Gudmestad_edit.qxp 24/10/08 11:43 Page 54
Selection of Support Vessels for Offshore Operations in Harsh Environments
Table 1: One-year Return Period Significant Wave Heights for a
experience with regards to typical intervals for maximum operating
Selection of Offshore Locations
sea state for different types of operation.
One-year Return H
s
(m)
Kikeh (offshore Malaysia) 3.5
Remotely operated vehicle (ROV) launch and recovery will typically have
Girassol (west coast of Africa) 3.4
a maximum limit of 3m H
s
for a simple ROV spread launched over the
Campos Basin (offshore Brazil) 5.7
side of a platform supply vessel (PSV) or similar. Using a more state-of-
White Rose (Grand Banks) 10.5
Oseberg (northern North Sea) 10.8
the-art launch and recovery system with a heave-compensated winch,
Ormen Lange (Norwegian Sea) 11.7 a maximum sea state of 4.5–5m H
s
may be achieved for an over-the-
Snøhvit (southern Barents Sea) 10
side launch when put onboard a large (~80m) PSV. For higher sea
Shtockman (eastern Barents Sea) 9.4
states, moonpool launch is recommended, and sea states of 6m H
s
Figure 1: All-year Wave Statistics are achievable.
In most cases and systems, in-field pipelay (typical pipe diameter up
1.1
to 16 inches) will have a maximum sea state of 2.5–3m H
s
when the
1
0.9
The maximum operational sea
0.8
state will have an impact on the
0.7
) 0.6
design and cost of vessels and
s
P(H
0.5
Ormen Lange
equipment, as well as on
0.4
DNV ERN
operational efficiency.
0.3
Snohvit
0.2
Campos Basin
pipe is laid from a 110–150m-long monohull lay vessel. Installation
0.1
of umbilicals will typically have the same limiting sea states, and
0
flexible flowlines may in some cases be laid in sea states up to 3.5m
01234567 8910
H
s
. Larger-diameter rigid pipes are usually laid from larger
H
s
(m)
monohulls, barges or semi-submersible platforms, and work can
DNV ERN = Det Norske Veritas environmental regulatory number. usually be carried out at a somewhat higher limiting design sea
state – typically 3–5m H
Figure 2: Seasonal Wave Statistics for the Visund Field in the
s
.
Northern North Sea
The limiting sea state for saturation diving will vary significantly with
diving system solutions, diving bell launch location and vessel size. For
1.1
80–150m-long monohulls with moonpool launch located not far
1
from amidships, the operating sea state should be in the interval of
0.9
4–6m H
s
.
0.8
Anchor handling with North Sea anchor handling tugs can usually be
0.7
Sheffield:
performed in up to 2.5–3m, or in some cases even 3.5m, H
s
. In lifting
)
s
0.6
0114 292 3711
operations through the splash zone, the wave forces dominate and
P(H
0.5
Visund – spring
the vessel motion will play a less important role (typically 10–20% of
0.4 the hydrodynamic forces are due to vessel motion). For most Vacancy Hotline:
Visund – summer
0.3
objects the maximum sea state will be somewhere between 1.5 and 0800 183 0730
Visund – autumn
2.5m H
0.2
s
.
Visund – winter
0.1
Lift operations from the deck of a supply boat will typically have a
0
maximum sea state of 2.5–3.5m H
s
, depending on the lifting speed,
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8
the stiffness of the system and the motion characteristics of the
H
s
(m)
vessels involved.
Graphs show fraction of time for which the sea state is less than the value indicated on the horizontal scale.
Table 1 and Figures 1 and 2 are based on information from internal and client-supplied unpublished Combined Operations
documents and reports.
In many offshore operations, several vessels will be involved. One
regions of operation. Furthermore, all elements and systems vital for example of an operation where a large spread of equipment is
the operations need to be considered. For example, it will not be involved is offshore fabrication of trunklines (installation of large-
possible to use a dive system with a high operational sea state if the diameter pipelines). Acergy operates the trunkline lay semi-
vessel is not capable of keeping position in the same sea state. Some submersible barge Acergy Piper (see Figure 3). This barge is
criteria for operations will be discussed below, reviewing the North Sea positioned using anchors, so anchor handlers are needed in the
54
EXPLORATION & PRODUCTION – OIL & GAS REVIEW 2008 – VOLUME 6 ISSUE II
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