We consider the problem of conditioning the Brownian excursion to have a fixed time average over the interval [0,1] and we study an associated stochastic partial differential equation with reflection at 0 and with the constraint of conservation of the space average. The equation is driven by the derivative in space of a space-time white noise and contains a double Laplacian in the drift. Due to the lack of the maximum principle for the double Laplacian, the standard techniques based on the penalization method do not yield existence of a solution.
Lorenzo Zambotti. "A conservative evolution of the Brownian excursion." Electron. J. Probab. 13 1096 - 1119, 2008. https://doi.org/10.1214/EJP.v13-525