SABAH- SARAWAK GAS PIPELINE (SSGP) – KP373
SABAH- SARAWAK GAS PIPELINE (SSGP), KP373
The Sabah–Sarawak Gas Pipeline (SSGP) is a 512km Malaysian natural gas pipeline that links Kimanis, Sabah to Bintulu, Sarawak (90km in Sabah and 422km in Sarawak) The pipeline is part of the Petronas development project of “Sabah–Sarawak Integrated Oil and Gas Project”, and started operating early 2014. Gas is transported from the SOGT to Petronas’ LNG Complex in Bintulu, Sarawak, where it is converted into LNG
The Pipeline Inspection Gauge (PIG) identified an abnormal pipeline curvature radius from joint KP 373H33 spanning approximately 100m with a displacement of 2.1m.
The section of the pipeline was located in a swampy area deep within the rainforest. Site access was extremely challenging and access to the pipeline even more so. The location was classed as hazardous (table 841-114A).
Subsequent to GTL’s involvement the following actions/methodologies had been put in place; • Original Contractor installed 6 slender screw anchors to 8m with nylon restraint straps to accommodate uplift due to buoyancy.
- The pipeline was depressurised.
- The excavation had taken place to expose the pipeline in an attempt to relieve stress and undertake further surveys and investigations.
- Dewatering of the excavation due to the high water table and persistent rainfall, however, dewatering was undertaken inconsistently with excavation being allowed to flood.
- Longitudinal & topography surveys and inspections
- Strain gauge monitoring
“Successful engineering is all about understanding why things break or fail” Henry Petroski
The challenge was to initially determine the failure mechanism, to prevent reoccurrence and to ensure that any remedial works undertaken would not have a detrimental effect of other parts of the pipeline (upstream/downstream), in paying the block value station which was in close proximity. Our solution would need to take into consideration the poor soil quality, combination loads (axial, lateral, compression & tension), restrictions in site access, material/plant access, constructibility, whilst working in close proximity to the existing pipeline, photographs embedded within this document are a testament to the environmental conditions.
Our initial calculations showed that the pipeline was highly stressed, 93% of allowable. It was also noted that the wall thickness was The local Axial load ranged from 400kg to 1MT. Vertical Load during startup condition could reach 30MT.
The pipeline had fixity at the block value station which acted as a thrust block. We also noted the previous undertakings, as above, which we evaluated and commented on as follows;
- Works were undertaken prior to understanding the cause of the failure and without a detailed mitigation strategy.
- The assumption that the pipeline would be subject to uplift due to buoyancy was correct, however, the design calculations were incorrect, and incorporated geotechnical assumptions. The piles themselves were slender with an installation depth of only 8m. These piles were never designed to accommodate compression and lateral loads.
- Exposing the pipeline caused the excavation to flood which, without any overburden or restraint caused the pipeline to become buoyant.
- Dewatering was inconsistent allowing the excavation to flood which caused the pipeline to become buoyant subsequently the excavation was dewatered which caused the pipeline to sink. This caused the pipeline to enter a cyclic condition which caused stress, strain and fatigue on the pipeline material especially at joints/welds.
- Additionally every time the excavation was dewatered causing the poor bearing soils to dissipate as such increased curvature.
- The pipeline was never designed to be exposed which increased the thermal designed loads which in turn caused an increase in axial loads.
ENGINEERING DESIGN PROCESS
GTL’s engineering design process is a specific set of steps our engineers use to organise our ideas and refine potential solutions to engineering challenges. Embarking on an engineering design project is about gaining an understanding of all the issues surrounding the particular design challenge. These issues included environmental issues, site access, mitigation of risk to manpower and materials, health and safety etc. whilst taking into account the project constraints and requirements. Working through these non-technical contextual factors helps GTL generate useful, appropriate and a successful design solution.
To establish the geology and explore the subsoil conditions of the site GTL undertook soil investigations in accordance with BS5930 & BS 1377. These results were utilised to determine ground conditions and soil parameters which in turn enable the design of a helical piled foundation to support and restrain the pipeline. Results were also used to determine geohazards and determine the reason for failure. N values of between 2-5 were recorded to depths of up to 20m due to the soft to very soft clay with decayed wood and rock fragments, occasionally peaty.
In accordance with ASME.B31.8.2003 (Gas Transmission and Distribution Piping Systems) stress analysis was undertaken to determine how the pipeline behaved based on its material, pressure, temperature, fluid, and support. This information was used to determine load combinations and support locations.
GTL established that the pipeline was not sufficiently supported by the soils and the uplift caused by buoyancy was not causing the failure. The initial stress-relieving works carried out previously had a negative impact on the situation. Additional thermal loads with the fixity at the block value station were causing the horizontal curvature. The weak bearing soil and buoyancy previously caused the pipeline to be subject to low cyclic loads, leaving the soils surrounding the buried pipeline uncompacted which could result in serious damage to the pipeline, especially during earthquake loading. We observed many other issues however, the majority were outside of our scope. The most important recommendation was to support the pipeline immediately.
HELICAL PILED REST & GLIDE SUPPORT
Fifteen (15) individual rest & glide supports were designed in conjunction with two (2) inclined helical (screw) piles, thirty (30) piles were installed in total. Each support consisted of an insulated pipe bracket which allowed for the pipeline longitudinal expansion and eliminated any fixity. The Helical pile, also known as the screw pile, consisted of 140mm dia central hollow shaft, with four (4) helical bearing plates. The piles were rotated into the ground using GTL’s bi-directional auger attached to a 20t long arm excavation. The structure was used to position pipeline into the required position by using a full system. All design and engineering conformed to International Standards.