Development of a quick estimation method for the dynamic hookload fluctuation
during upend operations.
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
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
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.
The most important conclusions of this study are:
- The impact of mass and stiffness variation on eigenfrequency shift;
- The impact of mass and stiffness variation on the dynamic hookload
- The energy transferred into the system by SSCV and Jacket.
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.
- 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
Reports on Transport Engineering and Logistics (in Dutch)
, TU Delft