Water Spray Well head Fire Protection Systems For Offshore Structures
Authors: Achenbach G.D. , Bourne H.A. , Ayres L.M. 
Published: October 2008

Previous laboratory tests revealed that pressurized API bolted wellhead flanges equipped with standard B-7 bolting began to leak after brief exposure to temperature above 850°F. Water spray systems offer a simple means of cooling and thereby protecting wellheads exposed to high temperatures. Theoretical minimum water rates needed to protect wellheads exposed to fire temperatures are low (e.g., 5–10 GPM). Field tests were conducted to determine actual water rates required to protect wellheads exposed to large gas, gas-condensate, and gas-crude oil flares. The work also established basic design criteria for effective application of the water. Results of this work revealed the ability of a properly designed, self-contained, on-board platform water spray system to protect wellheads in the event of a fire.

INTRODUCTION

Past offshore platform fires have demonstrated the vulnerability of API specification wellhead flanges to high temperatures. After relatively short periods of exposure to direct flame impingement or radiation from a nearby fire, shut-in wellheads began to leak. In some cases, these leaking wells then became new sources of fuel for the fire.

Laboratory experiments have closely defined the operating temperature limit (about 850°F) of API specification wellhead flanges and standard bolting, thereby confirming field observations. In addition, these tests have shown the improvement in high temperature "time to leak" performance that can be achieved through the use of high temperature creep resistant bolting. Even with exotic bolting materials, API specification flanges deteriorate within a few hours at temperatures in excess of l,200°F. Further, wellhead valves may leak or become inoperative after brief exposure to fire temperatures.

Protection of wellheads against high temperatures can be achieved by the use of insulation, flame barriers, reflective coatings, water sprays, or a combination of these. The purpose of this investigation was to determine the design criteria for a self-contained, on-board water spray system. Such a system would automatically respond to fire on the platform and protect the wellheads from overheating for an intermediate period of time (e.g., 36 hours). During that time, it is anticipated the fire would burn out or additional fire fighting equipment would arrive to combat a prolonged fire.

Using the design criteria developed by this investigation, water spray systems can be custom designed for multiple-well production platforms requiring this type of fire protection. The limited objective of this type of system must be kept in mind. The objective is not to extinguish the fire or protect the entire structure. Rather, the water spray is designed to cool and thereby assure the pressure containment capability of the wellheads.

BACKGROUND

Actual flame temperatures and total heat transfer rates are not absolutes for large gas and oil flares. The flame temperature is a function of heat content and volatility of the fuel, the quantity of air or oxidant available, and the mixing characteristics of the flame. In turn, heat transfer to an object in or near a large flare is a dynamic phenomenon dependent upon flame temperature, target temperature and surface emissivity, luminosity and thickness of the flare, local convection at the target surface, and view factor.