Sesam for subsea design analysis - ogj.com · Sesam for subsea design analysis - ogj.com
Transcript of Sesam for subsea design analysis - ogj.com · Sesam for subsea design analysis - ogj.com
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INTRODUCTION
This paper offers an introduction to subsea structures and shows how Sesam can offer a complete and
efficient tool to perform the design analysis of these structures.
The Sesam suite of programs offers several modules to meet the needs of designers of subsea structures.
These include GeniE, Wajac, Sestra, Framework, Sima, Wadam and Usfos. Each of them performs
specific tasks during the design process which are outlined in the sections that follow.
SUBSEA
In the oil and gas industry the term subsea relates to the underwater exploration, development and
drilling of oil and gas fields.
Subsea installations are used to develop reservoirs outside effective reach of existing platforms, they can
be installed, designed and constructed in a shorter time than traditional jacket type platforms making
them an attractive alternative. Subsea development can also offer a lower investment cost when
compared with installation of a jacket and associated topside facilities.
Subsea systems can range in complexity from a single satellite well connected to a fixed or floating
facility to several wells connected to a central manifold tied back to platforms or directly to an onshore
installation.
WHITEPAPER
SESAM FOR SUBSEA
AUTHOR: Mo Dessoukey, DNV GL - Software
DATE: August 2014
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Figure 1 : A typical subsea development layout
There has been an exponential growth in the subsea sector over the last decade mostly due to the
developments of fields in deep and ultra-deep waters where traditional surface facilities might be
economically or technically unfeasible. In 2010 the Norwegian Petroleum Directorate issued a statement
that “More oil and gas is now being produced from subsea wells on the Norwegian continental
shelf than from platform wells”.
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STRUCTURES AND ANALYSES
With the exponential growth in the subsea sector, a new set of challenges now face the industry. Mostly
due to equipment sizes growing as is the case with manifolds and templates. To keep up with demand,
the major operators have now classified such equipment as significant load carrying structures and not
just equipment. In the past these structures were small enough to be considered as non-structures and
a FEM (Finite Element Method) assessment was sufficient in most cases, but with the new classification a
FEM stress analysis alone is not adequate and a full design based on use of structural design Codes and
recommended practices is now required.
To ensure that the structures meet design criteria, relevant consideration is given to:
• In-place analysis, with the structure installed on the sea-bed and operating.
• Transportation analysis, when the structure is being transported to the location where it is to be
installed.
• Lifting through the splash zone, when the structure is being lowered through the water to its in-place
location on the sea-bed.
• FEM design, checking local details and connections for local hot spot stresses and possible
exceedance of allowable stresses.
• Accidental loads such as dropped objects, snag from fishing nets or ROV (Remote Operated Vehicles)
impacts.
One of the important strengths of Sesam is the extensive integration that exists between one program
module and another. This makes Sesam a complete and efficient tool for the design of subsea structures.
The structures are modelled and code checked in GeniE for the in-place, transportation and static lift
cases. Where necessary, finite element analysis of local details is also be carried out using GeniE.
The model is then automatically read by the other modules for the non-linear accidental analysis (Usfos)
and dynamic lift thru the splash zone analysis (Sima).
The model properties and loads are transferred without the user having to do any manual transformation
or additional load input from one module to the next in Sesam, and thus reducing the possibility of
erroneous input. In the post-processors all information of model property and loads are stored and these
are readily accessible. Typical information stored in the result file is model geometry properties,
displacements, accelerations, forces, moments and different types of stresses.
In the past subsea engineering companies met a challenge in finding one piece of software that could
perform all the above analyses using one model, with seamless data transfer from one analysis phase to
the next. In the sections that follow we will look at how Sesam is able to efficiently perform all of the
above analysis using just one model.
IN-PLACE ANALYSIS
Previous generation design and analysis software had architectural limitations prohibiting both FEM
assessment and Code based limit checking. Sesam GeniE provides both a full FEM tool and structural
code checking allowing the designer to switch between code checking and a full shell elements stress
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analysis with a minimum of effort. This is all done using the same software, eliminating the time, effort
and cost required in re-modelling into different software to fulfil both functions.
Figure 2 : A GeniE concept model of a subsea structure for use in all analysis phases
Figure 3 : Visualization of the FE model with labelling of Von-Mises stresses from the in-place analysis
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Figure 4 : Visualization of member Code check utilizations from the in-place analysis
Structures installed in high risk seismic regions can be designed against such loads based on the
earthquake spectrum or time history method using Sesam’s Framework program.
Another function that is only inherit to Sesam is the ability to model the lift, in-place and transportation
cases using the same model and utilizing the same basic concept model (Fig. 2). In other words, there is
no need to keep three separate models (databases) for each phase of the analysis and keep updating of
all files every time a member is re-designed.
