The Delta Tower: A Light, Compliant Tower for the Gulf of Mexico

ABSTRACT The concept which is described in the paper is applicable to fields located in the Gulf of Mexico in water depths ranging from 1 000 to 1 750 feet. The concept is a compliant tower which permits the use of conventional equipment for drilling and production and which is immediately applicable without any need for specific developments. Because of the low quantities of materials involved, and since conventional equipment can be used, the concept is particularly well adapted for marginal fields, as well as for larger ones. Material quantities are given for different water depths and deck loads, as well as dynamic behavior and the outline of the installation procedure. INTRODUCTION The Delta Tower is a concept which was originally developed through a joint industry study supported by 10 American oil companies, for hydrocarbon production in 2 500 feet of water depth. See reference 1. It is a comp1iant tower supported on a fixed base and having its stability brought by some axial tubes located at its periphery. The connection between the tower and the base is ach leveled by some tubu1ar members allowing the heeling of the tower and able to transfer the overall shear force to the foundation. The paper will present the latest works performed on this concept, which are related to water depth ranging between 1 000 and 1 750 feet. MAIN PRINCIPLE The Delta Tower is a concept which features a compliant structure, called lithe Tower", set on top of a fixed one, called lithe Base". The compliancy of the upper part is brought by three different means : the axial tubes, the buoyancy tank and the shear device. The axial tubes, located at the periphery of the tower, are rigidly connected to the tower and to the base at their top and bottom extremities respectively. In addition, they are simply guided along the tower and the base to prevent any possib1e buck1ing. These tubes act as vertical springs and, because of their eccentricity with regards to the axis of the tower, they provide the necessary righting moment in the tower when it heels under wave action. They also permit the transfer of all vertical loading between the tower and the base. The buoyancy tank, located below the water line at a low enough level to minimize the wave loads, produces also a part of the righting moment and permits to reduce the apparent weight of the structure which is supported by the axial tubes. The shear device, located at the bottom of the tower permits the transfer of all shear forces and torsion moment between the tower and the base. This shear device may be composed of a set of shear pins - shear guides or by a set of shear members. The high flexibility of these shear members combined to their axial resistance creates d pseudo-articulation between the tower and the base.