Malampaya Deep Water Gas-to-Power Project: Subsea Umbilical Challenge
Authors: Williams V.T. , Paton K.M. 
Published: October 2008

The Malampaya field development in the South China Sea comprises subsea wells in 820 metres water-depth producing via a subsea manifold and two 16 inch diameter inconel clad flowlines to a shallow water platform 30 km distant in 43metres water-depth. Condensate is removed on the platform and the dry gas is then transported via a 24 inch diameter, 504km long export pipeline to an onshore gas plant atTabangao (Batangas, Luzon Island) for extraction of H2S. The condensate is stored in the platform concrete gravity structure (CGS) caisson prior to export via shuttle tanker from a catenary anchor leg mooring (CALM) buoy located 3km from the platform.

Two umbilicals are installed between the Malampaya platform and the ten slot subsea production manifold. Each umbilical provides methanol injection, annulus vent service and electric and hydraulic control.

The dual umbilical solution for Malampaya is perhaps unique in that the cross sections are identical and the system design features redundancy within each umbilical and between the two umbilicals. The services are routed subsea to facilitate safe isolation and permit limited production in the event that all services in one umbilical are not available. This availability objective was driven by the high priority placed on cost effectively minimizing downtime across the entire system and the discipline to concentrate on lifecycle cost rather than capital expenditure (this subsea development is the sole source of gas to a major electrical power generation network and downtime is a major concern).

The majority of steel tubes in the cross section were required for methanol service to the subsea manifold for injection at the manifold headers and the trees. Unlike common methanol injection lines for hydrate prevention, the tubes were required to transport regenerated methanol with potential H2S carryover from the produced gas stream.

To prepare for the installation campaign in a region with minimal supporting infrastructure, contingency planning was emphasized and included an onshore full scale evaluation of vessel tensioner holding capacity in order to safely increase the operating envelope of the vessel lay spread.

The paper concentrates on the numerous challenges associated with design and logistics; material selection, the novel procurement strategy for steel tubes, delivery logistics for 768km of tubes, the extensive welding campaign and the remote installation.

Introduction

The service requirements for the Malampaya subsea umbilicals were to support the five Phase I subsea wells plus a contingency sixth well with redundancy for all services.

Table 1: Umbilical Service Requirements (SCM: Subsea Control Module)(Available in full paper)

Malampaya required dedicated methanol injection to each subsea xmas tree and both manifold headers for hydrate inhibition. Methanol is regenerated on the platform and contains up to 70ppm H2S. Annulus vent service was required for xmas tree annulus monitoring/maintenance, hydraulic service for valve control and power/communications for the subsea control system.