Figure 5 : A typical earthquake
response spectrum used is an
earthquake response analysis in Framework
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FEM ANALYSIS
The unique ability of Sesam to do both Code checks and FEM analysis from the same concept model is
also useful in checking local connections or details on the structure. It can relieve the end user from the
tedious task of extracting beam forces from the 3D beam model to apply as loading on refined finite
element models of local details. This often required the use of spread sheets and hand calculations to
define explicit modelling of loads on local details. Furthermore, with Sesam GeniE’s expansive modelling
and mesh generation, the user can create detailed and accurate finite element models with a minimum
of effort.
SPLASH ZONE LIFTING
Because of the nature of these structures, which are considered fixed structures when on the seabed,
but are non-fixed hydrodynamic sensitive structures when lifted thru the splash zone , there needs to be
communication between the engineers responsible for designing the structure during the installation and
their counter parts responsible for in-place analysis. Often this is two different contractors.
Sesam provides the tool to bridge that communication gap, in the form of Sima which enables the
engineering design firms and the installation contractors to determine more accurate dynamic
amplification factors (DAF) on their structures and thereby reduce the traditional conservative DAF’s
often appliedby designers. This has the added benefit of reducing the weight of these structures.
Figure 6 : A typical finite element
mesh of a structural detail showing
stress contours from the in-place analysis
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TRANSPORTATION
The transportation of these structures to their in-place locations should be taken into account, and
Sesam offers not just the simplified approach method, which is using a percentage of gravity approach
or angular accelerations, but a fully coupled hydrodynamic analysis of the structure on top of the
installation vessel.
This is of importance to engineering firms and installation contractors alike, who require more accurate
and less conservative analysis of their structures.
For the transportation analysis itself we utilize the same concept model of the structure together with a
concept model of the barge (as shown in Fig 8.).
The designer will need to consider the intact and damage stability of the unit particularly during the on-
loading and off-loading stages of the transport. Acceptable design criteria for roll/pitch angles will need
to be checked for both with and without flooding of barge tanks. This can be done using Sesam’s HydroD
program.
Figure 7 : Sima’s visualization of the
lifting and installation phase down
through the sea surface until contact with the seabed
Figure 8 : Concept model of the
subsea structure secured to a barge for transportation analysis
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For the transport itself the most important aspect is the motion response to the prevailing environmental
conditions. The waves in particuar can give the barge large accelerations leading to extreme loading in
the seafastenings used to secure the structure to the deck of the barge.
For this phase of the analysis we use Wadam to determine the maximum motions and vessel
accelerations to be applied as loads in GeniE when designing the seafastenings to be secure during the
transport phase.
Careful consideration is given during the offloading of the structure to ensure the seastate conditions are
within the design crireria, and if they are not it may then be necessary to wait for ‘acceptable’ weather
conditions.
NON-LINEAR ACCIDENTAL ANALYSES
Per code requirements, these structures are to be designed for accidental loads such as dropped object,
ROV impacts, fish net snag and of course for mating with the seabed. With Sesam the same model used
for the in-place linear structural analysis can then be used to run non-linear analysis without the need to
re-model using another software. This is done by importing the GeniE finite element model into the non-
linear solver Usfos with a minimum of effort on the part of the engineer.
The structure can be dynamically or statically checked. One of the cases to be checked is the ‘impact’
forces and induced stresses during installation on the seabed. With Usfos it is possible to simulate the
effect of ‘touch down’, looking at different scenarios of initial contact points between the structure and
the seabed.
Figure 9 : Simulation of dropped object
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THE AUTHOR
Mo Desoukey is chartered structural engineer with over 15 years experience in Subsea and Offshore
engineering. He is a DNV-GL employee.
ABOUT DNV GL
Driven by our purpose of safeguarding life, property and the environment, DNV GL enables organizations
to advance the safety and sustainability of their business. We provide classification and technical
assurance along with software and independent expert advisory services to the maritime, oil and gas,
and energy industries. We also provide certification services to customers across a wide range of
industries. Operating in more than 100 countries, our 16,000 professionals are dedicated to helping our
customers make the world safer, smarter and greener.
SOFTWARE
DNV GL is the world-leading provider of software for a safer, smarter and greener future in the energy,
process and maritime industries. Our solutions support a variety of business critical activities including
design and engineering, risk assessment, asset integrity and optimization, QHSE, and ship management.
Our worldwide presence facilitates a strong customer focus and efficient sharing of industry best practice
and standards.
Figure 10 : Usfos model
showing plastic strains in
the structure during
installation on the seabed