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Pipeline Planning, Sizing and Performance Criteria |
Pipeline performance requirements and design criteria are
defined in terms of the client's water needs. These criteria will include water
temperature, water purity, flow, reliability, lifetime, location and budget
restrictions. In most cases, all of these parameters are not fully defined at
the start of the design process; pipeline flow, operating budgets, and even
location are commonly dependent upon onshore facilities that are still in the
planning, design and permitting phase.
A study is performed to quantify the choice of best
pipeline length, diameter, wall thickness and operational lifetime for a given
project's flow rate, capital budget and operating budget. The results of the
study often present the client with tradeoff decisions such as saving money on
capital versus lowering operating costs. MOE assists the owner in finding the
pipe dimensions that best meet his objectives. Other tradeoff studies are performed
as necessary for each individual project to best economically and technically
fit the pipeline into the client's overall operation. Even the location may be
uncertain during the planning phase; since the pipeline costs are significantly
affected by the location, Makai identifies and provides critical information
relative to pipe site selection. Based on previous pipeline costs, Makai can
price various pipeline and pumping options such that the client can be well-informed
for these critical decisions.
Important site characteristics include bathymetry, bottom
roughness, soil conditions, slope, current profiles, obstacle location, wave
conditions, environmental restrictions and shoreline geometry; these and other
site conditions greatly influence the pipeline design and cost. A review of all
existing offshore data (including data from adjacent or related areas) plus a
site visit and dive in the shallow water will frequently suffice for an initial
survey. If bathymetry and bottom data are not available, it may be necessary to
conduct a survey for this information. In addition, depending upon how much is
known of the oceanographic characteristics in the area, it may be necessary to
commence current measurements at a few key locations.
A preliminary design is performed that concentrates on the
critical aspects of the project that most directly affect the pipeline's
performance and costs. The final output of the design includes preliminary
drawings, pipeline routing, an initial opinion of probable construction cost and a conceptual method for
the deployment. A quickly and easily deployed pipe is the key
to an economically viable pipeline.
The importance of the deployment on the offshore pipeline
cost and design cannot be overemphasized. The major pipeline costs occur during
deployment, and there is an accompanying high concentration of activity and
risk. This concentration of cost, structural loading of the pipeline and risk
all occur in a relatively few days at sea. As a result, the deployment of the
pipeline is often the major cost and design factor. Weights, loads, buoyancies
and material strains are carefully balanced during the deployment such that the
pipeline can both survive the deployment and serve properly once in place. Pipe
joints are placed at critical points that ease pipeline handling and are avoided
at points exposed to high loading.
An important aspect of the preliminary design is to
evaluate whether the pipeline is feasible and whether it will meet the
requirements of the client by delivering water within his performance and budget
restrictions. In order to complete this determination, a variety of pipeline
configurations may be studied. Makai often provides innovative but practical
solutions to difficult technical and budgetary problems. The preliminary design
gives the client a first cut at the overall performance and costs such that this
information can be integrated with his overall project.
The preliminary design identifies critical oceanographic
and site information that is needed for the final design and installation. These
needs may include the precise location of key obstacles on the ocean floor,
measurement of shoreline geometry, collection of ocean current data, and
verification of bottom slopes, soil conditions or roughness. Surveying equipment
may include SCUBA, manned submersibles or remote operated vehicles, ship
deployed bottom samplers, acoustic bathymetry, sub-bottom profilers, side
scan systems, and precision bottom roughness samplers. Makai has used all these
methods in the past to determine route information. Not all these methods are
required, nor desirable. Makai selects a combination that best fits the
information needed, survey equipment available and project costs. Makai directs
the survey to be sure that the quality is adequate for the design; whether Makai
performs the survey directly or through a marine survey company depends upon
whether the service is available and at what cost.
Surveys are not inexpensive, and they can be a key factor
in the overall budget. For this reason, the need for bottom information plays an
important part in the design of a economical pipeline. Makai has designed
pipelines for steep and rough bathymetry that are independent of the bottom
- thus minimizing surveying needs and lowering the overall project costs.
Such decisions are made on a case-by-case basis.
