Weather windows analysis incorporating wave height, wave period, wind speed and tidal current with relevance to deployment and maintenance of marine renewables M. O Connor, D. Bourke, T. Curtin, T. Lewis and G. Dalton Hydraulics and Maritime Research Centre, University College Cork, Pouladuff Road, Togher, Cork, Ireland E-mail: Michael.oconnor@ucc.ie Abstract This paper presents the results of a weather windows analysis in order to quantify the levels of access to marine renewables for installation and operations & maintenance (O&M) activities. The weather windows analysis was conducted at three sites. The Egmond aan Zee windfarm (OWEZ) off the Dutch North Sea coast, the Atlantic Marine Energy Test Site (AMETS) Belmullet on the Irish Atlantic coast and M in the Irish Sea. This paper builds on previous work which quantified weather windows by considering the significant wave height. This paper assesses weather windows by looking at a greater range of met-ocean parameters in addition to significant wave height, namely peak wave period (Tp), mean wind speed and tidal current speed as well as applying these parameters over a greater range of marine renewable locations. Met-ocean data from was used in the analysis. Keywords: Access, Deployment, Installation, Operations & Maintenance Introduction This paper presents the results of a weather windows analysis of wave data from three marine renewable locations.the Egmond aan Zee windfarm (OWEZ) off the Dutch North Sea coast, the Atlantic Marine Energy Test Site (AMETS) at Belmullet on the Irish Atlantic coast and at M in the Irish Sea. The analysis is carried out in order to quantify the levels of access for the installation and operation of marine renewables which may be deployed. Once marine renewable devices, such as offshore wind turbines or wave energy converters (WEC) have been deployed at sea, maintaining them will not be as simple as for maintaining similar devices onshore. There are many factors in an offshore environment that make installing and operating devices more difficult, costly and time consuming. The main factor is that of access. In order to deploy and operate marine renewable devices, a weather window will be required. This will involve a period of access where the met-ocean parameters must remain below a certain level, enough for the required operations to be conducted safely. This paper will inspect the following met-ocean parameters, significant wave height, peak wave period, wind speed and tidal current speed. Due to data limitations at the locations chosen, only the year of is analysed in this paper. The Weather Windows module of the Techno- Economic model NAVITAS was used for the analysis. Levels of access at each of the three sites were quantified based on the following four scenarios The significant wave height (Hs) only A Jack Up vessel with access limits based on wave height, wind speed and tidal current. A crew transfer Catamaran with access limits based on wave height and tidal current. And a Workboat with access limits based on both the wave height and peak period. The results of the levels of access are presented by showing the following:. A comparison of the met-ocean conditions at each site by showing the annual occurrence frequency of each parameter at each location.. Persistence tables and graphs which show the percentage access during the year for different window lengths. 3. The seasonality of the access levels by showing the number of hours each month that the met-ocean conditions are below each set of access limits.. The waiting periods between windows, by showing the est waiting period, or worst Developed in HMRC under the Charles Parsons Initiative and the Enterprise Ireland Commercialisation fund.
