Development of Fire-Resistant Wellhead and Christmas Tree Equipment
Authors: Hartley R. , Hamer P. , van Dort R. 
Published: December 2007

Sarawak Shell Berhad (SSB) has adopted the philosophy that wellhead equipment for platforms and other congested high-risk areas should be designed to withstand a fire in a neighboring well. This paper outlines the considerations leading to the development of the fire-resistant wellhead and Christmas tree and describes the weaknesses inherent in standard wellhead equipment exposed to fire.

Introduction

The need for fire-resistant wellhead equipment became apparent in the early 1970's after a multiwell fire that resulted in the loss of an offshore platform. Later investigations showed that the fire started with a blowout in one well, but before it could be controlled, the fire caused neighboring wellheads to fail and their wells to blow. A thorough examination of wellhead equipment recovered after the fire provided ample data about the damage a fire can cause.1. Wellheads with exposed flanges will not contain well fluids, and all exposed parts will be destroyed rapidly.2. Large metal masses act as a heat sink and withstand fire far better than thin, exposed items.3. The surface safety valve (SSV), if located on a wing valve, will fail quickly. (Six SSV's located on the flowline wing did not close because of excessive friction or were reopened by repressuring or failing debris.)4. A large external force may cause the clamp and the wellhead components to come apart as the bolts on the clamps come loose or fail.5. All external surfaces and pans of the overhead may be exposed to temperatures greater than 1100 degrees C [1832 degrees F], however, the larger internal sections are subject to lower temperatures, about 450 degrees C [842 degrees F]. Although the seal material in valves and in the tubing and casing hangers apparently did not leak, severe charring occurred. This made it difficult to conclude that the standard valves and tubing hangers were always reliable in fire conditions. As discussed later, further development work on behalf of SSB has included developing fire-resistant valves and fire-resistant tubing hanger neck seals.

Fire Testing of Equipment

Following the study of wellhead equipment recovered after the blowout. a series of tests was conducted to investigate the performance of flanges, clamps, and SSV's under fire conditions. The tests confirmed that.1. Flanges are prone to early failure under fire conditions.2. A shield with insulation material will protect a flange in a fire adequately for several hours.3. Large clamps do not experience extensive damage when subjected to a gas fire.4. Fire tests of several surface safety actuators showed that they will operate and close in a fire.

Recommendations for Improved Fire Resistance

As a result of the fire tests, it was possible to recognize design shortcomings in the standard wellhead equipment and to develop the first generation of fire-resistant wellheads. These are available from most manufacturers and generally have the following features.1. A forged integral block contains two master valves, a side outlet, and a swab valve for each tubing string. These multivalve components reduce installation cost and result in a stronger, more compact wellhead, which, because of its mass. acts as a heat sink and withstands fire far better. The disadvantages are that some flexibility is lost and replacement is more difficult and costly.2. The surface safety valve is on the top master valve. Moving the actuator from the wing to the top master valve was a basic change. Such a location is more positive in well control. The top master valve is far more positive in well control. The top master valve is far more likely to stay intact during a fire than a wing valve; and, in addition, there are fewer flanges that must contain well pressure. Also, the well can be remotely shut in if loss of wellhead integrity occurs in the top portion of the tree, or if the lubricator leaks or is knocked off. One disadvantage is that there is danger of accidentally cutting a wireline with this location of the surface safety valve. Thus, a reliable, fail-safe lockout device must be used or some other procedure followed during wireline operations.3. Any flange connection upstream of the surface safety valve (physically below it) subject to well pressure is either eliminated or protected by an insulated shield.4. The Christmas tree is designed to eliminate all small exposed parts.5. The outlet valves on the tubing spool utilize a studded connection.

JPT

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