This paper examines the ability of a Levingston Class 111-C jackup rig to withstand hurricane winds of 128 knots, which significantly exceeds the rig's 100-knot storm rating. It compares the analytical results of an engineering study focused on predicted rig attitude, areas of failure, and areas of high stress with the actual rig conditions observed.
Details of the shipyard repair show how an innovative chord repair method was used to dramatically decrease the cost of repairs and allow the rig to be returned to service in only 28 days.
On October 3, 2002, the eye of Hurricane Lili, a category IV storm, passed within 2 miles of the Noble John Sandifer, a Levingston Class 111-C jackup rig. The rig was located just off the Louisiana coast in the Eugene Island area, Block 305B, and was positioned adjacent to a platform owned by a major operator (Fig. 1). The drilling package was in the stowed position and the rig was evacuated 24 hours prior to the storm.
An initial helicopter fly-over revealed that this storm had resulted in a significant tilt downward to the bow end of the rig and a slight downward tilt to the starboard side. An engineering study was initiated immediately to determine if personnel could safely board the rig. Parameters considered for the study included water depth, wave height, wave period, wind speed, current, air gap, and leg penetration. Hurricane Lili delivered wind speeds of 128 knots and wave heights of 38 ft, well exceeding the 100-knot storm conditions for which the rig was classed.
On the Saffir-Simpson Hurricane Scale, a category IV classification is reserved for storms with winds of 131 to 155 mph (114 to 135 knots) and a storm surge of 13 to 18 ft above normal. Storms ranked category III and above are considered major hurricanes by the National Oceanic and Atmospheric Administration.
Initial reports from the fly-over indicated that the Noble John Sandifer was approximately 20 ft away from the platform (Fig. 2). Noble Corporation engineers quickly calculated that a gap of this size would require a tilt approaching 7°, which could result in the rig's collapse. Noble immediately dispatched a crew of three to investigate. Approaching the rig by helicopter, the crewmen observed that the tilt was not as severe as first reported, and the pilot determined that he could safely land on the heliport located on the bow.
To determine whether the rig was safe for personnel to board, Noble contracted the services of Zentech, Inc. to model the storm's impact on the rig's structural integrity.1 This endeavor not only provided the answers Noble needed to manage a critical safety issue, but also allowed Noble to evaluate the ability of a Levingston Class 111-C rig to endure severe offshore weather environments.
To further validate the safety of the rig following the storm, the spud cans were also subjected to a series of ultrasonic nondestructive tests. The results of these tests and the repairs made are also discussed in this paper.
The Noble John Sandifer was originally constructed as a slot drilling rig in 1975 by Levingston Shipbuilding Co. The design was based on ABS MODU 1973 rules.2 In 1995, the rig was upgraded by closing in the slot, adding cantilevers to improve access to operator platforms, and reinforcing the legs for added strength.
The rig's 208-ft by 178-ft hull is elevated above the water on three 425-ft legs. Each leg is constructed from four circular chords tied together with horizontal and diagonal bracing members. Chords and bracing members are fabricated from high-strength, 100-ksi steel (ASTM A514).3 Each leg extends through the hull and is supported by a 48-ft diameter steel foundation (spud can) bearing on the seafloor. Figs. 3 and 4 show the orientation of the legs and chords in the hull.