Risk of collision between service vessels and offshore wind turbines

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Abstract

Offshore wind farms are growing in size and are situated farther and farther away from shore. The demand for service visits to transfer personnel and equipment to the wind turbines is increasing, and safe operation of the vessels is essential. Currently, collisions between service vessels and offshore wind turbines are paid little attention to in the offshore wind energy industry. This paper proposes a risk assessment framework for such collisions and investigates the magnitude of the collision risk and important risk-influencing factors. The paper concludes that collisions between turbines and service vessels even at low speed may cause structural damage to the turbines. There is a need for improved consideration of this kind of collision risk when designing offshore wind turbines and wind farms.

Highlights

► We study the risk of collision between service vessels and offshore wind turbines. ► A new framework for analysis of the collision risk is proposed. ► We investigate the magnitude of the collision risk and risk-influencing factors. ► The results contribute to new knowledge on the impact of collisions on turbines.

Introduction

Current offshore wind farms are located close to the shore, typically within a distance of 20 km, where the density of shipping lanes and other traffic is relatively high. The maritime safety in the vicinity of offshore wind farms therefore raises concern. Several studies (e.g. [1], [2]) focus on the risk of collision between offshore wind turbines (OWTs) and passing vessels, due to maritime transportation, fisheries, and military activities, but limited attention is paid to the risk of collision from service vessels that approach the OWTs, for example, to carry out maintenance.

The trend is to move OWTs farther offshore and into deeper water to take advantage of the increased production potential and fewer conflicts with local human and animal populations. The future offshore wind farms may be located away from commercial ship traffic lanes. The need is therefore reduced for the analysis of collision risk between passing vessels and OWTs [3]. On the other hand, the more hostile environment farther offshore requires new and larger types of service vessels, and this increases the need for assessing the risk of collision between the service vessels and the OWTs.

The offshore oil and gas industry is much concerned about collisions of visiting vessels on assignment with offshore installations [4], [5]. In the last decade, 24 out of 26 reported collisions on the Norwegian Continental Shelf were caused by visiting vessels [6]. The underlying causes of the collisions include complex equipment, inadequately trained crew, and violation of procedures [6]. Due to the similarities in operational procedures and environmental conditions, the same causes may lead to collisions between service vessels and OWTs.

Offshore wind energy production does not involve hazards on the same level as the process hazards on oil and gas installations, but damage to the OWTs may pose risk to personnel onboard service vessels, increase the need for maintenance and repairs, and lead to costly production outage. To have a thorough understanding of the collision risk and the need for mitigating measures is, therefore, important in order to improve workers’ safety and cost-efficiency.

The main objective of this paper is to present a framework for analysis of the collision risk between service vessels and OWTs. For probability estimation and consequence analysis, methods are introduced with examples to illustrate their application. These examples can contribute to new knowledge about the impact of collisions on OWTs, and the importance of taking this kind of collision risk into consideration in the design of OWTs and offshore wind farms.

The collision risk may increase when offshore wind farms are moved farther from the coast and into more exposed areas and when, consequently, the service vessels become larger. These wind farms have yet not been designed to the level of detail necessary for enabling a complete risk analysis. The collision risk analysis framework in this paper is proposed for these offshore wind farms, but will need to be adapted to the specific cases. Therefore, the focus of this study is on the introduction of the framework and associated approaches, with simplified examples for illustration.

The remainder of this paper is arranged as follows: Section 2 provides an overview of the previous incidents in offshore wind energy industry and relevant statistics from the offshore oil and gas industry. The framework for collision risk analysis is presented in Section 3. Section 4 gives a brief introduction on current offshore wind farms and the development trend, as well as the service vessels which are categorized into four groups. Hazards related to collision between service vessels and OWTs are identified in Section 5. Section 6 focuses on causal analysis and presents methods for probability analysis. Section 7 applies a quasi-static simulation approach for collision consequences. The simulation results reveal the magnitude of collision between service vessels and OWTs. Section 8 discusses the strategies for risk reducing measures. Conclusions are drawn in the final section.

Section snippets

Previous incidents

Public data on accidents and incidents related to offshore wind farms is hard to find, but some brief incident descriptions based on news articles are collected in the Caithness Windfarm Information Forum (CWIF) database [7]. This database contains only one relevant record of a collision between a service vessel and an OWT (up to June 30, 2012):

A jack-up barge smashed into one OWT on October 6, 2006 in Scroby Sands wind farm, off the Norfolk coast in England. About 20 cm of the tip was broken

Risk analysis framework

A risk analysis provides answers to the three questions: (i) What can go wrong?; (ii) What is the likelihood of that happening?; and (iii) What are the consequences?. The OWT structure is normally designed to withstand collisions from dedicated service vessels at low speed. It is more uncertain whether the OWT is able to withstand collisions from the same vessels at high speed, or from larger service vessels. The threshold for damage to the OWT needs to be evaluated in each specific case, based

OWTs and types of vessels

Currently, there are 1247 OWTs installed and grid connected, totaling 3294 MW in 49 wind farms in nine European countries [11]. Experience from a few offshore wind farms in operation, for example, Tunø Knob, indicates a total number of five service visits to each OWT per year [12]. Tunø Knob consists of only 500 kW OWTs with relatively mature technology and extensive operational experience. Recent OWTs have a rated power of 2–3 MW, with a trend towards 5 MW and higher. These OWTs are developed

Hazard identification

To identify hazards related to collision risk, it is necessary to know the typical operations of the service vessels. A general procedure for service vessel access is adapted from the RenewableUK guideline [18], and is shown in Fig. 1.

The activities in Fig. 1 that may lead to collision impact are:

  • Service vessel approaches an OWT:

    • The service vessel fails to stop when it reaches the OWT and hits the OWT at high speed.

    • The vessel misjudges a turning or maneuvering, and hits the OWT at relatively

Collision probability

Due to the lack of historical data, it is not feasible to estimate the probability of the hazardous events and/or the consequences of the collision scenarios from data alone. The best alternative is to use a Bayesian approach where the collision probability is interpreted as our degree of belief based on all available knowledge. As part of this approach, it is necessary to identify the risk-influencing factors (RIFs) and evaluate their effects on the hazardous events and/or the development of

Assessment of collision consequences

The possible consequences resulting from collisions between service vessels and OWTs include environmental impacts, economic losses, and personnel injuries/fatalities. They are all dependent on the severity of the structural damage to the OWTs and the service vessels. Structural damage can be analyzed based on the principle of energy conservation. The basic formula for calculating the total collision energy E (J) is:E=12·a·m·vvessel2where m is the vessel displacement (kg), a is the added mass

Risk evaluation and risk reduction measures

Risk is usually measured and evaluated by the combination of probability and consequence. According to the simulation results, the OWT structure is damaged even when the vessel speed is low. Damage to the boat landing structure or the OWT support structure are costly to repair offshore. In addition, the collisions may lead to loss of human lives if, for example, the service vessel sinks after collision. Risk reduction measures can be grouped into two main categories: probability reducing

Conclusion

The offshore wind energy industry continues to grow fast and into more exposed areas which means that the safety of personnel and the structural integrity of the OWTs become more demanding issues. The development into more remote locations and deeper waters may require larger service vessels, increasing the potential impact energy and resultant severity of collisions with OWT structures. To investigate the risk of collision between service vessels and OWTs, the current paper proposes a specific

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