  Existing cooperative game theoretic studies of bargaining power in gas
pipeline systems are based on the so called characteristic function form (CFF).
This approach is potentially misleading if some pipelines fall under regulated
third party access (TPA). TPA, which is by now the norm in the EU, obliges the
owner of a pipeline to transport gas for others, provided they pay a regulated
transport fee. From a game theoretic perspective, this institutional setting
creates so called "externalities," the description of which requires partition
function form (PFF) games. In this paper we propose a method to compute
payoffs, reflecting the power structure, for a pipeline system with regulated
TPA. The method is based on an iterative flow mechanism to determine gas flows
and transport fees for individual players and uses the recursive core and the
minimal claim function to convert the PPF game back into a CFF game, which can
be solved by standard methods. We illustrate the approach with a simple
stylized numerical example of the gas network in Central Eastern Europe with a
focus on Ukraine's power index as a major transit country.
