A Tethered Submersible for Use as a Tool by a Diver
Authors: Marsland George E. , Marschner Karl Heinz 
Published: October 2008

Inspection, maintenance and repair of offshore installations and equipment essentially means going underwater to carry out usually pre-defined work programs. Although a number of remote controlled and manned submersibles are already available and doing useful work, for many tasks there is as yet no real substitute for the diver working on site.

The equipment desired in this paper provides a diver with tools, power and facilities which enable him to work with high efficiency and safety when carrying out the tasks associated with offshore equipment.

The complete system is self-contained and is intended for operation from the deck of a diver support ship. It consists of a tethered submersible, a launching and recovery unit, a control container and a power supply container. It is however portable and can be equally well installed on a platform.

The submersible unit has a computerized control and stabilization system and can be guided either remotely from the surface control container or locally by the diver. An integrated clamping arrangement enables the vehicle to be attached to a structure and positioned at any desired angle. The submersible also carries a moveable platform which can be erected to provide the diver with a stable base from which he can work. Other facilities include a range of hydraulic power tools, a power winch, equipment for NOT, water jetting and pumping equipment.

For inspection and diver observation purposes, the vehicle is equipped with cameras mounted on pan and tilt units, and obstacle sonar and tracking systems enable accurate remote control and positioning.

1. Introduction

The expansion of the offshore industry associated in particular with the development of North Sea oil and gas fields has produced the requirement for inspection, maintenance and repair work on a large number of submersed and semi-submersed offshore installations. As such installations become older, and as governments and classification organizations insist on ever more stringent regulations for safety and environmental protection the demand for inspection, maintenance and repair must increase.

The tasks involved in this work require special equipment and the system described in this report is designed to provide a diver with facilities which will enable him to work with high efficiency and safety when performing these tasks. The system is referred to as the "System DAVID".

2. Problems Associated With North Sea Diving Operations

Maintaining a diver working in deep water requires a dynamically positioned diving support ship specially equipped with a diving plant which includes the diving bell and handling equipment. Costs involved in bringing one man to a worksite on a submerged structure or on the sea-bed can be in excess of $ 20 000 per day. Cost savings resulting from improvements in working efficiency are therefore considerable and shorter task times enable better use of the periods of good weather when diving operations are possible.

The environment in which the diver has to work is difficult and dangerous. The water temperature is generally about 4° C and natural illumination is either poor or non-existent.