case scenario, between windows of a certain length that occur each year. 5. Sensitivity analysis by showing the impact on access levels of varying the tidal current limit and the wind speed limit. Once the levels of access are quantified, the implications of the observed levels of access with regard to installation and O&M activities of marine renewables deployed at the three locations are discussed. Literature review Quantifying the levels of access to marine renewable locations is important as they can have a major impact on the both the installation and operating costs of projects. According to Dalton [] more detailed research is required to determine specifics of device service times as well as weather windows before more reliable cost estimates for wave energy projects can be confidently assessed. Walker et al [] assessed the cost implications of weather windows for wave energy device deployment at UK test sites. Walker concluded that the primary influencing factor on the installation capital expenditure is the downtime due to weather windows and when planning operations, an understanding of weather windows is essential. According to O Connor et al [3] limited access for O&M operations may be a crucial barrier for future wave farm developments in aggressive wave climates such as the Irish west coast. Lack of access for O&M is already an issue for the offshore wind industry, even in benign wave regimes such as the North Sea [, 5]. Availability is defined as the amount of time the device is on hand to produce power and is affected by a number of factors including device reliability and the ability of the device to be accessed for maintenance []. Onshore wind turbines, with % access have availability levels of typically 9% or more [7]. Offshore wind farms in the North Sea have access levels typically between % [] and % [9] based on a wave height access limit of Hs.5m. As a result of the decreased levels of access, offshore wind farms have availability levels that are lower than onshore wind. Lyding et al [] for example quotes availability figures for various offshore wind farms in and 7 as between 7 to 9%. In a recent survey by PricewaterhouseCoopers (PwC) of the offshore wind industry, offshore wind operators reported typical availability levels of 9-97 % []. Some early work has been conducted which attempted to quantify impact that lack of access could have on availability for wave energy devices. O Connor et al [, ] assessed the levels of access to wave energy converter technologies at various marine renewable locations across Europe. From these levels of access and based on a methodology developed by Van Bussel et al[7], O Connor showed that the availability levels of wave energy devices could be as low as 3% for early stage deployments in high resource locations. The study found that when assessing the impact of access and availability that low access limits coupled with the higher failure rates associated with early stage technologies severely curtailed energy output for wave farms at high resource locations. At sites with lower resource, the negative impacts of poor reliability and/or access were less severe. The study concluded that high wave energy locations, which can produce high energy outputs, may be penalised by low access limits and poor device reliability, thus affecting economic returns. When assessing access levels, the significant wave height (Hs) is the primary met-ocean parameter [3]. Several studies by O Connor et al assessing weather windows [3,,,, 5] have to date based the levels of access on the significant wave height. This was also the case with the studies quoting offshore wind access levels [7-9]. Other studies which accounted for weather windows when assessing the economics of wave energy deployment [], as well as operations [], have also assessed weather windows based solely on the significant wave height. Even though Hs is the primary parameter, other metocean factors may have an impact on specific operations. Wind speeds will have an impact primarily where crane operations are involved. Tidal currents may have an impact on station keeping, and may be particularly impactful at tidal energy locations as these will tend to have very high tidal current flows. According to Walker et al [] the wave period should also be considered as when this is coupled with wave height, can impact on the dynamic response of vessels and the safety of those on deck. A review of procedures for estimating site accessibility for wave and tidal projects was conducted as part of the EU Equimar project [7]. Two approaches were identified, time series analysis and statistical analysis. The simpler statistical analysis methods were recommended to be used when only limited site data was available, usually in the form of a scatterplot of wave height and period at the site. This approach was adopted in the Equimar case study for assessing accessibility at a wave site where only one parameter, the significant wave height, was of interest. At tidal sites where the wave height, wind speed and tidal currents were of interest the time series approach was used. The time series approach is adopted in this case study as it is more thorough, allows for the inclusion of multiply parameters and provides for a more comprehensive assessment and more detailed results. In considering the tidal currents, the Equimar case study assessed access levels at a representative tidal site. None of the three locations in this case study would be suitable tidal locations. All three would be suitable wind farm sites and AMETS a wave site. Therefore when assessing tidal currents, we are looking at the potential effects they may have on weather windows at wind and
