Dynamic Response of Drill Floor to Fire Subsequent to Blowout

Explosions and fires on offshore drilling units and process plants, which cause loss of life and environmental damage, have been studied extensively. However, research on drilling units increased only after the 2010 Deepwater Horizon accident in the Gulf of Mexico. A major reason for explosions and fires on a drilling unit is blowout, which is caused by a failure to control the high temperatures and pressures upstream of the offshore underwater well. The area susceptible to explosion and fire due to blowout is the drill floor, which supports the main drilling system. Structural instability and collapse of the drill floor can threaten the structural integrity of the entire unit. This study simulates the behavior of fire subsequent to blowout and assesses the thermal load. A heat transfer structure analysis of the drill floor was carried out using the assessed thermal load, and the risk was noted. In order to maintain the structural integrity of the drill floor, passive fire protection of certain areas was recommended. Received 19 December 2019, revised 23 March 2020, accepted 9 April 2020 Corresponding author Jae-Myung Lee: +82-51-510-2342, jaemlee@pusan.ac. kr c 2020, The Korean Society of Ocean Engineers This is an open access article distributed under the terms of the creative commons attribution non-commercial license (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. Dynamic Response of Drill Floor to Fire Subsequent to Blowout 111 offshore structures or open space with consideration of the dispersion of flames and gas, and the results are illustrated (Vembe et al., 2014). For fire analysis following blowout, a blowout scenario is firstly established, and the entire drillship model including the drill floor is imported from the 3D CAD model using the KFX program. As shown in Fig. 1, it is converted to a 3D geometry model to be used in KFX, and in case of blowout, dispersion analysis of gas and fire subsequent to ignition is performed using CFD (Jin et al., 2016). From this, the heat flux distribution of the drill floor and the surrounding area is obtained for each fire scenario. The above procedure can be applied to fire analysis of offshore structures (Bai and Jin, 2016). The specifications of the drillship used for the analysis were 230 m in length, 42 m in width, and drill floor elevation 38 m. 2.2 Main assumptions Blowout flow rates of 10–35 kg/s for normal blowout and 50–200 kg/s for high pressure high temperature (HPHT) blowout are generally presented (Health and Safety Executive, 2017). In a study on deepwater drilling (ABS Consulting, 2015), a restricted flow rate of 35 kg/s and a full flow rate of 150 kg/s were presented. In this study, 35 kg/s and 150 kg/s were adopted. The gas flow composition was 80% methane and 20% pentane, and the environmental conditions were based on Meta ocean data (Santos Basin, 2008) of the Santos waters in Brazil, where the drillship is in operation. The ambient temperature used was the annual mean of 23 °C, wind speed was the annual mean 5.4 m/s, and 9 m/s, which is the upper value of the annual 10%, representing a rougher environmental condition than may occur during drilling. The wind is in the direction of the bow and the stern, because the direction of the wind is equal to the direction of the wave. Accordingly, the wind direction and the bow-stern direction are matched to reduce the roll motion of the ship during drilling operations. 2.3 Fire scenarios Fire analysis was performed using typical drilling scenarios. The results are shown in Table 1. As explained in 2.2 Main Assumptions, the annual mean values of a specific area of the sea from a previous Table 1 Fire analysis scenario considering blowout flow rate No. Release location Flow rate (kg/s) Wind speed (m/s) Wind direction 1 On drill floor 35 5.4 From forward 2 150 5.4 From forward 3 35 5.4 To forward 4 150 5.4 To forward 5 Under drill floor 35 5.4 From forward 6 35 5.4 To forward 7 35 9 From forward 8 150 5.4 From forward (a) Overview 3D geometry model of the drillship from starboard (b) 3D view on drill floor (c) 3D view under drill floor Fig. 1 Fire analysis model in KFX 112 Teak-Keon Kim, Seul-Kee Kim and Jae-Myung Lee study were used for the ambient temperature and wind speed, and the wind direction was selected in consideration of drilling operations. With the construction of the scenarios, the results of fires that may occur in a general drilling environment were obtained, and these were applied to the analysis of the drill floor structure to investigate the behavior, enabling the practical application of the results. 2.4 Analysis results The results of fire analysis in various scenarios are presented in Figs. 2–9. White represents values over 200 kW/m and bright yellow (a) Elevation view of heat radiation (b) Plan view of heat radiation Fig. 2 Fire analysis results : 35 kg/s release on the drill floor is ignited and 5.4 m/s wind blows from forward (scenario no.1) (a) Elevation view of heat radiation (b) Plan view of heat radiation Fig. 3 Fire analysis results : 150 kg/s release on the drill floor is ignited and 5.4 m/s wind blows from forward (scenario no. 2) (a) Elevation view of heat radiation (b) Plan view of heat radiation Fig. 4 Fire analysis results : 35 kg/s release on the drill floor is ignited and 5.4 m/s wind blows from aft (scenario no. 3) (a) Elevation view of heat radiation (b) Plan view of heat radiation Fig. 5 Fire analysis results : 150 kg/s release on the drill floor is ignited and 5.4 m/s wind blows from aft (scenario no. 4) represents 100–200 kW/m. With a blowout flow rate of 35 kg/s, the heat flux around the drill floor is approximately 100 kW/m, and at 150 kg/s, it is 200 kW/m. In case of a drill floor fire, the heat flux values under the drill floor are small as shown in Figs. 2–5. When Fig. 6 and Fig. 8 of scenario No. 5 and 7 were compared, the cases of wind speed 5 m/s and 9 m/s do not show significant difference in terms of the values and range of heat flux. Therefore, in the on drill floor case, the wind speed 9 m/s was not added. Since similar results are expected in the under drill floor case, the values of scenario No. 8 were used for the heat transfer analysis and the subsequent structural behavior analysis. Dynamic Response of Drill Floor to Fire Subsequent to Blowout 113 (a) Elevation view of heat radiation (b) Plan view of heat radiation (c) Section view of heat radiation Fig. 6 Fire analysis results : 35 kg/s release under the drill floor is ignited and 5.4 m/s wind blows from forward (scenario no. 5) (a) Elevation view of heat radiation (b) Plan view of heat radiation (c) Section view of heat radiation Fig. 7 Fire analysis results : 35 kg/s release under the drill floor is ignited and 5.4 m/s wind blows from aft (scenario no. 6) (a) Elevation view of heat radiation (b) Plan view of heat radiation (c) Section view of heat radiation Fig. 8 Fire analysis results : 35 kg/s release under the drill floor is ignited and 9 m/s wind blows from forward (scenario no. 7) 114 Teak-Keon Kim, Seul-Kee Kim and Jae-Myung Lee (a) Elevation view of heat radiation (b) Plan view of heat radiation (c) Section view of heat radiation Fig. 9 Fire analysis results : 150 kg/s release under the drill floor is ignited and 5.4 m/s wind blows from forward (scenario no. 8) 3. Heat Transfer Analysis 3.1 Analysis procedure Using the heat flux result, the temperature around the drill floor is obtained using Eq. (1), and the FAHTS program (USFOS A/S, 2013a) is used to predict the temperature change over time of the drill floor structure. In Figs. 2–9, the heat flux on the drill floor area was estimated at 200 kW/m, and applied in the analysis.