ENVIRONMENTAL CONDITIONS IN THE SOUTHERN BARENTS SEA
© Arne Kvitrud, Sondre Nordheimsgate 9, 4021 Stavanger.
Paper
presented in Stavanger in 1991, but put on Internet
25.9.2002.
The figures
are not presented here.
ABSTRACT
During the last
several years, exploration drilling have been performed in the Southern Barents
Sea. This paper describes the main conclusions from the experience concerning
the environmental conditions, as it concerns load bearing structures. The paper
is restricted to conditions south of 740N 30'N in the Norwegian Barents Sea.
DATA
COLLECTION
Since 1976 systematic
collection of environmental data have been performed in the Barents Sea. All
the data are publicly available and can be used without any restrictions. The
meteorological data are stored at the Environmental Data Center in Oslo, and
current data are stored at Oceanor in Trondheim.
WIND
CONDITIONS
Wind
velocity
If we look at measurements
carried out by several offshore locations and extrapolate them to an annual
probability of exceedence of 10¿², the result will be 30 - 36 m/s at a height
of 10 m, and averaged over 10 min.
Use of hindcast data from
Bjørnøya, Sentral- banken and Nordkappbanken gives approximately the same
extreme values as for Tromsøflaket (Torsethaugen, 1989).
In the guidelines (1987), a
recommended value of 41 m/s is suggested for the whole continental shelf. The
recommendation should be on the safe side as far as the Barents Sea is
concerned. This means that those who want to use other values may do so.
Polar
lows
A major research project
has been carried out by several Norwegian institutions and firms to examine
polar lows (Lystad, 1986). The highest measured wind velocity in polar lows was
35 m/s at a height of 10 metres, averaged over 10 minutes. One of the
conclusions was that it was not very likely that polar low pressures would give
higher wind speed than normal storms. Extreme value statistics gave 38 m/s for
Tromsøflaket, with an annual probability of exceedance of 10¿² (Houmb et al,
1986). For a given location the extreme values would be lower. As regards to
the design of platforms, polar low pressure is not a significant problem. It
will not give increased extreme estimates of wind speed, wave height, air
temperature or icing (Houmb and others, 1986).
On the other hand they
create problems for operations and planning of operations for a long period of
time, as is normal for other places on the continental shelf. One example is
Norsk Hydro's drilling on Block 7321/9 in the autumn of 1988. Two polar storms
resulted in drilling stoppage for 22 hours (Halleraker, 1988).
Conclusion
on wind
Exploration drilling
platforms designed in accordance with norwegian rules with a recommended value
of 41 m/s are not expected to have difficulties in the Barents Sea.
WAVES
A general trend on
the continental shelf is for mean wave heights to increase from the southern
part (56 degree N), to the largest value in the central part (630 - 650 N), and then decline further north (Eide,
1986). The same applies for wave heights with an annual probability of
exceedance of 10-².
On the basis of measured
data, and supplemented with data from the hindcast model WINCH where no
measurements exist, the 1987-guidelines for choice of wave heights to be used
in early development phases. This is shown in figure 1. Barstow and others
(1988) provide more information on WINCH.
Wave measurements have been
made at several places in the Barents Sea.
There is a clear difference
between the two western stations (Bjørnøya and Tromsøflaket) and the eastern
stations (Sentralbanken and Nordkappbanken). The wave climate becomes less
severe as we go east in the Barents Sea. This is also found by Torsethaugen
(1989), based on hindcast data.
Wave periods
At Tromsøflaket the wave
periods corresponding to a 100-year wave hight will be higher (17-19 sec) than
what is usual in the North Sea (15-17 sec), see figure 1.
During the drilling with
Ross Rig on 10 November 1988 near Bjørnøya, the wave period reached 18 seconds.
This was very close to the resonance period of 20.3 seconds for the platform.
This high wave period resulted in drilling stoppage for 8 hours (Halleraker,
1988).
