Landing String Analysis (including DST & SLS)
A landing string is used to run a Subsea Test Tree (SSTT) for well test (DST) or completion operations. The SSTT is run on a landing string inside the marine riser and is installed so that the SSTT spans the BOP with critical joints aligned to the BOP rams.
For well test operations the SSTT is normally run on a 3” landing string and is landed on the wellhead with an adjustable hanger which supports the weight of the test string. For completion operations the larger SSTT is typically run on a 7” landing string and is attached to the Tubing Hanger Running Tool (THRT) which lands in the wellhead or Horizontal Tree and supports the weight of the completion string. The diameter of the SSTT is relatively large, especially for completion operations. The 18.5” SSTT has to be run inside the 18.75” marine riser system and so there is very little clearance. This makes running operations difficult especially when the large diameter SSTT spans the upper and lower flex joints.
During running operations, it is very important to maintain low flex joint angles to ensure smooth passage of the equipment. Once the SSTT is run, a pressure test is performed and for completion operations this is generally done prior to landing the THRT to avoid damage to the Tubing Hanger seals if the test is unsuccessful and the Tubing Hanger has to be retrieved. Operations can be significantly limited for this condition because of the high tension resulting from the full suspended weight of the completion and the landing string. During landed and latched operations the flex joint angle limits can be relaxed a little, however, during a planned unlatch operation the Latch tool has to be disconnected and pulled through the lower flex joint. This can also result in restrictive angle limits and these must be maintained during all connected operations to allow a disconnect sequence to be performed at any time.
A summary of the typical challenges the Landing string system can face during operations are presented below:
- Maintaining Restrictive Flex Joint Angular Limits during:
> Running / Retrieval Operations while the SSTT spans the flex joints
- Stroke Limits due to:
> Compensator Stroke Out
> Available Stick Up of Landing String Surface Equipment
> Up Stroke of the Uppermost Landing String Connector
- Determination of Adequate Fatigue Life:
> Damage from wave and vessel motion
> Damage from Vortex Induced Vibration
- Accidental Events:
> Single Mooring Line Failure or DP Drift off
> Excessive Tension during Lock-Up of the crown block compensator
> Instability of landing string due to loss of tension
> Partial Loss of Marine Riser Tension
AS Mosley performs global riser analysis of the marine riser and landing string to determine the operating limits and fatigue life of the system. The operating limits are rationalised to ensure that stroke out will never occur even during accidental vessel excursions caused by a mooring line failure or drift off. Furthermore, the rationalised envelopes will accommodate the partial loss of marine riser tension.
Operating limits are typically established for the following design conditions:
- Running / Retrieval
- Pressure Test Prior to Landing
- Setdown prior to latching
- Landed and Latched Operations
- Storm Hangoff
> Unlatched landing string
> Disconnected landing string and marine riser
- Overpull to Remove Stuck Completion
AS Mosley performs detailed finite element analysis to determine accurate flex joint angle limits for all restrictive conditions. This is beyond normal global analysis, however, determining accurate angle limits ensures maximum operability which has a direct impact on savings in terms of costs and time.
For more information on Landing String Analysis, please contact us directly.