Analysis of the Spar Floating Drilling Production and Storage Structure

Abstract

ABSTRACT The Spar is a deep-draft floating steel caisson, which can be designed to drill, produce and store oil. It is very stable and relatively insensitive to deck loads. Drilling and production equipment are supported on the decks above the waterline. Oil may stored in the lower portion of the hull. The centerwell protects drilling and production risers down to 650 feet. The Spar can be moored with conventional or taut catenary mooring lines using embedment or pile anchors. Because of its deep draft and large displacement mooring line dynamic loads are relatively low. Fabrication and installation plans together with cost estimates are included for a 36-well design for Chevron in the Gulf of Mexico. Predicted motions under normal and maximum sea states are shown together with the results of global motion analysis using both frequency and time domain programs. The sensitivity of the design to mooring parameters, draft, diameter and Spar mass properties is discussed. Potential applications are briefly reviewed. INTRODUCTION The concept of a Spar (a slender vertical structure) stable floating ocean platform has been recognized for years. The Flip Ship (Fisher and Spiess, 1963) was built in 1962 as a stable platform for oceanographic measurements. Its favorable motion properties are well documented (Rudnick 1964, 1967, 1971). The potential for use of the Spar in the offshore industry was recognized in the 1970s with the construction of Shell's Brent Spar for oil storage and offloading in the North Sea. Other potential uses and the salient motion properties were discussed by van Santaan and deWerk (1976). Research on the Spar as a storage and/or production vessel was carried out by some oil companies in the mid 1970s, however additional use of the Spar concept did not materialize. It has again recently been proposed as a low cost production facility for remote subsea well sites (Kerckhoff and Pijfers, 1989). This paper describes a full drilling and production Spar designed for a 2,700 foot water depth in the Gulf of Mexico. Oil storage is not a requirement for this site, but could be a major advantage of the Spar for remote areas of the world. The proposed Spar is an order of magnitude larger (in displacement) than the Brent or Flip designs, however, it retains the stability and simplicity intrinsic to the concept. SPAR CONCEPT The Spar is a large, deep draft, cylindrical, floating platform (Figure 1) designed to support drilling and production equipment and to store oil. It is permanently moored with multipoint catenary anchor lines over a multiwell seafloor template. Wells can be drilled and completed with the BOP stack either on the surface or on the seafloor. Risers are held in tension by hydraulic tensioners or by buoyancy modules (Figure 2). The most obvious features of the Spar are its extreme draft, straight sides, large centerwell, and large displacement (Figure 1).