System for supplemental tensioning for enhanced platform design and related methods

A riser management system for maintaining a selected range of tension on a plurality of risers extending between subsea well equipment and a moored floating vessel, the riser management system comprising:

• a plurality of buoyancy cans operably coupled to a plurality of tensioner units, each of the plurality of buoyancy cans operably coupled together to form a mono-buoyancy can platform configured to be at least partially submerged and positioned within and circumscribed by substantial portions of a passageway extending through submerged portions of a hull of a moored floating vessel to provide tension to each a plurality of risers extending therethrough and between subsea well equipment and the moored floating vessel to collectively compensate for a relative vertical offset between the plurality of risers and the floating vessel during sustained operational employment of the riser management system, the relative vertical offset defining a first factor, the mono-buoyancy can platform substantially laterally constrained within at least the substantial portions of the passageway extending through the submerged portions of the hull of the moored floating vessel, the mono-buoyancy can platform being at least substantially decoupled from the hull of the moored floating vessel to allow independent vertical movement therewithin;
• and
• the plurality of tensioner units each connected to a top portion of a separate one of the plurality of risers to separately adjust stroke position and to separately provide tension within a selected range of tension values to each separate one of the plurality of risers to compensate for differential effects of one or more additional factors other than the first factor on one or more of the plurality of risers during sustained operational employment of the riser management system,
• each of the plurality of tensioner units comprising a plurality of cylinders having a first end portion operably coupled to a riser connector for a respective one of the plurality of risers and a second end portion operably coupled to the mono-buoyancy can platform,
• the plurality of risers each extending through interstitial space between the plurality of buoyancy cans of the mono-buoyancy can platform and operably coupled to and a respective different one of the plurality of tensioner units,
• each of the plurality of risers being de-coupled from movement of the floating vessel through coupling with the mono-buoyancy can platform via the corresponding plurality of tensioner units,
• the mono-buoyancy can platform and the plurality of tensioner units being configured to simultaneously provide tensioning responsive to a change in the relative vertical offset in conjunction with a change in the one or more additional factors.

A method of maintaining a selected range of tension on a plurality of risers extending between subsea well equipment and a moored floating vessel, the method comprising the steps of:

• coupling a plurality of risers to a corresponding different one of a plurality of tensioner units each configured to individually adjust stroke length in response to movement of the respective riser associated therewith in relation to a mono-buoyancy can platform during sustained operational employment thereof, the plurality of risers free to move independently within an internal structure of the mono-buoyancy can platform;
• coupling the plurality of tensioner units to the mono-buoyancy can platform, the mono-buoyancy can platform adapted to maintain tension on the plurality of risers within a certain range of tension values via the coupling with the plurality of tensioner units, the mono-buoyancy can platform positioned within and circumscribed by substantial portions of a passageway extending through submerged portions of a hull of a moored floating vessel, the mono-buoyancy can substantially laterally constrained within at least the substantial portions of the passageway extending through the submerged portions of the hull of the moored floating vessel, the mono-buoyancy can being decoupled from the vessel so as to allow vessel movement relative to a position of the mono-buoyancy can platform;
• and
• maintaining tension applied to each of the plurality of risers within a preselected range of tension during sustained operational employment thereof, tension being applied by a combination of both the plurality of tensioner units and the mono-buoyancy can platform via the coupling with the plurality of tensioner units to thereby both collectively account for relative vertical offset between the plurality of risers and the vessel and to separately account for differential effects of one or more additional factors on one or more of the plurality of risers.