Water-Spray Wellhead Fire Protection Systems For Offshore Structures
Authors: Achenbach G.D. , Bourne H.A. , Ayres L.M. 
Published: December 2007

Water-spray systems offer a simple way of cooling and protecting wellheads exposed to high temperatures. Field tests were conducted to determine the capability of water sprays in protecting wellheads from overheating when exposed to large gas, condensate, and crude-oil flares. Results demonstrate the ability of a properly designed, self-contained, water-spray system to protect offshore-platform wellheads during a fire.

Introduction

Offshore-platform fires have revealed the vulnerability of wellhead flanges to high temperatures. After relatively short periods of exposure to direct flame 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 degrees F) of API-specification wellhead flanges and standard bolting, thereby confirming field observations. In addition, these tests revealed the limited improvement in high-temperature "time to leak" performance that can be achieved with high-temperature, creep-resistant bolting. Even with exotic bolting materials, API-specification flanges leaked within a few hours at temperatures above 1,200 degrees F.

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

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

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 on flame temperature, target temperature and surface emissivity, luminosity and size of the flare, local convection at the target surface, and view factor. Theoretical flame temperatures are unrealistically high. Typical flame temperatures for gas or oil flares range from 1,500 to 2,500 degrees F.

Total heat transfer to a cold target (200 to 300 degrees F) engulfed in thick, luminous, buoyant flames has been measured at 12,000 to 46,000 Btu/sq ft/hr. This compares with heat transfer rates of 50,000 to 100,000 Btu/ sq ft/hr achieved in boiler furnaces .3 Convective heat transfer to a surface engulfed in flame primarily depends on flame temperature and turbulence at the surface. Hence, its contribution to total heat transfer is relatively independent of fuel, provided the fuel does not wet or blacken the surface. However, radiant heat transfer is highly dependent on flame luminosity.

JPT

P. 323