Design And Construction Of Deep Water Platforms
Authors: Lee Griff C. 
Published: December 2007

Lee, Griff C., Ray J. McDermott and Co. Inc., New Orleans, Louisiana

During the summer of 1976, a pile-supported steel jacket-type platform was installed in 850 feet of water and a concrete platform was installed in 850 feet of water and a concrete gravity structure in 500 feet water depth. Construction is now in progress for a jacket-type structure for installation in over 1000 feet water depth. Development is underway to extend these conventional-type platforms to deeper water. In addition, other concepts such as guyed towers and tension leg (or vertically moored) platforms are being developed and tested as alternative solutions.

The design of platforms for "deep" water is not unrelated to shallower depths. The same problems are encountered, although they may be somewhat more severe due to increased loading encountered. As the water depth increases, the dynamic response of structure to wave loading becomes increasingly important. In addition installation planning becomes more critical.

Launching has been the basic installation tool for deep water jackets. Also, a caisson or tower-type, self-buoyant structure for over 500 feet water depth was towed to location and unpended at the site. Concrete gravity structures have been cast in a vertical attitude, towed to the site and sunk in position.

For the very deep water depth, it is impractical to handle the jacket in one piece. For these installations, the jackets have been built in segments, launched separately and connected while floating in the horizontal position or stacked in the vertical position.

Deep water structures are entirely practical and are within the present construction capabilities of the South East Asia area. Fixed offshore platforms have a good safety record. There is no reason why deep water platforms have a good safety record. There is no reason why deep water structures will not be just as reliable.

Introduction

The definition of what constitutes "deep water" has been a variable parameter, gradually increasing with the age and the progress of the offshore industry. Traditionally it has been progress of the offshore industry. Traditionally it has been considered as that depth just beyond the deepest existing platform. During the summer of 1976, a pile-supported steel platform. During the summer of 1976, a pile-supported steel jacket-type structure was set up in 850 feet of water in the Santa Barbara Channel, off California, and a concrete gravity platform in 500 feet of water in the North Sea. This past platform in 500 feet of water in the North Sea. This past summer (1977), installation began on a structure in 1025 feet of water in the Gulf of Mexico. As shown in Figure 1, the industry has made a quantum jump in these past two years. "Deep Water" is certainly within the limits of the industry's current operating capabilities. This paper is intended to be an overview of the evolution of platform design and the current of state-of-the-art of deep water platform construction. platform construction.

Background

To appreciate the accomplishments of the offshore industry, it is desirable to review the history of offshore platform construction (illustrated in Figure 2). The first over-water drilling for crude oil probably took place along the California Coast, in the early 1900's, utilizing piers extending from the beach. Following this, drilling took place from structures constructed in several inland lakes and in Lake Maracaibo. By the early 30's, the oil industry moved into the marsh and swamplands of South Louisiana, and then, as a natural extension, into the Gulf of Mexico, using the existing technology for timber structures.

Following World II in 1947, the first steel platform was constructed in open water, out of the sight of land. This platform was installed off the Louisiana Coast in 20 feet of platform was installed off the Louisiana Coast in 20 feet of water, with another structure being constructed almost simultaneously in 50 feet of water. These first platforms were supported by a "forest" of small piling. By the mid-50's the average pile size had increased to 30 in. O.D. resulting in fewer piles with a more open bracing pattern. In 1955 the first platform was constructed in approximately 100 feet of water. Perform construction was extended to approximately 225 feet in 1965, and to 370 feet by 1970. In 1976 the first structure was set in 850 feet depth, and this past summer (1977), offshore installation began on a fixed platform in 1025 feet of water with completion scheduled for late summer 1978.

Design Considerations

The design of platforms for "deep" water is not unrelated to that for shallow water. The same problems inherent in any offshore platform design such as environmental loading, the design of foundation and tubular joints are encountered, although the problems may be somewhat more severe due to the increased loading encountered. However, deep water platform designs are dominated by factors that are less platform designs are dominated by factors that are less importance in shallow water structures. The deep water platform is more slender and therefore more susceptible to platform is more slender and therefore more susceptible to stress amplification due to wave dynamics, which could be safely ignored in shallow water. This also increases the significance of fatigue assessment.