Near-miss: Serious subsea burning incident

A member has reported an incident in which a saturation diver caused an underwater explosion during subsea burning operations. The incident occurred when the diver was in the process of burning drain holes in a toppled three-story quarters building. The diver was burning drain holes to allow water to escape more quickly when the structure was lifted to the surface, which would facilitate a safer recovery. The safety precautions taken during this procedure included flushing work areas with air, and ensuring that vent holes designed to remove all combustible gases were placed above the work areas. The explosion occurred when the diver changed locations to ‘enlarge a vent hole’.

The intent of this safety flash is to raise awareness of the safe work practices that should be employed during underwater burning operations. Failure to follow basic safe work practices can result in a serious incident with severe or even fatal consequences.

An investigation revealed the following:

• The immediate cause of the incident was that subsea burning operations introduced or encountered an explosive mixture of gases in a complex submerged structure (containing numerous interior walls and baffles) where it became entrapped. The explosive mixture was subsequently ignited by the underwater cutting operations;
• Contributory causes included the following:
  • It was assumed that flushing the structure with air through vent holes placed above the hot work areas would reliably remove all combustible gases produced by underwater burning operations. In reality, not all explosive gases were removed from the complex interior of the toppled building by this method. Once burning operations began sufficient accumulation of explosive gases must have occurred to create the potential for an explosion despite the flushing that was underway;
  • As water depth increases so must the volume and pressure of oxygen delivered to oxy-arc cutting gear. At saturation diving depths even apparen’tly small pockets of entrapped gas have the potential to produce large explosive forces;
  • The decision to select underwater burning as the preferred method for cutting drain holes in the structure, rather than consider alternative suitable cold cutting techniques;
  • By moving to enlarge a vent hole the diver moved closer to the entrapped explosive mixture of gases and increased the risk of ignition.

The following lessons were learnt:

• Special attention and consideration is to be given to operations that involve underwater cutting into enclosed spaces where gas entrapment can occur;
• Complex enclosed spaces may be particularly difficult to vent completely of explosive gases prior to and during underwater burning operations. If the configuration of the underwater structure is such that the complete absence of entrapped or accumulated explosive gases cannot be verified at all stages of the job then a suitable alternative to thermal cutting methods should be selected;
• Appropriate verbal communications and use of video monitoring should be employed to ensure the diving supervisor knows the precise position of the diver on the structure and to ensure the area is safe for burning;
• Avoid burning into an area containing anything but water. While it is prudent to use ‘pneumo’ or tool air as a method of verification that a vent hole is working as intended, or to flush unknown substances from an area, the air supply should be secured to allow water to fill the area before burning;
• Clear terminology and communications: Before beginning an operation, all personnel involved should understand the difference between a vent hole and a drain hole:
  • A vent hole is intended to allow pressure, suction, gas and/or hydrocarbons to escape from an area. A vent hole is never burned; it is always created using cold-cutting methods such as a reduced RPM drill;
  • A drain hole is what is created to allow water to escape from a structure when it is lifted and thus, should be in an area containing water. A drain hole should only be burned if the configuration of the underwater structure is such that the complete absence of entrapped or accumulated explosive gases around or above the area to be burned can be verified at all stages of the job. If the complete absence of explosive gases cannot be so verified a suitable alternative to thermal cutting methods should be selected to make the drain hole cut;
  • The two terms should not be used interchangeably, as it may create confusion in the communications between the diver and the dive supervisor;
• A risk assessment should always be used to determine if burning is the preferred method for the task at hand.

Members may also refer to Guidelines for oxy-arc cutting.