OTC-4868-MS-P

OTC 4868 Underwater Repairs Using Wet Welding in the North Sea by M.B. Green, Occidental Petroleum (Caledonia) Ltd. Copyright 1985 Offshore Technology Conference This paper was presented at the 17th Annual OTC in Houston, Texas, May 6-9, 1985. The material is sUbject to correction by the author. Permission to copy is restricted to an abstract of not more than 300 words. SUMMARY A description is given of the design and installation of several wet welded repairs made to appurtenances on 2 North Sea jackets. Details are given of the mechanical propertieE obtained for welds made in both the tank and offshore. Fatigue specimens were also prepared offshore and subsequently tested in 4 point bending. An S-N curve is developed and compared to data available for welds made in air. INTRODUCTION Underwater repairs in the North Sea have traditionally been made by either hyperbaric welding or installation of mechanical/grouted clamps. However, in the Gulf of Mexico wet welding has also been used successfully. Since several areas of secondary structural significance required repair on the Claymore platform's cathodic protection system it was decided to make repairs by wet welding prOVided it were demonstrat~d that adequate mechanical properties could be developed from such welds. The completion of these repairs would permit evaluation of both the perform- ance and cost of wet welds in a North Sea application versus other methods of repair. During the course of this repair work, furthex defects were discovered to appurtenances on the nearby Piper platform. The scope of repair work was therefore extended to cover these items. BACKGROUND The Piper and Claymore fields are situated some 100 miles north east of Aberdeen in Blocks 15/17 and 14/19 in the UK sector of the North Sea. The Piper field was discovered in January 1973 followed by Claymore in May 1974. References and illustrations at end of paper The Piper Platform is a 36 slot drilling and production facility located in 474 ft (144.475 m) water depth. It was launched in June 1975 and initial production was achieved in December 1976. Complete details of the installation are given in a paper by Duvivier and Henstock (1). The Claymore platform is a 36 slot drilling and production facility located in 360 ft (108.728 m) water depth. The jacket is conceptually similar to the Piper structure except for the removal of one bay of framing to accommodate the different water depth. The jacket was launched in June 1976 and first production was achieved in November 1977. DESCRIPTION OF THE CLAYMORE PLATFORM REPAIRS INTRODUCTION Corrosion protection of the underwater sections of the jacket was originally provide, by an impressed current cathodic protection system. 55 anodes were distributed over the jacket terminating at various levels. Cables to these anodes were run in pipes to the surface, where they terminated in junction boxes at the first level of jacket framing above water. Cabling was then routed from these junction boxes to the power supply located within the topside modules. The pipes or "anode risers" carrying the cablE were welded to the jacket at each structural member (either horizontal or diagonal) which they passed. This connection consisted of a cruciform plate fillet welded on both the jacket member and "riser" sides (see Figure- 1) There were 377 such connections distributed over the depth of the jacket. The anode riser was API 5L Grade B steel and 249 2 UNDERWATER REPAIRS USING WET WELDING IN THE NORTH SEA OTC 4868 1. Pump Casing PC 13 - A longitudinal defect was reported from approximately (-) 60 ft to (-) 70 ft. In 1979 during grouting operations to strengthen certain jacket members at the (-) 40 ft elevation, cracks were discovered in some jacket members at the anode riser 3. supports. Initial remedial action consisted of drilling 'crack stoppers' to prevent propagation. varied in diameter and wall thickness as follows: 10.75" dia. x 0.438" from (+) 20'to(-)40' 8.625" dia. x 0.375" from (-) 40'to(-)100' 6.625" dia. x 0.375" below (-)100' The cruciform plate was detailed as 0.375" thick plate fromBS 4360 Grade 50D steel or equivalent. However subsequent chemical analysis of a sample taken from one of the connections (46/10B 10) during the repair operation showed the steel to be compatible with the mild steel electrode (see Table 3) • 2. Shale Chute - Due to mud recirculation problems with the original chute a redundant anode riser had been adopted to serve as a shale chute. This chute had become blocked on one or two occasions where it tapers from 10.75" to 6.625" diameter at (-) 120 ft. To relieve these blockages it had been necessary to cut the chute and introduce a flexible coupling to connect the two sections of chute. However, this coupling had deteriorated to the point where it needed replacement. Anode Riser 23 - This is supported off anode riser 18 "piggy-back" style. Riser 23 had broken off above a support