wave sites and not a representative tidal site, as was done in the Equimar study. 3 Methodology. Data from was used in this case study. The methodology employed in this case study for the processing and analysis of the time series data is described in more detail in [3]. 3. Locations Three case study locations were chosen. The Offshore Wind Farm OWEZ located km off the Dutch coast, AMETS located off Belmullet on the Irish Atlantic coast and the M buoy located 37km off Howth head in the Irish Sea. These locations were chosen as all three had concurrent wave, wind and tidal current data available which allowed for the weather windows using multiple parameters to be quantified. A full weather windows analysis should be based on at least 5- years of data [3]. For this analysis, only one year s data was available where concurrent data for all the parameters was recorded at all sites. The OWEZ offshore wind farm site has a m offshore measurement mast. The mast mainly measures meteorological data. Data has been recorded by the mast since 5 and was sourced from the NoordzeeWind website. During some of this time, an ADCP was mounted on the wind mast and used to record wave and tidal current data. There was however only one full year at which concurrent wind, wave and tidal data was recorded at the wind mast, November 7 to November. A SWAN wave model has been developed by Cure for the AMETS site in Belmullet []. This contains 5 years of hindcast wind and wave data at the site from 995-. HMRC had access to ADCP tidal data recorded at the site during the summer of. An internal HMRC model used this ADCP data to produce a record of tidal currents at the site for the years that wind and wave data was available. The data for the M Irish Sea location was sourced from the Marine Institute 3. This data contained wave height and period measurements as well as wind speed. Tidal data is not measured by the M buoy. HMRC had access to ADCP data from the nearby Arklow Bank and this was used as an example of tidal data in the region. Both the AMETS and Arklow Bank (M) tidal data available at HMRC were in raw form. This enabled the data to be modelled for different years. The OWEZ data was not in raw form but processed and therefore the data could not be used to produce predictions for tidal currents for different years. As was the only year that concurrent wind, wave and tidal data was available http://www.noordzeewind.nl/ 3 http://www.marine.ie/home/publicationsdata/data/buoys/ at OWEZ, data from this year was used at M and AMETS. 3. Access limits For this analysis four main met-ocean access limits were chosen. These were the limits that the met-ocean conditions would have to be at or below in order for typical deployment/operation activities to be carried out. 3.. Based Hs only The first set of access limits were based on Hs only, as this is the primary parameter influencing offshore operations [3]. Results were produced for Hs access limits of Hs.m, Hs.5m, Hs.m, Hs.5m & Hs3.m. 3.. Based on additional parameters In order to investigate the impact of additional metocean parameters, together with the significant wave height, specific met-ocean limits for three vessels were used.. Jack-Up vessel: To observe the impact that wind speeds and tidal currents have in addition to wave height, the access limits associated with a Jack-Up vessel were used. These were based on the offshore wind installation vessel MPI Resolution and are a wave height limit of Hs 3.m, a wind speed limit of m/s [9]. The tidal current limit chosen was.5 m/s which was the limit used in the Equimar case study [7].. Catamaran: Catamarans are typically used to transport crew to and from offshore wind turbines for O&M tasks. These vessels typically have wave height access limits of Hs.m []. The tidal current limit of Hs.5m was again used. 3. Workboat: Walker et al [] provides access limits in terms of significant wave height and peak wave period (Tp) for a range of vessels, these are shown in Figure. Walker did not use the methodology to graph results. For this case study the Workboat vessel has been selected. No other limits were applied apart from the wave height and period limits from the graph. Figure : Access limits in terms of significant wave height (Hs) and peak period (Tp) for a range of different vessel types []. 3
For all locations, the height at which the wind speed limits were assessed was m above sea level. Wind data from M was recorded at 3.5m above sea level and wind data from OWEZ was recorded at m above sea level. The Power Law was used to produce wind speed values for both locations at m. The ADCP tidal data from each location was measured at various depths below the surface. A power law profile of the current speed as a function of depth was assumed based on [] Tidal current speeds measured at the different depths were then adjusted to surface current speeds at which the tidal access limits were assessed. Results. Occurrence frequency The occurrence frequency shows the number of hours each year that the individual values make up. Figure compares the wave height frequency occurrence during at all three locations. It can be seen that both M and OWEZ have similar profiles and generally low wave regimes. The much more exposed AMETS site has higher wave heights. Hours Figure : Wave Height Frequency at AMETS, OWEZ and M. A similar picture emerges when comparing the wave period occurrences in Figure 3 with both OWEZ and M having similar occurrences of wave periods typically between -9 seconds. Again the most exposed AMETS location typically has much er wave periods. Hours 3 5 5 5 5 3 AMETS OWEZ M Hs (m) AMETS OWEZ M 5 5 5 Tp (sec) Figure compares the wind speed occurrences at all three locations and shows that OWEZ typically has lower wind speeds than AMETS and M. AMETS and M have similar profiles with M slightly higher. Hours Figure : Wind Speed Frequency at AMETS, OWEZ and M. Figure 5 shows the tidal occurrences at both OWEZ and M. The tidal current speeds at AMETS were minimal with the tidal current speed never above.m/s. It was therefore not shown in Figure 5. It can be seen that OWEZ typically has higher tidal current flows than M. Hours AMETS OWEZ M 5 5 5 3 Wind Speed (m/s) OWEZ Figure 5: Tidal Current Frequency at OWEZ and M.. Persistence tables for Hs M.5.5.5 3 3.5 Current Speed (m/s) Persistence is calculated by recording whenever the wave height was at or below the wave height limit. Data results displayed were produced by summing up all the windows of at least a certain length at each wave height limit and finding what percentage of the year the total time represents. The AMETS persistence table is shown in Figure and shows that is has the lowest access levels, when Hs only is considered, of the three locations. It can be seen clearly that as the wave height limit is increased the levels of access also increase. Also as the window lengths are increased the access levels fall. Figure 3: Wave Period Frequency at AMETS, OWEZ and M.
