Motion Reponses of a Semi-submersible: Experimental Study 1
Y.Abbas1, V.J. Kurian1, N. Abu Bakar1 Civil Engineering Department, Universiti Teknologi PETRONAS Bandar Seri Iskandar, 31750 Tronoh, Perak, Malaysia
Abstract- Following a few catastrophic accidents involving mobile offshore drilling platforms, various studies were carried out to investigate the adequacy of stability criteria applied to offshore mobile platforms which was derived on an empirical basis considering service experience accumulated for ships over many years. In this study, a twin hulled semi-submersible model sea-keeping performance was studied successfully in the wave tank for regular and irregular waves in sea, quartering and beam orientations. Results showed that the model RAOs follow the same tendency for regular and irregular wave. Index terms: Response Amplitude Operator (RAO), Mobile Offshore Drilling Unit (MODU), Universal Tensile Machine (UTM), Fast Fourier Technique (FFT).Frequency Domain (FD). I. INTRODUCTION
As the search for oil & gas has extended to water depth beyond continental shelf with increasing exploitations and gas resources, many new types of floating productions and drilling platforms in deep or ultra-deep water have been developed in recent years [1,2]. An engineering idea is the minimization of the structure resistance to environmental loads by making the structure flexible (compliant). Semi-submersibles are compliant offshore structures. There are over 120 semisubmersible platforms worldwide operating primarily as exploration drilling vessels, although several semisubmersibles are now dedicated to other tasks such as diver support and fire fighting and as the surface vessels for offshore hydrocarbon production systems. Indeed, for marginal fields in deep water, a semi-submersible-based production system may be the only viable economic alternative to conventional bottom standing structures [3]. Semi-submersible platform consists of a deck, multiple columns and pontoons. They are “column-stabilized”, meaning that the centre of gravity is above the centre of buoyancy, and the stability is determined by the restoring moment of the columns. This contrasts with the spar platform, which achieves stability by having the centre of gravity below the centre of buoyancy, and the TLP, whose stability is derived from the tendons. Semi-submersible platform kept in position by system of mooring lines anchored at sea-floor as shown in Fig. 1. Following a few catastrophic accidents involving mobile offshore drilling platforms, various studies were carried out to investigate the adequacy of stability criteria applied to offshore mobile platforms which followed an empirical basis considering service experience accumulated for ships over many years [4]. Sea keeping performance is of significant
importance in vessel design due to the stationary nature of drilling and production platforms. For the purpose of the sea keeping design, its response assessment to environmental forces is evaluated using either physical experiments or computational simulations. Traditionally, the evaluation of a prototype vessel’s sea keeping performance was accomplished by physical experiments using scaled models in a towing or wave tank. This approach requires a detailed model be built incorporating the complete hull geometry and appropriate scaling of the mass properties. The primary purpose of this wave tank study is to obtain reliable results by minimizing scale effects and measurements error. Large scale is recommended to minimize the problem of scale effect when Reynolds effect (such as presence of drag force) is important. The common ranges of scale for studies such as breakwater stability are 1:150 to 1:20 in two dimensional (towing) tanks, and 1:150 to 1:80 in three dimensional wave tanks. The desired range of the scale for offshore structures in two dimensional wave tanks is 1:100 to 1:10 [5]. The 1:100 scale model designed for this study is considered similar in geometry and mass properties to prototype. The model composes of eight columns and twin pontoons with bracing members.
