Bed Wrap: Coiled Tubing Reel Disorder and Prevention

A bed wrap (also called a crossed wrap, over-wrap, or bridging wrap) is a condition where coiled tubing wound on the reel drum becomes physically disordered — an upper layer of tubing rides over, under, or between the turns of an adjacent or lower layer rather than laying smoothly in sequence against the preceding turn. On a properly wound coiled tubing reel, each revolution of tubing wraps tightly and evenly in a helical pattern beside the previous turn, building an orderly cylindrical spool of tubing from the core outward in successive concentric layers, with each layer also advancing progressively from one flange of the reel to the other in a controlled traverse. A bed wrap disrupts this ordered geometry: when the incoming tubing string fails to lay beside its predecessor and instead bridges over a gap between two turns of the lower layer, the resulting section has an irregular geometry that creates stress concentrations when the tubing is subsequently deployed from the reel. The mechanical consequences of deploying tubing through a bed wrap section include: permanent kinking or bending at the bridging crossover point where the tubing must be bent over the adjacent tube at a radius smaller than the design minimum bend radius; ovalization (collapse from circular to elliptical cross-section) at high-stress crossing points, reducing the tubing's internal pressure capacity and flow area; fatigue crack initiation at the outer surface of the kinked section; and groove wear on tubing where the higher layer repeatedly presses against the lower layer under the reel's containment forces during subsequent reeling cycles. In WCSB coiled tubing operations — which are extensively used for well stimulation, sand washing, nitrogen lift, acidizing, and remedial cementing on Viking, Cardium, Mannville, and Duvernay wells — a bed wrap discovered after a run into the wellbore requires the affected tubing section to be inspected, pressure-tested, and potentially cut out and retired from service, causing non-productive time and unplanned expenditure of CAD 15,000-50,000 depending on the extent of damage and replacement length required.

Key Takeaways

Causes and conditions that promote bed wraps: Bed wraps arise from any combination of factors that prevent the incoming tubing from being guided precisely onto the adjacent turn of the current winding layer. The primary mechanical cause is insufficient reel-in tension: when a coiled tubing string is retrieved from a well at high speed (rapid reel-in) or under low back-tension, the tubing arrives at the reel faster than the level-wind guide can position it accurately, and the tubing falls to the position of least resistance — which may be across a gap in the lower layer rather than beside the adjacent turn. Insufficient tension in the tubing between the injector head and the reel allows slack loops to form that spool onto the reel irregularly. Secondary causes include: level-wind guide malfunction (a mechanical failure of the traverse mechanism that guides the incoming tubing horizontally across the reel face between the two flanges, causing the tubing to pile in one area rather than distributing evenly); core distortion (if the reel's inner mandrel or core support structure is bent, deformed, or worn, the inner layers of tubing cannot maintain consistent geometry); damaged tubing section (a kinked, bent, or deformed section of tubing arriving at the reel causes a protrusion that prevents the next wrap from laying flat); and excessive reel speed differential between the rate of tubing return from the well and the reel drive motor speed, which can cause momentary slack that results in disordered wrapping. High ambient temperatures accelerating thermal expansion of the outer reel layer against a tight drum can also cause outer-layer tubing to buckle upward and ride over adjacent turns during a subsequent cool-down contraction cycle.
Detection methods and inspection protocols: Bed wraps are most reliably detected by visual inspection of the reel surface during and immediately after reeling operations, when the wound geometry is visible and anomalies are apparent before the reel cover (drum guard) is replaced. A bed wrap appears as a section of tubing that bridges across a gap between two adjacent turns rather than sitting in the groove between them — visible as a "hump" or elevated section on the reel surface. For preventive inspection, WCSB coiled tubing crews conduct a reel walk-down after each job, examining the outer two winding layers for visible crossovers, elevated bridges, or irregular spacing. When a bed wrap is suspected from an anomalous injector load signal during reel-in (a brief spike in reel drive motor current coincides with a bridging section arriving at the reel), the wound tubing is unwound by controlled reel-out until the suspected section passes the guide rollers, where the tubing is visually inspected and measured for ovalization using an inside micrometer or ovality gauge (maximum allowable ovalization is typically 3-5% of nominal OD per the manufacturer's specification or API RP 5C7). If ovalization exceeds the limit or if a kink is identified, that section is cut out and the ends are joined with a connector or the tubing is retired at that point.
Mechanical consequences and tubing retirement criteria: A bed wrap section that has been deformed into an ovalized or kinked geometry undergoes a step-change in fatigue life: the localized stress concentration at the deformed zone is 2-5 times higher than the nominal bending stress for the same applied load, meaning the fatigue endurance of the kinked section is 2-5 times shorter than the remainder of the string. WCSB coiled tubing strings are designed for a finite number of pressure cycles (each cycle being one deployment into a well under pressure) before fatigue failure probability reaches the acceptance threshold — typically 80-90% of the string's calculated fatigue life is consumed before the string is retired. A kinked or ovalized section within the string effectively means the full string's life calculation is invalid at that point, and the affected section must be retired (cut out) to restore the string's fatigue life integrity. API RP 5C7 (Coiled Tubing Service Life Prediction) and CTES (Coiled Tubing Engineering Solutions) fatigue models are used by WCSB CT contractors to track individual string fatigue cycles, and a bed-wrap event is recorded as a string anomaly that triggers an engineering review of the affected section's remaining fatigue capacity. Strings with bed-wrap damage within the working section (typically the innermost 800-1,500 m that is routinely deployed into wells) are usually retired at the damage point and the tail (reel end) is reused as a shorter string or for shallow applications.
Prevention: tension control systems and level-wind management: Prevention of bed wraps is primarily a tension management and level-wind maintenance challenge. Modern CT units (Hydra Rig, Stewart and Stevenson, Calfrac CT units widely used in the WCSB) incorporate closed-loop automatic reel-in tension control systems that maintain a user-set back-tension (typically 2-5 kN, specified in the manufacturer's reel-in procedure) on the tubing between the injector head and the reel regardless of reel speed — the hydraulic reel drive motor is commanded to maintain constant reel-in tension via a load cell on the guide roller, automatically adjusting motor torque as reel speed and tubing drag change. Proper level-wind adjustment ensures the traverse guide moves at exactly the right rate relative to reel rotation to advance the tubing one tube diameter per revolution — too fast and the guide skips to the next wrap position before the current wrap is complete; too slow and tubing piles up on the near-flange side. Level-wind guide rate is set by a mechanical gear ratio between the reel drive and the traverse drive on older units, and by electronic servo control on newer units, with the setting adjusted for each tubing OD size (2-3/8", 2-7/8", 3-1/2" CT is common in WCSB workover service, each requiring a different level-wind traverse rate per revolution). Pre-job reel inspection by the CT supervisor, including verification of level-wind gear setting, guide roller condition, and core support integrity, is required before every WCSB CT job to minimize bed wrap risk.
Bed wrap impact on coiled tubing fatigue life calculations: The API RP 5C7 fatigue life prediction model for coiled tubing accounts for the bending fatigue cycles imposed on the string during each deployment: the tubing bends around the reel (largest bend, constant throughout service life at the reel's core radius of typically 1.0-1.5 m), then unbends and rebends over the goose neck (a smaller radius bend at the injector head), then bends again in the wellbore curves (deviated wells). Each bend-unbend cycle at a given stress level consumes a fraction of the string's total fatigue capacity, and the fatigue life model integrates these consumption fractions over all cycles in the string's history to produce a remaining life percentage. A bed wrap adds an additional, uncontrolled high-stress bending event to the history: the kink at the bed wrap crossing point is equivalent to an instantaneous full-yield bend that consumes a disproportionate share of remaining fatigue life relative to the controlled bends over the reel and goose neck. Depending on the severity of the kink (mild ovalization consumes less life than a sharp kink approaching the tube wall's plastic limit), the fatigue life impact of a single bed wrap event ranges from 5-15% of remaining string life — a significant non-productive penalty that underscores the economic value of preventive tension and level-wind management.

