Design of the Baldpate Compliant Tower
Authors: Will S.A. , Edel J.C. , Kallaby J. , des Deserts L.D. 
Published: October 2008

The Baldpate compliant tower, owned by Amerada Hess Corporation as operator, and Oryx Energy Company as 50% joint interest owner, installed in Garden Banks 260 in the Gulf of Mexico, is the first free-standing, non-guyed compliant tower. "De-tuned" so that it has no natural period of vibration that resonates with the high energy seastates, the structure has sway periods in excess of 30 seconds.

In 1,648 feet of water, and measuring 1,900 feet from the flaretip to the seafloor, the tower, at the time of construction, was the tallest free-standing structure in the world. The tower is a 19 slot, one-rig drilling/production platform set over a 9-slot drilling template, has production facilities for 60,000 BOPD and 200 MMSCFD, and supports two steel catenary export risers and a steel catenary bundled flowline import riser. The tower was installed in the spring of 1998.

This paper discusses the design of the tower, focusing primarily on the jacket and foundations. Topics treated in the paper include: selection of the tower structural concept; preliminary design and decisions related to key design variables, including those that determine the dynamic characteristics of the structure and produce the compliant response; design of major components unique to this type of structure, such as axial tubes and hinges; in-place design and analyses; design for installation related conditions; foundation design; a summary of tonnages; a discussion of key design issues; and installation and fabrication considerations. Several other key issues, such as fatigue and wear, and development of response based criteria for extreme events, are discussed in companion papers.

Introduction

The detail design of the drilling and production tower was kicked-off in the last quarter of 1995. The project schedule called for loadout of the tower sections in March and April of 1998. Fabrication and installation of the tower were bid competitively, with separate awards granted for fabrication of the deck and jacket tower section (JTS), the jacket base section (JBS) and piling, and installation. As a result, a "fast-track" schedule was developed which supported "milestone" delivery of construction drawings and key construction related data to facilitate the two parallel fabrication projects and, eventually, the installation planning. The JTS and JBS were fabricated in Ingleside, Texas, and Morgan City, Louisiana, respectively.

The design of the "first of a kind" Baldpate compliant tower established the viability of the "third generation" compliant towers, which are much lighter and more efficient than their predecessors. The improved efficiencies result in less tonnages, reduced demands on fabrication and installation equipment and procedures, and ultimately costs, while providing the advantages of a conventional platform in the drilling and production phases.

This paper will emphasize the unique challenges presented to the design team in the preliminary and detail design phases. Global sizing was accomplished in the preliminary design phase, which included the integration of all key design issues related to in-place response due to environmental and operational events, as well as the initial constraints imposed by the fabrication and installation phases. This process is crucial for all deepwater structures, but especially so for "first-of-a-kind" systems, because constraints imposed by existing construction equipment and procedures, materials, and assembly techniques have not been fully defined.