THE HELICOPTER HAD SVRL INFLT ENG DECELERATIONS THAT MAINTENANCE EFFORTS DID NOT ISOLATE. AFTER REPLACING THE ENG FUEL CONTROLLER, A TEST FLT WAS MADE (FROM AN OIL DRILLING PLATFORM). DRG THE FLT, THE ENG SURGED, THEN DECELERATED. THE PLT WAS UNABLE TO REACH THE HELIPAD. HE ELECTED TO LAND ON THE PLATFORM, SHORT OF THE HELIPAD, RATHER THAN ACCEPT SEA CONDS OF 8 FT WAVES. DRG THE LANDING, THE HELICOPTER WAS DAMAGED. A POST-ACDNT EXAM REVEALED FUEL CONTAMINATION HAD OCCURRED. CONTAMINATION HAD BYPASSED FILTERS IN THE FUEL SYS & HAD CLOGGED & COLLAPSED THE ENG FUEL NOZZLE SCREEN. THE AIRFRAME FUEL FILTER CAP BYPASS VLV WAS FND STUCK OPEN IN THE BYPASS MODE. A PIN (CONNECTED TO THE FUEL FILTER CAP IMPENDING BYPASS TEST BUTTON) WAS FND BENT, HOLDING THE BYPASS VLV OPEN. AN OPEN BYPASS VLV SHOULD HAVE ILLUMINATED A RESPECTIVE ANNUNCIATOR LIGHT IN THE COCKPIT; HOWEVER, WHEN CHECKED, THE ANNUNCIATOR LIGHT OPERD ONLY INTERMITTENTLY DUE TO A FAULTYCOMPONENT IN THE SYS. DRG TESTS, THE CLOGGED/COLLAPSED FUEL SCREEN PREVENTED SUFFICIENT FUEL FLOW TO SUSTAIN ENG POWER.