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Assessing the consequences of sea-vessel collisions with offshore wind farms: Part I

Björn Kramer, (SFI), Offshore Wind Power department at TÜV SÜD Industrie Service in Hamburg

Calculation models and simulations are important tools in preventing accidents involving ships and offshore wind turbines.

Many aspects of ship collision analysis are currently the subject of discussion. Namely, how do wind turbines have to be designed and dimensioned to minimise the consequences of a collision or an impact during the landing operation of a service boat?

First and foremost, all offshore wind farms are obstacles that restrict the traffic areas of ships. Given this, collisions are a fundamental possibility. In the first instance, this applies to large sea-going vessels that must take evasive manoeuvres or may be unable to manoeuvre. The distance between their traffic routes and the wind farms must be as large as possible to reduce the risk of collision. 

However, in the second instance, this also concerns the routine landing operations of service boats for repair and maintenance work.
Their regular impacts and the periodic loads they cause may pose a risk – particularly, for instance, if the wind farm's boat-landing system is of a simplified, non-standardised design. The potential hazards involved for people, vessels, wind turbines and the environment should be examined in as much detail as possible in the run-up to the offshore project.

'Acceptable' risk

Competent approval authority for wind turbines installed between 12 and 200 nautical miles off the German coast (i.e., within Germany's Exclusive Economic Zone) is the Hamburg-based German Maritime and Hydrographic Agency (Bundesamt für Seeschifffahrt und Hydrographie, BSH). The authority ensures that Germany fulfils its international obligations concerning the safety of maritime transport. Wind farms, for example, must typically be sited at a safety distance of 4.5 km to any shipping routes. An “acceptable” risk is one collision in 100 years involving a larger vessel and tolerable consequences — which means, for example, that the collision does not cause any severe injuries or loss of life. 

Given the above, the consequences of a possible ship collision and the cyclic loads during landing operation of service boats are analysed in the run-up to a wind-farm project. However, which assumptions and parameters are relevant for analysis? Which model is the most suitable? Moreover, what does this mean for the design of the foundation structure and the offshore wind turbines? The answers to these questions are currently the subject of discussions in the offshore sector, since clear standards and regulatory requirements for offshore projects located in deep-sea areas have not yet been established for all areas concerned. 

It’s common knowledge that project owners, manufacturers, authorities and approval bodies are facing multifaceted challenges in the context of collision analysis. When calculating impact operations as load cases, experts must take many aspects into consideration that significantly influence wind turbine stability and service life. First experiences have shown that estimates of the operating loads imposed during routine landing operations of smaller service boats are as yet still very inaccurate. 

Key design considerations

Following are a few key considerations in wind-farm design when performing collision analysis:

Factor 1: Collision-friendly design preferred. Damaged boats and deformations or failure of the components of the "boat landing systems" of offshore wind farms significantly increase the risks for any members of the service crew during the critical point of landing at and accessing the platform. In some cases, this may even result in life-threatening situations. To that end, numerical models and analyses should aim at safe yet cost-efficient turbine design which fulfils the required safety criteria in case of a collision, whilst also ensuring functionality and providing reserves for reliable long-term routine operations. 

According to several experts, the currently established mathematical approaches to collision analysis are unable to realise both these objectives. The reason for this is that most models for the load case of ship collision are not accurate enough to permit exact assessment of the relatively weak impact during landing operations. This involves estimating the deformation and fatigue of the steel structure, but also possible damage to machine components in the nacelle. 

Factor 2: How realistic are calculations? The BSH requires collision analysis to give qualitative answers to the following questions: 

*Does wind-farm design offer sufficient flexibility to avoid causing tearing of the ship's inside hull, or are remedial measures required (proof of collision-friendly foundation design, CFFD)?

*Can the turbine or parts thereof fall onto the ship in the case of collision?

*Which of the four categories – minor, moderate, major or catastrophic – applies to the consequences of collision?

In this context, we must ask ourselves whether an exact, state-of-the-art numerical assessment of the severity of the possible consequences would be possible and more expedient. For a qualitative analysis of load limits and deformations in the case of ship collision, it may be sufficient to extensively simplify the complex calculations with the help of assumptions, provided these are based on conservative data and values. The offshore wind sector has seen a tendency to simplify calculations by using assumptions. However, in past analyses, the modelled ships were generally significantly more flexible (in the sense of "softer") than they actually were in reality.

Yet simplified ship models are not always suitable for calculating and assessing the cyclic operating loads caused by service boats and for preparing the fatigue analyses based thereon. Their use may be at the expense of the safety of people working on the wind farm, who must be able to get safely from the service boat onto the platform. In addition, simplified models that are based on differing assumptions and calculation methods do not permit quantitative comparison of various collision scenarios and analysis results. This shows that there is great need for discussion by all stakeholders and that harmonised standards are required in the medium term — to provide a clear and standardised description of the state of the art in the relatively new field of offshore wind technology and gradually catch up with other technologies that have decades of experience in this sector (think “shipbuilding or offshore oil and gas extraction”).  

Factor 3: Relevant aspects for calculation models. The choice of an adequate reference ship poses the first challenge, because the critical ship type for the wind farm in question must be identified before possible impact energy distribution in the case of collision can be calculated. The following aspects play a role in ship type selection: collision probability, environmental consequences of a collision (e.g. the possible amount of oil spilled into the Wadden Sea), potential hazards for the crew and passengers on the reference ship. 

Collision simulation

To simulate the collision, the experts then create a finite element model (FE model) of the chosen reference ship type. In most cases, the FE model is based not on a real ship, but on the typical design of a ship. A ship design commonly used as reference ship is a smaller, double-hull tanker with a length of approximately 175 metres and a displacement of 45,000 tonnes. The engineers now decide on a case-by-case basis whether — and to what extent — simplifications in the various ship sections are possible and acceptable. Instead of detailed consideration of the stiffening elements, for example, the thickness of the hull plating can be virtually increased. Note: The BSH standard does not contain any information about the required accuracy of the FE model. 

However, as a consequence of these types of simplification, the model may no longer correspond fully to real-life conditions. Yet, as assumptions are on the conservative side, we do not know the extent to which the model deviates from actual conditions. A comparison of the results would be easier, for example, if there was a standard that established the acceptable limits of a side-impact force-deformation ratio – as either normative or informative limits. These values could be used for the calibration of an FE model and as the basis of analysis where appropriate. 

he collision scenario can be based on the assumption that the wind farm will be empty of people at the time of collision, as mechanics only visit the wind farm for short periods during the day to carry out service and maintenance work. However, this assumption does not apply universally to transformer platforms, which can accommodate maintenance crews over several days. This, in turn, gives rise to further questions:

* When can a wind farm be considered staffed? (Consider the number of regular days of presence of how many people.)

* What should be designed more strongly in case of a collision – the ship or the staffed platform?

* How important is the issue of security of supply? 

Failure of a central substation on such a platform could mean that several other wind farms connected to the substation cannot supply any electricity until the substation has been repaired – which can take up to two years. So far, no satisfactory answers have been given to many of these questions, even though proposals have been submitted to the authorities.

Nevertheless, specific issues must be addressed: So far, solid reinforcing structures of the wind farm have not been taken into account in either the model or the direction of impact (number of load cases to be examined). Another issue concerns thick-walled boat landing systems that may cause extensive damage to the outer hull of a ship that passes by too closely. This, in turn, involves the risk of an oil spill with negative consequences for the ecosystem.

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Policy, investment and markets  •  Wind power




22 June 2014
Dr.A.Jagadeesh Nellore(AP),India


20 May 2014

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