deep water riser tech.pdf

8/18/2019 Deep water riser tech.pdf

1/8

Address:

Otto Nielsens veg 10

P.O.Box 4125 ValentinlystNO-7450 Trondheim, Norway

Phone: +47 7359 5500

Fax: +47 7359 5776

E-mail: [email protected]: www.marintek.sintef.no

NORWEGIAN MARINE TECHNOLOGY RESEARCH INSTITUTE

ISSN-801-1818

No. 1 - April 2005

Observations of marine riser responseshow current-induced vortex-inducedvibrations (VIV) to be a widely occurringphenomenon, with the potential to causecostly and environmentally damagingfatigue failures. In the deep waters of the

Gulf of Mexico, West Africa and Brazil,for example, where oil and gas explora-tion and production continue apace,VIV may make the largest contributionto overall riser fatigue damage. Waveand vessel motion-related damage mayremain at roughly the same level or evendiminish as water depth increases, butcurrents can act over the full depth of thewater column, which tends to make VIVmore important in deeper water. This,and the fact that VIV of long risers isgenerally less well understood than other

load effects, has led to an intensificationof research activity in recent years.

Interaction and collision between risersin arrays has been a concern in riserdesign. Up to date, the usual practicehas been to design the risers with suf-ficient spacing to avoid contact as theymove about in the ocean current. Avoid-ing interaction has become more difficultand expensive as production moves todeeper water. Therefore, riser arrays withsmall spacing is advantageous from both

a practical and economical point of view,and the industry is now considering toaccept the occurrence of riser clashing.

MARINTEK has recently carried outseveral research projects that havefocused on enhancing the understand-ing of the behaviour of long flexiblemarine risers in ocean currents.Innovative measurement techniques

and cutting-edge analytical techniqueshave been developed. The servicesMARINTEK provides are capable ofreducing the uncertainties in life-timeassessments of risers, pipelines andumbilicals, which could have a dra-matic impact on the cost of developingnew fields.

Recent projects on flow-induced vibra-tions include:

• High mode VIV

• Riser clashing• Efficiency of strakes• Galloping of riser bundles• Fluttering instability of faired risers• VIV tests on prototype umbilical• Improvements and validation of the

prediction tool VIVANA

In addition to deep-water riserresearch, this issue of MARINTEKReview also focuses, among others, onshallow water challenges. LNG tankersat offshore terminals in shallow water

are exposed to non-linear wave effectsthat are not fully taken into account innumerical simulations.

Deep-Water Riser Technology

As petroleum exploration and exploitation move to ever deeper waters, the focuson riser technology is sharpening. As water depth increases, the cost of the risersystem and the technological challenges both increase rapidly. Experimental studieson the behaviour of marine risers have recently been carried out at MARINTEK.Advanced model testing techniques and numerical analysis have been used toimprove our understanding of the complex behaviour of risers in ocean currents.

Riser modelused for high

mode VIV tests.

Contents:

High Mode VIV

Model Tests ............. 2

Interaction between

Tensioned Risers ..... 3

VIV Testing of

Full-Scale UmbilicalSection .................... 4

Stress and Fatigue

Analysis of Umbilicals 4

Systematic Investiga-

tion of Efficiency of

Helical Strakes ......... 5

MARINTEK Includes

PIV Measurements in

Model Testing .......... 5

Shallow Water Effects 6

WAVELAND JIP ....... 7

Taking the Effect of

Ocean Current Varia-

bility into Account .... 8


2/8

2

High Mode VIV Model Tests

VIV of long risers is generally less well understood than other load effectsand is a critical part of a floating production system, which means thatthis phenomenon is of great interest to the industry.

MARINTEK recently performed an extensive model test cam-paign in the Ocean Basin with a 38m long flexible riser, for theNDP (Norwegian Deepwater Programme). The model had an L/D(length-to-diameter ratio) of 1,400. The riser was tested without VIVsuppression and with various strake arrangements.

