Download AODC047.pdf PDF

TitleAODC047.pdf
Tags Scuba Diving Tide Diving Underwater Diving
File Size139.2 KB
Total Pages5
Table of Contents
                            1 Introduction
2 Definition of Currents
3 Effects of Currents on Divers
	Drag Forces
4 Effects of Current Conditions on Diving Operations
5 Special Devices for Operating in High Currents
References
                        
Document Text Contents
Page 1

AB
The International Marine
Contractors Association































































The Effects of Underwater
Currents on Divers’
Performance and Safety


www.imca-int.com

AODC 047
July 1987

Page 2

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.

Page 3

1 Introduction

1.1 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.

2 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.

Force on diver standing facing
current (max profile)

Force on diver horizontal
facing current (min profile) Current Speed

(Knots)
lbs kgs lbs kgs

0.5
1
1.5
2
2.5
3
3.5
4
4.5
5

6
23
52
92

144
207
282
369
467
567

3
10
23
41
65
94

128
167
212
260

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

Page 4

3.5 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

4 Effects of Current Conditions on Diving Operations

4.1 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 0.0 0.8 1.0 1.2 1.5 1.8 2.0 & Beyond
(knots)

Surf supply
in mid water

Normal
work

Observation * NB 1 ** NB 2

Surf supply
on bottom

Normal
work

Light
work

Observation * NB 1 ** NB 2

Bell or wet bell
in mid water

Normal work
Light
work

Observation * NB 1 ** NB 2

Bell or wet bell
on bottom

Normal work
Light
work

Observation * NB 1
** NB

2

Table 2 - Effects of current conditions on diving operations

Notes:

1 NB 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.

2 NB 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.

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

5 Special Devices for Operating in High Currents

5.1 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.

2 IMCA · AODC 047

Similer Documents