B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

1.4.4 Gas Hydrates Formation: Long Tiebacks 
Long tiebacks are commonly used in order to minimise capital and operational 
costs associated with offshore and deepwater production. A major cost in the 
development of an offshore field is the cost of the offshore platform. 
In offshore developments, selecting the location of the production platform is 
based on minimising the number of platforms and the length of subsea 
pipelines. Many marginal fields are only economical if they can be tied-back to 
an existing platform. 
Obviously, this has the extra benefit of extending the life of an existing 
platform. In most cases these platforms have extra capacity due to a reduction 
in their own reservoir production. Therefore, it is possible to negotiate 
reasonable tariffs which increase the platform and pipeline income (and delays 
their eventual decommissioning). 
From a flow assurance viewpoint, long tiebacks mean longer exposure to low 
seawater temperature for the untreated reservoir fluids. Also it means higher 
pipeline pressure for transportation of reservoir fluids (sometimes it is 
necessary to install a subsea booster pump and/or an electrical submersible 

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TOPIC 1: Gas Hydrates 
 
 
 
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pump to provide the necessary upstream pressure) – both of these result in 
favourable conditions for gas hydrate formation. 
It is known that kinetic inhibitors, to be discussed later, can prevent gas 
hydrate formation for a certain time limit under a maximum degree of 
subcooling at the system pressure. 
However, industrial experience shows that kinetic inhibitors have limited 
applications in some long tiebacks, as the pressure, temperature and (more 
importantly) the residence time are not suitable for existing inhibitors. 
Considerable research is underway to design new kinetic inhibitors for such 
applications. 
Furthermore high water cuts, potential liquid hold-ups and general remoteness 
of long tieback systems limit the applicability of thermodynamic inhibitors. In 
addition, the difficulties associated with removing a gas hydrate plug (in long 
tieback systems) increases flow assurance requirements. 
In some locations, due to prevailing ocean currents, the system temperature 
could drop below zero. In this situation, there is also a chance of ice formation 
in the pipeline. In addition, wax formation can further complicate matters. Salt 
and scale formation could also occur due to extreme operating conditions. 
Avoiding gas hydrate problems in long tieback is a new challenge in the oil and 
gas industry. Some potential options are: 
1. Implement downhole and/or subsea water separation. As discussed 
this would have a positive effect on minimising the amount of gas 
hydrates that can form – together with inhibitor injection and low water 
hold up, these factors increase gas hydrate transportability. 
2. Develop new low-dosage hydrate inhibitors with the aim of increasing 
the retention time, pressure range and degree of subcooling. 
3. Prevent temperature reduction – pipeline insulation, electrical heating 
and the use of tube bundles should be considered. 
4. Use smart inhibitor injection – monitor pipeline conditions and 
implement automatic inhibitor injection at certain locations, as deemed 
necessary.