Explanation of auto-submergence

Auto-submergence occurs when the submergence of one floating hose causes successive adjoining hose sections to also submerge; resulting in the eventual sinking of the complete hose string. This effect is caused by the floating hose closed-cell polyethylene floatation material being compressed in response to the increasing depth. It is therefore crucial to maintain reserve buoyancy of individual hose sections at or above minimum design levels to reduce the risk of local submergence in the first place.

 

GMPHOM states that ‘fully floating hoses will have a minimum reserve buoyancy of 20% when the hose, including buoyancy material and outer cover, is fully submerged in seawater and filled with seawater (density 1025 kg/m3) or product, whichever has the higher density’ (GMPHOM 2009).’

 

Auto-submergence is rarely caused by a single event and some possible contributory factors are:

• Crossed hoses in heavy, choppy sea.
• Excessive weight on the end of the hose string such as a heavy spool piece or poorly designed heavy Marine Breakaway Coupling.
• Propeller damage to the hose floatation material.
• Uncontrolled release of the hose string from a high position such as FSO or FPSO in ballast.

The cost of hoses sinking to the seabed following auto-submergence include submerged hoses (capital equipment) having to be replaced, terminal downtime, the expense of a support vessel required to remove damaged hoses and support vessel costs in placing new hoses.

GMPHOM recommends that ‘wherever ancillary equipment is to be fitted (tether spools, Y-reducers, concentric reducers and Marine Breakaway Couplings), it should be confirmed that the overall buoyancy configuration of the hose string is not disturbed. This may require the provision of additional buoyancy for either the ancillary equipment or the hose’ (GMPHOM 2009).

 

Given this information, Gall Thomson has always ensured its MBCs are as compact and lightweight as possible. Oil companies, Single Point Mooring and FPSO contractors have standardised on the use of Gall Thomson MBCs due to their long and successful track record and their minimum weight and dimension.

 

With 2000 MBCs supplied worldwide (the vast majority in floating applications), Gall Thomson MBCs have never caused auto-submergence of a hose string – even in the 280 reported violent MBC activations due to tanker breakout and extreme pressure surge events.

 

Field experience also shows that Gall Thomson MBCs will not cause auto-submergence even when floats have been lost due to operational circumstances or theft. Simulation studies conducted by a third party has confirmed that Gall Thomson MBCs will not cause auto-submergence throughout the typical lifespan of the hoses.

The reduced weight of the new Gall Thomson Petal17 MBC minimises the risk of auto-submergence even more. Its weight in air provides 457kg for a 16” Control Double Closure Petal Valve ANSI 150lb MBC (see Figures 1 and 2).