DEEPLINE is MARINTEK’s Strategic Institute Programme (SIP) for the development of new knowledge, technology and tools related to the design, installation and operation of subsea pipelines in deep waters and rough environments.

In recent years, more and more pipelines have been installed on uneven seabed, both in deepwater fields and in the inshore zone. This trend is continuing in new field developments, as can be seen in the seabed around the Ormen Lange gas field.

Designing cost-effective pipeline systems for transporting well flow in regions where the seabed is uneven and production temperatures are high requires new work procedures. The use of more advanced analytical methods in combination with a revised regulatory framework would offer significant savings. The probability of failure as a result of temperature or pressure gradients could be evaluated more accurately if such methods were adopted, and this in turn would reduce the need for seabed intervention.

Because of the high degree of uncertainty regarding loads and boundary conditions (currents, and interaction between pipeline and seabed), and because new pipeline concepts are being planned for new fields, new methods and tools will probably be required. Tools of this sort will be essential for selecting the optimal pipeline route, while they will deal with the regulations and take environmental considerations into account in a safe and professional manner.

New field developments are also highly uncertain with respect to environmental loading conditions. Current forces in particular may change dramatically, both with depth and time, as well as along the pipeline route.

Given the uncertainties related to loading conditions and pipe capacities, new procedures, tools and techniques are needed for planned deepwater field developments in order to take design, safety and environmental issues fully into account.

Objectives

Future pipelines for deep water applications will need to be designed to handle unprocessed well flow with aggressive constituents, free spans with point loads, large current loads which could cause dynamic excitation, and laying operations involving gross plastic deformations. These conditions offer significant challenges to pipeline strength and endurance.

The main objectives of the programme are as follows:

To develop improved methods and procedures for the analysis, design and operation of deepwater pipelines
To make the newly developed methods and procedures available as computer tools suitable for the cost-effective and safe design, installation and operation of deepwater pipelines

Key analysis functionality of the new computer tools will include:

Design and installation analysis of pipelaying operations
Stability on the seabed
VIV assessment of free spans, including effects of longitudinal pipeline expansion under thermal/pressure loads (snaking, upheaval buckling)
Reanalysis of offshore pipelines, taking relevant inspection data into account

Activities

DEEPLINE will prioritise the areas of PIPE CONCEPTS, FREE SPANS and LAYING.

However, efforts will also be put into transferring this new knowledge from the design and installation phase, as represented by the above activity areas, into the operational phase. Hence, the area of INSPECTION AND MONITORING will also be a point of focus, but this will require close cooperation with existing companies that are already operating at the leading edge of this business area.

DEEPLINE’s main areas of activity and tentative specific research topics are briefly described in the following paragraphs.

Pipe concepts

Pipe-in-pipe cross-section.

A range of new concepts for pipelines is being developed for severe conditions, using very different materials in various combinations.

The pipe-in-pipe cross-section, as illustrated in the figure to the right, is one example of such a new pipe concept.
Furthermore, in order to model and analyse installation and operation scenarios, the strength and endurance parameters of the different components in a given pipe cross-sectional design needs to be investigated, which is typically handled by small-scale laboratory tests.

Free spans

This activity will cover relevant aspects of the detailed design and analysis of free spanning pipelines.

In structural design, three factors are particularly important:

Material behaviour
Boundary conditions
Loads

Where pipelines are concerned, the last two factors relate in particular to a) pipe-seabed interaction and b) hydrodynamic loading, water kinematics and temperature and pressure effects. These aspects are also discussed in the following section.

Special-purpose tools may be developed for detailed analyses of free spans or to study, for example, pipeline snaking and local buckling during laying.

Laying

Local buckling during laying.

Taking Ormen Lange as an example, new field developments on the Norwegian continental shelf often involve subsea installation to ever more challenging seabed areas. It is becoming steadily more difficult to select the optimal pipeline route for bringing oil and gas ashore. For the Ormen Lange gas import pipeline several dozen alternative routes have been studied in detail.

For some time, the offshore engineering industry has been asking for an efficient tool for pipelaying analysis. Such a tool should, on the basis of the planned pipeline route, be capable of rapidly generating a series of static analyses that would permit the following aspects to be studied:

The route to be followed on the surface by the pipelaying vessel
Laying tension
The lateral stability of the pipeline during laying
The effects of plastic deformation of the pipe (reeling, deepwater laying) and its influence, e.g. on spiraling
The plastic strain history on any point of the pipe

The tool should make extensive use of 3D visualization.

Various analytical procedures may be considered, such as a fully interactive mode whereby the program presents the result of each analysis before the user decides how to proceed. A trial and error procedure may easily be implemented to allow the user to test a laying procedure and go back and try out alternatives if necessary.

The description of the as-laid pipe obtained from the laying simulation may be exported back to the pipeline routing tool, so that further analyses can be carried out on the preferred as-laid configuration.

Inspection and monitoring

State-of-the-art design procedures require accurate inspection techniques for the seabed, global pipeline geometry and internal corrosion effects. In addition, efficient and accurate tools are needed to check the status of the pipeline on the basis of the results of the inspection vis-à-vis the design criteria.

The current methodology for assessing pipeline condition by inspection can be summarized as follows:

The survey data are collected
The data are processed, and show:
• Free spans along the line
• Cross-section of pipeline relative to the seabed
• Video/photo of the pipe along its route
• Pipeline co-ordinates
• Condition of coating and anodes
• Pipeline wall thickness along the line
The survey data are compared with previous surveys in order to assess the development of free spans, pipe movement and pipeline damage

This method suffers from the following disadvantages:

The decision to request an additional survey has to be based on engineering assumptions from experienced personnel, as a probabilistic analysis is not easy to perform
The acceptance criteria are often based on initial design with regard to both boundary conditions and design criteria, e.g. different rules and requirements are involved

For the above reasons, we aim at transferring updated and new knowledge, technology and tools from the design and installation phase also into the operational phase of subsea pipelines.

MARINTEK contact: Egil Giertsen

(Article in MARINTEK Review No 1-2003)

Published January 29, 2005