SubseaDesign has performed an evaluation of Wellhead Load Relief (WLR) system for the Northern Lights project. The purpose was to find a feasible WLR system with an acceptable WLR efficiency to safely perform drilling operations on an existing well. Traditional tether lines were evaluated but could not meet the acceptance criteria. Instead, a HSBT (High Stiffness Buoyant Tether) system  was introduced.

The HSBT is a flexible tether with sections of high stiffness. This tether solution will increase the stiffness several times compared to existing WLR tether application and at the same time the flexibility of a wire/rope will be partly maintained.

The HSBT can be used in a WLR tether system where wells offer high bending stiffness in combination with long tether lines to achieve a high WLR efficiency. It can also be used to increase WLR efficiency on normal well stiffness wells when this is required.