Explanation of auto-submergence - Gall Thomson

Explanation of auto-submergence

Gall Thomson report

Explanation of auto-submergence

When and why auto-submergence might occur

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 consequences of auto-submergence

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.

Minimising the risk of auto-submergence

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.

Weight reduction of the Petal17 Marine Breakaway Coupling

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).

MBC Specification Weight in air (kg) Weight in water (kg) Weight immersed and full of seawater with 2 x Ultrafloats fitted (kg)
16″ Standard Controlled Double Closure ASME 150LB 457 398 -102

Figure 1. Recommended weight for 16” Petal Valve MBC

gall thomson petal17 mbc isometric gall thomson petal mbc isometric
Gall Thomson Petal17 Previous Gall Thomson MBC

Figure 2. Comparison between new Petal17 and the previous Gall Thomson MBC

Ultrafloat: the Dual Floatation Collar

The new Ultrafloat Dual Floatation Collar for a 16 inch MBC delivers an extra 102Kg of reserve buoyancy. This advantage adds to what is already the lightest MBC on the market. Stress to adjacent hoses is therefore eliminated.

Ultrafloat also negates the use of special hoses with increased reserve buoyancy adjacent to the MBC.

This extra 102Kg reserve buoyancy will prove to be useful throughout the lifetime of the terminal when adjacent floating hoses have lost some of their reserve buoyancy due to ageing, damage to the built-in hose floatation material or crossed hoses in heavy seas.

The securing mechanism of the Ultrafloat is also much improved on previous floatation options. The unique chamfered edges of Ultrafloat are designed to reduce drag and inspection windows allow the MBC to be inspected without the need to remove it from the line (see Figure 3).

marine coupling floatmarine breakaway coupling float

Figure 3. The Gall Thomson Ultrafloat Floatation Collar

The risk of auto-submergence occurring in your operation

Although auto-submergence was an identifiable risk when using MBCs, the risk of auto-submergence occurring with a Gall Thomson MBC was already minimal. The lighter weight Petal17 and the performance advantages of Ultrafloat virtually eliminates the risk of auto-submergence occurring in your operation.

Published: January 2018

All information provided in this Report was correct at the time of publishing. Gall Thomson recommends you obtain technical advice and confirmation of technical requirements before taking any actions regarding your operations or specifications.

Contact the Gall Thomson Technical Team for technical advice and assistance.

Tel:   +44 1493 857936

Fax:  +44 1493 850888

Email: support@gall-thomson.co.uk