Conclusion
waves
The wave climate in the Barents
Sea do not deviate significantly from our experience in other parts of the
shelf. Platforms for exploration drilling, which are designed for conditions on
Haltenbanken or Trænabanken for example, should be suitable for drilling in the
Barents Sea, too.
CURRENT
During the past few years
the operators and several public institutions have carried out measurements of
sea current in the Barents Sea. Measurements (closer to shore), over shorter
periods, show extreme currents exceeding 1 m/s. But, in general, the currents
are of the same magnitude as in other parts of the Norwegian continental shelf.
Conclusion
currents
Comparing with the current
speeds in the North Sea, which are often over 1 m/s, these currents are rather
small and should not present any major difficulties.
WATER
DEPTHS
Most of the seabed of
the Barents Sea is 200-400 m below the water surface. This is comparatively
deep, but it can be compared with Troll, Draugen, Gullfaks C and Snorre, where
development has been made or is planned further south.
TIDAL WATER
Measurements of the tidal
water level in the Barents Sea do not deviate much from values found further
south. However, in Vardø the M2 value is 104 m (Bjerke and Torsethaugen, 1989),
which is somewhat higher than found other places.
ICING
The recommendation in the
guidelines is based on a study made by the Otter group (1983) from Norsk Hydro
for use on Oseberg. The valuesin the reoprt has been multiplied with a factor
of two. Load case 1 gives the major contribution.
A theoretical analysis of
the models which have been used has been made by Horje and Vefsmo (1985). Since
the models:
a) have not taken into
account the heat properties of the rig
b) are based on experience
with small moving fishing boats,
the results of the models
are probably on the conservative side. Although Tromsøflaket is not the place
in the Barents Sea where the risk of icing is the largest (Bjerke and
Torsethaugen, 1989), the guidelines (1987) are probably on the conservative
side.
Conclusion
on icing
Ice loads due to icing
should not give major design problems, if they are considered at an early
stage.
SEA ICE
The most extreme known
expansion of sea ice occurred in June 1881 (Kvitrud and Hønsi, 1990). The sea
ice in a short period reached 20 km north of Berlevåg on the east coast of
Finnmark. For a major wintertime period the sea ice was at 710 30'N to 720N.
Statistics
The ice boundaries in the
Barents Sea have been and are recorded weekly by Geir Kjærnli at DNMI on the
basis of satellite observations.
Assuming that the annual
extreme values are statistical distributed, the southernmost distribution of
ocean ice with an annual probability of 10¿² of exceedance can be found
(Kvitrud and Qvale, 1989 or Vefsmo et al, 1990b). The results are presented in
figure 4. The southernmost position based on observations from 1898 to 1983
(Atlas of the Artic Ocean, 1985) correspond well with the extrapolations
(Kvitrud and Nilsson, 1989).
If we extrapolate to the
level of 10¿4, we will find
that in most parts of the Barents Sea sea ice must be taken into account in the
design of offshore structures.
Damage
potential
The damage potential of sea
ice depends on various parameters. The most important ones are the thickness of
the ice, the relative velocity between ice and platform, the physical ice
properties, and the size of the ice-fields (Mobil, 1988).
The oil companies in Norway
have through several years done research to investigate these factors. Several
years of future research must also be done before we have a safe way of
managing sea ice.
Conclusion
sea ice
In the most parts of
the Barents Sea, sea ice has to be taken into account in design. The damage
potential on the platforms is mainly on local elements in the splash zone. In
addition it will cause station keeping problems for floating structures.
ICEBERGS
Icebergs have been
observed at the coast of Norway a few times (Kvitrud and Hønsi, 1990 and Hønsi,
1988):
a) The first report of
icebergs in the Barents Sea south of 740N is in February 1881. Two icebergs reached the
coast at Kvaløya in Troms at 70013'N 19030'E. The
larger iceberg of the two was 7 metres high.
b) In June 1881 several
icebergs were observed at Gamvik, Berlevåg and Syltefjord at East- Finnmark.
