Design Study of Floating Crane Vessels for Lifting Operations in the Offshore Wind Industry

Hendrik Vorhölter1, Hannes Hatecke2 and Dag-Frederik Feder3

ABSTRACT

In the presented work a time-domain analysis of typical lifting operations for the offshore wind industry is performed. Three different vessels and two different loads variations of the lifting operations are considered in the analyses. The presented work is part of the joint research project “HoOK” for the development of simulation tools for offshore lifting operations.

KEY WORDS Heavy-Lifting Operation; Crane Vessel; Sea-keeping; Time-Domain Simulation; Renewable Energy

INTRODUCTION The construction and installation of offshore wind turbine generators (OWTG) requires a large number of lifting operations. Currently it is preferred to use specialized equipment for these lifting operations like crane jack-up vessels. In order to be competitive with conventional power generation the offshore wind industry is required to reduce the installation costs significantly. But, as the offshore oil and gas industry uses similar jack-up equipment, the two industries compete for the same equipment. The oil and gas industry is able to pay higher day rates. Therefore, the offshore wind industry is required to look for economic alternatives. One alternative is the use of floating crane vessels instead of jack-up vessels at least for the components of the OWTG which are below or in the level of the sea surface. These components are the substructures itself, foundation piles and suctions buckets as well as transition pieces. In this article a design study for different floating crane vessels is presented. Designs of crane vessels in a range from converted conventional heavy-lift carriers over offshore construction class vessels to crane derrick barges with crane capacities of up to 3000 t are compared to each other by simulating lifting operations typical for the offshore wind industry. The lifting operations are simulated with non-linear sea-keeping tools, which have been developed in the joined R&D project “HoOK”. The motion of the free hanging load is computed for operational setups. The results from these analyses will be used to derive recommendations both with regard to operational limits of existing crane vessels and for the design of future crane vessels. NEED FOR SIMULATION OF LIFTING OPERATIONS As pointed out above, one way for reducing installation costs in the offshore wind industry would be using floating crane vessels instead of jack-up vessels. By doing so capex, weight and space for the jacking systems could be spared. Time consuming jacking operations could be avoided. And by positioning the crane vessel with DP systems one would be more flexible for the approach of the OWTG. For jacking operations the flexibility for positioning of the vessel is usually very limited due to existing foot prints or cable routs. The other side of the coin is that lifting operations from floating vessels have to be planed even more carefully as from fixed platforms like jack-up vessels. This should include a detailed simulation of the complete lifting operation beginning from the moment the hook or the load is released from its sea-fastening until all parts of the lifting gear are fixed again. Simulations should not only be part of the planning of lifting operations. They should

1 MAREVAL AG, Hamburg, Germany 2 Hamburg University of Technology (TUHH), Institute of Ship Design and Ship Safety, Hamburg, Germany 3 Hamburg University of Technology (TUHH)

also be used for the design of new crane vessels or the selection of appropriate equipment for an installation campaign. The purpose of the simulation should be to address to major topics:

1. Ensure a sufficient level of safety for the crew working on the vessel and the construction site, limit the social impact and limit the risk for environmental damages.

2. Minimize the operational costs for the installation campaign.

The later can be achieved by selecting the right equipment, which is not over dimensioned, and by a precise definition of environmental limitations for the planned operation. In any case the master of the vessel, the supercargo engineer and the marine warranty surveyor are the persons, who finally decide offshore, if the operation is performed or not. But, no master would make a decision for a go of an operation, if he cannot rely on his own experience with the vessel or type of operation or at least on the competence of the operator’s engineering department. If new equipment should be used or even designed, or if the environmental limits should be pushed further, experience cannot exist. In these cases precise simulations for the lifting operations should be used. Up till now simulations for lifting operations are not very common although lifting operations need to be regarded as risky. Very often simulations are limited only to one phase of the operation, which was chosen as the most critical for the operation. In most cases only linear simulation are performed and the coupling of the hydrodynamics of the vessels and swinging loads is often neglected. Although swinging of the load needs to be avoided, a simulation should be able to handle this case in order to perform what-if simulations for risk analyses. There are also operations like the handling of upended monopiles where the possibility to use tugger lines in order to control the load are very limited. For these operations swinging of the load need to be considered without doubt. Currently there are no simulation tools available which are able to perform a quick and precise simulation for a lifting operation while fulfilling the following requirements:

Being suitable for the early design or early planning phase: no or only limited access to precise data about the equipment (i.e. vessel and crane) or the load

Simulating the complete lifting operation from hook-on until hook-off including ballast operations and moving of the crane

Performing a large number of simulations for different environmental conditions in a short time in order to define the environmental limitations.

