A Case History On the Way to Implementing Our Integrity Program We Discovered Pipeline Efficiencies Beyond Our Expectations

Transcription

1 A Case History On the Way to Implementing Our Integrity Program We Discovered Pipeline Efficiencies Beyond Our Expectations By Messrs. Al Michini, PE and Michael Mori, PE Northwest Pipeline, G.P. Salt Lake City, Utah USA And Mr. Randy L. Roberts Coastal Chemical Co., L.L.C. / Brenntag N-SPEC Pipeline Services Lafayette, Louisiana USA Key words: pipeline efficiency, ILI tool, dry mechanical pigging, liquid/mechanical pig cleaning, enhanced cleaners, slotted discs, dual diameter pigs, high-pressure water flex lance. ABSTRACT As part of its Integrity Management Program Northwest Pipeline, G.P. (NWP) targeted its 6-inch Vernal, Utah to Green River, Wyoming segment for internal inspection in 006. In addition to suspected cleanliness issues that complications associated with pigging included: a dual 0-inch river crossing, reduced port valves, unbarred tees, and a risky span inspection. Utilizing innovative pig trap designs and N-SPEC cleaning technology NWP successfully inspected this 61-mile segment. In so doing the internal roughness of the pipeline was reduced and pipeline efficiency was increased. FACILITY MODIFICATIONS AND PREPARATION FOR IN-LINE INSPECTION Williams NWP added three launcher and receiver sets, cleaned an unpiggable portion via water lance, and made various other modifications on this 61-mile (106 km) segment to prepare the pipeline for in-line inspection. The pipeline was broken down as follows: SEGMENT ONE: Segment one was the 41-mile 6 segment between NWP s Vernal Compressor Station and the south bank of the 34 Flaming Gorge pipeline span. Included in Segment One was a short 600-foot section where the 6-inch bifurcated into two 0-inch lines running parallel to each other under the Green River. The transitions from the single 6-inch to dual 0-inch lines under the river were modified on both sides of the river so that one of 1

2 the lines could be pigged as part of the larger segment using dual diameter pig technology. The other 0-inch line was modified to include its own launcher and receiver set. SEGMENT TWO: Segment two comprised the 100-foot long 34 pipeline span of the Flaming Gorge Reservoir which hangs suspended approximately six hundred feet above the average water level. Pigging this segment from one end to the other was considered very risky considering the weight of the tooling and back pressure required to move the pigs. Considering the ability to visually inspect the pipeline to ensure its integrity, the focus shifted to removing any liquids or solids trapped at either end of the span, which are low spots in the pipeline. To accomplish this NWP took the line out of service and depressurized it for 48-hours so taps on each end could facilitate the use of a highpressure water flex lance for cleaning. The flex lance removed any solid and liquid build up at the base of each end. Cleaning did not encompass the entire span; efforts were focused at the span s ends and a few hundred feet in each direction of the base taps. The 10K psi pressure flex lance consisted of a stinger combination nozzle attached to a high-pressure flex hose. The hose and nozzle were inserted inside the pipe at the hot tap locations and jettisoned in either direction of the insertion point a distance of approximately 50 feet. Once the inserted hose and nozzle head reached its end position in the pipe a solenoid lance valve was activated resulting in the water jet expanding to an umbrella shape that pointed back toward the point of entry. As the hose was retracted the emitted water cleaned the pipe walls while simultaneously forcing any spent solids, condensate, and water back toward the point of entry where they were removed via vacuum truck. Following the completion of the lancing on both sides the span was dried, purged with natural gas, and repressurized. SEGMENT THREE: The third and final segment consisted of the 0-mi 6 pipeline segment between the north side of the span and NWP s Green River Compressor Station. The major complication on this segment was an unbarred size on size 6 tee followed immediately downstream by a 6 x reduced port ball valve. After much analysis, NWP decided to leave the tee and valve in place and pig through them by shutting off the flow through the tee. Therefore, modifications to this segment were limited to new pig facility installations on both ends and the replacement of an intermediate reduced port plug valve. CLEANING & IN-LINE INSPECTION Internal cleanliness was an important issue as the in-line inspection (ILI) vendor warned of potential sensor lift off caused by solids left in the pipeline. The applicable dual diameter ILI tool was very sensitive to the magnetic and inorganic iron compounds (e.g. iron oxide, iron carbonate, iron sulfide, etc.) since the sensors in the ILI tool magnets were designed to come in very close contact with the pipe walls. Noting this sensitivity, sensor lift-off tolerances were of great concern to the ILI vender.