Figure 1: Semi-submersible & mooring lines systems. II. OBJECTIVES
Although the available numerical methods for analysing offshore structures provide acceptable results for the design of the offshore structures, it is desirable to assess the effects of the environmental forces such as wind, wave and current forces on the vessel prior to its construction. The primary objectives of this study were: 1) To determine the mooring line wire load extension relationship utilizing the universal tensile machine (UTM) to evaluate its modulus of elasticity and breaking strength. 2) To construct a semi-submersible model with appropriate scale factor to simulate an operating prototype platform. 3) To characterize the model responses in regular and irregular waves with different heading angles for the platform operating conditions. 4) To develop a computer code using Fast Fourier Technique (FFT) to transfer the model responses time histories to responses spectra in the tested frequency range. III. METHODOLOGY
In many fluid flow problems, the gravitational effects predominate, the effect of other factors, such as viscosity, surface tension, roughness …etc is generally small and can be neglected [6]. In this case, Froude’s model law is most applicable. A general assumption is made here that the model follows the Froude’s law. A twin hulled semi-submersible model was built to a scale of 1:100 in accordance with the drawings shown in Fig. 1. The model was placed in the wave tank by using 1.5 ton mechanical crane as shown in Fig. 2. The semi-submersible consisted of two rectangular pontoons each with four circular columns. The reason for choosing this particular geometry for the semisubmersible model was because it is the conventional type and has a similar configuration to the Ocean Ranger which had sunk to the bottom of the ocean with the loss of all 84 of its crew (Dudgeon, 1984), this model is for a Tankagi MODU prototype operating in the north sea. The model was painted in a high visibility colour (yellow) for video shots purposes and draft marks with measurements scale were added for accuracy and visual purposes. The twin rectangular hulled semi-submersible members are made of acrylic plastic sheets by Globe Plastic Industries (IPOH) SDN. BHD. The model members were cut using laser techniques and these members were connected by melting and cooling using chloroform compound. Special ballast containers were placed in the corner columns to ballast the model to the desired draft. The weights inside these ballast containers could be placed vertically so as to adjust the center of gravity of the model for the desired metacentric heights. The principal data for the prototype and the model are given in Table 6. Modelling of moored vessels involves modelling not only the floating structure but also the mooring system. Several types of mooring are used with floating structures. The most common of these are mooring chains, wires and hawsers. In this study, a multi component mooring system was utilized for
Stationing the model composed of aluminium alloy wire and distributed clump weight made of steel chain as shown in Fig. 3 having physical characteristics presented in Table 2. Four typical mooring lines were connected to the model at fairlead points according to the drawing shown in Figure 4. It is worth mentioning that the pretension on the mooring lines was maintained by small buoys designed to provide the desired net buoyancy and attached near the mooring fairleader as in the mooring line part. The stiffness of the wire was determined by placing a specified length of the mooring line in UTM and measuring its elongation at various loadings [7]. For the generation of regular and irregular waves, the wave maker paddles were oscillated with a constant and variable frequency and stroke. The range of stroke periods of regular oscillation varied from 0.4 s to 2.5 s [8]. The tests in regular waves were carried out in order to obtain RAOs of the semisubmersible applying following Equations 1 & 2 for linear system [9]. A high quality video camera was used to record the model motions in sea, quartering and beam waves. The data were collected in time domain for regular wave tests and all were filtered. The irregular wave responses were processed by FFT to represent collected data in FD.
ܴܱܣ =
ೃ
ೢ
ௌ ()
ೃ ܴܱܣௗ = ට ௌ()
… (1) … (2)
WhereAR = Response amplitude, A୵=wave amplitude , SR (f)= response spectrum energy at wave frequency (f) and S(f)=wave spectrum energy at wave frequency (f). I.
RESULTS AND DISCUSSION
The uni-axial tension tests were conducted for three specimens of the mooring line wire made of aluminium alloy in 1.55 mm diameter and 100 mm length to construct the loadextension and stress-strain relationships for the mooring line wire as shown in Fig. 5. It was found that the test specimens behave as perfectly elastic for lower strain values with average elasticity modulus of 3600 MPa and breaking tensile load of 800 N. Fig. 6 shows the surge response for a regular wave. Surge amplitude of 18 mm was obtained for a wave of 18 mm height and 2.5 s period. By comparing theoretical and observed wave profile for regular waves, it is noted that the wave diffraction effects is significant for wave frequency above 1.8 Hz. Fig. 7 shows the surge RAOs for irregular waves, a maximum of 4 m/m was noted. Fig. 8 shows the heave RAOs for irregular waves, a maximum of 2.5 m/m was shown. Finally, Fig. 9 shows the pitch RAOs for irregular waves with a maximum of 0.0105 rad/m. Regular wave RAOs were measured for the purpose of comparisons with RAOs of irregular waves. Results showed that the model ROAs follow the same tendency for regular and irregular wave.
a.
Figure 2: Perspective view of the semi-submersible model.
b. Figure 3: Mooring System configuration.
c. Fig.1: General arrangement of the semi-submersible model. a. Model plan. b. Section 1-1. C. Section 2-2. (All dimensions are in mm.)
Figure 4: Mooring System Setup plan.
TABLE 1
MULTI-COMPONENT MOORING LINE PROPERTIES
SEMI-SUBMERSIBLE GEOMETRY AND DYNAMIC DATA FOR PROTOTYPE AND MODEL (λ=100)
Variable
Prototype
Model Scaled
Scale
Actual
Description
Prototype
Model Scaled
Actual
Horizontal pretension component (Kg).
70000
70E-3
70E-3
30
30
30
factor
Length (m)
110
1.10
1.10
λ
Angle of inclination at fairlead point
Breadth (m)
015
0.15
0.15
λ
(Deg.).