Handling all types of crane vessels and potential loads Correct hydrodynamic properties of the vessel with non-linear lever arm properties for ship-shaped crane vessels.

In order to overcome the lack of simulation tools a research and development project was started by the consultancy office Mareval, the Institute of Ship Design and Ship Safety of the Hamburg University of Technology and the ship design office HeavyLift@Sea, all from Hamburg, Germany. The project was started in 2013 and is founded by the Federal Ministry of Economics. The purpose of the project is to develop simulation tools for lifting operations as part of the ship design system E4, which fulfill all requirements stated above (see also Shipbuilding Tribune 2013). In the first steps a crane model for the ship design system E4 was developed (Vorhölter et.al. 2014) and the existing sea-keeping tools were adapted for analysing lifting operations (Hatecke et. al. 2014). In the R&D project which is ongoing until 2016 also new simulation tools for multiple hook operations including tugger lines will be developed. For this work the tools developed in the first stage are used for analysing lifting operations for installing OWTG. The methodology therefore is described in the following. METHODOLOGY The planning and analysis of the crane operation is conducted within three basic steps. First of all, the crane operation must be described. Secondly, the motions of the load and the ship are calculated by a time-domain simulation for the crane configuration specified before. Thirdly, the calculated dynamics during the crane operation must be evaluated in order to allow acceptance or target-oriented modification of the operation. These three steps are shown in detail in Fig. 1 and are realized within the ship design environment E4 (Bühr et al. 1988).

Figure 1: Structure of planning and analysis method.

Description of Crane Operation First of all, the crane operation must be described. This is done by manual specification of certain description points. Each description point consists of the desired hook position and load with a corresponding time. Thus, the entire time-dependent path of the hook during a crane operation can fast and efficiently be described by a few distinct description points, as shown in Table 1. Furthermore, tank fillings can be specified in the description points for the main purpose of counteracting crane heeling moments. In order to prepare a time-domain simulation from the crane operation definition, an underlying crane model is required. The model contains data about the geometric, kinematic, mass, and control properties of the crane, as well as computational routines to process them. The first function of the crane model is to check if the operation is feasible. Feasibility means that the hook locations of all definition points can be reached by the crane and can be connected to a legal shortest path while complying with the speed limits of the crane. Due to the non-linear kinematics of the crane, the shortest path between two definition points may not be unique. For example, if the crane needs to do a 180° azimuth turn between two definition points, this can equally achieved clockwise or counter-clockwise. However, these problems can easily be manually solved by adding a further definition point in between the two existing definition points. Moreover, the crane model is used to calculate the suspended length and crane positions (azimuth, radius, etc.) to meet the previously defined desired hook locations. Based on the crane positions, the center of gravity and total inertia of the ship with the crane as well as the suspension point location can be calculated automatically. This information as well as the suspended length and load completely describe the dynamic model which is used for the time-domain analysis.

Table 1: Example of crane operation description

Description point Operation time Hook location Hook load Tank fillings x y z

[ ] [s] [m] [m] [m] [t] [%]

1 0 34 -3 27 301.5 …

2 150 34 -3 29 301.5 …

3 200 34 -3 29 301.5 …

4 600 48 -26 29 301.5 …

5 1800 48 -26 12 301.5 …

6 2000 48 -26 12 301.5 …

Simulation of Crane Operation The crane operation is simulated with an 8DOF time-domain sea-keeping method (Hatecke et al. 2014) with quasi-statically modified constraints. The underlying assumption behind this quasi-static approach is that the load and ship motions are much faster than the crane control motions. This assumption is certainly true for offshore crane operations and becomes even more satisfied when sea-keeping and wave induced motions are critical and therefore of interest. Hence, we neglect the time derivatives of the suspended length l, the load m the inertia of the ship sI and the suspension point location srT relative to the ship in the equations of motions of the system:

