Abstract

Offshore Borehole Seismic (BHS) surveys require the use of special equipment to generate a supply of high pressure air/gas, and high pressure air-guns deployed in the sea to generate the acoustic energy to propagate seismic waves through the formation. With offshore petroleum exploration occurring in deeper wells, more complex sub-surface formations, and with steadily higher expectations for the results of BHS surveys, there is an industry trend in the deployment of more powerful seismic air-gun sources to provide critical geophysical information of petroleum reservoirs.

Offshore BHS surveys also require the use of the drilling rig, supply ship or vessel to deploy the air-gun source. When conducting these surveys, the air-gun source is fired several hundred times over the duration of the survey and this paper describes how to mitigate the risks to As Low As Reasonably Practical (ALARP).

All well programs having, or planning to conduct a BHS or geophysical survey such as Check-Shot Survey (CSS) or Vertical Seismic Profile (VSP) can benefit from these safety recommendations.

Actual examples of unsafe seismic air-gun equipment, uncontrolled local practices, and lack of test or certification programs will be presented, to highlight the potential safety risks involved in BHS operations.

Recommendations for the testing & certification of critical high pressure air/gas equipment components, the training & certification program for personnel involved in BHS operations, the air-gun equipment setup and deployment methods, and an example of new air-gun controller safe shutdown feature will be presented, to show how safe operations can be maintained throughout the borehole seismic survey.

Failure to implement an HSE system can present a high risk of incident to people and equipment during borehole seismic operations.

Introduction

For many years the BHS industry has been operating high and low pressure pneumatic equipment. The risk associated with any pneumatic system whether at high or low pressure is far greater than a hydraulic system due to the expanding volume of gas remaining should a failure in the system occur.

Seismic air-guns are designed to release high pressure gas into the surrounding medium (usually sea-water) in a few milliseconds and therefore this design must be fully appreciated by all personnel involved in the operation of the air-guns and the high pressure equipment which supplies the energy to the air-guns. A bursting party balloon will make a large bang; however the pressure inside the balloon is only 1/10th of a bar. Imagine the balloon bursting with 140 bar inside! The effect would be very different.

By providing the correct training, examination and certification to operators of air-guns and associated equipment, the appreciation of the potential dangers can be better understood and therefore help to mitigate the hazards from developing into disasters which in the case of high or low pressure pneumatic systems can result in the serious injury or death of the people operating this equipment.

Air-gun theory

Seismic air-guns are operated by sending an electrical pulse for a set duration to a solenoid fitted to the air-gun. When the solenoid operates it opens a valve allowing the high pressure gas from the operating chamber to act on the top of the air-gun shuttle increasing the surface area thus forcing the shuttle to move away from seal. When the shuttle moves away from the seal, the operating chamber high pressure gas is released from the air-gun into the surrounding medium. This sudden release of high pressure gas generates the acoustic 'bang' (Figure 2).

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