Bed Wrap Discovery During a WCSB Acid Job

During a nitrogen-assisted acid cleanout operation on a Cardium oil well in the Pembina area, a 2-3/8 inch OD coiled tubing string is being retrieved from 820 m depth after completing the acid job. At approximately 340 m remaining depth (the tubing has been retrieved 480 m out of 820 m), the reel-in drive motor current spikes from its normal 18-22 amp range to 47 amps for approximately 3 seconds, then returns to normal. The CT operator reduces reel-in speed and calls the CT supervisor, who recognizes the spike as a possible bed wrap event. Reel-in is halted, and the supervisor inspects the reel surface, identifying a section of tubing approximately 1.5 reel turns in extent that is elevated approximately 35 mm above the surrounding wound layer — consistent with a bed wrap where the tubing has bridged across a gap in the lower layer under tension. The affected tubing section is deployed 3 m out of the reel (past the guide roller) for direct inspection, revealing a 40 cm section of tubing with 4.8% ovalization (OD measured at 60.45 mm versus nominal 60.33 mm — the 2-3/8 inch nominal OD of 60.33 mm with 4.8% ovalization = 63.2 mm measured OD in the minor axis, indicating the cross-section has been deformed toward elliptical). The manufacturer's maximum allowable ovalization for 2-3/8" CT is 5.0% of nominal OD, so the measured 4.8% is at the maximum boundary and requires engineering review.

The CT supervisor consults the string's fatigue record: the string has consumed 54% of its calculated fatigue life across 28 previous well runs. Adding the estimated 8% fatigue life impact of the bed wrap event (based on the ovalization severity), the remaining string life is estimated at 100% - 54% - 8% = 38%. The operator evaluates two options: (1) cut the string at the bed wrap section (losing approximately 340 m of tubing that would be left in the wellbore and recovered later, at a replacement cost of approximately CAD 18,500 for 340 m of 2-3/8" CT at CAD 54/m) or (2) continue using the string with the ovalized section, accepting the reduced remaining life and noting the anomaly in the string record. Since the well requires two more acid jobs in the coming weeks and the CT string's remaining 38% life is projected to support approximately 4-5 more jobs at this well's depth (820 m runs), the supervisor elects to continue with the string subject to an ultrasonic wall thickness measurement at the ovalized section before the next run — confirming that wall thickness has not been reduced below the minimum 4.5 mm required for the planned 45 MPa maximum operating pressure. The UT measurement shows 4.87 mm minimum wall, above the limit, and the string is returned to service with the anomaly logged in the string's operational history card, scheduled for retirement at the ovalized section after the next two runs.