There were three objectives:

(i) Acquire data to improve understanding of high-mode VIV of longrisers in different current profiles.

(ii) Provide benchmark information for calibration and validation ofcodes that predict riser response.

(iii) Assess suppression effectiveness of strakes with differentgeometries and different percentage coverage over the riserlength.

Testing was performed on an innovative new test rig that couldsimulate uniform and linearly sheared currents. The riser modelused was made of composite fibre pipe, see Figure 1. A total of 64strain gauges and 16 accelerometers were utilized, creating one ofthe most detailed instrumentation arrays used to date for measuringriser VIV response. The strain gauges were non-uniformly spaced,unlike in many earlier experiments.

During the planning of the test program, both inline and cross-flowresponses were considered to be of importance with respect tofatigue. Indeed, this study documented that inline fatigue damage isas severe as cross-flow fatigue damage, as shown in Figure 2. How-ever, industry analysis approaches generally ignore inline damagedue to VIV.

The findings also indicate that the response character of a bare risercan be quite distinct from that of a riser partially or fully coveredwith helical strakes. An example is shown in Figure 3, whichpresents fatigue in different riser configurations vs. tow speed. Thefigure shows that helical strakes of different types can be effectivein mitigating VIV fatigue of long risers; their performance is thus

dependent on their geometry.

Figure 1. Schematic of test rig (vertical view).

Figure 2: CF and IL fatigue vs. tow speed for a bare riser in uniformflow.

Figure 3. Maximum CF fatigue damage in uniform flow for a bare riserand two different strake geometries.

MARINTEK contacts: [email protected]

[email protected]


3/8

3

The behaviour of a riser array in currents is an extremely com-plex hydroelastic problem that can be characterized in terms ofmotions in different time scales. Long periodic translations occur

at low mode shapes (typically first mode, i.e. one half wave), whilehigh-frequency VIV motions occur at much higher modes. Theformer response is denoted WIO (Wake Induced Oscillations). Flowseparation and partial shielding between the cylinders can signifi-cantly modify the local fluid kinematics in amplitude, frequency andphase, compared to a single cylinder. The differences in excitationof adjacent cylinders may produce large relative motions that canbring them into contact.

Methods for predicting the probability of riser clashing and theresulting loads are welcomed by the industry. An important part ofthe process of developing a theoretical tool is validation. This mustbe done by comparing theoretical results with full-scale measure-

ments or scaled model tests. As a part of the Norwegian DeepwaterProgramme (NDP), MARINTEK has performed a research project onriser interaction, comprising model tests and analyses of dual riserinteraction in uniform flow. The tests were performed in Towingtank No. III at MARINTEK.

A sketch of the basic set-up is shown in Figure 1. Two 10 m-longflexible riser models with an L/D of 500 were tested simultaneously.Dense arrays of high-quality novel instrumentation were installedon the models. Both bare risers and risers with VIV suppressiondevices were tested, as were bumper elements attached to therisers. Riser spacing, inflow angle and current speed were system-atically varied. A total of 310 tests were performed.

Advanced analysis methods were used to interpret the measure-ments. In particular, the scope of the investigation includedi) describing the spatial distribution of riser clashing, ii) to computethe relative velocity at clashing, and iii) computing the riser VIV andWIO.

Figure 2 shows images of the VIV motion of straked risers in in-line(IL) and cross-flow directions (CF) for the upstream (blue) anddownstream riser (red). It was found that when a straked riserresides in the wake of another riser, the strakes lose some of theirability to suppress VIV.

MARINTEK contacts:[email protected]@marintek.sintef.no

Interaction between Tensioned Risers

Interactions and collisions between risers in arraysare a source of concern in offshore developments.Common practice is to design riser systems to avoid

contact between individual risers. In deep water,avoiding riser clashing becomes both expensive andtechnically cumbersome. MARINTEK has carried outa research project whose general objective was toimprove our understanding of the physical mecha-nisms that drive riser clashing. Our findings will beused as input to new industry guidelines on risersystem design.

