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Diving operations in exploration drilling

Every offshore drilling operation must include some possibility of underwater intervention. This may be required for a variety of routine tasks including re-entry, guide-wire or riser connection, riser inspection and scour examination. It may also be required in the case of accidental damage to the wellhead/BOP system or to the drilling unit itself, and to assist in the recovery of equipment from the sea-bed.

Only in exceptional circumstances should a manned diving operation be contemplated for an exploration operation. Most MODUs are fitted out for handling ROV equipment conveniently and efficiently.

1. The scope of work

The tasks required to be carried out in support of exploration drilling vary with the type of drilling unit.

Jack-ups

The main task is checking for scour and, if necessary, its correction, eg by sandbagging.

Floating units (drill-ships and semi-submersibles)

The main task will be guidance during re-entry operations prior to installation of guide wires, or in operations without guide wires. Other tasks may be guide wire replacement and observation of cement returns at sea-bed. If mud losses occur while drilling the cause will probably be down-hole, but the riser and its connection should be inspected by ROV to avoid the possibility of an incorrect assumption.

All types

General services required on all offshore units are to locate and recover equipment or debris lost overboard, and inspection in case of accidental damage to the units structure.

2. Equipment and techniques

A video recorder should be provided so that a permanent record can be made of any operations undertaken, or of damage, etc. Such records should be in a format usable with standard PC equipment.

There are three basic methods of intervention which have to be considered. These are described briefly in the notes below, in increasing order of cost and complication. In assessing which one to use consideration should be given to:

  • The scope of work.
  • The water depth.
  • Current and visibility.
  • Geographic location (remoteness).
  • The type of rig.
  • Space limitations on the rig for equipment, ease of access to the sea and hazards from thrusters, etc.

2.1 Guide frame run/through drill-pipe closed circuit television (CCTV)

These TV systems are normally provided by the drilling contractor as part of the standard equipment on a floating unit. They should provide a high quality picture, in colour, and are used routinely for re-entry and re-connection operations. However they are very clumsy, with restricted positioning possibilities. Assessment of the equipment offered should demonstrate that the required standard of picture definition is provided.

They are not normally provided with jack-ups as re-entry is not often necessary. If re-entry should be necessary, and there are difficulties finding the pre-drilled hole with a marine conductor, the ROV can be used

2.2 Remotely operated vehicles (ROVs)

ROVs are strongly favoured for company support. Their capabilities are broadening and their reliability improving steadily.

Advantages

  • Safety: no personnel are deployed underwater.
  • The piloting/maintenance crews may be suitable for integration into other multifunctional roles.
  • High quality underwater CCTV and still photographic inspection can be carried out from all angles and at a distance away from the spread if required.
  • Work class ROV types can carry out a wide range of planned/anticipated tasks, and many unplanned tasks can be completed.

Disadvantages

  • To be most effective ROVs should be installed on the rig for the duration of the drilling contract.
  • ROVs cannot work effectively in extremely turbid water other than with a scanning sonar.

Types

a. Low cost ROV (LCROV)

A very lightweight "suitcase" type, it may be flown by helicopter to site. Deployed by hand on a light tether, it has limited navigation, current, sea state capability and provides CCTV inspection only. Can be valuable for shallow inspections in kind environments.

b. Observation ROV

A control cabin and deployment system is required for this type (and those in Section 7.8.2.2.3.c and Section 7.8.2.2.3.d). It can provide CCTV and still photographs in more severe environments and to greater depths with a higher capability of navigation and control (e.g. UFO 300, RCV 225).

c. Light/medium work ROV

A larger, rigid frame ROV with one or two small manipulators, observation equipment as above plus perhaps sonar and light tools.

d. Heavy work ROV

The largest type of general purpose ROV with perhaps three heavy duty manipulators, a suite of tools and comprehensive instruments.

There are many ROVs offered and a careful selection will be necessary on a case by case basis - according to the scope of work and local circumstances; an important aspect will be the quality of the personnel in the contractor's team (one to three personnel required).

2.3 Manned diving

Every effort should be made to avoid the need for manned diving intervention. As drilling proceeds into deeper water, diving usually ceases to be cost effective; this may occur between 30-200 m depending on local circumstances.

Surface orientated air diving

One or more divers may be deployed with a continuous air supply provided by an umbilical. A two compartment recompression chamber and suitable means of deployment into the water for the diver and a stand-by diver are essential. The minimum team size for light work is five men. The absolute maximum depth for light work is 50 m, heavier work will be limited to about 30 m.

Self Contained Underwater Breathing Apparatus (SCUBA). This equipment is sometimes offered by contractors outside the North Sea for work down to 20 m. There are virtually no circumstances when use of the technique can be shown to be safer than surface orientated diving. Previous use of SCUBA has accounted for the majority of diver fatalities in offshore operations.

Saturation diving

This technique is the normal means of diving for all major work deeper than about 30 m. Underwater working times of up to eight hours per day per team of two divers in saturation can be achieved. The saturation diving system and bell deployment arrangements are extensive, and are usually built into a dynamically positioned Diving Support Vessel.

A minimum team size of fifteen persons is required to support four divers in the saturation chambers.

In areas where there is good access to international deep diving services, saturation diving operations in the range 30-200 m are considered routine. An observation ROV in support of the diving operation is strongly recommended.

For very deep dives between 200-300 m, some weeks/months of preparation may be required depending on the locality - and special selection of equipment, divers and diving crew is necessary. An observation ROV in support of the diving is essential. The life support of the divers becomes more critical at such depths and in addition the performance and working time of the divers reduces; normally this means of intervention would not be cost effective for drilling support at these depths when all requirements for safety are taken into account. Extreme depth diving deeper than about 300 m should not be contemplated.