Flowing sea ice has been known to induce vibrations in offshore structures such
... analysis of the ANSYS model and both free and forced vibration tests on the ...
Simulation and Laboratory Tests of Ice Induced Offshore Structure Vibrations Justin Hovland Department of Mechanical Engineering University of Wyoming Home address: 38 Marquette Dr. Cody, WY 82414 E-mail:
[email protected] Mentor: Dr. Yuefang Wang State Key Laboratory of Structural Analysis for Industrial Equipment Department of Engineering Mechanics Dalian University of Technology Dalian, Liao Ning, China 116024
Abstract This paper describes the dynamic testing of an offshore oil platform laboratory model outfitted with rubber-steel isolators and its accompanying computer model in ANSYS. Three different input excitation signals were used: harmonic sine waves, random ice loads, and fixed ice loads. The ice loads were composed of triangular force impulses and were designed to replicate a realistic loading situation for a structure subjected to flowing sea ice. The isolators were found to effectively reduce large accelerations onboard due to the jarring nature of an ice load. Also, the fixed ice load was deemed the best load to use in the testing of offshore structures subjected to ice loading.
Introduction Flowing sea ice has been known to induce vibrations in offshore structures such as lighthouses and oil platforms. The installation of ice breaking cones has been shown to effectively mitigate vibration in some cases, such as the Kemi I lighthouse in the Baltic Sea (Brown and Määttänen, 2002). But vibrations have been found to persist in more compliant structures with respectively narrow cones. Yue and Bi (1998, 2000) found that oil platforms in the Bohai Bay still experienced vibrations after cones were installed on the legs. These vibrations can subject the crew and equipment to uncomfortable and/or dangerous levels of displacement and acceleration. Also, the fatigue experienced due to vibration can compromise the integrity of a structure. It is for these reasons that a deeper understanding of how structures react to ice loading is desired. Also, it is desired to find ways to further mitigate the adverse effects of ice loading and ice induced vibrations. In a past study, a scale model of the JZ20-2 MUQ oil platform in the Bohai Bay was erected
at the State Key Laboratory of Structural Analysis for Industrial Equipment at the Dalian University of Technology. Also, Chris Hannemann (2005) generated a computer model of the structure in the ANSYS 9.0 finite element software package. Through comparison of a modal analysis of the ANSYS model and both free and forced vibration tests on the scale model, the ANSYS model was deemed acceptable for predicting the dynamic response of the laboratory structure. This is important because while laboratory tests are cheaper and faster than field tests, computer tests are even cheaper and faster. For the current study, the laboratory structure was modified by the addition of rubber-steel isolators between the frame and deck in hopes of mitigating vibrations. The ANSYS model was accordingly changed to include the isolators. Both of the models were subjected to harmonic and random loading in order to test the effectiveness of the isolators and to gain insight into the structure’s response to dynamic loading. The random loads were obtained with a random ice load model developed by Qu Yan (2006). A fixed (not random) ice load was also applied to the laboratory model. Following is the development of the models, experimental procedures, and the results of the testing. Comparisons between the computer and laboratory models are also made.
The Models Laboratory Model The laboratory model is a scale model of the JZ20-2 MUQ oil platform in the Bohai Bay. It is composed of a frame derrick structure loaded with a concrete filled crate as shown in Figure 1.
Figure 1. Two views of the laboratory model
The derrick structure is made of steel beams with annular cross sections, which are welded together. This frame measures 1.50 x 1.50 x 3.50 m. The base of each leg is welded into a flanged support that is securely bolted into the floor. Atop the derrick structure are stacked I-beams covered with a steel sheet measuring 2.00 x 2.40 x 0.01 m. Finally, a 1cm thick 1.05 x 1.20 x 2.20 m steel crate filled with concrete rests atop the steel sheet and weighs roughly 4.5 tons. This entire load assembly is isolated from the derrick structure with four rubber-steel isolators. Each isolator was constructed of alternating layers steel and rubber, as seen in Figure 2. Rubber layer
Steel plate
Figure 2. A sliced view of a rubber-steel isolator The top and bottom of each isolator is flanged and bolted to a corresponding flange on the structure. Overall, the laboratory model weighs between 5 and 6 tons and is 5.11 m in height. Excitation of the model is provided by an electro-hydraulic actuator, which can be seen attached to the structure in Figure 1. It acts in the horizontal direction and is secured to a 1-meter thick shear wall. The actuator is rated to have a +/- 200 kN dynamic output force, +/- 75 mm displacement range, and 10 mm/s maximum output velocity. An I-beam section is attached to the actuator arm and subsequently to both of the structure legs adjacent to the actuator, 2 meters above the ground. ANSYS Model The ANSYS model was designed to replicate the laboratory model. All elements except those used to model the isolators are visually accurate to the physical components they represent, as seen in Figure 3.
(a) full view (b) zoomed in view Figure 3. The ANSYS model and a close up of the isolator area All elements in the model were created by direct generation – not meshed into a previously created solid entity. The beams in the derrick structure were created using the BEAM189 element, meshing them with specified annular cross-sections. The I-beams were created with the same element, but with a different cross-section. The beams and I-beams were assigned a material model equivalent to steel: linear elastic isotropic material with a Young’s modulus of 210 3
GPa, Poisson’s ratio of 0.3, and density of 7850 kg m .
The concrete crate was modeled with
the SOLID185 element and was specified to be a linear elastic isotropic material with a density of 1890 kg m
3
to make it weigh 4.5 tons.
The isolators were modeled with the COMBIN14
element, which is essentially a linear spring/damper, whose spring and damping coefficients are input as real constants. The vertical stiffness was modeled with four elements placed in line with the legs to hold the load up. The horizontal stiffness was modeled by attaching the bottom of an element to one side of the derrick and the top of the element to the other side of the load assembly, as shown in Figure 3(b). To restrain the model, the bottom node of each leg was constrained from translation and rotation in all three directions. This is physically accurate to how the legs are bolted to the floor in the lab. Forces were applied to the model at two points corresponding to where the actuator is connected on the real structure.
Experimental Procedures Laboratory Model Three accelerometers and one displacement meter were mounted on the laboratory model for testing in the positions shown in Figure 4. All of the meters were aligned in the x-direction (actuator force direction) since all of the testing was unidirectional.
Accelerometer #1 (line 1)
Displacement meter (line 4) Accelerometer #2 (line 2) Accelerometer #3 (line 3)
Figure 4. Positions of measuring devices used for testing Three types of input signals were used to excite the structure: harmonic, random ice force, and fixed ice force. The input of the harmonic signals takes the form (Thomson, 1972)
U ( t ) = U o sin ( 2π ft )
(1)
where t is time, U 0 is the amplitude, and f is the frequency in Hertz. The random ice force is a series of triangular load spikes with an average amplitude and frequency. (2006), the random ice force is described by
N
F (t ) = ∑ f i (t − t i ) i =1
0
Developed by Qu et al
(2)
where
⎧ 6 F0i ⎪ T t ⎪ i ⎪ 6F fi (t ) = ⎨2 F0i − 0i t Ti ⎪ ⎪ ⎪0 ⎩
0