Height (m)
008
0.08
0.08
λ
Effective
the
150
1.50
1.55
Column
Longitudinal
024
0.24
0.24
λ
mooring/anchor lines (mm).
spacing
Transverse
060
0.60
0.60
λ
Effective area of the clump weight
1057
10.57
10.50
Pontoon
(m) Column diameter (m) No of bracing members Bracings diameter (m) Water depth (m) Draught (m) Displacement (Kg) GM values
Roll (m)
diameter
of
2
(mm ). 10&8
0.10 &
0.10
λ
of
20000
0.020
0.021
16
16
16
1
mooring/anchor lines (kg/m).
1.0
0.010
0.011
λ
Submerged unit weight of clump weight
83000
0.083
0.083
110.000
1.100
1.100
λ
kg/m. Mooring line length (m).
120
1.2
1.2
Submerged
unit
weight
16.000
0.160
0.160
λ
30000E+03
30.000
30.020
λ3
Anchor line length (m).
50
0.5
0.5
2.880
0.0288
0.0280
λ
Clump weight length (m).
100
1.0
1.0
110
1.1
1.1
Pitch (m)
2.360
0.0236
0.024
λ
Water depth (m).
Radii of
Roll (m)
34.300
0.343
0.340
λ
Height of fairlead point (m).
gyration
Pitch (m)
35.300
0.353
0.350
λ
Elasticity modulus of mooring/anchor
Yaw (m)
40.600
0.406
0.410
λ
lines KN/m2.
Regular wave height (m)
6.600
(1)
0.066
0.066
λ
Anchor average holding capacity (Kg).
Regular wave period (m)
8.700(1)
0.870
0.870
1/ 2
The max. water particle
2.385
0.239
0.240
λ λ1 / 2
3.300(2)
0.033
0.033
λ
105
1.045
1.045
360E+3
3.6E+3
3.594E +3
7250E+0
7.25
7.26
velocity (m/s) Significant wave height (m)
Notes: 1) 100 year storm criteria for Peninsular Malaysia Operation (PMO)-Appendix (G)-PETRONAS Technical Specifications PTS 20.073. 2) Regular and irregular waves were designed for wave headings of 0°, 45° and 90°.
Figure 6: surge response for regular sea wave (H=32 mm, f=0.4 Hz)
Surge RAO (m/m)
5 4 3 2 1 0 0.6
1.1
1.6
Wave period (s) RAO(regular) Figure 5: Universal Tension Machine (UTM) stress-strain curves for mooring wire specimens (d=1.55 mm.) TABLE 2
RAO(irregular)
Figure 7: Surge RAOs for sea waves
2.1
Heave RAO (m/m)
4
1)
3 2 1 0 0.55
1.05
1.55
2.05
Wave period (s) RAO(regular)
The response RAOs obtained by regular waves of different periods agreed very well with the RAOs obtained by a single irregular wave using FFT technique. 2) By comparing theoretical and observed wave profile for regular waves, it is found that the wave diffraction effect is significant for wave frequency above 1.8 Hz. 3) The maximum response RAOs were 4m surge, 2.5m heave and 0.0105 radians pitch for a unit sea wave.
RAO(irregular)
Figure 8: Heave RAOs for sea waves
ACKNOWLEGEMENT The support provided by the Universiti Teknologi PETRONAS is gratefully acknowledged.
REFERENCES [1]
[2] [3] [4] Figure 9: Pitch RAOs for sea waves [5] II. CONCLUSIONS [6]
The station-keeping experimental tests have been conducted in the Universiti Teknologi PETRONAS offshore laboratory for a semi-submersible model in scale of 1:100 using Froude’s modeling law. Model responses to regular and irregular waves were evaluated and assessed numerically. From this study, we can conclude the following:
[7] [8]
[9]
LI Run-Pei, XIE Yong-he and SHU Zhi. A review on the technical development of deep water offshore platform [J].China offshore platform, 2003, 18 (3):1-5. LI Yu-cheng. The new development of offshore engineering technology [J]. China Offshore platform, 1998, 13(1):9-12. Minoo H Patel and Joel A Witz, Compliant Offshore Structures, Butterworth-Heinemann 1991. M.soylemez, Motion tests of a twin-hulled semi-submersible, Ocean engineering, volume 22,No. 6,pp.643-660,1995. K. A. Anasri, Dynamics of offshore vessels, School of engineering, Washington USA-2001. S.K.Chakrabarti, Offshore structure Modeling, World scientific publications-1994. Hashemi, S. Foundations of materials science and engineering, 2006, 4th edition, McGraw-Hill, ISBN 007-125690-3. HR Wallingford Technical data, Multi-element wave generationsystem with AC drives and dynamic wave absorption, CQR 4187 Universiti Teknologi PETRONAS, January 2008. S. K. Chakrabarti, Hydrodynamics of Offshore structures. CBI Industries, Inc. Plainfield, Illinois 60544-8929, USA (2005).