3 Mechanical pigging alone would not have accomplished the cleanliness required by the ILI tool vender in the limited time that was allotted for cleaning. Continuous dry mechanical pigging would only have fractured the brittle iron compounds into sub particles causing downstream problems (e.g. at meter runs and turbine/compressor inlet filters) and would have embedded these iron sub particles in any internal anomalies if present. Considering these issues NWP opted to have N-SPEC Pipeline Services liquid clean the pipeline using enhanced cleaning products in a diesel base solution while utilizing specially designed mechanical pigs by Girard Industries in Houston, Texas USA. The special mechanical cleaning pigs were of two designs. The first was a steel body mandrel type with wire brushes designed to maneuver through the 6-inch nominal section and negotiate 19-inch valve bores. The second was a steel body mandrel type utilizing two rows of strong magnets with the same tolerances as the first. Both steel body mandrel types encompassed four sets of slotted urethane discs with each set made up of one 75- durometer slotted disc, one 85-durometer thin membrane disc, and a 65-durometer slotted disc rotated 45-degrees from the other discs (See Figure 1). (Figure 1) This design was durable, flexible, retractable, expandable, and allowed for good sealing and scraping along the internal pipe walls. Cleaning took place while the pipeline remained in service. Gas flow and pressure conditions were modified to ensure a pig speed of five to eight miles per hour (7.0 to

4 feet/second) for these cleaning runs. N-SPEC Pipeline Services provided a portable horizontal high-pressure separator at the receiver end of each segment of this 61-mile pipeline to separate out all liquids, injected cleaning solutions, and spent material (solids) while allowing the dry, clean gas to flow back into the downstream pipeline with uninterrupted gas service. CLEANING DETAILS: Cleaning of the pipeline consisted of progressive pigging from less aggressive pigs to aggressive pig types. (See Table 1) This allowed for removal of solids from the pipeline layer by layer minimizing the potential for plugging of the pipeline and separation equipment. In general, most pipelines can be liquid cleaned sufficiently enough in four runs to ensure a successful MFL tool run. A pipeline is generally considered clean when pigs and liquid samples received show six percent or less in solids per quantitative volume. 1 However no industry standard exists for pipeline cleanliness aside from efficiency tests and even they are subjective. In this case additional runs were necessary since the ILI vendor and their newly designed, ultra-sensitive, multi-diameter ILI tool required a greater degree of solids removal (internal pipeline cleanliness) than normal. The following procedures were used in Segment One (6 x 41 miles) and are typical of all segments: (Table 1) First Run/ Day 1: 1. Pump 1500 gallons of diesel into pipeline.. Launch YBS6 Swab pig with NO jetting ports. Second Run/Day : 1. Pump 40 gallons of N-SPEC cleaner and 1800 gallons of diesel into pipeline. Launch 6 RCCWBCAVJPR poly wire brush pig with jet ports; Third Run/Day 3: 1. Pump 40 gallons of N-SPEC cleaner and 1800 gallons of diesel into pipeline.. Launch 6 /0 Slotted discs Wire Brush Pig with jetting ports. 3. Pump 10 gallons of N-SPEC cleaner gallons of diesel into pipeline. 4. Launch GAUGE Pig with no jetting ports through pig. Forth Run/Day 4: 1. Pump 40 gallons of N-SPEC cleaner and 1800 gallons of diesel into pipeline.. Launch 6 /0 Slotted discs Wire Brush Pig with jetting ports. 3. Pump 10 gallons of N-SPEC cleaner gallons of diesel into pipeline. 4. Launch Two () 6/0 Slotted Discs MAGNET Sealing Pigs Fifth Run/Day 5: 1. Pump 40 gallons of N-SPEC cleaner and 1800 gallons of diesel into pipeline.. Launch 6 /0 Slotted discs Wire Brush Pig with jetting ports. 1 Roberts, Randy L., Coastal Chemical Co., L.L.C, N-SPEC Division, Preparation for In-Line Inspection page 5; (10-mil sample). 4

