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Key takeaways
- An FPSO hull bends (hogging and sagging) with loading and waves, and topside modules sit on supports that let the hull move under them.
- Piping that crosses between modules, or runs along the deck, must absorb these relative displacements.
- Vessel accelerations add inertia loads, and repeated wave cycles make fatigue a design check, not an afterthought.
The deck is not fixed
A ship’s hull acts like a long beam. When the cargo tanks are full or empty, or when a wave crest passes under the middle, the hull bends:
- Hogging: the middle rises relative to the ends.
- Sagging: the middle drops relative to the ends.
The deck stretches and shortens with this bending. Topside modules are usually mounted on stools with sliding or flexible supports so the module steel doesn’t take hull strains. That means adjacent modules move relative to each other, and so does the main deck relative to the modules.
Where piping gets caught
- Inter-module piping that connects one module to the next must take the relative displacement between the two support points.
- Long runs along the main deck, such as main headers and cargo lines, see the deck strain directly.
- Lines from the topsides to the turret or hull tanks cross from one structure to another.
These lines need flexibility built into the routing: loops, offsets or changes of direction, and supports that allow the right movements. Short straight connections between modules almost always overstress.
Motion loads
Roll, pitch and heave create accelerations at every point on the topsides. In the stress model they are usually applied as static equivalent accelerations in each direction, taken from the naval architect’s motion analysis for the design return period (and for the operating, transit and survival conditions).
Typical load cases combine:
- sustained loads (weight and pressure),
- thermal expansion,
- hull deflection displacements (hogging and sagging),
- motion accelerations,
- wind and, where relevant, blast.
Fatigue
A wave arrives every few seconds, so over a 20 to 25 year field life piping sees tens of millions of motion and deflection cycles. Small-bore connections, branch welds and support attachments are classic fatigue locations. Many projects run a fatigue assessment for lines with significant cyclic displacement, using methods such as those in DNV guidance.
Code reference
ASME B31.3 Stress limits for sustained, displacement and occasional loads.
DNV-RP-D101 Structural analysis of piping systems, including offshore load cases.
NORSOK L-002 Piping layout and stress analysis requirements.
What to take back to your desk
Ask the hull and structural teams early for three numbers at each module support: hogging and sagging displacements, and design accelerations. Without them, the inter-module piping can’t be designed, and late changes there are expensive.
Educational content. Always design to your project specification.