The largest iceberg was enormous, with a length said to be 10 km, and a sail
height of 30 m.
c) During the period of
April-June 1929, a number of icebergs reached the coast of Kola Peninsula and
eastern Finnmark (from 240 to 440 E). The local
newspapers in Finnmark reported that they reached up to 30 metres above sea
level.
d) In 1939 two icebergs
were observed at Koi- fjorden close to Gamvik.
Damage
potential and risk
To investigate or quantify
the risk of icebergs includes calculating the probability of collision and
studying the consequences of collisions.
The chance of collision is
closely connected with the frequency of icebergs appearing in the area of
interest. Vefsmo et al (1990) have developed a model computing the collision
probability. Based on this model and the historical data, at least two areas
are sensitive to iceberg collision, giving an annual probability of collision
higher than 10-4:
a) South of Bjørnøya based
on iceberg observation in recent years
b) East-Finnmark and the
sea north of the coast, based on historical data.
Present exploration rigs
(NMD, 1973) are designed for collisions with vessels with displacement of 5000
tonnes and a speed of 2 m/s. These collisions are assumed at sea level. The
energy formula = 1/2 m v² gives 10 MJ when hydro- dynamic additional mass is
not taken into account. A dynamical analysis of the condeep platform Gullfaks A
shows that this platform can take a collision with a tanker of 150,000 tonnes
at a speed of 2 m/s or an energy calculated the same way of 300 MJ. The energy
distribution between iceberg and platform in a collision will be different for
a vessel and an iceberg. The iceberg will be less rigid than the vessel, and a
smaller part of the energy will be absorbed by the platform.
Between Bjørnøya and Hopen
the mean speed of icebergs for a long period of time has been measured to be
approximately 0.1 m/s (Bercha, 1989). The currents may be somewhat larger in
this area than further south, but 0.45 m/s should be a reasonable estimate. To
get an energy of 10 MJ, one must have an iceberg of approximately 100.000
tonnes. To get 300 MJ, the iceberg must be 3 million tonnes.
Measurements between
Bjørnøya and Kong Karls Land in 1988 gave a mean value for icebergs of 570,000
tonnes, but there were large variations, (Løvås and Næss, 1989). Icebergs south
of 740N will most likely be smaller.
Drilling
experience north of 730N
During Mobil's drilling in
August 1988 a survey was made by flying the route shown in figure 4. Four
icebergs were found on this trip (Spring, 1988).
A similar survey carried
out by Norsk Hydro in November did not give observations of icebergs.
Nevertheless, on 23 November 1988 a remainder of an iceberg was found with a
freeboard of 1 m and a weight of approx 500 tonnes, 9 km north of the drilling
location (Engseth, 1989).
Conclusions
on icebergs
Icebergs may cause serious
damage to a platform. More information is needed about iceberg properties and
behavior.
TEMPERATURE
CONDITIONS
In the Barents Sea south of
740N temperatures down to -300 C have been registered (Iden and
Tønnesen, 1988), see figure 9. Extrapolation at an annual probability of
exceedence of 10-2 gives -350 C for Bjørnøya and -180 C for Tromsøflaket (Bjerke and Torsethaugen,
1989). These temperatures have an effect on the working environment,
operations, design, choice of materials and testing of materials. Provided the
temperature is taken into account, it should be a structural problem with the
material properties which are available today.
Changes in sea temperature
from the North Sea to the Barents Sea are minor. The differences have minor
consequences for most purposes.
FORECASTING
OF THE ENVIRONMENT
Weather
forecasts
When the Barents Sea was
opened for exploration drilling at winter time the operators provided reports
on their experience on how the weather forecasting had be carried out.
