Location

New York, New York

Session Start Date

4-13-2004

Session End Date

4-17-2004

Abstract

A new type of anchor, composed of steel caissons installed into the soil by suction under-pressure, presently finds a strong interest in the offshore oil industry, in particular for deepwater sites because of its vertical loading capability in taut-leg moorings, and the reduced installation equipment required. Two case histories are presented in the paper, describing the installation of the moorings for the Nkossa barge in Congo in 1995 and for the Girassol FPSO in Angola in 2001. In both cases, the paper gives the general soil conditions and design assumptions, together with the results of the installation operations, with a comparison between the predictions and the monitored site installation results. For the Nkossa barge anchors in about 200m of water, the results obtained in soft normally consolidated clays were in perfect agreement with the predictions, thus demonstrating the reliability of the design installation procedure. For the Girassol FPSO anchors in 1,400m of water, the measured anchor penetration resistance in soft highly plastic clays was lower than predicted, raising concern about the actual anchor holding capacity in a taut-leg mooring system with a significant vertical loading component. Following a third party review of the design assumptions, the holding capacity of the FPSO anchors was validated, and an improved installation prediction method was proposed.

Department(s)

Civil, Architectural and Environmental Engineering

Appears In

International Conference on Case Histories in Geotechnical Engineering

Meeting Name

Fifth Conference

Publisher

University of Missouri--Rolla

Publication Date

4-13-2004

Document Version

Final Version

Rights

© 2004 University of Missouri--Rolla, All rights reserved.

Document Type

Article - Conference proceedings

File Type

text

Language

English

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Apr 13th, 12:00 AM Apr 17th, 12:00 AM

Suction Anchors for Deepwater Moorings at Nkossa and Girassol in 200 and 1,400M of Water

New York, New York

A new type of anchor, composed of steel caissons installed into the soil by suction under-pressure, presently finds a strong interest in the offshore oil industry, in particular for deepwater sites because of its vertical loading capability in taut-leg moorings, and the reduced installation equipment required. Two case histories are presented in the paper, describing the installation of the moorings for the Nkossa barge in Congo in 1995 and for the Girassol FPSO in Angola in 2001. In both cases, the paper gives the general soil conditions and design assumptions, together with the results of the installation operations, with a comparison between the predictions and the monitored site installation results. For the Nkossa barge anchors in about 200m of water, the results obtained in soft normally consolidated clays were in perfect agreement with the predictions, thus demonstrating the reliability of the design installation procedure. For the Girassol FPSO anchors in 1,400m of water, the measured anchor penetration resistance in soft highly plastic clays was lower than predicted, raising concern about the actual anchor holding capacity in a taut-leg mooring system with a significant vertical loading component. Following a third party review of the design assumptions, the holding capacity of the FPSO anchors was validated, and an improved installation prediction method was proposed.