This paper describes the development of full-bore wellheads, a new 18-3/4 in.15,000 psi W.P. system, from conception to field installation. The wellheadwhen installed has a full-bore diameter without a casing hanger landingshoulder to allow one more casing string to be run below the mudline under BOPcontrol due to this major wellhead innovation.
Today wells are being drilled in deeper and deeper water which associates tohigher costs. More economical risk is taken to drill wells in deeper water andreaching the 'payzone' with a sufficient size production string to produce fromis more important than ever before. In addition to deeper water, there is aneed to find ways to drill in more difficult formations some of which includevarious shallow zone hazards1.
As a result of these concerns, there has been a demand to find ways to ensurethat the operators can reach the planned hole depth and/or size required toproduce from. Unforeseen hole problems that may have not been expected mayhinder reaching the depth requirements. By providing one more casing stringthan a conventional 18-3/4 in. wellhead system, a better opportunity to reachthe desired hole depth and size can be achieved.
To help solve these problems a new 18-3/4 in. wellhead system with a 36 in.conductor string was designed and implemented. A 36 in. × 18-3/4 in. wellheadsystem consists of the following casing program:
6 in. × 28 in./26 in. × 20 in. × 16 in. × 13-3/8 in. × 9-5/8 in. × 7 in.
36 in. × 28 in./26 in. × 22 in. × 18 in. × 16 in. × 13-3/8 in. × 9-5/8 in. × 7in.
The new system adds the 18 in. string which is run under BOP control, and the20 in. becomes 22 in. hanging from the high pressure housing as shown in FigureA. The 36 in. string is jetted or cemented in place using conventional methods. The 18-3/4 in. high pressure housing is run with 22 in. casing suspended fromthe bottom and landed in the 36 in. housing and cemented in place. The 18 in.string is landed in the 22 in. string below the mudline in its respective sub. The 16 in. string is run like the 18 in. string, but it will land in anindependent sub in the 22 in. string above the 18 in. string and below the highpressure housing. The load shoulder is then set in the high pressure housing,and 13-3/8 in., 9-5/8 in. and 7 in. casing can be run and set, respectively, with each subsequent casing string landing on top of the other.
The new system provides one more casing string to the above conventionalprogram, but with reduced radial clearances between several of the strings. Furthermore, due to the tight clearances, drives the requirement for highclearance or flush joint connections. This additional string of casing helpsoperators utilize existing rig equipment sizes and capabilities1.