Enrico Torselletti

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 JIP: Strength and Deformation Capacity of Pipes Passing Over the S-Lay Vessel Stinger

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 a market for using S-lay vessels 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 presently 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. The scope of the project is to produce a LRFD (Load Resistant Factor Design) design guideline to be used in the definition and application of design criteria for the laying phase e.g. to S and J-lay methods/equipment. The guideline covers D/t from 15 to 45 and applied strains over the overbend in excess of 0.5%. This paper addresses the failure modes relevant for combined high curvatures/strains, axial, external pressure and local forces due to roller over the stinger of an S-lay vessel and to sea bottom contacts, particularly: • Residual pipe ovality after laying, • Maximum strain and bending moment capacity. Analytical equations are proposed in accordance with DNV OS F101 philosophy and design format.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

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

Bending Capacity of Pipes Subject to Point Loads

Large bending moments may develop on free pipeline lengths in the proximity of pipe sections subject to a local force. Sometimes the local force is such as to cause a partial loss of the sectional strength capacity of the pipe. This is the case of a pipeline plastically bent over the stinger of an S-lay barge, or of a pipeline laid on the sea bottom and hooked by an anchor or trawling gear, or of a pipe subject to cold bending when it is made to cope with sharp bottom roughness etc. In such conditions, the limit bending capacity of the pipe section, subject to local load effects, is significantly influenced. This aspect is not covered by international design codes and the scope of this paper is to show that, in some circumstances, it must be taken into due account. In this paper: • The relevant literature as concerns experimental tests, interpretative models, analysis methodologies and design approaches, is reviewed; • The FE model and post-processing, purpose-developed to investigate the interaction between local and global effect, are discussed; • The findings of FE analyses, in particular the effect of load combination, load history, pipe geometric characteristics and loading — magnitude and shape of the contact area, are presented. It is concluded that the limit bending capacity reduces significantly when local effects are such as to develop stresses on the pipe wall that affect the activation of the sectional buckling mechanism.Copyright

ECA for Girth Welds of Offshore Pipelines: A Modified BS7910 Analytical Approach vs 3D FE Analyses

In the last decade new standards or revisions of existing guidelines have been launched on the subject. Among those, BS 7910 (now in edition 2013) [1] and DNV-OS-F101 (now in edition 2013) [2] are considered as reference in many world offshore districts of the Oil & Gas Industry.What is peculiar in offshore pipelines with respect to pressure vessel or nuclear plants, for which an engineering criticality assessment (ECA) was first established, it is the fact that in many circumstances offshore pipelines exceed the elastic limit (global pipe bending is a primary stress that causes mainly membrane stress through the pipe wall). This implies the extension of a stress based ECA into a strain based ECA, further including the bi-axial state of stress, caused by the presence of internal pressure and hoop stresses.An important step of ECA is the definition of loads and load effects at pipe girth weld, from global applied loads on the pipeline to the local effects at the crack.Finite elements (FE) are currently used to develop the relevant bending moment stress vs. strain relationship for the given pipe diameter, wall thickness and materials, both parent pipe and weld. Related longitudinal stress distribution on the pipe cross section without flaws in the weld is calculated for different pipe life stages (installation, pressure test and operation). The calculated global (or far from the flaw) longitudinal stress distribution is an input for the ECA analysis.For this aspect the new DNV OS-F101 (2013) has reviewed the appendix A requiring the use of 3D FE analyses to account for the effect of the internal pressure on the Crack Driving Force (CDF).In this paper it is discussed an analytical approach both to assess the pipeline strength in presence of flaws in the girth welds of offshore pipelines and to define defect acceptance criteria for specific new projects. The approach follows the framework of BS7910 and of DNV OS-F101 and includes load conditions under both installation and operation. In particular specific 3D FE analyses are presented to enforce the applicability of the proposed analytical approach.Copyright

HotPipe JI Project: High Pressure/High Temperature Pipelines Laid on Uneven Seabeds

The HotPipe Project is a Joint Industry Research and Development Project, whose overall objective is to prepare a DNV Recommended Practice, to be used in the verification and design of high temperature/high pressure pipelines. The design guideline will cover most practical cases where pipelines are subjected to high internal pressure and high temperature i.e. exposed pipeline laid in a flat or uneven seabottom and buried pipes. The design criteria are based on the application of reliability methods to calibrate the partial safety factors involved. In this paper, the analysis methodology, the design procedures and relevant design criteria/functional checks for pipeline laid on uneven seabed, developed in the HotPipe Joint Industry Project, are discussed.Copyright