Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
C.L. Schat Development of a quick estimation method for the dynamic hookload fluctuation during upend operations. Masters thesis, Report 98.3.TT.5118, Transport Engineering and Logistics.
The Dutch company Heerema Marine Contractors operates three Semi Submersible Crane Vessels (SSCV) used for various construction operations offshore. One of such operations is the installation of sub-structures as Jackets and Towers. Heerema's major contribution during the installation operation is the Heavy Lifting of the structures. Within Heerema in Leiden the engineering of the heavy lift operations is a major activity.
The main components of the system during a lift operation are the jacket, the SSCV, the waveloading and the interface (cranes, cables and other lift arrangements to connect the jacket with the cranevessel). One of the stages during the installation operation is when the jacket is partially submerged and connected to one or two cranes of the crane vessel. Besides gravity and buoyancy forces, both jacket and SSCV are then subjected to the waveloading that results in relative motions of SSCV and jacket. These forces and motions generate reaction forces in the interface. An important operational and engineering parameter is the maximum hookload, that equals the force in the hoistwire. This force is limited because of the crane capacity and the mean hookload must be big enough to avoid impact forces, due to slack slings, because of the hookload fluctuation.
The bid phase and early engineering stage of a heavy lift project mostly need accurate estimation of the maximum hookload. Generally the scoop of time is short and the amount of detailed information is small.
Because the waveloading is varying in time, the hookload is also varying in time and so is a dynamic parameter. It is common practice to assume a static hookload and a hookload fluctuation during stationary stages of the operation, i.e. when the mean hookload is not varying. System resonances are expected to be important with respect to the maximum hookload fluctuation.
Several methods are currently used to estimate the maximum hookload fluctuation. The relative simple methods do not account for resonances and are therefore, in most cases, not suitable to give an accurate estimation of the maximum hookload fluctuation. The more comprehensive methods, that do account for resonance are to time consuming and need to detailed description of the system parameters to be effective in early stage of the project. Because current engineering methods to estimate the maximum hookload do not meet the previous mentioned demands of time effectiveness and system information, there is a need for a new estimation method. A standard has to be developed that can estimate the hookload fluctuation taking into account the governing system properties.
A study is set up to find out which system parameters do govern the hookload fluctuation, and in which way these parameters can be used to estimate the magnitude of the maximum hookload in a simple and effective way.
Important system characteristics are the structural mass, hydrostatic stiffness, hydrodynamic mass, hydrodynamic damping and forcing of the floating objects, and the structural stiffness of the interface (cranes and cables).
A finite element model is build that represents these characteristics with structural elements.
For a 6.000 tons dual crane assisted lift jacket and a 19.000 tons single crane assisted launch jacket, two basis models are created with the Thialf as operating crane vessel. Based on these two models series of models are created in which the mass and stiffness parameters are varied systematically. Using these, some 64, models a frequency domain and response analysis is performed.
Surveyed are:
The impact of mass and stiffness variation on eigenfrequency shift;
The impact of mass and stiffness variation on the dynamic hookload fluctuation; The energy transferred into the system by SSCV and Jacket.
The most important conclusions of this study are:
Dynamic Amplification Factors used to multiply the mean hookload, as currently common in engineering, are not a good way to estimate the maximum dynamic hookload fluctuation. More reliable is to use a term for the hookload fluctuation that is added to the mean hookload. Resonance effects are important to account for.
The hookload fluctuati-on is majorly influenced by the variation of jacket mass. There may even be a almost linear relation between a jacket weight and the hookload fluctuation.
With respect to the excitation forces on the system can be concluded that the hookload fluctuation is governed by the wave loading on the SSCV. The wave energy that is transferred into the system via the jacket is negligible.
With these results a strong basis for a new estimation method is created. It is possible to construct a quick estimation method, but at this moment the range of application is very limited. With further research it can be possible to create a quite reliable method for a much larger range of structures.
Reports on Transport Engineering and Logistics (in Dutch)
