Kim Mo̸rk

Structural Response of Pipeline Free Spans Based on Beam Theory

One of the main risk factors for subsea pipelines exposed on the seabed is fatigue failure of free spans due to ocean current or wave loading. This paper describes how the structural response of a free span, as input to the fatigue analyses, can be assessed in a simple and still accurate way by using improved beam theory formulations. In connection with the release of the DNV Recommended Practice, DNV-RP-F105 “Free Spanning Pipelines”, the simplified structural response quantities have been improved compared to previous codes. The boundary condition coefficients for the beam theory formulations have been updated based an effective span length concept. This concept is partly based on theoretical studies and partly on a large number of FE analyses. The updated expressions are general and fit all types of soil and pipe dimensions for lower lateral and vertical vibration modes. The present paper focus on estimation of simplified response quantities such as lower natural frequencies and associated mode shapes. Hydrodynamical aspects of Vortex Induced Vibrations (VIV) are outside the scope of this paper.Copyright

Experiences Using DNV-RP-F105 in Assessment of Free Spanning Pipelines

Free spans often become a challenge in pipeline design and operation due to pipeline installation on uneven seabed or seabed scouring effects. The costs related to seabed correction and span intervention are in many projects considerable. Therefore it is relevant to investigate whether such intervention work is necessary or not. Despite the complexity inherent in free span response, the spans are often designed applying presumably conservative concepts and very simple analytical tools. The DNV guideline no 14 (GL14) for free spanning pipelines was issued in 1998 and has later been updated and issued as an Recommended Practice (DNV-RP-F105) to account for recent technical research and development and accumulated experience applying GL14 in pipeline projects. This code allows vortex induced vibrations (VIV) as long as the pipeline integrity is ensured, by for example checking that the fatigue life is sufficient. By giving design methodology and acceptance criteria for fatigue, the DNV-RP-F105 approaches the real physics of free spans in a better way than older codes and makes it possible to select cost-effective methods both in the design phase and later when re-assessing spans in the operational phase. This paper will briefly discuss some experiences obtained by using the DNV-RP-F105 in free span design/re-assessment. Some examples of pipeline failures due to free span and vortex induced vibrations will also be presented.Copyright

Assessment of VIV Induced Fatigue in Long Free Spanning Pipelines

Current design practice for free spanning pipelines is to allow free spans as long as the integrity with respect to potential failure modes are checked and found acceptable. The case study for Ormen Lange (OL) pipelines planned in the deep waters of the Norwegian Sea is associated with a large number of very long free spans, which requires significant intervention work if based on the state-of-practice acceptance criteria. The design philosophy of the state-of-the-art design code DNV-RP-F105 “Free Spanning Pipelines” is applied in combination with the experience gained from dedicated OL model tests. Updated project specific design guidelines with multi-mode behavior, typical for OL long free spans, is taken into account and an updated Cross-Flow (CF) response model has been developed. An approach to select the In-Line (IL) mode excited by CF response is suggested. Methods for combining stresses from multiple active modes have been proposed and tested, for both IL and CF Vortex Induced Vibrations (VIV). Fatigue analysis has also been performed on the stress series measured in the model tests and this has been successfully used to verify and validate the presented computational procedure. Uncertainty in the model test based fatigue estimates has been assessed and sensitivity studies have been carried out. Reasons for deviations and potential problem areas for long free spanning pipelines have been identified.Copyright

Hydrodynamic Coefficients From In-Line VIV Experiments

For subsea pipelines installed in areas with uneven seabed free spans may occur and fatigue failure due to vortex induced vibrations (VIV) is one of the main concerns related to these spans. In order to install pipelines in such areas the safety against fatigue failure from in-line (IL) and cross-flow (CF) VIV must be documented. Although maximum oscillation amplitudes in the IL direction are considerably smaller than the maximum amplitudes in the CF direction, the IL fatigue damage normally prevails and may limit the allowable span length. The reason for this is that the IL vibrations initiate at a lower current velocity (i.e., reduced velocity) than the CF vibrations and would hence be excited for a longer period of time. Prediction tools for VIV may be split into parametric Response Models such as described in DNV-RP-F105 and methods based on empirical coefficients such as SHEAR7 and VIVANA. Methods based on force coefficient have until recently been limited to CF VIV due to lack of hydrodynamic coefficients for IL response. This paper presents results from forced IL oscillation experiments of a smooth, rigid cylinder in uniform flow. The results are presented as dynamic in-line coefficients for the pure IL regime, i.e. reduced velocity between 1 and 4, at Reynolds number 24.000. The results are compared with IL results from free oscillation experiments found in the literature.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

Simplified Model for Evaluation of Fatigue From Vortex Induced Vibrations of Marine Risers

