Negative Planning LL with PGNiG

Tomcat (6501/10-1) – Pre-Lay

What happened:

We started on anchor 4. This anchor was together with anchor 5 and 6 supposed to be installed to 100y ULS load.
We tried to set this with 50 degrees fluke angle 2 times without success.
After 2 failed attempts the fluke angle was changed to 32 degrees and re-set for the third time. This time the anchor was successfully installed.

We moved on to anchor nr 3. This anchor had to be set with a fluke angle of 50 degrees according to DIGiN. This anchor also failed installation test load 2 times. We buoyed off the anchor and decided to come back to this anchor later.

Following this, the Marine Representative contacted Odfjell Drilling and explained the challenges and also how we experienced the seabed conditions at Tomcat.

Following this, Odfjell Drilling contacted DNV and new catenary and installation calculations were mede for new fluke angles of 41 degrees. Still DIGiN requirements.

We still had difficulties after re calculations.

Marine Representative again contacted Odfjell Drilling. During this conversation, both Odfjell Drilling and PGNiG Marine Representative agreed that Tomcat location was not suitable for DIGiN and therefore decided to scrap DIGiN analysis and recalculate catenary and installation loads to reflect 100y ULS loads.

What was planned:

Installation of anchors at Tomcat was planned performed using DIGIN analysis produced by DNV.
SoW was produced by Odfjell drilling reflecting this DIGiN analysis.
DIGiN analysis sets some absolute criteria for anchor installation. Anchor fluke angles was to be set to 50 degrees mud angle and all lines was to be set with defined chain and wire lengths.

What was learned:

– Always include 100y ULS load catenary and installation loads in the SoW even if DIGiN is supposed to be used.
– When working in an area with no history of anchoring such as Tomcat, we should always be prepared for the worst case scenario, e.g bring sufficient piggy back anchors.
– Preferably use single AHV, capable of installing anchors to 100y loads.
– Even if DP operation is planned, early planning should include planning for anchoring to secure good contingency if rig schedule changes and DP operation is less favourable.
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What actions are required:

Early marine department involvement during planning face (Site Survey etc) to flag requirements for planning purposes.
Clearly specify CPT locations to reflect most possible anchor locations for most accurate data collection.

What Was Done

Catenary and test tension calculations was re-calculated 2 times.
1st re-calculation was done for fluke angle of 41 degrees
2nd re-calculation was for 100y ULS load anchor installation.

All anchors were then successfully installed to 100y ULS loads. 5 anchor lines were installed with piggy back anchors.