Drone view of a single buoy mooring and a loading line.

| Case Study

Full Distance Loading Line Inspection

No more blind spots

A client engaged ROSEN for gauging, cleaning, and corrosion inspection of two parallel offshore pipelines – 10” and 24” – running from a platform to subsea Pipeline End Manifolds (PLEMs), and from there to a shared Single-Point Mooring Buoy (SPM).

The challenge

Both PLEMs had a short 20” section. In the 10” line, tools could drop into the oversized section, lose seal, and become unrecoverable. In the 24” line, the 20” reducer posed a risk of tool damage or jamming.

Schematic illustration of two parallel offshore pipelines running from a platform to subsea Pipeline End Manifolds (PLEMs), and from there to a shared Single-Point Mooring Buoy (SPM)

Figure 1: Schematic illustration of the assets.

Typically, bi-directional (bidi) inspection tools running toward critical PLEMs are either stopped at a safe distance – leaving part of the line uninspected – or monitored via subsea antennas, which require divers or remotely operated vehicles (ROVs). Subsea activities were not feasible and tethered inspection tools were ruled out due to the need for extensive cleaning. Leaving a section uninspected was no option either, as loading lines are particularly susceptible to localized corrosive environments due to limited maintenance pigging, potential product contamination, and intermittent stagnant flow, allowing water and solids to settle especially at low points which are often at the end of these lines (compare with figure 2). These factors made maximizing inspection coverage a top priority.

The main challenges identified were:

PLEMs not designed for inline tool entry
Inspection distance to be maximized
Subsea activities to be avoided
High levels of expected debris
No traps at either end of both lines
Tight time constraints due to a shutdown window

Graph of features accumulation at low point of a pipeline

Figure 2: Features accumulation at low point of a pipeline

Our solution

ROSEN identified access points topside and provided temporary piping to create a “pump loop” by connecting the parallel lines on both the SPM and the platform side. Bidi-capable tools were planned to be propelled by treated seawater toward the PLEMs and returned to the platform. A much greater concern was how to stop the tools entering the 20” sections.

For the 10” line, a careful pipeline assessment uncovered an interesting detail: There were bends with varying radii throughout the line, with the only 1.5D bend located inside of the PLEM. All other bends were of higher radius. ROSEN engineered tools specifically designed to become intentionally lodged in 1.5D bends, effectively using it as a natural stopping point (see figure 3 and 4). Despite the challenge, these tools were also built to safely traverse the heavy wall 3.0D bends on the platform’s topside piping, all while staying within the Maximum Allowable Operating Pressure (MAOP) limits of the temporary equipment. This approach turned a potential obstacle into a controlled endpoint, enabling full inspection coverage without compromising safety or tool integrity.

Pump tests including stress and durability tests (up to 16 bar differential pressure) confirmed the tools could safely enter and exit the bend without damage, thus ensuring operational safety at all times throughout the runs. The team also determined the flow rate at which a pressure increase would signal tool arrival at the endpoint – triggering flow reversal. A similar strategy was applied to the 24” line, using the 24”x20” reducer as the designated endpoint. In addition to that, the 24” tools were subject to extensive testing to ensure they perform exactly as intended and ensure maximum risk mitigation.

Tool stopper with subsequent return operation.

Figure 3: Tool „stopper” in 1.5D bend with subsequent return operation

Figure 4: Tool passing 3.0D bend

Your benefit

By combining attention to detail with creative problem-solving, ROSEN delivered a unique one-off solution that enabled a successful inspection under challenging conditions. The advantages of this approach are reflected in the following key outcomes:

Efficiency
No subsea pipeline modifications, no divers or ROVs, and no offshore teams waiting for transmitter signals as the tools approach the PLEM. A single team operated from the platform side without compromising inspection goals.

Safe & Simple
Tailored tool designs mitigate key risks through engineering controls. Procedures remained straightforward: approach the PLEM at a defined flow rate, monitor for pressure increase, stop, and reverse flow.

Full Inspection Distance
Both pipelines were inspected from the platform all the way to – and slightly beyond – the PLEM flange.