Mooring Spread Design Challenges for Stalk-On Riser Installations in Congested Offshore Fields

Stalk-on riser installation method is typically employed where flanged connections at the pipeline-riser interface are precluded by specific client or field requirements. Offshore field expansions frequently necessitate the installation of new subsea structures and platforms in proximity to existing ones. Often, risers for these new platforms are installed using the stalk-on method due to above mentioned stipulations. A dynamic positioning (DP) vessel is generally preferred for installation activities such as jacket installations in congested fields, due to easier maneuverability around multiple subsea assets and platforms. However, DP vessels have constraints when performing stalk-on riser installations near existing platforms, including vessel size, thruster capacity, excursion limits, operational sea state conditions etc., due to additional loads on the DP system from davit lifts. Moreover, uncertainties exists regarding DP vessel responses to thruster failures during stalk-on installations. Thus, a moored vessel with a meticulously designed mooring spread is preferred. This paper delineates the challenges encountered and solutions devised in designing and analyzing the mooring spread for one of the largest and critical stalk-on riser installations in the Arabian Gulf, adjacent to an active offshore production complex, characterized by significant external loads from pipeline lifting operations.