experimental investigation on internally ring-stiffened joints of offshore platforms

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Experimental Investigation on Internally Ring-stiffenedJoints of Offshore Platforms

Dr T S Thandavamoorthy, Non-member

This paper presents results of the experimental investigation on tubular joints that are stiffened internally with threeannular rings. The focus is mainly on the behaviour and strength of these types of joints subjected to axial brace compres-

sion loading. The nominal chord diameter of the tested joint was 324 mm and its thickness 12 mm. The correspondingdimensions of the brace were 219 mm and 8 mm, respectively. The joints tested were approximately one-fourth the sizeof the largest joints in the platforms built in a shallow water depth of 80 m in the Bombay High field. Some of the jointswere actually fabricated by a leading offshore agency directly involved in the fabrication of prototype structures. Bend-ing of the chord as a whole was observed to be the predominant mode of deformation of the internally ring-stiffened

joints in contrast to ovaling and punching shear of the unstiffened joints. Strengths of the internally ring-stiffened jointswere found to be almost twice that of unstiffened joints of the same dimensions.

Keywords: Offshore platforms; Tubular joints; Internally ring-stiffened; Strength; Bending

INTRODUCTION

Steel tubular framed structures are installed on the sea bed forthe exploration and production of oil from the sea bottom.They are called offshore platforms which serve as artificialbases, supporting drilling and production facilities above thelevel of waves. While a variety of platforms have been utilizedoffshore for exploration and production1, the most popularstructure for shallow water depth is the jacket or templateplatform that is fabricated mostly out of cylindrical steel

tubular sections because of their merit over other structuralshapes2. In the past four decades thousands of large tubularstructures have been built for offshore oil drilling andproduction. At present there are more than 7000 offshoreplatforms worldwide3. There are about 148 platforms4 in theBombay and other fields in the Arabian Sea and 10 platformsin the Ravvaa field of Bay of Bengal.

The typical structure consists of a deck, a substructure, andfoundation piles. The substructure is a prefabricated tubularspace frame, which extends from the sea floor to just abovethe sea surface, and is usually fabricated in one piece onshore,transported by barge, launched at sea, and upended on site by

partial flooding. Tubular pilings are driven through the mainlegs to fix the structure to the sea bottom, provide support forthe deck, and resist the lateral loads due to wind, waves andcurrents.

In the tubular frame, the intersection between two or moremembers, at least one of which is a tubular member, is called atubular connection. In the tubular connection, theintersection between various members is welded and thus

Dr T S Thandavamoorthy is with Shock and Vibration Laboratory,Structural Engineering Research Centre Madras, CSIR Campus, Chennai600 113.

This paper (revised) was received on August 2, 2002. Written discussion on thepaper will be entertained till October 31, 2003.

forms a joint called welded tubular joint. Because of welding,enormous heat is generated and hence the intersectionbecomes a heat-affected zone. Welding process involvesdeposition of metal at the intersection. Therefore, the joint inaddition to the segments of various members, also consists ofthe weld deposit, heat-affected zone and the base metal at theintersection5. The main member is denoted as chord and thesecondary member as brace or branch. The outside diameterof the brace is less than or equal to that of the chord. A joint

reinforced with welding of annular rings inside the chord atthe welding intersection is called internally ring-stiffened

joint.

Internally ring-stiffened tubular joints are used widely in theconstruction of fixed steel platforms6 and it is estimated thatthere are at least 2000 ring-stiffened joints in North Sea struc-tures7. However, techniques for assessing the capacity ofinternally ring-stiffened joints are still lacking. Therefore,more research is needed over a wide range of geometricparameters before formulating guidelines to assess the capacityof internally ring-stiffened tubular joints.

This paper presents in detail the experimental investigationcarried out on tubular joints that are stiffened internally withthree annular rings. Results of the static tests on internallyring-stiffened T and Y joints under axial brace compressionloadings are presented. Comparison of the behaviour of theinternally ring-stiffened joints under axial brace compressionloadings has been made with that of the unstiffened joints ofthe same dimensions and configurations under identicalloadings, published elsewhere8.

EXPERIMENTAL INVESTIGATION

The geometrical configuration of the internally ring-stiffenedtubular Tjoint is shown in Figure 1 while Figure 2 shows the

geometry of the Yjoint. In each joint, three annular rings, eachof 12 mm thickness and 75 mm width, have been welded to the

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Indian oil and gas field in the Arabian Sea. Compared with thelargest joints in these platforms, the actual test joints wereapproximately one-fourth in size.

