15. Termination of mandate and substitution of arbitrator.- (1) In addition to the circumstances referred to in section 13 or section 14, the mandate of an arbitrator shall terminate- (a) where he withdraws from office for any reason; or (b) by or pursuant to agreement of the parties.

(2) Where the mandate of an arbitrator terminates, a substitute arbitrator shall be appointed according to the rules that were applicable to the appointment of the arbitrator being replaced.

(3) Unless otherwise agreed by the parties, where an arbitrator is replaced under subsection (2), any hearings previously held may be repeated at the discretion of the arbitral tribunal.

(4) Unless otherwise agreed by the parties, an order or ruling of the arbitral tribunal made prior to the replacement of an arbitrator under this section shall not be invalid solely because there has been a change in the composition of the arbitral tribunal.