Repair of a Gulf of Mexico Platform Using API RP 2A Section 17 Guidelines and Multiple Undenvater Repair Techniques
Authors: Daniel Kenneth P. , Couch Jack , Puskar Frank J. 
Published: October 2008

This paper describes a significant underwater repair program for the Vastar South Pass (SP) 60 D platform located in the Gulf of Mexico. The eight-leg drilling and production platform is located in 187 ft water depth, and is a key facility for Vastar. The platform was damaged by tropical storm Josephine in the fall of 1996. The damage consisted of the loss of several members and severe cracking of the conductor trays located at -40 ft and -140 ft, as well as damage to major horizontal framing at these locations. In addition, an inverted K-joint at the -140 ft elevation was cracked. Damage likely occurred due to a failed conductor guide which allowed one of the conductors to move freely among the other conductors, shock-loading the conductor tray, and damaging the tray as well as several support braces. This paper presents the structural assessment of the repaired platform using API RP 2A Section 17, and the implementation of the repair using a variety of underwater construction techniques.

Background

The general configuration of SP60D is shown in Figure I. SP60D is an eight-leg platform located in 187 feet of water with 24 conductors and 3 risers. The platform was designed in 1971 and installed in 1976. The structure is continually manned, supporting both drilling and production operations. The jacket of SP60D is of typical Gulf of Mexico design during that era, with diagonal bracing in the longitudinal direction and inverted K-bracing in the transverse direction. There are joint cans at all of the jacket leg nodes. However, there are no joint cans at the K-nodes; but these joints are overlapped, which provides greater joint capacity than K-joints with a gap on the chord (with no can). The foundation system of SP60D consists of eight 48 inch diameter steel piles of 1.75 inches in typical wall thickness at the mudline. These piles have a penetration of 398 feet below the mudline. Per API RnA Section 17 guidelines, the platform exposure category is L-2 for life-safety (manned-evacuated) and L-3 for consequence of failure (low-consequence - wells shut-in during the design event and the wells have sub-surface safety valves).

Platform Damage

Detailed underwater inspections of SP60D were conducted in May 1997. These inspections were initiated following the discovery of surface damage (a loose conductor) attributed to tropical storm Josephine, which passed through the platform area in the fall of 1996. The underwater inspections revealed the existence of extensive damage to the conductor tray bracing and to some of the horizontal jacket bracing at the -40' and -140' levels of the jacket. A flooded member check was also conducted on the entire jacket. All members were found to be intact, except the flooded members associated with the above damage. An overall summary of the key damage is as follows:Conductor tray at -40' was basically disconnected from the jacket due to missing, broken and cracked members which frame the tray back to the jacket. This is the first conductor tray below the waterline.Conductor tray at -140' was basically disconnected from the jacket due to missing, broken and cracked members which frame the tray back to the jacket (see Figure 2). This is the intermediate conductor tray between -40' and the mudline.Horizontal member at Row I, -40' was severely cracked.Horizontal member at Row 2, -40' was separated at Row A.Horizontal member at Row 2, -40' was missing at Row B.Vertical post at Row I, -40' was separated.Horizontal member at Row 2, -140' was separated at Row B.