Research Papers: Ocean Engineering

Equilibrium Position Analysis for Offloading Operations With Turret-Moored FPSO

[+] Author and Article Information
Alex S. Huang

Numerical Offshore Tank Laboratory (TPN-USP),
Universidade de São Paulo,
Av. Professor Mello Moraes, 2231,
São Paulo 05508-030, Brazil
e-mail: alex.huang@usp.br

Felipe M. Moreno

Numerical Offshore Tank Laboratory (TPN-USP),
Universidade de São Paulo,
Av. Professor Mello Moraes, 2231,
São Paulo 05508-030, Brazil
e-mail: felipe.marino.moreno@usp.br

Eduardo Aoun Tannuri

Numerical Offshore Tank Laboratory (TPN-USP),
Universidade de São Paulo,
Av. Professor Mello Moraes, 2231,
São Paulo 05508-030, Brazil
e-mail: eduat@usp.br

Joselito G. A. Câmara

PETROBRAS/Shipping New Operations
Av. Henrique Valadares, 28,
Rio de Janeiro 20.231-030, Brazil
e-mail: joselito.camara@petrobras.com.br

1Corresponding author.

Contributed by the Ocean, Offshore, and Arctic Engineering Division of ASME for publication in the JOURNAL OF OFFSHORE MECHANICS AND ARCTIC ENGINEERING. Manuscript received July 7, 2018; final manuscript received December 30, 2018; published online February 18, 2019. Assoc. Editor: Xinshu Zhang.

J. Offshore Mech. Arct. Eng 141(5), 051102 (Feb 18, 2019) (9 pages) Paper No: OMAE-18-1095; doi: 10.1115/1.4042531 History: Received July 07, 2018; Revised December 30, 2018

With the expansion of oil exploration in deep waters, assessing the risks associated with offloading operations becomes essential in preventing accidents that may cause huge environmental disasters. In this paper, the system that composed of a turret-moored floating production storage and offloading (FPSO) connected to a conventional shuttle tanker, which is assisted by a tug boat to maintain its position during an offloading operation, will be studied. Using environmental data collected over a period of 6 years, from 2004 to 2009, from the Campos Basin in Brazil, the equilibrium positions of the system were calculated, considering its constraints (operational criteria defined by Petrobras) and verifying the stability of those equilibrium points. The hydrodynamic and aerodynamic static forces were calculated using models validated in the literature. Dynamic effects and oscillations are taken into account by adding safety margins to the operational sectors. With this analysis, we calculated the FPSO heading probabilities during an offloading operation and the expected downtime of operation in Campos Basin. We concluded that the downtime of the offloading operation with a conventional shuttle tanker is close to that with a dynamic positioned (DP) shuttle tanker (10% downtime). Furthermore, the results from the stability analysis were used to generate a simplified set of rules to classify the environmental conditions into four classes of operational risk by applying an unbiased decision tree. This method obtains practical rules based on measurements of wind, wave, and current, allowing the operator to quickly evaluate the risk level before starting the operation.

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Fig. 1

Vessel horizontal dynamics

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Fig. 2

Representation of the system

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Fig. 3

Campos Basin—average environmental conditions

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Fig. 4

Operational sector for offloading in turret moored platforms [13]

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Fig. 5

Example of analysis for viability of the offloading operation

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Fig. 6

Expected downtime for offloading operation

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Fig. 7

FPSO heading distribution—configuration 4 (tugboat applying 20 tf)

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Fig. 8

Decision tree for an applied tugboat force of 0 tf

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Fig. 9

Decision tree for an applied tugboat force of 20 tf

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Fig. 10

Decision tree for an applied tugboat force of 50 tf

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Fig. 11

Stable, but nonacceptable situation. Forces and moment of forces (MoF) due to environmental agents are shown for each vessel—configuration 1.

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Fig. 12

Typical stable situation, vessels align almost in the same direction—configuration 4



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