The Evolution of a New Mooring System for Ultra-Deep Waters By Charles N. White Strategic Offshore Solutions – Houston Fall 2001

Introduction to the Disconnectable Compliant Vertical Tension-Leg Mooring Concept The “Disco-VTL” mooring concept is based on the proven principles of compliant vertical tension-leg mooring systems. The “disconnectable” feature of a Disco-VTL mooring system means that the moored vessel/structure may be disconnected and driven (or taken) out of the path of severe weather while the elements of the mooring system left behind are submerged below the most severe wave (& wind) wind action. This means that the mooring system generally experiences much lower loads, resulting in a lower cost design than a mooring that must attempt to hold the vessel on station against the extreme loadings. This means substantially lower costs in ultra-deep waters!

“Mooring” Structures Heavy Chain w/ piles Chain w/ Drag Anchors HS Chain w/ Drag Anchors WireRope + Chain w/ Drag Anchors WireRope w/ Drag or Suction Anchors Tendons Buoyant Legs SynRope w/ Drag or Suction Anchors Compliant Vertical Tension-Leg Moorings (CVTL)

Water Depth Drives Mooring Concept Shallow Waters –All Chain Moorings –Articulated Towers Moderate –High Strength Chain Deep Waters –Chain & Wire –All Wire Systems Ultra-deep –Synthetic Fiber Rope Moorings –Compliant Vertical Tension-Leg Moorings –Dynamic Positioning (“DP” means no moorings) Italics indicates lack of “splayed” or spread mooring legs Any of these mooring systems can be designed as disconnectable systems To reduce costs

Taut-Leg Spread Moorings Drag Anchor Suction Anchor Synthetic Rope

Broken Lines have big impact on spread mooring line design as adjacent lines must pick up loads (and resist dynamic response induced) from the broken line.

Why chose a CVTL mooring and why make it disconnectable? Synthetic ropes (e.g., polyester) are considered attractive for deepwater mooring systems because they are almost buoyant. However, the reliability of the product is so low that designing to compensate for broken line conditions drives designers of spread moorings to use very large diameter, very costly ropes. The Compliant Vertical Tension-Leg mooring was conceived many years ago because it avoids the costly necessity of having to “over-design” all legs to cover the high load sharing that occurs in the legs adjacent to a broken line in a spread mooring. The CVTL can be comprised of many small diameter lines which are relatively more available, less expensive and easier to install than the large lines for spread moorings. In a CVTL mooring all lines are vertical and parallel to each other. This means all lines share the load distribution much more evenly and the breaking of an individual line does not force the platform to offset. The broken line event has little consequence on extreme loads calculated for the remaining lines in a parallel arrangement. Since the CVTL was never intended to provide tight watch-circle mooring, the cost of a CVTL can be dramatically reduced by making it a disconnectable system that only holds the vessel on station during targeted conditions. Disconnectability is an innovative wrinkle on the well-known CVTL mooring concept that brings great value for ultra-deep development (esp. to shuttle loading systems). When the Disco-VTL is disconnected, it supporting buoy drops well below the surface out of the way of extreme wave & current loads from a severe storm.

Compliant Vertical Tension-Leg Mooring CVTL Many small diameter ropes in parallel Stretch to match & resist vessel movement The stretching and angular deflection of the mooring line cluster generates a horizontal restoring force that tends to keep the vessel centered on station. Anchor

Disconnectable Compliant Vertical Tension- Leg (Disco-VTL) Mooring Disco-VTL The Disco-VTL mooring buoy is pulled well below the sea surface (until its buoyancy matches the load from the stretched mooring lines and the apparent weight of any attached flowlines), while the ship sails away to safety. The equilibrium point is usually selected at a depth accessible to divers.. The Disco-VTL buoy and mooring line designs need only be checked against operating criteria for the metocean conditions under which the vessel will be attached and the ability to survive extreme events when the vessel is not attached. Optional buoyant pendant line for recovery