Whitepaper DNVGL Sesam for Subsea

WHITEPAPER

SESAM FOR SUBSEA AUTHOR: Mo Dessoukey, DNV GL - Software DATE: August 2014

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.

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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. Figure 5 : A typical earthquake response spectrum used is an earthquake response analysis in Framework

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.



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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.

Figure 6 : A typical finite element mesh of a structural detail showing stress contours from the in-place analysis

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.



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Figure 7 : Sima’s visualization of the lifting and installation phase down through the sea surface until contact with the seabed

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.).

Figure 8 : Concept model of the subsea structure secured to a barge for transportation analysis

The designer will need to consider the intact and damage stability of the unit particularly during the onloading 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.



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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 nonlinear 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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Figure 10 : Usfos model showing plastic strains in the structure during installation on the seabed

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.



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Whitepaper DNVGL Sesam for Subsea

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