Figure 1. Experimental set-up.

Figure 2.Snapshots of in-line and cross-flow VIV motion of straked risers.

Upstream riser

Downstream riser


4/8

4

The umbilical consists of several internal

hydraulic pipes bound together with electri-cal and fibre-optic control cables and willbe used to control the subsea gas field fromshore. During a period before burial, freespans are believed to be critical with respectto VIV-induced fatigue. We therefore carriedout a test programme whose primary objec-tive was to verify VIV-induced stresses inthe most critical pipe.

The umbilical was heavily instrumented toacquire records of bending and axial strainand lateral acceleration in both cross-flow and

in-line directions at several stations. Two sepa-rate instrumented cores were installed insidethe full-scale umbilical, in each hydraulic tube(see Figure 2) one with accelerometers andone with fibre-optic strain gauges.

VIV Testing of Full-Scale Umbilical SectionMARINTEK has recently performed a free/span VIV test of a 20 m full-scaleprototype-section of the Ormen Lange umbilical for Norsk Hydro.

Figure 3. Example of cross flow response from one towing test. Figures from right: i) Standarddeviation of modal weight factors to diameter ratio, ii) Snapshot of displacement to diameterratio along umbilical length, iii) Standard deviation of displacement to diameter ratio vs. umbilicallength, iv) Standard deviation of curvature times diameter vs. umbilical length.

Figure 2. Umbilical showing instrument wiresbefore they were cast-in by epoxy.

Figure 1. Test set up for the tow tests of prototype umbilical.

In cooperation with Nexans Norway a.s.,MARINTEK has already developed the

state-of-the-art software tool UFLEX2D fordetailed 2D-analysis of complex umbilicalcross-section designs. One of the primaryobjectives of the new project is to extendthe 2D description to 3D, allowing the studyof longitudinal effects that are of specialimportance for stress and fatigue analysis ofdeepwater umbilicals, such as:

• End effects and friction interactions

• Radial reaction forces at bending stiffen-ers/restrictors

• Kinking due to coupling between ten-sion/compression, torsion and bending.

• Load-sharing between structural ele-

ments during installation and in riserconfigurations

Planned start-up is June 2005 and the JIPwill run for three years.

Stress and Fatigue Analysisof UmbilicalsIn cooperation with Nexans Norway a.s. and several oil companies,MARINTEK is currently launching a joint industry project (JIP) called Stressand Fatigue Analysis of Umbilicals, which focuses on software development,verification testing and case studies of deepwater umbilicals.

MARINTEK contact:[email protected]

2D and 3D umbilical models.

signals as input, bending moment signals

as input or both. The next figure shows thedisplacements along the umbilical, where itcan clearly be seen that 2nd mode domi-nates in this case. The next figure showsthe standard deviation of the displacementsalong the umbilical; the solid line indicatesthe results obtained from a modal analysis,while the circles are results found by doubleintegration of the acceleration signals. Thedisplacement was also measured directly by

means of a linear spring/force transducerat one location (green circle), also showingexcellent agreement with the other results.The last figure shows the standard devia-tion of curvature versus umbilical length.

The curvature is proportional to the bendingstress and indicates thus the influence onfatigue. The results obtained from modalanalysis (solid line) agree well with theresults found directly from the bendingmoment tranducers (stars).


The umbilical was towed from a carriageabove the Ocean Basin (Figure 1) at speedsranging from 0.3 to 2.5 m/s, correspond-ing to Reynolds numbers 30 000 - 260 000.The test programme gave results valid forprototype conditions, with high qualityand consistent measurements that gave avery good insight into the behaviour of theumbilical. Figure 3 shows an example of

cross-flow response. The figure to the leftshow the standard deviation of the modalweight factors, based on using accelerations


5/8

Helical strakes are often installed in orderto mitigate VIV. The strakes are screw-likeprotrusions that are wrapped around thecylinders to suppress flow vortices byshortening their correlation lengths. Theefficiency of the helical strakes is dependenton the height of the protrusion and the pitchof the helical, i.e. the length of a full wraparound the cylinder. A drawback of the heli-cal strakes is that they may increase dragon pipe sections. They will also increase thefabrication and installation cost.

