Shipping safety - Ingenia

MARINE MATTERS
●
●
●
select a survival storm in terms of
the significant wave height H s
as a
function of ship length and type
this wave should approach limiting
steepness conditions just before
wave breaking, defined by its peak
period T p
= √(13H s
); for Derbyshire
this leads to a most probable
wavelength of 294 m, which is close
to her 284 m length and ensures
extreme pitching motions
a maximum extreme wave height of
2.5H s
is recommended, with a
severe wave crest to wave height
ratio of about 0.65 to 0.7 (recent
measurements have reached 0.74);
this is particularly important for wave
impact loads on vertical structure
and fittings.
Non-linear wave and ship response
theory is then applied, preferably also
checked by high quality tank
measurements, over a range of nearby
wave periods to establish the
conditions for maximum relative
motions and maximum pressure on the
hatch covers being considered. This
leads to required improvements in
hatch cover strength of between 2 and
3.5 times the present standards to
effectively eliminate the weakness.
Recent ship damage and
developments in horizontal wave
impact theory have shown that present
ship rules are ‘an order of magnitude
inadequate’ (a recent Health &Safety
Executive report). This huge gap arises
from three sources which are not
presently considered:
●
●
●
insufficient wave height and crest
elevation
no allowance for near breaking wave
crests reaching velocities up to two
times that of the wave
insufficient consideration of water
impact coefficients.
This has led to many failures and losses
of deck equipment (including the loss of
the 21 ton starboard windlass on the
foredeck of Derbyshire), damage to
bridge fronts and bursting of bridge
windows (25 m or more above sea
level), damage to bulwarks, hatch
coamings and to ships’ sides in bulkers
and tankers, etc. The procedure
outlined above illustrates a survival
approach to prevent or considerably
reduce such damage.
A similar survival design approach
has been advanced by William Buckley
to improve ship capsize resistance. It is
sobering to recall that the Queen Mary
nearly capsized in the Western
approaches with 15,000 American
troops in 1942 when struck on the
beam by a long crested elevated wave
which rolled her to just short of her
vanishing stability limit.
Implementing ship safety
improvements
Other political rocks ahead are that not
every classification society or flag of
convenience country will fall in line with
anything that costs money, plus the
long tail of substandard ships to be
worked through. Ideally, to overcome
this, international legislation is required.
Failing this, EU ports could consider
adopting tough docking approval
criteria to prevent use of port facilities
by shipowners/operators who do not
comply with new tougher safety
requirements. Whilst IMO’s present
proactive stance to improve ship safety
is unlikely to impede an agreed survival
approach to design, the same cannot
be said of the present understandably
cautious stance of IACS. High level
intervention is being considered to
ensure this new knowledge (when
validated) is implemented sensibly in
ship design and operation. It would also
help to ensure that the MaxWave
research and development may
continue to provide further data.
Some present and future
ship trends
Cruise market ships are increasing in
size. The French-built Queen Mary 2
will be twice the gross tonnage of
Queen Elizabeth 2. The World, recently
built in Norway, is the first residential
apartment cruise ship. This floating
‘resort’ offers 110 luxury spacious
residences ranging from 335 m 2 to
975 m 2 , as well as 88 guest suites for
rental. The World is scheduled to visit
46 countries, and will spend 250 days
in port each year at such events as the
Monaco Grand Prix, Cannes Film
Festival, the British Open (which it
visited this year) and the Americas Cup.
Container ships now dominate the
dry cargo trades and continue to grow
in size. Among the largest are P&O
Nedlloyd’s 6802 TEU (20 foot equivalent
units) trans-Pacific ships. A design for a
330 m long × 46 m beam 9,000 TEU
design is being developed by Daewoo
in anticipation of Chinese interest.
Meanwhile, IHI and others anticipate a
generation of 10,000 TEU ships, and
P&O’s new container port on the
Thames is being designed to handle this
size. A remarkable concept of a 15,000
TEU ship of 69 m beam and
28–30 knots has recently been reported.
Over the past decade, high speed
cruise-ferry ships have become popular,
notably in Europe. Many superfast
ro–pax ferries have entered service,
typically powered by large propeller
drives developing 40–60 MW to give
28 knots. Figure 5 shows a 38 knot
monohull ro–pax ferry with T-hydrofoils
fore and aft and 4 lateral fins for roll
control. Electric drive is to 4 Lips
steerable water jets under the transom
stern. Meanwhile, Japan plans a 49 knot
hydrofoil assisted techno-superliner.
Larger ocean going fast ships have
yet to be proven. Nevertheless, a
40 knot multiple gas turbine ship with
five 50 MW water jets is due to start
trans-Atlantic service next year. A trans-
Pacific concept of a foil assisted semi-
SWATH container ship (small
waterplane area twin hull) capable of
64 knots is being considered.
There must be some anxiety about
such developments since increased
ship speed and/or size increase the risk
of heavy weather damage, especially in
open ocean conditions where abnormal
steep elevated waves are most likely to
occur.
ingenia
19