In the final design phase the design plans (drawings),
specifications, and an opinion of probable construction cost are prepared. Careful attention is paid
to every detail such that the hardware designed can be successfully deployed and
operated over the desired lifetime. Details include wave loading, corrosion,
pipeline fatigue, water flow dynamics on pump start-up and shutdown,
maintenance, electrical routing, and deployment loads.
If the client wishes a formal contractor bidding process,
the design plans, specifications, and deployment guidelines are suitable. Makai
has also worked for clients who have selected the contractor prior to or during
the design process. Either process has its advantages and disadvantages depending upon the size of
the overall project, the client's relationship with the contractor, and whether
it is a public or private sector project.
During the design phase, Makai utilizes a variety of
computer programs developed for deep water pipeline analysis and deployment. The
drawings are also prepared and plotted using computer aided drafting (Autocad).
Most deep water pipeline designs use polyethylene as the
pipe material. Its main advantages are its buoyancy in seawater, flexibility for
installation, high strength, resilience to shock, corrosion resistance, high
fatigue strength, and almost unlimited lifetime underwater. Polyethylene design,
however, is quite different from design with other materials: polyethylene is a
viscoelastic material with properties that are a strong function of both
temperature and time. Acceptable design loads for a cool pipe for a short time
are considerably different from design loads for a warm pipe over a long time. A
design approach for a plastic pipeline that must reliably survive the deployment
process must take into account these unique characteristics; this is a design
strength of Makai Ocean Engineering.
The risks and costs of the at-sea portion of the
installation can be quite high because of the concentration of critical tasks,
the quantity and variety of equipment involved, and the number of personnel
working. An unplanned delay results in significant additional costs and some
mistakes could end up causing the loss of the pipeline. These problems are
inherent with all marine construction; proper planning of the deployment is a critical
step resulting in lower risks and cost savings.
A detailed deployment plan, written by the contractor, is
necessary. The guidelines prepared by Makai form a basis for this plan, but the
exact personnel, equipment, and scheduling of the contractor need to be
included. A step-by-step procedure is written detailing each
individual's responsibility, materials required, and the major equipment
involvement. Makai reviews this deployment plan for the owner to assure that the
pipe is properly handled and that it will be correctly placed on the bottom.
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Fabrication and Deployment |
Since the deployment process usually
causes the highest stresses on the pipeline, Makai
includes the deployment process as an integral part of the design, and is
concerned that it be installed properly. Although relatively few contractors
have installed pipelines using Makai's methods, most properly equipped and
conscientious contractors could do so. Using the considerable experience gained
during our previous pipeline deployments, Makai can help guide the contractor to
reduce the cost and equipment risk inherent during the installation process. The final role that Makai serves
during the deployment is dependent upon the contractual relationship between the
owner and the marine contractor, and the contractor's confidence and experience. Makai
typically serves as the owner's representative, inspecting and observing the
contractor's work, and checking the adequacy of the contractor's deployment
planning.
Other construction tasks are involved in addition to the
deployment. Pump installation, shoreline installation, pipeline component
fabrication, etc. are equally important but less critical relative to risks.
Makai usually performs construction observation for these tasks.
Pipeline inspection starts with the pipe fabrication by
checking each component and ends with the overall performance check of the
installed pipeline. As sections of the pipeline are completed, the pipe is
checked relative to meeting the specifications and whether it will perform
satisfactorily for the client. Onshore, shoreline, and nearshore portions of the
pipeline are visually inspected as they are assembled and completed. The deep
portion of the pipeline can be inspected with an undersea submersible or ROV,
although this may not be necessary for all pipeline designs. The final
pipeline's performance is checked in detail by pumping water and observing water
flow rate, power consumption, water temperature, water quality, and pump start
and stop dynamics.
The demand for deep water marine intake pipelines is
increasing as the cold water resources find greater uses in energy, aquaculture,
and desalination. CWP technology has improved in the recent years to the point
that there are numerous operating pipelines, and this technology is continuing to
improve.
The deep water polyethylene pipeline techniques that have
been developed and demonstrated for OTEC and aquaculture intake pipelines are
applicable to outfalls and transmission pipelines in both moderate and deep
depths.
Makai Ocean Engineering has been a key contributor to deep
water pipeline technology development and demonstration, and Makai is continuing
to work on larger and lower cost pipelines. No company has more deep water
polyethylene pipe experience. Makai is eager to pursue any challenging pipeline project at
any location.
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