When helical strakes are being installed, westrive to combine the best possible VIV sup-pression with the least possible increase indrag. As a part of the Norwegian DeepwaterProgramme, MARINTEK has studied theefficiency of different helical strake configu-rations in terms of 2D model tests. Rigid

models were elastically mounted in a set-upwhere they were free to move in the cross-flow direction, but restricted in the in-linedirection. The cylinders investigated were2 m long, with diameters varying from76 mm to 114 mm. A systematic parametricstudy was performed, with strake pitch-to-diameter ratio ranging from 5 to 17.5, andstrake height-to-diameter ratio from 0.10 to0.25. Both 3-start and 4-start strakes weretested. Each configuration was tested fora wide range of reduced velocities rangingfrom 3 to 30. Figure 1 shows some of thecylinders tested.

The measurements focused on cross-flowvibrations as well as drag forces on thecylinders. In order to investigate the physicsof straked cylinders in current, PIV measure-ments were made.

Systematic Investigation ofEfficiency of Helical StrakesVortex-induced vibrations play a critical role in inducing fatigue in offshorerisers, pipelines and other tubular members. The suppression of VIV is

essential to minimize the risk of premature failure. MARINTEK has carriedout a systematic parametric study of the efficiency of different helical strakeconfigurations. The results will provide valuable input for the selection of VIVsuppression devices for flexible risers and umbilicals.

Although all strake configurations mitigatedthe VIV, large differences in efficiency werefound. The most important parameter forstrake efficiency is the strake height. At thehighest reduced velocities, a “galloping”type of behaviour was seen.


Figure 1. Some of the test cylinders.

MARINTEK Includes PIV

Measurements in Model Testing

PIV: Particle Image Velocimetry for assessing fluid kinematics

Particle Image Velocimetry (PIV)is a measurement technique thatprovides velocity fields from con-sequent digitized images. Digitalcameras and laser lighting syn-chronized with a computer providehigh accuracy and repeatability.PIV, as of today, is regarded asa well-established measurementtechnique capable of bringing sig-nificant progress to applied fluidmechanics. 3D velocity vectors

may be obtained by using twocameras at different relativeangles (Figure 1).

PIV is useful in many areas, and providesboth qualitative and quantitative informa-

tion. Examples of direct usage are:

• Evaluation of wave loads and sloshingloads – important for design of shipsand offshore structures.

Figure 2. Sloshing experiments.

• Validation and verification of theoreti-cal/numerical models – important forimprovement of simulation tools.

Examples of more theoretical character are:

• Improved understanding of highly non-linear flows, such as steep waves andVortex Induced Vibrations (VIV).

• Studies of boundary layer flow as wellas turbulence.

NTNU, in cooperation with MARINTEK, has

Cont. on page 6

5

Figure 1. VIV experiments.


6/8

6

Shallow Water EffectsLNG Terminal Hydrodynamics

New offshore LNG terminals are being developed worldwide. They areusually installed in shallow waters, for which reason special effects must

be taken into account. In particular, floater hydrodynamics, e.g. for a LNGcarrier, need to be paid special attention. This includes such influences asnonlinear wave effects, increased slow-drift forces, and changes in addedmass and damping.

Furthermore, multi-body interactions add tothe complexity of the problem, and numeri-cal modelling is not straightforward. For thisreason, model tests play an important rolein software validation as well as operationand design verification. During the past fewyears, MARINTEK has carried out experi-mental studies on a number of terminalprojects.

