Neptune Project: Spar Hull and Mooring System Design and Fabrication

Abstract

Abstract The Spar hull applied on the Neptune Project is 72 ft in diameter and 705 ft in length. It provides the buoyancy for the support ofthe production 'facilities and well work capability for the production and processing of the hydrocarbons. in Viosca Knoll 826 unit. The hull was designed and fabricated in Pori, Finland. This paper describes the design and fabrication techniques that were utilized to construct and handle the large hull sections. The assembly and inspection processes will be addressed. Due to the large diameter and length of the hull, special transportationtechniques were required. The hull was assembled in two large main sections and then dry transported to the Gulf of Mexico forfinal assembly. Methods used in alignment, and welding these large sections, while floating, will be presented. The special mooring system consists of a six leg taut mooring system composed of wire rope and chain. This marks first use of studless chain for a fixed mooring system in the Gulf of Mexico. Introduction The Neptune Spar hull was successfully engineered and built in Finland to two main sections, transported to the Gulf of Mexico for joining, joined floating in water, and the hull was then delivered to Oryx Energy Company (Oryx) floating horizontally at quay side after a 19.5 months contract period. The project was executed on the schedule originally set in an EPC contractbetween Aker Rauma Offshore (ARO) and Oryx as a part of the Neptune field development1. The Neptune Spar project was a pilot milestone for the long cooperation between Deep Oil Technology (DOT), ARO and J. Ray McDermott (JRM) for developing the Spar type floating production platform. The Neptune project proved the new Spar hull construction techniques and feasibility of a taut mooring system for deep water developments in the Gulf of Mexico (GOM). Project Execution Outline . The technical basis for the Neptune hull and mooring contract was built in a preliminary engineering effort mobilized by Oryxafter initial concept feasibility studies. The preliminary engineering phase started in January 1994 and lasted until October 1994. The work included definition of the main dimensions and characteristics for the hull, topsides, and mooring system. ARO was responsible for the development of the hull and mooringspecifications, hull main scantling plans, hull marine system diagrams and arrangements. The best construction method was developed together with the Mantyluoto Works (MW) construction yard. Documentation for the hull structures was submitted to ABS for review and major elements of the hull were approved by ABS in late 1994. Based on the preliminary engineering work a firm bid was submitted for the hull and mooring system covering project management, engineering, procurement and construction. Oryx decided to go ahead and on 2 February, 1995 issued ARO a lump sum contract for delivery of the Spar hull and the mooring system components excluding the anchor piles (Fig. 1). Conceptual design work and analysis was carried out by DOT.