Safety and Reliability

Optimal Offloading Configuration of Spread-Moored FPSOs

[+] Author and Article Information
Steven M. Wilkerson

 Haynes Whaley Associates, Inc., 2000 West Sam Houston Parkway South, Suite 1800, Houston, TX 77042

Satish Nagarajaiah

Department of Civil and Environmental Engineering, Rice University, P.O. Box 1892, MS318, Houston, TX 77251-1892

Internet photo from http:∕∕www.offshore-technology.com∕projects∕texaco∕texaco7.html.

Swells and local waves are considered independently.

The depth is not reported by Codiglia et al. (11), but the value given here is a typical value for VLCCs and is consistent with the results.

J. Offshore Mech. Arct. Eng 131(2), 021603 (Mar 24, 2009) (9 pages) doi:10.1115/1.2783886 History: Received December 06, 2005; Revised January 19, 2007; Published March 24, 2009

As the oil offloading operations of floating production storage and offloading (FPSO) units become more routine, the desire grows to increase the availability for offloading and thus decrease production downtime. Experience with these operations is the main tool available to increase the efficiency of this aspect of deepwater production. However, it is clear that a formal optimization approach can help to fine tune design parameters so that not only is availability increased but the significance of each design parameter can be better understood. The key issue is to define the environmental conditions under which the vessels involved in offloading are able to maintain position. By this, we reduce the notion of availability to a set of operating criteria, which can or cannot be met for a particular set of environmental conditions. The actual operating criteria such as relative vessel heading depend on selection of design parameters, such as the direction and magnitude of external force applied by thrusters or tugs. In the earliest offloading operations, engineering judgment was used to determine the feasibility of offloading at a particular time. For example, if wind and current were not expected to exceed a 1year return period, offloading may be considered safe. This approach can be both conservative and unconservative, depending on the nuances of the particular environmental conditions. This study will propose a formal approach to choosing the design parameters that optimize the availability of a FPSO for offloading. A simple analysis model will be employed so that optimization can be performed quickly using a robust second order method. The proposed analysis model will be compared to model test data to demonstrate its agreement with the more complex system.

Copyright © 2009 by American Society of Mechanical Engineers
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Figure 1

Offloading of a FPSO by shuttle tanker and tugs

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Figure 2

Schematic diagram of the offloading procedure

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Figure 3

Plan view of FPSO and tanker in bow-to-bow tandem offloading

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Figure 4

Free body equilibrium of tanker and FPSO

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Figure 5

Comparison of model tests (11) with optimal solution

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Figure 6

Optimal equilibrium configuration of Test No. 1059 Codiglia (11)



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