They had all experienced
the weather forecasting as being variable in quality. The general opinion was
that winter forecasting in the Barents Sea was of a somewhat poorer quality
than that of the North Sea (Aanstad, 1988 and Askedal, 1989). A verification of
daily forecasts to "Ross Rig" in the Barents Sea and
"Petrojarl" at Oseberg did not, however, show any appreciable
differences. Comparisons made between forecasts from DNMI and forecasts from
private companies do not show any systematic difference in quality (Aanstad,
1988). Bera (1988) was of the opinion that the use of a meteorologist onboard
the rig gave a better forecast than those that came from shore.
Ice
forecasts
In connection with
exploration drilling north of the 73rd parallel, the operators established
systems for monitoring ice that could represent a danger during drilling. This
included both icebergs and sea ice.
The elements that Mobil
(Armstrong, 1988) and Norsk Hydro (Engseth, 1989) employed in monitoring
were:
a) satellite observations
of the ice borders, conducted by DNMI
b) plane and helicopter
missions to look for sea ice and icebergs
c) supply ship that went to
the ice border and patrolled the area (Norsk Hydro)
d) satellite buoy on the
ice to see how it moved (Norsk Hydro)
e) ice forecasting from
DNMI and others
f) use of reports from the
coast guard (Norsk Hydro)
One of the conclusions was
that ice forecasting had a variable quality (Engseth, 1989).
Forecasting of icebergs is
not possible unless each iceberg is equipped with instruments so that one may
know where they are at any given time. What one must rely on are good radar
systems that can give warning of icebergs when they are at a certain distance
from the platforms. Knowledge of experience from other countries seems
essential.
Conclusions
on forecasting
The forecasting service in
the Barents Sea is not as good as that found further south. This gives a lower
safety level than what one is used to for operations sensitive to
weather.
EARTHQUAKES
There are only few
registrations of earthquakes in the Barents Sea (Bungum, 1988). Since 1987
there have, however, been local measurements that show that the Ringvassøy -
Loppa fracture is earthquake-active. However, the earthquakes here are so small
that they are not registered by the regular measuring stations.
The Barents Sea has low
values for design earthquakes compared with other parts of the shelf, (Bungum
and Selnes, 1988).
SOIL
CONDITIONS
Topography
and geotechnical conditions
Major parts of the seabed
in the Barents Sea show signs of ploughing caused by icebergs. Local pockmarks
have been recorded along some of the plough marks.
Surveys of the geotechnical
conditions in and outside the plough marks on Tromsøflaket show that the
ploughing process produces plastic deformations and stirring of the earth, and
thus local changes in the geographical characteristics. Installation of
platforms in plough marks may be difficult. It may also be difficultto establish
the correct geotechnical characteristics of the soil.
As in other parts of the
shelf there is stratification with sediment from clay to stones (Gunleiksrud,
1986 and Elverhøy and others, 1988). Drilling done by Statoil at Askeladden
shows low shear strength in the upper 2 m, and then harder clay further down.
From 10-25 m the soils are very stiff. The geotechnical characteristics of the
clay that were examined at Askeladden correspond to what is found in the North
Sea (Tjelta and others, 1983).
Sedimentary
strata
At Tromsøflaket there
are sedimentary strata at about 100-200 m (Gunleiksrud, 1986). Further north
and east it seems that the sediment depth diminishes (Solheim and Elverhøy,
1988).
Conclusion
on soil conditions
The Barents Sea differs
from the North Sea in that it has large plough marks on the seabed. This is a
problem also found at the Haltenbanken. The sediment strata in the Barents Sea
are considerably smaller than what is usual. This could provide simpler
foundation solutions.
FINAL
CONCLUSION
Exploration drilling and
possible future production of oil and gas in the Barents Sea has presented on
the shelf. With regard to knowledge about the environmental conditions, it is
probably in connection with icebergs that we have the largest gaps. Icebergs
also may cause the greatest damage. Wave loads, however, will dominate design
in the southern Barents Sea as they do further south.
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