Significant Wave Height (m) AMETS Persistence Table 3 3 5 57 5 55 5 53 5 5 5 9 7.9 59 57 55 55 53 5 5 9 9 9 7. 59 5 57 5 5 5 9 7 5.7 5 57 5 55 53 5 9 7 7 3. 5 55 5 53 5 7 5 5 3 39 3.5 5 53 53 5 7 5 5 3 3 37. 5 5 5 9 3 3 39 3 37 3 3 35.3 5 9 3 39 3 37 3 35 35 3. 5 3 37 37 3 35 3 3 33. 39 3 3 3 35 33 33 33 33 3 39 3 37 3 35 3 3 3 3 3 3.9 39 39 3 3 3 3 33 3 3 3 7 7 7 7. 39 3 37 3 3 35 3 33 3 3 3 7.7 37 3 35 3 3 33 3 3 3 9 7 3. 35 3 3 3 3 3 3 9 7 5 3.5 3 3 3 3 9 9 9 9 9. 3 9 9 7 5 3 9 9 9 9.3 7 5 9 7 7 7 3. 5 3 3 3 7 5 3. 5 3 3 3 3 9 7 5 9 9 9 9 3 3 5 7 7 9 9 Figure : AMETS Hs persistence table. It can be seen in Figure 7 OWEZ and Figure M that they both have very similar levels of access when Hs only is considered. At the high wave height limit of Hs 3.m there is almost full of access at both sites. It can be seen that at the lowest access limit considered, Hs.m, there is similar levels of access at both OWEZ and M as there is at AMETS at a wave height limit of Hs.m. Significant Wave Height (m) Significant Wave Height (m) OWEZ Persistence Table 3 95 9 9 93 93 93 93 93 93 93 93 9 9 9 9 9.9 93 93 9 9 9 9 9 9 9 9 7 7 5 5. 93 9 9 9 9 9 9 9 9 5 5 5 5 5.7 9 9 9 9 9 9 9 9 9 5 3 3 3. 9 9 9 9 9 7 7 7 3.5 9 9 7 7 5 3 79. 9 7 5 5 3 3 7 77 77 7 7.3 7 5 3 79 7 7 77 7 7 75 73. 5 5 79 79 77 77 75 75 75 75 73 7. 3 79 7 77 7 7 7 73 7 7 7 7 7 77 77 7 75 73 7 7 7 7 9 5 3.9 79 77 75 75 73 7 7 9 9 7 7 5. 75 73 7 7 7 7 7 3 5 5.7 73 7 7 9 5 3 3 57 5 5. 7 7 3 59 5 55 55 53 53 5.5 7 5 5 57 55 5 5 5 9. 5 5 5 55 5 5 5 7 5 3.3 57 5 55 53 5 5 5 7 5 3 3. 5 5 5 5 9 7 3 39 39 39 39 3 3. 7 5 39 3 3 3 33 33 3 9 9 7 3 37 35 3 33 3 9 3 3 5 7 7 9 9 Figure 7: OWEZ Hs persistence table. M Persistence Table 3 9 97 97 97 97 9 9 95 95 9 93 93 93 9 9 9.9 97 97 97 97 95 9 93 93 93 9 9 9 9 9. 9 9 9 9 9 9 9 9 9 9 7 7 7 5 5.7 9 9 9 9 9 9 9 9 9 9 7 7 7 5 5. 95 9 93 93 9 9 9 9 7 3.5 9 9 93 9 9 9 3 3 3. 9 9 9 9 7 5 5 3 7 7 77.3 9 9 7 5 3 77 77 77 7 7 73. 9 9 5 5 3 79 77 77 77 7 7 73. 7 5 7 77 7 7 73 7 9 7 5 7 3 79 77 7 7 73 7 9 7.9 79 7 75 7 7 9 7 5 3 5 57 5 53 5. 7 77 7 7 7 9 5 3 5 57 5 53 5.7 7 75 73 7 7 5 3 5 5 55 55 5 7. 7 7 5 5 57 55 5 5 5 37.5 7 57 5 5 5 5 5 5 39 3 33 3. 3 59 57 5 5 5 9 5 3 3 33 3 9.3 5 5 5 5 5 9 5 3 3 33 3 9. 5 5 5 5 3 3 3 3 3 7 3. 9 3 3 3 3 7 5 3 7 3 37 35 3 3 7 3 3 3 3 5 7 7 9 9 Figure : M Hs persistence table. Figures 9 to show the persistence graphs for all three vessels at each location. The persistence graphs are similar to the persistence tables but present the data in graphical rather than numeric form. It can be seen that % Occurence % Occurence % Occurence at all three locations the Jack-Up has the highest levels of access followed by the Catamaran and the Workboat. For the Jack-Up and Catamaran, the highest levels of access are achieved at M followed by OWEZ and AMETS. For the Workboat the highest levels of access are at OWEZ. For lower window lengths using the workboat, there is a higher level of access at M than AMETS, however as the window lengths increase, there is a substantial drop off in access levels at M. This results in AMETS having higher access levels, for er window lengths, than M using the workboat. Figure 9: Persistence graph for all three vessels at AMETS. Figure : Persistence graph for all three vessels at OWEZ. 9 7 5 3 9 7 5 3 9 7 5 3 AMETS Jack Up AMETS Catamaran AMETS Workboat 3 3 5 7 7 9 9 OWEZ Jack Up OWEZ Catamaran OWEZ Workboat 3 3 5 7 7 9 9 M Jack Up M Catamaran M Workboat 3 3 5 7 7 9 9 Figure : Persistence graph for all three vessels at M..3 Monthly access levels The monthly access levels display the number of hours that the met-ocean parameters were at or below each set of access limits during each month. Figure to Figure 5 show the monthly access levels at each location using the Jack-Up, Catamaran, Workboat and using a 5
