1
INSTALLATION OF PIPELINE AND BURIAL USING
JETTING
This Annex presents information on the potential
installation method for the proposed submarine water main from South Soko
Island to Shek Pik. The submarine
water main is proposed as part of the construction of a Liquefied Natural Gas
(LNG) Terminal on South Soko Island.
The intention of the paper is to provide background
information on typical jetting works associated with the follows:
·
Pipelay
and pre-test;
·
Jetting
“spead”;
·
Deployment
and operation of jetting; and
·
Post
trench rock dumping.
In sections of the route where burial is proposed
using a water jetting technique, the pipeline will first be laid on the seabed
using the pipe lay barge. Prior to
trenching, the pipeline will be pre-tested to ensure its strength and leak
integrity, enabling easier rectification (rarely necessary) of any faults
exposed by the test. The pipeline
will be left flooded with water to increase its weight and aid its settlement
into the trench as the jetting progresses.
The jetting “spread” is typically based on a flat
barge that will be towed into position over the pipeline at the start point and
then anchored using a 4-anchor pattern.
The forward anchors will be deployed some distance ahead of the barge
and the stern anchors will be positioned close to the stern of the barge. These distances are variable depending
on water depth, current conditions and the presence of any constraints. The contractor will determine these
details in the course of its detailed design and will continually verify/update
its practices and procedures in response to the results being obtained on site
and any unforeseen or changed conditions.
The jetting machine will be chosen by the contractor
based on its past experience with site conditions comparable to those existing
along the pipeline route. There are
many different designs of jet sled that have been developed by a number of
general and specialist contractors operating in the pipeline industry. Whilst they all operate using the same
principles, each one features several variables including:
·
number
and positioning of water jets;
·
water
pressure operating range;
·
arrangement
of eductor tubes;
·
airlift
or water venturi eductor tubes;
·
depth
of “cut” per pass and number of passes required for required depth;
·
length
of route “blocks”; and
·
speed
of forward travel.
These variables are adjusted to suit specific site
conditions. While Contractors can
predict reasonable starting values for the site based on site investigation
results, it is probable that adjustments will be required on commencement of
work and even in the course of work to obtain optimum trenching performance
and/or to ensure that water quality standards are met.
Because designs of jetting machines are proprietary
to the various specialist contractors, design details are not given out for
fear of copying of designs by competitors.
CAPCO consulted several contractors, including Saipem and Leighton, both
of which constructed offshore pipelines in
Known:
Pump pressure:
400 psi
Pump flow rate:
4000 USG min-1 => 15,140 L min-1
Eductors output:
20,000 L min-1 with 1.5 m head @~3.5 psi
Assumed:
That all liquefied soils are ejected from the trench.
Trench area:
2 m2 => 1 m deep and 2 m wide per pass
Jetting speed:
0.35 m min-1
Trench Volume:
0.7 m3 min-1
Eductor discharge Soil/water %: 10 %=> 2 m3 min-1
soil
Output to environment:
Pump pressure:
400 psi
Pump flow rate:
4000 USG min-1 => 15,140 L min-1
Eductors output:
20,000 L min-1
Water output:
36,000 L min-1
Eductor discharge height above seabed: 2 m
Volume of soil in water column: 0.7 m3 min-1
Potential volume @ 10% soil in water column: 2 m3 min-1
It is found that the monitoring results of jetting
operations were comparable to the model results predicted for the EIA for the
Towngas project. The LNG project
used the similar approach to the Towngas project in the modelling and hence it
is expected that the assumptions in the water quality assessment and model
would be realistic.
From previous experience in HKSAR the soils were
dispersed in a 70 m radius with 80% of the material falling from the water
column. By the 150 m radius the SS
concentrations were consistently below the EPD requirements, even when there
was a 1.5 m s-1 current effecting greater dispersion.
1.4
Deployment and Operation of Jetting Spread (including
anchoring)
The jetting spread will be deployed at the start
location of the section to be jetted and the support barge will be pulled
forward on its forward anchors, towing the jet sled behind. The position of the barge will be
controlled by maintaining tension on the stern anchors as the forward anchor
cables are winched in. The progress
of this activity is similar to that of pipe laying, with the exception that
there is no need to pause at each pipe joint. The barge can be moved ahead
continuously, with anchor handling tugs working to recover and relocate bow and
stern anchors (duplicate sets of anchors are used) as the limits of anchor
cable movement are reached. The
contractor may elect to perform jetting over limited “block” lengths of the
route, completing a number of passes to achieve the required depth in one area
before moving to the next.
Alternatively, it is possible that for the site conditions on this
project, completing each pass over the entire section will produce better
overall results.
The pipeline protection design concept for this
project identifies the need to protect the pipeline from third party contact,
particularly dropped and dragged anchors.
Where jetting is used for trenching, there are some areas where it was
assessed that natural backfill may be either insufficient or might not occur
sufficiently rapidly after construction, to provide the necessary
protection. Accordingly,
supplementary protection may be provided by partially backfilling the jetted
trench with a layer of rock, nominally 1m over the top of the pipe. The requirement for all subsea
constructions in
This design has been assessed as suitable to protect
the pipeline from small vessel activity.
In areas where larger vessels (and therefore larger anchors) are
expected, a completely different design will be utilized, incorporating a
pre-dredged trench and a larger rock mound protecting the pipeline. In such cases, the design requirement to
finish the construction level with the surrounding seabed will be maintained.
1.2
HKLNG Project Construction Approach
The pipeline construction will be contracted to an
experienced, reputable offshore pipeline contractor on the basis of a detailed
construction specification.
Included in the specification will be the requirement to follow the
environmental management plan developed for the project and, particularly, an
obligation to meet the defined water quality standards (and other standards)
mandated by the relevant HKSARG departments. Also included will be a requirement for
the contractor to utilize proven equipment and work methods.
Water quality modelling was performed as part of the
EIA to enable a selection of acceptable technologies in terms of meeting water
quality standards and to provide guidance on developing environmental
management plans and mitigation methods.
Ultimately, it will be the responsibility of CAPCO and its Contractor/s
to monitor the work closely and adjust the work procedures as required to
ensure that water quality standards are met to the satisfaction of EPD and
other relevant authorities.
A series of images show the variations in design of
jetting equipment.
Figure 1.1 “Arabian
Leopard” 12 – 30 inch pipeline Jet Sled (OES Equipment)
Figure 1.2 “
Figure 1.3 “Sumatran Tiger” 20 to 42 inch pipeline
Jet Sled (OES Equipment)
Figure 1.4 “Canyon Horizon” Pipe Jetting
Barge(Horizon Offshore equipment)
Figure 1.5 “Canyon Horizon” Pipe Jetting Barge
(Horizon Offshore equipment)
Figure 1.6 “Castoro 10” Pipelay and trenching barge
(Saipem Equipment)
Figure 1.7 “DJS 1” Diverless Jet Sled for up to 60
inch pipelines (Saipem Equipment)