Gudmestad_edit.qxp 24/10/08 11:43 Page 52
Selection of Support Vessels for Offshore Operations in Harsh Environments
Operations
a report by
Erlend Hovland
1
and Ove Tobias Gudmestad
2
1. OH Meling, Stavanger; 2. University of Stavanger, Norway
The selection of support vessels for offshore operations is often based impact on the design and cost of vessels and equipment, as well as on
on experience from similar operations in regions with similar operational efficiency. Figure 2 shows that seasonal variations at a
environmental and geographical conditions. In areas with a lack of typical North Sea field can be significant. It may be very challenging to
operational experience and in areas with special challenges it is carry out operations at certain times of the year, and the core
important to evaluate environmental, geographical and operational installation season may in some regions be limited to six to eight months
challenges to establish a basis for selection of support vessels and of operation.
design criteria for new vessels. In addition to information about the
physical environment, it is important to have knowledge about Waves are not the only weather phenomenon challenging offshore
the expected operational profile of the vessel in order to decide on operations. In Arctic and sub-Arctic seas, ice features may be present to a
values such as speed, operational sea state, loading capacity and more. varying degree throughout the year. Surface ice poses very special
demands on vessels and equipment operating in the area. Winterisation
The vessel’s day rate is often used as a selection criterion by major for operations in low-temperature environments, including measures for
operators. In areas dominated by harsh environmental conditions de-icing, may also be an important aspect in the design of vessels and
and remoteness, efficiency becomes an important factor, and a day- equipment, and ice class strengthening and winterisation may increase
rate comparison will in many cases not provide correct information the vessel’s capital costs significantly. Some parts of the world are also
about the optimal design. This article suggests some criteria that can subject to special weather phenomena such as cyclones, hurricanes and
be used in the process of selecting vessels to take part in offshore polar low atmospheric pressures, all of which will cause operations to halt
operations in harsh environments. and may need to be given attention in the selection of designs/methods.
The Physical Environment Location
The physical environment differs significantly from region to region. The development rate in regions defined as ‘remote areas’ is often low.
Table 1 shows one-year extreme significant wave heights (H
s
) for a It is hard – if not impossible – to optimise vessels for offshore operations
selection of basins with offshore oil and gas activity. The design in some less-developed areas, simply because the amount of work may
conditions vary significantly from region to region, with the harshest be insufficient to defend a large asset investment. On the other hand,
wave conditions being present in the areas situated around the some regions viewed as remote areas contain significant amounts of
North Atlantic. From an operational point of view, the maximum sea planned and potential work, such as the offshore African West Coast.
state is perhaps not so interesting, but it gives an indication of the
areas in which the roughest wave conditions can be expected. Some regions and certain operations involve significant amounts of
transit time. The asset solution, in particular the ideal transit speed
Figure 1 shows the wave statistics presented as fraction of time (vertical and ideal loading capacity, varies with the remoteness, transit
scale) for which the H
s
is less than the value shown on the horizontal distance and amount of local infrastructure. For operations involving
scale for some selected offshore locations and for definition of the Det large amounts of transit and transportation time, the efficiency of
Norske Veritas (DNV) environmental regulatory numbers (ERNs).
1
For the the transportation solution is increasingly important. In general, for
Campos Basin offshore Brazil, the H
s
is <2m 90% of the time, whereas remote locations faster and larger vessels will be favoured. Also, the
at the other locations presented in Figure 1 the sea state is <2m ability to operate without needing a large number of support vessels
approximately 40–50% of the time, which is a significant difference. It and support functions may be an advantage in some remote areas
is apparent that the desired maximum operational sea state varies from due to the low availability of support vessels (no spot market).
region to region, and thus vessel and equipment solutions and best
practices also vary. The maximum operational sea state will have an The quality of weather forecasts is generally poorer for remote areas
due to the lack of weather observations and stations. This implies
that more safety factors will need to be added to the design sea
Ove Tobias Gudmestad is an Adjunct Professor of
Marine Technology at the University of Stavanger and
state to account for uncertainty in the weather forecasts.
of Arctic Technology at the Norwegian University of
Alternatively, wave radars or buoys and even weather radars and a
Science and Technology, Trondheim. He is also an
meteorologist on watch can to be included in the operation spread
advisor in Arctic technology for StatoilHydro. He
holds degrees from Gubkin University, Moscow and
to increase the quality of the weather forecasts.
Murmansk State Polytechnical University, as well
as a PhD in hydrodynamics from the University of
Bergen; he also undertook post-graduate study at the
Operating Criteria
Massachusett’s Institute of Technology (MIT). In order to decide the operational sea state of a vessel, the sea state
needs to be compared with the actual weather conditions of the
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© TOUCH BRIEFINGS 2008
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