Hs.5m only access limit. It can be seen that for the Jack-Up at AMETS there is a far higher level of seasonality than at either OWEZ or M. In general OWEZ and M have a less of a seasonal variation than AMETS. This is due to the more extreme conditions at AMETS impacting on access which is more pronounced during the winter months. As the met-ocean limits become more constrained, there is a greater element of seasonality in the levels of access observed as the impact of adverse winter conditions has a greater impact. It can be seen that for vessels with relatively lower access limits there is both an overall lower level of access and a greater seasonality to the access that is available. Access Figure : Monthly access levels of the Jack-Up barge at the three locations. Access Figure 3: Monthly access levels for the Workboat at the three locations. Access % 9% % 7% % 5% % 3% % % % % 9% % 7% % 5% % 3% % % % % 9% % 7% % 5% % 3% % % % AMETS -Jack Up OWEZ - Jack Up M -Jack Up Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AMETS Workboat OWEZ Workboat M Workboat Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec AMETS CATAMARAN OWEZ CATAMARAN M CATAMARAN Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure : Monthly access levels for the Catamaran at the three locations. Access % 9% % 7% % 5% % 3% % % % AMETS Hs.5m OWEZ Hs.5m M Hs.5m Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 5: Monthly access levels for and access limit of Hs.5m at all three locations.. Longest waiting period The est waiting period between windows observed over the four years at each site and for each vessel is shown in Figures to Figure 9. The waiting periods were calculated by recording the length of time between windows at least,, and hours. The overall pattern of access is very visible here as higher wave height limits have much shorter waiting periods. Similarly the waiting periods increase if er windows are required, as these tend to be rarer. The results for the Jack-Up show that for low window lengths, AMETS has the est waiting periods of around 3 weeks with M and OWEZ around -3 days. As the window length is increased, the est waiting period increases at all locations but there is a relatively much larger increase at OWEZ resulting in a er waiting period for a hour window at OWEZ than AMETS of 7 weeks. This may be due to the impact of the relatively larger tidal currents at OWEZ, which have the impact of reducing the occurrences of er windows. Similar results are observed for the Catamaran, although the waiting periods have increased. For the workboat, the waiting periods have increased again, but in this case there is a large increase in waiting periods at M, as well as OWEZ, for er windows. When Hs of.5m only is considered, the relative position of the est waiting period is similar for different window lengths. There is not the same phenomenon of a larger increase in est waiting periods at either M and OWEZ which closes the gap to AMETS. At some locations there appears to be very little change in the est waiting period when the wind length is increased. These results show the individual est waiting period. It may be the case that a large storm caused the est waiting period for a hour window at AMETS using a Jack Up vessel to be 3 weeks. It may be the case that the same storm result in the est waiting period for and hour windows, resulting in the same 3 week result.
