The Tensioned Riser Compliant Tower (1994)

ABSTRACT The Tension Riser Compliant Tower (TRCT)is a new concept in deepwater compliant towers which utilizes top-tensioned production risers instead of conventional laterally-supported drivepipes to provide individual well communication to the surface. A detailed design has been performed for 3000 foot water depth and compared with conventional designs to identify weight savings. Weight correlations have been developed for 15 and 30 well cases over a water depth range of 1350 to 3250 feet, Payload sensitivity has been examined, and detailed riser interference checks have been performed to determine minimum riser spacing. INTRODUCTION Patent applications have been filed for a new deepwater platform concept (Shell Oil Company, 1993) which provides the benefits of both conventional compliant towers and tension leg platforms. The Tensioned Riser Compliant Tower (TRCT) employs Top-tensioned well risers with surface trees inside an own tower framework to achieve breakthrough cost savings for certain field development opportunities, The TRC'T provides cm economically competitive platform alternative for deepwater development, Relative to a Compliant Piled Tower (CPT) with conventional wells, the introduction of top-tensioned riser:Eliminates well guide framingReduces environmental load on the structureProvides improve dynamic characteristics through decoupled well massPermits a "Wide-body" configuration for more advantageous fatigue characteristicsSimplifies fabrication; reduces high-cost tonnageImproves buoyancy for installationReduces installation time by eliminating conductor driving operationsReduces cathodic protection requirements Figure 1 shows a CPT and a TRCT for 3000 foot water depth. Like a CPT, wells can be pre-drilled prior to setting the TRCT. Since there are no guides in the structure, alignment tolerances are not an issue, Alternatively, wells can be drilled and complete from the deck using a conventional platform chilling rig. CONFIGURATION DEVELOPMENT A simplified design method was developed which used a closed-form solution for tower acceleration caused by on impulse-type wave load to determine level shear and moment envelopes. The technique was calibrated against detailed 3D time-domain analyses (Morrison, 1994). Input applied force was estimated from previous tower designs. The structure was defined by number of piles, size, length and location; number and size of legs; tower width; bay heights; payload; and distributed weights and masses to account for non-structural items such as joint cans, anodes, launch cradle, etc, as well as unmodelled launch framing. The following parameters were set for all water depths:Four comer legsThree piles per comer -20,000 kip maximum axial load (working)Two different well counts and payloads:15 wells -50,000 barrels per day (b/d) produce while drilling (PWD)facility30 wells -100,000 b/d PWD facility Iterations were made on pile size and length, and tower stiffness as a function of the elevations for each water depth and payload to achieve desirable performance characteristics. Members were sized for inplace load combined with hydrostatics, providing sufficient excess buoyancy for installation.