Zhiwen Chen and Romke Bijker
Alkyon Hydraulic Consultancy and Research
P.O. Box 248, 8300 AE Emmeloord
The Netherlands
E-mail: bijker@alkyon.nl; chen@alkyon.nl
tel. 00 31 527 248100; fax 00 31 527 248111
Abstract: Self-lowering of pipelines has
for many years been considered as a feasible and cost effective way to achieve
pipeline stability with reduced weight coating and to reduce trenching costs. In
the Joint Industry between 1991 and 1994, the numerical model PIPESIN was
developed for predictions of free span development and pipeline lowering. The
model simulates the pipeline seabed interactions in the time domain for the
period from installation until a stable pipeline seabed configuration has
occurred. All important physical processes are modelled integrally.
Nevertheless, the PIPESIN model has a number of limitations in practical
applications. Firstly, in the model the pipeline is assumed to be placed on a
horizontal seabed and the global seabed level is assumed to be constant. The
model is therefore not able to describe the behaviour of pipelines on (stable or
migrating) sand waves. Secondly, the model simulates the pipeline behaviour from
the moment of pipe laying. The model does not enable predictions of the
behaviour of an existing pipeline and the development of an individual free
span. These limitations can be important in practical applications, for example,
if free spans are observed in an annual survey, we would like to predict the
further behaviour in order to assess the requirement for remedial measures.
Further, validation of the PIPESIN model has been limited to pipelines with
large diameters.
This paper presents an update of the original PIPESIN model which
overcomes the above limitations. The following improvements have been carried
out. Actual pipeline levels and seabed levels can be specified in the model.
They can vary along the pipeline. This enables simulations to be started at any
time during the pipeline operational phase. For example, the model can simulate
how an existing free span will develop. Another major improvement is that the
global seabed level may not only vary along the pipeline but may also vary in
time. This enables simulations of interactions between offshore pipelines and
migrating sand waves. The model has further been calibrated with additional
field data from both large and small diameter pipelines.
The paper will present a few examples of practical applications.
Keywords: offshore pipelines,
dynamic seabed, self-lowering of pipelines, free spans, sand waves
The PIPESIN model was originally developed on
the basis of the PICASS model by a joint venture between Delft Hydraulics and
Danish Hydraulic Institute, in a Joint Industry Project between 1991 and 1994 ‘Free Span Development and
Self-lowering of Offshore Pipelines’. The model was developed for
predictions of free span development and pipeline lowering, see Bijker et al
(1991), Hansen et al (1995) and Klomp et al (1995).
It is a three dimensional model, simulating
self-lowering and free span development of a pipeline on a horizontal seabed.
The model simulates a section of pipeline with a length typically of 500 m to
1000 m. The model
simulates the pipeline seabed interactions in the time domain for the period
from installation until a stable pipeline seabed configuration has occurred.
All important physical processes are modelled integrally in the model. The model predicts initial lowering of a pipeline due to its weight and laying forces. Input wave and current fields are transformed to the seabed and the resulting sediment transport is calculated. Onset of scour is determined based on the wave and current conditions and pipeline embedment. The model includes generation and development of free spans, tunnel erosion, leeside erosion and upstream backfilling. Pipeline deflection is calculated dependent on the seabed configuration and pipeline mechanics. Geotechnical failure of pipeline support is calculated dependent on soil conditions and pipeline forces. The model was calibrated with field data from the Zeepipe and Nogat pipelines.
In the practical applications, the original PIPESIN model has the following limitations:
l Simulations in the model start from the moment of pipeline installation. In practice, one would like to start the simulation from any moment during the operational phase. For example, if free spans were observed in an annual survey, one would like to know whether these free spans would further develop and disappear so that the necessity of remedial measures can be assessed (Bijker et al 1990).
l It is assumed in the model that a pipeline is placed on a horizontal seabed. In practice, the seabed is often irregular. Sand waves and mega-ripples are present in some areas.
l The global seabed level is fixed and does not change in time. In practice, the seabed level may change as the result of seabed erosion due to storms or migrations of sand waves.
l The validation of the model was confined to pipelines with large diameters (24” and 36”).