In the fabrication of the tubular joint test specimens, thefabrication procedures, dimensions, materials, welding,

quality control, etc, correspond as precisely as possible toactual offshore structures. Some of the tested joints were, infact, fabricated with the same grade of steel used for offshorestructures by M/s Mazagaon Dock Ltd, Bombay, which isdirectly involved in the fabrication of the prototypestructures. This means that the tubular joints tested can beconsidered to be representative of a large number of platformsbuilt in the Bombay High in shallow water depths of up to80 m and also similar other environments elsewhere in theworld.

The joints were fixed to the pedestals by bolting. The pedestalswere, in turn, fixed to the strong concrete floor by means of 60

mm size mild steel bolts. The entire assembly was placedunder a reaction frame (Figure 3). On the flange of the bracemember, and between the horizontal cross beam of thereaction frame and the flange, a built-up steel joist assemblyand two numbers of 2000 kN Enerpack hydraulic jacks wereplaced as shown in Figure 3. Another 1000 kN hydraulic jackand a 1000 kN Proceq load cell were placed in a self-strainingframe that was kept by the side of the reaction frame on thetest floor. All the three hydraulic jacks were connectedthrough distributors to the electrical pumping unit by meansof high pressure rubber hoses. Axial brace compressionloading was applied on the joints by means of the hydraulic

jacks mounted on the brace. Load was applied on the specimen

in equal increments11.

Three dial gauges, with a least count of 0.01 mm, weremounted beneath the joint (Figure 3), one directly at mid-span

under the load point and others approximately at third points,to facilitate measurement of the deflection under load. In thecase of Y joint, dial gauges were fixed perpendicular to thechord member. For each load increment, deflection readingsof all the three gauges were recorded. Load was monotonicallyincreased till the joint reached its ultimate strength. Theultimate loads sustained by the T joint (UDT1) and Y joint(UDY1) are given in Table 2. Typical loadmid-spandeflection curve for UDT1 is depicted in Figure 4. In the caseof Y joints, the component of the deflection along thedirection of loading was resolved from the measured values.Corresponding load-deflection curve for the mid-span for

UDY1 is illustrated in Figure 5.

RESULTS AND DISCUSSION

The experimentally measured ultimate load for UDT1 was1887.6 kN. The maximum load sustained by UDY1 was1834.0 kN. When compared to the already published8 ultimateload of unstiffened joints, the internally ring stiffened joint isalmost twice strong as the unstiffened joint of the sameconfiguration and dimensions. With the addition of threeinternal rings to the chord member the capacity of joint hasincreased to a great extent.

The shapes of the load-deflection curve of the stiffened joints issimilar to that of a typical prismatic beam subjected to flexuralloading. They clearly exhibit the strain hardeningcharacteristics of steel. The load-deflection curve is linear upto

Figure 3 Typical set-up for axial brace compression loading

Table 2 Measured loads

Specimen No Experimental Ultimate Loads, kN

UDT1 1887.60

UDY1 1834.00

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about 900 kN in the case ofTjoint (Figure 4). Afterwards thecurve is non-linear with larger increase in deflection with load.Near ultimate load the curve shows a plateau indicating

excessive deflection of the joint. The chord member as a wholewas bent like a prismatic member. Figure 6 shows clearly thebending and excessive deflection of the chord of an internallyring-stiffened joint tested under a simply supported conditionas part of a preliminary investigation carried out at the FatigueTesting Laboratory of the Structural Engineering ResearchCentre. Absolutely no deformation of the chord wall andconsequent ovaling in the vicinity of the welded intersectionof the internally ring-stiffened joints were observed in thisexperimental investigation. In the case of Y joint, the load-deflection curve is linear upto about 600 kN (Figure 5), andthen the curve is non-linear upto ultimate load. The plateauobserved in the case ofTjoint is absent in this case. Because ofthe inclination of the brace, a horizontal force is created in thechord, which limits the deflection of the chord.

Load-deflection curves of both Tand Yjoints are compared inFigure 7. It is observed that, in general, there is a closeagreement in the behaviour of both the joints. Initially upto aload of 1000 kN the behaviour of the Yjoint is slightly stifferthan that of the Tjoint. Above the load of 1000 kN and uptoabout 1800 kN, the Tjoint behaves in a stiffer manner than theYjoint. Right from the start of the loading till failure, bendingwas observed to be the predominant mode of deformationunder axial brace compression loading. This predominant

flexural behaviour of the ring-stiffened joints, quite differentfrom that of the unstiffened joints, truly represents the

realistic global behaviour of the structure which is quiteevident from global frame responses obtained from thecollapse tests conducted by Bolt et al12 on large scale tubular

frames representative of offshore jacket structures initiatedunder the Frames project in 1987.

