Roberto Bruschi

Minimum Wall Thickness Requirements for Ultra Deep-Water Pipelines

The offshore pipeline industry is planning new gas trunklines at water depth ever reached before (up to 3500 m). In such conditions, external hydrostatic pressure becomes the dominating loading condition for the pipeline design. In particular, pipe geometric imperfections as the cross section ovality, combined load effects as axial and bending loads superimposed to the external pressure, material properties as compressive yield strength in the circumferential direction and across the wall thickness etc., significantly interfere in the definition of the demanding, in such projects, minimum wall thickness requirements. This paper discusses the findings of a series of ultra deep-water studies carried out in the framework of Snamprogetti corporate RD • The line pipe material i.e. the effect of the shape of the actual stress-strain curve and the Y/T ratio on the sectional performance, under combined loads; • The load combination i.e. the effect of the axial force and bending moment on the limit capacity against collapse and ovalisation buckling failure modes, under the considerable external pressure. International design guidelines are analysed in this respect, and experimental findings are compared with the ones from the application of proposed limit state equations and from dedicated FE simulations.Copyright

Submarine Pipeline Installation Joint Industry Project: Global Response Analysis of Pipelines During S-Laying

The development of deep water gas fields using trunklines to carry the gas to the markets is sometime limited by the feasibility/economics of the construction phase. In particular there is market for using S-lay vessel in water depth larger than 1000m. The S-lay feasibility depends on the applicable tension at the tensioner which is a function of water depth, stinger length and stinger curvature (for given stinger length by its curvature). This means that, without major vessel up-grading and to avoid too long stingers that are prone to damages caused by environmental loads, the application of larger stinger curvatures than allowed by current regulations/state of the art, is needed. The work presented in this paper is a result of the project “Development of a Design Guideline for Submarine Pipeline Installation” sponsored by STATOIL and HYDRO. The technical activities are performed in co-operation by DNV, STATOIL and SNAMPROGETTI. This paper presents the results of the analysed S-lay scenarios in relation to extended laying ability of medium to large diameter pipelines in order to define the statistical distribution of the relevant load effects, i.e. bending moment and longitudinal strain as per static/functional, dynamic/total, and environmental load effects. The results show that load effects (longitudinal applied strain and bending moment) are strongly influenced by the static setting (applied stinger curvature and axial force at the tensioner in combination with local roller reaction over the stinger). The load effect distributions are the basis for the development of design criteria/safety factors which fulfil a predefined target safety level.Copyright

Buckle Propagation and Its Arrest: Buckle Arrestor Design Versus Numerical Analyses and Experiments

Buckle propagation under external pressure is a potential hazard during offshore pipeline laying in deep waters. It is normal design practice to install thicker pipe sections which, in case of buckle initiation and consequent propagation, can stop it so avoiding the lost of long pipe sections as well as threats to the installation equipment and dedicated personnel. There is still a series of questions the designer needs to answer when a new trunkline for very deep water applications is conceived: • What are the implications of the actual production technology (U-ing, O-ing and Expansion or Compression e.g. UO, UOE and UOC) on the propagation and arrest capacity of the line pipe, • How formulations for buckle arrestors design can be linked to a safety objective as required in modern submarine pipeline applications. The answers influence any decision on thickness, length, material and spacing of buckle arrestors. This paper gives an overview of buckle propagation and arrest phenomena and proposes a new design equation, applicable for both short and long buckle arrestors, based on available literature information and independent numerical analyses. Partial safety factors are recommended, based on a calibration process performed using structural reliability methods. Calibration aimed at fulfilling the safety objectives defined in DNV Offshore Standards OS-F101 and OS-F201.Copyright