The original model needed to be modified in order to account for the effects of dynamic seabed like migrating sand waves.
This paper
presents an update of the PIPESIN model which overcomes the above limitations.
The paper will also present a few examples of practical applications.
To overcome the above shortcomings, the PIPESIN model has been upgraded by Alkyon in the last a few years. Important improvements are the following:
l Initial actual pipeline levels and seabed levels can be specified in the model. They can vary along the pipeline. This enables simulations to be started from any time during the pipeline operational phase. For example, the model can simulate how an existing free span will develop.
l In the model, the global seabed may vary in time. The effect of migrating sand waves can be simulated.
l The model was further calibrated and adjusted with field data from pipelines with smaller diameters.
The initial conditions for the model are defined by the user and can be arbitrary. The configuration of a pipeline relative to the seabed is defined by global seabed level, local seabed level, scour level and pipeline position (see Figure 1). The global seabed level is the seabed level with some distance from the pipeline and it is not influenced by interactions of pipeline and the seabed. The local seabed level is the level of seabed next to the pipeline which may vary as the result of leeside erosion and upstream backfilling. The scour level is the seabed level underneath the pipeline which may vary as the result of tunnel erosion below the pipeline (2D vertical scour) and free span development (3D longitudinal scour). The initial global seabed level, local seabed level, scour level and pipeline position may vary along the pipeline.
The global seabed levels are input in the model and can vary along the pipeline and in time. For migrating sand waves, the global seabed level as a function of time can be calculated from the initial seabed profile and an user-defined sand wave migration rate (constant or varying in time). The global seabed level is checked at each time step. The local seabed level is calculated on the basis of the global seabed level at each time step and the leeside erosion and upstream backfilling.
The sequence of calculations is shown in the
flow diagram in Figure 2.
The model was further
calibrated using additional field data including survey data for smaller
diameter pipelines. The data was collected from pipeline surveys in the North
Sea. The data covered major offshore pipelines in the North Sea which have been
installed during the last 10 years and provides the self-lowering data for a
wide variety of pipelines under various environmental and soil conditions:
l
Pipeline diameter ranging between 2” and 40”
l
Water depth ranging between 15 m and 45 m
l
Sediment size ranging between 0.06 and 0.5 mm
l
Silt fraction of sediment ranging between 0% and 40%.
A comparison between the
model and the field data shows that the predictions agree generally well with
the measurements except for a few cases which will further be discussed below.
The differences between the model and the measurements are generally within 20%
or 0.1 m.
In the following cases,
considerable differences are found between the predictions and the measurements:
l
Seabed with coarse sediments (D50 larger than 0.3 mm)
l
Dominant current direction almost parallel to the
pipelines
The results
indicate that the scour process for large grain sizes is not well described in
the model. Possible aspects include e.g. sediment transport module, onset module
and 2D/3D module (the 2D scour depth is not very much affected by the grain size
in the model formulations. In the present model formulations, the scour depths
are assumed to be proportional to sinα to the power 1.4-1.5 with α
being the pipeline flow angle. The laboratory experiments show that the scour
depth is probably related to sinα to the power 0.5. Further, in the present
formulation, the leeside erosion geometry is fixed with a constant slope. Field
data show that steep scour slopes may develop for a small pipe flow angle. These
aspects need to be considered in the future development.
The PIPESIN model can be used in many practical applications, for example:
l
To assess the feasibility of self-lowering as an
alternative to traditional mechanical trench;
l
To assess free span development and calculate the
maximum potential free span length and related duration;
l
To simulate interactions of a pipeline and migrating
sand waves;
l
To assess feasibility of use of a spoiler to increase
self-lowering and improve pipeline stability.
The updated model can be
used as a tool in the design of pipelines and cables crossing a sand wave area.
