own crane and attendant support vessel are considerably lower than using a jack-up, there will be occasions when component size require the greater capability of a jack-up vessel. Deployment stages involve: mobilisation, load-out, sea transit, positioning on site, elevated operations and demobilisation.

Many variables are involved in planning the operation. On one side of the equation; the cost of chartering a jack-up, can vary depending on availability with day charter rates influenced by demand from other sectors including offshore oil and gas and civil engineering. There is also the question of where the nearest available unit is located and its distance from mobilisation and load-out sites.  On the other side of the equation is the lost production from a failed turbine. Condition monitoring systems may provide advance warning of a fault: should this affect more than one turbine a so called batch repair may be considered rather than a quick, vessel mobilisation. There will be a break-even point between the cost of a quick repair, and additional lost revenue risks waiting for further turbines to fail before bringing in the jack-up.

Detailed project planning is complex and coordination essential in achieving an effective and efficient outcome. Replacement components or spare parts will need to be delivered to the load-out port to coincide with the jack-up’s arrival. The turbine will need to be shut down, prepared and be ready just in time. The paperwork planning will also be complex, involving: project consent, contract negotiation, technical information exchange, contractor audit and vessel inspection, and detailed planning and project approval. And of course the programme may be subject to weather delays. Marine licencing is not required for O&M jack-up operations but there may be site specific environmental considerations. Knowledge of seabed conditions will be required for the jack-up, and while geophysical and geotechnical survey data will be held from the construction phase, changing conditions may require additional surveys.

Jack-ups are often suited for particular types of seabed, the study finding that currently no single vessel is capable of accessing every offshore turbine installed or planned. One variable is the extent to which different types of ‘feet’ sink into different types of seabed: the specified air-draft (dictated by expected weather conditions) will influence the depth of water the vessel can work in. The study includes a detailed list of 35 currently available jack-ups, dividing them into four groups, from vessels previously adapted for the industry to the modern breed of dedicated installation vessels.

Charter costs for jack-ups can range from £45k to over £100k per day, rates being influenced by long-term charters compared with unplanned spot-market arrangements with up to 40% difference in the two being reported.

As mentioned there are many variables for the operator to consider when balancing cost of intervention against loss of income, including the turbine’s capacity factor and the level of subsidy. One study identifies a typical period of between two and six months for planning once a vessel has been identified. Savings can be made here using vessels (and crews) familiar with the location, perhaps from involvement in the construction phase and where site-specific assessment applies to vessels of the same design.

The relevance of windfarm locations is another important factor. Looking at the geographical spread of UK windfarms, three distinct clusters are identified: from the Thames estuary to the southern and central North Sea including Dogger Bank, Hornsea and East Anglia Zones, the cluster from around Sunderland to Moray Firth with the third cluster covering the Irish Sea and SW Scotland. Mainland Europe windfarms can also be considered ‘North Sea centric’ and when demand from oil and gas activities in this region are factored in, it is clear that mobilising vessels for the Irish Sea region involves significant extra costs in transit times to an area perhaps not containing a pool of available vessels. A selection of typical transit times by three classes of vessels from four ports in the North Sea region, to two windfarms in the Irish Sea is presented. Minimum transit times vary from 2.9 days for a ten knot, self-propelled ship (Great Yarmouth to Robin Rigg) to eight days for a towed barge achieving five knots (Esbjerg to Robin Rigg), weather delays of several weeks may be experienced during winter months. The economic case for suitable vessels available in the region is clear to see, especially where components can be transported to load-out ports in the region by more flexible conventional shipping.

As the time to extract legs deeply embedded into the seabed cannot be accurately predicted, early warning of bad weather is required if safe limits are not to be exceeded. Taking advantage of benign seasons is part of the game although campaigns involving more than one windfarm can be affected if delays see work for those at the end of the queue wandering into unfavourable weather seasons.

The aim of the study is to put forward opportunities for gaining efficiencies relative to the issues included above. Planning is key, predictive failure modelling enables a proactive replacement strategy for components approaching the end of their life. As windfarm sizes increase, turbine availability-based warranty agreements can be met even with a number of turbines stopped for long periods. Jack-up availability (or lack of) can be a factor for warranty providers planning repair campaigns and some windfarm owners are now taking on responsibility for providing jack-ups. Percentage energy yield (rather than availability-based) contracts are options now being offered.

Windfarm owners entering into framework agreements with jack-up operators and collaborating with others for their use have potential for efficiencies and case studies are included supporting this approach. Adapting jack-ups from windfarm to oil and gas work involves significant work and crew training, the study considering that apart from multi-purpose vessels, vessel availability will be unaffected by any surge in oil and gas activity.

Options for collaboration with sourcing jack-ups, benefiting from economies of scale include: a ‘do nothing’ approach, full-time vessel charter club of windfarm owners, part-time club covering a certain number of months annually and flexible charter club or 'club without commitments'. An actual and a hypothetical case study (covering UK east coast and Irish Sea respectively) demonstrating advantages with a collaborative use of vessels is included. Barriers to collaboration are also explored including: confidentiality/intellectual property concerns, site specific constraints, uncertainty about future failure rates, financing arrangements and procurement policies.

To date, the biggest jack-up campaigns addressed early-life serial defects and retrofits of main components with serial defects, estimated to make up around 70% of all jack-up vessel operations. The study concludes with recommendations for improving repair times, minimising repair cost, and addressing risk. Collaborative sharing of vessels between site owners during the O&M phase has potential to deliver tens of millions of pounds of value per year, with potential for significantly higher benefits if long term failure rates rise or vessel availability is reduced. A table translates benefits in financial terms from three value sources: improvements in planning campaigns, reduced mobilisation costs through collaborative campaigns and reduced production downtime through faster deployment of vessels from collaborative arrangements with existing and future sites. The best figures estimate savings of between £100m and £400m per year from the third source (and future sites).

Recommendations fall into three categories. The first two concern facilitating increased efficiency and collaboration in the use of jack-ups and eliminating barriers to such. The third takes a wider look, beyond actual optimisation of vessels themselves aimed at reducing main component repair risks. The points made are: to promote the need for advances in condition-based monitoring techniques and “remaining life” predictions to improve the earlier detection of faults and enable proactive O&M strategies to be more widely implemented; develop improved turbine reliability models to increase confidence in predictions of vessel demand and finally, encourage innovation to develop alternative repair options not requiring jack-up vessels including more widespread in-situ repair techniques and innovative main component replacement methods.

The list of vessels includes three jack-ups and a leg stabilised vessel particularly suitable for O&M projects:Wind, Wind Pioneer, Wind Server (all operated by DBB) and A2SEA’s Sea Power, a converted cargo vessel.The DBB vessels are specifically suited for windfarm O&M duties. They may be restricted for future deepwater offshore projects, but these four can be described as ‘handy-sized’, with potential to achieve efficiencies of scale compared to the third group of self-propelled multi-role heavy lift jack-ups featuring large deck space to accommodate the maximum number of turbine components. Sitting in between are the second group, also installation vessel listed as 'could also be used during O&M' and the fourth group of seven non-propelled jack-up barges, five owned by Jack-Up Barge BV and two by A2SEA.

The 35 listed, of course overlap, and if available and logistically feasible, the biggest could find work in the O&M market. It will be interesting to see if this niche market attracts more newbuilding vessels, but perhaps not to the scale of the order rush that filled the vacuum of installation vessel requirements a few years ago.

By Peter Barker 



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