at approximately (-) 10 ft. This was followed by further inspection and an engineering study to define the possible cause and extent of the problem. Full details of this work are given in a paper by Nicholson et a1 (2). In some instances cracks were restricted to the 'riser side' of the connection only. Considerable experience had already been gained in the design and installation of mechanical or grouted clamps (3 and 4) and alternative methods of strengthening and repair were constantly under review. Since these particular cracks were not in primary structural members it was decided to make wet welded repairs in order to gain experience of this technique from both the aspects of time/cost and quality. DESIGN OF REPAIRS The connections selected for welded repair are shown in Table 1. Design of the repairs (5) was based on forces generated in the computer model that had been used in the earlier defect assessment study (2). One location was selected for repair using a small prefabricated habitat so that the technique could be compared with the wet welded solution for both cost and quality. This "mini" habitat provided localised shielding OL the repair area from the surrounding environment without the usual disadvantages of high cost and problems of handling. Various alternative repair configurations were also selected to permit evaluation of their ease of installation and subsequent performance. Details of the various repair schemes and completed repairs are shown in Figures 2 to 6. DESCRIPTION OF THE PIPER PLATFORM REPAIRS INTRODUCTION Routine inspection on Piper had also found significant damage to certain appurtenances which appeared suitable for wet welded repair. The items affected were:- DESIGN OF REPAIRS 1. Pump Casing - Crack stopping holes were drilled at the ends of the 'defect' to prevent propagation. A mild steel doubler plate was then fillet welded over the defect (Figure 7). The doubler plate was rolled to the correct I.D. onshore. 2. Shale Chute - A transition piece was detailed similar to the original design, but made up in two sections to allow ease of handling and installation (Figure 8) • The transition pieces were prefabricated onshore. 3. Anode Riser 23 - A replacement section of riser and new support to riser 18 were made up using materials available on the MSV 'Tharos" (Figure 9). This particular repair illustrated the versatility of wet welding since it had taken only 10 days between first discovering the damage to completion of the repair. A summary of the repairs is given in Table 2. OFFSHORE OPERATIONS INTRODUCTION Sea Con undertook the wet welding as nominated sub contractor to Wharton Williams (2W) who were the main surface diving contractor for the platforms. This arrangement was used because the scope of the wet welding work did not warrant a separate contract with the inherent problem of split responsibility. A welding procedure qualification document was prepared on the basis of the anode riser pipe being API 5L Grade B and the cruciform plates BS 4360 Grade 50 D steel. For the proposed repairs it was unnecessary to obtain welds with minimum notch toughness 250 TC 4868 M.B. Green 3 conforming to the Department of Energy's Guidance Notes (8). However, a minimum requirement of 25 ft Ibs average at (-) 20°C was specified so that any procedure passing this requirement could be subsequently considered for more structurally significant items. MOBILISATION/DEMOBILISATION accordance with AWS D3.6-83(6). The results of these tests are given in Table 4. However in the case of the wet welded specimens it was believed that the water temperature in the tank (80°F) could have a significant effect on the results obtained. Furthermore, conditions in the tanks, for both the wet and habitat welding, would also be less severe than those expected offshore. FATIGUE TESTING Further procedure plates were therefore welded offshore at the actual work site and subsequently tested. Results of these tests are given in Table 5. However, because of logistical and scheduling problems the results of these tests were not used as a basis for acceptance of the repairs. In order to make a preliminary assessment of the fatigue performance of wet welds versus similar specimens welded in air twelve plate specimens (Figure 10) were welded at (-) 40 ft. The steel was BS4360 Grade SOD and it was welded in the 3G position. The specimens were prepared with run on/run off plates on either side of the weld. The plates were tested under 4 point bending (7) at a nominal stress ratio R = 0 (ratio of minimum to maximum stress) • Loading was applied using a servohydraulic actuator of 400 KN capacity. The load was measured using an associated loadcell. The specimens were supported in the test rig on rollers and load applied through a spreader plate to rollers at quarter points (Figure 11) • ashore where the removed by The specimens were in Figure 10. The specimens were brought run on/run off