Semi-submersible/Drill Ship Riser Analysis

The benefits of using riser analysis against semi-submersible and drill ships are vast, but include the ability to reduce unnecessary costs, whilst providing valuable insight in to the operations of using a floating vessel when subjected to global dynamic loads.

Some common semi-submersible/drill ship analysis includes:

• Operating envelope analysis — to predict the limiting environmental conditions for the planned drilling campaign.
• Wave fatigue analysis — to determine the wellhead and conductor fatigue lifetimes from repeated wave loading.
• VIV fatigue assessment — to compute VIV fatigue damage of the wellhead and conductor if onerous VIV lock-on was predicted from the VIV assessment. Provide recommendations for using VIV suppression devices.
• Weak point analysis — to determine the maximum limiting rig offset that the riser can stay connected without damaging the well or rig equipment. This can be compared with a broken mooring line analysis or incorporated into emergency disconnect procedures.
• Capping stack analysis —to ensure the well is structurally acceptable with a capping stack installed, as the additional subsea equipment can increase loads on the conductor.
• Conductor axial load assessment — to verify if the surrounding soil has sufficient capacity to support the well loads. Often the soil conditions are soft and the conductor needs to be suitable designed to avoid the well subsiding (sinking).
• Conductor jetting analysis — to calculate the axial capacity of the soil during jetting and provide recommendations for when further equipment can be installed on the conductor.
• Riser hang-off analysis — to determine if the hung-off riser is acceptable during storm conditions to mitigate against costly damage to riser joints.
• Installation analysis —to find the maximum allowable environmental conditions for installing the conductor, surface casing and riser to minimise potential rig downtime waiting on weather, providing significant cost savings to the drilling operations.
• Cement optimisation analysis — to determine the lowest acceptable cement top elevation. This can reduce risk to drilling operations and reduce costs by avoiding the need for costly equipment and rig time to remedy cement issues.
• Soil sensitivity analysis — to determine the influence of the soil properties on the results and conclusions. Gauging the impact of the soil profile has previously enabled operators to better understand their risk and reduce operational costs.