Polyester synthetic mooring lines are used widely in deepwater moorings for MODU (Mobile Offshore Drilling Units). However, practical and technical limitations in the deployment of polyester tethers beyond water depths of 2,000m for short-term MODU moorings has prompted the development of alternative synthetic mooring materials. This paper describes the application of Low Creep, High Modulus Polyethylene (HMPE) mooring systems for deepwater MODUs.
HMPE has many practical offshore handling benefits compared with polyester. The ropes are lighter, easier to handle and have a smaller diameter than comparable polyester ropes for the same minimum breaking load (MBL). From a technical perspective, the neutral buoyancy of HMPE, low creep rate and stiffer rope properties are ideal for ultra deepwater mooring systems. In addition, HMPE ropes are easier and quicker to deploy than polyester, leading to operational cost savings when moving MODU rigs between locations for drilling and work over.
Mooring system storm survivability is of increasing concern to operators. Hybrid mooring lines containing individual sections of rope made with HMPE and polyester yarns have been developed and shown to comply with the latest American Petroleum Institute (API) requirements. These hybrid lines combine the attributes of polyester and HMPE enabling naval architects to more accurately engineer the mooring systems to better suit prevailing offshore and storm conditions.