Heavy lift: Direct simulation of offshore lifting operations

In recent years more and more heavy lifting crane operations in the offshore industry have been performed by floating vessels. On the one hand, the oil & gas industry is exploring deeper waters where the use of jack-up or fixed platforms are not possible. On the other hand, the offshore wind industry requires cost efficient solutions for heavy lifting operations. The use of floating crane vessels is an interesting alternative when compared to jack-ups as there is less competition for these vessels on the charter market. 

Crane operations for wind farm installations

The installation of a typical wind farm with 60 to 80 offshore wind turbine generators (WTG) requires over 500 separate lifting operations of components weighing over 50 tonnes and have lengths of over 50 meters. Due to the large number of repeated lifting operations, the reduction of cycle times while maintaining the safety is an important issue. Therefore, special emphasis should be put on the choice and design of the crane equipment in the early planning phase.

Not all of these crane operations can be based on floating platforms, such as the delicate installation of the rotor. But more robust components, such as foundation structures and transitions pieces, have been successfully installed by floating vessels in the past. It is this type of heavy lifting operation, which is in the focus of this article.

Besides the actual heavy lifting operations, transportation to the installation site is another important issue. This can be done either with feeder barges or vessels, or by the crane vessel itself. In line with the second approach, several wind farm transport and installation vessels have been purpose-built in recent years.

Most of these vessels are equipped with jack-up systems which increase the building costs and reduce the transported load capacity. An alternative approach is the use of specialised conventional heavy lift carriers. These vessels, such as the SAL Heavy Lift vessel MV LONE, provide the necessary crane and transport capacity. Typically, they are equipped with a dynamic positioning system. These vessels are not dependent on time consuming mooring or jacking operations which saves operational time and eventually money.

A significant issue for the use of floating crane vessels is to assess their actual operational capabilities or limiting conditions, respectively. In order to reduce overall installation time and costs of large offshore wind farms, the installation window needs to be stretched as far as possible throughout all the seasons. Currently, most European wind farms are erected in the North, Baltic and Irish Seas, therefore there is a requirement for crane lifting operations in harsh weather conditions. In order to maintain a reliable cost and time planning for the wind farm installation, the operational capability has to be determined as precisely as possible, without increasing the analysis efforts excessively.

Operational capability

When determining the operational capability, the focus must always be on the safety level, i.e. the strict control of risks to personnel, equipment and the environment. Hence, the focus is to determine the limiting values of the environmental parameters such as wave height and period, wind speed and current speed.

The traditional approach for the planning of floating crane operations is commonly based on sequential 2D-drawings, load charts and empirical formulas for the determination of dynamic amplification factors. These factors can be used to determine by what level the allowable load of the crane has to be reduced for offshore operations when compared to harbour operations.

The aspect of time for the execution of the crane lifting operation is based on experience. As long as one operates inside the field of experience with regard to the vessel, the lifting operation and the environmental conditions, this is a fast and reasonably reliable method. But this approach is not as effective when coping with new operating conditions or equipment, or if the task is the optimisation of the overall process. In such a case, direct simulation of a lifting operation provides a better approach.

Motion simulations of floating vessels in frequency or time domain are standard industrial tasks nowadays. For lifting operations, however, multi-body simulations have to be performed, because the vessel motions influence the motions of the free hanging crane load and vice versa.

Several different software solutions for multi-body simulations are available, such as SIMO by DNV. Normally they are only capable of simulating discreet phases of the operation, meaning crane parameters, including the outreach and hook height or the ballast condition, cannot be altered during the simulation. Therefore, usually one or two critical phases are selected and analysed. Hence, it is not proven that there is no critical resonance between the motions of the load and the vessel which occur at any other operation stage.

Another aspect is that for blunt bodies like crane barges, linear computations are sufficient in most cases. However, non-linear computations should be performed if slender, ship shaped heavy lift vessels are analysed in sea conditions.

R&D project for simulation tools

Although there are a large number of different tools for the simulation of different aspects of transport and installation operations, there is a lack of comprehensive, fast and reliable simulations tools that meet the above described criteria. In order to fill this gap, the research and development project ‘Hochsee-Operationen mit Kranen’ – in short ‘HoOK’ – has been started in spring 2013. It aims at the development of new simulation tools for offshore lifting operations.

The project consortium consists of the ship design office HeavyLift@Sea, the Institute of Ship Design and Ship Safety of the Hamburg University of Technology (TUHH) and the marine consultancy office MAREVAL as project coordinator. In this consortium, the different strengths of the three partners are combined: MAREVAL’s experience in offshore operations and offshore technology, HeavyLift@Sea’s experience in the design of heavy lift vessels and the competence of TUHH in the development of software for dynamic ship simulations.

The project is scheduled for a three year period and is funded by the German Federal Ministry of Economics and Technology in its program “Maritime Technologien der nächsten Generation” (Next generation maritime technologies).

The project’s goal is to perform the complete dynamic simulation of a floating crane operation in the time domain within one single software package. This will be able to perform a fully-integrated simulation of a complete lifting operation, including all motions of the crane and its load, and the necessary ballasting and dynamic positioning operations of the vessel. Hydrostatic stability checks and assessments of the motion in various sea conditions will be provided at any stage. This approach also opens the door to manual or even algorithmic optimisation, once the specific boundaries of the operation are defined.

Simulations will be either purely hydrostatic for stability checks and scheduling of ballast operations, or fully dynamic for analysing sea-keeping and DP-capabilities. The hydrostatic computations will be performed with the hydrostatic model of the vessel with correct consideration of the load induction of the crane and load in their respective positions, even for multi-hook operations.

The dynamic simulations will be performed with the non-linear sea keeping simulation E4ROLLS. This code is capable of simulating several hours of real time motion in sea conditions within only a few minutes. Thus, the influence of the whole range of environmental parameters can be analysed within a few hours.

By integrating the new simulation tools developed by HoOK into the existing ship design environment RDE/E4, the modelling effort for the application of the simulation tools is reduced. Instead of remodelling the crane and the vessel in different tools only one data base is used. This addresses all the necessary questions.

Conclusion

The planning and approval of offshore crane operations should be based on direct simulations as far as possible and feasible. This is even more import if innovative technology is used or new fields of applications are explored. But a major hindrance for the application of direct simulation for floating crane operations exists.

This is that there is a lack of simulation tools which can analyse the complete sequence of a lifting operation in all necessary details. In order to fill this gap, the R&D project HoOK was started by MAREVAL, HeavyLift@Sea and TUHH. With the help of the new developed software tools, the direct simulation of the complete sequence of offshore heavy lift operations will be possible and the operational capabilities for the operation can be determined more precisely.

This will result in more reliable planning, reduced downtimes and ensure proper safety levels. The new tools will also strengthen each of the project partner’s competences in the design of offshore equipment and the planning of offshore operations. For MAREVAL this means that costumers can be better assisted than ever before in their selection or design of the correct equipment for their specific offshore lifting operations.

Thanks to Hendrik Vorhölter and Jan-Henning Günther, Naval Architects at MAREVAL AG