Malampaya DCPDESIGN FOR THE RELAXATION OF SEABED PREPARATION TOLERANCES

EMMA STEERARUP

Background

• Malampaya field, offshore Palawan,

Philippines

• Malampaya gas to power project - Shell,

Chevron, PNOC – Shell operators

• Existing platform at site – Shallow Water

Platform (SWP) – concrete gravity structure

installed in 2000

• Supplies gas to four power stations

providing ~45% of Luzon’s power needsSource: malampaya.com

Depletion Compression Platform

• Field requires depletion compression to continue operating

effectively i.e. for future expected decrease in well pressure

• Depletion Compression Platform – DCP

• Maintain flow of gas to shore at acceptable rate to end of

field life

• Topside equipment (above deck): 3500 – 5000t, +/- 2m COG

variation

ACE Platform Range

• 5 - 140m water depth range

• Up to 15,000te topsides equipment payload (equivalent to 30,000te integrated deck)

• Wellhead / Minimum Facilities Platforms

• Drilling / Workover & Production

DCP – Key Project Drivers• Form suited to local

construction capability → steel

• No special transport vessel resident in the region → Self-installing

• Ground conditions → gravity based foundation

• Resists typhoon and earthquake events

• Safety

DCP – Key Structural Elements

• Four 21 x 19 x 4m stiffened

hexagonal pad footings

linked by horizontal truss

• Four Φ 3.8m stiffened

cylindrical legs

• 62 x 42 x 7.5m stiffened

rectangular barge

Geotechnical Aspects• In-Situ Ground

Conditions• 0.5 – 6m of carbonate

sand (partially cemented)• 3 – 15m of reef limestone• 35m of calcarenite

• Relatively poor cyclic characteristics of insitusoil

• Remove and replace approach

• Material sourced locally

Geotechnical Aspects• Challenge: Eliminate

dedicated scour protection

layer i.e. size seabed

preparation to resist scour

Soil-Structure Interaction• Develop a solution to accommodate platform installation on

an undulating seabed

• Globally accommodate differential vertical levels in the

prepared seabed at footing locations

• Perform plastic analysis to assess local behavior of the

structure for a range of potential seabed undulations

• Relax tolerances for undulation shape and location – resulting

in reduced installation time

Seabed Preparation Contractor Consultation

Mound Squash – Bounding Shapes

2:1 radius to height ratio

10:1 radius to height ratio

Legend

Mound Squash cont.

Mound Squash cont.

Pad Footing Design

• Four individual hexagonal pad

footings, integral with leg

• Six compartments

• Filled with iron ore slurry to

provide weight

• Connected by pin ended

horizontal truss to

accommodate relative levels

during installation and allow

lateral load share

Pad Footing Design

• Prepared seabed includes possibility

for local mounds of finite stiffness

• Mound squash loads quantified –

associated pressure/contact area

• Local mounds apply high localised

pressure to underside of base

• Base stiffened to resist

Pad Footing Design

Radial T-stiffeners

Concentric T-stiffeners

Tubular leg

Vertical propping

columns between

top and bottom plateHorizontal T-

stiffeners on

external walls

Vertical T-

stiffeners on

bulkheads

Bottom Plate

Critical Load Cases

• Environmental and seismic in-place loading

• 10,000 year Abnormal Level Earthquake (ALE)

• Identify possible mound types

• Steep mounds (1:2 slope)

• Small gradual mounds (1:10 slope, up to 100mm)

• Large gradual mounds (1:10 slope, 100-300mm height)

• One or two discrete mounds most critical

Local EquilibriumAxial N*

Uniform lateral pressure from seismic inertia

Ballast applied as distributed pressure load

Moment M*

Shear V*

Lever arm determined from moment equilibrium

Equivalent mound reaction applied as distributed pressure over squash area

Design Cases

Case 1: Single mound

Case 2: Two mounds (opposite edges)

Case 3: Two mounds (near edge)

Case 1: Single mound

Case 2: Two mounds (opposite edges)

Case 3: Two mounds (near edge)

Case 1: Single mound

Case 2: Two mounds (opposite edges)

Case 3: Two mounds (near edge)

Case 1: Single Mound Case 2: Two mounds

(opposite edge)

Case 3: Two mounds

(near edge)

Mound Parameters

VERTICAL

FORCE (KN)

DISPLACEMENT

(MM)

AREA (M2) PRESSURE (KPA)

2506 60 5.655 443.1

8640 120 11.310 763.9

17928 180 16.965 1056.8

29437 240 22.619 1301.4

46027 300 28.274 1627.9

• 300mm height, 10H:1V

• Mound located away from edge of footing

• Vertical load and area halved for mound at edge

• Limit max pressure based on complete squash load

yLoad in-line with bulkhead

x

Squashed mound footprint

Load perpendicular to face

Squashed mound footprint

y

x

Case A - load in line with bulkhead

Case B - load perpendicular to face

Mound Parameters

yLoad in-line with bulkhead

x

Squashed mound footprint

Load perpendicular to face

Squashed mound footprint

y

x

Case A - load in line with bulkhead

Case B - load perpendicular to face

Case A: Load in-line with bulkhead Case B: Load perpendicular to face

Structural Analysis

• Linear analysis in DNV

SESAM

• Non-linear analysis in Strand7

• Plate - 2D quad-8 shell

elements

• Stiffeners – beam elements

• Fixed boundary conditions at

tubular leg

Structural Analysis

• Strength - Von Mises stress

Structural Analysis

• Plastic behaviour acceptable

• Plastic strain limits from ISO

19902 and API-RP-2A• Base plate 5%

• Compact stiffener flange/web

5%

• Non-compact stiffener

flange/web 1%

Structural Analysis

• Buckling assessment of stiffened

plates and girders using GeniE

PULS (Part 2 DNV-RP-201)

• Longitudinally stiffened panels

• Local elastic buckling and non-

linear post-buckling behaviour

• Define initial imperfections to

account for permanent plastic set

Stiffener

Girder

Axial / Longitudinal direction

Longitudinally Stiffened Panel

GeniE 'sub-panel '

Transverse direction

Secondary stiffenerLongitudinally Stiffened Panel

Transverse direction

Axial/longitudinal direction

Primary girder

GeniE sub-panel

Girder being checked

Stresses averaged over highlighted panels

Assessment for primary girder

In-plane stresses averaged over highlighted sub-panels

• In-situ material removed within nominated footprint

• Backfilled with rock fill material

• Local surface tolerances achieved

Offshore Work – Seabed Preparation

Offshore Work – Seabed Preparation

As-built Prepared Seabed

As-built Prepared Seabed

South-West Pad Footing

South-East Pad Footing

Offshore Work – Prepared Seabed

Offshore Work - DCP Installation

• DCP successfully installed

February 2015

• Self-installing system

performed as anticipated

• Barge jacked into position

within 2 days, allowing

rapid access to commence

weld-outSource: Shell Philippines Exploration B.V.

Source: Shell Philippines Exploration B.V.

Questions?