Rehabilitation Of A 96-Slot Platform
Authors: Craig M.J.K. , DoIan D.K. , Hopper D.M. 
Published: October 2008

This work affirms the premise that in certain cases hyperbaric or clamped repair of all known cracks in a jacket structure is not necessary. The work highlights the need for qualified diving inspectors; the use of a new subsea inspection technique; and the potential use of marine growth cleaning devices. Also highlighted is the need for consensus on regional seismic hazard definitions; the use of modulated time histories; and the development of simplified damaged joint characteristics. The work describes the use of elastic simulations to help identify effective repair strategies; and illustrates through inelastic analyses the robustness of a remediated structure to accommodate loads 50% greater than were used in original design. Definitions of â??failure' are discussed. The importance of vigilant inspection is emphasized.

BACKGROUND

Platform Gilda is a 96-slot drilling and production platform in 210' of water offshore Ventura, CA. The platform was installed in 1980. Present production comes from 59 wells. Storm conditions were encountered during the platform's trans-Pacific tow. No damage was detected by selective visual and nondestructive inspections performed prior to launch.

Cracks were discovered during regular subsea inspection in 1985 in the joints of secondary conductor support framing members in the two horizontal bays directly below sea level. This prompted an evaluation of the effects of these cracks on structural integrity. The results of this evaluation led to a program of regular marine growth removal and inspection for crack growth.

In 1992, inspections indicated crack growth in some joints. This initiated a second evaluation phase which is the subject of this paper. This work resulted in limited remediation using clamps, and the implementation of a Long-term program to monitor crack growth and maintain reduced levels of marine growth. The results of the engineering evaluation work, the repair/strengthening work and the comprehensive ongoing field monitoring program, as described here, comprise the key elements to assure the continued structural fitness-for purpose of this platform.

DESIGN

The platform jacket's primary framing was sized to accommodate site-specific 100-year return interval storm forces and 200-year return interval earthquake forces as defined in 1979, using API RP 2A 10thedition criteria. Ductility was verified by explicit nonlinear analysis. The resulting structure is robust and ductile as inferred by the primary bent framing illustrated in Figure 1.

Primary joints were sized to prevent development of Long-term fatigue cracking. Figure 2 illustrates framing of the -28' horizontal bay where the mitigation of in-place fatigue effects were a primary design focus.

Some member and joint sizes were controlled by the tow criteria of 15 degrees at a 9 second period or approximately 0.4g at 100' from the center of rotation.

INITIAL REHABILITATION PLAN

Discovery in 1985 of cracking in joints of secondary conductor support members prompted the implementation of a plan comprising regular framing inspections, marine growth removal, selective crack size monitoring and detailed engineering evaluations. These initial engineering evaluations confirmed 1) that the extant cracking had no effect on the global structural reliability of the platform, 2) that the limited damage was induced by tow conditions, and was not due to in-service fatigue, and 3) crack growth was not expected.