Background Information An Offshore Windfarm Operator in the southern North Sea is looking to improve the operational and safety aspects of transferring technicians from Crew Transfer Vessels (CTVS) to offshore structures. CTVs are primarily used for transporting crew and tools for common maintenance operations offshore. The day rate for chartering a CTV is typically £1750 and the vessels can typically operate to a wave height level of 1.5m. In practice, a very urgent repair may warrant the charter of a more expensive vessel, which can withstand greater wave heights. Due to their flexibility, CTVs are used extensively by the Operator. At this time, the Operator plans to use the CTV 365 days per year and is capable of making 5 transfers from the boat to structures per day when the sea conditions are within normal operating conditions. The Operator expects the wind farm to be operational for another 15 years, but expects the number of transfers to increase as the system ages and more failures are realized. Currently the Operator charters a CTV for maintenance. Since the maintenance crew are employees of the Operator, who is ultimately responsible for the safe operation of the offshore windfarm, the Health and Safety Executive (HSE) requires evidence that the risks within the process of transferring crew to offshore structures are understood and are being managed. Since there are very little data available on this crew transfer process, the HSE has requested the Operator conducts a risk analysis to support an assessment of process safety. System Information The current transfer process requires the CTV to approach the offshore structure head on and requires technicians to transfer by stepping off the CTV onto an access ladder whilst being tied on with a safety rope. A detailed overview of the process is given in Appendix A. Schematics and further illustrations of a standard CTV and the operations associated with transferring crew to offshore structures is included in Appendix B. The current process has a number of issues; 1. From a safety point of view there is a risk that a rogue wave¹ may strike the vessel at the point of transfer and cause the CTV to drop leaving the technician hanging on the safety rope. The CTV will then come back up on the wave and the technician can then be crushed between the CTV and the ladder suffering severe crushing injuries or even death. 2. To mitigate risk, there is a restriction in transfer operations. CTVs are restricted to transferring technicians in significant wave heights of 1.5 meters or less. This then limits the ability of the operator to conduct maintenance on the Wind Turbine Generators (WTG). Due to this, there are times when a failure has occurred but access to the site is blocked. For every time a CTV is unable to access a turbine, the expected cost impact is £10,000 per day. The Operator is aware of risks existing within individual steps of the current process, but is unaware of the overall level of risk. As well as the safety concerns, the Operator is also unaware of the risk that they will be unable to carry out repairs and hence lose income through loss of turbine availability. A rogue wave is defined as any wave that is twice the wave height at that particular time. A transcript of a recent conversation with the Captain of a CTV has been included in Appendix C. During this conversation, the Captain has provided quantitative estimates of some of events. However, you should not assume that this list is exhaustive or that all items are relevant. Some data you want or need may be missing - for this you should make assumptions based on other data relevant to the associated failure rate or probability. In addition, note that some failure data are given as "failures per hour" and some are given as "probabilities". Thus, you should make assumptions about the meaning of these. The Operator is aware of potential control systems to protect the crew in the case of an accident. Some of these are already adopted as part of their process, e.g. the yoyo system (see Appendix A). The Operator would like to have an understanding of the potential impact of these safety control systems and the effect of adding or removing them within the transfer process. As mentioned previously, the current system only operates at wave heights of 1.5 meters or less. Appendix D contains data on the wave height at the wind farm location and the likelihood of a rogue wave. The Operator would like to be able conduct safe transfers in higher sea states. There are a number of crew transfer systems (see example image on the right) in development that aim to operate in more harsh seas states by providing the following desirable characteristics: For vessels with stable platforms and good station-keeping • Ability to operate in high sea states • Fast to deploy/recover • Robust fail-safe mechanisms • Ability to operate on variety of vessels The Operator has identified the following systems that could operate at higher sea states. System Projected sea state limit of system Capital Cost per vessel (£k) Maintenance cost per year per vessel for 10 years (£k) S1 S2 S3 1.75 2.25 2.8 1200 10 1400 20 2600 40 The Operator would like advice on the cost-benefits of using each of the above systems and give a recommendation on which system would be the best to implement at their site. The following assumptions should be made: • For every rogue wave, there is a 50% chance that it will result in either significant injury or death, depending on rogue wave height. All year round, there is a 3% chance that during any transfer, a rogue wave will occur. • The value per fatality (VPF) is taken to be £1.6million The value per significant injury is taken to be £250k. • The discount factor is assumed to be 8% per annum. • • The discount factor for VPF is to be taken as 4% per annum The Operator forecasts that there could be 200 transfers a year on site for 365 days of operation. This covers annual maintenance (which every turbine requires once per annum) and reactive maintenance. However, this figure will change over time, and the Operator is not confident of his assessment. For every lost day of WTG operation it costs the Operator £10,000 The breakdown rate for WTGs on this site is projected to be 5% per year • There are 100 wind turbines on site