5 3. Pump 10 gallons of N-SPEC cleaner gallons of diesel + 5 gallons of N-SPEC 304 Inhibitor into pipeline. 4. Launch Two () 6/0 Slotted Discs MAGNET Sealing Pigs; 5. Launch 6 /0 MFL Vender Scraper Blade Pig; Sixth Run/Day 6: 1. Pump 50 gallons of N-SPEC cleaner and 1100 gallons of diesel into pipeline.. Launch ():6 /0 Slotted discs MAGNET Pigs with jetting ports. 3. Launch 6 /0 MFL Vender Scraper Blade Pig Seventh Run/Day 7: 1. Launch 6 /0 Slotted discs MAGNET Pig with NO jetting ports.. Launch 6 /0 MFL Vender Scraper Blade/Magnet Pig; 3. Launch 6 /0 Vender CALIPER TOOL CLEANING RESULTS: Following the completion of the cleaning operations of all segments the dual diameter MFL tool was run successfully without sensor lift-off. (Table ) N-SPEC Total Cleaning Results (6 x 61 Miles) Description Standard US Metric Volumes Volumes N-SPEC Cleaners: 4,556 Gallons ~17,45 litres Diesel as diluent: 0,400 Gallons ~77,15 litres Total Cleaning Solution Injected: 4,956 Gallons ~94,460 litres Total Liquids Received: 5,43 Gallons ~96,5 litres Total Estimated Material ~-tons ~0 metric tons Removed: Cleaning Duration: 15 Days INCREASE IN PIPELINE EFFICIENCIES REALIZED Based on the N-SPEC results noted above NWP expected an increase in pipeline efficiency. Several months after the completion of the pigging operations analyses were completed to quantify the magnitude of efficiency improvement as well as establish a baseline value representing a clean pipeline. The efficiency data was based on separating the 61-mile pipeline into two segments utilizing the existing intermediate metering station as a breaking point. The testing sections were: (1) Vernal Station to Clay Basin (six miles south of the Flaming Gorge Span) and () Clay Basin continuing north to Green River Station. The analysis utilized a transient computer model of the segment and flow data (volume, pressure and temperature) compiled from the SCADA system. Three time periods before the cleaning operations and three time periods after the cleaning operations were selected and duplicated in the model. 5

6 For the study, roughness was used as the measure of efficiency. Roughness is a term contained in the pipeline friction factor calculation 1, which in turn is used in the fundamental flow equation used by the model. Roughness is typically measured in micro-inches and pertains to the internal finish of the pipe. But in this study, the roughness is essentially an effective roughness, one that accounts for any foreign material adhering to the pipe wall, any liquid accumulations of hydrocarbons or compressor oils, or any bends or valve losses. Tables 3 and 4 (below) summarize the analyses: (Table 3) Before Cleaning Pig Operations Analysis Run 1 3 (Table 4) Time Period :00 to : :30 to : :30 to :30 After Cleaning Pig Operations Analysis Run Time Period :30 to : :00 to : :00 to :30 Clay Basin Mode Shut-in Shut-in Injection Clay Basin Mode Shut-in Shut-in Withdrawal Segment Roughness Results (inches) Vernal to Clay Clay Basin to Basin Green River Segment Roughness Results (inches) Vernal to Clay Clay Basin to Basin Green River The average roughness for the Vernal to Green River pipeline segment prior to the cleaning operations is inches, and after the cleaning operations the average roughness of the segment was reduced to inches. Based on the repeatability of these results, NWP accepts that the roughness values determined in the analysis are close approximations of the actual roughness values and the baseline roughness for this segment of pipeline could possibly be as low as inches. 6

7 To provide a perspective on the effect lower roughness values play in pipeline operations NWP ran steady-state models to demonstrate the improvement in capacity, the reduction in pressure loss, and the decrease in horsepower required: Flow Capacity Aspect: Pressure Loss Aspect: Decreased HP Aspect: Keeping the pressures constant at the segment endpoints the segment capacity increased by 8%. Keeping the pressure at inlet constant and segment flow the pressure drop decreased by 39%. Utilizing the pressure loss aspect assumptions above the HP requirement at the downstream compressor station dropped by 3%. This resultant drop in HP required can be correlated to a direct fuel savings. Although these are illustrative cases only, they clearly demonstrate the effect of decreasing roughness in pipeline segments. It is important to note, however, that at the current time it is not known how long this increase in efficiency will persist. 1 Friction Factor Equation - The equation used for determining the pipeline friction factor known as the Colebrook-White equation takes the form: 1 f e d.51 = log Re f Where: e = Effective Surface Roughness (in.) d = Inside Pipe Diameter (in.) Re = Reynolds Number (dimension less) Fundamental Flow Equation The equation used for determining pipeline flow: T Q = D Pb b Where: 7

8 P1 = P G ( H H Z T f G T L Z f f 1 ) P a Q = Flow Rate (scf/d) T b = Temperature Base ( R) P b = Pressure Base (psia) D = Pipe Inside Diameter (in.) P 1 = Upstream Pressure (psia) P = Downstream Pressure (psia) T f = Flowing Gas Temperature ( R) L = Pipe Length (miles) P a = Average Pressure (psia) f = Friction Factor (dimensionless) H 1 = Upstream Elevation (feet) H = Downstream Elevation (feet) Z = Compressibility at Pa (dimensionless) G = Specific Gravity 8