Design and Operation of Coupled Mooring System for Tender-Assisted Drilling at Malikai TLP

Abstract

Malikai field, located offshore Malaysia at a water depth of 500 m, was brought on stream in December 2016. The field is developed using tender-assisted drilling and completions from a Tension Leg Platform (TLP) – a first in Malaysia. The TLP is a conventional 4-column, ring-pontoontype hull structure that connects to asemi-submersible type TAD vessel through nylon hawser ropes which together with chain-polyester-wiremoorings on the TAD and TLP allow for station-keeping during drilling operations. This paper describes the design, analysis and operational experience of the coupled TLP-TAD mooring system. Design of the coupled mooring-hawser system was driven by the need to not only keep TLP excursion limits for risers but to also maintain the gangway within its stroke limits. In addition, there was a need to move the TLP over short distances by winching the TLP and TAD moorings to facilitate latching of the top-tensioned risers and to maximize drilling uptime. Understanding the strongly nonlinear stiffness behavior of nylon rope was crucial in ascertaining these safety and functional requirements. Several numerical simulations were performed and were supplemented by wave-basin and rope stiffness tests during design validation. Marine instrumentation systems were incorporated on the TLP from which real time environmental data, TLP and TAD conditions, and mooring and hawser tensions are obtained. These measurements are used to calibrate numerical models for mooring advisory. Station keeping during drilling operations were managed through daily interface between offshore and onshore teams for winching plans recommendations for TLP repositioning and to predict station keeping responses based on short-term weather forecasts. Collaborative interface with the risers, wells and facility-operations teams has proved valuable in safe and effective station keeping during the drilling campaign.