WorldOil

Evaluation of hybrid riser towers as deepwater drill centers

As water depth increases, the risers’ loads on an oil-producing Floating Processing Unit (FPU) increase. Hybrid Riser Towers (HRTs) have been proposed to alleviate this issue for FPSOs and semisubmersibles. They generally fall into two main categories: monobore and multibore. ExxonMobil’s Kizomba Field in West Africa is an example of a monobore HRT. Total’s Girassol and Rosa Fields and BP’s Greater Plutonio development are examples of multibore HRTs. Both single-bore and multibore systems are effective. Nevertheless, improvements are necessary. In deep water, riser hydrodynamic interference becomes a problem, especially for monobore HRTs. For single-line risers or riser bundles sharing parallel paths in a cross current, the upstream riser or bundle influences the movement of the downstream riser or bundle beyond previously acknowledged distances, thereby making riser interference more acute for single-bore HRTs. Large multibore HRTs servicing large fields may lack the flexibility required by the operating company.