Offshore mooring

John Mannisto, Supervisor Structural/Thermal Engineering Services, CD-adapco

Delmar Systems recently contracted CD-adapco to perform a design validation of the world's largest suction pile anchor system. These anchors are used for "parking" large offshore structures such as mobile drilling units.

The anchoring of a drilling rig is somewhat more sophisticated than anchoring a boat. Installing these units is equivalent to towing an eight story building out into the middle of the Gulf of Mexico, tipping it over the side of a large vessel, and burying it two miles down into the sea floor.

The structure is constructed from massive welded sections of plate steel. Internal ribbing and mounting flanges are designed to provide the necessary rigidity and strength. Not surprisingly, the shear weight of this structure makes its transport a major undertaking.

The most critical of these is the " overboarding event", when the pile is launched overboard using a sled and winch system. There is a potential for damage to the structure during deployment, when the massive weight is cantilevered off the ship.

Proper design of the sled structure and cable mounting system are essential; damage to the pile during this phase would be catastrophic.

Another "event" that requires investigation is the sinking of the structure. Suction piles drive themselves part way into the sea floor under their own weight. A remotely operated vehicle (ROV) is then used to pump the seawater from the tower, creating a vacuum that draws the tower further into the seabed. During this process, adequate internal ribbing is necessary to avoid buckling of the outer shell.

All aspects of the installation were verified with FEA analysis. Analytical models were constructed in pro-fe, the FEA pre and postprocessor supported by CD-adapco. High quality hex meshing was used for all components of the pile, to ensure good stress values at the mount points. pro-STAR’s automatic trimmed cell meshing capability was used to easily fill the soil volume of the seabed surrounding the anchor.

The structural analyses included G-loadings during transport and over-boarding. Particular emphasis was placed on the skid support structure, as well as the cable mounting system. During the initial design phase, the analysis determined that the winch attachment

needed to be redesigned for adequate margin. Additional analysis addressed pressure loadings during installation, when the ROV creates a negative pump pressure on the inside of the pile. The seabed was modeled as a bi-phasic material, using soil properties developed for that particular location. Eigenvalue buckling calculations were also performed to check susceptibility during installation.

The solver used to perform the analysis was ANSYS, a commercially available FEA code. All post-processing of the model was performed in pro-fe, which has the ability to directly access the ANSYS solution files with impressive speed.

The anchor was successfully installed earlier this year. The anchor will remain at its installed location for approximately six months to allow the soil to settle, at which time it will become active as a parking anchor for mobile drilling units.