Caisson Wellheads Allow For Future Use Of Offshore Exploration Wells
Authors: McIntosh I. , Birarda G. , Jonasson W.B. 
Published: March 2010

Exploration wells on the iceberg infested Grand Banks have traditionally been abandoned after testing, mainly because the wellheads have no protection against iceberg impact. The cost of drilling wells on the Grand Banks and the likelihood of future subsea development now justify retaining certain exploration wells for further testing or completion. Caissons have recently been used to place wellheads submudline below iceberg scour depth.

This paper discusses the reasons for employing caisson wellheads, mentions other iceberg protection methods, explains the decision tree analysis method used to identify candidate wells and outlines the drilling and completion equipment installed.

Introduction

Typical offshore drilling practice is to plug and cement exploration and delineation wells on the completion of drilling and testing operations. They may then be abandoned by removing the subsea wellhead, or they may be left suspended to allow for future re-entry.

In most areas of the world, subsequent development wells are drilled from fixed platforms. The exploration and delineation wells are therefore abandoned rather than re-entered for production because a replacement development well is cheaper to drill and maintain, However, as smaller offshore fields are being developed, floating production systems have become more common (Fig. 1). Such a development system requires subsea wells, making suspended delineation wells candidates for production.

On the Grand Banks, other than the Hibernia field, no fields have yet been discovered which would warrant a fixed production platform. This is due to the high reserve threshold required to justify a fixed platform designed to withstand iceberg impact. Floating production systems are likely to be the only means by which these smaller discoveries can be developed.

Unfortunately, icebergs pose major problems for subsea developments. Floating production systems will have to be designed to disconnect to avoid iceberg collision. Also, seafloor equipment should be placed below scour depth, where possible, to avoid damage to equipment such as manifolds, flowlines and wellheads. This is crucial for wellheads as an iceberg collision could result in a subsea well blowout if the only remaining means of well control is the subsurface safety valve. Furthermore, ice conditions would hamper immediate well control operations. Delineation wells with conventional subsea wellheads are therefore of little value as ‘keeper’ wells without wellhead protection.

Various conceptual methods to protect wellheads from iceberg scour damage exist. For reasons of flexibility and compatibility with the drilling program and cost, the most appropriate system for protection of delineation wells is an arrangement whereby the wellhead (and ultimately the "Xmas " tree) is placed below scour depth in a caisson. Caisson wellhead systems were originally conceived to protect wellheads from fishing gear damage and are relatively recent developments.

There are additional costs associated with using a caisson wellhead, both in equipment and rig time. A decision analysis must therefore be undertaken for each delineation well to justify the use of a caisson wellhead on that specific location. This involves estimating future cost savings and determining the risks associated with the delineation well and ultimate field development.