Structural qualification and design challenges of the Atlantis deep water drilling concept
Authors: Gudmann S. , Adams A.J. , Nedergaard H. , Magnussen Terje 
Published: October 2008

Abstract
The basic principle of the Atlantis deep water drilling concept is to establish an Artificial Buoyant Seabed, the ABS, at a convenient water depth at which drilling can take place with a floating drilling installation using conventional subsea drilling equipment. The BOP and wellhead are located on the artificial buoyant seabed, which is placed at moderate water depth, 200 - 300 m. Anchoring of the ABS is achieved by extending the 20" surface casing from the subsea well. By moving the termination point of the well up to moderate water depth a number of problems and hazards normally associated with deep water drilling are reduced or eliminated. By applying sufficient net buoyancy to the ABS, the lateral offset and the bending moment of the surface casing string at mudline is kept within acceptable limits. The ABS can be compared to a submerged TLP and the tension applied to the casing string is well below the tensile strength of typical casing grades. The artificial buoyant seabed is an open steel structure, and buoyancy is provided by injection of compressed air. A main concern for all slender constructions in a sea current and wave regime are the potential vortex induced vibrations. The paper describes the analyses performed in this respect and why the concept is robust relative to this phenomenon. Many deep water well locations have very loose seabed conditions and foundation behavior becomes a problem. The pper soil layers can hardly provide the lateral support needed for the conductor. Contrary to the conventional use of a conductor, the foundation of Atlantis is based on the cemented surface casing and is therefore less dependent onthe soil strength in the upper layers. Actually, soft soil conditions become an advantage since it reduces the mudline moment. The casing - soil interaction was investigated and integrated in the casing string design, eliminating the need for both conductor and special stress joints. Instead of using buoyancy elements distributed along the riser combined with tension from the drilling vessel, the integrity of the casing string depends here on the adjustable tension provided by the ABS. A flexible and reliable ballast control system had to be designed.

Introduction
The Atlantis concept is a patented system for deep water drilling and field development. It was invented by Terje Magnussen, who is a senior consultant with Proffshore.

Applications.
The concept can be used in a range of applications from exploration drilling to field development. The exploration drilling version has a simple, single well ABS while the field development version uses a multi-well type and more advanced ABS. The concept enables the use of established FPU and FPSO technology also in deep waters. Drilling advantages. The operational and drilling related merits of the concept is not the main issue of the paper and is therefore described in Appendix 1. However, some key features are:deep water riser and BOP not needed;space and variable load capacity for long riser not needed;no time consuming deep water riser running and wellhead work runs;normal riser margin and improved well control;less possibility for formation of hydrates in the BOP