Subsea hydraulic control system

A system for supplying hydraulic fluid to operate subsea equipment having a fluid opening port and a fluid closing port, the system comprising:

• a subsea hydraulic fluid storage vessel for storing at least 80 gallons of water based hydraulic fluid;
• a storage vessel fluid separation member separating water from the hydraulic fluid in the fluid storage vessel while pressurizing the hydraulic fluid in response to the pressure of the water at the depth of the fluid separation member;
• a hydraulic fluid supply line extending from the surface to the fluid storage vessel for supplying hydraulic fluid to the fluid storage vessel;
• an equipment supply line fluidly interconnecting the fluid storage vessel and both the fluid opening port and the fluid closing port on the subsea equipment;
• a subsea hydraulic fluid reservoir vessel for receiving hydraulic fluid from the subsea equipment;
• a fluid exhaust line fluidly interconnecting the subsea equipment and the fluid reservoir vessel;
• a vent line extending from the fluid reservoir vessel to the surface;
• a subsea pump for exhausting fluid from the fluid reservoir vessel to the water;
• and
• a subsea control system for selectively controlling the flow of hydraulic fluid from the fluid storage vessel to the fluid opening port and the fluid closing port.

A method of hydraulically operating subsea equipment having first and second fluid input ports, the method comprising:

• storing a selected quantity of hydraulic fluid in a subsea hydraulic fluid storage vessel;
• separating water from the hydraulic fluid in the fluid storage vessel while pressurizing the hydraulic fluid in response to the pressure of the water;
• fluidly interconnecting the fluid storage vessel and the first and second input ports of the subsea equipment;
• fluidly interconnecting the subsea equipment and a subsea hydraulic fluid reservoir vessel;
• extending a vent line from the fluid reservoir vessel to the surface;
• receiving hydraulic fluid from the subsea equipment in the subsea hydraulic fluid reservoir vessel;
• and
• exhausting fluid from the fluid reservoir vessel.

A system for supplying pressurized seawater to operate subsea equipment having first and second fluid input ports, the system comprising:

• the first and the second fluid input ports on the subsea equipment being selectively in fluid communication with seawater pressurized by its hydraulic head;
• a subsea hydraulic fluid reservoir vessel for receiving seawater from the subsea equipment;
• a fluid exhaust line fluidly interconnecting the subsea equipment and the fluid reservoir vessel;
• a vent line extending from the fluid reservoir vessel to the surface;
• a subsea pump for exhausting water from the fluid reservoir vessel;
• and
• a subsea control system for selectively controlling the flow of water fluid from the subsea equipment to the fluid storage vessel.

A method of hydraulically operating subsea equipment having first and second fluid input ports, the method comprising:

• selectively exposing the first and second input ports of the subsea equipment to seawater pressurized by its hydrostatic head;
• fluidly interconnecting the subsea equipment and a subsea hydraulic fluid reservoir vessel;
• extending a vent line from the fluid reservoir vessel to the surface;
• receiving seawater from the subsea equipment in the subsea hydraulic fluid reservoir vessel;
• and
• exhausting seawater from the fluid reservoir vessel.