The variation of these parameters is only accounted for quasi-statically by using the instantaneous suspended length l(t), the load m(t), the inertia of the ship sI(t) and the suspension point location srT(t) relative to the ship in the equations of motions. In the time-domain simulation, these parameters are prescribed by an interpolation in a look-up-table, which includes the parameters at discrete time steps and is called simulation model table here. The simulation method itself (Hatecke et al. 2014) uses an 8DOF model for the dynamic system of the ship and the suspended load. Six degrees-of-freedom describe the rigid body motions of the ship. Three of them describe the translatory motions surge, sway and heave and three describe the rotational motions roll, pitch, and yaw in terms of Tait-Bryan angles. The suspended load is considered as a point mass. Hence there are two degrees-of-freedom required to completely describe the motions of the suspended load when the suspended length is a prescribed constraint and rope elasticity is neglected. These two degrees-of-freedom are the roll and pitch angle and of the load as shown in Fig. 2.

Figure 2: Dynamic model of ship and load.

The concept of the dynamic analysis of Hatecke et al. (2014) is based on the sea-keeping code E4-ROLLS (IMO 2007, 2008) which was first established by Söding (1982) and further improved by Kröger (1986, 1987). The focus of the method lies on fast and efficient roll and load motion computations. This intention led to the following separated treatment of the degrees-of-freedom: The heave, sway, yaw, and pitch motions of the ship are assumed to be small and less influenced by non-linear effects than the load motions or the roll motion of the ship. Therefore, these motions are computed in the frequency-domain by means of a linear 8DOF strip method. Their response amplitude operators (RAOs) are then used to create motion history signals in irregular seas by superposition of reactions in regular waves. The load motions and the roll motion of the ship, by contrast, are strongly coupled, depend much more non-linearly on the wave as well as on the motion amplitude and thus are computed non-linearly. The equations of motion are coupled and thus solved as a system of ordinary differential equations considering all coupling effects between these motions. The remaining degree-of-freedom is the surge motion, which is computed non-linearly but uncoupled. In E4-ROLLS the non-linear treatment was done to account for surf-riding and added wave resistance effects. In the method of Hatecke et al. (2014) this non-linear computation has been adopted although there is no physical motivation for it during analysis of crane operations. The sea-keeping model accounts for various external forces. These forces are summarized in table 2. Further and detailed explanations can be found in Hatecke et al. (2014).

Table 2: External loads included in the dynamic model.

Acting on the ship Acting on the suspended load Gravity Non-linear Froude-Krylov force Heave added mass Linear wave excitation of irregular, short crested seas Non-viscous and viscous roll damping

Gravity Tugger lines (5% of critical damping)

Evaluation of Crane Operation After the motions of the ship and the load during the operation are calculated, both can be assessed. Critical phases of the operation can be identified and statistical results can be obtained. The latter are often used to define safety criteria which can be checked during the evaluation as well. In a final step the knowledge about the motions, the fulfillment or non-fulfillment of safety criteria, and potential critical phases is used to assess the operation as safe or to modify and improve the operation. ANALYSED VESSELS AND OPERATIONS The simulation and modelling tools are in principal able to handle any type of vessel or lifting operation. For this article three different vessels have been selected with two different loading operations to show the possible range of application for the time being. Up till now the time-domain sea-keeping analysis is limited to single hook operations. The three vessels are a converted heavy-lift carrier with DP2 capability (HLV), a 130 m offshore construction vessel (OCV) and 3000 t crane derrick vessel (CDV). The main particulars for the three vessels are shown in Table 3.

