Getting into Ship Shape for Far Offshore

Round 3, the world’s largest offshore wind energy generation programme, signalled the start of a new era for the energy, engineering and maritime industries. In developing Round 3, the UK government has adopted an aggressive schedule for construction and implementation of offshore wind, aiming to install over 30 GW in 10 years. Whilst industries are rallying, most of the technical focus is currently on fabrication and installation of the towers and turbines. A key area that has yet to be fully addressed is the absence of a suitable fleet of seagoing vessels that will enable the safe and efficient operation and maintenance of the subsequent energy assets.

The consistent delivery of power to the grid relies on moving people on and off turbines throughout the lifecycle. Clearly there are safety issues related to the physical operation of stepping onto a turbine. Furthermore, the sheer size and scale of the Round 3 offshore installations in terms of distance from shore, as well as turbine density, brings a new challenge to the renewables value chain, from ship designer to wind farm developer.

For an energy company to ensure the profitability of a mechanical asset, it must firstly be sure that the asset can be properly maintained. In terms of offshore wind energy generation this means that the turbines must be serviced throughout the year as part of scheduled maintenance programmes and repaired as necessary following failure or damage. Currently, a standard 5 MW wind turbine requires approximately 15 man-days of planned maintenance per annum. If this level of support is to be delivered cost-effectively and safely, the entire marine transport operation must be optimised. Vessel capabilities must be scrutinised and optimised to provide a support fleet which can be relied upon to deliver capabilities that exceed the strategic maintenance requirements. Any shortfall in capability is likely to result in financial loss. Therefore, it is imperative that maintenance strategies are modelled and costs evaluated during the early stages of site development.

An illustration of an active vessel motion compensation system (Source: BMT)

To date, the vast majority of the wind farms constructed in Northern Europe are relatively close to land, within about 20 nautical miles of port, and typically comprise only relatively few turbines. In the UK, which leads offshore wind development, the current fleet of vessels operating at these wind farms is mostly small catamarans designed within the existing UK MCA (Maritime and Coastguard Agency) or EU regulatory regime, but these vessels will not be adequate to support the operational requirements needed for transfer further offshore, where wave heights significantly increase.

In the UK, elements of Round 2 and, in particular, Round 3 are taking wind farms significantly further offshore – in some cases 150 miles (240 km) from the coast – as well as greatly increasing the numbers of turbines. Without a fundamental redesign, existing transfer vessels would face considerable operational challenges from both factors. Current regulations including the UK MCA Small Commercial Vessels code (SCV) also need to be reviewed so that they meet the requirements of far-shore wind farm maintenance. Wind farm developments in North America and China are also gathering pace and the more exposed sites in these regions will certainly require larger, more sea capable vessels than the marine industry is currently offering. These vessels will also need to be designed to meet different regulatory requirements such as US Coastguard Code of Federal Regulations (USCG CFR).

There have been moves within the offshore industry to apply a more uniform approach to regulating these vessels. The HSC (High Speed Craft) Code developed by the IMO (International Maritime Organization) has now been widely adopted by many flag states as the regulatory framework for high speed ferries, even where vessels are only operating on domestic routes. Interesting developments include new classification rules from DNV (Det Norske Veritas), specifically designed for wind farm and other special service vessels. In the future national authorities may enforce these rules in vessel construction, establishing a more uniform international industry standard.

The key challenge for the ship designer is to comply with the operational needs of the wind farm developer without compromising the safety of the technicians onboard the offshore vessel. If developers decide to imitate the oil and gas industry and introduce accommodation platforms or flotels, then a much larger vessel will be the most cost effective type of transportation for transferring larger numbers of technicians from shore to offshore accommodation.

But such craft would not suit for transporting personnel to individual turbines. Ideally, organisations will want a fleet of smaller intra-field vessels to transfer technicians on a daily basis between the platform and individual wind turbines, as the number of technicians will be much lower, about 10 to 12 passengers. However, these smaller vessels will require careful hull selection so that they can tackle rougher sea conditions and minimise sea sickness through high quality sea keeping capabilities.