Longest waiting time (Weeks) 7 5 3 AMETS Jack-up OWEZ Jack-up M Jack-up At least hours At least hours Window Length At least hours At least hours.5 Sensitivity to tidal and wind speed limit. Figure shows the sensitivity of accessibility to changes in the tidal access limit. Results are shown for the Jack Up at M and the Catamaran at OWEZ. Similar profiles occur for these vessels at each location. It can be seen that for the tidal access limit chosen in this case study,.5 m/s, there is very little change in access levels in or around this tidal access limit. However when the tidal access limit is reduced to below.3 m/s there is a sharp drop off in access levels which continues as the tidal access limit is lowered. Figure : Longest waiting periods for windows using a Jack- Up vessel at all three locations Longest waiting time (Weeks) Figure 7: Longest waiting periods for windows using a Catamaran vessel at all three locations. Longest waiting time (Weeks) Figure : Longest waiting periods for windows using a Workboat vessel at all three locations Longest waiting time (Weeks) 7 5 3 9 7 5 3 AMETS Catamaran OWEZ Catamaran M Catamaran At least hours At least hours Window Length At least hours AMETS Workboat OWEZ Workboat M Workboat At least hours At least hours Window Length At least hours AMETS Hs.5m OWEZ Hs.5m M Hs.5m At least hours At least hours Window Length At least hours At least hours At least hours At least hours 9 7 5 3 Jack Up M Catamaran OWEZ.5..7..9...3..5..7..9 Tidal Limit (m/s) Figure : Sensitivity of accessibility to tidal current limit. Results are shown for a Jack-Up at M and the Catamaran at OWEZ. In both cases the other met-ocean limits have been held stationary. Figure shows the sensitivity of accessibility to changes in the wind speed limit for the Jack Up at all three locations. It can be seen that at high wind speed limits, OWEZ and M have similar levels of access and both are above AMETS. As the wind speed limits are reduced, the higher wind resource at M results in it having less access than OWEZ and similar levels to AMETS. As an example, the wind speed limit chosen for the Jack-Up vessel in this study, m/s, is highlighted in Figure. It can be seen that at this level there are similar levels of access at M and OWEZ and less access at AMETS. This is due to OWEZ and M having similar wave resource and AMETS having a higher wave resource. If a more restrictive wind speed limit were applied for example m/s, also shown in Figure, then the overall levels of access are substantially reduced at all locations. As well as this, M and AMETS having similar access levels with OWEZ having higher access levels. This is due to the wind speed now being the predominant factor with OWEZ having the lowest wind speeds and M and AMETS having similar wind speeds. Figure 9: Longest waiting periods for windows using an access limit of Hs.5m at all three locations. 7
9 7 5 3 AMETS OWEZ M 5 7 9 3 5 7 9 Wind Speed Limit (m/s) Figure : Sensitivity of accessibility to wind speed limit. Results shown for the Jack-Up at all three locations. The sensitivity to different wave height access limits can be observed from the wave height persistence tables (Figures to Figure ). 5 Summary The aim of this paper was to assess the impact on access levels of considering additional factors in addition to the primary factor, the significant wave height. The frequency graphs gave a site comparison for each met-ocean parameter considered. It was shown that M and OWEZ have similar wave heights, wave period and tidal current levels, but M has higher winds levels similar to those at AMETS. AMETS has much higher wave heights and periods but has low tidal current flows. The persistence tables showed that for Hs only, M and OWEZ have similar access levels and both have more access than AMETS. This is the same conclusion when comparing the access levels for the Jack-Up, Catamaran and Workboat at each location. In terms of vessel comparison, at