A Numerical Lab to Predict the Strength Capacity of Offshore Pipelines

In the recent years, the offshore pipeline industry has been under pressure to provide solutions for demanding material and line pipe technology problems, installation technology to safely tackle the ultra-deep waters challenge, quantitative prediction of reliable operating lifetime for pipelines under high pressure/high temperature conditions and remedial measures to tackle considerable geo-morphic and human activity related hazards. Future pipelines are being planned in very difficult environments, i.e. crossing ultra-deep water and difficult geo-seismic-morphic conditions. In these circumstances, it is of crucial importance (1) to adopt advanced design procedure and criteria, possibly based on limit state principles recently implemented in the design codes, and (2) to use advanced engineering tools for predicting the strength capacity and the pipeline behaviour during the installation and operational phase, in order to design the pipeline safely and to assess properly the technic-economical feasibility of the project. This paper discusses the relevant failure modes for offshore pipelines, the FE analysis results relevant to the sectional capacity of thick-walled pipes, and the FE analysis results relevant to the global and local response effect of a pipeline, laid on the sea bottom, and subject to a point-load force.Copyright

Bending Capacity of Girth-Welded Pipes

In the last ten years, several studies were completed with the aim to define a design format for the local buckling of pipes subjected to differential pressure, axial load and bending moment. Experimental tests were carried out and simplified analytical solutions were developed in order to predict the pipe bending moment capacity and the associated level of deformation. Standard finite element (FE) structural codes, such as ABAQUS, ADINA, ANSYS, etc., were and are used as a “numerical testing laboratory”, where the model is suitably calibrated to few experimental tests. The outcomes of these research efforts were implemented in the design equations enclosed in international design rules, as DNV OS-F101. The local buckling design formats, included in these rules, give the limit bending moment and associated longitudinal strain as a function of the relevant parameters. The effect of the girth weld is introduced with a reduction factor only for what regards the strain at limit bending moment. This paper addresses the effects of the presence of the girth weld on both limit bending moment and corresponding compressive longitudinal strain. A 3-dimensional (3D) FE model developed in ABAQUS has been developed to perform a parametric analysis. The FE model results are shown to compare reasonably well with full scale experiments performed for on-shore pipelines. The limit bending moment is reduced by the weld misalignment and this reduction is also dependent on both internal pressure load and linepipe material mechanical strength. The FE results are compared with the limit bending moment calculated with DNV OS-F101.Copyright

VIV Basics for Subsea Spool/Jumper Design

Coming and future Deep and Ultra-Deep Water project developments involve the use of many Subsea Rigid Jumpers used to connect well heads, manifolds or riser base with Flowline End Terminations.Generally, Subsea Rigid Jumpers are short and flexible pipe sections assembled in a variety of spatial configurations to accommodate the installation tolerances, the Flowline End Terminations translation and settlement guaranteeing the continuity and the flexibility needs of the subsea pipe layout. These Subsea Rigid Jumpers are critical components as they are subject to fatigue damage due to Vortex Induced Vibrations induced by the bottom currents and/or Flow Induced Vibrations induced by the high internal flow rate, often coupled with slugging flow conditions.In this paper, a Subsea Rigid Jumper design approach based on basics of Vortex Induced Vibrations is presented, and outcomes on a few typical multi-planar Subsea Rigid Jumpers discussed.Copyright

A Neural Network Predictive Model of Pipeline Internal Corrosion Profile

Internal corrosion is a crucial issue for the safe operation of oil&gas pipelines. This is a phenomenon due to interaction of different mechanisms. Water and electrochemistry, protective scales, flow velocity, steel composition and localized bacteria attacks are relevant. Despite the large number of models proposed in literature, the corrosion process is very complex and rarely reproduced by existing models. For this reason, an artificial neural network (ANN) based model is investigated, with the aim to correctly predict the presence of metal loss and corrosion rate along a pipeline. In this paper, a case study is considered, based on real field data. The model integrates the geometrical profile of a real pipeline, flow simulations and the most important deterministic corrosion models. It is shown that the ANN model outperforms the deterministic ones.