Table 3: Main particulars of the analyzed vessels

Dimension Unit HLV OCV CDV LBP m 150. 140. 140. B moulded m 27.5 30.5 40. T (operation) m 8.0 8.8 8.5 Displacement (operation) t 22,300 21,300 41,000 Max. deadweight t 13,000 10,000 18,000 Max crane capacity t 1,000. 400. 2,600 @ outreach m 16. 13.5 20. Crane type - Derrick w/o

counterweight Knuckle-boom Derrick with

counter weight Number of cranes - 2 1 1 The first two vessels are used to lift a transition peace for a wind turbine generator foundation with a weight of 300 t and a height of 18 m. The distance between top of the load and the hook for the rigging is assumed to be 2.5 m. The CDV is used to lift a tripod foundation with a weight of 900 t and a height of 35 m and a half diameter of 16 m. The time for the operation is

limited on the one hand by the speed of the crane and on the other hand by the amount of water ballast to be moved in order to limit heel and trim and the respective water ballast pump capacity. The operations performed are described in detail in the following. Hereby only the corner steps for the operation are described. The positions of the crane in between and the amount of pumped water are computed by the crane operational model. Heavy-Lift Vessel For the HLV the following operation is performed (see also Figure 3).

1. The load is hooked up to the aft crane (crane 2), while the crane is slewed about 40° inboards. The forward crane (crane 1) is slewed outboard and used as counter weight.

2. The load is lifted about 1 m. 3. Both cranes are slewed in longitudinal direction. The outreach for crane 2 is reduced and for crane 1 it is increased

in order to minimize and respectively maximize the resulting heeling moment. 4. Crane 2 is slewed outboards and crane 1 is slewed inboards as counterweight. 5. The load is lowered about 9 m using the main hoist.

Figure 3: Operation for the HLV.

Offshore Construction Vessel For the OCV vessel basically the same operation is performed but with two different setups. In the first setup the height of jib tip above deck remains constant. In the second setup the suspend length from the jib tip to the hook is minimized.

1. The load is hooked up to the crane. The crane is slewed about 30° inboards in aft direction. Outreach is about 15.6 m.

2. The load is lifted about 2 m: by using the main hoist (setup 1) or wiping the jibs (setup 2). 3. The crane is slewed outboards until 4. The crane is on the abeam position. 5. The load is lowered about 17 m using the main hoist (setup 1) or wiping the jibs (setup 2)

Figure 4: Crane operation for the OCV with constant tip height (setup 1).

Figure 5: Operation for the OCV with minimized suspended length (setup 2).

Crane Derrick Vessel For the CDV the following operation is performed.

1. The crane is hooked up to the load which is positioned at the forward end of the deck. 2. The load is lifted about 3 m. 3. The load is shifted outboards to starboard side, until it is free of the deck. 4. The crane is slewed towards the stern, whilst the jib is toped up. Hence, the heeling moment is kept constant while

slewing the crane. 5. The load is lowered about 28 m with the crane slewed to the abeam position.

Figure 6: Operation for the CDV.

Environmental Conditions The analyses for this work are performed in irregular seas with a significant wave height of 1.5 m, and a mean period of 7.5 s. The used spectrum is of JONSWAP type with a directional cosine spreading. The mean encounter angle of the short crested seas is 30°. It is measured counter-clockwise about the vertical ship axis starting at zero if the waves are coming directly from the stern of the vessel. Hence, all operations are performed in following quartering seas from the starboard side.

RESULTS AND DISCUSSION This section presents the results and discussion of the crane operation analysis in time-domain. The results are visualised in terms of time series of ship, crane and load motions. The crane and load motions are measured relative to the ship here. In contrast to absolute load motions, the load motions relative to the ship give a good indication of the hazard for deck personnel and the requirement for tugger lines. Moreover, the crane (tip) motions relative to the ship are exactly the time-dependent constraints which are prescribed by the operation description and thus can easily relate automatically computed results to manual operation descriptions. The ship motions are not shown for every degree-of-freedom here for the sake of brevity. Only the roll motion is presented because it is of superior interest during side loading operations which are solely analysed in this study. Heavy-Lift Vessel The ship, crane, and load motions during the operation of the HLV are shown in Fig 7. The figure clearly shows how the load follows and oscillates around the crane tip position in longitudinal x- and transverse y-direction. The simulation also captures the imperfect counterbalancing of the crane heeling moment which results in a considerable heel from approximately +3° to -5°. Moreover, the sway motion of the load and the roll motion of the ship often show similar phase and excitation which indicates strong coupling.