Slow-drift forces in shallow water

In shallow waters, slow-drift forces inirregular waves increase. This is due tothe greater interaction between waves withdifferent periods, which are known as off-diagonal contributions in Quadratic TransferFunctions (QTFs). Thus modelling by thefrequently used Newman’s approximationwill be non-conservative. Validation bymodel tests is recommended, especially for

multi-body cases, but also in general.

Low-frequency bound waves andkinematics

Nonlinear contributions in wave elevationand particle kinematics increase in shallowwater. It is well known that wave crestsbecome sharper and troughs flatter. Forfloaters, the low-frequency (LF) group-induced wave components are of greatestinterest. In steep waves, LF contributionscan become quite large, with a correspond-ing large set-down current.

Common offshore hydrodynamic analysismethods assume an infinitely wide hori-zontal bottom. To simulate this accuratelythrough wave generation in a limited wavebasin is generally challenging; second-order

corrections to the wave-maker signal maytherefore be needed in order to reduce so-called “parasitic” long waves. A method ofdealing with this need has been developedat MARINTEK.

Barge motions in shallow water

Motion characteristics can change signifi-

cantly in shallow waters. MARINTEK hascarried out model tests for Statoil to verifythe in-docking operation of a barge to beinstalled at the Snøhvit Melkøya terminal.The change in wave-induced heave androll motion characteristics with the changein water depth was of particular interest.As well as the model of the barge itself, amodel of the dock and of the nearby shoreand sea bottom were also included.

MARINTEK Includes PIVMeasurements in ModelTesting ... Cont. from page 5

Slow-drift surge force quadratic transfer func- tions – QTFs – for 200 m monohull in (upper)84 m and (lower) 25 m water depths.

Laboratory irregular wave in shallow water(Tp=1.4s, Hs=0.16m, d=0.40m), total & LF.

invested in state-of-the-art PIV equip-ment. The set-up includes software, twohigh-speed cameras and a pulsing laser.MARINTEK has used PIV measurements inseveral model testing projects, and plans to

increase the use in future.

The figures illustrate examples of its use at

MARINTEK and NTNU. Figure 1 shows the3D velocity field behind a horizontal cylinderunder towing (VIV tests). The vortices shedby the cylinder are seen. Colours indicatethe velocity component perpendicular to theplane. Figure 2 shows velocity vectorsobtained from sloshing experiments. Figure3 presents run-up on a fixed, vertical circu-lar column.

MARINTEK contacts:[email protected] [email protected]

Snøhvit barge in-docking model tests (forStatoil).


Figure 3. Run-up experiments.


7/8

7

In the MARINTEK WaveLand JIP Phase 2,state-of-the-art engineering tools have beensystematically validated against a widerange of experimental data, and new know-ledge has been implemented into improvedguidelines for industry use. IntroductoryCFD studies have also been carried out. TheJIP participants included Statoil, HSE (UK),NDP, ABB (USA), Deepwater TechnologyGroup Pte, DNV, AkerKværner, ComplexFlow Design AS and MARINTEK.

Wave amplification and air-gap

Predictions of wave amplification aroundand between platform legs were made bysecond-order diffraction-radiation panelmodelling. Convergence studies were

made to assure numerically stable results.A single fixed column, an array of fixedcolumns, a three-column GBS, and a float-ing semi were considered. Comparisonswere made to model test data in regular andirregular waves, in various wave steep-

nesses. Linear diffraction theory was foundto significantly under-predict the ampli-fication, while improved agreement wasobtained with the second-order models. Insteep waves, additional effects grow larger,and further corrections are needed in somecases to avoid obvious errors. Systematictrends have been interpreted and docu-mented in guidelines.

Wave-in-deck impact

A simplified and efficient method for theprediction of deck-impact loads has beenestablished. The approach is based onthe conservation of momentum, similarto Kaplan’s approach but extended to 3Dand including effects from second-orderdiffraction due to the submerged hull. The

diffracted vertical wave kinematics areessential. Comparisons with experimentswith a GBS show good agreement.

