PREVENTING CRUSHED CABLES ON DEEP LOGGING JOBS, INSTALLATION AND SPOOLING

	Español\| Home \| Our Story \| Distributors \| News \| Contact Us \|
	\| Catalog \| Manual \| Features \| Designation \| Warranty \| Calculator

CABLE DAMAGE DUE TO DRUM CRUSH

A common cause of shorts in the middle of a wireline where there is no obvious operational cause is that the cable core has been compressed or crushed by the adjoining cable wraps above and below the short. This is easily spotted by making line diameter measurements differing by 90 degrees at any point on the line near the short. An elliptical or "egg shaped" cable cross section over a region containing the short strongly suggests drum crush. The cable, and thus the core, has been compressed or crushed by the surrounding high tension cable exerting extreme forces on the low tension section, eventually resulting in electrical leakage or a short.

Normally, when a cable is spooled onto a truck or skid winch, it goes onto the winch under the tension that is exerted on the cable when it is extracted from the borehole. An approximate tension profile for a 25,000 ft. 7J46RP multi-conductor cable operated in a location with 18,000 ft wells is shown in figure 1. Such a tension profile is naturally built as the cable is extracted from a borehole. An experienced technician in a spooling shop attempts to match this profile as closely as possible as he installs a line on a winch to be used in that region.

With respect to logging cables, the higher the tension on the cable, the less likely the cable core will suffer damage from any external forces. The basic idea is demonstrated in figure 2. Tension in cable armor wires results in less armor wire deflection by forces external and perpendicular to the cable axis As is well known, the armor wires do not run parallel to the cable axis as illustrated in figure 2 but, rather spiral In a helix around the core. The tension in the wire has two components - one parallel to the cable axis, and the second encircling that axis. The component of tension parallel to the axis is the component acting to protect the core. The armor wires under tension act as a cage to protect the core. When the cage wires are pulled taut, the cage is less easily deflected by forces perpendicular to the cage. Alternately one can think of the cable surface as "hardened" under cable tension and "softened" under relaxed or low tension conditions.

Problems with cable shorts can arise when the cable tension profile is interrupted due to an operational problem (figure 3). If the cable is accidentally relaxed anytime there is a significant amount of tension on the cable, then the core is not protected by the taut armor wire cage around the core over this relaxed section of cable. The cable surface becomes "soft" due to the absence of cable tension. Experience has shown that cable crushing can occur

anytime there is a Interruption in the normal .tension profile whether the interruption occurs in a field situation with cable in the borehole, or in a spooling shop with cable going from a shipping reel through capstans onto a winch.

An intervening low tension section of cable can result in the cable being damaged at particular periodic locations over that low tension section. These locations are usually separated by a length corresponding to the circumference of the drum plus the underlying cable or some multiple of this length (figure 4). As the cable is level wound onto the winch, the cable will tend to lie in the groove formed by the two adjacent wraps in the-layer directly beneath. Each layer of cable will have a directional change or break corresponding to the flange location where the cable hops up to the next layer. Breaks on successive layers are in the opposite direction. Due to alternating break directions, there is one point in each complete wrap of the cable where the cable crosses the cable underneath it (see figure 5). These are points of very high external forces because the pressures applied are localized over a very small surface area. Thus, at any given tension the external forces will always be highest at the break or crossover point for each wrap. The core of the low tension "soft" relaxed cable section can be squeezed or crushed at these high pressure points by the adjacent high tension "hardened" cable above and below this relaxed region.

In order to prevent shorts due to "drum crush", care should be exercised every time cable spooling is halted. If for any reason tension on the cable is lowered during this halt, cable tension must be increased to the previous spooling tension before proceeding with cable spooling. In some situations, this is easier stated than accomplished.

Consider the situation where the line supports a logging tool resting at the bottom of the borehole. Suppose the line is supported at the borehole entrance. If the line goes slack between the winch and the well head, then a logging truck would have to move away from the wellhead to build tension before resuming spooling. Offshore such movement would be impractical.

As a second example, consider a shop spooling a line off a shipping reel through capstans to build tension and then onto a winch. Such a shop lacking the ability to reverse spooling direction would be in a bad situation when slack is accidentally introduced between the winch and the tensioning capstan during line installation.

Regardless of the operational situation, risk of drum crush, and thus electrical leakage or shorts, exists in situations where it is necessary to insert low tension sections of cable within an otherwise normally spooled cable.

Often the electrical leakage or short resulting from the cable crushing of the core does not appear immediately after cable compression takes place. In fact electrical problems may only appear days or weeks later. The non-symmetrical radial forces on the compressed core cause the plastic insulation to slowly migrate or cold flow. Over a period of days or weeks, if the migration is such that any copper conductor comes in contact with another conductor or the armor, the cable user then experiences a short or leakage. Thus user electrical problems often become evident days or weeks after the spooling operational problem that caused the short took place. This is unfortunate because it makes it very difficult to pinpoint exactly what situation or operation led eventually to the electrical leakage or short.

Previous Page | Print Page


Camesa, Inc · 1615 Spur 529 · P.O. Box 1048 · Rosenberg, Texas 77471 Phone: (281) 342-4494 · Fax: (281) 342-0531