AODC047.pdf

AB The International Marine Contractors Association

The Effects of Underwater Currents on Divers’ Performance and Safety

www.imca-int.com

AODC 047 July 1987

AB

The International Marine Contractors Association (IMCA) is the international trade association representing offshore, marine and underwater engineering companies. IMCA promotes improvements in quality, health, safety, environmental and technical standards through the publication of information notes, codes of practice and by other appropriate means. Members are self-regulating through the adoption of IMCA guidelines as appropriate. They commit to act as responsible members by following relevant guidelines and being willing to be audited against compliance with them by their clients. There are two core activities that relate to all members: ‹ Safety, Environment & Legislation ‹ Training, Certification & Personnel Competence The Association is organised through four distinct divisions, each covering a specific area of members’ interests: Diving, Marine, Offshore Survey, Remote Systems & ROV. There are also four regional sections which facilitate work on issues affecting members in their local geographic area – Americas Deepwater, Asia-Pacific, Europe & Africa and Middle East & India.

www.imca-int.com/diving

The information contained herein is given for guidance only and endeavours to reflect best industry practice. For the avoidance of doubt no legal liability shall attach to any guidance and/or recommendation and/or statement herein contained.

1 1.1

2

Introduction The subject of diving in currents has been dealt with by a number of bodies and there are many sources of published information available for reference. It is unfortunate, however, that this documentation is often contradictory and does not always reflect commercial practice. The Safety and Technical Committee have reviewed the published literature together with a combination of personal experience and anecdotal evidence in the preparation of this Information Note, the object of which is to provide assistance in assessing the acceptability of certain conditions to ensure the safety of divers, while maintaining operations in a realistic manner.

Definition of Currents

2.1

Currents are flowing masses of water within a body of water and can be divided into the following groups: major ocean currents; tidal currents (which may augment or reduce existing currents); rip currents; river currents in the proximity of the estuary.

2.2

The direction and speed of a current varies with water depth, tide and bottom contour. Current readings near the surface may not reflect the actual speed and direction of a current in deeper water.

3

Effects of Currents on Divers

3.1

The effects of currents on divers varies with the individual, the work being done and the diving method used. Currents produce forces which affect not only the diver's body but also his umbilical, together with the various lines and pieces of equipment he may have at the work depth. A proportion of the diver's energy is thus devoted to overcoming these forces until the point is reached when he will eventually become unproductive. As an increasing amount of energy is devoted to combating the effects of current as well as carrying out productive operations, it follows that the greater the speed of the current, the shorter will be the period during which the diver will be effective before the onset of significant fatigue.

3.2

A diver operating from a bell or wet bell is better able to operate in currents than a surface orientated diver since his umbilical is shorter, is deployed in the horizontal plane and therefore attracts much less resistance to water movement. Thus, a diver operating down or upstream from a bell has to contend with the effect of the current on his person only and not on his umbilical.

Drag Forces 3.3

The force exerted on a diver by a current is proportional to the water velocity squared i.e. if the current doubles, the force against the diver increases four times (Ref 1).

3.4

Table 1 below (Ref 1), shows approximate drag forces exerted on the "average" diver in both a vertical and horizontal position. Current Speed (Knots)

0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Force on diver standing facing current (max profile) lbs

6 23 52 92 144 207 282 369 467 567

kgs

3 10 23 41 65 94 128 167 212 260

Force on diver horizontal facing current (min profile) lbs

kgs

1 4 9 15 24 36 47 61 78 96

0.5 2 4 7 11 16 21 28 35 44

Table 1 - Approximate drag forces exerted on a diver in the vertical and horizontal modes IMCA · AODC 047

1

3.5

4 4.1

How much current, or force, a diver can withstand and still continue to work effectively depends on various factors: a)

Individual physical strength and endurance to fatigue

b)

Type of equipment being used

c)

Length of umbilical, its deployment either vertical or horizontal, orientation in the current, and whether floating or negatively buoyant

d)

Whether the work is being carried out on the seabed or in mid-water

e)

The type of work being carried out and the tools used to do it

f)

Whether the use of both hands is required to perform a task

g)

Current fluctuation and changes of direction

h)

The possibility of using aids such as underwater tender, swim and downlines, screening etc.

i)

The ability of the standby diver to go to the aid of the diver in the event of an emergency

Effects of Current Conditions on Diving Operations In Table 2 below, personal experience and anecdotal evidence have been extrapolated in order to: a)

indicate the increasing restrictions placed on diving operations by increasing current strength, and

b) facilitate identification of certain conditions beyond which it would be unwise to continue operations on a purely routine basis It must be appreciated, however, that it is impractical to be definitive in identifying the restrictions imposed by certain current conditions since these are affected by so many variable factors as indicated in para 3.5 above. The extrapolated values shown in this table should, therefore, be applied with a degree of flexibility taking account of diver feedback and operational requirements. Current (knots)

0.0

0.8

1.0

1.2

1.5

1.8

Surf supply in mid water

Normal work

Observation

* NB 1

Surf supply on bottom

Normal work

Light work

Observation

* NB 1

Light work

Observation

* NB 1

Light work

Observation

Bell or wet bell in mid water Bell or wet bell on bottom

Normal work Normal work

2.0 & Beyond

** NB 2 ** NB 2 ** NB 2 * NB 1

** NB 2

Table 2 - Effects of current conditions on diving operations Notes:

5 5.1

2

1 – Diving by means of this method in these currents should not be a routine operation. The Diving Supervisor should consult with the divers involved and any other person he judges necessary about the best way to conduct such an operation.

1

NB

2

NB

3

The information contained above will be revised in the light of feedback from the field.

2 – Diving by means of this method in these currents should not be considered unless the operation has been pre-planned taking account of the presence of high current from the early stages of the project. Special solutions involving equipment techniques and procedures should have been evolved to overcome – or protect the diver from – the effects of currents and to provide contingencies for foreseeable emergencies.

Special Devices for Operating in High Currents For operations in high currents, special devices and arrangements have been proposed to assist the diver for example a device to enable the umbilical to be clipped to the guideline and which is capable of quick release in an emergency. When planning such special operations, consideration should be given to the consequences to the diver if such a special device fails or is released, or if a standby diver is required to intervene. IMCA · AODC 047

5.2

In tidal diving operations, identification of slack water periods is essential, the accuracy of tidal charts cannot be relied upon for precise planning of such operations. The use of a current meter will provide a valuable aid to increasing performance and improving safety.

References 1

Diver's Handbook of U/W Calculations, W Tucker, 1980, page 159

2

NOAA Diving Manual, 1979, Section 6, page 5

3

Professional Divers Handbook, Submex, 1982, page 203

4

Royal Navy Diving Manual, 1980, Chapter 3 pages 7 and 33

5

Standard Handbook for Mechanical Engineers, 1967, Section 3, pages 48-59

6

The Underwater Handbook, C. Shilling, M. Werts and N. Schandelmeir, 1976, pages 77-80

7

US Naving Diving Manual, 1979, Chapter 4, pages 9, 17-19

IMCA · AODC 047

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