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Posts by Samuel Haugum:
Two fullsize 75ton Deep Penetrating Anchors™ were fabricated at the Western Shipyard in Klaipeda, Lithuania and delivered at the Central Coastal Base in Ågotnes near Bergen, Norway in late July 2009. These anchors are part of a qualification program for the DPA concept for installation at the Gjøa field in the North SeaWest of Bergen off the coast of Norway. These anchors are 13m in length with four meters wide fins. Maximum pullout capacity is approximately 700tons and will be used by the mobile drilling rig unit (MODU) TransOcean Searcher for completing the wells at the Gjøa field. The anchors are designed to take larger loads than what will be realized by the present MODU, which may be operating on the field in the future where 84mm chain is manditory with MBL in excess of 700tons.
The anchors were sucessfully installed at the Gjøa field in mid August 2009 using the anchor handling vessel M/S Island Vanguard (photo below right) with less than two degrees tilt and anchor tip penetration in excess of 30m. Water depth was 360m. Maximum velocity when they hit the seabed was approximately 27m/sec (nearly 100kmh) dropped from a height of 75m above the sea floor. The sucessful installation and qualification of these anchors at the Gjøa field have subsequently resulted in an anchor solution that can now be offered to the oil and gass industry as a vialble alternative to present day solutions for anchoring of FPSOs and similar structures in soft seabed sediments. The Deep Penetrating Anchor™ was qualified by DNV in the last quarter of 2009.
TECHNOLOGY QUALIFICATION PLAN (TQP)
1:3 Scale Prototype Testing at Troll East Area 2
The Troll field test, together with the test performed in Trondheimsfjord (2003), is a part of a Technology Qualification Plan (TQP) model testing (small prototype) scale 1:3.
Two anchors of scale 1:3 were used in this testing program (12 in all). Both anchors have instrumentation: one has its instrumentation installed on the inside of the anchor while the other has external containers where the instrumentation is mounted. Each anchor has 500m of 38mm wire laid out on the seabed for later loading and retrieval.
A total of 12 anchor drop tests have been performed from different heights ranging from 15m to 75m over the seabed at the Troll Field Area 2 approximately 3.8 kilometers West of the Troll A platform. These tests comprised the following:
• 10 vertical pullout tests directly after anchor installation at t = 0 i.e. where no consolidation had taken place,
• 1 inclined load test six months after anchor installation,
• 1 vertical pullout test four months after installation and when primary consolidation was assumed complete.
While the observation from this test has shown excellent results in term of marine operation procedure, hydrodynamic behavior during drop, penetration into the seabed, and pull out capacity; probably the most important finding from the 1:3 scale tests was that the hydrodynamic stability was verified. In Trondheimsfjord the current can be extreme in relation to normal sea tidal currents. Therefore, due to the current, the anchor starts off with a rotation but quickly recovers due to the hydrodynamic design. The final anchor tilt is approximately one degree.
This can be seen in the following animation:
Small prototype testing was performed in Trondheimsfjord in August 2003. Two anchors of approximately 1/3 the expected average full size anchor i.e. 4.4m long and approximately 2.8 tons, were machined and fabricated at REINERTSEN Orkanger for testing and verification of the procedures involving prototype installation and anchor behavior during deployment and service. The test was carried out on borehole G08 in a proposed gas pipeline route through Trondheimsfjord to Skogn, about 5,5 nautical miles from Port of Trondheim.
The test program is divided into to parts i.e.
- an installation part comprising preparation for anchor launching, anchor launching and drop to seabed, and
- anchor pullout capacity as a function of drop height (penetration), consolidation time and direction of loading from chain/mooring line.
The anchor was instrumented with:
- one accelerometer for monitoring the anchor velocity changes throughout the drop and soil penetration phase in addition to determining the total penetration depth through the sediments,
- two pressure sensors for measurement of pore water dissipation with time at two points on the anchor,
- an inclinometer to determine verticality
- one depth sensor.
The tests were well carried out and satisfied its goals which are to implement and execute the proposed procedures and associated equipment for fabrication, transportation and installation of the anchor. Furthermore, the geotechnical assessment to verify the anchor penetration depth and holding capacities was conducted. The pull-out-tests were carried at time 0 (right after drop), 1 week after drop, and 2 weeks after drop, both laterally and vertically. In general, the results from the test conformed the expected capacity of such DPA.
