Natural Ventilation in Offshore Modules Using Wind Wall and Double Louvres: A Wind Tunnel Study
Authors: Tam V.H.Y. , Dawson R.P.M. 
Published: October 2008

ASBSTRACT
BP Research carried out a series of wind tunnel experiments to measure the internal flow characteristics of an offshore module which was naturally ventilated using double louvres and wind walls. This work showed that there was significant spatial variation of flow velocities and dispersion characteristics inside the module. In this paper, a qualitative description of these flow characteristics and simple guidelines on maximizing ventilations inside a module is given.

INTRODUCTION
Adequate ventilation is essential for safe operations throughout hazardous areas on offshore platforms. It ensures that small leakages of flammable or toxic gases do not accumulate, and are diluted and removed quickly from the platform. In the North Sea Continental Shelf, platforms are commonly constructed from modular units - mcdules. These modules represent different functional areas on the platform. There are a range of sizes of modules depending upon crane lifting capacity and platform design. qically, a module measures 30 m long, 15 m wide and 10 m tall. Hazardous areas are naturally ventilated on the majority of BPrs platforms. Mechanical stirrers are sometimes used to supplement natural ventilation to produce a more even distribution of ventilation inside the module, the risk of leakages is high. There is also a need to protect personnel against adverse weather conditions, and equipment against the elements. Weather cladding such as wind walls and lowres have been used for these purposes. However, these cladding affect the flow of air inside the module and hence the distribution of ventilation rates inside. It is important that the effect of cladding is understood and quantified. This will allow more efficient use of mechanical ventilation, more effective layout of gas detectors, and the establishment of a technical basis for the zoning of hazardous areas. This paper describes results from anexperimental study carried out by BP Research using a scaled model of an isolated offshore module in a wind tunnel.

VENTILATICN RARATES
Ventilation rate is commonly used to describe the average rate the air inside the module is renewed and measured in air changes per hour. It is used to describe the air flow rate for the whole area. For a hazardous area offshore in the North Sea, the minimum designed ventilation rate is 12 air changes per hour and that this must prevail for 95% of the year to allow for the seasonal variation of the wind. This measure does not provide information on the distribution of ventilation rate inside the module. It is the local rate of ventilation that governs whether a leak of hazardous gas at that location could be dispersed safely and carried out of the module We measured the local ventilation rate by the local dispersion characteristics. The local air flow speed cannot be used as there is recirculation inside the module; a locally high flow speed does not necessarily mean high dispersion rate as the material can be advected back. The local ventilation rate (n) is given by the following expression when the gas and air are assumed to be well mixed within the volume of interest.