Large amount of work on the seabed is often required to install a pipeline
through such an area. The specification of this work, i.e. pre-sweep dredging
and post-lay trenching is generally a major technical challenge and has an
important influence on the cost-effectiveness and long term safety of the
pipeline. Large scale movement of sand waves could result in pipe exposure and
generation of free spans. Large costs will be incurred if subsequent
intervention is required.
The updated PIPESIN model
facilitates optimisation of the vertical pipeline position relative to the
seabed. Reduced dredging and trenching requirements result in considerable cost
saving (Bijker et al 1994, Bijker et al 1995, and Chen & Bijker, 1998).
This study was carried out for the Dutch Authority to continue development on PIPESIN in relation with a pipeline management aspect: what to do with abandonment of pipelines. The program was adjusted in such a way that one can forecast the future behaviour of the pipeline on the basis of a hindcast of same pipeline. The primary goal of the model is to predict long term (20 to 30 years) behaviour of offshore pipelines which are to be abandoned. The results of the model enables determinations of required survey frequency after abandonment. It also enables proper formulations of required maintenance strategy.
The model can also be used
to predict the development of an existing free span. Free span field data are
being collected. These data contain information of free span development either
over a winter period or over a few years. The data will be used to validate the
free span module in the PIPESIN model. Until now, it has not been possible to
directly calibrate the free span module due to the absence of this type of field
data. It has been assumed that the development of free spans may be reasonably
predicted since the integrated self-lowering behaviour is well predicted.
Although it is an indication that the individual processes may be well modelled,
if the program is to be used to predict free span development, it is required to
validate the individual processes independently.
This paper
presents an update of the PIPESIN model which simulates the interactions of
offshore pipelines and irregular and dynamic seabed.
The following
improvements have been carried out. Actual pipeline levels and seabed levels can
be specified in the model. They can vary along the pipeline. This enables
simulations to be started at any time during the pipeline operational phase. For
example, the model can simulate how an existing free span will develop. Another
major improvement is that the global seabed level may not only vary along the
pipeline but may also vary in time. This enables simulations of interactions
between offshore pipelines and migrating sand waves. The model has further been
calibrated with additional field data from both large and small diameter
pipelines.
The upgraded
model can be used in many practical applications.
The model will be further developed by Alkyon. Emphasis of the next phase will be:
l
Improvement of the model performance under specific
soil and environmental conditions: for example, coarse sediments and parallel
flow conditions;
l
Effect of seabed with inhomogeneous soil conditions. It
is assumed in the present model that the soil conditions are homogeneous.
l
Accurate and reliable predictions of individual free
span development.
References
Bijker, R, R.J. Van Foeken, A.H. Van Der Pal, D. Schaap, C.A. Staub (1990), Pipeline Sandwave Interactions. Pipeline Technology Conference, Ostend, Belgium.
Bijker, R., C. Staub, F. Silvis, R. Bruschi (1991), Scour Induced Free Spans, Offshore Technology Conference, OTC 6762, Houston.
Bijker, R., Z. Chen, H. Moshagen, (1994), Morphology and Pipeline Design Through a Tidal Inlet, the Europipe Case, OMAE 1994.
Bijker, R., Z. Chen, J. Spiekhout (1995), Cross the Eems With a 42” Gas Pipeline, 20 Years Learning Experience, OMAE 1995.
Chen, Z. and R. Bijker (1998), Probabilistic Design of Burial Depth for Offshore Pipelines and Cables, OMAE 1998, Lisbon.
Hansen, E.A., W.H.G. Klomp P.F. Smed, Z. Chen, M.B. Bryndum, , and R. Bijker (1995), Free Span Development and Self-lowering of Pipelines, OMAE 1995, Copenhagen.
Klomp, W.H.G., E. A. Hansen, Z. Chen, R. Bijker, M.B. Bryndum (1995), Pipeline Seabed Interaction, Free Span Development. ISOPE 1995, The Hage.
Fig. 1 Seabed and pipeline definitions perpendicular to (upper) and along (lower) the pipeline
Fig. 2 Model flow diagram