160
H.Grann
basis chosen for the burying of the pipeline was an exposure probability of the top of
the pipeline of 5% in the lifetime, i.e. once in 1000 years. This implied that the minimum cover over the pipeline was to be 2 m. For the purpose of burying the pipeline
permanently under the sea floor, a trench of varying depth and with a trench bottom
width of 12 m had to be dredged. In addition, a loo-m-wide vessel access channel had
to be cut through the sandbank. In total some 3.4 million m 3 of material containing
sand, clay and peat was removed to be used for beach nourishment, disposal on-shore,
or to be brought to an interim storage area off-shore for later backfIlling, dependent on
the type of the dredged material and its environmental impact.
Depending on water depth and local conditions, the pipeline route was divided in
sections, each requiring different construction methods and equipment.
The near-shore section is 3 km long with water depths varying between 9 and 18 m.
This area constitutes a natural shelf in front of the sandbank and is exposed to severe
wave conditions. In this section, large seagoing trailer suction hopper dredgers were
used. The material was loosened mechanically or by waterjets, hydraulically extracted
and loaded into the hopper. In areas where peat-containing material was expected, a
bucket dredger was used in order to avoid peat from being set adrift, contaminating the
beaches or, even worse, causing problems for the local fishermen. Peat-containing material was loaded in barges and brought on-shore in Emden 100 krn away.
The sandbank section has a typical profile with depths less than 9 m, in some areas
only 3 m, forming a shallow water barrier at the ocean side of the islands. This section
is about 2.8 km long and is characterized by the prevailing presence of breaking waves
and by tide-dependent restrictions on marine traffic. In the sandbank section, a cutter
suction dredger was used. The cutter suction dredger loosens the material by means of
a cutting head and conveys it hydraulically to seagoing barges for transport out of the
area. This dredging method is particularly effective in areas where wide trenches are to
be dredged. In addition, trailer suction hopper dredgers were used in parts of the sandbank and for maintenance dredging. The tidal channel is about 4 km long and is the
most narrow passage between the islands, with a water depth of up to 22 m and high
current velocities which require special construction methods and equipment. In the
tidal channel various size trailer suction hopper dredgers were used. A large stiff clay
ridge in the tidal channel was dredged using a cutter suction dredger.
The Accumersieler Balje section is 2 km long and is the inner shallow part of the
landfall with water depths of 2-5 m and strong tidal currents. In the Balje section two
spud pontoon dredgers equipped with backhoes were employed. This method was chosen due to the shallow water depth and large inclusions of peat in the dredged material.
All peat-containing material was transported to Emden and used in the construction
of an artificial biotop, which Statoil made as an environmental compensation measure.
About 400000 m 3 of clay material was disposed of 60 krn off-shore on a permanent
dumping site. All sandy material was brought out of the National Park area to an interim off-shore storage area for later use as pipeline trench backfilling material.
15.5.2
Pipelaying in the Tidal Channel
The off-shore pipelaying was performed by large pipelaying barges with a capacity to
lay up to 3.5 km of large diameter pipe per day, virtually independently of seasonal
H.Grann
basis chosen for the burying of the pipeline was an exposure probability of the top of
the pipeline of 5% in the lifetime, i.e. once in 1000 years. This implied that the minimum cover over the pipeline was to be 2 m. For the purpose of burying the pipeline
permanently under the sea floor, a trench of varying depth and with a trench bottom
width of 12 m had to be dredged. In addition, a loo-m-wide vessel access channel had
to be cut through the sandbank. In total some 3.4 million m 3 of material containing
sand, clay and peat was removed to be used for beach nourishment, disposal on-shore,
or to be brought to an interim storage area off-shore for later backfIlling, dependent on
the type of the dredged material and its environmental impact.
Depending on water depth and local conditions, the pipeline route was divided in
sections, each requiring different construction methods and equipment.
The near-shore section is 3 km long with water depths varying between 9 and 18 m.
This area constitutes a natural shelf in front of the sandbank and is exposed to severe
wave conditions. In this section, large seagoing trailer suction hopper dredgers were
used. The material was loosened mechanically or by waterjets, hydraulically extracted
and loaded into the hopper. In areas where peat-containing material was expected, a
bucket dredger was used in order to avoid peat from being set adrift, contaminating the
beaches or, even worse, causing problems for the local fishermen. Peat-containing material was loaded in barges and brought on-shore in Emden 100 krn away.
The sandbank section has a typical profile with depths less than 9 m, in some areas
only 3 m, forming a shallow water barrier at the ocean side of the islands. This section
is about 2.8 km long and is characterized by the prevailing presence of breaking waves
and by tide-dependent restrictions on marine traffic. In the sandbank section, a cutter
suction dredger was used. The cutter suction dredger loosens the material by means of
a cutting head and conveys it hydraulically to seagoing barges for transport out of the
area. This dredging method is particularly effective in areas where wide trenches are to
be dredged. In addition, trailer suction hopper dredgers were used in parts of the sandbank and for maintenance dredging. The tidal channel is about 4 km long and is the
most narrow passage between the islands, with a water depth of up to 22 m and high
current velocities which require special construction methods and equipment. In the
tidal channel various size trailer suction hopper dredgers were used. A large stiff clay
ridge in the tidal channel was dredged using a cutter suction dredger.
The Accumersieler Balje section is 2 km long and is the inner shallow part of the
landfall with water depths of 2-5 m and strong tidal currents. In the Balje section two
spud pontoon dredgers equipped with backhoes were employed. This method was chosen due to the shallow water depth and large inclusions of peat in the dredged material.
All peat-containing material was transported to Emden and used in the construction
of an artificial biotop, which Statoil made as an environmental compensation measure.
About 400000 m 3 of clay material was disposed of 60 krn off-shore on a permanent
dumping site. All sandy material was brought out of the National Park area to an interim off-shore storage area for later use as pipeline trench backfilling material.
15.5.2
Pipelaying in the Tidal Channel
The off-shore pipelaying was performed by large pipelaying barges with a capacity to
lay up to 3.5 km of large diameter pipe per day, virtually independently of seasonal