Development of an Advanced Subsea Wellhead System Incorporating All Metal-to-Metal Sealing

ABSTRACT This paper describes the design, analysis, testing and the resources devoted to the development of an advanced sub sea wellhead system. This wellhead system significantly improves casing suspension/load capacities, annulus sealing and rig floor operations. INTRODUCTION Wells are now being drilled in water depths over 7,500 ft. with reservoir pressures approaching 15,000 psi, and temperatures approaching 180°C. A high-performance sub sea wellhead system providing simple rig operations, reliable sealing and improved load carrying capacity is an important element in safely and cost effectively drilling and developing deepwater fields. In developing this system, major emphasis was placed on:passive load transfer.All metal SealingSimple weight set operation. These elements, combined with other specific system performance goals, provided the benefits necessary to warrant this project. The approach was to define a specification based on historical perspective and present day customer input, to design and analyze the elements using the most modern technique: available, to rigorously and thoroughly test the equipment under laboratory and field-simulated conditions, and to perform field trials. The wellhead resulting from this research is shown in Figure 1. Specification The detailed specification is a lengthy document covering all associated equipment from spud-in to tieback and sub sea completion interfaces. The basic requirements for the wellhead are:18-3/4 in. -15,000 psi rated working pressure.compliance with industry specifications: API 17D, NACE MR-01 75, and Certifiable for North Sea operations.Temperature range - 4°C thru + 120°C.Simple reliable installationWeight set operation.Passive casing suspension load carrying system.Single trip installation of hanger and seal assembly.Minimization of the number of trips and tools.Load carrying capacity -7.1 million lb.1 million lb. per hanger (13-3/8 in., 9-5/8 in., 7 in.).4.1 million lb. end-load due to 15 Ksi full bore test.Highly reliable all metal sealing.Maximized flow-by area and particle size.Compatibility with sub sea production systems as well a platform or TLP tieback systems. DESIGN Wellhead Housing The external design of the housing is standard and can be fitted with a mandrel or a hub profile to interface with the BOP stack connector. The load transfer system from casing hanger to wellhead housing is completely passive as shown in Figure 2. Unlike previous systems, no mechanisms, shear pins or addition trips are required to provide over 7 million lb. of load carrying capacity. The high-strength load shoulder insert in the wellhead housing has bore clearance for a 17-1/2 in. bit. The pre-installed wert provides additional bearing area on the NACE-rated housing material while maintaining 360° of contact with the first casing anger. This hanger-to-housing interface allows the hanger to simply land upon a conventional angular shoulder while maintaining the ability to support a load-tested 8 million lbs.