This paper presents a novel approach for approximate calculation of the fatigue damage from vortex-induced vibrations (VIV) of marine risers. The method is based on experience from a large number of laboratory tests with models of full-length risers, large-scale tests and also full-scale measurements. The method is intended to provide a conservative result and be used for screening purposes at the early design stage. The model is in particular aimed at predicting fatigue for risers that respond at very high mode orders (above 10), but may as well yield valid results for lower mode numbers. The model will, however, not be adequate for free span pipelines or other structures that normally will respond at first and second mode. The riser will be defined in terms of some key parameters like length, weight, tension, hydrodynamic diameter and stress diameter. A current profile perpendicular to the riser in one plane must be known. The program will apply a simple model for calculation of eigenfrequencies and mode shapes, and these are sorted into in-line (IL) and cross-flow (CF) bins. An effective current velocity and excitation length can be defined from the profile and will be applied to identify the dominating cross-flow response frequency and the total displacement rms value. The dominating in-line response frequency is taken as twice the cross-flow frequency, and inline response rms is taken as a given portion of the cross-flow rms value. A set of contributing modes is defined from an assumed frequency bandwidth that reflects observed bandwidths, but also modal composition for cases with discrete frequency response. A simple mode superposition technique is then used to find the set of modes that gives the identified rms values. Bending stresses will be found directly from the curvature of the mode shapes. Fatigue damage will be found from stress rms values, user defined stress concentration factor and given SN curves. The model has been implemented in a simple computer program and verified by comparing results to measurements. The ambition has not been to obtain an exact match between computed results and observations, but to verify that the model gives reasonable but conservative results in almost all cases. However, an unrealistic over prediction of the fatigue damage is not desired. The results are promising, but the need for more information from measurements and response analyses with programs like VIVANA and SHEAR7 is still obvious.Copyright

Evaluation of Free Spanning Pipeline Design in a Risk Based Perspective

The Ormen Lange subsea pipeline shall be designed to meet a specified risk acceptance criterion, established by consideration of failure probability and consequences of failure. Traditional design for Vortex Induced Vibrations (VIV) of free spans limits the allowable free span length and implies that interventions work may be required. Through a risk based approach the probability of fatigue failure of free spanning pipelines is quantified, and the governing uncertainties identified. A sensitivity analysis of different risk control options is performed. The outcome facilitates to focus in the design process such that a preferred design solution can be identified and implemented via testing campaign in the design stage, prelaid rectification activities and inspection programs. The aim is to obtain a cost efficient design that comply with the given acceptance criterion. Best practices as reflected in DNV-RP-F105 “Free Spanning Pipelines” and updated field specific design guidelines form the basis for the analysis. A probabilistic module is implemented on top of DNV-RP-F105 methodology, which allows application of a dedicated uncertainty modeling for a specific project. Parameters considered include: Pipeline properties, effective axial force, span length and gap, soil properties, ocean current (distribution, depth and directional variation), multiple mode response analysis (VIV response models, natural frequencies, damping, effect of concrete, static deflection), different SN curves, strakes and monitoring. Both the As Laid phase and the Operational phase are considered for different locations along the pipeline route.Copyright

System Effects in Fatigue Design of Long Pipes and Pipelines

In fatigue design of steel structures the hot spot area is normally limited in size. In pipelines and cylinders used for transportation and storage of compressed natural gas the hot spot area can extend to several kilometers along a seam weld subjected much to the same stress range. The fatigue capacity for large diameter pipes is reduced as the weld length is increased. The seam weld in pipes is subjected to a significant stress range normal to its weld toes when it is subjected to varying internal pressure from start and stop of gas transportation through pipelines and from filling and emptying of cylinders. A reliable methodology is required for fatigue design of long pipes in order to meet target safety level at an acceptable cost. This includes description of physical models that represents actual long term fatigue capacity of the pipes subjected to varying internal pressure. The design methodology includes length of welds, fabrication tolerances, fabrication methodology and non destructive testing. It also involves definition of characteristic values for loading and capacity in addition to a recommended Design Fatigue Factor to be used in fatigue design. Alternative fatigue design procedures require different Design Fatigue Factors to achieve a required target safety level for pipelines and cylinders used for transportation of gas. These issues are further considered in this paper.Copyright

Updated Design Procedure for Free Spanning Pipelines DNV-RP-F105: Multi-Mode Response

In search for new gas and oil fields, the trend in offshore development points towards deeper waters, harsher environment, increased use of subsea installations and use of pipelines to transport the hydrocarbons to processing facilities onshore or in shallower waters. This also implies installation of pipelines at very uneven seabed causing a high number of spans that can be difficult and very costly to intervene. Conventional free span design according to the DNV Recommended Practice DNV-RP-F105 (2002) allows for vortex induced vibrations (VIV) as long as the integrity of the pipeline is within acceptable limits. However, the 2002 issue of the design code mainly covers short and moderate spans. As the knowledge about very long and/or multiple spans, where several vibration modes may be activated, has been limited, such cases have been treated in an assumed conservative way. This paper discusses the technical advancements in free span design in general and with respect to both long free spans and multi-spanning sections where several vibration modes may be activated simultaneously in particular. These advancements form the basis for the updated DNV-RP-F105 (2006). Changes from the former 2002 version are illustrated by an examples and the technical background is discussed.Copyright

HotPipe JIP: HP/HT Buried Pipelines

The HotPipe Project is a Joint Industry Project, whose overall objective is to prepare a DNV Recommended Practice to be used in structural design of high temperature/high pressure pipelines. The developed design criteria are based on the application of structural reliability methods to calibrate the partial safety factors involved. One of the three scenarios covered in this DNV-RP is buried pipes subjected to upheaval buckling which is discussed in this paper. The most significant factor in this scenario is uncertainty in the pipeline configuration and uncertainty in the pipe-soil interaction. The paper presents the background of the proposed soil capacities and the associated uncertainties for both uplift resistance and downward resistance in cohesive and non-cohesive soil. The paper links these soil models with the design requirements to upheaval buckling including: - Functional requirements i.e. survey data accuracy, smoothing of survey data, modeling of the pipeline, design conditions, soil cover etc.; - Trenching technology; - Qualification of the minimum soil cover, natural or artificial, with the aim to guarantee pipeline stability; - Assessment of pipeline response; - Pipe integrity checks and design criteria. The internal confidential project guideline has been completed and is currently in the process of being converted into an official DNV-RP-F110, to be published later this year.Copyright ?? 2005 by ASME