The horizontal added mass of a suction bucket jacket (SBJ)

Abstract

The new rapidly growing wind energy market has been moving to offshore waters the last few years. New techniques and growing experiences make it possible to move to challenging offshore domains and towards harsher conditions and deeper waters. From the oil & gas industry a lot of expertise is already available on support structures.
The focus in this thesis is on the installation of a jacket type sub-structure for an offshore wind turbine.
In more detail: the horizontal added mass for suction bucket jackets during installation was studied, from the moment the bucket touches the water till the buckets are fully submerged. This is a period during installation with a lot of uncertainties and unknowns, especially inside the buckets.
A suction bucket jacket is identical to the commonly known jacket type support structure, however, at the bottom of each leg a suction anchor is attached. For a structure of about 60[m] in height and 1000[tonne], these anchors have a rough dimension of 8-9[m] in diameter and have similar heights. Note that the combined mass of the enclosed water inside the buckets can almost reach twice the weight of the dry structure.

A benchmark suction bucket with dimensions 10[m] in diameter, 10[m] in height and a wall thickness of 10[cm] was used to design a scale model of 50[cm] diameter, 62.5[cm] in height and a wall thickness of 0.5[cm]. This corresponds to a Froude scaling factor of 1:20.

The experiments were executed at the hydrolab at Boskalis, Papendrecht. A tank of 20x3.5x3[m] was available there to execute the experiments.
The water-depth in the tank was 1.12[m].
During the experiments a vertical pipe segment was forced to oscillate at a known frequency. The pipe was attached to a guiding system, restraining all motions but the surge direction.
The applied force by the actuator was measured by a load-cell attached at the point where the crankshaft was connected to the pipe. Simultaneously a LBT-sensor measured the position of the pipe versus time.

The experiments were done for the following settings for draft and scale model wave period range, each experiment was executed three times to reduce the error:

Periods: 2.46 2.24 2.01 1.79 1.57 1.34 1.12 0.89 seconds.
Drafts: 0, D/4, D/2 3D/4 D.

The data was filtered and analyzed with the use of the linear Least Squares Method (LSM). The raw data was fitted by adjusting the amplitude and phase of a sinusoidal fit signal. The amplitude and phase provide information on the relation between damping and added mass in the system.
Finally the results added mass (and damping) were checked with the theoretical potential theory.

While for all frequencies the added mass stayed more or less constant for each draft, during sloshing the added mass decreased. The damping was more or less constant for all drafts at low frequency, but at a wave period of 9[s] the damping start to increase for deeper drafts.
Sloshing occurred at the shortest two periods. This had a significant impact on the damping and added mass terms.
At sloshing the added mass decreased rapidly with 40%, while the damping increased up to 300%.