LOAD HISTORY FOR SSFU UNDER MULTIMODAL WAVE SPECTRA

This paper presents results from an ongoing development project which aims at increasing the knowledge for the estimation of extremes conditions and fatigue assessment of structural element (i.e. topside) of Ship Shaped Floating Units (SSFU) in a complex environment-load conditions. The method can be extended for the calculation of total bending moment to connections and hull-girder system. In some world geographical context, such as those of West Africa and Offshore Brazil, the environmental conditions are characterized by co-existence of waves and swells propagating in different directions. In these conditions load history for SSFU are strictly linked to comprehension knowledge of site specific environment. Loads pertain to various frequency ranges, for instance the long-period response of a morning, quasi-static response to swell, dynamic response to shorecrested sea and structural resonance of flexible components as flares etc. This paper aims at developing a new method for establishing load history at site in case of metocean climate including combination of several (up to 3 or 4) sea state components, such as those of main swell, secondary swell and wind sea. The method is applicable to both extreme conditions and fatigue assessment as results of a “combined-event approach”. The results are compared and discussed. They show that the use of different approaches, long and a short term analysis with deterministic and probabilistic computation of vessel heading, provide a reasonably conservative estimate of the vessel responses. Differences from the conventional method, i.e. unimodal spectrum and equivalent sea state, are also presented.

Strain Based Design: Crossing of Local Features in Arctic Environment

In arctic pipeline projects, seismic risk and differential settlements are common, whether local or distributed across long stretches. For buried pipelines, seismic hazards are generally classified as wave propagation hazard (WP) or permanent ground deformation (PGD) hazard. Below ground crossing of seismic faults has been the real challenge in a series of pipeline projects. STress Based Design (STBD) criteria has been used in the past. Application of this method is straightforward as simple linear elastic analysis is required to calculate the load effects in the specified conditions. In the assessment of the structural integrity of a pipeline, load effects are compared with allowable states of stress. Unfortunately, unsatisfactory design, both from economic and safety points of view, may result. StraiN Based Design (SNBD) is an attractive option in these situations.The use of SNBD in pipeline technology has been widely discussed during the last decade, particularly for offshore applications. In many instances the offshore pipeline engineer can adopt SNBD to avoid onerous measures necessary to meet the traditional STBD criteria. First introduced to make allowance for crossing bottom roughness and harsh environments, more recently for High Pressure/High Temperature (HP/HT) applications, SNBD is currently used in a series of strategic project developments in North America and East Siberia, for both offshore and land pipelines crossing regions affected by ice gouging and geo-hazards from seismic activity such as land slides, active faults, soil lateral spreading due to soil liquefaction etc. Conditions for which SNBD are applicable, as well as permissible deformations in relation to line pipe material and safe operation of the pipeline in the long run, are of major concern.In this paper, the following is discussed:• Relevant hazards for arctic land and offshore pipelines such as ice scouring, permafrost thaw, frost heave etc..• The design approach and design philosophy for Buried Pipeline Crossing active faults. In particular:○ The Pipeline Crossing Layout of local features to minimize Load Effects;○ Material and Steel Wall Thickness Selection vs. Crossing Location;○ Pipeline Deformation Capacity (PDC) Assessment;○ Pipeline Strain Demand (PSD) Assessment;○ Pipeline Trench Design including Shape, Back-filling etc. vs. Pipe-Soil and Temperature Effects.Copyright

Deformation Capacity of Induction Bends

In the last two decades, reliability methods have been extensively used to calibrate rationally-based Load and Resistance Factored Design (LRFD) equations for the design of offshore and onshore pipelines. Several experimental and theoretical studies have been carried out with the aim to assess the strength and deformation capacity of induction bends, when buried and subject to severe and extreme loading conditions such as the ones caused by an earthquake crisis. Design criteria for induction bends do not reflect the R & D efforts of the recent years and working stress design in generally accepted by the Industry. The scope of this paper is: • Review the relevant literature on bends discussing analytical solutions, numerical analyses and experimental tests carried out aiming to predict the limit loads/deformations of induction bends and to define design criteria for induction bends. • Discuss the ABAQUS Finite Element Model (FEM) developed; • Present the FEM development, calibration and validation; • Show the parametric study results considering relevant parameters including bend geometry (wall thickness and bend angle), fabrication tolerances (thickness variation, bend cross section etc.), steel material characteristics (hardening and shape), and loading conditions (inner pressure, steel axial force, bending moment under closing and opening mode).Copyright