Page 304 - Pipeline Pigging Technology

Cable-operated and self-contained ultrasonic pigs

CABLE-OPERATED AND
SELF-CONTAINED ULTRASONIC PIGS

IN ORDER to establish the integrity of ageing pipelines, intelligent pigging
has become of increasing interest. For several decades, pigs> using magnetic
stray flux were the only tools available for this purpose on the market. The
need for more accurate tools was an incentive to develop ultrasonic systems
to measure metal loss.
This paper provides an overview of special ultrasonic pigging systems and
methods. Conventional cable-operated ultrasonic field-proven tools for dis-
tances up to 2000m are described, as well as those using long glass-fibre cables
up to 6000m in length.
Such tools can be propelled either by reversible wheel-driven crawlers, or
by differential pressure, as applied for self-contained intelligent pig propul-
sion. Self-contained liquid-propelled intelligent pigs are used for on-stream
inspection of pipelines; a field-tested system (RPIT) to inspect riser pipes is
also described.

INTRODUCTION

Long-distance pipelines are often equipped with launch and receive traps
to operate cleaning pigs; most of these traps are long enough also to handle
intelligent pigs. Propulsion of such is by the pumped liquid.
Short pipelines, most of the time, are not provided with traps; if such lines
are on land, and local excavation is possible, spot checks may be sufficient to
ensure their integrity.
For short offshore pipelines, which are often weight-coated with concrete
and buried, inspection from the outside is impractical, and is prohibited by
the costs involved. In this case, inspection from the inside seems more
practical; this also can provide information over the full length, and not just
as spot checks. A typical example is the off-loading line illustrated in Fig.l.

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