Figure 7: Crane tip and load motion of HLV operation.

Offshore Construction Vessel The operation of the OCV with the knuckle boom crane is defined according to two different crane control strategies. One control strategy is to keep the crane tip height constant during the operation. The other strategy is to keep the suspended length constant and minimal. Two operations are described based on these strategies and the associated simulation results are shown in Fig. 8 and 9. Both operations are fully comparable because the same deterministic short-crested seas are used.

Figure 8: Crane tip and load motion of OCV operation with constant tip height.

A comparison of the roll motion of the ship during both operations shows that load motions are reduced by minimizing the suspended length. The comparison also shows another interesting fact. The roll motion of the vessel can also be reduced considerably by minimizing the suspended length. This fact is insofar particularly interesting because the suspended load is small (<1.5%) with respect to the vessel displacement, here. Current classification standards consider lifts of loads less than 1-2% of the vessel displacement not as heavy-lifts and approve uncoupled analyses (DNV 2011). These uncoupled analyses cannot account for any influence of the suspended load on the ship motions because they require modeling of the suspended load in the crane tip.

Figure 9: Crane tip and load motion of OCV operation with minimized suspended length.

Crane Derrick Vessel The necessity for coupled lifting analyses is further demonstrated by the operation of the CDV. During this operation, the coupling effect between the load and the ship motions becomes very obvious, because it shifts the eigenfrequency of the roll motion very close to the peak frequency of the used sea spectrum. This fact is illustrated in Fig. 10, where the response amplitude operators (RAOs) from a linear, coupled and a linear, uncoupled analysis in frequency-domain are shown together with the used sea spectrum in the non-linear analysis in time-domain.

Figure 10: RAO of roll motion for zero forward speed and wave encounter angle of 30° based on coupled and uncoupled analysis.

Here, the resonance peak of the roll motion determined by the uncoupled analysis is at very long wave lengths higher than 300 m which are practically not represented in the sea spectrum. The coupling effect shifts this resonance peak to wave lengths of 150 m. Thus, the roll resonance is shifted close to the spectrum peak wave length of 126 m. The resulting higher excitation of the roll motion is captured by the non-linear analysis in time-domain and clearly shown in Fig. 11 and 12.

Figure 11: Crane tip and load motion of CDV operation based on RAOs from uncoupled analysis.

In contrast to the operation of the OCV, the load motion becomes smaller at higher suspended lengths during the operation of the CDV (Fig. 12). This contradiction once again underlines the fact that dynamics of heavy-lift operations are so complex that critical phases cannot be identified in advance or by rules of thumb. Rather robust, first-principle based methods without simplifications or ill-founded linearisations must be used to identify them.

Figure 12: Crane tip and load motion of CDV operation based on RAOs from coupled analysis.

CONCLUSIONS In the presented work non-linear sea-keeping simulations have been performed for lifting operations with three different crane vessels. From these analyses the following main conclusions are drawn:

It is absolutely necessary to perform an analysis which considers the full coupling between the load motion and the motion of the vessel.

Even for rather small load masses, i.e. less than 2% of the displacement of the ship, already a significant impact on the motion behaviour can occur. This means that the vessel or load motion response to the sea way can be shifted from a safe into an unsafe region and vice versa.

Longer suspended length does not necessary means that the sway motions of the load are increased, as the example of the CDV shows.

Without additional measures to control the motion of the load, like tugger lines or active motion compensation of the crane the sway and surge motions of the load are too large in order to perform a safe lifting operation. This is valid at least for the investigated operations and conditions.

In order to address the last topic further developments and investigations will be done in the near future. In particular the following tasks are planned:

Development of the simulation tools: incorporating tugger lines, multi-hook operations, ship roll damping measures. Analyses of more vessels, operations and environmental criteria. Accident investigations in order to develop criteria for unsafe operations.

ACKNOWLEDGEMENTS The authors would like to thank the Federal Ministry for Economic Affairs and Energy (BMWI) for the provision of the founding of the project “HoOK-Offshore Lifting Operations”. The founding was granted from the program “Next generation maritime technologies” which is managed by the team from Projektträger Jülich (PTJ), whom the authors would also like to thank for the support of the project.

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