Figure 4 Load-midspan deflection curve for Tjoint UDT 1

Figure 5 Load-midspan deflection curve forYjoint UDY 1

Figure 6 Bending of internally ring stiffened joint

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CONCLUSIONS

Two internally ring-stiffened joints were tested under axialbrace compression loadings. The normal chord and bracediameters of the joints were 324 mm and 219 mm. The

nominal thickness of these joints were 12 mm and 8 mm,respectively. From this experimental investigation, it has beenobserved that the strength of internally ring stiffened jointswas almost twice that of the unstiffened joints of the samedimensions and configuration, the results of which have beenpublished elsewhere8.

Experimental results, obtained from the testing internallyring-stiffened tubular joints under axial brace compressionloading, clearly show bending to be the predominant mode ofdeformation. This is similar to that of a prismatic membersubjected to flexural loading and is in contrast to the primarymode of failure of ovaling and punching of the chord member

of the unstiffened joints. However, the change in behaviour ofthe internally ring-stiffened tubular joints has not beenreported earlier.

It has been observed from the experimental investigation thatwelding of three annular ring stiffeners to the inside of thechord member has resulted in completely eliminating the localbending and ovaling of the chord wall in the vicinity of thewelded intersection. This arrangement has also impartedenormous bending stiffness to the chord as a whole with theresult the behaviour of the internally ring-stiffened joints hasdrastically changed from the punching shear to flexure. Thepredominant bending behaviour of the chord member truly

represented the realistic global behaviour of the structure as

the prototype structure behaves in a similar manner, as isevident from the results of tests on large scale tubular frames,representing offshore structures, published in the literature12.

ACKNOWLEDGEMENTS

This paper is published with permission of the Director, Struc-tural Engineering Research Centre, Chennai, India.

REFERENCES

1. D V Reddy, A S J Swamidas and S Reddy. Introduction in Offshore

Structures. D V Reddy and M Arockiasamy, (eds), Krieger Publishing Co,

Florida, vol 1, 1991, pp 1-65.

2. W J Chen and D J Han. Tubular Members in Offshore Structures. Pitman

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3. K A Digre, W Krieger, D J Wisch and C Petrauskas. API RP 2A Draft

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Structures, BOSS 94, C Chryssostomidis, et al, (eds),Pergamon, Oxford, 1994,

pp 467-478.

4. S Y Kekre. Engineering and Construction Division of ONGC Bombay A

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94.

5. P W Marshall. Design of Welded Tubular Connections. Elsevier Science

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6. Wimpey Offshore. In-service Database for Ring-Stiffened Tubular Joints.

Report No WOL 035/91, Wimpey Offshore Ltd, London, 1991.

7. N Tahan, N W N Chols, T V Sharp and S Y A Ma. Uncertainties Associated

with the Behaviour of Ring-Stiffened Joints Derived from an In-service

Database. Behaviour of Offshore Structure, BOSS 92, M H Patel and R Gibbins,

eds, BPP Technical Services, London, Supplement, 1992.

8. T S Thandavamoorthy. Behaviour of Unstiffened Tubular Joints ofOffshore Platforms. Journal of The Institution of Engineers (India), vol 82,

pt CV, February 2002, pp 224-228.

9. API RP 2A-LRFD. Recommended Practice for Planning, Designing and

Constructing Fixed Offshore Platforms Load and Resistant Factor Design,

American Petroleum Institute, Washington DC, 1993.

10. IS: 226. Indian Standard Specification for Structural Steel (Standard

Quality),Indian Standards Institution, New Delhi, 1975.

11. T S Thandavamoorthy. Assessment and Rehabilitation of Damaged Steel

Offshore Structures. Ph D Thesis,Anna University, Chennai, 1998.

12. H M Bolt, C J Billington and J K Ward. Results from Large-scale Ultimate

Load Tests on Tubular Jacket Frame Structures. Offshore Technology

Conference, OTC 94, May 2-5, vol 2, 1994, pp 303-312.

Figure 7 Superposition of deflections of stiffened Tand Yjoints

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experimental investigation on internally ring-stiffened joints of offshore platforms

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