Oil Offloading Lines (OOL)

For large offshore fields in benign environments such as West Africa, a common field-development option is a large spread-moored FPSO with high production
rates and regular offloading operations which, for safety and availability reasons, are usually performed via a Single Point Mooring (SPM) terminal.

Until recently, two types of Oil Offloading Lines (OOL) were used for the ransfer of stabilized crude oil between the FPSO and the SPM: steel pipe or unbonded flexible pipe. SBM Offshore and Trelleborg have jointly developed and qualified an attractive alternative: a new type of OOL, the TRELLINE™.

The TRELLINE™ is made of a new generation of bonded rubber hoses, developed for a specific deepwater offloading application where the OOL is hanging, either in a U or lazy-W configuration, from the FPSO to the Single Point Mooring (SPM) offloading terminal. The OOL is of large ID to enable an offloading rate of 7,500 m3/hrwhich corresponds to the export of 1,000,000 barrels within 24 hours. The main objective of this development was an OOL type that could be installed by a lighter and more flexible installation vessel spread than the existing OOL types (i.e. un-bonded flexible or steel pipes).

The successful application of this OOL at the Bonga field (installed in December 2006) illustrates the flexibility in project execution offered by an OOL made of bolted flexible sections. This anticipated flexibility in offshore execution was a key factor in the TRELLINE™ selection. Performing the assembly of the individually bolted hose sections offshore allowed for the full capacity and uninterrupted production and crude oil export from the Bonga FPSO system (1,000,000 bbl offloading every 4-5 days) to the crude oil export tanker via the SBM Offshore-supplied SPM terminal.

Key Features:

• Lower pressure drop, therefore less pumping power needed on the FPSO
• Superior fatigue performance Significantly lower suspended weight and horizontal pull on the SPM, reducing its size and cost
• Repairable on site by changing one hose section, unlike the conventional OOL types

The TRELLINE™ is a bonded hose made of rubber, steel rings and reinforcement layers, end-fitted with a unique integrated gasket flange system. The TRELLINE™ comes in sections of (usually) 40 ft, which are bolted together in situ to make an OOL of the required length.

The (FPSO and SPM) end connections of the OOL are achieved through gimbal tables (double articulated uni-joints decoupling tension and bending and allowing free rotation). The export line is typically arranged in lazy wave configuration thanks to the installation of distributed buoyancy modules.

The TRELLINE™ has been developed for a typical 25-year design life. The need for gimbal tables at each extremity is a consequence of the following facts:

• The range of content specific gravity (from sea water to light oil) gives
• large variations of departure angles at the attachment points.
• The combination of the high tension and large bending at the attachment is not compatible with a service life of 25 years.

To demonstrate the 250 years fatigue life of the system, Finite Element Analysis (FEA) tools developed during the TRELLINE™ qualification program have been used to calculate the stress in any steel wires or bonding rubber layers. This FEA tool was also used to optimize the bonded structure for its service.