Research Papers: Ocean Engineering

Full Scale Measurements and Theoretical Predictions of 2nd Order Pitch and Roll Slow Motions of a Semisubmersible Platform

[+] Author and Article Information
Vinicius L. F. Matos

Petrobras, E&P,
Av. Santa Monica 120,
Macaé, RJ, 27943-625, Brazil
e-mail: vmatos@petrobras.com.br

Eric O. Ribeiro

Petrobras, Cenpes/PDEP/TEO,
Av. Horácio Macedo 950,
Rio de Janeiro, RJ, 21941-915, Brazil
e-mail: eric_oliveira@petrobras.com.br

Alexandre N. Simos

University of São Paulo,
Av. Prof. Mello Moraes 2231,
São Paulo, SP, 05508-030, Brazil
e-mail: alesimos@usp.br

Sergio H. Sphaier

COPPE/Universidade Federal do Rio de Janeiro,
LabOceano—Parque Tecnológico do Rio,
Rua Paulo Emídio Barbosa,
485, Quadra 07-A,
Cidade Universitária, Rio de Janeiro, RJ, 21.941-907, Brazil
e-mail: sphaier@peno.coppe.ufrj.br

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received October 6, 2010; final manuscript received June 30, 2012; published online June 6, 2013. Assoc. Editor: Moo-Hyun Kim.

J. Offshore Mech. Arct. Eng 135(3), 031106 (Jun 06, 2013) (10 pages) Paper No: OMAE-10-1102; doi: 10.1115/1.4024206 History: Received October 06, 2010; Revised June 30, 2012

In Oct. 2007, the semisubmersible platform PETROBRAS 52 (P-52) was installed in Campos Basin (Roncador Field) offshore Brazil, in a depth around 1800 m through 16 lines in taut-leg con. The maximum production capacity is 180.000 bpd with a displacement of 80,986t at the operational draft of 27.5 m. Slow drift motions in the vertical plane (heave, roll, and pitch) were observed in a model test performed in a wave basin during the design phase. As resonant responses vary considerably with the damping loads, slow motion could be affected by scale effects. To observe the phenomena, by that time, it was a doubt if this phenomenon would happen during the platform operation. Since June 2008, PETROBRAS has been monitoring P-52 motions with the use of accelerometers and rate-gyros. Through spectral analysis of the measured signals, it was possible to verify the presence of slow motions with frequencies around the natural frequencies of roll and pitch during almost the whole monitoring period. Sometimes, the 2nd order amplitudes were even greater than the 1st order ones. Furthermore, the environmental conditions have also been monitored through wave radars, ADCPS (current) and meteorological stations (wind) in the vicinity of P-52 location, making the excitation loads identification possible. A comparative study confronting full-scale measurements and theoretical predictions was performed. First and second-order forces and responses were calculated using Wamit® second order module. This study permitted the estimation of the full scale damping values of this offshore system (hull plus mooring and riser lines) for one of the environmental conditions measured. The results indicate the importance of considering the resonant roll and pitch motions in the seakeeping analysis of large-volume semisubmersible platforms, contributing with an important feedback to future designs.

Copyright © 2013 by ASME
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Fig. 1

Model test with P-52 (see Ref. [1])

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Fig. 2

Scheme of the positioning of the platform P52 showing wave spectrum radar plot

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Fig. 3

Displacement (a) and draft (b) of PETROBRAS semisubmersibles

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Fig. 4

Spectral density functions of pitch movement (DOF = 16) for different mooring configurations and same wave sea condition (a); same results, giving more emphasis to the resonant range (b)

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Fig. 5

P-52 picture after the topside installation (deck mating) in Angra dos Reis

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Fig. 6

Picture of the equipment OCTANS® installed at P-52

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Fig. 7

Statistics of P-52 roll (a) and pitch (b) measured with OCTANS®: June 2008

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Fig. 8

P-52 roll (a) and pitch (b) 1st and 2nd order significant amplitudes: June 2008

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Fig. 9

P-52 roll, pitch and heave natural periods: June 2008

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Fig. 10

P-52 time series for roll (a) and pitch (b)

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Fig. 11

Current velocity measured in (m/s) in the vicinity of P-52 June 15–19, 2008

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Fig. 12

Wind velocity measured in P-52

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Fig. 13

Spectral density function for the time series showed in Fig. 12

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Fig. 14

Wave spectrum map: directional wave spectrum measured in the first hour of June 17, 2008

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Fig. 15

Force coefficients by direction of wind incidence

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Fig. 16

Wind moment spectral density functions

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Fig. 17

SSTAB model prepared for P-52

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Fig. 18

Spectral density function for the time series presented in Fig. 10: roll (a) and pitch (b)

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Fig. 20

Spectral density functions of second order roll and pitch moments caused by waves (smwaroll and smwapitch): first hour of June 17, 2008

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Fig. 21

Comparison of resonant second order roll (a) and pitch (b). P-52 theoretical and measured movements in the first hour of June 17, 2008 (DOF—degrees of freedom used in the spectral analysis).

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Fig. 22

Unidirectional wave spectrum density function measured in the first hour of June 17, 2008

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Fig. 23

Comparison of resonant second order roll (a) and pitch (b). P-52 theoretical and measured movements in the first hour of June 17, 2008: theoretical calculation with directional wave spectrum (DOF—degrees of freedom used in the spectral analysis).



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