each location the Jack-Up had the most access followed by the Catamaran and the Workboat. The monthly access levels showed that there is a greater level of seasonality for lower access levels due to the increased impact of poor winter weather on the access levels. As a result there is a greater level of seasonality to the access levels of the catamaran and workboat than the Jack-up, due to their relatively lower access limits. Longest waiting period results showed that, in general, the relative access levels when additional parameters are included are similar to those when Hs-only is considered. However, the inclusion of additional parameters may lead to different results if er window lengths are required, as the there is less access than when Hs only is considered. The sensitivity results showed that, for the limits chosen, Hs is still the predominate factor. As stricter tidal or wind limits are enforced, the access levels drop significantly and different trends are observed than when Hs-only is considered. Conclusion This study s results were based on one year s worth of data at each location due to data limitations. It is recommended to use 5- years of data for a full weather windows analysis. Given this caveat, the results showed that, in general, the inclusion of the additional factors did not significantly affect observed levels of access at each location. However, if more restrictive wind speed or tidal current limits were to be applied then different results would be observed. Also different results were observed when er window lengths were required. In conclusion, the results at M in the Irish Sea were similar to those at OWEZ in the North Sea. Therefore the issues with accessibility, together with the mitigation measured employed by North Sea offshore wind developers, would apply to the Irish Sea. Access levels at AMETS off the Irish Atlantic coast were significantly lower than M and OWEZ. This reinforces earlier conclusions that limited access for installation and O&M operations may be a crucial barrier for future marine renewable developments in aggressive wave climates such as the Irish west coast. Vessel suitability for operations are site dependent and very sensitive to access limits chosen. Future work will expand this study by including more data years at locations where it is available. 7 Acknowledgments The authors would like to thank Dr Jimmy Murphy of HMRC for his work in processing the tidal data. References [] Dalton GJ, Alcorn R, Lewis T. Operational expenditure costs for wave energy projects; O/M, insurance and site rent. International Conference on Ocean Energy (ICOE) Bilbao, Spain.. www.icoebilbao.com. [] Walker RT, Johanning L, Parkinson R. Weather Windows for Device Deployment at UK Test Sites: Availability and Cost Implications.. http://www.ewtec.org/. [3] O'Connor M, Dalton G, Lewis T. Weather windows analysis of Irish west coast wave data with relevance to operations and maintenance of marine renewables. Renewable Energy ; In Review. [] Leske S. Momac-Offshore Access System. European Offshore Wind Stockholm. 9. www.eow9.info/. [5] DONG Energy. Offshore O&M Experience in DONG Energy. European Offshore Wind Berlin. 7. www.eow7proceedings.info/allfiles/9_eow7present ation.ppt. [] Lyding PF SH, B; Callies, D. Offshore - WMEP : Monitoring Offshore Wind Energy Use. European Offshore Wind Stockholm. 9. http://www.eow9.info/. [7] Van Bussel GJW. Offshore Wind Energy, The Reliability Dilemma... http://www.lr.tudelft.nl/fileadmin/faculteit/lr/organisatie/af delingen_en_leerstoelen/afdeling_aewe/wind_energy/res earch/publications/publications_/doc/bussel_offshore_w ind_energy.pdf. [] Salzman DJCvdT JG, F.W.B ; Gobel, A.J ; Koch, J.M.L. Ampelmann - The new offshore access system. European Offshore Wind. Stockholm. 9. www.eow9.info/. [9] EWEA. Wind Energy The Facts: Wind Turbine Technology for Offshore Locations. http://www.wind-
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