plates were hacksawing and grinding. then instrumented as shown The mini habitat weld was undertaken by 2W personnel during JUly 1984. The wet welding team was mobilised to Claymore on the 6th June 1984. The team consisted of:- The team was transferred from Claymore to Piper on the 22nd June 1984. Demobilisation from Piper occurred' on' 11th Jul-y.19 8.4 • SITE PREPARATION 1 Diver/Welder Supervisor 1 Drilling Supervisor 4 Diver/Welders 1 Trainee The wet welding contractor had expressed concern regarding the effect of residual magnetism on the welding process, particularly as a result of the impressed current system. Should the level of residual magnetism exceed 40 gauss it had been proposed that welding be suspended until such time as the magnetic field decayed to allowable levels by either shutting down the cathodic protection system and/or locally demagnetising the weld area using local magnets. In the event it proved unnecessary to shutdown the C.P. system, although residual magnetism did cause welding problems in several instances. 2W divers were used to clean the areas to be repaired, drill crack stoppers if required, and undertake initial fit up. Sea Con completed final fit up. WELDING The wet welded repairs were made by Sea Con. Mini habitat repairs were undertaken by 2W divers. A complete breakdown of the times required for each repair is given in Tables 1 and 2. INSPECTION The specimens were initially subjected to a few cycles to ensure adequate 'shakedown' of strain gauges. The strain gauges and crack probes were calibrated and the specimens sUbjected to fatigue loads at a frequency between 5 Hz to 9 Hz depending on the level of load. Continuous strain measurements and crack probe measurements were maintained for the duration of the test. Failure was defined as the life at which the crack had grown through 50% of the material thickness. This failure coincided with the point at which complete loss of specimen stiffness was observed. All welds were inspected 100% by 2W divers. Welds made using mild steel electrodes were inspected both Visually and by magnetic particle. Welds made using austenitic electrodes were inspected visually only. RESULTS OF PROCEDURE QUALIFICATION TESTS Before proceeding with the offshore repairs, procedure plates were welded in tanks onshore and sUbjected to mechanical tests in Results of the tests are given in Table 6. Specimens 1 and 9 showed no signs of failing after 2.14 million and 1.2 million cycles respectively. They were designated "runners" and the tests terminated. ~.linear regression analysis was performed on the results (omitting the 2 runners) and the resulting curve (Figure 12) compared with the Department of Energy's 'F' curve (8) which had been developed from testing of similar specimens welded "in air" as part of the United Kingdom Offshore Steels Research 251 UNDERWATER REPAIRS USING WET WELDING IN THE NORTH SEA OTC 4868 Project UKOSRP (9). The analysis of the the designer because of potential problems data gave the following mean line equation with electrode compatibility. for the wet welded Specimens:- FUTURE DEVELOPMENTS Loglo N = 12.592 - 3.1 Loglo Af where N = cycles to failure The recent underwater welded repairs to Piper A f = stress range (N/mm2) and Claymore have shown that both wet and habitat welds can be made satisfactorily and A statistical analysis of the data about the cost effectively. Because of constraints mean line gave a standard deviation of of depth and welding position the use of 0.093; a design curve based on the mean austenitic electrodes for wet welded repairs minus two standard deviations would is limited at present. Further development therefore be defined by:- work on electrode make up is required to improve workability, visual appearance and Loglo N = 12.409 - 3.1 LogloAf mechanical properties. CONCLUSIONS No mechanical or fatigue data was obtained for the mild steel electrode during the Mild steel electrodes have alrea”dy been used present project. Because .of the good extensively in the Gulf of Mexico for wet workability and weld appearance achieved with welded repairs. During the repairs this electrode, further work may be warranted described here consistently good visual in obtaining this information. Also trials quality was achieved and the integrity could be undertaken for welding of BS4360 confirmed by magnetic particle inspection. Grade SOD steel with a “modified” mild steel This electrode did not suffer from the electrode. positional constraints exhibited by the austenitic electrode. ACKNOWLEDGEMENTS The austenitic electrode used for welding This work was carried out as part of routine the anode riser supports was difficult to repairs to the Piper and Claymore platforms. work and generally had a poor appearance. The author is grateful to Occidental Since subsequent magnetic particle Petroleum (Caledonia) Limited and their inspection is impossible the poor visual Partners for permission