The Neptune Spar - Performance Over the First Two Years of Production
Authors: Glanville Roger S. , Vardeman R. Don 
Published: October 2008

This paper reviews the performance of the Neptune Spar for the fwst two years of its producing history. Equipment operation and required modifications together with causes and extent of downtime are discussed. Facility capacity proved to be limited so a facility expansion was undertaken coinciding with the shutdown scheduled for installing the remaining buoyancy cans. As called for in the original field development plan, additional wells have been drilled under the Spar and tied back with dry trees to the existing production deck manifold. The Spar was moved aside 250 feet to accommodate a MODU drilling the new wells into the existing well pattern. Challenges of designing the MODU mooring to prevent contact between it and the Spar mooring are detailed. A remote subsea well has been drilled and tied back to the Spar with a flexible pipe riser. Installation of the remaining buoyancy cans and completion of the new wells are discussed together with the concerns raised in operating a moored MODU close to the moored Spar.

Introduction

Oryx Energy Company installed a production Spar at Viosca Knoll Block 826 in September 1996. The location is in the Gulf of Mexico about 90 miles due South of Mobile, Alabama, where the water depth is 1,930 feet. Seven wells were pre-drilled to total vertical depths ranging from 8,000 feet to 13,000 feet with semi-submersible rigs. A packaged 650 hp workover rig was set on the Spar and used to run the production risers and complete the wells. Production started on March 11, 1997, building to 26,000 barrels of oil and 22 million cubic feet of gas per day by September. In February 1998 the Spar was moved aside with its mooring system and a moored semisubmersible drilled three additional wells into the existing seafloor pattern after which the MODU was moved to an adjacent block to drill and complete one more well which was tied back to the Spar. Again using a packaged workover rig set on the Spar, the new wells were tied back and completed, bringing the number of producing platiorm wells to ten. At the end of January, 1999, with two of the new wells completed, production was 27,500 barrels of oil and 30 million cubic feet of gas per day.

Background

The Neptune Spar produces oil and gas from a four block unit operated by Oryx Energy Company with CNG Producing as an equal partner. To develop the 50–75 million BOE reserves a phased development based on a Spar platform was selected. Seven wells were predrilled with MODUs, before and during construction of the Spar hull and topsides, as Phase I of the drilling effort.

The 72 foot diameter steel hull (Figure 1) was built in two sections which were assembled afloat into a single, 705 foot long cylinder with buoyancy compartments at one end and a 32 foot square centerwell throughout the length. A short tank at the other end of the hull provided temporary buoyancy to help float the hull out to the installation site at Viosca Knoll Block 826.

The hull was towed to the site and upended by flooding the temporary buoyancy tank after which the six preinstalled mooring lines were connected to the Spar and tensioned up. Each mooring leg (Figure 2) consists of a 180 foot long 84 inch diameter pile, approximately 200 feet of 4-3/4 inch chain, 2,400 feet of 4-3/4 inch spiral strand wire rope and 1050 feet of 4-3/4 inch chain running from 300 feet outside the fairlead up the side of the hull through a chain jack and down into a chain locker.