Global Analysis of Wellhead Protection Glory Holes For Terra Nova

Abstract

Abstract The Terra Nova wellheads are sited in depressions ("Glory Holes") made in the seabed to protect the wellhead equipment from iceberg impact. This paper describes the construction of the Terra Nova Glory Holes including the choice of protection system, the choice of excavation method, onshore preparation and offshore operations. It concludes with a discussion on the use of the chosen excavation technology for other applications in the offshore oil and gas industry. Introduction The Terra Nova Oil Field Development is located offshore Newfoundland, Canada, some 350 km ESE of the provincial capital, St. John's. The field is located in an area, which is subject to iceberg migration. Bottom scouring icebergs are known to have traversed the region in the past, as evidenced by scour marks of up to 1.5 metres deep on the seafloor. The water depth at the Terra Nova field is 95 metres and a bottom-scouring iceberg has a mass in the order of several million tonnes. If such an iceberg were to collide with subsea wellhead equipment, the consequences are potentially severe. Terra Nova has a comprehensive ice management strategy based on detection, monitoring and deflection, to prevent iceberg encroachment into the vicinity of the field. However, the application of such an ice management strategy does not guarantee that the encroachment of icebergs into the area will be completely avoided. It is thus, necessary, to protect the subsea wellhead equipment from bottom scouring icebergs. Detailed studies conducted by Terra Nova Alliance member Coflexip Stena Offshore concluded that the optimum way of protecting the subsea wells from bottom scouring icebergs is to site the wellhead equipment in excavated depressions (termed "Glory Holes") in the seabed (Fig.1). The Terra Nova Glory Holes were constructed in the summer of 1999 using trailer suction dredging technology. A Glory Hole was constructed at each drill centre; five in all, each being 10 metres in depth. The largest Glory Hole has a base area of 65 metres by 25 metres, and is roughly equivalent in size to a tennis stadium. Wellhead Protection Method Selection Whilst Terra Nova is using open Glory Holes as a means to provide protection to the subsea equipment, a number of wellhead protection methods were considered Cased Glory Hole: This method involves placing the wellhead and Christmas tree in a steel or concrete silo typically 8 metres in diameter. Installation would normally be carried out from a drilling rig. The silo is installed prior to the commencement of drilling operations. The silo has a weak-point at a pre-determined elevation below sea level. In the case of iceberg impact, the silo is sheared at the weak point and the upper part of the silo is sacrificed, leaving the lower part of the silo, the wellhead and the Christmas tree in tact. The Cased Glory Hole is particularly suited to single wellheads. Caisson Completion Method: This method involves a seabed level Xmas tree with the wellhead situated below the iceberg scour line. Installation is carried out from the drilling rig.