Downhole Operations From a Dry Wellhead
Authors: Engelberg Arieh , Suchy Les 
Published: December 2007

This paper discusses the conditions, limitations, and requirements for conducting downhole operations on a live well from within a dry, 1-atm (100-kPa) production chamber. The paper begins with a description of Can Ocean Resources Ltd.'s subsea wellhead cellars (WHC's). The offshore service system and diving capsule are also described. Various kinds of downhole work are discussed together with the impact of four main operating factors: available space, mission time, safety, and payload. To illustrate better the advantages and limitations of conducting downhole work subsea, downhole operations conducted in the Garoupa field offshore Brazil are recounted. This experience included 5 hot tap operations, 26 polished rod lubricator operations, and 26 wirelining operations-all conducted from dry, 1-atm (100-kPa) WHC's on live wells.

Introduction

Over the past decade, CanOcean Resources has pioneered the development of "dry," 1-atm (100-kPa) production chambers for subsea wells. Today there are more than a dozen such WHC's in the Gulf of Mexico and offshore Brazil. This paper is concerned with the advantages and limitations of conducting downhole operations such as wirelining, polished rod lubricator operations, and hot tapping from within a dry WHC.

Dry, 1-Atm (100-kPa) Production System

The dry production system is based on the use of 1-atm (100-kPa) WHC's to encapsulate petroleum production equipment installed on the seafloor. It allows the use of conventional oilfield equipment and techniques and permits trained technicians to work subsea in a safe, dry, "shirtsleeve" permits trained technicians to work subsea in a safe, dry, "shirtsleeve" environment.

The WHC's are serviced by offshore service vessels (Fig. 1). Each vessel is equipped with a diving capsule capable of transporting men and equipment to the subsea chambers. The capsule is linked to the vessel at all times with an umbilical that supplies a variety of needs, including air supply and exhaust, and electrical power. The capsule is positively buoyant so that in an emergency it can float freely to the surface. To get to the WHC, therefore, the capsule must winch itself down a cable established between the two. The capsule and subsea chambers are maintained at 1-atm (100-kPa) pressure during all manned operations. regardless of water depth. This pressure during all manned operations. regardless of water depth. This simplifies diving operations and allows personnel with no diving experience to be taken down. Equipment and tools required for work inside the chamber are brought down from the service vessel in the capsule. Bulky or heavy equipment is transported in a wireline module (WLM). The WLM is a steel cylinder attached to the skin of the capsule to carry the extra equipment. When the capsule and WLM are mated to the WHC, it creates a single, large subsea chamber consisting of capsule, WLM, and WHC. The WLM is left attached to the WHC as long as required. The internal volume of the capsule-plus-WLM-plus-WHC chamber is approximately 2,500 cu ft (70 m3), This is the internal space from which most downhole work must be conducted. It can be increased, however, by the addition of a second WLM to provide not just additional volume but, more important, additional height above the wellhead.

Life Support Systems

The breathable, 1-atm (100-kPa) environment of the service capsule is maintained from the service vessel through the umbilical. Fresh air is pumped down a supply hose and exhaust air is removed by suction through pumped down a supply hose and exhaust air is removed by suction through exhaust hoses and discharged on the surface.

JPT

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