to publish the result$ appearance of the weld may prove a hindrance of this work. in assessing its subsequent performance. Howeverr the fatigue strengths obtained REFERENCES using this electrode (in 3G position) show a good comparison with properties obtained 1. Duvivier S. and Henstock P.IL. for similar welds made in air. “Installation of the Piled Foundations and Production Modules on Occidental’s Overhead .welding of butt welds (4G position) Piper ‘A’ Platform” Paper 8237 with the austenitic electrode proved Proceedings of the Institution of Civil extremely difficult in the tank. Because of Engineers, London, August 1979. this, every care was taken in the subsequent design of repairs to eliminate any 2. Nicholson R., Cotter K.H. and Green M.B. requirement for making butt welds in this “Defect Assessment of the Claymore ‘A’ position. However, the problems with Platform” Paper 356 Fourth International making overhead butt welds may limit the Symposium on Offshore Mechanics and scope for future repair design. Overhead Arctic Engineering, Dallas, February fillet welds (4F position) do not present 1985. the same problem and can be made to a satisfactory quality. 3. Green M.B. “Experience with Fatigue Analysis and Inspection Results in the The application of mini-habitat welding North Sea” Paper OTC 4524, Offshore proved successful in terms of both quality Technology Conference, Houston, May 1983. and cost. Subsequent magnetic particle inspection of the welds was possible and 4. Fern D.T. and Shear L. “Bolted Repair of therefore the overall confidence level when Tubular Joints” Paper 23 Second welding BS 4360 Grade 50D steel is higher International Symposium on Integrity of than for a similar wet weld. Because of the Offshore Structures, Glasgow, July 1981. time required for fit up of the habitat (and its initial cost) habitat welds are more 5. “Underwater Welded Riser Repairs expensive than equivalent wet welds and their Claymore ‘A’ Platform” Wimpey Offshore use may be restricted to a “one off” Report WOL 122/83A, July 1983. situation or where a higher confidence level is required regarding quality. 6. American Welding Society “Specification for Underwater Welding” ANSI/AWS D3.6-83. It is important to obtain comprehensive material documentation when making welded 7. “Testing of Underwater Wet Welded Plate repairs. Any material substitutions (i.e. Specimens” Wimpey Offshore Report use of high grade steels when a lower grade WOL 320/84, December 1984. has been specified) must first be cleared with %7 8. Offshore Installations: Guidance on Design and Construction; U.K. Department of Energy 3rd Edikion, April 1984. 9. “Background to New Fatigue Design Guidance for Steel Welded Joints in Offshore Structures” U.K. Department of Energy, 1984. TABLE 1 Claymore Platform Repairs I I I I Approx. I I / lApprox. Lengthl Welding { I [ Depth I I Type of I of Weld I Time* I Connection I (feet)! Type of Wpair [ Electrode I (inches) ~ (Hours) I I I I I I I I 52/04B9 I 22 I Wet Weld I Austenitic I 120’’-3/8”FW I 12 I I I I I 5/9/0412 I 30 I Habitat Weld I AWS A5.1-691 149’’-3/8”FW [) 15 I I I I Class E70181 75” - Butt !) / I I ! I 30/0452 I 30 I Wet Weld I Austenitic ~ ~7’’-8°8°FW ! ) 12 I I I - Butt ~) I II I I I 46/10BIO I 100 ! Wet Weld I Mild Steel [ 40’’-3/8”FW I 5 I I I I I I I 47/10B12 I 100 I Wet Weld I Austenitic I 39’’-Butt I 5 I I I I NOTE: * Welding time is strictly on bottom time actually welding. It does not include fit up, habitat installation (where appropriate) or =pection. FW = fillet weld TABLE 2 Piper Platform Repairs I I I I I Approx. I lApprox. Lengthl Welding I I Depth \ I Type of I of Weld Time* I Item ~ (feet)~ Type of Repair I Electrode I (inches) \ (Hours) I I I I I I I I I Pump Casing! 70 I Wet Weld [ MildSteel ~ 350’’-3/8”FW I 7 1 I PC13 I I I I I I I I I Shale Chutel 120 I Wet Weld [ Mild Steel I 87’’-3/8”FW I 7 I I I I I I I I I I Austenitic I 18” ) 1) I I Anode Riserl 12 I Wet Weld )FwI)6 I I AR23 I I Mild Steel I 18” ) ~) I I / NOTE : * Welding Time is “on bottom” time welding. It does not include prior cleaning, fit up or inspection. Fw = fillet weld 962 I I I I I I M..,iptio” 1T.., IW.,”weld P1.t.. ! [Plate, 1 & 2} I mbitat ..ld I (Plate 3) I F. C19U. P1.t*. ~ clay - I AR 5/9/0412 -,. I (Ubit.t) ~ AR 30/0452 I I AR 52/0489 I I AR 47/10B12 I Extg. Plate I I AR 46/LOB1O I RW’air I AR23 ! P+lacemallt I I I -..1 COmfa. iciar, s “..hmlic.l ,rop.rtia. I 1 I I I I I I I I 1111 ch.rpy rapmt, [Joules) I Ill! I II 1111 I P1.t* I steal I l%i.kn..s I I De.$Fatlcm I [=) ,84360 I 12 I 0.15 ~ 0.43 ~ 1.41 / 0.015 / 0.0o6 I I 4>3 / 563 I Grad. 50S I II I I 1 ,,, , ,,, , 11 ,s4360 i 10 i 0.~6 ~ 0.37 j 1.36 j 0.015 \ 0.008 / 0.40 i 420 i SS0 i Gr,ti 50. 1 1 I I I I I I I I I 8s4360 ( 38 I 0.13 / 0.41~ 1.47 ~ 0.017 ~ 0.007 ~ 0.41 ~ 399 I G,.d. 50. I S30 ! 186 185 184 i 1S5 II I (-140 ; I BS4360 I 10 I l! I I 1 I I I 1 I G,ade 50. I II / II II ,s4360 I 10 I Grade 500 1 [ l! BS4360