WAVELAND JIPWave Amplification and Deck Impact

Air-gap, run-up and deck impacton offshore platforms are prob-lems frequently encounteredby the offshore industry. Waveamplification due to interactionwith large volume structuressuch as columns, pontoons and

caissons adds significantly to theimpacts. This includes structuressuch as semis, TLPs, Spars, GBStowers and LNG terminals. In

steep and extreme waves, thereare strongly nonlinear effects notpredicted by standard theoreti-cal tools, and model test data/ empirical methods are necessaryin design.

Further WorkPhase 3 of the JIP is currently in prepara-tion. Findings from Phase 2 will form thebasis for the development of robust andpractical procedures, and more compari-sons to model test data on impact loadswill be included. Systematic studies on theuse and validation of CFD codes are alsoplanned, where the cases from Phase 2 willbe considered. Validation against new 3-DPIV flow measurements will be included.CFD results will also provide useful addi-tional input to the practical procedures.

MARINTEK contact:[email protected] amplification and deck impact.

Photo from model test of semi in extreme storm.

Predicted vs. measured wave near a column.

Predicted vs. measured deck impact loadon GBS.


8/8

8

During the past few years, the offshoreindustry has moved from the continentalshelf to ever deeper waters. Along with thisshift in water depth, there has also beena shift in technology. Solutions for field

development have changed from varioustypes of fixed structures in shallow water,via different types of floating or tetheredstructures in intermediate to deep waters,to pure sub-sea developments at somedeepwater locations. It is believed thatthis tendency will increase the demand foradvanced marine operations for installa-tion and maintenance. Due to the com-plexity of such operations, and the longdistance from the surface to the sea bed,marine operations in deep water will beconsiderably more time-consuming than in

shallower waters. It is quite possible thatlocal current flows will have time to changesignificantly in the course of such opera-tions. Abrupt changes in the flow couldalso result in unforeseen and, possibly,unwanted events when a difficult task nearthe seabed is being performed.

Traditionally, it has been common practice

in the offshore engineering community totreat the current flow on a field as a profilewhich is constant. This simplification isacceptable for shallow to intermediate waterdepths (50-500 meters), where the effect

of the current tends to be considerably lessthan that of waves. As depth increases,however, the loading imposed by the currentwill assume greater importance, and thecurrent modeling should be more refined insuch cases.

This background makes it clear that thereis a need to establish well formulated andrelevant design conditions for marinestructures/operations in variable currentenvironments. Methods for predictingstrong current events, or estimating the

probability that such events occur, shouldalso be sought. These tasks offer a consider-able challenge to the offshore engineeringcommunity. Contributions from physicaloceanography, mathematics/statistics andinformation technology must be combinedto meet this challenge, and MARINTEKis currently in the process of preparing aresearch project for this purpose. Some

Taking the Effect of Ocean CurrentVariability into Account

There is wide variability in the current flow near

continental slopes around the world. Strongcurrent events have been recorded at severallocations. The mechanisms responsible for thishigh variability are still far from being fullyunderstood, but internal waves, boundary layer

activities which will be considered for inclu-sion in the project are listed below:

• Collection and analysis of ocean currentdata (speed, direction, temperature).

• Studies of critical current conditions forvarious types of structures, and variouscombinations of environmental loading(current, wind, waves).

• Synoptic observations of geophysi-cal parameters (waves, wind, current,atmospheric pressure, etc.).

• Techniques for establishing joint prob-ability distributions of waves, wind andcurrent conditions.

• Further development and improve-ment of numerical tools and methods(algorithms and computer technology)for direct calculations and prediction ofocean currents.


Illustrations of various current phenomena

Time and space varying current. Travelling “turbulence vortex” passingthe floater.

Current downdraft.

turbulence, surface winds and turbidity currents

are all processes capable of causing such condi-tions. There are also variations in the flow higherin the water column. This variability is causedby jets, meanders, eddies and various types ofinstabilities.

deep water riser tech.pdf

Documents