Safer alternatives to the traditional friction method will be required as wind farms go further offshore and become exposed to significant higher waves (Source: BMT)

Developers must consider that technicians travelling to these sites will often experience wave heights significantly higher than they are currently experiencing on more sheltered water installations. Stepping from the vessel onto the turbine is the most dangerous part of the transfer and risks are heightened by rougher offshore sea conditions. Existing vessels push up against turbine foundations and use the thrust of the engine to maintain position, giving the technicians the opportunity to step across. This procedure is likely to be totally unsuitable further offshore where higher wave conditions could cause sudden movement of the vessel resulting in injury or even fatality. An active transfer system could help to eradicate such risks, helping to achieve the required level of operability for safely allowing personnel on and off turbine installations. Existing access systems such as the Ampelmann are designed to provide a stabilised access gangway from a vessel that is dynamically positioned off an offshore platform. An alternative, developed by BMT and Houlder Ltd, uses an actively stabilised gangway and a damped cylindrical bow fender to compensate for the movement of the vessel and provides a safer step-across than the current friction lock-on method.

Developers also need to consider a more specialist hull form with exceptional sea keeping capabilities such as a SWATH (Small Waterplane Area Twin Hull). Although more expensive, this twin-hull design minimises the ship’s volume nearer the surface, which reduces forcing on the hull, improving vessel motions. The other constraint that must be considered is that contact with the turbine’s support structure may be prohibited on larger vessels by the excessive load a vessel could impart in unfavourable wave conditions, requiring future vessels to be fitted with a Dynamic Positioning (DP) system. Rather than pushing up against the turbine as current crafts do, they will be required to enable the DP mode which will hold the vessel at a set position away from the actual turbine foundation – an approach regularly used for offshore oil industry vessels. Once in position, an access system can be used to provide safe transfer.

Another area that demands careful consideration is the landing arrangement on the turbine foundation. Existing UK turbine foundations often use a pair of vertical structural tubes stood off from the main turbine foundation. The transfer vessel then pushes up against these tubes, between which an access ladder is recessed. This arrangement means that the vessel can only complete transfer on one heading, which may be disadvantageous in certain wave conditions. On more exposed sites it will be important to develop a landing arrangement that can allow docking on different headings in order to minimise vessel motion.

Furthermore, as part of the pre-construction planning phase, wind farm developers must carry out detailed analysis of the hostile environments they plan to operate within, and consider if and how planned and unexpected maintenance can be executed by technicians in a safe way.

In terms of development, the lead times for building such a fleet of appropriate vessels is significant. To be ready in time for the proposed construction phase, the industry must begin to determine the correct strategy for designing and building these service vessels today. Unless such measures are taken now, either the developer or the energy company is going to find it cannot ensure the profitability of its assets. Wind turbines in need of repair will stand dormant due to lack of accessibility, while turbines in need of maintenance will be operating well below par for the same reason. The marine transport system as it currently stands is not optimised for the new era of offshore wind farms and to ignore this fact could seriously impact this type of future development.

Installation Vessel Oversupply Forecast

With 23 new and specialised installation vessels due to be delivered to the offshore wind market between 2011 and 2013 an oversupply of turbine and foundation installers in the global market has been forecast between 2012 and 2015.

According to new analysis from ODS-Petrodata annual installation capabilities will increase from 672 turbines in 2011 to some 1960 turbines in 2013. On the financial side, the company predicts a drop in day rates as the current bottleneck in the turbine and foundation market disappears, as well as the return of less specialised vessels to their original sectors and the retirement of less capable vessels to offshore wind operations and maintenance.

Bottlenecks, however, are likely in the cable installation market. With a 1300% increase in the volume of export cable needing to be installed between 2011 and 2019, along with increasing demand for inter-array cabling installation, the analysis forecasts. More than 30 cable installers equipped with dynamic positioning, are likely to be required by the global market by 2020. The European market alone is likely to require up to 24 cable installers with dynamic positioning by 2016, the company concludes. Because three vessels are working outside the wind market, the current fleet size is 25 vessels.

Current global offshore wind installation fleet:

10 turbine installation vessels

12 foundation installation vessels (9 in Europe)

6 turbine and foundation installation vessels

Source: ODS-Petrodata

John Bonafoux is managing director of BMT Nigel Gee, a subsidiary of BMT Group.

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