Standards Development

Scope

This document gives requirements for the design, setting depth and installation of conductors used by the offshore petroleum and natural gas industries. This document covers:

⎯ design of the conductor, i.e. determination of the diameter, wall thickness, and steel grade;

⎯ determination of the setting depth for three installation methods, namely, driving, drilling/cementing, and jetting;

⎯ installation requirements for the installation methods, i.e. selection principles, operating procedures and parameters.

This document is applicable to:

⎯ Platform conductors: installed through a guide hole in the platform drill floor and then through guides attached to the jacket at appropriate intervals through the water column to support the conductor withstand metocean actions and prevent excessive displacements.

⎯ Jack-up supported conductors: a temporary conductor used only during drilling operations, which is installed by a jack-up drilling rig. In some cases, the conductor is tensioned by tensioners attached to the drilling rig.

⎯ Free-standing conductors: a self-supporting caisson in cantilever mode installed in shallow water, typically depths of about 10 m to 20 m. It provides sole support for the well and sometimes supports a small access deck and boat landing.

⎯ Subsea wellhead conductors: a fully submerged conductor extending only a few metres above the seafloor.

This document does not apply to drilling risers.

Purpose

Offshore drilling conductor is an integral part of every offshore well whether in shallow water or deep water, which is the first casing penetrated into seabed. It has three important functions (i.e. isolating seawater, circulation channel, and sustaining loads) and three operation methods (i.e. hammering, drilling & cementing, and jetting in).

The core issue of conductor is the shoe setting depth. If the depth is too shallow, wellhead instability or subsidence maybe occur due to insufficient bearing capacity, which may lead to serious accident and huge economic loss. There were many occurrences around the world, such as wellhead subsidence accident of F2 well in West Africa, which caused wellhead scrapped and wasted 320 mm$. On the contrary, if the depth is too deep, it may waste time and cost, or lead to structure deformation during operation due to the hammer refused in the condition of pile-group effect. For example, a large oil field in Bohai Bay of China, with about 300 wells and 1,5m×1,7m spaced slots, had taken place serious deformation of cross-sectional shape during hammering operation in the initial two platform of phase I project, which wasted more than 1 200 mm$.

Why did this happen? The most likely reason is that there is no suitable standard for offshore drilling conductor setting depth design and operation guideline around the world. Although there are two standards (i.e. API RP16Q and API RP 2A-WSD) addressing drilling conductor, these two standards are not given the calculation model for conductor shoe setting depth and the operation requirements, Page 2 of 5 etc. Therefore, the traditional design only takes the drilling conductor as a sustaining loads structure, the setting depth calculation method is carried out with reference to the platform legs.

In order to solve this core issue, China National Offshore Oil Corporation (CNOOC) has done lots of work for more than 10 years, and the enterprise standard named "Specifications for design and operation of conductor of offshore well" was published in 2011. A series of calculation models were established for different three operation methods in this standard, which considers two functions of the circulation channel and sustaining loads of drilling conductor, the group-pile effect also be considered. At the end of 2016, this standard has been successfully used in China, West Africa, Indonesia, etc. It has saved tens of millions dollars due to the application in about 58 offshore oil and gas fields and more than 2 300 wells.

So this guideline of conductor setting design and operation will play a major part in avoiding safety accidents and saving cost for every offshore well. It will also fill the standard blank in this area.